Chen Weijie

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.

Kesterite Cu₂ZnSnSe₄ (CZTSe) thin‐film solar cells with independently confirmed 12.5% total area efficiency are demonstrated using a novel strategy to effectively control the formation of intrinsic defects and defect clusters in CZTSe by carefully engineering the local chemical environment (e.g., suitable local chemical composition, oxidation states of cations) during film growth.

Abstract

Kesterite‐based Cu₂ZnSn(S,Se)₄ semiconductors are emerging as promising materials for low‐cost, environment‐benign, and high‐efficiency thin‐film photovoltaics. However, the current state‐of‐the‐art Cu₂ZnSn(S,Se)₄ devices suffer from cation‐disordering defects and defect clusters, which generally result in severe potential fluctuation, low minority carrier lifetime, and ultimately unsatisfactory performance. Herein, critical growth conditions are reported for obtaining high‐quality Cu₂ZnSnSe₄ absorber layers with the formation of detrimental intrinsic defects largely suppressed. By controlling the oxidation states of cations and modifying the local chemical composition, the local chemical environment is essentially modified during the synthesis of kesterite phase, thereby effectively suppressing detrimental intrinsic defects and activating desirable shallow acceptor Cu vacancies. Consequently, a confirmed 12.5% efficiency is demonstrated with a high V _OC of 491 mV, which is the new record efficiency of pure‐selenide Cu₂ZnSnSe₄ cells with lowest V _OC deficit in the kesterite family by E _g/q‐Voc. These encouraging results demonstrate an essential route to overcome the long‐standing challenge of defect control in kesterite semiconductors, which may also be generally applicable to other multinary compound semiconductors.

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.

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.

The photo‐effect on ion conduction in mixed cation and halide perovskites is studied. Unlike A‐site substitution, anion replacement is of great influence. In I‐Br mixtures the differences in hole localization and defect formation favor (reversible) photo‐demixing (the situation in the right part is simplified as the interstitial neutral iodine is further stabilized by ionic rearrangement, and the hole in the bromide is delocalized over several regular anions).

Abstract

Lead halide perovskites are considered to be most promising photovoltaic materials. Highest efficiency and improved stability of perovskite solar cells have been achieved by using cation and anion mixtures. Experimental information on electronic and ionic charge carriers is key to evaluate device performance, as well as processes of photo‐decomposition and photo‐demixing which are observed in these materials. Here, we measure ionic and electronic transport properties and investigate various cation and anion substitutions with a special eye on their photo‐ionic effect, following our previous study on CH₃NH₃PbI₃, where we found that light enhances not only electronic but also ionic conductivities. We find that this phenomenon is very sensitive to the nature of the halide, while the cationic substitutions are less relevant. Based on the observation that the ionic conductivity enhancement found for iodide perovskites is significantly weakened by bromide substitution, we provide a chemical rationale for the photo‐demixing in mixed halide compositions.

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.

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.

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.

Nature Energy, Published online: 11 November 2020; doi:10.1038/s41560-020-00732-2

The power conversion efficiency of organic solar cells has rapidly increased, yet significantly less attention has been paid to materials stability and device longevity. For organic solar cells to make an impact in the marketplace, researchers, funding agencies and journals should do more to address this crucial gap.

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.

Publication date: February 2021

Source: Nano Energy, Volume 80

Author(s): Yun-Ming Sung, Meng-Zhen Li, Dian Luo, Yan-De Li, Sajal Biring, Yu-Ching Huang, Chun-Kai Wang, Shun-Wei Liu, Ken-Tseng Wong

Significant progress has been made in non‐fullerene organic solar cells (OSCs) in recent years, including in materials development, device engineering, and mechanistic understanding. This review summarizes progress and offers some reflections on the emerging methods for enabling high efficiency and improved stability for non‐fullerene OSCs.

Abstract

The past three years have witnessed rapid growth in the field of organic solar cells (OSCs) based on non‐fullerene acceptors (NFAs), with intensive efforts being devoted to material development, device engineering, and understanding of device physics. The power conversion efficiency of single‐junction OSCs has now reached high values of over 18%. The boost in efficiency results from a combination of promising features in NFA OSCs, including efficient charge generation, good charge transport, and small voltage losses. In addition to efficiency, stability, which is another critical parameter for the commercialization of NFA OSCs, has also been investigated. This review summarizes recent advances in the field, highlights approaches for enhancing the efficiency and stability of NFA OSCs, and discusses possible strategies for further advances of NFA OSCs.

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.

Two well‐regular polymer acceptors (PY‐IT and PY‐OT) with different polymerization sites are developed. For comparison, a random ternary copolymer (PY‐IOT) with the same ratio of the two acceptors is synthesized. All‐polymer solar cells (PSCs) based on PM6:PY‐IT achieve an excellent PCE of 15.05%, which is significantly higher than those based on PY‐OT (10.04%) and PY‐IOT (12.12%).

Abstract

Recent advances in the development of polymerized A–D–A‐type small‐molecule acceptors (SMAs) have promoted the power conversion efficiency (PCE) of all‐polymer solar cells (all‐PSCs) over 13%. However, the monomer of an SMA typically consists of a mixture of three isomers due to the regio‐isomeric brominated end groups (IC‐Br(in) and IC‐Br(out)). In this work, the two isomeric end groups are successfully separated, the regioisomeric issue is solved, and three polymer acceptors, named PY‐IT, PY‐OT, and PY‐IOT, are developed, where PY‐IOT is a random terpolymer with the same ratio of the two acceptors. Interestingly, from PY‐OT, PY‐IOT to PY‐IT, the absorption edge gradually redshifts and electron mobility progressively increases. Theory calculation indicates that the LUMOs are distributed on the entire molecular backbone of PY‐IT, contributing to the enhanced electron transport. Consequently, the PM6:PY‐IT system achieves an excellent PCE of 15.05%, significantly higher than those for PY‐OT (10.04%) and PY‐IOT (12.12%). Morphological and device characterization reveals that the highest PCE for the PY‐IT‐based device is the fruit of enhanced absorption, more balanced charge transport, and favorable morphology. This work demonstrates that the site of polymerization on SMAs strongly affects device performance, offering insights into the development of efficient polymer acceptors for all‐PSCs.

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.

Nature Energy, Published online: 09 November 2020; doi:10.1038/s41560-020-00721-5

Two-dimensional Ruddlesden–Popper layered metal-halide perovskites show better performance over three-dimensional versions, but are typically based on quantum wells with random width distribution. Liang et al. show that introducing molten salt spacers gives phase-pure quantum wells and improved solar cell performance.