awn

Dimethylammonium (DMA) is incorporated in controlled, incremental amounts into the A‐site of CsPbI₃ perovskite materials. Confirming that the stabilization afforded from the DMA iodide precursors in CsPbI₃ perovskites comes from an alloy of the A‐site with an organic cation. The limit to DMA incorporation is ≈25%, making a Cs_0.75DMA_0.25PbI₃ material that is more stable than neat CsPbI₃.

All‐inorganic perovskite materials are attractive alternatives to organic–inorganic perovskites because of their potential for higher thermal stability. Although CsPbI₃ is compositionally stable under elevated temperatures, the cubic perovskite α‐phase is thermodynamically stable only at >330 °C and the low‐temperature perovskite γ‐phase is metastable and highly susceptible to non‐perovskite δ‐phase conversion in moisture. Many methods have been reported which show that the incorporation of acid (aqueous HI) or “HPbI₃”—recently shown to be dimethylammonium lead iodide (DMAPbI₃) —lowers the annealing temperature required to produce the black, perovskite phase of CsPbI₃. Herein, the optical and crystallographic data presented show that dimethylammonium (DMA) can successfully incorporate as an A‐site cation to replace Cs in the CsPbI₃ perovskite material. This describes the stabilization and lower phase transition temperature reported in the literature when HI or HPbI₃ is used as precursors for CsPbI₃. The Cs–DMA alloy only forms a pure‐phase material up to ≈25% DMA; at higher concentrations, the CsPbI₃ and DMAPbI₃ begin to phase segregate. These alloyed materials are more stable to moisture than neat CsPbI₃, but do not represent a fully inorganic perovskite material.

Low‐temperature solution‐processed perovskite solar cells (PSCs) based on organic‐inorganic hybrid perovskites have emerged as a low‐cost and high‐efficiency thin‐film photovoltaic technology. The reported power conversion efficiency (PCE) of laboratory produced PSCs with an active area less than 0.1 cm² has already excessed 25%, which, however, decreases significantly into about 16% for large device area of about hundred centimeters. Therefore, the scalability has become one of the most significant limits on a successful commercialization of perovskite photovoltaics. This includes realizing a homogenous and compact electron transport layer (ETL), facing with issues of defects, energy level mismatch and high temperature annealing requirements over large area development. Therefore, an exploration of effective and low‐cost charge transport materials is crucial to scalable fabrication of highly efficient perovskite devices. Two‐dimension (2D) materials have drawn wide‐attention in PSCs community with tunable electronic energy level function and high carrier mobility. So far, the searching of a wide range of novel 2D materials for use in PSCs has documented considerable progress, however, a lot remains to be done in this field. This review summarizes recent advancements in the application of emerging 2D materials as effective ETL, thus provides direction for future development toward efficient and large‐scale perovskite devices.

This article is protected by copyright. All rights reserved.

Thermal cross‐linking of new enamine‐based hole‐transporting materials is shown to provide an advantage in p–i–n perovskite solar cells. Due to the improved resistance to organic solvents, the cross‐linked films manage to withstand solution processing of the perovskite absorber layer. This leads to an improved open‐circuit voltage and over 18% efficiency for the devices with the V1187 material.

The development of the simple synthesis schemes of organic semiconductors can have an important contribution to the advancement of related technologies. In particular, one of the fields where the high price of the hole‐transporting materials may become an obstacle toward successful commercialization is perovskite solar cells. Herein, enamine‐based materials that are capable of undergoing cross‐linking due to the presence of two vinyl groups are synthesized. It is shown that new compounds can be thermally polymerized, making the films resistant to organic solvents. This can allow the use of a wet‐coating process for the deposition of the perovskite absorber film, without the need for orthogonal solvents. Cross‐linked films are used in perovskite solar cells, and, upon optimization of the film thickness, the highest power conversion efficiency of 18.1% is demonstrated.

Quasi‐Fermi level splitting (QFLS), which sets the maximum value of the open‐circuit voltage (V _OC) of a photovoltaic device, in state‐of‐the‐art organic solar cells is evaluated using spectroscopic and semiconductor device physics approaches.

The power conversion efficiency (PCE) of state‐of‐the‐art organic solar cells is still limited by significant open‐circuit voltage (V _OC) losses, partly due to the excitonic nature of organic materials and partly due to ill‐designed architectures. Thus, quantifying different contributions of the V _OC losses is of importance to enable further improvements in the performance of organic solar cells. Herein, the spectroscopic and semiconductor device physics approaches are combined to identify and quantify losses from surface recombination and bulk recombination. Several state‐of‐the‐art systems that demonstrate different V _OC losses in their performance are presented. By evaluating the quasi‐Fermi level splitting (QFLS) and the V _OC as a function of the excitation fluence in nonfullerene‐based PM6:Y6, PM6:Y11, and fullerene‐based PPDT2FBT:PCBM devices with different architectures, the voltage losses due to different recombination processes occurring in the active layers, the transport layers, and at the interfaces are assessed. It is found that surface recombination at interfaces in the studied solar cells is negligible, and thus, suppressing the non‐radiative recombination in the active layers is the key factor to enhance the PCE of these devices. This study provides a universal tool to explain and further improve the performance of recently demonstrated high‐open‐circuit‐voltage organic solar cells.

Defect passivation is an effective strategy to adjust the energy band structure, reduce the density of defect states, and suppress the nonradiative recombination of carriers. Herein, the recent progress in the passivation strategy for perovskite films is summarized and the development direction of passivation strategies to further improve the performance of perovskite solar cells (PSCs) is proposed.

Organic–inorganic halide perovskite photovoltaic devices have advanced rapidly in recent years, and the photoelectric conversion efficiency of perovskite solar cells (PSCs) has exceeded 25%. However, the defects from the crystallization process become nonradiation recombination centers and hinder the performance and the stability of PSCs. Defect passivation by tuning grain size and grain boundary (GB) is an effective strategy to reduce the defects on GBs and film surface. Herein, recent progress in the passivation strategy for perovskite films is summarized, including nonstoichiometric passivation, iodide vacancies filling, dimensional engineering, passivation with crosslink, physical passivation, and other passivation methods. These passivation strategies play an important role in improving the quality of perovskite films, adjusting the energy band structure, reducing the density of defect states, and suppressing the nonradiative recombination of carriers. Finally, this review puts forward the development direction of passivation strategies to further improve the performance of PSCs.

Layer‐by‐layer solution‐processed organic solar cells optimize the donor layer and acceptor layer separately to make the two components ideally distribute in the vertical direction, which facilitates charge transport and collection. This bilayer structure has less dependence on donor/acceptor ratio, solvent concentration, and so on. It is easy to prepare high‐performance devices with good stability and a high repetition rate.

Organic solar cells (OSCs) have attracted wide attention due to their economy, environmental protection, and potential for large‐scale commercial production. The layer‐by‐layer (LbL) solution processing method, where donor solution and acceptor solution are coated sequentially, is a simple and effective way to fabricate OSCs, achieving a high power conversion efficiency (PCE) of up to 17%. Compared with bulk‐heterojunction (BHJ) OSCs, LbL solution‐processed OSCs separately adjust different layers, making the components distribute ideally in the vertical direction that is beneficial for exciton dissociation, charge transport, and charge collection. Moreover, the LbL approach has better potential in the preparation of large‐area devices, which is a key link in the commercialization of OSCs. Herein, the basic principles and the latest research progress of LbL solution‐processed OSCs are summarized, and the existing challenges and prospects of the LbL solution processing method in industrial production are discussed.

A novel structural design of donor–acceptor dyads, in which an oligothiophene donor and fullerene acceptor are covalently linked by flexible spacer of variable length, is presented. Favorable optoelectronic, charge transport, and self‐organization properties of dyads are the basis for reaching power conversion efficiencies of 4.26% in single‐material organic solar cells, which promise advantages for printed large‐area solar foils.

Single‐material organic solar cells (SMOSCs) promise several advantages with respect to prospective applications in printed large‐area solar foils. Only one photoactive material has to be processed and the impressive thermal and photochemical long‐term stability of the devices is achieved. Herein, a novel structural design of oligomeric donor–acceptor (D–A) dyads 1–3 is established, in which an oligothiophene donor and fullerene acceptor are covalently linked by a flexible spacer of variable length. Favorable optoelectronic, charge transport, and self‐organization properties of the D–A dyads are the basis for reaching power conversion efficiencies up to 4.26% in SMOSCs. The dependence of photovoltaic and charge transport parameters in these ambipolar semiconductors on the specific molecular structure is investigated before and after post‐treatment by solvent vapor annealing. The inner nanomorphology of the photoactive films of the dyads is analyzed with transmission electron microscopy (TEM) and grazing‐incidence wide‐angle X‐ray scattering (GIWAXS). Combined theoretical calculations result in a lamellar supramolecular order of the dyads with a D–A phase separation smaller than 2 nm. The molecular design and the precise distance between donor and acceptor moieties ensure the fundamental physical processes operative in organic solar cells and provide stabilization of D–A interfaces.

The “double dipole step” model is used to explain the physicochemical property of the indium tin oxide (ITO)/polyvinylpyrrolidone (PVP) interface that the W _F reduction is caused by the directional intrinsic molecular dipole moments and the image dipole moments. Moreover, high‐performance inverted organic solar cells (OSCs) are achieved by introducing a self‐assembled ultrathin PVP layer using a simple immersion method.

Polyvinylpyrrolidone (PVP) has been successfully used as the cathode interfacial layer (CIL) in organic solar cells (OSCs) for work function (W _F) modification. However, detailed insight into the effect of a PVP interlayer on the physicochemical properties of the indium tin oxide (ITO) electrode in inverted OSCs (I‐OSCs) is still largely absent. Herein, the ITO/PVP interface is investigated by photoelectron spectroscopy and the mechanisms for the energy level alignment of PVP on different substrates in general are unraveled. The results indicate that the dipole formation that reduces the W _F is driven by not only the directional intrinsic molecular dipole moments associated with the γ‐lactam of PVP, but also an additional dipole step with the same direction created by the image charges in the contacting (semi‐)conductor layer. In addition, high‐performance inverted OSCs (I‐OSCs) are achieved by introducing a self‐assembled ultrathin PVP layer using a simple immersion method. This work provides enhanced understanding of the PVP‐based CIL and demonstrates its great potential in I‐OSC fabrication, which can pave the way to simplified manufacturing of low‐cost and large‐area devices in organic electronic technologies.

Herein, a small amount of the ionic liquid methylammonium difluoroacetate is introduced to anchor the organic cations via hydrogen bonding and to enhance the Pb–O interaction in perovskite precursors for efficient and stable solar cells.

The instability of organic cations in lead halide perovskite materials is a major obstacle for the commercial breakthrough of perovskite photovoltaics due to desorption of organic cations during the thermal annealing and device operation. Herein, a novel strategy is reported to improve the performance and stability of organic halide perovskite solar cells containing organic cations by adding a small amount of the ionic liquid methylammonium difluoroacetate (MA⁺DFA⁻). Nuclear magnetic resonance and Fourier‐transform infrared spectroscopy measurements show that MA⁺DFA⁻ can anchor the organic cations via hydrogen bonding and enhance the Pb–O interaction in perovskite precursors, leading to the retardation of the perovskite crystallization and improved stability of the perovskite precursor solution. Dynamic light scattering and scanning electron microscopy verify the defect‐passivation effect of MA⁺DFA⁻ on the perovskite precursors and films. The passivated perovskite film shows superior photo carrier dynamics as investigated by time‐resolved photoluminescence and transient absorption spectra. Moreover, the hydrogen bonding of the perovskite with MA⁺DFA⁻ imparts excellent ambient and thermal stability to the film as revealed by X‐ray diffraction measurements. As a result, devices with a high efficiency of 21.46% and excellent stability over 180 days in nitrogen atmosphere at room temperature are achieved with the ionic liquid.

A facile and efficient strategy of gradient doping is adopted for optimizing inverted CsPbI₂Br‐based perovskite solar cells (PeSCs) using a bicationic iodine salt, namely BFBAI₂, as the dopant. The doped PeSCs exhibit significantly improved photovoltaic performance and stability, which is attributed to efficient defect passivation and enhanced electric field upon the gradient doped BFBAI₂.

Cesium‐based all‐inorganic perovskites (PVKs) are prized for their high thermal stability and wide bandgap suitable for the top layer of tandem solar cells. To further boost the photovoltaic performance of inorganic PVK solar cells (PeSCs), a variety of strategies aiming to either passivate defects or enhance the electric field are developed. Nevertheless, a double‐aim strategy is less explored. Herein, a facile strategy of gradient doping is adopted for optimizing the inverted CsPbI₂Br PeSCs. Particularly, a bicationic iodine salt, namely 2,2′‐bis(trifluoromethyl)‐[1,1′‐biphenyl]‐4,4′‐diamine iodine (BFBAI₂), is used to realize gradient doping in PVK and ZnO layers, respectively. As a result, the inverted PeSCs with the doped PVK/ZnO bilayer deliver improves power conversion efficiency (PCE) up to 14.38% along with enhanced device stability under ambient or thermal aging conditions, greatly surpassing the pristine devices. The improvements are attributed principally to the low‐defect PVK layer as well as enhanced electric field across the inverted PeSCs upon gradient doping. This work thus demonstrates an efficient bifunctional strategy toward highly efficient and stable CsPbI₂Br PeSCs with inverted configuration.

The perovskite solar cells have emerged as one of the most promising candidates for next‐generation solar cells. However, their instability remains a grand challenge for practical applications. Here, we aim to enhance the stability and efficiency simultaneously by tuning the organic components in Ruddlesden−Popper perovskites (2D‐RPPs). Four groups of 2D‐RPPs are prepared and the influence of 4‐fluorophenethylammonium (FPEA) and formamidinium (FA) cations on the film properties and device performances are investigated. The (FPEA)₂(FA)₈Pb₉I₂₈ film is found to be exceptionally vertically orientated, showing enhanced charge transport and lower defect density. Its absorption edge substantially extends in infrared region, which greatly increases the photocurrent. A high efficiency of 16.15% along with a V _oc of 1.07 V and a J _sc of 20.88 mA cm⁻² is achieved for the (FPEA)₂(FA)₈Pb₉I₂₈ solar cell. Notably, the (FPEA)₂(FA)₈Pb₉I₂₈ film exhibits good humidity stability and remarkably enhanced thermal stability. Its unencapsulated device maintains 95% of its stating PCE after 2112 h when exposed to ambient air with 30‐70% RH, which is more superior than the reported (PEA)₂(MA)₈Pb₉I₂₈ and (FPEA)₂(MA)₈Pb₉I₂₈ solar cells. Our study demonstrates that enhanced performances of 2D‐RPPs can be obtained by strategically designing organic compositions, which paves an avenue towards the commercialization of 2D‐RPP devices.

This article is protected by copyright. All rights reserved.

Based on a modified method, selenium‐based solar cells with polymers as both hole and electron transport layers are presented, which show a great water‐stable property and long‐term and thermal stability. The working principles under water are carefully explored and discussed.

Solar cells with varied absorbers, ranging from crystalline silicon to perovskite materials, are very vulnerable to water. Expensive and complicated encapsulation is needed to protect the devices. Thus, it is highly desirable to achieve encapsulation‐free solar cells with high water tolerance. Herein, encapsulation‐free selenium (Se)‐based solar cells (SSCs) with hydrophobic polymers as electron and hole transport layers are presented and can successfully tolerate the aqueous conditions. Moreover, it is found that the photocurrent–time curve under the aqueous/ambient environment can be smoothly fitted into the first‐order exponential models. The photocurrent of water‐soaking SSCs can recover to its original value after drying in the air, which proves the great water tolerance of fabricated SSCs. Furthermore, the SSCs also show good long‐term and thermal stability. The viability of encapsulation‐free SSCs under a harsh environment provides opportunities to further explore the interactions among light, water, and solar cells as well as make large‐scale industrialization applications of SSCs much easier.

A novel donor polymer PBTVT comprising a bis(carboxylic ester)‐substituted thiophene–vinylene–thiophene acceptor unit and a benzodithiophene donor unit is designed and synthesized. It displays a wide bandgap of 1.87 eV and good solubility in halogen‐free solvents such as o‐xylene. The corresponding devices exhibit a high PCE of 11.04%, though the highest occupied molecular orbital (HOMO) level offset is small (0.08 eV).

Herein, a novel wide‐band‐gap donor polymer, PBTVT, which comprises a bis(carboxylic ester)‐substituted thiophene–vinylene–thiophene (TVT) acceptor unit and benzodithiophene donor unit, is designed and synthesized. PBTVT displays good solubility in halogen‐free green solvents such as o‐xylene at elevated temperature. Grazing‐incidence wide‐angle X‐ray scattering (GIWAXS) measurement revealed that PBTVT can form the desired face‐on orientation in the blend films. Organic solar cells are fabricated with PBTVT as the donor and ITOIC‐2F as the acceptor. Although the highest occupied molecular orbital (HOMO) level offset between PBTVT and 2,2′‐((2Z,2′Z)‐(((4,4,9,9‐tetrakis(4‐hexylphenyl)‐4,9‐dihydro‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene‐2,7‐diyl)bis(3,4‐bis(hexyloxy)thiophene‐5,2‐diyl))bis(methanylylidene))bis(5,6‐difluoro‐3‐oxo‐2,3‐dihydro‐1H‐indene‐2,1‐diylidene))dimalononitrile (ITOIC‐2F) is only 0.08 eV, devices still exhibit an overall power conversion efficiency (PCE) of 11.04%. Especially, devices fabricated with the green solvent o‐xylene outperform those fabricated with halogenated solvents such as 1,2‐dichlorobenzene (o‐DCB).

Perovskite solar cells with the 2D passivation layer display excellent photovoltaic performance and superior stability via introducing hydrophobic alkyl molecules with polyfunctional groups.

The charges stuck in trap sites hinder charge transport and lead to V _oc below the radiative limit, which seriously restrict the performance and stability of organic–inorganic halide perovskite solar cells (PSCs). Chemical passivation is an effective method to reduce defects and suppress nonradiative recombination. Herein, a new passivation molecule l‐cysteine methyl ester hydrochloride (CME) with thiol and ester groups is designed to modify the interface between the perovskite layer and hole transport layer (HTL). It reveals that thiol possesses outstanding moisture resistance and ester suppresses nonradiative recombination by coordinating with undercoordinated Pb²⁺. Furthermore, the 2D modified layer at the grain boundaries and surface passivates surface defects and promotes hole extraction. As a result, the CME device achieves the highest PCE of 20.33% with an enhanced open‐circuit voltage (V _oc) of 1.11 V. Due to the barrier of highly hydrophobic 2D perovskites, the modified devices show excellent stability while exposed to humidity and high‐temperature environment. A facile and effective strategy to design organic molecular structures with polyfunctional groups to passivate trap‐assisted nonradiative recombination at the surface and grain boundaries is provided.

Current density–voltage (J–V) hysteresis in perovskite solar cells (PSCs) is a major challenge in this field. Herein, the possible origins and factors of J–V hysteresis behavior in PSCs are focused and the strategies to suppress the hysteresis are summarized. Finally, insights on the future development of the J–V hysteresis in PSCs are also provided.

The power conversion efficiency (PCE) of perovskite solar cells (PSCs) has exceeded 25%, showing great potential in the photovoltaic field. However, PSCs often show anomalous current density–voltage (J–V) hysteresis behavior in the forward and reverse scanning directions, which makes it impossible to accurately evaluate the performance of PSCs. Therefore, it is necessary to clearly understand the mechanism of hysteresis and suppress the hysteresis. Herein, the J–V hysteresis behavior in PSCs and strategies to suppress hysteresis is focused: first, the various factors that affect J–V hysteresis in PSCs are summarized. And the mechanism behind the various possible origins of hysteresis and the challenges encountered are explored. Then, the strategies to suppress or eliminate the hysteresis are summarized, including optimizing the perovskite light‐absorbing layer, improving the performance of the carrier transport layer and interface engineering. Finally, insights on the future development of the hysteresis are also provided.

The defect landscape in metal–halide perovskites is described. This Review highlights the promise of the compounds, explains defects as an outstanding problem, and discusses the background of defects, methods to probe defects, and various passivation strategies used successfully to date.

The rise of hybrid metal–halide perovskites as potential solar energy materials has revolutionized research on next‐generation solar cells. According to recent studies, the rationale behind such success is the rich defect physics of materials. Studies on the origin of different types of prevailing defects, their formation, and mechanism of defect passivation have hence become decisive avenues. Herein, the possible origins of defects and different defect analysis techniques in hybrid halide perovskites are discussed. While initiating the discussion with the archetypal methylammonium lead halide, perovskites beyond the conventional ABX₃ structure are included. In this direction, some major advancements to date on defect formation in the bulk of hybrid halide perovskites, at the grains and grain boundaries, are summarized. Numerous effective methods to passivate the defects and the adverse effect of defects on device efficiency are further highlighted. Hence, the prospect of defect engineering in perovskite materials is pointed toward improving the power conversion efficiency and long‐term stability of perovskite solar cells (PSCs). The discussion rightfully addresses that the in‐depth exploration of defect engineering is anticipated to have a gigantic impact toward the achievement of predicted efficiency in metal–halide PSCs.

B‐site doping provides a new approach to improve the optoelectronic properties and stability of CsPbX₃ inorganic perovskite. By judiciously selecting B‐site dopants and optimizing their concentration, B‐site doping strategy remarkably enhances the stability, tunes the bandgap, reduces the defects of CsPbX₃ inorganic perovskites, and thereby improves the photovoltaic performance of inorganic perovskite solar cells.

CsPbX₃ (X = I, Br) inorganic perovskite solar cells (PSCs) have been considered as one of the most appealing research topics in the fields of photovoltaic technologies in the past several years due to their excellent thermal stability and booming conversion efficiency. Nevertheless, there are still a large number of critical challenges and issues for inorganic PSCs, such as unstable phase structure of I‐rich inorganic perovskites at ambient condition, the wide bandgap of Br‐rich inorganic perovskites, and serious defect traps, hindering further development of inorganic PSCs. Recently, partially substituting Pb²⁺ with other metal ions has been shown to enhance the stability, tune the bandgap, reduce the defects of CsPbX₃ inorganic perovskites, and thereby improve the photovoltaic performance of inorganic PSCs. Herein, the recent progress in improving the photovoltaic performance of inorganic PSCs through the B‐site doping strategy is summarized, and the influence of the alternative metal ions on the stability and optoelectronic properties of inorganic perovskites and photovoltaic characteristics of CsPbX₃‐based PSCs is discussed. Finally, the issues that need to be understood in more detail are presented. It is believed that B‐site doping offers a practical strategy to gain high‐performance perovskite photovoltaic devices.

Carbon nanotube electrode–laminated perovskite and n‐type tunnel oxide–passivated contact (TOPCon) silicon solar cells exhibit 24.42% efficiency when stacked in tandem. Both semitransparency and power conversion efficiency are important for top subcells of tandem solar cells. The carbon nanotube‐based perovskite solar cells demonstrate record high efficiency among the reported four‐terminal tandem solar cells while exhibiting good semitransparency.

Carbon nanotube electrode–laminated perovskite solar cells in combination with n‐type tunnel oxide–passivated contact silicon solar cells demonstrate a high power conversion efficiency (PCE) of 24.42% when stacked in tandem. This is compared with conventional indium tin oxide/MoO _x ‐deposited perovskite solar cells which give an efficiency of 22.35% when stacked in the same four‐terminal tandem system. Despite higher transmittance of the carbon nanotube electrode than that of the indium tin oxide/MoO _x in the infrared range, the carbon nanotube electrode‐laminated devices show lower transmittance in the same region due to the total internal reflection and scattering as evidenced by optical simulation. Yet, the exceptionally high PCE of the carbon nanotube electrode‐laminated semitransparent devices far exceeding than that of the indium tin oxide/MoO _x ‐deposited semitransparent top cell outweighs the effect of the optical transparency. Four types of silicon solar cells are compared as the bottom subcells, and the n‐type tunnel oxide‐passivated contact silicon solar cells are the best choice mainly due to their high absorption in the long‐wavelength region. The obtained 24.42% efficiency is one of the high PCEs among the reported four‐terminal perovskite–silicon solar cells, and this article is the first demonstration of the carbon nanotube electrode application in tandem solar cells.

A novel donor polymer PBTVT comprising a bis(carboxylic ester)‐substituted thiophene–vinylene–thiophene acceptor unit and a benzodithiophene donor unit is designed and synthesized. It displays a wide bandgap of 1.87 eV and good solubility in halogen‐free solvents such as o‐xylene. The corresponding devices exhibit a high PCE of 11.04%, though the highest occupied molecular orbital (HOMO) level offset is small (0.08 eV).

Herein, a novel wide‐band‐gap donor polymer, PBTVT, which comprises a bis(carboxylic ester)‐substituted thiophene–vinylene–thiophene (TVT) acceptor unit and benzodithiophene donor unit, is designed and synthesized. PBTVT displays good solubility in halogen‐free green solvents such as o‐xylene at elevated temperature. Grazing‐incidence wide‐angle X‐ray scattering (GIWAXS) measurement revealed that PBTVT can form the desired face‐on orientation in the blend films. Organic solar cells are fabricated with PBTVT as the donor and ITOIC‐2F as the acceptor. Although the highest occupied molecular orbital (HOMO) level offset between PBTVT and 2,2′‐((2Z,2′Z)‐(((4,4,9,9‐tetrakis(4‐hexylphenyl)‐4,9‐dihydro‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene‐2,7‐diyl)bis(3,4‐bis(hexyloxy)thiophene‐5,2‐diyl))bis(methanylylidene))bis(5,6‐difluoro‐3‐oxo‐2,3‐dihydro‐1H‐indene‐2,1‐diylidene))dimalononitrile (ITOIC‐2F) is only 0.08 eV, devices still exhibit an overall power conversion efficiency (PCE) of 11.04%. Especially, devices fabricated with the green solvent o‐xylene outperform those fabricated with halogenated solvents such as 1,2‐dichlorobenzene (o‐DCB).

An interfacial layer of triphenylamine–polystyrene blend is used between the perovskite layer and charge‐transporting layer to concurrently suppress energy loss and improve device stability. The energy loss is reduced from 0.49 to 0.35 eV, along with a large open‐circuit voltage of 1.18 V and a high power conversion efficiency of 22.1% in air‐stable perovskite solar cells.

Energy loss induced by nonradiative recombinations plays a critical role in determining power conversion efficiencies in perovskite solar cells, whereas device stability impacts their long‐time reliability in the ambient environment. It is an important challenge to suppress energy loss and improve device stability simultaneously. Herein, an interfacial layer of triphenylamine (TPA):polystyrene (PS) blend coated on the hybrid perovskite layer to concurrently suppress energy loss and improve device stability is reported. The energy loss is suppressed from 0.49 to 0.35 eV by passivating surface defects in hybrid perovskites via Lewis acid–base interactions with the combination of electron‐donating aromatic nucleus in PS and tertiary amine in TPA, leading to perovskite solar cells with a high open‐circuit voltage of 1.18 V, a fill factor of about 80%, and a power conversion efficiency of 22.1%. Meanwhile, the device stability in the ambient environment is improved significantly by the TPA:PS blend due to its superior hydrophobicity which is suggested by its high contact angle of 91.1° as compared to 64.0° for the pristine perovskite film. Herein, an efficient interfacial engineering approach with the TPA:PS blend to suppress energy loss and improve device stability simultaneously towards realistic applications is demonstrated.

Novel asymmetric alkoxy and alkyl substitutions on the well‐known nonfullerene acceptor Y6 yield a molecule named Y6‐1O, and its photoelectric properties and photovoltaic performance are systematically compared with the two related symmetric molecules (Y6 and Y6‐2O), which suggests that this design strategy is promising and effective.

Abstract

In this paper, a strategy of asymmetric alkyl and alkoxy substitution is applied to state‐of‐the‐art Y‐series nonfullerene acceptors (NFAs), and it achieves great performance in organic solar cell (OSC) devices. Since alkoxy groups can have a significant influence on the material properties of NFAs, alkoxy substitution is applied to the Y6 molecule in a symmetric manner. The resulting molecule (named Y6‐2O), despite showing improved open‐circuit voltage (V _oc), yields extremely poor performance due to low solubility and excessive aggregation properties, a change that is due to the conformational locking effect of alkoxy groups. In contrast, asymmetric alkyl and alkoxy substitution on Y6, yields a molecule named Y6‐1O that can maintain the positive effect of V _oc improvement and obtain reasonably good solubility. The resulting molecule Y6‐1O enables highly efficient nonfullerene OSCs with 17.6% efficiency and the asymmetric side‐chain strategy has the potential to be applied to other NFA‐material systems to further improve their performance.

Through investigation of the underlying thermodynamic and kinetic aspects of non‐fullerene acceptor crystallization, the importance of diffusion coefficients and melting enthalpies in controlling the crystal growth rates is demonstrated, and it is revealed and that differences in halogenation can drastically change crystallization kinetics and device stability.

Abstract

With power conversion efficiency now over 17%, a long operational lifetime is essential for the successful application of organic solar cells. However, most non‐fullerene acceptors can crystallize and destroy devices, yet the fundamental underlying thermodynamic and kinetic aspects of acceptor crystallization have received limited attention. Here, room‐temperature (RT) diffusion coefficients of 3.4 × 10⁻²³ and 2.0 × 10⁻²² are measured for ITIC‐2Cl and ITIC‐2F, two state‐of‐the‐art non‐fullerene acceptors. The low coefficients are enough to provide for kinetic stabilization of the morphology against demixing at RT. Additionally profound differences in crystallization characteristics are discovered between ITIC‐2F and ITIC‐2Cl. The differences as observed by secondary‐ion mass spectrometry, differential scanning calorimetry (DSC), grazing‐incidence wide‐angle X‐ray scattering, and microscopy can be related directly to device degradation and are attributed to the significantly different nucleation and growth rates, with a difference in the growth rate of a factor of 12 at RT. ITIC‐4F and ITIC‐4Cl exhibit similar characteristics. The results reveal the importance of diffusion coefficients and melting enthalpies in controlling the growth rates, and that differences in halogenation can drastically change crystallization kinetics and device stability. It is furthermore delineated how low nucleation density and large growth rates can be inferred from DSC and microscopy experiments which could be used to guide molecular design for stability.

With synergistic optimization of the active layer morphology, flexible substrate properties, and processing temperature, large‐area flexible organic solar cells with high performance are achieved by the slot‐die coating process. The 1 cm² flexible devices produce an excellent power conversion efficiency (PCE) of 12.16%, and, for modules with an area of 25 cm², an extraordinary PCE of 10.09% is observed.

Abstract

Slot‐die coating is generally regarded as the most effective large‐scale methodology for the fabrication of organic solar cells (OSCs). However, the corresponding device performance significantly lags behind spin‐coated devices. Herein, the active layer morphology, flexible substrate properties, and the processing temperature are optimized synergistically to obtain high power conversion efficiency (PCE) for both the flexible single cells and the modules. As a result, the 1 cm² flexible devices produce an excellent PCE of 12.16% as compared to 12.37% for the spin‐coated small‐area (0.04 cm²) rigid devices. Likewise, for modules with an area of 25 cm², an extraordinary PCE of 10.09% is observed. Hence, efficiency losses associated with the upscaling are significantly reduced by the synergistic optimization. Moreover, after 1000 bending cycles at a bending radius of 10 mm, the flexible devices still produce over 99% of their initial PCE, whereas after being stored for over 6000 h in a glove box, the PCE reaches 103% of its initial value, indicating excellent device flexibility as well as superior shelf stability. These results, thus, are a promising confirmation the great potential for upscaling of large‐area OSCs in the near future.

The role of methylammonium chloride (MACl) in sequentially deposited bromine (Br)‐free formamidinium lead iodide (FAPbI₃)‐based perovskite is systematically demonstrated to regulate the PbI₂/FAI reaction, tune the phase transition at room temperature, and adjust the PbI₂ residual through an intermediate‐related perovskite decomposition during thermal annealing. The resulting optimized solar cells achieve a remarkable efficiency of 23.1% with considerably improved photostability.

Abstract

So far, the combination of methylammonium bromide/methylammonium chloride (MABr/MACl) or methylammonium iodide (MAI)/MACl is the most frequently used additives to stabilize formamidinium lead iodide (FAPbI₃) fabricated by the sequential deposition method. However, the enlarged bandgap due to the addition of bromide and the ambiguous functions of these additives in lead iodide (PbI₂) transformation are still worth considering. Herein, the roles of MACl in sequentially deposited Br‐free FA‐based perovskites are systematically investigated. It is found that MACl can finely regulate the PbI₂/FAI reaction, tune the phase transition at room temperature, and adjust intermediate‐related perovskite crystallization and decomposition during thermal annealing. Compared to FAPbI₃, the perovskite with MACl exhibits larger grain, longer carrier lifetime, and reduced trap density. The resultant solar cell therefore achieves a champion power conversion efficiency (PCE) of 23.1% under reverse scan with a stabilized power output of 23.0%. In addition, it shows much improved photostability under 100 mW cm⁻² white illumination (xenon lamp) in nitrogen atmosphere without encapsulation.

The fabrication challenges of monolithic all‐perovskite tandem photovoltaics are detailed in a step‐by‐step, choose‐your‐own‐adventure fashion. The trade‐offs between sub‐cell efficiency and processing stability are highlighted and pros and cons are weighed. Through this detailed analysis, a few routes to reach >30% power conversion efficiency and the necessary work are identified.

Abstract

Metal halide perovskites (MHPs) have transfixed the photovoltaic (PV) community due to their outstanding and tunable optoelectronic properties coupled to demonstrations of high‐power conversion efficiencies (PCE) at a range of bandgaps. This has motivated the field to push perovskites to reach the highest possible performance. One way to increase the efficiency is by fabricating multijunction solar cells, which can split the solar spectrum, reducing thermalization loss. Low‐cost all‐perovskite tandems have a real chance to soon exceed 30% PCE, which could transform the PV industry. Achieving this goal requires the identification of perovskite sub‐cells that are both highly efficient and can be effectively integrated. Herein, it is discussed how to navigate the multiple‐choice adventure in choosing between the myriad of options and considerations present when deciding what perovskite materials, contact layers, and processing tools to use. Some of the potential fabrication pitfalls often encountered in MHP based tandem PVs are highlighted, so that they can hopefully be avoided in the future.