Efficient Photovoltaic Devices Research Articles

Strain analysis and engineering in halide perovskite photovoltaics.

Halide perovskites are a compelling candidate for the next generation of clean-energy-harvesting technologies owing to their low cost, facile fabrication and outstanding semiconductor properties. However, photovoltaic device efficiencies are still below practical limits and long-term stability challenges hinder their practical application. Current evidence suggests that strain in halide perovskites is a key factor in dictating device efficiency and stability. Here we outline the fundamentals of strain within halide perovskites relevant to photovoltaic applications and rationalize approaches to characterize the phenomenon. We examine recent breakthroughs in eliminating the adverse impacts of strain, enhancing both device efficiencies and operational stabilities. Finally, we discuss further challenges and outline future research directions for placing stress and strain studies at the forefront of halide perovskite research. An extensive understanding of strain in halide perovskites is needed, which would allow effective strain management and drive further enhancements in efficiencies and stabilities of perovskite photovoltaics.

Ga-doped Czochralski silicon with rear p-type polysilicon passivating contact for high-efficiency p-type solar cells

The use of Ga-doped Czochralski (CZ) silicon wafers in Passivated Emitter and Rear Cells (PERC) has been confirmed to have a prominent advantage in suppressing light-induced degradation (LID), which will attract considerable attention for the application of Ga-doped wafers in more efficient photovoltaic devices. In this work, we investigate the passivation quality and address the issue of LID in Ga-doped CZ Si wafers equipped with p-type polysilicon passivating contact that consists of an ultrathin SiO x and a heavily doped polysilicon. We also present the modeling results for solar cells using this type of contact. The experiments show that samples with Ga-doped CZ Si wafers have superior anti-LID properties when compared to the samples with B-doped wafers. An excellent passivation performance with a high implied open-circuit voltage ( iV oc ) of 705 mV and a low single-sided saturation current density ( J 0,s ) of 9 fA/cm 2 was achieved. Moreover, with the help of the numerical simulations, we predict that the p-type Ga-doped CZ Si solar cells with p-type polysilicon passivating contacts have the potential to achieve a high efficiency of 23.8%, an ~0.5% absolute efficiency improvement over that of PERC solar cells. The results demonstrated in this study suggest that Ga-doped CZ Si wafers combined with p-type polysilicon passivating contacts could resolve the LID issue while maintaining good passivation properties, providing a promising alternative for p-type solar cells in the photovoltaic industry. • The p-type TOPC on featuring Ga-doped CZ Si wafers achieved an excellent passivation quality with iV oc > 705 mV. • The passivation improvement sources of Ga-doped CZ Si-based structures were attributed to the suppression of B- and O-related defects. • Ga-doped TOPCon solar cells show the LID-free performance. • Simulation results show a comparable efficiency of 23.8% for Ga-doped TOPCon solar cells.

Extensive Enhancement in Charge Collection Efficiency of Ferroelectric Cr-Doped BFO-Based Solar Cells by Using TiO2 Nanotube Arrays

In this article, 100 times or above enhancement in current has been reported in comparison to any of the previously reported BFO-based PV device. This was achieved by fabricating a novel heterostructure junction between ferroelectric Cr-doped BFO (BFCrO) and TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanotubes (T-NTs). The device performance is also compared with BFO/T-NTs junction as well. The Ag/BFO (or BFCrO)/T-NTs/Ti PV cells were developed with BFO (or BFCrO) as the absorber layer, T-NTs were employed as the electron transport layer, with Ti substrate and Ag as bottom and top electrodes, respectively. The remarkable performance of the p-BFCrO/n-T-NTs junction was due to higher absorption in the visible spectrum owing to the stronger ferroelectricity in BFCrO and the large surface area-to-volume ratio of T-NTs. The photovoltaic (PV) performance revealed enhanced current density (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SC</sub> ) and photovoltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> ) of 7.6 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and 0.85 V, respectively. The structural, morphological, and optical properties of both the fabricated structures have been investigated and compared. A well-fitted band structure was realized for the fabricated devices illustrating the band dynamics and carrier flow generation at the interface. The impedance properties confirmed a substantial improvement in the interfacial charge transfer kinetics with an amended driving force for electron-hole pair separation. The enhanced PV performance can be ascribed to the synergetic effect between the polarization induced switching of Cr doping in BFO and the reduced carrier recombination stimulated by highly conductive T-NTs. The structure presented here is promising for developing low cost and efficient PV devices suitable for green energy harvesting applications.

Design and Theoretical Study of a Metamaterial Absorber-Emitter Pair Matched with a Low-Bandgap PV Cell for an STPV System

The conversion efficiency of conventional semiconductor photovoltaic (PV) devices is restricted to a low level due to the Shockley-Queisser (S-Q) limit. A solar thermophotovoltaic (STPV) system, which consists of a spectrally selective absorber, narrowband emitter, and low-bandgap PV cell, can serve as an alternative to overcome this theoretical limit. However, the stringent requirements of absorber and emitter (like angular and polarization insensitivity and high-temperature resistance) require a careful selection of appropriate material and optimization of structures. To further improve the efficiency of the overall STPV system and ensure high performance, we designed a metamaterial-based absorber-emitter pair matched with InGaAsSb cells for an STPV system. Our numerical simulations demonstrate that the designed absorber provides ultrahigh broadband absorption in the spectral range of 300-2000 nm with a sharp decrease in absorptivity beyond 2000 nm, improving the spectral efficiency to 98%. For the emitter, narrowband emission is ultimately achieved, which prevents both sub-bandgap emission and thermalization loss in the PV cell. Moreover, the pair composed of refractory materials is insensitive to the incident angle and polarization angle. According to detailed balance calculations, the system efficiency can exceed the S-Q limit (41%) since a low temperature (1453 K) and the maximum efficiency (45.94%) outperforms previously reported demonstrations of STPV systems with InGaAsSb cells.

Rare earth doping in perovskite luminescent nanocrystals and photoelectric devices

AbstractHalide perovskites have shown great promise in optoelectronics and photovoltaics owing to their outstanding photoelectrical properties. However, their inherent shortcomings, such as lack of near‐infrared (NIR) photoluminescence (PL), instability and low efficiency, limit their practical applications. It is notable that doping and ion substitution represent effective strategies for tailoring the optoelectronic properties and stabilities of perovskite nanocrystals (NCs). Rare earth (RE) ions exhibit unique electronic and optical properties; the combination of lead halide perovskite and RE ions can combine the excellent optoelectronic properties of the host with the f‐f electronic transitions of the dopants and promote the further development of perovskite in the field of optoelectronics. For example, the efficient quantum cutting phenomenon in Yb3+‐doped CsPb(Cl/Br)3 NCs yields a photoluminescence quantum yield (PLQY) close to 200%, which have enabled promising proof‐of‐principle demonstration of solar photovoltaics. This review comprehensively summarizes the crystal structure, synthesis strategy, and applications of RE‐doped halide perovskites. We systematically discussed the challenges of RE ions in halide perovskite photovoltaic and light‐emitting devices, which provided a leading direction for the development of efficient and stable perovskite photovoltaic devices in the future.

Truth and Myth of Phase Coexistence in Methylammonium Lead Iodide Perovskite Thin Film via Transmission Electron Microscopy.

Hybrid organic-inorganic perovskites have attracted substantial interest as the most favorable prospective material for efficient photovoltaic and optoelectronic devices. However, their extreme sensitivity to electron beam radiation makes it difficult to obtain their intrinsic structure by transmission electron microscopy and can even lead to significant misidentifications. In 2018, the coexistence of methylammonium lead iodide (MAPbI3 ) in the cubic and tetragonal phase using electron microscopy and electron diffraction techniques was reported in article "Self-Organized Superlattice and Phase Coexistence inside Thin Film Organometal Halide Perovskite". Herein, however, that claim is challenged by comparing their experimental data to simulated diffraction patterns and arguing that their perovskite samples may have been damaged due to excessive electron beam irradiation. Consequently, true phase coexistence was not observed in that previously reported work, rather merely the decomposition products of MAPbI3 .

Synthesis of Group II-VI Semiconductor Nanocrystals via Phosphine Free Method and Their Application in Solution Processed Photovoltaic Devices.

Solution-processed CdTe semiconductor nanocrystals (NCs) have exhibited astonishing potential in fabricating low-cost, low materials consumption and highly efficient photovoltaic devices. However, most of the conventional CdTe NCs reported are synthesized through high temperature microemulsion method with high toxic trioctylphosphine tellurite (TOP-Te) or tributylphosphine tellurite (TBP-Te) as tellurium precursor. These hazardous substances used in the fabrication process of CdTe NCs are drawing them back from further application. Herein, we report a phosphine-free method for synthesizing group II-VI semiconductor NCs with alkyl amine and alkyl acid as ligands. Based on various characterizations like UV-vis absorption (UV), transmission electron microscope (TEM), and X-ray diffraction (XRD), among others, the properties of the as-synthesized CdS, CdSe, and CdTe NCs are determined. High-quality semiconductor NCs with easily controlled size and morphology could be fabricated through this phosphine-free method. To further investigate its potential to industrial application, NCs solar cells with device configuration of ITO/ZnO/CdSe/CdTe/Au and ITO/ZnO/CdS/CdTe/Au are fabricated based on NCs synthesized by this method. By optimizing the device fabrication conditions, the champion device exhibited power conversion efficiency (PCE) of 2.28%. This research paves the way for industrial production of low-cost and environmentally friendly NCs photovoltaic devices.

Improved hole injection/extraction using PEDOT:PSS interlayer coated onto high temperature annealed ITO electrode for efficient device performances

We demonstrate the improved device performances by using the structure of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) film coated onto the indium tin oxide (ITO) anodic electrode annealed at 400 °C under the normal ambient. The ITO thin films show the improved film quality with decreased dislocation density and lattice strain as annealing temperature increases. The spin-coated PEDOT:PSS film smoothens the wrinkle kind of surface morphology of the ITO film annealed at 400 °C. The annealed ITO (400 °C) with PEDOT:PSS interlayer improves the hole-current density in the hole-only devices (HODs) having the device structure of ITO/PEDOT:PSS/N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine as hole-transporting layer/Al. It enhances the efficiency of organic photovoltaic devices [ITO (annealed)/PEDOT:PSS/P3HT:PCBM (active layer)/LiF/Al] by three times higher (1.69 %) when compared to that (0.48 %) of pristine ITO based OPV device. These results show that the annealing of ITO film at the high temperature of 400 °C under the normal ambient improves the film quality and lowers the potential energy barrier at ITO/PEDOT:PSS interface for effective hole injection/extraction process, resulting in the enhanced device electrical performances. • Annealed ITO films improve film quality with decreased dislocation density and lattice strain. • PEDOT:PSS smoothens the wrinkle kind of surface morphology of ITO annealed at 400 °C. • 400 °C annealed ITO with PEDOT:PSS interlayer lowers the potential barrier and improves hole-current density in HODs. • It enhances the efficiency of OPVs by three times (1.69%) compared to that (0.48%) of pristine ITO based OPV. • 400 °C annealed ITO film lowers potential barrier at ITO/PEDOT:PSS interface for effective hole injection/extraction process.

High-efficiency organic solar cells prepared using a halogen-free solution process

Although the power conversion efficiency (PCE) of organic photovoltaic (OPV) devices has recently improved to more than 16%, halogenated solution processes are typically employed to obtain optimal performance. However, halogenated processing is harmful to health and the environment, which can be a significant obstacle to commercialization. Therefore, development of active materials processable under halogen-free conditions is of great importance in this field. In this study, a 16.04% OPV device, processed under halogen-free conditions, is developed by employing a new active-blend system, PBDTTPD-HT:BTP-2F-BO. It is highly soluble in halogen-free solvents, forming a preferential bulk-heterojunction morphology. The PBDTTPD-HT:BTP-2F-BO device achieves a PCE comparable with current state-of-the-art devices based on PBDB-TF:BTP-4F (also known as PM6:Y6) using a conventional halogenated process (16.40% versus 16.33%). Furthermore, it demonstrates a significantly higher PCE than the PBDB-TF:BTP-4F device with a halogen-free process (16.04% versus 9.70%). • A halogen-free processable OPV device with a PCE of 16.04% is demonstrated • The device achieves a comparable PCE under halogenated and halogen-free processing conditions • The reported device with an active area of ∼1 cm 2 achieves a PCE of 14.93% • Preferential energy levels, solubility, and miscibility enhance performance of device In many cases, processing of organic photovoltaics uses conventional halogenated solvents, such as chloroform and chlorobenzene, which are significantly harmful to health and the environment. Wibowo et al. report a photoactive system that is processable under halogen-free conditions, showing that the active-blend system achieves similar performance under halogenated and halogen-free processing (16.40% versus 16.04%).

Ultrafast Charge Transfer and Relaxation at a Donor-Acceptor Interface.

The efficiency of charge separation in organic photovoltaic materials is crucially determined by the underlying dynamics of the charge transfer (CT) excitons and their dissociation into free electrons and holes. To unravel the main principles of the underlying mechanism on a molecular level, we construct a toy model of electronically coupled donors interacting with a manifold of CT exciton states. In particular, we set up a ladder of CT site energies to model the exciton dissociation. To mimic the complexity of the exciton dynamics at the donor-acceptor interface, the electronic CT manifold is designed to include two vibrational modes that are vibronically coupled to the excitons. We examine the impact of the electronic and vibrational coherences and the structure of the vibronic manifold on the transfer efficiency and charge recombination. Optimal configurations of the vibronic CT manifold are revealed. In particular, the rate of charge recombination can be minimized when the transient dynamics are carefully explored. Such a toy model can be used as a guide for the design of organic materials for efficient photovoltaic devices.

Graphene quantum dots doping SnO2 for improving carrier transport of perovskite solar cells

SnO2 is recognized as an excellent electron transport layer. However, it does not possess the optimal morphology required by perovskite photovoltaic devices, and the conduction band of SnO2 cannot match well with perovskite conduction bands. In the present study, we introduced graphene quantum dots (GQDs) into SnO2. The results show that the introduction of graphene quantum dots effectively improved the morphology of the SnO2 film, reduced the roughness of the surface, and increased the ohmic contact between the SnO2 and perovskite layer. In addition, the conductivity of the SnO2 film was improved, thereby facilitating the transfer of electrons. The optimized SnO2: GQDs layer reduced the density of defect state, suppressed the pinholes appearance to a certain extent, and facilitated the extraction of electrons, thereby ultimately improving the FF, and increasing the efficiency of the perovskite photovoltaic device.

Optoelectronic properties of naphthalene bis-benzimidazole based derivatives and their photovoltaic applications

In the current study, the four molecules (F1-F4) have been designed to augment the efficiency of organic photovoltaic (OPV) devices by employing different acceptor moieties named as 2-methylenemalononitrile (F1), 2-(2-methylene-3-oxo-2,3-dihydroinden-1-ylidene) malononitrile (F2), 3-methyl-5-methylene-2-thioxothiazolodin-4-one (F3) and methyl 2-cyanoacrylate (F4). All the designed molecules shared same acceptor core named as naphthalene bis benzimidazole linked with acceptor groups at peripheral positions through thiophene. Comparative analysis has been conducted using selected hybrid functional WB97XD/6-31G (d, p) between designed molecules and reference molecule to interpret their optoelectronic properties such as Frontier molecular orbitals, Density of States, Reorganization energies, Transition density matrix analysis. Optical properties at TD-DFT predicted that F2 molecule has better UV-absorption as compared to reference molecule R. Reorganization energies, lower binding energies and higher open circuit voltage values have been predicted which showed that these designed molecules could be the best substitute for charge transporting materials. All these theoretical calculations showed that the designed acceptor molecules have outstanding optoelectronic properties in future utilization as compared with reference molecule.

Direct Quantification of Quasi-Fermi-Level Splitting in Organic Semiconductor Devices

Non-radiative losses to the open-circuit voltage are a primary factor in limiting the power conversion efficiency of organic photovoltaic devices. The dominant non-radiative loss is intrinsic to the active layer and can be determined from the quasi-Fermi level splitting (QFLS) and the radiative thermodynamic limit of the photovoltage. Quantification of the QFLS in thin film devices with low mobility is challenging due to the excitonic nature of photoexcitation and additional sources of nonradiative loss associated with the device structure. This work outlines an experimental approach based on electro-modulated photoluminescence, which can be used to directly measure the intrinsic non-radiative loss to the open-circuit voltage; thereby, quantifying the QFLS. Drift-diffusion simulations are carried out to show that this method accurately predicts the QFLS in the bulk of the device regardless of device-related non-radiative losses. State-of-the-art PM6:Y6-based organic solar cells are used as a model to test the experimental approach, and the QFLS is quantified and shown to be independent of device architecture. This work provides a method to quantify the QFLS of organic solar cells under operational conditions, fully characterizing the different contributions to the non-radiative losses of the open-circuit voltage. The reported method will be useful in not only characterizing and understanding losses in organic solar cells, but also other device platforms such as light-emitting diodes and photodetectors.

Synthesis and Formation Mechanism of Colloidal Janus-Type Cu2-xS/CuInS2 Heteronanorods via Seeded Injection.

Colloidal heteronanocrystals allow for the synergistic combination of properties of different materials. For example, spatial separation of the photogenerated electron and hole can be achieved by coupling different semiconductors with suitable band offsets in one single nanocrystal, which is beneficial for improving the efficiency of photocatalysts and photovoltaic devices. From this perspective, axially segmented semiconductor heteronanorods with a type-II band alignment are particularly attractive since they ensure the accessibility of both photogenerated charge carriers. Here, a two-step synthesis route to Cu2–xS/CuInS2 Janus-type heteronanorods is presented. The heteronanorods are formed by injection of a solution of preformed Cu2–xS seed nanocrystals in 1-dodecanethiol into a solution of indium oleate in oleic acid at 240 °C. By varying the reaction time, Janus-type heteronanocrystals with different sizes, shapes, and compositions are obtained. A mechanism for the formation of the heteronanocrystals is proposed. The first step of this mechanism consists of a thiolate-mediated topotactic, partial Cu+ for In3+ cation exchange that converts one of the facets of the seed nanocrystals into CuInS2. This is followed by homoepitaxial anisotropic growth of wurtzite CuInS2. The Cu2–xS seed nanocrystals also act as sacrificial Cu+ sources, and therefore, single composition CuInS2 nanorods are eventually obtained if the reaction is allowed to proceed to completion. The two-stage seeded growth method developed in this work contributes to the rational synthesis of Cu2–xS/CuInS2 heteronanocrystals with targeted architectures by allowing one to exploit the size and faceting of premade Cu2–xS seed nanocrystals to direct the growth of the CuInS2 segment.

Tuning optical and electronic properties of 2D ZnI2/CdS heterostructure by biaxial strains for optical nanodevices: A first-principles study

The structural and optoelectronic properties of a novel ZnI2/CdS van der Waals (vdW) heterostructure are studied under the effect of biaxial strain based on the density functional theory. Our results show that the ZnI2/CdS vdW heterostructure is dynamically and thermally stable depending on the molecular dynamics simulation and phonon dispersion curve. The results also indicate that the ZnI2/CdS heterostructure exhibits type-II band alignment with an indirect energy gap of 0.886 and 1.336 eV according to the Perdew–Burke–Ernzerhof and Heyd–Scuseria–Ernzerhof methods, respectively. Besides, the biaxial strain has a significant impact on the electronic properties. The energy bandgap of the ZnI2/CdS heterostructure decreases gradually as the compressive strain increases, reaching a minimum value of 1.162 eV at −6%. Also, a transformation from indirect bandgap to direct bandgap appears at strains of 4% and 6%. Broadly, it has been found that the optical properties of the ZnI2/CdS vdW heterostructure improve under the influence of strain, and the absorption coefficient can reach 105 cm−1 with the emergence of a shift phenomenon that expands the absorption capacity. Therefore, the application of strain will drastically improve the optical and electronic properties of the ZnI2/CdS vdW heterostructure, providing a roadmap for enhancing optical efficiency in photocatalytic and photovoltaic devices.

Development of interlayers based on polymethacrylate incorporating tertiary amine for organic solar cells with improved efficiency and stability

Nonfullerene acceptors have played a critical role in achieving record efficiency of organic photovoltaic (OPV) devices, and ITIC-based materials are an important type of nonfullerene acceptor. Herein, we developed new interlayer polymers for nonfullerene acceptor-based OPVs: PDMA(N)EMA and PDMA (N–O)EMA. In particular, PDMA(N)EMA effectively lowered the work function at the interface with the bulk heterojunction active layer and thus enabled efficient electron collection by the OPV device. Therefore, the OPV device incorporating PDMA(N)EMA showed a power conversion efficiency (PCE) of 8.19%, which was substantially enhanced compared with the 4.54% PCE of an ethoxylated polyethylenimine (PEIE)-based device. Whereas PEIE chemically attacked ITIC and degraded the performance of the OPV device, the PDMA(N)EMA formed a stable interface with ITIC, resulting in a dramatic increase of the VOC of the PDMA(N)EMA-based devices. Furthermore, the suppression of degradation at the PDMA(N)EMA interface also improved device stability at elevated temperatures. This work highlights the importance of the design and development of appropriate interlayer materials for realizing stable and high-efficiency nonfullerene-based OPVs.

Over 17.6% Efficiency Organic Photovoltaic Devices with Two Compatible Polymer Donors

Efficient ternary blend organic photovoltaic devices (OPVs) are built based on a D18‐Cl:Y6 host system and star polymer donor PM6 as the third component. A power conversion efficiency (PCE) of 16.89% is achieved in D18‐Cl:Y6 host binary OPVs, with a short‐circuit current density (J SC) of 25.31 mA cm−2, an open‐circuit voltage (V OC) of 0.878 V, and a fill factor (FF) of 75.81%. Upon incorporating appropriate PM6 in active layers, the PCE of optimal ternary blend OPVs can be increased to 17.61%, benefiting from a J SC of 26.35 mA cm−2, a V OC of 0.871 V, and an FF of 76.82%. Polymers D18‐Cl and PM6 have good compatibility and similar highest occupied molecular orbital (HOMO) levels, which enable to form D18‐Cl:PM6 alloyed states for efficient hole transport in ternary blend active layers. Meanwhile, trap density in ternary blend active layers is decreased by incorporating PM6, which is conducive to weaken charge recombination in ternary blend active layers. The gradually varied V OC values of ternary blend OPVs can be well explained from the varied HOMO levels of D18‐Cl:PM6 alloyed states. The results indicate that two compatible polymer donors with similar HOMO levels have great potential in achieving efficient ternary blend OPVs.

Upconversion Photovoltaic Effect of WS2/2D Perovskite Heterostructures by Two-Photon Absorption.

Photovoltaic devices work by converting sunlight energy into electric energy. The efficiency of current photovoltaic devices, however, is significantly limited by the transmission loss of photons with energies below the bandgap of channel semiconductors, which can be circumvented by photon energy upconversion. Energy upconversion has been widely employed to improve the efficiency of traditional solar cells. However, the employment of energy upconversion in two-dimensional (2D) heterostructure photovoltaic devices has not been investigated yet. Here, we report the upconversion photovoltaic effect of WS2 monolayer/(C6H5C2H4NH3)2PbI4 (PEPI) 2D perovskite heterostructures by below-bandgap two-photon absorption via a virtual intermediate state. An open circuit voltage of 0.37 V and short circuit current of 7.4 pA are obtained with a photoresponsivity of 771 pA/W and current on/off ratio of 130:1. This work demonstrates that upconversion by two-photon absorption may potentially be a strategy for boosting the efficiency of 2D material-based photovoltaic devices by virtue of the absorption of photons below the bandgap energy of channel semiconductors.

Study on the physical properties of Cu2ZnSnS4 thin films deposited by pneumatic spray pyrolysis technique

The acquisition of new materials for the manufacturing of high efficiency and low-cost photovoltaic devices has currently become a challenge. Thin films of CuInGaSe and CdTe have been widely used in solar cell of second generation, achieving efficiencies about 20 %; however, the low abundance of In and Te as well as the toxicity of Cd is the primary obstacles to their industrial production. Compounds such as Cu2ZnSnS4, Cu2ZnSnSe4 and Cu2ZnSn(SSe)4 have emerged as an important and less costly alternative for efficient energy conversion in the future. In addition, these compounds have the required characteristics to be used as an absorber material in solar cells (band-gap close to 1.4 eV, an absorption coefficient greater than 104 cm-1 and a p-type conductivity). In this work, we present a study of the structural, compositional, morphological and optical properties of Cu2ZnSnS4 thin films deposited by spray pyrolysis technique as well as their dependence on temperature.

High-Lying 31Ag Dark-State-Mediated Singlet Fission.

Singlet fission (SF), the conversion of one high-energy singlet to two low-energy triplets, provides the potential to increase the efficiency of photovoltaic devices. In the SF chromophores with C2h symmetry, exemplified by polyenes, singlet-to-triplet conversion generally involves a low-lying 21Ag dark state, which serves as either a multiexciton (ME) intermediate to promote the SF process or a parasitic trap state to shunt excited-state populations via internal conversion. This controversial behavior calls for a deep understanding of dark-state-related photophysics involving the higher-lying singlet state. However, the optical "dark" and "transient" nature of these dark states and strong correlation feature of double exciton species make their characterization and interpretation challenging from both experimental and computational perspectives. In the present work combining transient spectroscopy and multireference electronic structure calculations (XDW-CASPT2), we addressed a new photophysical model, i.e., a high-lying 31Ag dark-state-mediated ultrafast SF process in the benzodipyrrolidone (BDPP) skeleton. Such a 31Ag dark state with distinctive double excitation character, described as the ME state, could be populated from the initial 11Bu bright state on an ultrafast time scale given the quasi-degeneracy and intersection of the two electronic states. Furthermore, the suitable optical band gap and triplet energy, high triplet yield, and excellent photostability render BDPP a promising SF candidate for photovoltaic devices. These results not only enrich the arsenal of SF materials but also shed new insights into the understanding of dark-state-related photophysics, which could promote the development of new SF-active materials.