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(a) Negative-ion TOF–SIMS depth profile of a C 60 (40 nm)/ P3HT(35 nm)/PEDOT:PSS(80 nm)/ITO(125 nm)/glass multilayer before annealing. Data of the marker fragments for the individual layers were recorded under conditions described in the text. Note that due to the lower sputter rate of C 60 , the depth scale starts at 1500 s. (b) TOF–SIMS profile of negative secondary ions from the same layer stack after thermal annealing at 150 1C for 30 min. The chemical structures corresponding to the detected masses are indicated in the graph.
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The performance of heterojunction organic solar cells is critically dependent on the morphology of the donor and acceptor components in the active film. We report results of photovoltaic devices consisting of bilayers and bulk heterojunctions using poly(3-hexylthiophene) (P3HT) and Buckminsterfullerene C60. White light power efficiencies of η∼2.2%...
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... not present in P3HT. Although also recorded and analyzed, the positive ion TOF-SIMS spectra were more complex, and signal mismatch due to the absence of C 60 -specific carbon signals after sputtering with O 2 À made marker selection in the positive ion mode difficult. Consequently, only negative-ion secondary ion TOF-SIMS data are discussed here. Fig. 6 shows the depth profiles of a C 60 /P3HT/ PEDOT:PSS/ITO/glass multilayer structure before (as-produced, Fig. 6a) and after (Fig. 6b) annealing at 150 1C for 30 min. We also tried to extract numerical values for sputter rates from crater depth measurements using a stylus profilometer. However, while such measurements can be applied to ...
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... and signal mismatch due to the absence of C 60 -specific carbon signals after sputtering with O 2 À made marker selection in the positive ion mode difficult. Consequently, only negative-ion secondary ion TOF-SIMS data are discussed here. Fig. 6 shows the depth profiles of a C 60 /P3HT/ PEDOT:PSS/ITO/glass multilayer structure before (as-produced, Fig. 6a) and after (Fig. 6b) annealing at 150 1C for 30 min. We also tried to extract numerical values for sputter rates from crater depth measurements using a stylus profilometer. However, while such measurements can be applied to inorganic layers with some confidence (particularly in dynamic SIMS experiments), the much more pronounced ...
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... due to the absence of C 60 -specific carbon signals after sputtering with O 2 À made marker selection in the positive ion mode difficult. Consequently, only negative-ion secondary ion TOF-SIMS data are discussed here. Fig. 6 shows the depth profiles of a C 60 /P3HT/ PEDOT:PSS/ITO/glass multilayer structure before (as-produced, Fig. 6a) and after (Fig. 6b) annealing at 150 1C for 30 min. We also tried to extract numerical values for sputter rates from crater depth measurements using a stylus profilometer. However, while such measurements can be applied to inorganic layers with some confidence (particularly in dynamic SIMS experiments), the much more pronounced roughening of the sputter ...
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... and depth profiles are presented with a starting time of 1500 s to clarify experimental details. Data collection was performed at a location without an aluminum layer belonging to the cathode. By this way we could avoid conflicting effects from the metal. Aluminum is only present in the thin film stack in a very small amount as shown in Figs. 6a and b. The organic layer system was removed from the ITO film and measured from the backside with PEDOT:PSS as top layer as well. No striking difference was found in the reverse depth profile in the negative secondary ion mode as compared with data shown in Fig. 6. Following the C 60 marker signals (C 6 À and C 9 À ) in the as- deposited ...
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... is only present in the thin film stack in a very small amount as shown in Figs. 6a and b. The organic layer system was removed from the ITO film and measured from the backside with PEDOT:PSS as top layer as well. No striking difference was found in the reverse depth profile in the negative secondary ion mode as compared with data shown in Fig. 6. Following the C 60 marker signals (C 6 À and C 9 À ) in the as- deposited sample, a decrease of the signals is visible around 2100 s. A steep increase of the previously absent C 4 HS À signal indicates the presence of the C 60 /P3HT interface. While the sulfur containing signal continues to increase, the C 60 markers show a small ...
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... This hybrid route of LbL OPV fabrication enabled the incorporation of C 60 and C 70 acceptors with donor polymers or soluble small molecule such as ADPM, cyanines, squaraines and porphyrins. 62,63,66,68,72,73,75,77,78,83,84,86,87,[91][92][93][94][95][96][97][98][99][100][101][104][105][106][107][108]110,112,166,[194][195][196][197][198][199][200][201][202][203][204][205][206][207] Some examples of evaporated phthalocyanine-based NFAs have also been reported. 88,109,208 A summary of device performances and processing conditions from hybrid LbL OPVs prepared from thermally evaporated acceptors can be found in Table 9. ...
Layer-by-layer (LbL) processing, otherwise known as sequential deposition, is emerging as the most promising strategy for fabrication of active layers in organic photovoltaic (OPV) devices on both laboratory and industrial scales. In comparison to the bulk heterojunction (BHJ) configuration, LbL facilitates separate and sequential deposition of each layer, enabling greater control and optimization of interfaces and final donor/acceptor morphology. Furthermore, this process encourages formation of an efficient vertical phase separation, where the acceptor and donor aggregations are largest at their respective electrodes, increasing exciton dissociation and transport while reducing unwanted charge recombination. Compared to BHJ OPVs, LbL OPVs are more robust, with less dependence on processing conditions, resulting in increased photo, thermal, and mechanical stability and greater power conversion efficiency retention when applied to large area modules. These advantages have resulted in significant interest in the LbL process and its potential to displace BHJ as the dominant process for large-scale OPV manufacturing. This review summarizes recent developments in OPV fabrication through LbL, with particular emphasis on material selection and thin film processing conditions.
... Kumar et al. managed to tune the position of the I-V curve kink in P3HT:PCBM solar cells by introducing Bathocuprine (BCP) and V 2 O 5 at the charge extraction interfaces [95]. Together with a theoretical model, they concluded that the s-shaped characteristics in their cells and similar devices [108] are caused by interface dipoles [95]. Ecker et al. also investigated P3HT:PCBM solar cells and found, with impedance spectroscopy, that a TiO x selective charge transport layer causes s-shapes that disappear after UV soaking because of the decreased resistivity of the TiO x [2]. ...
S-shaped current–voltage (I–V) characteristics are a frequently occurring hurdle in the development of new solar cell material combinations and device architectures. Their presence points to the existence of a charge transport bottleneck that needs to be removed in order to unlock high fill factors and power conversion efficiencies. In this review, examples of studies in which s-shaped I–V curves have appeared are presented, and the cause and mitigation are discussed. Different solar cell material systems are often treated by separate communities, thereby, also the physics of s-shaped I–V curves have been treated separately. This review covers the main solar cell technologies—silicon, thin film, organic, hybrid—with the aim to provide an overarching picture of the common mechanisms and universal guidelines for mitigation of s-shaped I–V characteristics in emerging solar cell technologies. Except for a few studies on organic solar cells, s-shaped I–V curves are reported to result from charge transport barriers at one of the (selective) contact layers that can be overcome by interface engineering and doping.
... An important process that develops π stacking is the polymer component used in creation of hetero-junctions of organic solar cells being, for instance, a mixture of P3HT and C 60 [15,20]. One particularly interesting problem, both from the simulation and the experimental point of view, is to understand the process of formation of the interface between the two components; as it affects the performance of the capture of the energy by the solar cell [5]. For instance, some experimental groups have studied the relationship between the molecule characteristics with the final behavior of the cell is of special relevance [13]. ...
By means of a computational model, we study the relation between two complementary views of gelation, rheological tests against the characterization of a network of consecutive particles. The model we propose consists of slender, plane, colloidal sized particles, which we name laths, which self-assemble into long ordered aggregates of several particles; called whiskers in the literature. Within a whisker, the interaction potential is a minimum when: the planes of two consecutive laths are aligned, thus favoring their alignment; when the center of three consecutive laths lie in a straight line, thus favoring stacking; and when the center of two consecutive laths are located at a certain distance, which mimics excluded volume. A threshold value of the potential gives a condition for sticking free laths into whiskers, and for the breaking of whiskers. The simplicity of the model allows the simulation to reach large enough times, of the order of minutes, needed to simulate numerical rheology tests. We are able to characterize the whisker formation, as well as to simulate the gel transition, by means of an oscillatory shear numerical experiment. We conclude that according to the usual rheological definition a gel transition occurs at about 250K, even though there is no branching and less than 10% of whiskers are long enough as to percolate the system.
... 150 P3HT has high charge carrier mobilities which is one of the disadvantages that organic solar harvesting materials often face. 151 It is relatively easy to synthesize and the regioregularity can be easily controlled. Regioregularity is important for the absorption and charge carrier mobilities. ...
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... 7 In many cases, active materials have been deposited as bilayers, 14,15,19,20 or interdigitated heterojunctions, usually using soluble fullerene derivatives [21][22][23] rather than pristine C 60 . 24 Nanoporous C 60 lms with morphological features in the range of the typical exciton diffusion length, $10-20 nm in comparable systems, 16,25,26 would offer an attractive alternative to bilayers in bicomponent systems that do not phase-separate spontaneously to form an optimal bulk heterojunction morphology. ...
... Reports on C 60 /P3HT solar cells are scarce, because of the low solubility of C 60 in organic solvents and because the two components do not spontaneously phase-separate to form an optimal bulk-heterojunction morphology, limiting drastically the cell performance. 24 One solution found by Geiser et al. was to anneal the whole device to force the morphology to evolve favorably, reaching over 2% in PCE. 24 Even if the nal PCE shown here does not reach that of bulk-heterojunction fullerene/P3HT devices, the solution proposed, using a C 60 lm stabilized in an optimal morphology, can be seen as rather universal and opens important improvement possibilities for other working systems. ...
... 24 One solution found by Geiser et al. was to anneal the whole device to force the morphology to evolve favorably, reaching over 2% in PCE. 24 Even if the nal PCE shown here does not reach that of bulk-heterojunction fullerene/P3HT devices, the solution proposed, using a C 60 lm stabilized in an optimal morphology, can be seen as rather universal and opens important improvement possibilities for other working systems. ...
Nanoporous surface morphologies often perform better than their planar or bulk counterparts in phenomena such as catalysis and charge transport. In organic photovoltaics, for example, fullerene films structured on a scale corresponding to the exciton diffusion length, typically a few tens of nanometers, could improve charge separation. Existing methods to form such films, however, typically do not reach features smaller than 100 nm. Here, we propose a simple method based on interfacial nucleation and growth to produce large and flexible 2D percolating assemblies of C60 nanoparticles with diameters below 50 nm that cover up to 60% of the surface. These films can be rendered insoluble by photopolymerization and infiltrated with other functional molecules. We illustrate the benefit of such stabilized nanoporous films on the example of organic solar cells of the architecture ITO/TiO2/C60/P3HT/MoO3/Ag. Structured C60 films with interconnected morphological features perform better than planar C60 films by 50% on average, reaching a power conversion efficiency of 1.3%. Due to the ease of film fabrication and small dimensions of the C60 morphological features, these results may find application beyond organic photovoltaics, in a variety of fields where an enhanced interface with a fullerene surface is needed.
... For several decades, polymer based organic photovoltaic cells (OPVCs) have been intensively investigated due to their high potential for low-cost, simple, and large-scale processability, in particular, from solution. [1][2][3][4][5][6][7][8][9] Key material parameters that impact both the short-circuit current density (J SC ) and the open-circuit voltage (V OC ) in OPVCs 10 are the hole and electron mobilities of the donor and acceptor materials, respectively. On the one hand, too high mobilities can be detrimental to device performance, which is reflected in a decrease in V oc . ...
The electronic, optical, and morphological properties of molecularly p-doped polyfuran (PF) films were investigated over a wide range of doping ratio in order to explore the impact of doping in photovoltaic applications. We find evidence for integer-charge transfer between PF and the prototypical molecular p-dopant tetrafluoro-tetracyanoquinodimethane (F4TCNQ) and employed the doped
polymer in bilayer organic solar cells using fullerene as acceptor. The conductivity increase in the PF films at dopant loadings ≤2% significantly enhances the short-circuit current of photovoltaic devices. For higher doping ratios, however, F4TCNQ is found to precipitate at the heterojunction between the doped donor polymer and the fullerene acceptor. Ultraviolet photoelectron spectroscopy reveals that its presence acts beneficial to the energy-level alignment by doubling the open-circuit voltage of solar cells from 0.2 V to ca. 0.4 V, as compared to pristine PF.
... According to the height bar of figure 3(b), the vertical length of CuMePc crystal could be as long as 80 nm. Considering the thin thickness (27 nm) of CuMePc layer, direct contact between ITO and C 60 could be expected in the cell without MoO 3 layer after thermal annealing [20], which could lead to the strong leakage current as depicted in figure 2(b). On the other hand, the surface morphology of MoO 3 layer was almost unchanged with RMS from 4.696 to 5.302 nm (figures 3(c) and (d)) after annealed at 190 • C. ...
... Besides the improved FF, both J sc and V oc could be enhanced by annealing treatment. In literatures [20,21], significantly improved performance in OPV cells with polymer PHJ structure by post annealing has been reported, which was attributed to the broader and less distinct organic-organic interface caused by the annealing treatment. In our case, as shown in figure 3(b), after thermal annealing, the morphology of CuMePc layer became very rough, and hence broader and less distinct CuMePc/C 60 interface could be expected. ...
The fabrication of planar heterojunction (PHJ) organic photovoltaic (OPV) cells using tetramethyl substituted copper(II) phthalocyanine (CuMePc) as an electron donor and C60 as an acceptor is described. The impact of post-fabrication thermal annealing upon the performance of these cells has been examined. Atomic force microscopy (AFM) images and UV–visible absorption spectra of CuMePc thin films revealed crystallization of CuMePc induced by thermal annealing at 190 °C. The crystallized CuMePc films accounted for improved hole mobility, broadened absorption spectrum, and increased donor/acceptor interface in the as-fabricated cells after thermal annealing. AFM images also revealed that the surface of MoO3 film was smooth and close-packed after thermal annealing, which efficiently blocked the leakage current in the annealed cells, leading to dramatic improvement of performance for the PHJ cells using CuMePc as the electron donor and MoO3 or V2O5 as the anode buffer layer. The power conversion efficiency of the thermal-annealed PHJ cell with a configuration of ITO/MoO3/CuMePc/C60/Bathocuproine/Al was higher than that of the bulk heterojunction (BHJ) cell fabricated by co-depositing CuMePc and C60. It was mainly because the isolated clusters of CuMePc and/or C60 molecules formed during the fabrication of the BHJ cell was avoided in the PHJ cell.
... [1][2][3] The system consisting of regioregular poly(3-hexylthiophene) (P3HT) and fullerene derivative [6,6]-phenyl C 61 butyric acid methyl ester (PCBM) has shown power conversion efficiencies of about 4-5% under standard solar conditions. [4][5][6][7] Precise control of the molecular structure of such complex organic multilayers, e.g. chemical purity, 8 layer uniformity, morphology, 9 phase separation and interfaces, 10,11 diffusion, 12,13 and effects of annealing, 14,15 is required in order to improve the performance of the devices and the characterization remains a challenge. To address these questions, molecular analysis by secondary ion mass spectrometry (SIMS) constitutes a method of choice. ...
... 22 Low energy Cs + ions provide interesting results for a series of polymers which are problematic for SF 5 + and C 60 n+ sputtering, such as polystyrene and polycarbonate, 23 but crater bottom roughening and probable degradation of fullerenes were reported for P3HT:C 60 heterojunction solar cells. 15 Large noble gas cluster ions have already proven to deliver better performance than SF 5 + and C 60 n+ when the other beams failed 24 and there are some reports concerning organic electronic layers. 25 In particular, OLED multilayers including Alq3 and other molecules could be successfully depth proled with large Ar clusters. ...
With the recent developments in secondary ion mass spectrometry (SIMS), it is now possible to obtain molecular depth profiles and 3D molecular images of organic thin films, i.e. SIMS depth profiles where the molecular information of the mass spectrum is retained through the sputtering of the sample. Several approaches have been proposed for "damageless" profiling, including the sputtering with SF5(+) and C60(+) clusters, low energy Cs(+) ions and, more recently, large noble gas clusters (Ar500-5000(+)). In this article, we evaluate the merits of these different approaches for the in depth analysis of organic photovoltaic heterojunctions involving poly(3-hexylthiophene) (P3HT) as the electron donor and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) as the acceptor. It is demonstrated that the use of 30 keV C60(3+) and 500 eV Cs(+) (500 eV per atom) leads to strong artifacts for layers in which the fullerene derivative PCBM is involved, related to crosslinking and topography development. In comparison, the profiles obtained using 10 keV Ar1700(+) (∼6 eV per atom) do not indicate any sign of artifacts and reveal fine compositional details in the blends. However, increasing the energy of the Ar cluster beam beyond that value leads to irreversible damage and failure of the molecular depth profiling. The profile qualities, apparent interface widths and sputtering yields are analyzed in detail. On the grounds of these experiments and recent molecular dynamics simulations, the discussion addresses the issues of damage and crater formation induced by the sputtering and the analysis ions in such radiation-sensitive materials, and their effects on the profile quality and the depth resolution. Solutions are proposed to optimize the depth resolution using either large Ar clusters or low energy cesium projectiles for sputtering and/or analysis.
... For these thicknesses, achieving suitable percolation pathways and phase separation simultaneously in the range of the exciton diffusion length (approximately 10 nm) is challenging,567 so great effort has been invested into controlling the morphology of the blends by choosing appropriate solvents or by employing annealing treatments891011 . Despite these optimizations, discontinuous pathways to the external electrodes are still a problem and result in the recombination of photogenerated charges, limiting charge extraction and efficiency1213141516. Although more 'ideal' geometries consisting of interdigitated donor and acceptor phases have been proposed as an alternative to bulk heterojunctions17181920, these structures are difficult to achieve and low carrier mobilities would still inhibit charge collection from their thick active layers. ...
We introduce hybrid solar cells with an architecture consisting of an electrodeposited ZnO nanorod array (NRA) coated with a conformal thin layer (<50 nm) of organic polymer-fullerene blend and a quasi-conformal Ag top contact (Thin/NR). We have compared the performance of Thin/NR cells to conventional hybrid cells in which the same NRAs are completely filled with organic blend (Thick/NR). The Thin/NR design absorbs at least as much light as Thick/NR cells, while charge extraction is significantly enhanced due to the proximity of the electrodes, resulting in a higher current density per unit volume of blend and improved power conversion efficiency. The NRAs need not be periodic or aligned and hence can be made very simply.
... In this work, we have a systematic study on the effect of variations in anode surface characteristics on charge collection efficiency. The study shows that total internal efficiency in single layer organic solar cell is also limited by the work function of used electrodes and HOMO (highest occupied molecular orbital)-LUMO (lowest occupied molecular orbital) levels of polymer [18,19]. ...
Arylenevinylene-co-pyrrolenevinylene (AVPV) is polymer oligomer system derived from arylbridged bispyrroles which has been explored for photovoltaic devices. In this paper, we show the dependence of the photovoltaic device parameters on the anode surface treatment in an organic single layer photovoltaic device based on AVPV as an electron donor. Since the total quantum efficiency includes the charge collection efficiency at the electrodes, experiments were carried out to vary the anode (ITO) characteristics, achieved by using ITO with or without ozonization and with or without PEDOT:PSS (Polyethylene dioxythiophene:Polystyrene sulphonic acid) layer. Devices fabricated on ITO anode (without ozonization and without PEDOT:PSS) exhibited the maximum current density (J sc = 1.3 µA·cm –2) as compared to those devices where ITO was ozonized as well as had a PEDOT:PSS layer (J sc = 0.1 µA·cm –2) measured under 1 sun illumination of AM 1.5 through a calibrated solar simulator.
