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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.

An electron transport layer is one of the essential components for most of the efficient perovskite devices. This review focuses on 2D materials as the electron transport layer in perovskite solar cells with tunable work function and high carrier mobility.

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 of less than 0.1 cm² has already exceeded 25%, which, however, decreases significantly to about 16% for a large device area of about 100 cm². Therefore, the scalability has become one of the most significant limits on 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. Therefore, an exploration of effective and low‐cost charge transport materials is crucial for scalable fabrication of highly efficient perovskite devices. The 2D materials have drawn wide attention in the PSC community with tunable bandgap and high carrier mobility. So far, the search for 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 providing direction for future development toward efficient and large‐scale perovskite devices.

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.

Comprehensive experimental and theoretical evidence is presented to elucidate the charge carrier recombination in mesoscopic perovskite solar cells (PSC). The spatially decoupled electron and hole migration inside the nanoporous charge extraction scaffold is examined by a 2D electrical model that explains the remarkably high photovoltages achieved in hole extraction layer‐free, carbon–graphite‐based PSCs.

A physical model to explain the 2D charge recombination in mesoscopic graphite‐based perovskite solar cells (PSCs) having a highly selective front electrode and a nonselective back electrode is presented. Steady‐state photovoltage and photoluminescence (PL) as well as transient PL are studied for a wide range of device configurations, providing insights in the interface recombination at the front and back contact, namely, the mesoporous titania (m‐TiO₂) and the graphite layer. Combining experimental evidence with the first 2D simulation of a perovskite solar cell, it is found that the characteristic thick absorber layer of mesoscopic graphite‐based PSC is a necessity to enhance the photovoltage. This is because the interface recombination at the back contact is a diffusion‐limited process. The electrode spacing should, however, not be enhanced by increasing the perovskite/m‐TiO₂ thickness as this increases surface recombination losses at this interface. The study determines design rules for the optimal geometry of the mesoporous layers and helps to identify the limiting recombination pathways for an improvement of future device architectures.

Fe(III) ion grafted metal–organic complexes (Fe(III)⊂MOCs) are introduced as p‐type dopants into hole transport layers (HTLs), contributing to outstanding n–i–p perovskite solar cells (PSCs) with improved power conversion efficiency (PCE, 20.46%), reduced J–V hysteresis, and enhanced air stability, which is significantly superior to the reference devices.

Realizing the rapid/controllable oxidation of 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene (Spiro‐OMeTAD) under an inert atmosphere, reducing the J–V hysteresis and enhancing the air stability of devices is extremely significant to fabricate high‐performance perovskite solar cells (PSCs). Herein, the Fe(III) ion grafted metal–organic complexes (Fe(III) ⊂ MOCs) are assembled and used as the p‐type dopants of hole transport layers (HTLs) to prepare efficient and stable PSCs. Consequently, the optimal Fe(III) ⊂ In‐2‐bpydc‐doped device presents a significantly enhanced power conversion efficiency (PCE) of 20.46%, benefitting from the improved hole extraction and weakened carrier recombination at the interface between HTLs and perovskite films. More importantly, the modified device possesses a reduced J–V hysteresis index (HI) of 0.094, and can maintain nearly 90% of its initial PCE value after being exposed to the air at ≈25 °C and relative humidity (RH) of ≈35% for 4 weeks, which is attributed to the restrained detrimental penetration behavior by the MOC carrier part of dopants. This work is of important guiding significance for the application of MOC materials in photovoltaic fields.

State‐of‐the‐art perovskite and organic solar cells use inorganic hole transport materials (HTMs) due to their superior electronic properties. These HTMs are, however, expensive and prone to degradation. A range of robust inorganic HTMs are emerging, that provide a trade‐off between efficiency, stability, and cost, and are critically reviewed herein.

Interfaces in perovskite and organic solar cells play a central role in advancing efficiency and prolong device durability. They improve charge transport/transfer from the absorber layer to the collecting electrodes, while also blocking the opposite charge carriers, minimize voltage losses by suppressing charge recombination. and may act as buffer/protective layers and nanomorphology regulators for the absorber layer. One such interface is formed by the hole transport layer (HTL) and the organic/perovskite absorber. These HTLs typically consist of organic semiconductors, which, although are solution processable at low temperatures and allow perfect energy‐level alignment with the absorber layer and therefore efficient charge collection, are prone to degradation in ambient conditions and under continuous light exposure. In a quest for robust alternatives, inorganic materials such as metal oxides, graphene oxide, bronzes, copper thiocyanate, and transition metal dichalcogenides are actively investigated. However, their hole extraction capability is inferior compared with organic semiconductors as they possess specific energetics leading to significant charge extraction barriers and moderate charge collection. To achieve further advancements in their hole transporting capabilities, strongly interconnecting knowledge of their synthesis, electronic properties, and device performance metrics is required.

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 non‐annealed, ultrathin, amorphous metal oxyhydroxide was introduced to suppress interfacial charge recombination and reduce energy loss in electron‐transport‐layer (ETL)‐free perovskite solar cells. The cells achieve a record efficiency of 21.1 %, outperforming their ETL‐containing metal oxide counterparts (18.7 %).

Abstract

The performances of electron‐transport‐layer (ETL)‐free perovskite solar cells (PSCs) are still inferior to ETL‐containing devices. This is mainly due to severe interfacial charge recombination occurring at the transparent conducting oxide (TCO)/perovskite interface, where the photo‐injected electrons in the TCO can travel back to recombine with holes in the perovskite layer. Herein, we demonstrate for the first time that a non‐annealed, insulating, amorphous metal oxyhydroxide, atomic‐scale thin interlayer (ca. 3 nm) between the TCO and perovskite facilitates electron tunneling and suppresses the interfacial charge recombination. This largely reduced the interfacial charge recombination loss and achieved a record efficiency of 21.1 % for n‐i‐p structured ETL‐free PSCs, outperforming their ETL‐containing metal oxide counterparts (18.7 %), as well as narrowing the efficiency gap with high‐efficiency PSCs employing highly crystalline TiO₂ ETLs.

A new type of methylammonium‐free formamidinium (FA) based perovskites is reported. The low‐dimensional perovskite films are obtained in the presence of the FACl additive, and the role of Cl is investigated through grazing‐incidence X‐ray diffraction. Solar cell devices based on (PDA)(FA)₃Pb₄I₁₃ films show extremely high thermal stability and a remarkable PCE of 13.8 %.

Abstract

Currently, most two‐dimensional (2D) metal halide perovskites are of the Ruddlesden–Popper type and contain the thermally unstable methylammonium (MA) molecules, which leads to inferior photovoltaic performance and mild stability. Here we report a new type of MA‐free formamidinium (FA) based low‐dimensional perovskites, featuring a general formula of (PDA)(FA) _n−1Pb_nI_3n+1 with propane‐1,3‐diammonium (PDA) as the organic spacer cation. The perovskite films with well‐oriented crystal grains are attained under the assistance of the FACl additive, where the role of Cl is investigated through the grazing‐incidence X‐ray diffraction technique. The photovoltaic device based on the optimized (PDA)(FA)₃Pb₄I₁₃ film demonstrates a remarkable power conversion efficiency of 13.8 %, the highest record for the FA‐based 2D perovskite solar cells. In addition, compared to (PDA)(MA)₃Pb₄I₁₃, the MA‐containing analogue and a renowned stable 2D perovskite, both the (PDA)(FA)₃Pb₄I₁₃ films and their derived devices exhibit exceedingly higher thermal stability.

We disclosed a key finding to modulate the crystallization kinetics of FASnI₃ through a non‐classical nucleation mechanism based on pre‐nucleation clusters. A direct link between the colloids in the perovskite precursor solution and final optoelectronic quality of the perovskite films was established. Finally, power conversion efficiency of 11.39 % was obtained for FASnI₃‐based perovskite solar cells.

Abstract

Tin halide perovskites are rising as promising materials for lead‐free perovskite solar cells (PSCs). However, the crystallization rate of tin halide perovskites is much faster than the lead‐based analogs, leading to more rampant trap states and lower efficiency. Here, we disclose a key finding to modulate the crystallization kinetics of FASnI₃ through a non‐classical nucleation mechanism based on pre‐nucleation clusters (PNCs). By introducing piperazine dihydriodide to tune the colloidal chemistry of the FASnI₃ perovskite precursor solution, stable clusters could be readily formed in the solution before nucleation. These pre‐nucleation clusters act as intermediate phase and thus can reduce the energy barrier for the perovskite nucleation, resulting in a high‐quality perovskite film with lower defect density. This PNCs‐based method has led to a conspicuous photovoltaic performance improvement for FASnI₃‐based PSCs, delivering an impressive efficiency of 11.39 % plus improved stability.

In article number 2001759, Kilwon Cho and co‐workers report the novel design of an organic spacer as a multifunctional additive for formamidinium lead tri‐iodide (FAPbI₃) perovskite solar cells. Low dimensional (LD) perovskites assembled by organic spacers not only protect the grain boundary of FAPbI₃ from moisture but also facilitate the nucleation and growth of FAPbI₃ at low temperature. An LD/ FAPbI₃ composite based solar cell exhibits power conversion efficiency of 21.25% retaining 80% of the initial efficiency after 500 hours without encapsulation.

The capability to deposit perovskite materials by either thermal evaporation or solution processing offers intriguing possibilities for mass production of perovskite solar cells. This Progress Report describes the current state of research in both fields, discusses the challenges faced by these methods and their future opportunities.

Abstract

The last decade has seen remarkable advancements in the field of perovskite materials and photovoltaic technologies. One of their most extraordinary characteristics is the high quality of layers that can be obtained by “dirty processing” from solution at low temperatures. Alternatively, perovskites can also be deposited by thermal evaporation, a clean, solvent‐free process, which is well established for many industrial applications. Although the vast majority of research reports focus on solution‐processing as the deposition method for perovskite solar cells, thermally evaporated perovskite solar cells are closing in the performance gap with several reports of efficiencies above 20%. In this Progress Report, the two deposition methods are briefly introduced, the key developments in photovoltaic devices based on each deposition technique are outlined, and the challenges and future possibilities are discussed.

Recent progress is reviewed in applying self‐assembled monolayers in perovskite solar cells to improve surface morphology, energy band alignment, reduced interfacial charge recombination, and the trap passivation mechanism. The opportunities for molecular design of self‐assembled monolayers in enhancing the power conversion efficiency and stability of perovskite solar cells are discussed.

Abstract

Due to a certified 25.2% high efficiency, low cost, and easy fabrication; perovskite solar cells (PSCs) are the focus of interest among the next‐generation photovoltaic technologies. Long‐term stability is one of the most challenging obstacles to bring technology from the lab to the market. In this review, applications of self‐assembled monolayers (SAMs) to enhance the power conversion efficiency (PCE) and stability of PSCs is discussed. In the first part, the introduction of SAMs, and deposition techniques applied to different PSC architectures are described. In the middle section, current efforts to utilize SAMs to fine‐tune the optoelectronic properties to enhance the PCE and stability are detailed. The improvements in surface morphology, energy band alignment, as well as reduced interfacial charge recombination induced by SAMs, and the trap passivation mechanism allowing optimal PCE and stability are described. A general outlook summarizing the importance of SAMs to the improvement of PSCs performance is also given, alongside a discussion of future opportunities and possible research directions.

An additive‐involved leaching method is proposed to reduce the preparation temperature of CsPbI₃ to 100 °C. The CsPbI₃ perovskite film with high crystallinity is formed by an ion exchange reaction between DMAPbI₃ and Cs₄PbI₆. More than 16% photoelectric conversion efficiency can be achieved and the inencapsulation device exhibits remaekable stability.

Abstract

Inorganic CsPbI₃ perovskite with an optical bandgap ranging from 1.67 to 1.75 eV is a promising light‐harvesting material as a top cell in tandem solar cells, but its high fabrication temperature can damage the middle layers or the bottom subcells. Here, an additive‐involved leaching method to fabricate CsPbI₃ perovskite films is demonstrated, which can decrease the preparation temperature to 100 °C. The CsPbI₃ perovskite films with high crystallinity are achieved by a solution assisted reaction between DMAPbI₃ and Cs₄PbI₆ with the leaching of DMA⁺, Cs⁺, and I⁻. The as‐prepared CsPbI₃ perovskite films exhibit much superior stability compared to their high‐temperature counterparts. As a result, a power conversion efficiency of over 16% is obtained, and the unencapsulated device maintains over 93% of the initial efficiency after aging for 30 days in air with a relative humidity of 10%.

SnO₂ has been applied as an efficient electron transport layer for perovskite solar cells over the past few years. In this progress report, recent advances in SnO₂ modification toward high efficiency and stability are summarized from the perspective of the optimization strategies, and the remaining challenges as well as opportunities for future research are also discussed.

Abstract

The electron transport layer plays a key role in affecting the charge dynamics and photovoltaic parameters in perovskite solar cells. Compared to other counterparts, SnO₂ has unique advantages such as low temperature fabrication and high electron extraction ability, and it receives extra attentions from the research community since the first report. Planar‐type perovskite solar cells based on SnO₂ exhibit a simple architecture and state of art device can achieve a power conversion efficiency of over 23%, which can compete with traditional devices using mesoporous TiO₂. The modification engineering of SnO₂ has contributed significantly to the enhanced device performance during the past years. There is still great potential for further improvement in the efficiency and long‐term stability. Herein recent advances toward modifying the optoelectronic properties of SnO₂ from the perspective of the optimization strategies are summarized and the remaining challenges as well as opportunities for future research are discussed. The continuous efforts dedicated to this exciting field may pave the way for developing commercial perovskite solar cells.

This review summarizes the isomerization strategy of nonfullerene small‐molecule acceptors for organic solar cells, and discusses the key structure–property relationships in depth.

Abstract

Nonfullerene acceptors (NFAs) are a current focus of research on bulk‐heterojunction organic solar cells (OSCs), as they can exhibit strong absorption, suitably matched energy levels, and good stability. Isomerization affords a new material design strategy for nonfullerene small‐molecule acceptors (SMAs). In this article, the development of isomeric nonfullerene SMAs, including isomeric perylene diimide (PDI)‐based nonfullerene SMAs and isomeric acceptor–donor–acceptor (A–D–A)‐type nonfullerene SMAs, is reviewed. The general design principles for isomeric SMAs and the key structure–property relationships are comprehensively surveyed and discussed. The remaining challenges and promising future directions of isomeric nonfullerene acceptors are presented.

A clear solid‐state aggregation transition of the acceptor N3 is discovered, which enables a double‐annealing method that can fine‐tune aggregation and morphology. Compared with the 16.6% efficiency for PM6:N3:PC₇₁BM‐control devices, a higher efficiency of 17.6% is obtained through the improved protocol. The results provide a molecular design and engineering conundrum to achieve simultaneously low annealing temperatures, high efficiency, and stability.

Abstract

Thermal transition of organic solar cells (OSCs) constituent materials are often insufficiently researched, resulting in trial‐and‐error rather than rational approaches to annealing strategies to improve domain purity to enhance the power conversion efficiency. Despite the potential utility, little is known about the thermal transitions of the modern high‐performance acceptors Y6 and N3. Here, by using an optical method, it is discovered that the acceptor N3 has a clear solid‐state aggregation transition at 82 °C. This unusually low transition not only explains prior optimization protocols, but the transition informs and enables a double‐annealing method that can fine‐tune aggregation and the device morphology. Compared with 16.6% efficiency for PM6:N3:PC₇₁BM control devices, higher efficiency of 17.6% is obtained through the improved protocol. Morphology characterization with x‐ray scattering methods reveals the formation of a multilength scale morphology. Moreover, the double‐annealing method is illustrated and easily transferred and validated with Y6‐based devices, using the transition of Y6 at 102 °C. As a result, the PCE improved from 16.0% to 16.8%. Design of high‐performance acceptors with yet lower aggregation transitions might be required for OSCs to successfully transition to low thermal budget industrial processing methods where annealing temperatures on plastic substrates have to be kept low.

Molecular packing and thermodynamic properties of D18‐based fullerene‐free organic solar cells are studied. The D18 polymer exhibits strong chain extension in films, which is beneficial to charge transport. Miscibility and other characterizations explain the disparate performance of three systems and the processing procedures.

Abstract

Organic solar cells (OSCs) based on D18:Y6 have recently exhibited a record power conversion efficiency of over 18%. The initial work is extended and the device performance of D18‐based OSCs is compared with three non‐fullerene acceptors, Y6, IT‐4F, and IEICO‐4Cl, and their molecular packing characteristics and miscibility are studied. The D18 polymer shows unusually strong chain extension and excellent backbone ordering in all films, which likely contributes to the excellent hole‐transporting properties. Thermodynamic characterization indicates a room‐temperature miscibility for D18:Y6 and D18:IT‐4F near the percolation threshold. This corresponds to an ideal quench depth and explains the use of solvent vapor annealing rather than thermal annealing. In contrast, D18:IEICO‐4Cl is a low‐miscibility system with a deep quench depth during casting and poor morphology control and low performance. A failure of ternary blends with PC₇₁BM is likely due to the near‐ideal miscibility of Y6 to begin with and indicates that strategies for developing successful ternary or quaternary solar cells are likely very different for D18 than for other high‐performing donors. This work reveals several unique property–performance relations of D18‐based photovoltaic devices and helps guide design or fabrication of yet higher efficiency OSCs.

Incorporation of a poly[N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)] (PCDTBT) fibrils as an efficient hole transfer layer (HTL) is demonstrated as an effective approach to significantly enhance air and mechanical stability of perovskite solar cells (PSCs) as an alternative to widely used doped HTLs, resulting from the interlocking effect of PCDTBT fibrils formed at the interface between perovskite and PDCTBT layers.

Abstract

Atmospheric and mechanical stability of perovskite solar cells (PSCs) must be guaranteed for successful commercialization. A fibrillar polymer, poly[N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)] (PCDTBT), is reported as an efficient hole transfer layer (HTL) which significantly improves air and mechanical stability of perovskite solar cells (PSCs). PCDTBT fibrils formed at the grain boundaries of perovskite layer induce the highest fracture energies in the PSCs, which provide extrinsic reinforcement and shielding for enhanced mechanical and chemical stability. Debonding energy increases by 30% for the PSCs with PCDTBT fibrils, which fractures at 2.66 J m⁻², compared to the devices without PCDTBT fibrils at 2.09 J m⁻²; more importantly, the threshold debonding driving force of the PCDTBT fibril‐based devices is greatly improved by twofold under ambient conditions.