awn

2,3,6,7,10,11‐Hexaacetoxytriphenylene (HATP) as a discotic liquid crystal with high mobility can aggregate into a column structure on poly(3,4‐ethylene‐dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) in organic solar cells. HATP columns facilitate the formation of a 3D charge transportation, which increases the intermolecular charge transport and mobility. In addition, triplet excitons, trap states, and bimolecular recombination are suppressed. Thus, the short‐circuit current density is increased significantly.

In this work, a discotic liquid crystal (DLC) 2,3,6,7,10,11‐hexaacetoxytriphenylene (HATP) is used as the interlayer between poly(3,4‐ethylene‐dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and the active layer to achieve 3D charge transportation for organic solar cells (OSCs). HATP exhibits a columnar structure with a dominantly edge‐on orientation. For a (non‐)fullerene OSC system based on face‐on orientation, HATP columns are favorable for the expansion of edge‐on and face‐on crystallite in the active layer. According to the surface energy, surrounding the HATP columns are mainly acceptors. The significantly improved electron mobility indicates that it is easier for the electron to hop into HATP columns to transport, which represents the formation of the 3D pathway. This transport mode typically can enhance the intermolecular charge transport and effectively suppress the generation of triplet excitons and recombination. Thus, the short‐circuit current density (J _SC) is increased by 14% and 19% for fullerene and non‐fullerene systems, respectively. The power conversion efficiency is improved for non‐fullerene OSCs with different active layer thicknesses (≥150 nm) and fullerene OSCs with an active layer thickness of 140 nm. Overall, this work demonstrates an approach to introduce HATP columns on a PEDOT:PSS layer that has great potential to form a 3D pathway for achieving high‐performance (non‐)fullerene OSCs.

A sulfonyl fluoride‐functionalized phenethylammonium salt (SF‐PEA) is demonstrated as an efficient crystallization modulator for fabricating formamidinium lead iodide (FAPbI₃) perovskite films with high optoelectronic quality and phase stability. A champion power conversion efficiency (PCE) of 21.25% (certified PCE of 20.70%) is achieved, which is the highest among “methylammonium‐free” FAPbI₃ perovskite solar cells.

Bulky organic ammonium cations have been widely used to stabilize lead halide perovskites via surface passivation or dimensionality modulation. Herein, a sulfonyl fluoride‐functionalized phenethylammonium salt (SF‐PEA) is reported as a bifunctional additive to stabilize the formamidinium lead iodide (FAPbI₃) perovskite. The sulfonyl group is found to interact with PbI₂ in the precursor and slow down the crystallization of FAPbI₃ during thermal annealing, leading to improved crystalline quality and decreased structural defects. After annealing, the spontaneous assembly of SF‐PEA on the crystal surface of FAPbI₃ not only passivates the surface defects, but also protects the perovskite from phase transition that is caused by strain or moisture invasion. The resulting FAPbI₃ films are extremely stable, which can maintain their black phase for more than 3 months in air with 40–50% relative humidity, much better than pristine and unsubstituted phenethylammonium (PEA)‐based samples. Because of the greatly improved phase stability and crystallization quality, a champion power conversion efficiency (PCE) of 21.25% (certified PCE of 20.70%) is achieved in planar n–i–p structured solar cells, which is the highest one among “methylammonium‐free” FAPbI₃ perovskite photovoltaics.

Publication date: May 2020

Source: Nano Energy, Volume 71

Author(s): Mi-Jung Choi, You-Sun Lee, In Hwa Cho, Seok‐Soon Kim, Do-Hyung Kim, Sung-Nam Kwon, Seok-In Na

Publication date: May 2020

Source: Nano Energy, Volume 71

Author(s): Cheng Chen, Cheng Wu, Xingdong Ding, Yi Tian, Mengmeng Zheng, Ming Cheng, Hui Xu, Zhiwen Jin, Liming Ding

Sn‐based perovskite light‐emitting diodes with ultra‐high red color purity, a brightness of 70 cd m⁻², and 24 nm linewidth are prepared. The devices show excellent color stability under different temperatures, power, and operating voltage. Based on the oxidation pathway of Sn, H₃PO₂ is chosen to suppress the oxidation of Sn²⁺ and slow down the crystal growth, simultaneously.

Abstract

Perovskite‐based light‐emitting diodes (PeLEDs) are now approaching the upper limits of external quantum efficiency (EQE); however, their application is currently limited by reliance on lead and by inadequate color purity. The Rec. 2020 requires Commission Internationale de l'Eclairage coordinates of (0.708, 0.292) for red emitters, but present‐day perovskite devices only achieve (0.71, 0.28). Here, lead‐free PeLEDs are reported with color coordinates of (0.706, 0.294)—the highest purity reported among red PeLEDs. The variation of the emission spectrum is also evaluated as a function of temperature and applied potential, finding that emission redshifts by <3 nm under low temperature and by <0.3 nm V⁻¹ with operating voltage. The prominent oxidation pathway of Sn is identified and this is suppressed with the aid of H₃PO₂. This strategy prevents the oxidation of the constituent precursors, through both its moderate reducing properties and through its forming complexes with the perovskite that increase the energetic barrier toward Sn oxidation. The H₃PO₂ additionally seeds crystal growth during film formation, improving film quality. PeLEDs are reported with an EQE of 0.3% and a brightness of 70 cd m⁻²; this is the record among reported red‐emitting, lead‐free PeLEDs.

Folding‐flexible semitransparent organic solar cells with over 10% efficiency and 21% average visible light transmission are realized by using xylitol microdoping and acid treatment on poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate transparent electrodes for supplying power and promoting plant growth in future multifunctional self‐powered greenhouses.

Abstract

Semitransparent organic solar cells (ST‐OSCs) have attracted extensive attention for their potential greenhouse applications. Conventional ST‐OSCs are typically based on indium tin oxide (ITO) electrodes which suffer from mechanical brittleness. Therefore, alternatives for ITO are required for realization of foldable‐flexible ST‐OSCs (FST‐OSCs). Herein, flexible poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) electrodes are prepared as ITO alternatives via polyhydroxy compound (xylitol) microdoping and acid treatment. As a result, flexible opaque OSCs based on PBDB‐T‐2F:Y6 photoactive system yield a high efficiency of 14.20%. The desirable optical properties of modified PEDOT:PSS electrodes in the visible light region and PBDB‐T‐2F:Y6 photoactive layer in the near‐infrared region facilitate the fabrication of FST‐OSCs with over 10% efficiency and 21% average visible light transmittance. Those FST‐OSCs also display excellent mechanical stability against bending and folding due to the xylitol doping, where over 80% of the initial efficiency can still be maintained even after 1000 folding cycles. Meanwhile, parallel comparisons between plants grown under direct sunlight with a FST‐OSCs roof and those under direct sunlight yield very similar results in terms of branch sturdiness and hypertrophic leaves. The results pave the way for realizing high‐performing FST‐OSCs based on PEDOT:PSS electrodes that could utilize visible light for plant growth and infrared light for power generation.

A novel benzodithiophene (BDT)‐based small molecule (BDTID‐Cl) is used as an electron donor in small molecules solar cells (SM‐SCs). A record fill factor of 78.0% in SM‐SCs is achieved using BDTID‐Cl as a novel SM donor. In addition, a two‐terminal tandem solar cell is designed with a remarkable power conversion efficiency of 15.1% by complementary absorption of up to 1000 nm.

Abstract

Small molecules have been recently highlighted as active materials owing to their facile synthesisis method, well‐defined molecular structure, and highly reproducible performance. In particular, optimizing bulk heterojunction (BHJ) morphologies is important to achieving high performance in solution‐processable small molecule solar cells (SM‐SCs). Herein, a series of benzodithiophene‐based active materials with different halogen atoms substituted at the end‐group, are reported, as well as how these halogen atoms affect the morphology of BHJ architectures through microstructure analyses. Materials with chlorine atoms show a well‐mixed morphology and interpenetrating networks when blended with [6,6]‐phenyl‐C₇₁‐butyric acid methyl ester, facilitating effective charge transportation. This controlled morphology helps attain excellent performance with a power conversion efficiency (PCE) of 10.5% and a highest fill factor of 78.0% without additives. In addition, it can be applied to two‐terminal (2T)‐tandem solar cells, attaining an outstanding PCE of up to 15.1% with complementary absorption in the field of the 2T‐tandem solar cells introducing the SM‐SCs. These results suggest that tailoring interactions with halogen atoms is an effective way to control BHJ architectures, thereby achieving remarkable performance in SM‐SCs.

A new nonfullerene acceptor (NFA) with acceptor–donor–acceptor (A–D–A) architecture, i‐IEICO‐2F, is designed and synthesized. Devices based on i‐IEICO‐2F exhibit optimized photovoltaic performance with a power conversion efficiency (PCE) of 11.28%. Devices are found to be thermally stable and maintain 44% of their initial PCE after 184.5 h of continuous thermal annealing treatment at 150 °C.

Abstract

A nonfullerene acceptor (NFA) with acceptor–donor–acceptor (A–D–A) architecture, i‐IEICO‐2F, based on 4,9‐dihydro‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene as an electron‐donating core and 2‐(6‐fluoro‐2,3‐dihydro‐3‐oxo‐1H‐inden‐1‐ylidene)‐propanedinitrile as electron‐withdrawing end groups, is designed and synthesized. i‐IEICO‐2F has a twist structure in the main conjugated chain, which causes blueshifted absorption and leads to harmonious absorption with a high bandgap donor. The bandgap of i‐IEICO‐2F compliments the bandgap of suitable wide bandgap donor polymers such as J52, leading to complete light absorption throughout the visible spectrum. Devices based on i‐IEICO‐2F exhibit optimized photovoltaic performance including an open‐circuit voltage of 0.93 V, a short‐circuit current density of 16.61 mA cm⁻², and a fill factor of 73%, and result in a power conversion efficiency (PCE) of 11.28%. The i‐IEICO‐2F‐based devices reach PCEs of >11% without using any additives or post‐treatments. Devices are found to be thermally stable and maintain 44% of their initial PCE after 184.5 h of continuous thermal annealing (TA) treatment at 150 °C. Based on UV, atomic force microscopy (AFM), and grazing incidence wide angle X‐ray scattering (GIWAXS) results, i‐IEICO‐2F devices show almost identical morphology and molecular orientation throughout the TA treatment and excellent stability compared to other IEICO derivatives.

A generic guideline for accurately controlling phase purity and arrangement in 2D perovskite films is provided by utilizing coordination engineering of a single‐crystal precursor solution. The resulting films with narrow distribution and preferentially perpendicular crystal orientation result in a significant improvement in device performance and stability, which is not typically found in conventional stoichiometric precursors.

Abstract

2D Ruddlesden–Popper perovskites (RPPs) have recently drawn significant attention because of their structural variability that can be used to tailor optoelectronic properties and improve the stability of derived photovoltaic devices. However, charge separation and transport in 2D perovskite solar cells (PSCs) suffer from quantum well barriers formed during the processing of perovskites. It is extremely difficult to manage phase distributions in 2D perovskites made from the stoichiometric mixtures of precursor solutions. Herein, a generally applicable guideline is demonstrated for precisely controlling phase purity and arrangement in RPP films. By visually presenting the critical colloidal formation of the single‐crystal precursor solution, coordination engineering is conducted with a rationally selected cosolvent to tune the colloidal properties. In nonpolar cosolvent media, the derived colloidal template enables RPP crystals to preferentially grow along the vertically ordered alignment with a narrow phase variation around a target value, resulting in efficient charge transport and extraction. As a result, a record‐high power conversion efficiency (PCE) of 14.68% is demonstrated for a (TEA)₂(MA)₂Pb₃I₁₀ (n = 3) photovoltaic device with negligible hysteresis. Remarkably, superior stability is achieved with 93% retainment of the initial efficiency after 500 h of unencapsulated operation in ambient air conditions.

Time of flight secondary ion mass spectrometry (TOF‐SIMS) is a versatile characterization technique which can provide key insights into the spatial location of all components of perovskite solar cell materials, and how those distributions change with performance/degradation. The technique is summarized here, past uses from the literature are covered, and example data and mitigation of known measurement artifacts are described.

Abstract

Time‐of‐flight secondary‐ion mass spectrometry (TOF‐SIMS), a powerful analytical technique sensitive to all components of perovskite solar cell (PSC) materials, can differentiate between the various organic species within a PSC absorber or a complete device stack. The ability to probe chemical gradients through the depth of a device (both organic and inorganic), with down to 100 nm lateral resolution, can lead to unique insights into the relationships between chemistry in the absorber bulk, at grain boundaries, and at interfaces as well as how they relate to changes in performance and/or stability. In this review, the technique is described; then, from the literature, several examples of how TOF‐SIMS have been used to provide unique insight into PSC absorbers and devices are covered. Finally, the common artifacts that can be introduced if the data are improperly collected, as well as methods to mitigate these artifacts are discussed.

This review surveys the current body of work related to scanning electron microscope based cathodoluminescence (CL) analysis of halide perovskite materials for energy applications. In addition to the comprehensive literature survey, a detailed discussion of the origin of the CL signal in terms of experimental conditions and material properties is also provided.

Abstract

Halide perovskite solar cells have achieved a certified efficiency of 25.2%, surpassing CdTe and CuInGaSe₂, which have long been regarded as the most‐efficient thin‐film photovoltaic materials. As this exciting class of materials continues to mature, researchers will require characterization techniques capable of exposing the interplay among structure, chemistry, and optoelectronic properties to inform processing strategies and increase both device efficiencies and long‐term stability. Cathodoluminescence microscopy is an ideal technique to provide such information due to the high spatial resolution and robust optical information acquired. Here, the current body of work related to cathodoluminescence analysis of halide perovskite materials for optoelectronic applications is surveyed. This review demonstrates how cathodoluminescence can monitor degradation due to environmental stressors, phase segregation resulting from material processing, and other halide perovskite‐centric material issues. A persistent concern associated with e‐beam‐based analysis of halide perovskites is what effect the electron beam has on the material properties being probed. Addressing this, a detailed discussion is provided on the origin of the cathodoluminescence signal and a review of studies focused on revealing changes in the properties of halide perovskites resulting from e‐beam excitation. Finally, a perspective on future opportunities to expand the role of cathodoluminescence analysis for halide perovskites is provided.

This article aims to present an in‐depth review of the current understanding of metal halide perovskite devices and module stability by outlining how basic material intrinsic and extrinsic degradation mechanisms as well as additional complications from the presence of other layers and nonequilibrium conditions impact device and module performance over time.

Abstract

Metal halide perovskite solar cells (PSCs) have risen in efficiency from just 3.81% in 2009 to over 25.2% today. While metal halide perovskites have excelled in efficiency, advances in stability are significantly more complex and have progressed more slowly. The advance of efficiency, which is readily measured, over stability, which can require literally thousands of hours to demonstrate, is to be expected given the rapid rate of innovation in the field. In the face of changing absorber composition, synthetic approaches, and device stack components it is necessary to understand basic material properties to rationalize how to enable stability in devices. In this article the aim is to present an in‐depth review of the current understanding of metal halide perovskite devices and module stability by focusing on what is known retarding intrinsic and extrinsic degradation mechanisms at the material, device, and module level. Once these considerations are presented the discussion then moves to connecting different degradation mechanisms to stresses anticipated in operation and how they can impact efficiency of cells and ultimately modules over time.

Folding‐flexible semitransparent organic solar cells with over 10% efficiency and 21% average visible light transmission are realized by using xylitol microdoping and acid treatment on poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate transparent electrodes for supplying power and promoting plant growth in future multifunctional self‐powered greenhouses.

Abstract

Semitransparent organic solar cells (ST‐OSCs) have attracted extensive attention for their potential greenhouse applications. Conventional ST‐OSCs are typically based on indium tin oxide (ITO) electrodes which suffer from mechanical brittleness. Therefore, alternatives for ITO are required for realization of foldable‐flexible ST‐OSCs (FST‐OSCs). Herein, flexible poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) electrodes are prepared as ITO alternatives via polyhydroxy compound (xylitol) microdoping and acid treatment. As a result, flexible opaque OSCs based on PBDB‐T‐2F:Y6 photoactive system yield a high efficiency of 14.20%. The desirable optical properties of modified PEDOT:PSS electrodes in the visible light region and PBDB‐T‐2F:Y6 photoactive layer in the near‐infrared region facilitate the fabrication of FST‐OSCs with over 10% efficiency and 21% average visible light transmittance. Those FST‐OSCs also display excellent mechanical stability against bending and folding due to the xylitol doping, where over 80% of the initial efficiency can still be maintained even after 1000 folding cycles. Meanwhile, parallel comparisons between plants grown under direct sunlight with a FST‐OSCs roof and those under direct sunlight yield very similar results in terms of branch sturdiness and hypertrophic leaves. The results pave the way for realizing high‐performing FST‐OSCs based on PEDOT:PSS electrodes that could utilize visible light for plant growth and infrared light for power generation.

A novel benzodithiophene (BDT)‐based small molecule (BDTID‐Cl) is used as an electron donor in small molecules solar cells (SM‐SCs). A record fill factor of 78.0% in SM‐SCs is achieved using BDTID‐Cl as a novel SM donor. In addition, a two‐terminal tandem solar cell is designed with a remarkable power conversion efficiency of 15.1% by complementary absorption of up to 1000 nm.

Abstract

Small molecules have been recently highlighted as active materials owing to their facile synthesisis method, well‐defined molecular structure, and highly reproducible performance. In particular, optimizing bulk heterojunction (BHJ) morphologies is important to achieving high performance in solution‐processable small molecule solar cells (SM‐SCs). Herein, a series of benzodithiophene‐based active materials with different halogen atoms substituted at the end‐group, are reported, as well as how these halogen atoms affect the morphology of BHJ architectures through microstructure analyses. Materials with chlorine atoms show a well‐mixed morphology and interpenetrating networks when blended with [6,6]‐phenyl‐C₇₁‐butyric acid methyl ester, facilitating effective charge transportation. This controlled morphology helps attain excellent performance with a power conversion efficiency (PCE) of 10.5% and a highest fill factor of 78.0% without additives. In addition, it can be applied to two‐terminal (2T)‐tandem solar cells, attaining an outstanding PCE of up to 15.1% with complementary absorption in the field of the 2T‐tandem solar cells introducing the SM‐SCs. These results suggest that tailoring interactions with halogen atoms is an effective way to control BHJ architectures, thereby achieving remarkable performance in SM‐SCs.

Fill factor losses in nonfullerene‐acceptor‐based organic solar cells under illumination are caused by morphological traps due to diffusion limited aggregation of the nonfullerene acceptors in the mixed matrix. To achieve stable and high‐performance organic solar cells under illumination, it is essential to engineer the mixed regions from both thin‐film formation kinetics and materials intrinsic properties, e.g., materials compatibility and diffusion constant.

Abstract

As the power conversion efficiency (PCE) of organic solar cells (OSCs) has surpassed the 17% baseline, the long‐term stability of highly efficient OSCs is essential for the practical application of this photovoltaic technology. Here, the photostability and possible degradation mechanisms of three state‐of‐the‐art polymer donors with a commonly used nonfullerene acceptor (NFA), IT‐4F, are investigated. The active‐layer materials show excellent intrinsic photostability. The initial morphology, in particular the mixed region, causes degradation predominantly in the fill factor (FF) under illumination. Electron traps are formed due to the reorganization of polymers and diffusion‐limited aggregation of NFAs to assemble small isolated acceptor domains under illumination. These electron traps lead to losses mainly in FF, which is in contradistinction to the degradation mechanisms observed for fullerene‐based OSCs. Control of the composition of NFAs close to the thermodynamic equilibrium limit while keeping adequate electron percolation and improving the initial polymer and NFA ordering are of the essence to stabilize the FF in NFA‐based solar cells, which may be the key tactics to develop next‐generation OSCs with high efficiency as well as excellent stability.

A novel bifunctional (anti)solvent system is developed for regulating the perovskite crystallization procedure. It can perform not only as an antisolvent at the spin‐coating step to rapidly generate crystal seeds, but also as a solvent for ripening the precursors to large crystal grains during the thermal‐annealing process. Therefore, it can significantly enhance the efficiency, stability, and reproducibility of perovskite solar cells.

Abstract

The preparation of high‐quality perovskite films is important for achieving high‐performance perovskite solar cells (PSCs). The effective balance between solvent and antisolvent is an essential factor for regulating high‐quality perovskite film during the spin‐coating and thermal‐annealing steps. In this work, a greener, nonhalogenated, nontoxic bifunctional (anti)solvent, methyl benzoate (MB), is developed not only as an antisolvent to rapidly generate crystal seeds at the perovskite spin‐coating step, but also as a digestive‐ripening solvent for the perovskite precursors, which can prevent the loss of organic components during the thermal‐annealing stage and effectively suppress the formation of miscellaneous lead halide phases. As a result, this novel bifunctional (anti)solvent is employed in planar n–i–p PSCs for engineering high‐quality perovskite layers and thus achieving a power conversion efficiency up to 22.37% with negligible hysteresis and >1300 h stability. Moreover, due to the high boiling point and low‐volatility characteristic of MB, high‐performance PSCs are achieved reproducibly at different operating temperatures (22–34 °C). Therefore, this developed bifunctional solvent system can provide a promising platform toward globally upscaling and commercializing PSCs in all seasons and regions.

All‐solution‐processed and printable nonfullerene organic solar cells are fabricated. All layers from the substrate to the top electrode are solution‐processed. Hydrogen molybdenum bronze is introduced to solve the charge extraction issue and the wetting issue of the top electrode (PEDOT:PSS) on a hydrophobic active layer. Efficiency over 10% (>1 cm²) is obtained for the all‐solution‐processed nonfullerene solar cells.

Abstract

All‐solution‐processed organic solar cells (from the bottom substrate to the top electrode) are highly desirable for low‐cost and ubiquitous applications. However, it is still challenging to fabricate efficient all‐solution‐processed organic solar cells with a high‐performance nonfullerene (NF) active layer. Issues of charge extraction and wetting are persistent at the interface between the nonfullerene active layer and the printable top electrode (PEDOT:PSS). In this work, efficient all‐solution‐processed NF organic solar cells (from the bottom substrate to the top electrode) are reported via the adoption of a layer of hydrogen molybdenum bronze (H_XMoO₃) between the active layer and the PEDOT:PSS. The dual functions of H_XMoO₃ include: 1) its deep Fermi level of −5.44 eV can effectively extract holes from the active layer; and 2) the wetting issues of the PEDOT:PSS on the hydrophobic surface of the NF active layer can be solved. Importantly, fine control of the H_XMoO₃ composition during the synthesis is critical in obtaining processing orthogonality between H_XMoO₃ and the PEDOT:PSS. Flexible all‐solution‐processed NF organic solar cells with power conversion efficiencies of 11.9% and 10.3% are obtained for solar cells with an area of 0.04 and 1 cm², respectively.

A “welding” transparent flexible electrode, with respect to both the upper electrode and the underlying substrate, for fabricating high‐performance flexible OSCs is proposed, resulting in a record power conversion efficiency of single‐junction flexible organic solar cells (OSCs) with excellent mechanical properties.

Abstract

The power conversion efficiencies (PCEs) of flexible organic solar cells (OSCs) still lag behind those of rigid devices and their mechanical stability is unable to meet the needs of flexible electronics at present due to the lack of a high‐performance flexible transparent electrode (FTE). Here, a so‐called “welding” concept is proposed to design an FTE with tight binding of the upper electrode and the underlying substrate. The upper electrode consisting of solution‐processed Al‐doped ZnO (AZO) and silver nanowire (AgNW) network is well welded by utilizing the capillary force effect and secondary growth of AZO, leading to a reduction of the AgNWs junction site resistance. Meanwhile, the poly(ethylene terephthalate) is modified by embedding the AgNWs, which are then used to link with the AgNWs in the upper hybrid electrode, thus enhancing the adhesion of the electrode to the substrate. By this welding strategy, critical bottleneck issues relating to the FTEs in terms of optoelectronic and mechanical properties are comprehensively addressed. The single‐junction flexible OSCs based on this welded FTE show a high performance, achieving a record high PCE of 15.21%. In addition, the PCEs of the flexible OSCs are less influenced by the device area and display robust bending durability even under extreme test conditions.

Fill factor losses in nonfullerene‐acceptor‐based organic solar cells under illumination are caused by morphological traps due to diffusion limited aggregation of nonfullerene acceptors in the mixed matrix. To achieve stable and high‐performance organic solar cells under illumination, it is essential to engineer the mixed regions from both thin film formation kinetics and materials intrinsic properties, e.g., materials compatibility and diffusion constant.

Abstract

As the power conversion efficiency (PCE) of organic solar cells (OSCs) has surpassed the 17% baseline, the long‐term stability of highly efficient OSCs is essential for the practical application of this photovoltaic technology. Here, the photostability and possible degradation mechanisms of three state‐of‐the‐art polymer donors with a commonly used nonfullerene acceptor (NFA), IT‐4F, are investigated. The active‐layer materials show excellent intrinsic photostability. The initial morphology, in particular the mixed region, causes degradation predominantly in the fill factor (FF) under illumination. Electron traps are formed due to the reorganization of polymers and diffusion‐limited aggregation of NFAs to assemble small isolated acceptor domains under illumination. These electron traps lead to losses mainly in FF, which is in contradistinction to the degradation mechanisms observed for fullerene‐based OSCs. Control of the composition of NFAs close to the thermodynamic equilibrium limit while keeping adequate electron percolation and improving the initial polymer and NFA ordering are of the essence to stabilize the FF in NFA‐based solar cells, which may be the key tactics to develop next‐generation OSCs with high efficiency as well as excellent stability.

Layered Cu–Cl perovskites show an electronic conductivity of 10⁻⁴ S cm⁻¹ only above 50 GPa. In contrast, an analogous Cu‐Br perovskite exhibits conductivity as high as 10⁻³ S cm⁻¹ at only 2.6 GPa. Substitution of Br for Cl brings the compression‐induced conductivity of layered copper‐halide perovskites to more technologically accessible pressures.

Abstract

We show that the onset pressure for appreciable conductivity in layered copper‐halide perovskites can decrease by ca. 50 GPa upon replacement of Cl with Br. Layered Cu–Cl perovskites require pressures >50 GPa to show a conductivity of 10⁻⁴ S cm⁻¹, whereas here a Cu–Br congener, (EA)₂CuBr₄ (EA=ethylammonium), exhibits conductivity as high as 2×10⁻³ S cm⁻¹ at only 2.6 GPa, and 0.17 S cm⁻¹ at 59 GPa. Substitution of higher‐energy Br 4p for Cl 3p orbitals lowers the charge‐transfer band gap of the perovskite by 0.9 eV. This 1.7 eV band gap decreases to 0.3 eV at 65 GPa. High‐pressure X‐ray diffraction, optical absorption, and transport measurements, and density functional theory calculations allow us to track compression‐induced structural and electronic changes. The notable enhancement of the Br perovskite's electronic response to pressure may be attributed to more diffuse Br valence orbitals relative to Cl orbitals. This work brings the compression‐induced conductivity of Cu‐halide perovskites to more technologically accessible pressures.

A proof‐of‐concept tandem solar cell with Sb₂S₃ and Sb₂Se₃ as subcells shows perfectly matched spectral utilization and delivers higher efficiency than the individually optimized subcells.

A proof‐of‐concept tandem solar cell using Sb₂S₃ and Sb₂Se₃ as top and bottom cell absorber materials is demonstrated. The bandgaps of Sb₂S₃ and Sb₂Se₃ are 1.74 and 1.22 eV, perfectly satisfying the requirement of tandem solar cells. The application of few‐layer graphene enables high transmittance and excellent interfacial contact in the top subcell. By controlling the thickness of the top cell for maximizing the spectral application, the tandem device delivers a power conversion efficiency of 7.93%, which outperforms the individually optimized top cell (5.58%) and bottom cell (6.50%). Mechanistical investigation shows that the tandem device is able to make up voltage loss in the subcells, which is a critical concern in the current antimony chalcogenide solar cells. This study provides an alternative approach to enhancing the energy conversion efficiency of antimony selenosulfide.

Sb₂Se₃/AgSbSe₂ alloying thin films are fabricated by using a close spaced sublimation technique. The doping density of the (Sb₂Se₃) _x (AgSbSe₂)_1−x alloy is about one order of magnitude higher than that of pure Sb₂Se₃ films. The alloying approach boosts the solar cell open‐circuit voltage (V _OC) from lower than 400–475 mV, and the V _OC further increases up to more than 500 mV after 1100‐h illumination.

A simple binary inorganic antimony selenide (Sb₂Se₃) compound is attractive as a promising light absorber for low‐cost and high‐efficiency photovoltaics. The external quantum efficiencies of Sb₂Se₃ solar cells are now approaching the optical limit values, which are comparable with the traditional well‐developed solar cells (such as Si, CuInGaSe₂, CdTe, etc). However, the power conversion efficiency of the Sb₂Se₃ devices is constrained by the open‐circuit voltage (V _OC) deficit, due to the intrinsic high resistivity and low element‐doping efficiency in such one‐dimensional (1D) crystals. Herein, a highly conductive, three‐dimensional (3D) crystal‐structure AgSbSe₂ phase, formed by phase transition from low symmetry binary Sb₂Se₃, is introduced to control the doping density in the alloyed (Sb₂Se₃) _x (AgSbSe₂)_1−x films utilizing configurational entropy. Guided by this alloying concept, 1D–3D (Sb₂Se₃) _x (AgSbSe₂)_1−x alloy films with tunable doping densities are obtained. As a consequence, a noticeable improvement in V _OC by >18% is observed in solar cells based on the (Sb₂Se₃) _x (AgSbSe₂)_1−x alloy absorber layer, compared with the reference cell with a pure Sb₂Se₃ absorber, leading to a high conversion efficiency of 7.8%. This alloying model provides a universal approach to control the photoelectrical properties for high‐efficiency Sb₂Se₃‐based solar cells.