CuSbSe2 as a Potential Photovoltaic Absorber Material: Studies from Theory to Experiment

CuSbSe₂ appears to be a promising absorber material for thin‐film solar cells due to its attractive optical and electrical properties, as well as earth‐abundant, low‐cost, and low‐toxic constituent elements. However, no systematic study on the fundamental properties of CuSbSe₂ has been reported, such as defect physics, material, optical, and electrical properties, which are highly relevant for photovoltaic application. First, using density functional theory calculations, CuSbSe₂ is shown to have benign defect properties, i.e., free of recombination‐center defects, and flexible defect and carrier concentration which can be tuned through the control of growth condition. Next, systematic material, optical, and electrical characterizations uncover many unexplored fundamental properties of CuSbSe₂ including band position, temperature‐dependent band gap energy, Raman spectrum, and so on, thus providing a solid foundation for further photovoltaic research. Finally, a prototype CuSbSe₂‐based thin film solar cell is fabricated by a hydrazine solution process. The systematic theoretical and experimental investigation, combined with the preliminary efficiency, confirms the great potential of CuSbSe₂ for thin‐film solar cell applications.     

Ultrasmall Nanoplatelets: The Ultimate Tuning of Optoelectronic Properties

2D semiconducting nanoplatelets (NPLs) are an emerging class of photoactive materials. They can be used as building blocks in optoelectronic devices thanks to their large absorption coefficient, high carrier mobility, and unique thickness‐dependent optical transitions. The main drawback of NPLs is their large lateral size, which results in unfavorable band energy levels and low quantum yield (QY). Here, ultrasmall lead chalcogenide PbSe_1–x S_x NPLs are prepared, which exhibit an unprecedented QY of ≈60%, the highest ever reported for this structure. The NPLs are applied as light absorber in a photoelectrochemical system, leading to a saturated photocurrent density of ≈5.0 mA cm^?2 (44 mL cm^?2 d^?1), which is a record for NPL‐based photoelectrodes in solar‐driven hydrogen generation. Ultrasmall NPLs hold the potential for breakthrough developments in the field of optically active nanomaterials.     

Low Temperature Synthesis of Stable γ‐CsPbI3 Perovskite Layers for Solar Cells Obtained by High Throughput Experimentation

The structural phases and optoelectronic properties of coevaporated CsPbI₃ thin films with a wide range of [CsI]/[PbI₂] compositional ratios are investigated using high throughput experimentation and gradient samples. It is found that for CsI‐rich growth conditions, CsPbI₃ can be synthesized directly at low temperature into the distorted perovskite γ‐CsPbI₃ phase without detectable secondary phases. In contrast, PbI₂‐rich growth conditions are found to lead to the non‐perovskite δ‐phase. Photoluminescence spectroscopy and optical‐pump THz‐probe mapping show carrier lifetimes larger than 75 ns and charge carrier (sum) mobilities larger than 60 cm² V^?1 s^?1 for the γ‐phase, indicating their suitability for high efficiency solar cells. The dependence of the carrier mobilities and luminescence peak energy on the Cs‐content in the films indicates the presence of Schottky defect pairs, which may cause the stabilization of the γ‐phase. Building on these results, p–i–n type solar cells with a maximum efficiency exceeding 12% and high shelf stability of more than 1200 h are demonstrated, which in the future could still be significantly improved, judging on their bulk optoelectronic properties.     

From Recombination Dynamics to Device Performance: Quantifying the Efficiency of Exciton Dissociation,Charge Separation,and Extraction in Bulk Heterojunction Solar Cells with Fluorine‐Substituted Polymer Donors

An original set of experimental and modeling tools is used to quantify the yield of each of the physical processes leading to photocurrent generation in organic bulk heterojunction solar cells, enabling evaluation of materials and processing condition beyond the trivial comparison of device performances. Transient absorption spectroscopy, “the” technique to monitor all intermediate states over the entire relevant timescale, is combined with time‐delayed collection field experiments, transfer matrix simulations, spectral deconvolution, and parametrization of the charge carrier recombination by a two‐pool model, allowing quantification of densities of excitons and charges and extrapolation of their kinetics to device‐relevant conditions. Photon absorption, charge transfer, charge separation, and charge extraction are all quantified for two recently developed wide‐bandgap donor polymers: poly(4,8‐bis((2‐ethylhexyl)oxy)benzo[1,2‐b:4,5‐b′]dithiophene‐3,4‐difluorothiophene) (PBDT[2F]T) and its nonfluorinated counterpart poly(4,8‐bis((2‐ethylhexyl)oxy)benzo[1,2‐b:4,5‐b′]dithiophene‐3,4‐thiophene) (PBDT[2H]T) combined with PC₇₁BM in bulk heterojunctions. The product of these yields is shown to agree well with the devices' external quantum efficiency. This methodology elucidates in the specific case studied here the origin of improved photocurrents obtained when using PBDT[2F]T instead of PBDT[2H]T as well as upon using solvent additives. Furthermore, a higher charge transfer (CT)‐state energy is shown to lead to significantly lower energy losses (resulting in higher V_OC) during charge generation compared to P3HT:PCBM.     

Evaluation of Built‐In Potential and Loss Mechanisms at Contacts in Organic Solar Cells: Device Model Parameterization,Validation, and Prediction

Maximizing the power conversion efficiency of organic solar cells requires the simultaneous optimization of its short‐circuit current density, fill factor, and open‐circuit voltage V_oc. Several key parameters of the device model needed to understand these quantities have not been reliably determined, even for the prototypical poly(3‐hexylthiophene):phenyl‐C₆₁‐methylbutyric ester (P3HT:PCBM) photoactive layer (PAL). Detailed analysis of the loss mechanisms at contacts and their rational optimization have not been possible. Here, using crosslinked P3HT network:PCBM cells with predefined ultrafine donor–acceptor morphology and very high internal quantum efficiencies, the built‐in potential V_bi is measured to decouple and reliably extract other key parameters of the cells. Using the refined device parameters, the general optimization of organic solar cells is evaluated and the following is established: i) The PAL composition of the first optical absorption optimum is displaced towards the more strongly absorbing component due to thin film effects. ii) The optimal cell configuration is the one in which the slower carrier travels on average the shorter distance to the collection contact, a consequence of the asymmetric photogeneration profile. iii) The absorption thickness optima follows a simple λ_p/n_PAL scaling law, where λ_p is its absorption center wavelength and n_PAL is the corresponding refractive index.     

Correlation of Absorption Profile and Fill Factor in Organic Solar Cells: The Role of Mobility Imbalance

We investigate the role of the spatial absorption profile within bulk heterojunction small molecule solar cells comprising a 50 nm ZnPc:C₆₀ active layer. Exploiting interference effects the absorption profile is varied by both the illumination wavelength and the thickness of an optical spacer layer adjacent to the reflecting electrode. The fill factor under 1 sun illumination is observed to change from 43 to 49% depending on the absorption profile which approximately equals the charge‐carrier generation profile. It is shown by varying the mixing ratio between ZnPc and C₆₀ that the importance of the generation profile is correlated with the imbalance of mobilities. Therefore, it is concluded that non‐geminate recombination is the dominating loss mechanism in these devices. Numerical drift‐diffusion simulations reproduce the experimental observations showing that charge carrier extraction is more efficient if charge carriers are generated close to the contact collecting the less mobile charge carrier type. Furthermore, this effect can explain the dependence of the internal quantum efficiency measured at short circuit on wavelength and implies that the spectral mismatch for a given solar simulator and device depends on the applied voltage.     

Highly Efficient Aqueous‐Processed Hybrid Solar Cells: Control Depletion Region and Improve Carrier Extraction

Environmental friendly aqueous‐processed solar cells have become one of the most promising candidates for the next‐generation photovoltaic devices. Researchers have made lots of progress in designing active materials with novel structures, manipulating the defects in active materials, optimizing device architecture, etc. However, it has long been a challenge to control the width of the depletion region and enhance carrier extraction ability. Fabrication of a thick bulk heterojunction (BHJ) film is an effective strategy to address these issues but difficult to realize. Herein, the thicker BHJ film of ZnO:CdTe is successfully fabricated and incorporated into CdTe‐poly(p‐phenylenevinylene) hybrid solar cells. As expected, this BHJ film enhances light absorption, extends the width of the depletion region, prolongs carrier lifetime, and promotes carrier extraction ability. Moreover, the electron transport layer of sol–gel ZnO with excellent transmittance and electrical conductivity boosts electron generation, transport, and injection, which further improves the device performance. As a result, the highest short current density (J_sc) of 19.5 mA cm^?2, power conversion efficiency of 6.51%, and the widest depletion region (177 nm) are obtained in aqueous‐processed hybrid solar cells.     

Disorder‐Induced Open‐Circuit Voltage Losses in Organic Solar Cells During Photoinduced Burn‐In

The photoinduced open‐circuit voltage (V_oc) loss commonly observed in bulk heterojunction organic solar cells made from amorphous polymers is investigated. It is observed that the total charge carrier density and, importantly, the recombination dynamics are unchanged by photoinduced burn‐in. Charge extraction is used to monitor changes in the density of states (DOS) during degradation of the solar cells, and a broadening over time is observed. It is proposed that the V_oc losses observed during burn‐in are caused by a redistribution of charge carriers in a broader DOS. The temperature and light intensity dependence of the V_oc losses can be described with an analytical model that contains the amount of disorder broadening in a Gaussian DOS as the only fit parameter. Finally, the V_oc loss in solar cells made from amorphous and crystalline polymers is compared and an increased stability observed in crystalline polymer solar cells is investigated. It is found that solar cells made from crystalline materials have a considerably higher charge carrier density than those with amorphous materials. The effects of a DOS broadening upon aging are suppressed in solar cells with crystalline materials due to their higher carrier density, making crystalline materials more stable against V_oc losses during burn‐in.     

Device Physics of Hybrid Perovskite Solar cells: Theory and Experiment

Perovskite solar cells (PSCs) exhibit a series of distinctive features in their optoelectronic response which have a crucial influence on the performance, particularly for long‐time response. Here, a survey of recent advances both in device simulation and optoelectronic and photovoltaic responses is provided, with the aim of comprehensively covering recent advances. Device simulations are included with clarifying discussions about the implications of classical drift–diffusion modeling and the inclusion of ionic charged layers near the outer carrier selective contacts. The outcomes of several transient techniques are summarized, along with the discussion of impedance and capacitive responses upon variation of bias voltage and irradiance level. In relation to the capacitive response, a discussion on the J–V curve hysteresis is also included. Although alternative models and explanations are included in the discussion, the review relies upon a key mechanism able to yield most of the rich experimental responses. Particularly for state‐of‐the‐art solar cells exhibiting efficiencies around or exceeding 20%, outer interfaces play a determining role on the PSC's performance. The ionic and electronic kinetics in the vicinity of the interfaces, coupled to surface recombination and carrier extraction mechanisms, should be carefully explored to progress further in performance enhancement.     

14.1% CsPbI3 Perovskite Quantum Dot Solar Cells via Cesium Cation Passivation

Surface manipulation of quantum dots (QDs) has been extensively reported to be crucial to their performance when applied into optoelectronic devices, especially for photovoltaic devices. In this work, an efficient surface passivation method for emerging CsPbI₃ perovskite QDs using a variety of inorganic cesium salts (cesium acetate (CsAc), cesium idodide (CsI), cesium carbonate (Cs₂CO₃), and cesium nitrate (CsNO₃)) is reported. The Cs‐salts post‐treatment can not only fill the vacancy at the CsPbI₃ perovskite surface but also improve electron coupling between CsPbI₃ QDs. As a result, the free carrier lifetime, diffusion length, and mobility of QD film are simultaneously improved, which are beneficial for fabricating high‐quality conductive QD films for efficient solar cell devices. After optimizing the post‐treatment process, the short‐circuit current density and fill factor are significantly enhanced, delivering an impressive efficiency of 14.10% for CsPbI₃ QD solar cells. In addition, the Cs‐salt‐treated CsPbI₃ QD devices exhibit improved stability against moisture due to the improved surface environment of these QDs. These findings will provide insight into the design of high‐performance and low‐trap‐states perovskite QD films with desirable optoelectronic properties.     

The Emergence of the Mixed Perovskites and Their Applications as Solar Cells

The halide perovskite (PVSK) materials (with ABX₃ formulation) have emerged as “dream materials” for photovoltaic (PV) applications due to their remarkable physical properties such as high optical absorption coefficient, carrier mobility, long carrier diffusion lengths, etc. These properties have enabled the PV devices to reach higher than 20% power conversion efficiencies (PCE) in record time. The further pursuit of higher PCE and improved stability brings forth increasing interests in so‐called “mixed composition” PVSK materials, consisting of partial substitution of the A, B, and/or X‐sites with alternative elements/molecules of similar size. Herein, we highlight the recent advances in developing mixed PVSK for PVs and their relevant optoelectronic properties. We mainly focus on mixed PVSK materials in the form of polycrystalline thin films, but also discuss nanostructured and two‐dimensional (2D) PVSK materials due to the increasing interest of broad readership. Efforts are exerted to elucidate the design principles of mixed PVSK and fabrication techniques for high performance optoelectronic devices, which help deepen our fundamental understanding of mixed PVSK systems. We hope this review will shed light onto the design and synthesis of mixed PVSK materials to further the progress of PVSK photovoltaics towards higher efficiencies and longer lifetimes.     

Linking Vertical Bulk‐Heterojunction Composition and Transient Photocurrent Dynamics in Organic Solar Cells with Solution‐Processed MoOx Contact Layers

It is demonstrated that a combination of microsecond transient photocurrent measurements and film morphology characterization can be used to identify a charge‐carrier blocking layer within polymer:fullerene bulk‐heterojunction solar cells. Solution‐processed molybdenum oxide (s‐MoO_x) interlayers are used to control the morphology of the bulk‐heterojunction. By selecting either a low‐ or high‐temperature annealing (70 °C or 150 °C) for the s‐MoO_x layer, a well‐performing device is fabricated with an ideally interconnected, high‐efficiency morphology, or a device is fabricated in which the fullerene phase segregates near the hole extracting contact preventing efficient charge extraction. By probing the photocurrent dynamics of these two contrasting model systems as a function of excitation voltage and light intensity, the optoelectronic responses of the solar cells are correlated with the vertical phase composition of the polymer:fullerene active layer, which is known from dynamic secondary‐ion mass spectroscopy (DSIMS). Numerical simulations are used to verify and understand the experimental results. The result is a method to detect poor morphologies in operating organic solar cells.     

Morphology and Efficiency: The Case of Polymer/ZnO Solar Cells

The performance of polymer solar cells critically depends on the morphology of the interface between the donor‐ and acceptor materials that are used to create and transport charge carriers. Solar cells based on poly(3‐hexylthiophene) and ZnO were fully characterized in terms of their efficiency and three‐dimensional (3D) morphology on the nanoscale. Here, we establish a quantitative link between efficiency and morphology by using the experimental 3D morphology as direct input for a 3D optoelectronic device model. This model includes the effects of exciton diffusion and quenching; space‐charge; recombination, generation, drift and diffusion of charge carriers; and the injection/extraction of carriers at the contacts. The observed trend in internal quantum efficiency as a function of layer thickness is reproduced with a single set of parameters. Several morphological aspects that determine the internal quantum efficiency are discussed and compared to other organic solar cells. This first direct use of morphological data in an optoelectronic device model highlights the importance of morphology in solar cells.     

Atomic Layer Deposition of Tin Monosulfide Thin Films

Thin film solar cells made from earth‐abundant, non‐toxic materials are needed to replace the current technology that uses Cu(In,Ga)(S,Se)₂ and CdTe, which contain scarce and toxic elements. One promising candidate absorber material is tin monosulfide (SnS). In this report, pure, stoichiometric, single‐phase SnS films were obtained by atomic layer deposition (ALD) using the reaction of bis(N,N′‐diisopropylacetamidinato)tin(II) [Sn(MeC(N‐ⁱPr)₂)₂] and hydrogen sulfide (H₂S) at low temperatures (100 to 200 °C). The direct optical band gap of SnS is around 1.3 eV and strong optical absorption (α > 10⁴ cm^?1) is observed throughout the visible and near‐infrared spectral regions. The films are p‐type semiconductors with carrier concentration on the order of 10¹⁶ cm^?3 and hole mobility 0.82–15.3 cm²V^?1s^?1 in the plane of the films. The electrical properties are anisotropic, with three times higher mobility in the direction through the film, compared to the in‐plane direction.     

Vacuum‐Assisted Growth of Low‐Bandgap Thin Films (FA0.8MA0.2Sn0.5Pb0.5I3) for All‐Perovskite Tandem Solar Cells

All‐perovskite multijunction photovoltaics, combining a wide‐bandgap (WBG) perovskite top solar cell (E_G ≈1.6–1.8 eV) with a low‐bandgap (LBG) perovskite bottom solar cell (E_G < 1.3 eV), promise power conversion efficiencies (PCEs) >33%. While the research on WBG perovskite solar cells has advanced rapidly over the past decade, LBG perovskite solar cells lack PCE as well as stability. In this work, vacuum‐assisted growth control (VAGC) of solution‐processed LBG perovskite thin films based on mixed Sn–Pb perovskite compositions is reported. The reported perovskite thin films processed by VAGC exhibit large columnar crystals. Compared to the well‐established processing of LBG perovskites via antisolvent deposition, the VAGC approach results in a significantly enhanced charge‐carrier lifetime. The improved optoelectronic characteristics enable high‐performance LBG perovskite solar cells (1.27 eV) with PCEs up to 18.2% as well as very efficient four‐terminal all‐perovskite tandem solar cells with PCEs up to 23%. Moreover, VAGC leads to promising reproducibility and potential in the fabrication of larger active‐area solar cells up to 1 cm².     

The Effect of PCBM Dimerization on the Performance of Bulk Heterojunction Solar Cells

Increasing the lifetime of polymer based organic solar cells is still a major challenge. Here, the photostability of bulk heterojunction solar cells based on the polymer poly[4,4′‐bis(2‐ethylhexyl)dithieno[3,2‐b:2′,3′‐d]silole)‐2,6‐diyl‐alt‐[2,5‐bis(3‐tetradecylthiophen‐2‐yl)thiazole[5,4‐d]thiazole)‐1,8‐diyl] (PDTSTzTz) and the fullerene [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PC₆₀BM) under inert atmosphere is investigated. Correlation of electrical measurements on complete devices and UV‐vis absorption measurements as well as high‐performance liquid chromatography (HPLC) analysis on the active materials reveals that photodimerization of PC₆₀BM is responsible for the observed degradation. Simulation of the electrical device parameters shows that this dimerization results in a significant reduction of the charge carrier mobility. Both the dimerization and the associated device performance loss turn out to be reversible upon annealing. BisPC₆₀BM, the bis‐substituted analog of PC₆₀BM, is shown to be resistant towards light exposure, which in turn enables the manufacture of photostable PDTSTzTz:bisPC₆₀BM solar cells.     

Benzo[1,2‐b:4,5‐b′]Dithiophene–6,7‐Difluoroquinoxaline Small Molecule Donors with >8% BHJ Solar Cell Efficiency

Solution‐processable small molecule (SM) donors are promising alternatives to their polymer counterparts in bulk‐heterojunction (BHJ) solar cells. While SM donors with favorable spectral absorption, self‐assembly patterns, optimum thin‐film morphologies, and high carrier mobilities in optimized donor–acceptor blends are required to further BHJ device efficiencies, material structure governs each one of those attributes. As a result, the rational design of SM donors with gradually improved BHJ solar cell efficiencies must concurrently address: (i) bandgap tuning and optimization of spectral absorption (inherent to the SM main chain) and (ii) pendant‐group substitution promoting structural order and mediating morphological effects. In this paper, the rational pendant‐group substitution in benzo[1,2‐b:4,5‐b′]dithiophene–6,7‐difluoroquinoxaline SMs is shown to be an effective approach to narrowing the optical gap (E_opt) of the SM donors ( SM1 and SM2 ), without altering their propensity to order and form favorable thin‐film BHJ morphologies with PC₇₁BM. Systematic device examinations show that power conversion efficiencies >8% and open‐circuit voltages (V_OC) nearing 1 V can be achieved with the narrow‐gap SM donor analog ( SM2 , E_opt = 1.6 eV) and that charge transport in optimized BHJ solar cells proceeds with minimal, nearly trap‐free recombination. Detailed device simulations, light intensity dependence, and transient photocurrent analyses emphasize how carrier recombination impacts BHJ device performance upon optimization of active layer thickness and morphology.     

Self‐Assembly Atomic Stacking Transport Layer of 2D Layered Titania for Perovskite Solar Cells with Extended UV Stability

A novel atomic stacking transporting layer (ASTL) based on 2D atomic sheets of titania (Ti_1?δO₂) is demonstrated in organic–inorganic lead halide perovskite solar cells. The atomically thin ASTL of 2D titania, which is fabricated using a solution‐processed self‐assembly atomic layer‐by‐layer deposition technique, exhibits the unique features of high UV transparency and negligible (or very low) oxygen vacancies, making it a promising electron transporting material in the development of stable and high‐performance perovskite solar cells. In particular, the solution‐processable atomically thin ASTL of 2D titania atomic sheets shows superior inhibition of UV degradation of perovskite solar cell devices, compared to the conventional high‐temperature sintered TiO₂ counterpart, which usually causes the notorious instability of devices under UV irradiation. The discovery opens up a new dimension to utilize the 2D layered materials with a great variety of homostructrual or heterostructural atomic stacking architectures to be integrated with the fabrication of large‐area photovoltaic or optoelectronic devices based on the solution processes.     

From Pb to Bi: A Promising Family of Pb‐Free Optoelectronic Materials and Devices

Lead‐based organic–inorganic hybrid perovskite materials are widely used in optoelectronic devices due to their excellent photophysical properties. However, the main issues which hinder its commercialization are the toxicity caused by lead and the intrinsic instability of the material. Recently, many lead‐free halide materials with good intrinsic stability have been reported, among which bismuth‐based halide materials have attracted extensive research due to their structure and promising optoelectronic properties. In this review, bismuth‐based materials are divided into binary BiX₃ (X = I, Br, Cl), ternary A_aBi_bX_a+3b (A = Cs, Rb, MA, Ag, etc.), and quaternary A₂AgBiX₆ (A = Cs, Rb, MA, etc.) according to its elemental composition. The structure and optoelectronic properties of bismuth‐based halide materials, which may be helpful for the development of bismuth‐based halide materials and lead‐free perovskites in the future, are summarized and highlighted.     

Highly Efficient and Stable Solar Cells with 2D MA3Bi2I9/3D MAPbI3 Heterostructured Perovskites

Organic–inorganic hybrid lead halide perovskites are emerging as highly promising candidates for highly efficient thin film photovoltaics due to their excellent optoelectronic properties and low‐temperature process capability. However, the long‐term stability in ambient air still is a key issue limiting their further practical applications. Herein, the enhancement of both performance and stability of perovskite solar cells is reported by employing 2D and 3D heterostructured perovskite films with unique nanoplate/nanocrystalline morphology. The 2D/3D heterostructured perovskites combine advantages of the high‐performance lead‐based perovskite 3D CH₃NH₃PbI₃ (MAPbI₃) and the air‐stable bismuth‐based quasi‐perovskite 2D MA₃Bi₂I₉. In the 2D/3D heterostructure, the hydrophobic MA₃Bi₂I₉ platelets vertically situate between the MAPbI₃ grains, forming a lattice‐like structure to tightly enclose the 3D MAPbI₃ perovskite grains. The solar cell based on the optimal 2D/3D (9.2%) heterostructured film achieves a high efficiency of 18.97%, with remarkably reduced hysteresis and significantly improved stability. The work demonstrates that construction of 2D/3D heterostructured films by hybridizing different species of perovskite materials is a feasible way to simultaneously enhance both efficiency and stability of perovskite solar cells.