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Spectroscopic Evaluation of Mixing and Crystallinity of Fullerenes in Bulk Heterojunctions
Advanced Functional Materials
Abstract
The microstructure of blend films of conjugated polymer and fullerene, especially the degree of mixing and crystallization, impacts the performance of organic photovoltaic devices considerably. Mixing and crystallization affect device performance in different ways. These phenomena are not easy to screen using traditional methods such as imaging. In this paper, the amorphous regiorandom poly(3-hexylthiophene) is blended with the potentially crystalline fullerene [6,6]-phenyl-C61-butyric acid methyl ester PCBM and the amorphous bis-adduct. First, the degree of mixing of polymer: fullerene blends is evaluated using UV–Vis absorption, steady-state and ultra-fast photoluminescence spectroscopy. The blue-shift of the polymer emission and absorption onset are used in combination with the saturation of the polymer emission decay time upon fullerene addition in order to infer the onset of aggregation of the blends. Second, the crystallinity of the fullerene is probed using variable angle spectroscopic ellipsometry (VASE), electroluminescence and photoluminescence spectroscopy. It is shown that the red-shift of charge transfer emission in the case of PCBM based blends cannot be explained solely by a variation of optical dielectric constant as probed by VASE. A combination of optical spectroscopy techniques, therefore, allows to probe the degree of mixing and can also distinguish between aggregation and crystallization of fullerenes.
... 12,26 Further, device studies in parallel with Raman 27 or spectroscopic ellipsometry measurements 1 have shown that the increase in poly-3-hexylthiophene (P3HT) polymer order achieved during P3HT:PCBM blend thermal annealing is correlated with an increase in device PCE. Optical measurements including luminescence have also shown a direct correlation between fullerene crystallization and device performance, 25 while other studies have shown that fullerene domains affect the ordering of the polymer component in polymer:PCBM and related the composition-dependent device performance with the percolation threshold in the fullerene phase. 28 Optical techniques such as absorption, photoluminescence (PL), Raman spectroscopy, and spectroscopic ellipsometry have also proved valuable as probes of the material, composition, and process-dependent microstructure of polymer:molecule blends. ...
... 28 Optical techniques such as absorption, photoluminescence (PL), Raman spectroscopy, and spectroscopic ellipsometry have also proved valuable as probes of the material, composition, and process-dependent microstructure of polymer:molecule blends. 1,25,28 Recently, the introduction of so-called nonfullerene acceptors (NFAs), which offer highly tunable redox potentials and strong optical absorption in the visible range, has advanced the power conversion efficiency (PCE) of BHJ devices. 29 In particular, calamatic acceptor−donor−acceptor structures such as O-IDTBR and ITIC appear to produce a favorable microstructure when mixed with commonly used polymers. ...
The performance of photovoltaic devices based on blends of conjugated polymers with non-fullerene acceptors depends upon the phase behaviour and microstructure of the binary, which in turn depends on the chemical structures of the molecular components and the blend composition. We investigate the correlation between molecular structure, composition, phase behaviour and device performance of a model system comprising semi-crystalline poly-3-hexylthiophene (P3HT) as the donor polymer and three non-fullerene acceptors, two of which (O-IDTBR/EH-IDTBR) have a planar core with different side-chains, and one (O-IDFBR) has a twisted core. We combine differential scanning calorimetry with optical measurements including UV-Vis, photoluminescence, spectroscopic ellipsometry and Raman, and photovoltaic device performance measurements, all at varying blend composition. For P3HT:IDTBR blends, the crystallinity of polymer and acceptor are preserved over a wide composition range and the blend displays a eutectic phase behaviour, with the optimum solar cell composition lying close to the eutectic. For P3HT:IDFBR blends, increasing acceptor content disrupts the polymer crystallinity, and the optimum device composition appears to be limited by polymer connectivity rather than being linked to the eutectic. The optical probes allow us to probe both the crystalline and amorphous phases, clearly revealing the compositions at which component mixing disrupts crystallinity.
... 40 The CT state EL also red-shifts with the addition of extra PCBM and is red-shifted from the excitonic neat polymer EL. 16,39 An analogous composition dependent CT state EL red shift has been commonly observed, 15 which has been suggested to be caused by the stabilization of the PCBM's LUMO upon aggregation, as well as a rise of the film's dielectric constant. 41 These data clearly indicate that PL and EL sample the distribution of CT states in these blends differently. The same behavior has been observed previously, 17 for example for AnE-PV (anthracene-containing poly(p-phenylene-ethynylene)-alt-poly(p-phenylene-vinyl ene) co-polymers 42 and as discussed further below. ...
... We remark briefly on the comparison of CT state PL and EL spectra. In comparison with CT PL, the CT EL from the PCDTBT:PCBM blends ( Figure 2) is red-shifted from the CT PL by at least > 0.1 eV, and shows a distinct red shift in peak emission energy as PCBM content increases, both as previously reported for several other systems, 15,17,41 (although recently reported EL and PL spectra of PCPDTBT:PCBM devices overlapped in energy suggesting material specific behavior). 54 The results presented here can be explained by the fact that in CT EL injected carriers populate the lowest available energy interfacial CT states in the density of states (DoS) of the conduction and valence bands, and that the electron states will tend to shift to lower energy with increasing PCBM content as PCBM domains enlarge. ...
Despite performance improvements of organic photovoltaics, the mechanism of photoinduced electron-hole separation at organic donor-acceptor interfaces remains poorly understood. Inconclusive experimental and theoretical results have produced contradictory models for electron-hole separation, in which the role of interfacial charge-transfer (CT) states is unclear with one model identifying them as limiting separation and another as readily dissociating. Here, polymer-fullerene blends with contrasting photocurrent properties and enthalpic offsets driving separation were studied. By modifying composition, film structures were varied from consisting of molecularly-mixed polymer-fullerene domains to consisting of both molecularly-mixed and fullerene domains. Transient absorption spectroscopy revealed that CT state dissociation generating separated electron-hole pairs is only efficient in the high energy offset blend with fullerene domains. In all other blends (with low offset or predominantly molecularly-mixed domains), nanosecond geminate electron-hole recombination is observed revealing the importance of spatially-localized electron-hole pairs (bound CT states) in the electron-hole dynamics. A two-dimensional lattice exciton model was used to simulate the excited state spectrum of a model system as a function of microstructure and energy offset. The results could reproduce the main features of experimental electroluminescence spectra indicating that electron-hole pairs become less bound and more spatially-separated upon increasing energy offset and fullerene domain density. Differences between electroluminescence and photoluminescence spectra could be explained by CT photoluminescence being dominated by more bound states, reflecting geminate recombination processes, whilst CT electroluminescence preferentially probes less bound CT states that escape geminate recombination. These results suggest that apparently contradictory studies on electron-hole separation can be explained by the presence of both bound and unbound CT states in the same film, as a result of a range of interface structures.
... Although we measured blends with 50 wt % of PCBM, in this study, the simulated amorphous mixture of P3HT:PCBM contains 20 wt % of PCBM to take into account the finite miscibility of PCBM with P3HT, the other 80 wt % of PCBM being considered as crystalline. The miscibility limit of PCBM in P3HT has been estimated to be between 10 and 25 wt % by various groups, 2,9,13,14 and the model of the microstructure comprising solely a neat polymer and a neat fullerene phase has been ruled out experimentally in favor of a three-phase model with the fullerene being mixed with the polymer in the amorphous domain. 2 Phase separation is not achievable within accessible time scales using fully atomistic MD simulations; thus, the signal is reconstructed from the following three phases studied separately: crystalline P3HT, crystalline PCBM, and the amorphous mixture leading to an overall PCBM loading of 50 wt %. ...
... As mentioned previously, in the simulation, we consider separately the three phases (crystalline P3HT, crystalline PCBM, and an amorphous mixture of P3HT:PCBM) of the blend with experimental results for a blend of 1:1 weight ratio. The crystalline phase of P3HT accounts for about 20% by volume of the P3HT in the blend, 24 the miscibility of PCBM with P3HT is about 20 wt %, 2,9,13,14 and the rest consists of crystalline PCBM. For the blend of P3HT:PCBM with a 1:1 weight ratio considered here, this means that the crystalline P3HT phase accounts for 10 wt %, the amorphous P3HT phase accounts for 40 wt %, the amorphous PCBM phase accounts for 10 wt %, and the crystalline PCBM phase accounts for 40 wt %. ...
The optoelectronic properties of blends of conjugated polymers and small molecules are likely to be affected by the molecular dynamics of the active layer components. We study the dynamics of regio-regular poly(3-hexylthiophene) (P3HT): phenyl-C61-butyric acid methyl ester (PCBM) blends using molecular dynamics (MD) simulation on time scales up to 50 ns and in a temperature range of 250-360K. First, we compare the MD results with quasi-elastic neutron scattering (QENS) measurements. Experiment and simulation give evidence of the vitrification of P3HT upon blending, and the plasticization of PCBM by P3HT. Second, we reconstruct the QENS signal based on the independent simulations of the three phases constituting the complex microstructure of such blends. Finally, we found that P3HT wrap itself around PCBM in the amorphous mixture of P3HT and PCBM; this molecular interaction between P3HT and PCBM is likely to be responsible for the observed frustration of P3HT.
... 4. Interband transitions and the band gap [87]. 5. Surface and interfacial roughness [88]. 6. Crystallinity [89,90]. 7. Inhomogeneity [91,92]. ...
The ongoing development of nanofabrication capabilities ever increases our ability to exploit the light-matter interactions occurring on the nanoscale, promising radical breakthroughs in many technological sectors. This research field, namely plasmonics, faces a major roadblock in respect to realistic applications. Specifically, to translate the potential of plasmonics into practical optical devices, the right set of materials are required; materials that can overcome the loss issue and expand the operational window towards the infrared and (IR) near-IR (NIR) spectral ranges. Transparent conductive oxides (TCOs) are appealing alternatives due to their transparency, refractory character, and maturity in electronic devices. Importantly, their non-stoichiometric character allows for the modulation of their optoelectronic properties through the manipulation of their defects. This research developed the knowledge of the optoelectronic properties of TCOs, via spectroscopic ellipsometry (SE) while examining high-throughput methods to tune their transport properties towards the requirements of IR plasmonics. Specifically, this research investigated, for the first time, the utility of reactive laser annealing (ReLA) to probe the crystal structure and donor state variations of sputtered tin-doped indium oxide (ITO) thin films. The demonstration of ReLA comprised an investigation of the role of the laser processing parameters (e.g., laser fluence, number of pulses and ambient composition) with an extensive characterisation methodology comprising: four-point probe, Hall Effect, X-ray diffractometry, energy-dispersive X-ray spectroscopy and X-ray photon spectroscopy. In this way, ReLA is established as a novel and versatile tool to tailor the properties of TCO films towards the requirements of potential plasmonic applications. Furthermore, SE in the wide spectral range between 0.034−6.5 eV was employed to get insights on the properties of seed TCO films and the laser-induced modifications, revealing how and why TCOs present challenges both for further elucidating the still not fully known mechanisms that govern their optoelectronic behaviour and for their integration into far-IR plasmonic devices.
... These values are within the range found by means of other techniques. 1,4,25,26 Furthermore, we found that as expected and supported by other techniques, the miscibility is increasing slightly with temperature. 1 The observed difference in the miscibility limits obtained at 296 K for h-RRa-P3HT:h-PCBM and d-RRa-P3HT:h-PCBM can be attributed to factors like deuteration, difference in molecular weight of the two polymers and the difference in regioregularity ( Table 1). The same batch of materials have been studied in depth previously, and the reader is referred to ref. 27 and references therein for more details about the polymers. ...
We present a neutron spectroscopy based method to study quantitatively the partial miscibility and phase behaviour of an organic photovoltaic active layer made of conjugated polymer:small molecule blends, presently illustrated with the...
... A similar reduction of PCBM absorption has been noted for annealed blends of poly(3hexylthiophene) (P3HT) with PCBM [14]. The effect is possibly related to enhanced long-range ordering in the neat PCBM regions [15]. Rise in PCBM absorption below 400 nm in films with 1 wt. ...
Organometallic halide perovskite based solar cells are considered as the foundation of future photovol-
taic technology. In these types of solar cells, it has been emphasized that the bulk heterojunction active
layer architecture may show superior performance than the bilayer active layer architecture due to the in-
crease in the interfacial area by intermixing both donor and acceptor phases in the bulk heterojunction.
Organometallic halide perovskite with suitable acceptor in bulk heterojunction architecture can be a prom-
ising active layer in perovskite solar cells. Conventionally, the perovskite and acceptor are mixed together
in a single solvent before thin film formation. Though this offers a one-step synthesis way, limited solubili-
ty of perovskite and acceptor in single solvent puts major constraint on the formation of bulk heterojunc-
tion through one-step solution processable method. This paper describes a new way of one-step synthesis of
bulk heterojunction using surfactant free microemulsion in slot die method, which removes the constraint
of limited solubility of the two phases in a single solvent. Emulsion of DMSO (solvent for CH3NH3PbI3) and
cyclohexane (solvent for PCBM) stabilized with acetone was used for making perovskite:fullerene bulk het-
erojunction. Solvent evaporation dynamics has been simulated to get deeper understanding of emulsion so-
lidification leading to bulk heterojunction formation. Structural and optical studies support the formation
of bulk heterojunction for efficient charge separation at donor:acceptor interfaces. A perovskite solar cell
employing this bulk heterojunction has also been reported.
... www.advancedsciencenews.com of fullerene crystallites has been associated with: i) enhanced charge generation due to the presence of an energetic cascade between mixed amorphous domains and pure crystallites, as well as ii) enhanced charge transport due to the improved charge mobility in crystalline domains. [104][105][106] Similarly, enhancing the crystallinity of nonfullerene acceptors has been associated with improved OPV performance in some systems, such as those involving ITIC or IDTBR acceptors. [71,107] However, the tendency of some acceptors to aggregate excessively induces undesirable large-scale phase separation in OPV blends, as in the case of some PDI acceptors. ...
Organic semiconductors require an energetic offset in order to photogenerate free charge carriers efficiently, owing to their inability to effectively screen charges. This is vitally important in order to achieve high power conversion efficiencies in organic solar cells. Early heterojunction‐based solar cells were limited to relatively modest efficiencies (<4%) owing to limitations such as poor exciton dissociation, limited photon harvesting, and high recombination losses. The development of the bulk heterojunction (BHJ) has significantly overcome these issues, resulting in dramatic improvements in organic photovoltaic performance, now exceeding 18% power conversion efficiencies. Here, the design and engineering strategies used to develop the optimal bulk heterojunction for solar‐cell, photodetector, and photocatalytic applications are discussed. Additionally, the thermodynamic driving forces in the creation and stability of the bulk heterojunction are presented, along with underlying photophysics in these blends. Finally, new opportunities to apply the knowledge accrued from BHJ solar cells to generate free charges for use in promising new applications are discussed.
... In this regard, the morphology of the active layers has an essential influence on the device performance [8][9][10][11][12]. So as to tackle the morphological matters, some approaches have been introduced including cross-linking [13,14], electrical annealing [15,16], mixed solvents [17,18], additives [19,20], functionalized derivatives [21,22], molecular weight manipulation [23], regioregularity [24], active layer thickness [25], etc. Among several multi-choices, the pre-developed ordered structures could be considered as best candidates. ...
Semiglobular and globular nanostructures were designed by conducting the isothermal crystallization on dilute solutions of Y-type poly(ethylene glycol) (PEG)-b-polythiophene (PTh)2 and PEG-b-poly(3-dodecyl thiophene) (PDDTh)2 copolymers. The semiglob(PEG-star-PTh) (6–25 nm) and glob(PEG-star-PDDTh) (5–16 nm) nanostructures were then employed as the morphology compatibilizers in active layers of poly(3-hexylthiophene) (P3HT):phenyl-C71-butyric acid methyl ester (PC71BM) solar cells. The incorporation of pre-designed glob(PEG-star-PDDTh) (11.25 mA/cm², 55%, 0.66 V, 8.7 × 10⁻⁵ cm²/V.s and 7.9 × 10⁻⁴ cm²/V.s) and semiglob(PEG-star-PTh) (13.03 mA/cm², 60%, 0.69 V, 2.1 × 10⁻³ cm²/V.s and 1.5 × 10⁻² cm²/V.s) nanostructures significantly improved the morphological and photovoltaic characteristics, leading to the efficacies of 4.08 and 5.39%, respectively. The ordered assemblies of polythiophenes, in particular the PTh backbones without any side chains, were capable of controlling the morphology in a better manner compared with the individual polymer chains. Although the PEG-b-(PDDTh)2 (8.7 ns) and PEG-b-(PTh)2 (9.8 ns) block copolymers increased the life time, the pre-designed glob(PEG-star-PDDTh) (14.5 ns) and semiglob(PEG-star-PTh) (19.1 ns) scrolled nanostructures further affected the reduction of charge trap states. The P3HT:PC71BM:semiglob(PEG-star-PTh) systems also demonstrated the lower charge transfer resistance (Rtr = 158 Ω cm²) with respect to the glob(PEG-star-PDDTh) based devices (309 Ω cm²).
... Experimental measurement of ϵ ∞ by ellipsometry for the PCBM molecule was done by Guilbert et al. 51 Their result of 3.4 is in good agreement with our obtained value of 3.57. For C 60 , there are two ellipsometry measurements with somewhat different values (3.6, ...
The low efficiency of organic photovoltaic devices has often been attributed to the strong Coulombic interactions between the electron and hole, impeding the charge separation process. Recently, it has been argued that by increasing the dielectric constant of materials used in OPVs, this strong interaction could be screened. In this work, we report the application of periodic density functional theory together with the coupled perturbed Kohn-Sham method to calculate the electronic contribution to the dielectric constant for fullerene C60 derivatives, a ubiquitous class of molecules in the field of organic photovoltaics. The results show good agreement with experimental data when available, and also reveal an important undesirable outcome when manipulating the side chain to maximize the static dielectric constant: In all cases, the electronic contribution to the dielectric constant decreases as the side chain increases in size. This information should encourage both theoreticians and experimentalists to further investigate the relevance of contributions to the dielectric constant from slower processes like vibrations and dipolar reorientations for facilitating the charge separation, because electronically, enlarging the side chain of conventional fullerene derivatives only lowers the dielectric constant and consequently, their electronic dielectric constant are upper-bound by the one of C60.
... As reported previously, this result is consistent with PC 61 BM molecules being less aggregated in the lower wt % blend films. 49,50 The PS film has a negligible absorbance over this spectral range. Panels b and c of Figure 1 compare the photobleaching of a neat PC 61 BM film and a 50 wt % PC 61 BM film as a function of photoaging time in ambient air under AM1.5 irradiation over 1950 min (32.5h). ...
The photochemistry and stability of fullerene films is found to be strongly dependent upon film nanomorphology. In particular, PC61BM blend films, dispersed with polystyrene, are found to be more susceptible to photobleaching in air than the more aggregated neat films. This enhanced photobleaching correlated with increased oxygen quenching of PC61BM triplet states, and the appearance of a carbonyl FTIR absorption band indicative of fullerene oxidation. sug-gesting PC61BM photo-oxidation is primarily due to triplet-mediated singlet oxygen generation. PC61BM films were observed to undergo photo-oxidation in air for even modest (≤ 40 mins) irradiation times, degrading electron mobility substantially, indicative of electron trap formation. This conclusion is supported by observation of red shifts in photo- and electro-luminescence with photo-oxidation, shown to be in agreement with time-dependent density functional theory calculations of defect generation. These results provide important implications on the environmental stability of PC61BM-based films and devices.
... Poly(3-hexylthiophene-2,5-diyl) P3HT.-P3HT is a conjugate polymer in which the orientation of the polymer chains can be easily controlled 3 and can form stable composites when mixed, for instance, with fullerenes. 19 P3HT has attracted attention from the thermoelectric community because there were different approaches suggesting that by optimizing the p-doping or mixing it with CNTs it will be competitive with other thermoelectric polymers. Nevertheless, the obtained power factors were too small and showed that a new approach was necessary. ...
In this review article we present the most relevant outcomes of the joint project "Tailoring Electronic and Phononic Properties of Nanomaterials: Towards Improved Thermoelectricity (nanoTHERM)", a Spanish research Consolider project focused on the understanding of the thermoelectric materials and the tailoring of both their electronic and phononic properties toward the optimization of the efficiency of thermoelectric devices working at low and high temperatures.
... [15b] Finally, there is a noticeable reduction in the relative absorption of PCBM below 450 nm in the predominantly phaseseparated blends (Fig. 2, phase morphology D), although they contain the same amount of fullerene as the fully intercalated samples with phase morphology A. This is possibly related to enhanced long-range ordering in the neat PCBM regions. [16] ...
Organic solar cells consist of thin films combining an electron donor (often a conjugated polymer) with an electron acceptor (often a fullerene derivative), in a blend commonly referred to as bulk heterojunction material. Charge separation between the donor and the acceptor leads to
the generation of carriers, which can be extracted from photovoltaic devices in the form of photocurrent. The generation mechanism of free, extractable charges has caused a lot of controversial discussion in literature. Our research has shown that all the steps involved in charge generation are strongly dependent on the arrangement of the donor and the acceptor (i.e. the structure) of the bulk heterojunction.
... Such studies cannot evaluate the role of intermolecular delocalisation on charge separation. The concept of delocalised states is relevant to a number of studies that have shown that the well established redshift of CT state emission with increasing fullerene content in organic solar cells, 18,[83][84][85] can be explained partly in terms of fullerene crystallisation. Higher ratios of fullerene have also been shown to correlate with higher charge generation yield, and with a larger photocurrent and PCE in devices. ...
Efficient charge pair generation is observed in many organic photovoltaic (OPV) heterojunctions, despite nominal electron-hole binding energies which greatly exceed the average thermal energy. Empirically, the efficiency of this process appears to be related to the choice of donor and acceptor materials, the resulting sequence of excited state energy levels and the structure of the interface. In order to establish a suitable physical model for the process, a range of different theoretical studies have addressed the nature and energies of the interfacial states, the energetic profile close to the heterojunction and the dynamics of excited state transitions. In this paper we review recent developments underpinning the theory of charge pair generation and phenomena, focussing on electronic structure calculations, electrostatic models and approaches to excited state dynamics. We discuss the remaining challenges in achieving a predictive approach to charge generation efficiency.
Polymer-fullerene blends based on poly(3-hexylthiophene-2,5-diyl) (P3HT) and phenyl-C61-butyric-acid methyl ester (PCBM) have been extensively studied as promising bulk heterojunction materials for organic semiconductor devices with improved performance. In these donor–acceptor systems where the bulk morphology plays a crucial role, the generation and subsequent decay mechanisms of photoexcitation species are still not completely understood. In this work, we use femtosecond transient absorption spectroscopy to investigate P3HT:PCBM diodes under the influence of applied static electric fields in comparison to P3HT:PCBM thin films. At the same time, we try to present a detailed overview about work already done on these donor–acceptor systems. The excited state dynamics obtained at 638 nm from P3HT:PCBM thin films are found to be similar to those observed earlier in neat P3HT films, while those obtained in the P3HT:PCBM devices are affected by field-induced exciton dissociation, resulting not only in comparatively slower decay dynamics, but also in bimolecular deactivation processes. External electric fields are expected to enhance charge generation in the investigated P3HT:PCBM devices by dissociating excitons and loosely bound intermediate species like polaron pairs (PPs) and charge transfer (CT) excitons, which can already dissociate only due to the intrinsic fields at the donor–acceptor interfaces. Our results clearly establish the formation of PP-like transient species different from CT excitons in the P3HT:PCBM devices as a result of a field-induced diffusion-controlled exciton dissociation process. We find that the loosely bound transient species formed in this way also are reduced in part via a bimolecular annihilation process resulting in charge loss in typical donor–acceptor P3HT:PCBM bulk heterojunction semiconductor devices, which is a rather interesting finding important for a better understanding of the performance of these devices.
Morphological and photovoltaic stabilities of poly(3‐hexylthiophene) (P3HT):phenyl‐C61‐butyric acid methyl ester (PC71BM) solar cells were investigated in pristine and modified states. To this end, four types of patterned/assembled nanostructures, namely reduced graphene oxide (rGO)‐g‐poly(3‐dodecylthiophene)/P3HT patched‐like pattern, rGO–polythiophene/P3HT/PC71BM nanofiber, rGO‐g‐P3HT/P3HT cake‐like pattern and supra(polyaniline (PANI)‐g‐rGO/P3HT), were designed on the basis of rGO and various conjugated polymers. Intermediately covered rGO nanosheets by P3HT crystals (supra(PANI‐g‐rGO/P3HT)) performed better than sparsely (patched‐like pattern) and fully (cake‐like pattern) covered ones in P3HT:PC71BM solar cell systems. Supra(PANI‐g‐rGO/P3HT) nanohybrids largely phase‐separated in active layers (root mean square = 0.88 nm) and also led to the highest performance (power conversion efficiency of 5.74%). The photovoltaic characteristics demonstrated decreasing trends during air aging for all devices, but with distinct slopes. The steepest decreasing plots were obtained for the unmodified P3HT:PC71BM devices (from 1.77% to 0.28%). The two supramolecules with the most ordered structures, that is, cake‐like pattern (10.12 mA cm⁻², 51%, 0.58 V, 2.2 × 10⁻⁶ cm² V⁻¹ s⁻¹, 4.3 × 10⁻⁵ cm² V⁻¹ s⁻¹, 0.69 nm and 2.99%) and supra(PANI‐g‐rGO/P3HT) (12.51 mA cm⁻², 57%, 0.63 V, 1.2 × 10⁻⁵ cm² V⁻¹ s⁻¹, 3.4 × 10⁻⁴ cm² V⁻¹ s⁻¹, 0.82 nm and 4.49%), strongly retained morphological and photovoltaic stabilities in P3HT:PC71BM devices after 1 month of air aging. According to the morphological, optical, photovoltaic and electrochemical results, the supra(PANI‐g‐rGO/P3HT) nanohybrid was the best candidate for stabilizing P3HT:PC71BM solar cells. © 2020 Society of Chemical Industry
Strong electron-phonon coupling leads to polaron localisation in molecular semiconductor materials and influences charge transport, but is expensive to calculate atomistically. Here, we propose a simple and efficient model to determine the energy and spatial extent of polaron states within a coarse-grained representation of a disordered molecular film. We calculate the electronic structure of the molecular assembly using a tight-binding Hamiltonian and determine the polaron state self-consistently by perturbing the site energies by the dielectric response of the surrounding medium to the charge. When applied to fullerene derivatives, the method shows that polarons extend over multiple molecules in C60, but localise on single molecules in higher adducts of phenylC61-butyric-acid-methyl-ester (PCBM) due to packing disorder and the polar side chains. In PCBM, polarons localise on single molecules only when energetic disorder is included or when the fullerene is dispersed in a blend. The method helps to establish when a hopping transport model is justified.
Processing additives are commonly used to optimize morphology and power conversion efficiency (PCE) in the bulk heterojunction (BHJ) organic photovoltaics (OPVs), however their exact effects on morphology are not well understood. The bulk-heterojunction OPV system with the model bithiophene imide-benzodithiophene copolymer (PBTIBDT): phenyl-C71-butyric-acid-methyl ester (PC71BM) exhibits a maximum PCE of 5.4% with addition of 3.0 vol% 1,8-diiodooctane (DIO). The effects of increasing DIO concentrations (0 → 5 vol%) on the PBTIBDT:PC71BM solutions and thin films are studied by several X-ray scattering methodologies. As the concentration of DIO is increased, the radius of gyration of the PC71BM aggregates falls from 17 Å to 9 Å in solution, and TEM indicates the formation of increasing PC71BM charge percolation pathways in the thin BHJ films. Increased PBTIBDT + PC71BM intermixing not only affects BHJ film charge transport, but also enhanced the initial exciton splitting yield in the PBTIBDT cation population as measured by fs transient absorption spectroscopy. In contrast, the hole carrier (as presented by the cation in the polymer) population detected several nanoseconds after the photoexcitation is greatest with 3.0 vol% DIO, agreeing well with the corresponding BHJ composition for the highest OPV short circuit current density (Jsc) and fill factor (FF). The increase in initial cation lifetime with the DIO concentration is attributed to enhanced donor-acceptor interfacial area while the increase in long-lived cation population is attributed to formation of a bicontinuous donor polymer - PC71BM acceptor network that promotes large spatial separation of free charges in the device active layer. The results demonstrate the importance of OPV function on the correct balance, as tuned by processing additives, between a high initial donor cation formation yield and high carrier transport efficiency with minimized charge recombination rate.
In this chapter we will start by briefly summarizing the basic concepts of the electronic structure of conjugated polymers. This will enable the discussion of the relevant descriptions of the dielectric function. We will relate these descriptions to the model parameterizations which are used in advanced ellipsometric analysis of thin films such as those used in devices for organic photovoltaics (OPVs) and light emitting diodes (OLEDs). Amongst other things, such parametric descriptions are useful to deal with structural changes in conjugated polymer thin films. Once the models are presented, we will provide representative examples of the nexus between morphology and optical constants, and how the latter can be employed to infer aspects of the former. First, we will discuss how chain conformation affects the optical properties. Then, we will explain the anisotropic behavior of conjugated polymer films due to their intrinsic molecular anisotropy and review different cases (f. i., oriented films or semicrystalline polymers). We will also describe structural changes that occur upon blending polymers with fullerenes and concomitant variations of the optical properties. Here we will focus on state of the art low band gap polymers mixed with fullerenes. Finally, real-time ellipsometric experiments in which these structure-property relationships can be exploited will be presented.
Electronic polarisation contributes to the electronic landscape as seen by separating charges in organic materials. The nature of electronic polarisation depends on the polarisability, density, and arrangement of polarisable molecules. In this paper, we introduce a microscopic, coarse-grained model in which we treat each molecule as a polarisable site, and use an array of such polarisable dipoles to calculate the electric field and associated energy of any arrangement of charges in the medium. The model incorporates chemical structure via the molecular polarisability and molecular packing patterns via the structure of the array. We use this model to calculate energetics of charge separation in crystalline fullerene lattices of different chemical and crystal structures. We calculate mean dielectric constants in good quantitative agreement with those measured experimentally in C$_{60}$ and phenyl-C$_{61}$-butyric acid methyl ester (PCBM), but find significant differences in dielectric constant depending on packing and on direction of separation, which we rationalise in terms of density of polarisable fullerene cages in regions of high field. In general, we find lattices containing molecules of more isotropic polarisability tensors exhibit higher dielectric constants. By exploring several model systems we conclude that differences in molecular polarisability (and therefore, chemical structure) appear to be less important than differences in molecular packing and separation direction in determining the energetic landscape for charge separation. We propose that the model could be used to design molecular systems for effective electronic screening.
A key challenge in achieving control over photocurrent generation by bulk-heterojunction organic solar cells is understanding how the morphology of the active layer impacts charge separation and in particu-lar the separation dynamics within molecularly-intermixed donor-acceptor domains versus the dynamics between phase-segregated domains. This paper addresses this issue by studying blends and devic-es of the amorphous silicon-indacenodithiophene polymer SiIDT-DTBT and the acceptor PC70BM. By changing the blend composition, we modulate the size and density of the pure and intermixed domains on the nanometre lengthscale. Laser spectroscopic studies show that these changes in morphology cor-relate quantitatively with the changes in charge separation dynamics on the nanosecond timescale, and with device photocurrent densities. At low fullerene compositions, where only a single, molecularly in-termixed polymer-fullerene phase is observed, photoexcitation results in a ~30% charge loss from gem-inate polaron pair recombination, which is further studied via light intensity experiments showing that the radius of the polaron pairs in the intermixed phase is 3-5 nm. At high fullerene compositions (≥ 67%), where the intermixed domains are 1-3 nm and the pure fullerene phases reach ~4 nm, the geminate recombination is suppressed by the reduction of the intermixed phase making the fullerene domains accessible for electron escape.
Distinct multi-thermal treatments comprising cycling, aging, and seeding were introduced to prepare very thick bulk heterojunction (BHJ) active layers (ca. 800 nm) of poly(3-hexylthiophene) (P3HT):phenyl-C71-butyric acid methyl ester (PC71BM) photovoltaic cells. To this end, various P3HT48800-based rod-coil block copolymers having the coily blocks of polystyrene (PS), poly(methyl methacrylate) (PMMA), and poly(ethylene glycol) (PEG) were synthesized. The grazing incidence X-ray scattering, field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM) analyses proved that the dielectric coily blocks, which were excluded from the P3HT crystalline structure, accumulated on the crystals surface without decreasing the crystal quality and formed hairy crystals. The multi-thermal techniques facilitated stacking of the growth planes in π-π direction for the P3HT crystals, thereby, this dimension was improved from 5 to 27 nm for conventionally prepared BHJs to 53–265 nm for multi-thermally developed ones. The hydrophobic coily blocks were capable of neutralizing the influence of the PCBM molecules presence in the growth environment, which resulted in the larger P3HT crystals in a similar condition. By switching the conventional spin coating approach to the cycling, aging, and seeding methods, the P3HT crystals and the PCBM clusters were gradually coarsened and the respective d-spacings decreased. This trend enhanced the hole mobility (=8.8×10−5 cm2/Vs), electron mobility (=2.5×10−3 cm2/Vs), short circuit current density (Jsc=12.02 mA/cm2), fill factor (FF=69%), and power conversion efficiency (PCE=4.39%) up to the maximum values for seeding approach. Moreover, the higher percentages of face-on orientation were detected in the BHJs with lower d-spacings in the hexyl side chain direction. Hairy P3HT48800-b-PS crystals developed by seeding method possessed the highest face-on orientation (~5.5%).
We report the impact of the ternary solution phase behaviour on the film morphology and crystallization of a model polymer:fullerene system. We employ UV-Vis absorption spectroscopy, combined with sequential filtration and dilution, to establish the phase diagram for regio-random poly(3-hexylthiophene-2,5-diyl) and phenyl-C61-butyric acid methyl ester (PCBM) in chlorobenzene. Films are systematically cast from one- and two-phase regions decoupling homogeneous and heterogenous nucleation, and the role of pre-formed aggregates from solutions. Increasing annealing temperature from 120 to 200 °C reveals a highly non-monotonic nucleation profile with a maximum at 170 °C, while the crystal growth rate increases monotonically. UV ozonolysis is employed to vary substrate energy, and found to increase nucleation rate and to promote a binary crystallization process. As previously found, exposure to light, under an inert atmosphere, effectively suppresses homogeneous nucleation; however, it has a considerably smaller effect on heterogeneous nucleation, either from solution aggregates or substrate-driven. Our results establish a quantitative link between solution thermodynamics, crystallization and provide insight into morphological design based on processing parameters in a proxy organic photovoltaic system.
The poly(3-hexylthiophene) (P3HT)-based block copolymers were synthesized with hydrophilic and hydrophobic dielectric blocks including poly(ethylene glycol) (PEG), poly(methyl methacrylate) (PMMA), and poly(styrene) (PS), and employed as morphology modifiers (10-80 wt%) in P3HT: phenyl-C71-butyric acid methyl ester (PC71BM) bulk heterojunction (BHJ) solar cells. The advantages and drawbacks of the coily blocks were also outlined from perspective of photovoltaic characteristics. A novel matrix-bridged disperses model was proposed to map out well-connected BHJ networks, which was consistent with the scattering data. Rod-coil block copolymers simultaneously controlled some main properties, i.e., crystallinity, d-spacing, domain sizes, and stability. Hydrophobic-based block copolymers (P3HT-b-PS and P3HT-b-PMMA) increased crystallinity, P3HT crystallite size, and PC71BM cluster size and decreased d-spacings. This reflected larger and denser P3HT crystallites and coarser and more packed PC71BM clusters. P3HT crystallites grew up to ∼ 60 and 30 nm in (100) and (020) directions, respectively. In 30 wt% of P3HT7150-b-PS, PC71BM clusters were also the largest (= 38.87 nm). A fully interdigitated hexyl chains (10.05 Å) was acquired in 80 wt% of P3HT7150-b-PS. Impressing trends of the P3HT-b-PMMA copolymers resembled that of P3HT-b-PS ones, but with a smoother slope. In contrast, hydrophilic-based block copolymers (P3HT-b-PEG and P3HT-b-PEG-b-P3HT) resulted in finer and looser P3HT crystallites and PC71BM clusters. The largest d-spacings in the directions of the hexyl side chains (= 19.97 Å) and π-π stacking (= 4.95 Å) were detected in 80 wt% of P3HT7150-b-PEG750. The lowest (= 3.51 Å) and highest (= 5.63 Å) d-spacings for PC71BM clusters appeared in 80 wt% of P3HT7150-b-PS and P3HT7150-b-PEG750, respectively. Unlike PEG-based block copolymers, in P3HT-b-PS and P3HT-b-PMMA ones, lower P3HT molecular weights conduced to higher power conversion efficiencies (PCE = 4.43%). The short current density (Jsc = 11.68 mA/cm²), open circuit voltage (Voc = 0.63 V), and fill factor (FF = 63%) were maximizd in well-modified photovoltaic devices.
We report a systematic study into the effects of cyano substitution on the electron accepting ability of the common acceptor 4,7-bis(thiophen-2-yl)-2,1,3-benzothiadiazole (DTBT). We describe the synthesis of DTBT monomers with either zero, 1 or 2 cyano groups on the BT unit, and their corresponding co-polymers with the electron rich donor dithienogermole (DTG). The presence of the cyano group is found to have a strong influence on the optoelectronic properties of the resulting donor-acceptor polymers, with the optical band gap red-shifting by approximately 0.15 eV per cyano substituent. We find that the polymer electron affinity is significantly increased by ca. 0.25 eV upon addition of each cyano group, whilst the ionization potential is less strongly affected, increasing by less than 0.1 eV per cyano substituent. In organic photovoltaic (OPV) devices power conversion efficiencies (PCE) are almost doubled from around 3.5% for the unsubstituted BT polymer to over 6.5% for the mono-cyano substituted BT polymer However, the PCE drops to less than 1% for the di-cyano substituted BT polymer. These differences are mainly related to differences in the photocurrent, which varies by one order of magnitude between the best (1CN) and worst devices (2CN). The origin of this variation in photocurrent was investigated by studying the charge generation properties of the photoac-tive polymer:fullerene blends using fluorescence and transient absorption spectroscopic techniques. These measure-ments revealed that the improved photocurrent of 1CN in comparison to 0CN was due to improved light harvesting properties whilst maintaining a high exciton dissociation yield. The addition of one cyano group to the BT unit opti-mized the position of the polymer LUMO level closer to that of the electron acceptor PC71BM, such that the polymer’s light harvesting properties were improved without sacrificing either exciton dissociation yield or device VOC. We also identify that the drop in performance for the 2CN polymer is caused by very limited yields of electron transfer from the polymer to the fullerene LUMO levels, likely caused by poor orbital energy level alignment with the fullerene acceptor (PC71BM). This work highlights the impact that small changes in chemical structure can have on the optoelectronic and device properties of semiconducting polymer. In particular this work highlights the effect of LUMO-LUMO offset on the excited state dynamics of polymer:fullerene blends.
Intermolecular charge transfer (CT) states at the interface between electrondonating (D) and electron-accepting (A) materials in organic thin films are characterized by absorption and emission bands within the optical gap of the interfacing materials. CT states efficiently generate charge carriers for some D-A combinations, and others show high fluorescence quantum efficiencies. These properties are exploited in organic solar cells, photodetectors, and light-emitting diodes. This review summarizes experimental and theoretical work on the electronic structure and interfacial energy landscape at condensed matter D-A interfaces. Recent findings on photogeneration and recombination of free charge carriers via CT states are discussed, and relations between CT state properties and optoelectronic device parameters are clarified. Expected final online publication date for the Annual Review of Physical Chemistry Volume 67 is April 01, 2016. Please see http://www.annualreviews.org/catalog/pubdates.aspx for revised estimates.
To fulfill the requirement of energy sustainability, organic solar cells (OSCs) should combine the use of solar energy as a renewable energy source with materials that guarantee a clean energy solution based on an evaluation of the full lifecycle from cradle to grave. In this study, we investigated the possibility of using combustion-produced nanoparticles (CPNs) in blends with regioregular poly (3-hexylthiophene) (P3HT), as active layer in OSCs. We present a detailed characterization of the CPNs using Raman spectroscopy, light absorption, cyclic voltammetry and differential mobility analysis, showing their optimal features of band gap and size for use as electron acceptor materials. Moreover we present a spectroscopic investigation of the P3HT:CNPs blends, using static and dynamic fluorescence and transient absorption spectroscopy. P3HT exhibits strong quenching and shortened fluorescence lifetimes when mixed with CNPs. Interestingly, an efficient charge transfer was observed when a high loading of CNPs was added to the blend.
Conjugated polymers may be used as thermoelectric materials due to their low thermal conductivity and
have the advantageous characteristics of conventional polymers, such as low weight, non-toxicity and
low cost. Here, a detailed investigation into the thermoelectric properties of PCDTBT films is reported.
Moreover, in order to improve the thermoelectric properties of this polymer, FeCl3 is used as a doping
agent. For the most optimally doped film reported in this work, a power factor value of 24 mW m�1 K�2 is
obtained at 150 C. The different films were characterized by wide-angle X-ray scattering (WAXS)
experiments at different temperatures. In order to see the temperature effect, the thermoelectric power
factor is measured as a function of temperature from (from RT to 150 C). Thermal conductivity at room
temperature is calculated with two independent methods which give values in agreement within the
margin of uncertainty. The results obtained show promise and give insight to motivate future
investigation into these types of carbazole derivates.
Conjugated polymers may be used as thermoelectric materials
due to their low thermal conductivity and have the
advantageous characteristics of conventional polymers, such
as low weight, non-toxicity and low cost. Here, a detailed
investigation into the thermoelectric properties of PCDTBT
films is reported. Moreover, in order to improve the
thermoelectric properties of this polymer, FeCl3 is used as a
doping agent. For the most optimally doped film reported in
this work, a power factor value of 24 μW m-1 K-2 is obtained at
150 °C. The different films were characterized by Wide-angle
X-ray scattering (WAXS) experiments at different
temperatures. In order to see the temperature effect, the
thermoelectric power factor is measured as a function of
temperature from (from RT to 150 °C). Thermal conductivity at
room temperature is calculated with two independent
methods which give values in agreement within the margin of
uncertainty. The results obtained show promise and give
insight to motivate future investigation into these types of
carbazole derivates.
Polymer solar cells are fabricated with systematic variation of the phase purity. Photovoltaic tests demonstrate that devices with ≈10% of mixed phases outperform pure phase devices. Photophysical studies reveal the effects of mixed phase on charge generation and recombination. These results show a promising strategy for the optimization of organic electronic materials.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
We reveal some of the key mechanisms during charge generation in polymer:fullerene blends exploiting our well-defined understanding of the microstructures obtained in pBTTT:PCBM systems via processing with fatty acid methyl ester additives. Based on ultrafast transient absorption, electro-absorption and fluorescence up-conversion spectroscopy, we find that exciton diffu-sion through relatively phase-pure polymer or fullerene domains limits the rate of electron and hole transfer, while prompt charge separation occurs in regions where the polymer and fullerene are molecularly intermixed (such as the co-crystal phase where fuller-enes intercalate between polymer chains in pBTTT:PCBM). We moreover confirm the importance of neat domains, which are essential to prevent geminate recombination of bound electron-hole pairs. Most interestingly, using an electro-absorption (Stark effect) signature, we directly visualize the migration of holes from intermixed to neat regions, which occurs on the sub-picosecond time scale. This ultrafast transport is likely sustained by high local mobility (possibly along chains extending from the co-crystal phase to neat regions) and by an energy cascade driving the holes towards the neat domains.
Recently, an intermixed phase has been identified within organic photovoltaic (OPV) bulk heterojunction (BHJ) systems that can exist in addition to relatively phase-pure regions, highlighting the need for a refined picture of the solid-state microstructure of donor-acceptor blends and for gaining further understanding of the exact nature and role such intermixed phases play in such devices. Here we manipulate the microstructure of polymer-fullerene systems via processing means and the selection of the molecular weight of the donor polymer. This manipulation is used as a tool to vary the fraction of intermixed phase present and its effects on the structure and subsequently the opto-electronic processes. We find clear relationships between the state of mixing and amount of exciton quenching and number of polarons generated per absorbed photon. Furthermore, we observe that blend systems incorporating higher molecular weight polymer result in a greater yield of dissociated polarons, likely due to the increase of the intermixed fraction.
Control over the structure of donor/acceptor blends is essential for the development of solution processable organic solar cells (OSCs). We have used time-resolved neutron reflectometry (NR) and in situ annealing to investigate the nanoscale structure and interdiffusion of sequentially spin-coated thin films of poly(3-n-hexylthiophene-2,5-diyl) (P3HT)/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and correlated the evolving structure with the device performance. While the as-prepared film shows a clear two-layer structure it is evident that (19 wt%) PCBM has percolated throughout the lower P3HT layer. Upon heating, analysis of time-resolved NR data shows that the diffusion process is dependent on the annealing temperature. At temperatures up to 110 °C, the two-layer structure is retained and only a small amount of PCBM diffuses from the interface into the lower layer, increasing the total PCBM content throughout the P3HT layer to 26 wt%. Significantly, this small change in acceptor content leads to a profound increase in device performance; with the power conversion efficiency (PCE) of the OSCs increasing from 0.47% (unannealed, 19 wt% PCBM) to 3.23% (annealed, 26 wt% PCBM) with the latter showing a similar efficiency to devices prepared from a blend containing 50 wt% PCBM. Further annealing at 120 and 130 °C sees rapid interdiffusion between the two layers, along with an overall expansion in the thickness of the bilayer film. Despite the complete intermixing of the PCBM and P3HT to form a structure resembling a bulk heterojunction, essentially no improvement in device performance was observed for annealing at temperatures above 110 °C.
The thermal behaviour of an organic photovoltaic (OPV) binary system comprised of a liquid-crystalline fluorene-based polymer and a fullerene derivative is investigated. We employ variable-temperature ellipsometry complemented by photo- and electroluminescence spectroscopy as well as optical microscopy and scanning force nanoscopy to explore phase transitions of blend thin films. The high glass transition temperature correlates with the good thermal stability of solar cells based on these materials. Furthermore, we observe partial miscibility of the donor and acceptor together with the tendency of excess fullerene derivative to segregate into exceedingly large domains. Thus, for charge generation less adequate bulk-heterojunction nanostructures are poised to develop if this mixture is exposed to more elevated temperatures. Gratifyingly, the solubility of the fullerene derivative in the polymer phase is found to decrease if a higher molecular-weight polymer fraction is employed, which offers routes towards improving the photovoltaic performance of non-crystalline OPV blends.
We developed a new method to accurately extract the singlet exciton diffusion length in organic semiconductors by blending them with a low concentration of methanofullerene[6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM). The dependence of photoluminescence (PL) decay time on the fullerene concentration provides information on both exciton diffusion and the nanocomposition of the blend. Experimentally measured PL decays of blends based on two narrow band gap dithiophene– benzothiadiazole polymers, C–PCPDTBT and Si–PCPDTBT, were modeled using a Monte Carlo simulation of 3D exciton diffusion in the blend. The simulation software is available for download. The extracted exciton diffusion length is 10.5 AE 1 nm in both narrow band gap polymers, being considerably longer than the 5.4 AE 0.7 nm that was measured with the same technique in the model compound poly(3-hexylthiophene) as a reference. Our approach is simple, fast and allows us to systematically measure and compare exciton diffusion length in a large number of compounds.
The effect of poly(3-alkylthiophene) (P3AT) crystallinity in (nanofiber P3AT):PCBM photovoltaic devices on the energy of the charge-transfer state (E(CT)) and on the open-circuit voltage (V(oc)) is investigated for poly(3-butythiophene), poly(3-pentylthiophene) and poly(3-hexylhiophene). P3AT crystallinity, expressed as the crystalline nanofiber mass fraction f to the total P3AT mass in the spin-coating dispersion, is varied between similar to 0.1 and similar to 0.9 by temperature control. E(CT), as obtained by Fourier-transform photocurrent spectroscopy decreased with f as E(CT)=E(CT)(0)-0.2f eV. Alkyl side-chain length only influences E(CT)(0). V(oc) relates to E(CT) as V(oc)=E(CT)/q-0.6 V. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3232242]
We present a detailed investigation on the effective dimensionality associated with the degree of delocal-ization of electronic excitations in thin organic films using the dielectric function as obtained from ellipsom-etry. To this end, we study first the best analytical representation of the optical dielectric function of these materials and compare different approaches found in the literature: i the harmonic oscillator approximation, ii the standard critical-point model SCP, iii the model dielectric function MDF, and iv the Forouhi-Bloomer model. We use these models to analyze variable angle spectroscopic ellipsometry raw data for a thin poly9,9-dioctylfluorene PFO film deposited on quartz taken as an archetypal sample. The superiority of the SCP model for PFO films and a wide range of other spin-coated conjugated polymers and guest-molecules in polymers is demonstrated. Moreover, we show how the SCP model can be used to gain physical informa-tion on the microscopic structure. As an example, we show that the delocalization of excitons decreases for nonconjugated polymers, such as polymethylmethacrylate and polyimide, while the conjugation length and exciton delocalization are, respectively, enhanced in cases where a planar conformation e.g., phase of PFO or a high degree of crystallinity e.g., poly3-hexylthiophene is achieved. As an additional example, we employ the SCP excitonic model to investigate the temperature dependence of the dielectric function of crystalline and glassy PFO films. We propose that the SCP excitonic model should be adopted as the standard choice to model the optical properties of polymer thin films from ellipsometry data.
An understanding of aggregation effects in organic semiconductors is essential for their effective use in optoelectronic devices. Typically, the electronic dynamics in such systems are heavily dependant upon the aggregation state, and dynamics often occur on subnanosecond timescales. Here, we determined the singlet exciton population within isolated and aggregated P3HT regions using time-resolved photoluminescence measurements, and find a strong decay pathway in the aggregated case only. Comparison of the emission from the lowest two vibronic bands demonstrates a changeover from isolated chain to aggregate-like emission within ~14 ps corresponding to timescales for torsional relaxation in these materials. We conclude that formation of an aggregate excited state in conjugated polymers is mediated by vibrational relaxation from a low-symmetry to a high-symmetry, ordered state for the ensemble.
Emission quenching is studied in systems composed of [9,9-dioctylfluorene-co-N-(4-butylphenyl)-diphenylamine] (TFB) and various concentrations of three different types of acceptors: [6,6]-phenyl-C61 butyric acid methyl ester (mono-PCBM) and its multiadduct analogues bis-PCBM and tris-PCBM. We find that the degree of emission quenching for a given fullerene concentration decreases as the PCBM adduct number increases, while the microstructure, as observed with transmission electron microscopy, becomes more coarse. These observations are rationalized in terms of possible differences in the miscibility of fullerenes in the polymer and different excitation dissociation rates. We also extract a value for the exciton diffusion length in TFB of 9.0 ± 2 nm from ultrafast fluorescence decay measurements. The results have been confirmed with independent measurements.
Conjugated polymers are attracting worldwide attention due to their potential for use as the active layer in advanced electronic, optoelectronic, and energy harvesting applications, and their cost-effective and low thermal budget processing traits. As the technologies based on these materials develop, new and more sensitive characterization techniques are needed. Recent progress on the use of spectroscopic ellipsometry as a highly sensitive and non-invasive method to obtain fundamental information about conjugated polymer films is reviewed. After a brief introduction to the practical details of the technique, the use of ellipsometry to determine optical parameters that provide insight into film morphology is described, including physical phase and molecular orientation, and resulting electronic structure. The characterization of layered systems and the use of in-situ ellipsometry as a means to gain understanding of the kinetics that occur during film deposition and post-deposition thermal and solvent vapor treatment is also discussed.
Charge transfer (CT) states at a donor-acceptor heterojunction have a key role in the charge photogeneration process of organic solar cells, however, the mechanism by which these states dissociate efficiently into free carriers remains unclear. Here we explore the nature of these states in small molecule-fullerene bulk heterojunction photovoltaics with varying fullerene fraction and find that the CT energy scales with dielectric constant at high fullerene loading but that there is a threshold C60 crystallite size of ~4 nm below which the spatial extent of these states is reduced. Electroabsorption measurements indicate an increase in CT polarizability when C60 crystallite size exceeds this threshold, and that this change is correlated with increased charge separation yield supported by CT photoluminescence transients. These results support a model of charge separation via delocalized CT states independent of excess heterojunction offset driving energy and indicate that local fullerene crystallinity is critical to the charge separation process.
Natural photosynthetic complexes accomplish the rapid conversion of photoexcitations into spatially separated electrons and holes through precise hierarchical ordering of chromophores and redox centers. In contrast, organic photovoltaic (OPV) cells are poorly ordered, utilize only two different chemical potentials, and the same materials that absorb light must also transport charge; yet, some OPV blends achieve near-perfect quantum efficiency. Here we perform electronic structure calculations on large clusters of functionalized fullerenes of different size and ordering, predicting several features of the charge generation process, outside the framework of conventional theories, but clearly observed in ultrafast electro-optical experiments described herein. We show that it is the resonant coupling of photogenerated singlet excitons to a high-energy manifold of fullerene electronic states that enables efficient charge generation, bypassing local-ized charge-transfer states. In contrast to conventional views, our findings suggest that fullerene cluster size, concentration, and dimensionality control charge generation efficiency, independent of exciton delocalization.
The energy of the charge-transfer state formed between electron-donating and electronaccepting materials, a state that directly absorbs, largely determines the limit of the open-circuit voltage in organic photovoltaic devices. This is described in work by Aram Amassian, Michael D. McGehee and co-workers on page 6076.
We explore the interrelation between density of states, recombination kinetics, and device performance in efficient poly[4,8‐bis‐(2‐ethylhexyloxy)‐benzo[1,2‐b:4,5‐b']dithiophene‐2,6‐diyl‐alt‐4‐(2‐ethylhexyloxy‐1‐one)thieno[3,4‐b]thiophene‐2,6‐diyl]:[6,6]‐phenyl‐C71‐butyric acid methyl ester (PBDTTT‐C:PC71BM) bulk‐heterojunction organic solar cells. We modulate the active‐layer density of states by varying the polymer:fullerene composition over a small range around the ratio that leads to the maximum solar cell efficiency (50–67 wt% PC71BM). Using transient and steady‐state techniques, we find that nongeminate recombination limits the device efficiency and, moreover, that increasing the PC71BM content simultaneously increases the carrier lifetime and drift mobility in contrast to the behavior expected for Langevin recombination. Changes in electronic properties with fullerene content are accompanied by a significant change in the magnitude or energetic separation of the density of localized states. Our comprehensive approach to understanding device performance represents significant progress in understanding what limits these high‐efficiency polymer:fullerene systems. The high performance PBDTTT‐C:PC71BM BHJ system is limited by recombination of a nongeminate and non‐Langevin origin. Both the mobility and carrier lifetime are increased with higher PC71BM content around the optimal range of 50–67 wt.% PC71BM. The likely origin of this observation is a significant shift in density of states with increased PC71BM concentration.
A series of well-defined perfluoroalkyl end-functionalized poly(3-hexylthiophenes) (P3HT) were synthesized by Stille coupling of stannylated 2-perfluoralkylthiophene with the bromine end of P3HT. The length of the perfluoroalkyl end group was varied from −C4F13 to −C8F17. These polymers were fully characterized and tested in bulk heterojunction solar cells with phenyl-C61-butyric acid methyl ester (PCBM) as the acceptor. Performance of the solar cells was highest for the unmodified P3HT and decreased as the length of the perfluoroalkyl end increased. The most affected device parameters were the short-circuit current density (Jsc) and series resistance, pointing to lower charge carrier mobility and poor morphology as the cause for the lower performance. While the morphology of blends did not significantly change with perfluoroalkyl end modification, analysis of blended films by energy-filtered transmission electron microscopy (EF-TEM) revealed wider P3HT domains, consistent with the perfluorinated end groups segregating to the edge or exterior of P3HT domains, causing two domains to join. This study demonstrates that the perfluoroalkyl end group can be detrimental to polymer solar cell device performance, and further work toward understanding the interface between the donor and acceptor phases is required to fully understand this effect.
The maximum open-circuit voltage VOC of bulk-heterojunction solar cells is limited by the effective HOMO(donor)-LUMO(acceptor) gap of the photoactive absorber blend. We investigate blend layers comprising zinc-phthalocyanine (ZnPc) and the buckminster fullerene C60 with ultraviolet, x-ray, and inverse photoelectron spectroscopy. By varying the volume mixing ratio ZnPc:C60 from 6:1 to 1:6, we observe a linear increase of the HOMO(ZnPc)-LUMO(C60) gap by 0.25 eV. The trend in this gap correlates with the change in the charge transfer energy measured by Fourier-transform photocurrent spectroscopy as well as with the observed open-circuit voltage of solar cells containing ZnPc:C60 as the photoactive absorber layer. Furthermore, the morphology of different ZnPc:C60 blend layers is investigated by grazing-incidence x-ray diffraction. As physical origins for the changed energy levels, a suppressed crystallization of the C60 phase in the presence of donor molecules as well as concentration-dependent growth modes of the ZnPc phase are suggested.
Solution processed polymer/fullerene blend films are receiving extensive attention as the photoactive layer of organic solar cells. In this paper we report a range of photophysical, electrochemical, physicochemical and structural data which provide evidence that formation of a relatively pure, molecularly ordered phase of the fullerene component, phenyl-C61-butyric acid methyl ester (PCBM), may be the key factor driving the spatial separation of photogenerated electrons and holes in many of these devices. PCBM crystallisation is shown to result in an increase in its electron affinity, providing an energetic driving force for spatial separation of electrons and holes. Based upon our observations, we propose a functional model applicable to many organic bulk heterojunction devices based upon charge generation in a finely intermixed polymer/fullerene phase followed by spatial separation of electrons and holes at the interface of this mixed phase with crystalline PCBM domains. This model has significant implications for the design of alternative acceptor materials to PCBM for organic solar cells.
Recombination in poly-3-hexylthiophene (P3HT) blends with five fullerene acceptors was resolved with temperature-dependent transient absorption spectroscopy. Recombination rates were temperature and acceptor dependent with differing timescales originating from acceptor functionalization and fullerene size. Acceptors with increasing numbers of sidechains (bis > mono > C 60) or decreasing fullerene size (C 60 < C 70 < C 80) exhibit slower recombination. The recombination kinetics was correlated to average distance between the fullerene cage and donor site with functionalized moieties acting as dielectric shields to hinder recombination. Three commonly used differential models were evaluated to describe the data. The quantitative failure of these models suggests a microscopic approach (e.g., Monte-Carlo) is needed to quantitatively model the dynamics.
The optimization of the polymer solar cells based on regioregular poly(3-hexylthiophene) (P3HT) and the bisadduct of phenyl C61-butyric acid methyl ester (bisPCBM) is studied thoroughly as a role of solvent-annealing effect as well as different concentration of bisPCBM. In the case of P3HT:bisPCBM of 1:0.8 w/w, more balanced electron and hole mobilities are observed, resulting in better performance of the solar cells. Under the best balance conditions such as P3HT:bisPCBM of 1:0.8 w/w, the solventannealing is employed to further clarify the optimization of the devices. Such a treatment leads to the formation of crystalline P3HT domains in the blend films, which is determined by X-ray diffraction, UV-vis spectroscopy, and atomic force microscopy. From our experiment, one can conclude that the best power conversion efficiency of 3.75% is achieved in a layered structure of P3HT:bisPCBM of 1:0.8 w/w for a solvent-annealing time of 24 h.
We demonstrate that 1H spin diffusion NMR is a valuable method to estimate the domain size distribution in bulk heterojunction (BHJ) active layers composed of poly-3-hexylthiophene (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM). Bulk samples were annealed at several temperatures, and a distribution of domain sizes was observed that ranged over hundreds of nanometers. Unannealed samples exhibited a large amount of PCBM in small domains (1 to 5 nm) and smaller amounts in moderate (tens of nm) and large (>100 nm) domains. Annealing the samples at 100 °C had no effect on the morphology as evidenced from 1H spin diffusion NMR, grazing-incidence diffraction, and calorimetry, but phase separation was observed after annealing at 150 °C. Even with this higher temperature processing, the 1H NMR showed conclusively that phase separation remained incomplete; this finding was confirmed with photoluminescence quenching measurements.
In this report the effect of solvent to control the degree of mixing of the polymer and fullerene components, as well as the domain size and charge transport properties of the blends were investigated in detail using P3HT:C60 films. The polymer blend films spin coated from faster evaporating, non-aromatic solvents demonstrated an improved ordering and optical absorption in P3HT and blended films, indicating that the limited solubility of P3HT:C60 in a marginal solvent can lead directly to optimal morphologies on the films. The PL quenched by a factor of 3 after blending the P3HT with C60 in a 1:1wt. ratio using CB, xylene, DCB, and toluene as solvents, indicating a partially charge transfer from P3HT to C60. A complete reduction in the PL intensity was observed in the film spin-coated from chloroform.
The device function of polymer bulk heterojunction (BHJ) solar cells has been commonly interpreted to arise from charge separation at discrete interfaces between phase-separated materials and subsequent charge transport through these phases without consideration of phase purity. To probe composition, the miscibility of poly(3-hexylthiophene) (P3HT) and poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene) (MDMO-PPV) with phenyl-C61-butyric acid methyl ester (PCBM) has been determined, while the effects of polymer crystallinity on miscibility are probed using P3HT grades of varying regioregularity. It is found that, while no intercalation occurs in P3HT crystals, amorphous portions of P3HT and MDMO-PPV contain significant concentrations of PCBM, calling into question models based on pure phases and discrete interfaces. Furthermore, depth profiles of P3HT/PCBM bilayers reveal that even short annealing causes significant interdiffusion of both materials, showing that under no conditions do pure amorphous phases exist in BHJ or annealed bilayer devices. These results suggest that current models of charge separation and transport must be refined.Keywords (keywords): moleculear miscibility; polymer-fullerene blends; organic solar cells; thermodynamic phase diagram; near-edge X-ray absorption fine structure (NEXAFS); grazing-incidence wide angle X-ray scattering (GIWAXS or GIXRD); dynamic secondary ion mass spectrometry (SIMS)
The influence of fullerene side chain functionalization on both the morphology and electro-optical properties of bulk-heterojunction polymer:fullerene solar cells is discussed through a systematic investigation of material blends consisting of the conjugated polymer poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) as donor and fullerene molecules with different side chains as the acceptor. The varying side chain of the fullerenes was found to induce morphological changes as confirmed by different analytical techniques such as Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD), and Nuclear Magnetic Resonance (NMR). The fullerene with the shorter side chain, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), forms crystalline nanophases in the blend, while this is not the case for the alternative diphenylmethanofullerene acceptor, [6,6]-1,1-bis(4,4′-dodecyloxyphenyl)methanofullerene (DPM-12). The introduction of NMR allows us to estimate the fraction of crystalline fullerene. The morphological changes have a profound effect on the characteristics of charge transfer states (CT) formed at the polymer:fullerene interfaces. Crystallization of fullerene molecules shifts the energy of the CT state. This shift in energy is directly manifested in the open-circuit voltage of solar cells based on the fullerene acceptors under investigation.
The improvement of the power conversion efficiency (PCE) of polymer bulk heterojunction (BHJ) solar cells has generally been achieved through synthetic design to control frontier molecular orbital energies and molecular ordering of the electron-donating polymer. An alternate approach to control the PCE of a BHJ is to tune the miscibility of the fullerene and a semiconducting polymer by varying the structure of the fullerene. The miscibility of a series of 1,4-fullerene adducts in the semiconducting polymer, poly(3-hexylselenophene), P3HS, was measured by dynamic secondary ion mass spectrometry using a model bilayer structure. The microstructure of the bilayer was investigated using high-angle annular dark-field scanning transmission microscopy and linked to the polymer-fullerene miscibility. Finally, P3HS:fullerene BHJ solar cells were fabricated from each fullerene derivative, enabling the correlation of the active layer microstructure to the charge collection efficiency and resulting PCE of each system. The volume fraction of polymer-rich, fullerene-rich, and polymer-fullerene mixed domains can be tuned using the miscibility leading to improvement in the charge collection efficiency and PCE in P3HS:fullerene BHJ solar cells. These results suggest a rational approach to the design of fullerenes for improved BHJ solar cells.
Here, it is shown how carrier recombination through charge transfer excitons between conjugated polymers and fullerene molecules is mainly controlled by the intrachain conformation of the polymer, and to a limited extent by the mesoscopic morphology of the blend. This experimental result is obtained by combining near-infrared photoluminescence spectroscopy and transmission electron microscopy, which are sensitive to charge transfer exciton emission and morphology, respectively. The photoluminescence intensity of the charge transfer exciton is correlated to the degree of intrachain order of the polymer, highlighting an important aspect for understanding and limiting carrier recombination in organic photovoltaics.
A simple rationale for selecting the optimum composition of binary organic photovoltaic crystalline/crystalline polymer/small molecule blends was proposed. A unique relationship between the composition dependence of photocurrent generation and microstructure, controlled by the phase behavior of the two crystalline components, was also demonstrated. The rate of solidification is found to be critical for the P3HT/PC61BM binary when processed both from solution and the melt. The devices display a maximum short circuit photocurrent, which is maximized for blend films after thermal treatment at 140°C for 45 min. The binary systems are also found to be of simple eutectic in nature, featuring different eutectic compositions and temperatures. Optical measurements of dielectric function indicate that the effect of blend composition on the Columbic binding energy for a polaron pair in an effective medium is small.
The relation between the nanoscale morphology and associated device properties in conjugated polymer/fullerene bulk-heterojunction "plastic solar cells" is investigated. We perform complementary measurements on solid-state blends of poly[2-methoxy-5-(3,7-dimethyloctyloxy)]-1,4-phenylenevinylene (MDMO-PPV) and the soluble fullerene C-60 derivative 1-(3-methoxycarbonyl) propyl-1-phenyl [6,6]C-61 (PCBM), spin-cast from either toluene or chlorobenzene solutions. The characterization of the nanomorphology is carried out via scanning electron microscopy (SEM) and atomic force microscopy (AFM), while solar-cell devices were characterized by means of current-voltage (I-V) and spectral photocurrent measurements. In addition, the morphology is manipulated via annealing, to increase the extent of phase separation in the thin-film blends and to identify the distribution of materials. Photoluminescence measurements confirm the demixing of the materials under thermal treatment. Furthermore the photoluminescence of PCBM clusters with sizes of up to a few hundred nanometers indicates a photocurrent loss in films of the coarser phase-separated blends cast from toluene. For toluene-cast films the scale of phase separation depends strongly on the ratio of MDMO-PPV to PCBM, as well as on the total concentration of the casting solution. Finally we observe small beads of 20-30 nm diameter, attributed to MDMO-PPV, in blend films cast from both toluene and chlorobenzene.
The photophysical properties of blends of fluorene copolymer and the fullerene derivative PCBM are analyzed with a particular attention to photovoltaic applications. The properties of the blends are determined by the relative alignment of the HOMO energy levels. In the blend where the HOMO levels of the copolymer and the fullerene are aligned there is not signature of charge stabilization and photovoltaic effect. While in the blend where there is an offset between the HOMO levels the charge stabilization is accompanied by good photovoltaic performances. The photoluminescence spectrum of the latter blend is characterized by the appearance of a new peak at low energy with a lifetime of a few ns that red-shifts with the increase of the PCBM percentage. The feature is attributed to the emission from a charge-transfer exciton that is red-shifted by the change of dielectric constant of the medium.
Developing a better understanding of the evolution of morphology in plastic solar cells is the key to designing new materials and structures that achieve photoconversion efficiencies greater than 10%. In the most extensively characterized system, the poly(3-hexyl thiophene) (P3HT):[6,6]-phenyl-C61-butyric-acid-methyl-ester (PCBM) bulk heterojunction, the origins and evolution of the blend morphology during processes such as thermal annealing are not well understood. In this work, we use a model system, a bilayer of P3HT and PCBM, to develop a more complete understanding of the miscibility and diffusion of PCBM within P3HT during thermal annealing. We find that PCBM aggregates and/or molecular species are miscible and mobile in disordered P3HT, without disrupting the ordered lamellar stacking of P3HT chains. The fast diffusion of PCBM into the amorphous regions of P3HT suggests the favorability of mixing in this system, opposing the belief that phase-pure domains form in BHJs due to immiscibility of these two components.
The bis and tris adducts of [6,6]phenyl-C(61)-butyric acid methyl ester (PCBM) offer lower reduction potentials than PCBM and are therefore expected to offer larger open-circuit voltages and more efficient energy conversion when blended with conjugated polymers in photovoltaic devices in place of PCBM. However, poor photovoltaic device performances are commonly observed when PCBM is replaced with higher-adduct fullerenes. In this work, we use transmission electron microscopy (TEM), steady-state and ultrafast time-resolved photoluminescence spectroscopy (PL), and differential scanning calorimetry (DSC) to probe the microstructural properties of blend films of poly(3-hexylthiophene-2,5-diyl) (P3HT) with the bis and tris adducts of PCBM. TEM and PL indicate that, in as-spun blend films, fullerenes become less soluble in P3HT as the number of adducts increases. PL indicates that upon annealing crystallization leads to phase separation in P3HT:PCBM samples only. DSC studies indicate that the interactions between P3HT and the fullerene become weaker with higher-adduct fullerenes and that all systems exhibit eutectic phase behavior with a eutectic composition being shifted to higher molar fullerene content for higher-adduct fullerenes. We propose two different mechanisms of microstructure development for PCBM and higher-adduct fullerenes. P3HT:PCBM blends, phase segregation is the result of crystallization of either one or both components and is facilitated by thermal treatments. In contrast, for blends containing higher adducts, the phase separation is due to a partial demixing of the amorphous phases. We rationalize the lower photocurrent generation by the higher-adduct fullerene blends in terms of film microstructure.
The photovoltaic action of plastic solar cells based on P3HT/PCBM- (poly[3-hexylthiophene-2,5-diyl]/[6,6]-phenyl C61 butyric acid methyl ester)-composites depends strongly on structure of the active layer. In this work, the impact of P3HT-crystallinity on optical absorption of P3HT/PCBM-films was investigated. We observed, that both P3HT-crystallinity and optical absorption of thermally annealed films are increased in comparison to the not annealed ones. The highest crystallinity was achieved for films annealed at 125 °C. Further increase of annealing temperature leads to decrease of P3HT crystallinity. It is shown that the absorption coefficient of the films at low photon energies is proportional to the area under the X-ray-diffraction peak, which is a measure for the degree of film crystallinity.
We study the appearance and energy of the charge transfer (CT) state using measurements of electroluminescence (EL) and photoluminescence (PL) in blend films of high-performance polymers with fullerene acceptors. EL spectroscopy provides a direct probe of the energy of the interfacial states without the need to rely on the LUMO and HOMO energies as estimated in pristine materials. For each polymer, we use different fullerenes with varying LUMO levels as electron acceptors, in order to vary the energy of the CT state relative to the blend with [6,6]-phenyl C61-butyric acid methyl ester (PCBM). As the energy of the CT state emission approaches the absorption onset of the blend component with the smaller optical bandgap, E(opt,min) ≡ min{E(opt,donor); E(opt,acceptor)}, we observe a transition in the EL spectrum from CT emission to singlet emission from the component with the smaller bandgap. The appearance of component singlet emission coincides with reduced photocurrent and fill factor. We conclude that the open circuit voltage V(OC) is limited by the smaller bandgap of the two blend components. From the losses of the studied materials, we derive an empirical limit for the open circuit voltage: V(OC) ≲ E(opt,min)/e - (0.66 ± 0.08)eV.
Organic photovoltaics (OPVs) have attracted increasing interest as a lightweight, low-cost, and easy to process replacement for inorganic solar cells. Moreover, the morphology of the OPV active layer is crucial to its performance, where a bicontinuous, interconnected, phase-separated morphology of pure electron donor and acceptor phases is currently believed to be optimal. In this work, we use neutron scattering to investigate the morphology of a model OPV conjugated polymer bulk heterojunction, poly[3-hexylthiophene] (P3HT), and surface-functionalized fullerene 1-(3-methyloxycarbonyl) propyl(1-phenyl [6,6]) C(61) (PCBM). These results show that P3HT and PCBM form a homogeneous structure containing crystalline P3HT and an amorphous P3HT/PCBM matrix, up to ca. 20 vol % PCBM. At 50 vol % PCBM, the samples exhibit a complex structure containing at least P3HT crystals, PCBM crystals, and a homogeneous mixture of the two. The 20 vol % PCBM samples exhibit behavior consistent with the onset of phase separation after 6 h of thermal annealing at 150 °C, but appear to be miscible at shorter annealing times. This suggests that the miscibility limit of PCBM in P3HT is near 20%. Moreover, for the 50 vol % PCBM sample, the interface roughens under thermal annealing possibly owing to the growth of PCBM crystals. These observations suggest a different morphology than is commonly presented in the literature for optimal bulk heterojunctions. We propose a novel "rivers and streams" morphology to describe this system, which is consistent with these scattering results and previously reported photovoltaic functionality of P3HT/PCBM bulk heterojunctions.
We have investigated the charge photogeneration dynamics at the interface formed between single-walled carbon nanotubes (SWNTs) and poly(3-hexylthiophene) (P3HT) using a combination of femtosecond spectroscopic techniques. We demonstrate that photoexcitation of P3HT forming a single molecular layer around a SWNT leads to an ultrafast (∼430 fs) charge transfer between the materials. The addition of excess P3HT leads to long-term charge separation in which free polarons remain separated at room temperature. Our results suggest that SWNT-P3HT blends incorporating only small fractions (1%) of SWNTs allow photon-to-charge conversion with efficiencies comparable to those for conventional (60:40) P3HT-fullerene blends, provided that small-diameter tubes are individually embedded in the P3HT matrix.
Predicting Energetic Disorder: A quantum chemical method is used to calculate the LUMO energies of all possible isomers of the bis and tris adducts of the fullerene, [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM). The calculated energy level distributions agree well with the observed mean and spread of LUMO energies as determined using solution differential pulse voltammetry (DPV). We propose this method as a powerful tool for the design and functional optimisation of novel fullerenes, as well as other classes of pi‐conjugated molecules with multiple isomers.
Solution-processed bulk heterojunction organic photovoltaic (OPV) devices have gained serious attention during the last few years and are established as one of the leading next generation photovoltaic technologies for low cost power production. This article reviews the OPV development highlights of the last two decades, and summarizes the key milestones that have brought the technology to today's efficiency performance of over 7%. An outlook is presented on what will be required to drive this young photovoltaic technology towards the next major milestone, a 10% power conversion efficiency, considered by many to represent the efficiency at which OPV can be adopted in wide-spread applications. With first products already entering the market, sufficient lifetime for the intended application becomes more and more critical, and the status of OPV stability as well as the current understanding of degradation mechanisms will be reviewed in the second part of this article.
The spectral evolution of an intrachain neutral singlet exciton toward a charge-transfer (CT) state in solvents of increasing polarity has been monitored by time-resolved photoluminescence and ultrafast transient-absorption spectroscopy in a model conjugated random copolymer composed of electron donor and electron acceptor units. In polar solvents, a charge-like absorption superimposes the region of stimulated emission and leads to a dramatic reduction in gain implying that CT states can be detrimental for light amplification and lasing.
Blends of poly(3-hexylthiophene) (P3HT) and the bis-adduct of [6,6]-phenyl-C(61)-butyric acid methyl ester (bisPCBM) show enhanced performances in bulk-heterojunction solar cells compared to P3HT:PCBM thin films due to their higher open-circuit voltage. However, it is not clear whether the decrease of the short-circuit current observed in P3HT-bisPCBM blends originates from the 100 mV reduction of the offset between the lowest unoccupied molecular orbitals of the donor and the acceptor or from a change in the morphology. The analysis of the photoluminescence dynamics of the various bulk heterojunctions provides information on the dependence of the electron transfer process on their microstructure. We find that in solution, where the donor-acceptor distribution is homogeneous, the photoluminescence dynamics is the same for the bis- and PCBM-based blends, while in thin films the first shows a slower dynamics than the second. This result indicates that the reduction of the LUMO offset of approximately 100 meV does not influence the electron transfer efficiency but that the diversity between the photoluminescence dynamics in thin films should be ascribed to the different microstructure of the bulk heterojunctions fabricated with the two acceptors.
In this article we report the weak but omnipresent electroluminescence (EL) from several types of organic polymer:fullerene bulk heterojunction solar cells biased in the forward direction. The light emitted from blends of some commonly used polymers and the fullerene molecule is significantly different from that of any of the pure materials comprising the blend. The lower energy of the blend EL is found to correlate with both the voltage onset of emission and the open-circuit voltage of the photovoltaic cell under solar illumination. We accordingly interpret the emission to originate from interfacial charge transfer state recombination and emphasize EL as a very valuable tool to characterize the charge transfer state present in donor/acceptor organic photovoltaic (OPV) cells.
Pi-conjugated polymers and oligomers show charge transfer (CT) absorption bands when mixed with electron acceptors in chloroform solution. This is attributed to the formation of (ground state) donor-acceptor complexes in solution. By varying the concentration of the donor and acceptor, the extinction coefficient for the CT absorption and the association constant of donor and acceptor are estimated. The spectral position of the CT bands correlates with the electrochemical oxidation potential of the pi-conjugated donor and the reduction potential of the acceptor.
The electro-optical properties of thin films of electron donor-acceptor blends of a fluorene copolymer (PF10TBT) and a fullerene derivative (PCBM) were studied. Transmission electron microscopy shows that in these films nanocrystalline PCBM clusters are formed at high PCBM content. For all concentrations, a charge transfer (CT) transition is observed with absorption spectroscopy, photoluminescence, and electroluminescence. The CT emission is used as a probe to investigate the dissociation of CT excited states at the donor-acceptor interface in photovoltaic devices, as a function of an applied external electric field and PCBM concentration. We find that the maximum of the CT emission shifts to lower energy and decreases in intensity with higher PCBM content. We explain the red shift of the emission and the lowering of the open-circuit voltage (V(OC)) of photovoltaic devices prepared from these blends with the higher relative permittivity of PCBM (epsilon(r) = 4.0) compared to that of the polymer (epsilon(r) = 3.4), stabilizing the energy (E(CT)) of CT states and of the free charge carriers in blends with higher PCBM concentration. We show that the CT state has a short decay time (tau = ca. 4 ns) that is reduced by the application of an external electric field or with increasing PCBM content. The field-induced quenching can be explained quantitatively with the Onsager-Braun model for the dissociation of the CT states when including a high electron mobility in nanocrystalline PCBM clusters. Furthermore, photoinduced absorption spectroscopy shows that increasing the PCBM concentration reduces the yield of neutral triplet excitons forming via electron-hole recombination, and increases the lifetime of radical cations. The presence of nanocrystalline domains with high local carrier mobility of at least one of the two components in an organic heterojunction may explain efficient dissociation of CT states into free charge carriers.
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