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Simulation of current–voltage characteristics at 10K of ITO/PEDOT:PSS/MAPbI3/Au with active layer thickness 200, 300, and 450 nm. (a) Full model including volume and interface traps. (b) Model with volume trap only. Models are the solid lines, experimental data symbols. Dot line represent pure SCLC model with no traps. Fitting parameters are presented in Table 1.
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Electron injection by tunneling from a gold electrode and hole transport properties in polycrystalline MAPbI3 has been investigated using variable temperature experiments and numerical simulations. The presence of a large and unexpected band bending at the Au/MAPbI3 interface is revealed and attributed to the trapping of holes, which enhances the i...
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... For the undoped MAPI samples, the absolute value of this difference is around 0.83 eV, placing the EF in the middle of the bandgap. 30 The position of EF increases along with the dopant concentration, reaching ~0.3 eV below the EC at dopant concentrations ≥ 0.2 mg/mL. This clearly demonstrates the ntype character of the doped samples. ...
Electrical doping of metal halide perovskites (MPHs) is a key step towards the use of this efficient and cost-effective semiconductor class in modern electronics. In this work, we demonstrate n-type doping of methylammonium lead iodide (CH3NH3PbI3) by the post-fabrication introduction of Sm2+. The ionic radius of the latter is similar to that of Pb2+ and can replace it without altering the perovskite crystal lattice. It is demonstrated that once incorporated, Sm2+ can act as a dopant by undergoing oxidation to Sm3+. This results in the release of a negative charge that n-dopes the material, resulting in an increase of conductivity of almost 3 orders of magnitude. Unlike substitution doping with heterovalent ions, furtive dopants do not require counterions to maintain charge neutrality with respect to the ions they replace and are thus more likely to be incorporated into the crystalline structure. The incorporation of the dopant throughout the material is evidenced by XPS and ToF-SIMS, while the XRD pattern shows no phase separation at low and medium doping concentrations. A shift of the Fermi level towards a conduction energy of 0.52 eV confirms the doping to be n-type with a charge carrier density, calculated using the Mott-Schottky method, estimated to be nearly 1017 cm-3 for the most conductive samples. Variable-temperature conductivity experiments show that the dopant is only partially ionized at room temperature due to dopant freeze-out.
... Combined with the result of the Tauc plot obtained from the UV-Vis spectra of the CABB film, the band gap of CABB film was determined to be 2.33 eV and the valence band (VB) maxima as 2.12 eV. This indicates that the Fermi level is located near the conduction band (CB), which shows that a larger number of hole-trap states are distributed above the VB [34,35]. As shown in Figure 5a, the Br 3d peak is composed of a doublet with 3d 3/2 and 3d 5/2 signals at 69.38 and 68.33 eV. ...
... Combined with the result of the Tauc plot obtained from the UV-Vis spectra of the CABB film, the band gap of CABB film was determined to be 2.33 eV and the valence band (VB) maxima as 2.12 eV. This indicates that the Fermi level is located near the conduction band (CB), which shows that a larger number of hole-trap states are distributed above the VB [34,35]. Figure 6 displays the conductive process in the Pt/CABB/ITO/glass device. ...
Light-modulated lead-free perovskites-based memristors, combining photoresponse and memory, are promising as multifunctional devices. In this work, lead-free double perovskite Cs2AgBiBr6 films with dense surfaces and uniform grains were prepared by the low-temperature sol-gel method on indium tin oxide (ITO) substrates. A memory device based on a lead-free double perovskite Cs2AgBiBr6 film, Pt/Cs2AgBiBr6/ITO/glass, presents obvious bipolar resistive switching behavior. The ROFF/RON ratio under 445 nm wavelength light illumination is ~100 times greater than that in darkness. A long retention capability (>2400 s) and cycle-to-cycle consistency (>500 times) were observed in this device under light illumination. The resistive switching behavior is primarily attributed to the trap-controlled space-charge-limited current mechanism caused by bromine vacancies in the Cs2AgBiBr6 medium layer. Light modulates resistive states by regulating the condition of photo-generated carriers and changing the Schottky-like barrier of the Pt/Cs2AgBiBr6 interface under bias voltage sweeping.
... The calculated theoretical Φ B values were determined to bẽ 680 and 440 meV for Gr-perovskite and Ag-perovskite interfaces, respectively, where the work function ϕ of Gr and Ag are ϕ Gr = 4.5 eV and ϕ Ag = 4.26 eV, respectively, and the perovskite electron affinity χ~3.82 eV 41 . The difference between the theoretical Φ B of the two contact metals is Δ ΦB (theoretical) = Φ B (Gr) − Φ B (Ag) = 680 − 440 meV = 240 meV, whereas at a 0 V bias voltage, the difference between the experimental Φ B values is Δ ΦB (experimental) = Φ B (Gr) − Φ B (Ag) = 920 − 712 meV = 208 meV, which closely matches Δ ΦB (theoretical) . ...
Silver (Ag) and graphene (Gr) inks have been engineered to serve as efficient electrical contacts for solution-processed two-dimensional (2D) organo-halide (CH3(CH2)3NH3)2(CH3NH3)n−1PbnI3n+1 (n = 4) layered perovskites, where all inkjet-printed heterostructure photodetectors (PDs) were fabricated on polyimide (PI) substrates. To date, limited studies exist that compare multiple contacts to enable high-performance engineered contacts to 2D perovskites. Moreover, of these few reports, such studies have examined contacts deposited using vapor-based techniques that are time-consuming and require expensive, specialized deposition equipment. In this work, we report on the inkjet printed, direct contact study of solution-processed, 2D perovskite-based PDs formed on flexible PI substrates. Solution processing offers a cost-effective, expedient route for inkjet printing Gr and Ag using a dispersion chemistry developed in this work that is compatible with the underlying 2D perovskite layer to construct the PDs. The wavelength λ-dependent photocurrent Ip peaked at λ ~ 630 nm for both PDs, consistent with the bandgap Eg ~ 1.96 eV for our semiconducting 2D perovskite absorber layer. The external quantum efficiency was determined to be 103% for Ag-perovskite PDs, where strain-dependent bending tests were also conducted to reveal the opto-mechanical modulation of the photocurrent in our devices.
... In view of that the detectivity of a photodetector is simultaneously determined by the responsivity and dark current, it can be further increased if the dark current of our devices were decreased. Based on these findings, a mechanism is proposed for the photocurrent amplification of the inverted HTL-free photodetector, as shown in Fig. 4. The work function of ITO is 4.72 eV [29], while it is 4.65 eV for CH 3 NH 3 PbI 3 [30]. Thus a downward shift energy level at the ITO/CH 3 NH 3 PbI 3 interface can be anticipated due to the electron transfer from CH 3 NH 3 PbI 3 to ITO (Fig. 4b). ...
... Thus a downward shift energy level at the ITO/CH 3 NH 3 PbI 3 interface can be anticipated due to the electron transfer from CH 3 NH 3 PbI 3 to ITO (Fig. 4b). The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of CH 3 NH 3 PbI 3 are 5.45 and 3.82 eV, respectively [30]. According to these energy levels, an electron injection barrier from ITO to CH 3 NH 3 PbI 3 is about 0.9 eV and the dark current under negative bias of the photodetector should be primary contributed from the electron Fig. 4. Schematic mechanism of the photocurrrent amplification in the ITO/CH 3 NH 3 PbI 3 /C 60 /Bphen/Ag photodetectors. ...
Perovskite photodetectors processing a photocurrent amplification have attracted much attention due to their high response to light illumination and have been exploited in a regular device geometry. Here, photocurrent amplification is demonstrated in an inverted perovskite photodetector, and the effect of the indium tin oxide (ITO)/perovskite interface on the photocurrent amplification factor are exploited. It is found that the photocurrent amplification is limited by the ITO/perovskite interface, which can be proved by the devices with bare ITO, Cu2O particles partially covered ITO, and poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) fully covered ITO as the anodes. A photocurrent amplification factor of about 27 is demonstrated in the perovskite photodetector with a bare ITO as the anode, which is one of the highest ones among the reported perovskite photodetectors. It also exhibits a maximum response of 4.1 A/W and a detectivity of 10¹¹ Jones. Compared with the bare ITO device, the photocurrent amplification of the devices decreases gradually with the reduced contact area between ITO and perovskite and eventually disappears when the direct contact is totally removed. The photocurrent amplification is attributed to the long-lived accumulated holes as well as the positively charged CH3NH3⁺ and I vacancy at the ITO/perovskite interface, which dramatically lowers the injection barrier of electrons and leads to a multiple electron injection.
... From the S as a function of V plot, ΦB was extracted using the following Eqn., where the fit to the data is shown in Figure 5 The calculated theoretical ΦB value for the Au-perovskite interface is ~ 460 meV where the work function of Au, ϕAu = 5.1 eV was used, and the perovskite electron affinity χ and Eg were ~ 3.9 eV [188] and ~ 1.66 eV, respectively; the Eg value used was extracted from our data in Figure 5.2-9. ...
This dissertation is devoted to the development of novel devices for optoelectronic and photovoltaic applications using the promise of inkjet printing with two-dimensional (2D) materials. A systematic approach toward the characterization of the liquid exfoliated 2D inks comprising of graphene, molybdenum disulfide (MoS2), tungsten diselenide (WSe2), and 2D perovskites is discussed at depth. In the first study, the biocompatibility of 2D materials --
graphene and MoS2 -- that were drop cast onto flexible PET and polyimide substrates using mouse embryonic fibroblast (STO) and human esophageal fibroblast (HEF) cell lines, was explored. The
polyimide samples for both STO and HEF showed high biocompatibility with a cell survival rate of up to ~ 98% and a confluence rate of 70-98%. An inkjet printed, biocompatible, heterostructure photodetector was constructed using inks of photo-active MoS2 and electrically conducting graphene, which facilitated charge collection of the photocarriers. The importance of such devices stems from their potential utility in age-related-macular degeneration (AMD), which is a condition where the photosensitive retinal tissue degrades with aging, eventually compromising vision. The biocompatible inkjet printed 2D heterojunction devices were photoresponsive to broadband incoming radiation in the visible regime, and the photocurrent scaled proportionally with the
incident light intensity, exhibiting a photoresponsivity R ~ 0.30 A/W. Strain-dependent measurements were also conducted with bending, that showed Iph ~ 1.16 μA with strain levels for curvature up to ~ 0.262 cm-1, indicating the feasibility of such devices for large format arrays printed on flexible substrates. Alongside the optoelectronic measurements, temperature-dependent (~ 80 K to 573 K) frequency shifts of the Raman-active E12g and A1g modes of multilayer MoS2 exhibited a red-shift with increasing temperature, where the temperature coefficients for the E12g
and A1g modes were determined to be ~ - 0.016 cm-1/K and ~ - 0.014 cm-1/K, respectively. The phonon lifetime τ was determined to be in the picosecond range for the E12g and A1g modes,
respectively, for the liquid exfoliated multilayer MoS2.
Secondly, an all inkjet printed WSe2-graphene hetero-structure photodetector on flexible polyimide substrates is also studied, where the device performance was found to be superior
compared to the MoS2-graphene photodetector. The printed photodetector was photo responsive to broadband incoming radiation in the visible regime, where the photo responsivity R ~ 0.7 A/W and conductivity σ ~ 2.3 × 10-1 S/m were achieved at room temperature.
Thirdly, the synthesis of solution-processed 2D layered organo-halide (CH3(CH2)3NH3)2(CH3NH3)n-1PbnI3n+1 (n = 2, 3, and 4) perovskites is presented here, where inkjet printing was used to fabricate heterostructure flexible photodetector devices on polyimide substrates. The ON/OFF ratio was determined to be high, ~ 2.3 × 103 while the photoresponse time 𝛕 on the rising and falling edges was measured to be 𝛕rise ~ 24 ms and 𝛕fall ~ 65 ms, respectively. The strain-dependent measurements, conducted here for the first time for inkjet printed perovskite photodetectors, revealed the Ip decreased by only ~ 27% with bending (radius
of curvature of ~ 0.262 cm-1). This work demonstrates the tremendous potential of the inkjet printed, composition tunable, organo-halide 2D perovskite heterostructures for high-performance
photodetectors, where the techniques are readily translatable toward flexible solar cell platforms as well. Fourthly, metal contacts and carrier transport in 2D (CH3(CH2)3NH3)2(CH3NH3)n-1PbnI3n+1 (n = 4) perovskites is a critical topic, where the use of silver (Ag) and graphene (Gr) inks as metallic contacts to 2D perovskites was investigated. The all inkjet printed Gr-perovskite and Agperovskite photodetectors were found to be photo-responsive to broadband incoming radiation where measurements were conducted from λ ~ 400 nm to 2300 nm. The photoresponsivity R and detectivity D were compared between the Gr-perovskite and Ag-perovskite photodetectors, which revealed the higher performance for the Ag-perovskite photodetector. The superior performance of the Ag-perovskite photodetector was also justified with the Schottky barrier analysis using the thermionic emission model through temperature-dependent transport measurements.
Finally, this dissertation ends with the description of the first steps for using solutionprocessed, inkjet printed perovskites for solar cells. The preliminary investigations include the discussion of the chemical formulations for the carrier separation layers, dispersion route, and the variation of solar cell figures of merit with processing.
... Direct perovskite/Au contacts are commonly regarded as an anode, but it is also known that the Au can become the cathode upon voltage bias. 9,[39][40][41][42] In XPS measurements, we observe the Au to be cathodic in our ITO/MAPbI 3 /Au (5 nm) devices. Figures 5(a) and 5(b) show the Au 4f 7/2 and valence XPS spectra of an ITO/MAPbI 3 /Au (5 nm) sample in the dark and under illumination (the data in Fig. 5 were obtained after 20 min of X-ray irradiation followed by 60 min of resting in the analysis chamber). ...
Chemical reactivity of halide perovskites coupled with a low energy of formation makes it a challenge to characterize material properties and achieve long-term device stability. In this study, we elucidate electrochemical reactions occurring at the methylammonium lead triiodide (MAPbI3)/Au interface. X-ray photoemission spectroscopy is used to identify a type of reduction/oxidation reaction termed underpotential deposition (UPD) involving lead, iodine, and hydrogen occurring at interfaces with noble metals. Changes in surface compositions and oxidation states suggest that UPD derived adsorbates at MAPbI3/Au interfaces lower the energy barrier for release of volatile HI and/or I2 catalyzing degradation at exposed contacts. Additionally, comparison to PbI2/Au interfaces demonstrates that the presence of methylammonium/methylamine accelerates the formation of a Pb⁰ adlayer on the Au. Reactions involving UPD Pb⁰ can transform the typically anodic (hole collecting) Au to a cathode in a photovoltaic measurement. Cyclic voltammetry reveals electrochemical reaction peaks in indium tin oxide (ITO)/MAPbI3/Au devices occurring within voltage ranges commonly used for perovskite characterization. The electrochemical stability window of this device architecture is measured to be between −0.5 V and 0.9 V. Voltage induced interfacial reactions contribute to reversible electrochemical peaks, hysteresis, switchable perovskite diode polarity, and permanent degradation at larger voltages. These types of surface reactions alter the interface/interphase composition beyond ion accumulation, provide a source for the diffusion of defects, and contribute to electrode material dependent current-voltage hysteresis. Moreover, the results imply fundamental limitations to achieving high device stability with noble metals and/or methylammonium containing perovskites.
... 294 Fig. 15a. 295 They found that photoactivated or electroactivated ions accumulated at localized sites and changed carrier dynamics, thus enhancing nonradiative recombination. Besides, ion migration from perovskite films can lead to corrosion of metal electrodes. ...
In the last few years, organic-inorganic halide perovskites (OIHP) have gained significant attention in optoelectronic community and became one of the most popular choice of research topics. Thanks to their fascinating properties, including a wide-range tunable band gap, long charge carrier diffusion length, high absorption coefficient and easy solution processability, they become one of the most promising low-cost and easily scalable semiconductor materials for various optoelectronic devices such as solar cells, photodetectors, and light-emitting diodes. In order to achieve the best performance and make them viable technologies for future energy harvesting, display and light sensing prototypes, in addition to the OHIP active layer, the use of interfacial charge transporting layers is crucial. These interfacial charge-transporting layers not only enhance the performance of these devices but also effectively protect the active environmentally unstable OHIP layer. In this review, we summarize the development and utilization of organic interfacial materials and OIHP in solar cells, photodetectors and light-emitting diode. In each part, the working principle and the development of wide range of hole/electron transporting materials are discussed. Finally, an outlook and further research directions as well as useful rules for the design of novel hole/electron transporting materials are proposed.
... Lapremì eré etape de la fabrication consistè a préparer la solution de pérovskite en dissolvant un mélange de iodure de méthylammonium (MAI) et d'acétate de plomb (PbAc) au ratio de 1 pour 3 dans du N,N-diméthylformamide (DMF) anhydre pour obtenir une concentration massique de 40%. Ce ratio est choisi car il a permis, au sein de notre laboratoire, d'obtenir des couches homogènes permettant d'atteindre des rendements de 15% dans des cellules solaires 285 . La solution est agitée durant une heure pour permettre la dissolutioncompì ete des solides avant d'ˆ etre filtrée (0,2 µm PTFE). ...
L’opinion publique est consciente que l’électronique qui nous entoure présente un coût de développement et de production important en plus d’un impact environnemental non négligeable. C’est dans le but de résoudre ces inconvénients que l’électronique organique est étudiée et développée. L’électronique organique a été introduite par la découverte de polymères conducteurs, par les prix Nobel de chimie de l’année 2000, Alan J. Heeger, Alan G. MacDiarmid et Hideki Shirakawa. Depuis lors cette technologie c’est grandement développée, on note ainsi de nos jours la commercialisation des écrans OLED (Organic Light Emitting Diode) mais aussi d’autres composants organiques comme les MEMS (Micro ElectroMechanical System), des systèmes liant l’électronique et la mécanique. Ces MEMS organiques sont de plus en plus étudiés et développés dû à une plus grande flexibilité des semi-conducteurs organiques par rapport à leurs homologues inorganiques. Cependant, même si la recherche sur la mécanique des polymères et l'électronique des semi-conducteurs organiques est avancée, l'interaction électromécanique de ces semi-conducteurs n'est que peu étudiée. Néanmoins, il est nécessaire de comprendre cette interaction pour développer l'électronique flexible de demain. L'objectif de ces travaux est donc d'approfondir les connaissances sur l'interaction électromécanique au sein des semi-conducteurs organiques et de développer des outils/méthodes facilement transposables à l'étude de nouvelles molécules. Pour mieux comprendre l'interaction entre la déformation de la structure des semi-conducteurs et leur réponse électrique, ces derniers sont fabriqués sous forme de monocristaux pour étudier un arrangement moléculaire parfait, sans défauts, dans les trois dimensions de l'espace. Ainsi donc dans un premier temps, l'influence de la structure moléculaire sur la mobilité des charges a été étudiée dans le cas du rubrène. Même s'il est majoritairement avancé que la distance intermoléculaire est la raison de la variation de mobilité dans le rubrène, il s'avère que la réponse électrique dépend en réalité d'un réarrangement moléculaire et de la variation d'une multitude de paramètres intra/intermoléculaires modifiant le couplage électronique entre molécules. Dans un deuxième temps, la réponse électromécanique de transistors, à diélectrique d’air, à base de rubrène a été étudiée. Dans ces systèmes plus complexes, plusieurs paramètres sont modifiés lors de la déformation. A l'aide du facteur de jauge, il est possible de mettre en évidence que la réponse électromécanique de ces transistors dépend majoritairement de la modification mécanique et électrique du contact entre le semi-conducteur et les électrodes. La forte amélioration de la réponse électrique des transistors a permis la fabrication de capteurs de forces capables de mesurer des forces de l'ordre de 230 nN. Finalement, les méthodes développées et utilisées lors de ces travaux ont été utilisées pour amorcer la fabrication et caractérisation électrique de transistors à base de pérovskites hybrides, dans le but d'étudier l'interaction électromécanique de ces matériaux émergents.
... using different methods. 39 This band bending produces that electronic charge injection at electron selective contact occurs via tunneling. 39 As a result the electrical field formed at the interface due to the cation and hole accumulation that produce the band bending 14 also influence the charge transfer properties via tunneling and consequently the photocurrent, see Figure S6, explaining the enhanced of J sc observed when Au@SiO 2 NPs are added at the interface. ...
... 39 This band bending produces that electronic charge injection at electron selective contact occurs via tunneling. 39 As a result the electrical field formed at the interface due to the cation and hole accumulation that produce the band bending 14 also influence the charge transfer properties via tunneling and consequently the photocurrent, see Figure S6, explaining the enhanced of J sc observed when Au@SiO 2 NPs are added at the interface. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 ...
The photoconversion efficiency of perovskite solar cells (PSCs) has been enhanced by the deposition of inorganic nanoparticles (NPs) at the interface between the compact TiO2 electron selective contact and the mesoporous TiO2 film. The NPs used have been core/shell Au@SiO2, where a thin SiO2 coating protects the Au core from the direct chemical interaction with CH3NH3PbI3 halide perovskite used as light harvesting material. Samples prepared with the Au@SiO2 NPs exhibits higher external quantum efficiency in all the complete wavelength range at which perovskite presents light absorption and not just at the wavelengths at which Au@SiO2 NPs presents their absorption peak. This fact rules out a direct plasmonic process as the responsible on cell performance enhancement. A detail characterization by photoluminescence, impedance spectroscopy and open circuit voltage decay unveil a modification of the interfacial properties with an augmentation of the interfacial electrostatic potential that increases both photovoltage and photocurrent. This work highlights the dramatic role of interfaces in PSC performance. The use of reduced quantities of highly stable inorganic compounds to modify the PSC interface instead of the extensively used organic compounds opens the door to a new surface engineering based on inorganic compounds.
... This is far from surprising, as the previously formulated SPM incorporates a kinetic delay of charge at the surface but not the energy loss process across the interface such as electron transfer by tunneling suggested in Figure 1. 22 The departure of b from the value 1 indicates that the spectra possess additional physical characteristics that need to be described by a more general model. We suggest that a resistive process for charge transfer across the interface may be needed, in agreement with the usual observation of a major decrease in photocurrent in the presence of poorer contacts, which causes a substantial decrease in solar cell performance. ...
The analysis of perovskite solar cells by impedance spectroscopy has provided a rich variety of behaviours that demand adequate interpretation. Two main features have been reported, firstly, different impedance spectral arcs vary in combination, secondly, inductive loops and negative capacitance characteristics appear as an intrinsic property of the current configuration of perovskite solar cells. Here we adopt a previously developed surface polarization model, based on the assumption of large electric and ionic charge accumulation at the external contact interface. Just from the equations of the model the impedance spectroscopy response is calculated and explains the mentioned general features. The inductance element in the equivalent circuit is the result of the delay of the surface voltage, and depends on the kinetic relaxation time. The model is therefore able to quantitatively describe exotic features of the perovskite solar cell and provide insight to the operation mechanisms of the device.
