Efficient,Hysteresis‐Free,and Stable Perovskite Solar Cells with ZnO as Electron‐Transport Layer: Effect of Surface Passivation

The power conversion efficiency of perovskite solar cells (PSCs) has ascended from 3.8% to 22.1% in recent years. ZnO has been well‐documented as an excellent electron‐transport material. However, the poor chemical compatibility between ZnO and organo‐metal halide perovskite makes it highly challenging to obtain highly efficient and stable PSCs using ZnO as the electron‐transport layer. It is demonstrated in this work that the surface passivation of ZnO by a thin layer of MgO and protonated ethanolamine (EA) readily makes ZnO as a very promising electron‐transporting material for creating hysteresis‐free, efficient, and stable PSCs. Systematic studies in this work reveal several important roles of the modification: (i) MgO inhibits the interfacial charge recombination, and thus enhances cell performance and stability; (ii) the protonated EA promotes the effective electron transport from perovskite to ZnO, further fully eliminating PSCs hysteresis; (iii) the modification makes ZnO compatible with perovskite, nicely resolving the instability of ZnO/perovskite interface. With all these findings, PSCs with the best efficiency up to 21.1% and no hysteresis are successfully fabricated. PSCs stable in air for more than 300 h are achieved when graphene is used to further encapsulate the cells.     

Emerging Semitransparent Solar Cells: Materials and Device Design

Semitransparent solar cells can provide not only efficient power‐generation but also appealing images and show promising applications in building integrated photovoltaics, wearable electronics, photovoltaic vehicles and so forth in the future. Such devices have been successfully realized by incorporating transparent electrodes in new generation low‐cost solar cells, including organic solar cells (OSCs), dye‐sensitized solar cells (DSCs) and organometal halide perovskite solar cells (PSCs). In this review, the advances in the preparation of semitransparent OSCs, DSCs, and PSCs are summarized, focusing on the top transparent electrode materials and device designs, which are all crucial to the performance of these devices. Techniques for optimizing the efficiency, color and transparency of the devices are addressed in detail. Finally, a summary of the research field and an outlook into the future development in this area are provided.     

Bulk‐Heterojunction Organic Solar Cells: Five Core Technologies for Their Commercialization

The past two decades of vigorous interdisciplinary approaches has seen tremendous breakthroughs in both scientific and technological developments of bulk‐heterojunction organic solar cells (OSCs) based on nanocomposites of π‐conjugated organic semiconductors. Because of their unique functionalities, the OSC field is expected to enable innovative photovoltaic applications that can be difficult to achieve using traditional inorganic solar cells: OSCs are printable, portable, wearable, disposable, biocompatible, and attachable to curved surfaces. The ultimate objective of this field is to develop cost‐effective, stable, and high‐performance photovoltaic modules fabricated on large‐area flexible plastic substrates via high‐volume/throughput roll‐to‐roll printing processing and thus achieve the practical implementation of OSCs. Recently, intensive research efforts into the development of organic materials, processing techniques, interface engineering, and device architectures have led to a remarkable improvement in power conversion efficiencies, exceeding 11%, which has finally brought OSCs close to commercialization. Current research interests are expanding from academic to industrial viewpoints to improve device stability and compatibility with large‐scale printing processes, which must be addressed to realize viable applications. Here, both academic and industrial issues are reviewed by highlighting historically monumental research results and recent state‐of‐the‐art progress in OSCs. Moreover, perspectives on five core technologies that affect the realization of the practical use of OSCs are presented, including device efficiency, device stability, flexible and transparent electrodes, module designs, and printing techniques.     

Metal-Organic Framework Materials in Perovskite Solar Cells: Recent Advancements and Perspectives

Organic–inorganic hybrid perovskite solar cells (PSCs) are among the most promising candidates for the next generation of photovoltaic devices because of the significant increase in their power conversion efficiency (PCE) from less than 10% to 25.7% in past decade. The metal-organic framework (MOF) materials owing to their unique properties, such as large specific surface area, abundant binding sites, adjustable nanostructures, and synergistic effects, are used as additives or functional layers to enhance the device performance and long-term stability of PSCs. This review focuses on the recent advancements in the applications of MOFs as/in different functional layers of PSCs. The photovoltaic performance, impact, and advantages of MOF materials integrated into the perovskite absorber, electron transport layer, hole transport layer, and interfacial layer are reviewed. In addition, the applicability of MOFs to mitigate leakage of Pb²⁺ from halide perovskites and corresponding devices is discussed. This review concludes with the perspectives on further research directions for employing MOFs in PSCs.     

High‐Performance Long‐Term‐Stable Dopant‐Free Perovskite Solar Cells and Additive‐Free Organic Solar Cells by Employing Newly Designed Multirole π‐Conjugated Polymers

Perovskite solar cells (PSCs) and organic solar cells (OSCs) are promising renewable light‐harvesting technologies with high performance, but the utilization of hazardous dopants and high boiling additives is harmful to all forms of life and the environment. Herein, new multirole π‐conjugated polymers (P1–P3) are developed via a rational design approach through theoretical hindsight, further successfully subjecting them into dopant‐free PSCs as hole‐transporting materials and additive‐free OSCs as photoactive donors, respectively. Especially, P3‐based PSCs and OSCs not only show high power conversion efficiencies of 17.28% and 8.26%, but also display an excellent ambient stability up to 30 d (for PSCs only), owing to their inherent superior optoelectronic properties in their pristine form. Overall, the rational approach promises to support the development of environmentally and economically sustainable PSCs and OSCs.     

Nexuses Between the Chemical Design and Performance of Small Molecule Dopant-Free Hole Transporting Materials in Perovskite Solar Cells

Perovskite solar cells (PSCs) have grabbed much attention of researchers owing to their quick rise in power conversion efficiency (PCE). However, long-term stability remains a hurdle in commercialization, partly due to the inclusion of necessary hygroscopic dopants in hole transporting materials, enhancing the complexity and total cost. Generally, the efforts in designing dopant-free hole transporting materials (HTMs) are devoted toward small molecule and polymeric HTMs, where small molecule based HTMs (SM-HTMs) are dominant due to their reproducibility, facile synthesis, and low cost. Still, the state-of-art dopant-free SM-HTM has not been achieved yet, mainly because of the knowledge gap between device engineering and molecular designs. From a molecular engineering perspective, this article reviews dopant-free SM-HTMs for PSCs, outlining analyses of chemical structures with promising properties toward achieving effective, low-cost, and scalable materials for devices with higher stability. Finally, an outlook of dopant-free SM-HTMs toward commercial application and insight into the development of long-term stability PSCs devices is provided.     

Tailoring interface of lead-halide perovskite solar cells

Lead-halide perovskite solar cells (PSCs) have attracted tremendous attention during the past few years owing to their extraordinary electronic and photonic properties.To improve the performances of PSCs,many researchers have focused on the compositional engineering,solvent engineering,and film fabrication methodologies.Interfacial engineering of PSCs has become a burgeoning field in which researchers aim to deeply understand the mechanisms of cells and thereby increase the efficiency and stability of PSCs.This review focuses on the interface tailoring of lead-halide PSCs,including the modification of each layer of the cell structure (i.e.,perovskite absorber,electron-transport layers,and holetransport layers) and the interfacial materials that can be introduced into the PSCs.     

Buried Interface Passivation: A Key Strategy to Breakthrough the Efficiency of Perovskite Photovoltaics

Owing to the merits of low cost and high power conversion efficiency (PCE), perovskite solar cells (PSCs) have become the best candidate to replace the commonly used silicon solar cells. However, PSCs have been slow to enter the market for a number of reasons, including poor stability, high toxicity, and rigorous preparation process. Passivation strategies including surface passivation and bulk passivation have been successfully applied to improve the device performance of PSCs. The passivation of the defects at the buried interface, which is regarded as a key strategy to breakthrough the device efficiency and stability of PSCs in the future, is ongoing with challenge. Herein, in detail the recent passivation of the buried interface is introduced from three aspects: perovskite layer, buried interlayer, and transport layer. The passivation effect of the buried interface is clearly demonstrated through three categories of salts, organics, and 2D materials. In addition, the transport layer is classified into electron transport layer (ETL) and hole transport layer (HTL). These classifications can help to have a clear understanding of substances which generate passivating effect and guide the continuous promotion of the follow-up buried interface passivating work.     

倒置钙钛矿太阳能电池的空穴传输层的研究进展

倒置钙钛矿太阳能电池(PSCs)具有器件结构简单、吸光系数高、迟滞效应小、良好的缺陷容忍性等优点,受到了广泛的关注。但倒置器件光电转换效率(PCE)尚有待提高,究其原因是空穴传输层(HTL)和钙钛矿层界面处的能量损失表现出相对较小的开路电压。文章综述了包括有机聚合物、无机物、尖晶石氧化物等作为空穴传输材料的相关研究进展,进一步分析了通过调节电极/空穴传输层能级使之与钙钛矿价带匹配,及通过界面修饰促进器件对载流子的注入与收集,从而提高光电转换效率的研究现状。对提高倒置钙钛矿太阳能电池性能的研究具有一定的指导意义,最后对倒置器件的应用前景进行了展望。     

In Situ Back‐Contact Passivation Improves Photovoltage and Fill Factor in Perovskite Solar Cells

Organic–inorganic hybrid perovskite solar cells (PSCs) have seen a rapid rise in power conversion efficiencies in recent years; however, they still suffer from interfacial recombination and charge extraction losses at interfaces between the perovskite absorber and the charge–transport layers. Here, in situ back‐contact passivation (BCP) that reduces interfacial and extraction losses between the perovskite absorber and the hole transport layer (HTL) is reported. A thin layer of nondoped semiconducting polymer at the perovskite/HTL interface is introduced and it is shown that the use of the semiconductor polymer permits—in contrast with previously studied insulator‐based passivants—the use of a relatively thick passivating layer. It is shown that a flat‐band alignment between the perovskite and polymer passivation layers achieves a high photovoltage and fill factor: the resultant BCP enables a photovoltage of 1.15 V and a fill factor of 83% in 1.53 eV bandgap PSCs, leading to an efficiency of 21.6% in planar solar cells.     

20.67%-Efficiency Inorganic CsPbI3 Solar Cells Enabled by Zwitterion Ion Interface Treatment

All-inorganic CsPbI₃ perovskite solar cells (PSCs) have been extensively studied due to their high thermal stability and unprecedented rise in power conversion efficiency (PCE). Recently, the champion PCE of CsPbI₃ PSCs has reached up to 21%; however, it is still much lower than that of organic–inorganic hybrid PSCs. Interface modification to passivate surface defects and minimize charge recombination and trapping is important to further improve the efficiency of CsPbI₃ PSCs. Herein, a new zwitterion ion is deposited at the interface between electron transporting layer (ETL) and perovskite layer to passivate the defects therein. The zwitterion ions can not only passivate oxygen vacancy (V_O) and iodine vacancy (V_I) defects, but also improve the band alignment at the ETL-perovskite interface. After the interface treatment, the PCE of CsPbI₃ device reaches up to 20.67%, which is among the highest values of CsPbI₃ PSCs so far. Due to the defect passivation and hydrophobicity improvement, the PCE of optimized device remains 94% of its original value after 800 h storing under ambient condition. These results provide an efficient way to improve the quality of ETL-perovskite interface by zwitterion ions for achieving high performance inorganic CsPbI₃ PSCs.     

Combining Efficiency and Stability in Mixed Tin–Lead Perovskite Solar Cells by Capping Grains with an Ultrathin 2D Layer

The development of narrow-bandgap (E_g ≈ 1.2 eV) mixed tin–lead (Sn–Pb) halide perovskites enables all-perovskite tandem solar cells. Whereas pure-lead halide perovskite solar cells (PSCs) have advanced simultaneously in efficiency and stability, achieving this crucial combination remains a challenge in Sn–Pb PSCs. Here, Sn–Pb perovskite grains are anchored with ultrathin layered perovskites to overcome the efficiency-stability tradeoff. Defect passivation is achieved both on the perovskite film surface and at grain boundaries, an approach implemented by directly introducing phenethylammonium ligands in the antisolvent. This improves device operational stability and also avoids the excess formation of layered perovskites that would otherwise hinder charge transport. Sn–Pb PSCs with fill factors of 79% and a certified power conversion efficiency (PCE) of 18.95% are reported—among the highest for Sn–Pb PSCs. Using this approach, a 200-fold enhancement in device operating lifetime is achieved relative to the nonpassivated Sn–Pb PSCs under full AM1.5G illumination, and a 200 h diurnal operating time without efficiency drop is achieved under filtered AM1.5G illumination.     

Critical review of recent progress of flexible perovskite solar cells

Perovskite solar cells (PSCs) have emerged as a ‘rising star’ in recent years due to their high-power conversion efficiency (PCE), extremely low cost and facile fabrication techniques. To date, PSCs have achieved a certified PCE of 25.2% on rigid conductive substrates, and 19.5% on flexible substrates. The significant advancement of PSCs has been realized through various routes, including perovskite composition engineering, interface modification, surface passivation, fabrication process optimization, and exploitation of new charge transport materials. However, compared with rigid counterparts, the efficiency record of flexible perovskite solar cells (FPSCs) is advancing slowly, and therefore it is of great significance to scrutinize recent work and expedite the innovation in this field. In this article, we comprehensively review the recent progress of FPSCs. After a brief introduction, the major features of FPSCs are compared with other types of flexible solar cells in a broad context including silicon, CdTe, dye-sensitized, organic, quantum dot and hybrid solar cells. In particular, we highlight the major breakthroughs of FPSCs made in 2019/2020 for both laboratory and large-scale devices. The constituents of making a FPSC including flexible substrates, perovskite absorbers, charge transport materials, as well as device fabrication and encapsulation methods have been critically assessed. The existing challenges of making high performance and long-term stable FPSCs are discussed. Finally, we offer our perspectives on the future opportunities of FPSCs in the field of photovoltaics.     

离子液体在有机光电转换器件中的应用研究进展

近年来,离子液体因具有不易挥发、性质稳定、透光性好、导电率高、可设计性,以及易于在界面处形成双电层等物理化学性质,而展现出广阔的应用潜力和前景,逐渐成为国际科学研究的前沿和热点之一。其中,将离子液体应用于染料敏化太阳能电池(Dye-sensitized solar cells,DSSCs)、钙钛矿太阳能电池和有机光电探测器等有机光电转换器件的研究备受关注。 在有机光电转换器件中,离子液体在染料敏化太阳能电池方面的应用最为广泛且完善。高效DSSCs主要是基于有机溶剂的液态电解质结,但有机溶剂在带来较高光电转换效率的同时,其本身存在的易挥发汽化、光热稳定性差等缺点,导致DSSCs的器件寿命与长期稳定性受到影响,离子液体的引入能有效解决以上问题。此外,离子液体还以电子传输层以及界面修饰层的形式引入,具有高电荷迁移率、低功函数以及高稳定性等优点,能在一定程度上改善器件的短路电流、填充因子和光电转换效率等。因此,离子液体成为在DSSCs的实际应用中兼具性价比高、封装难度低、性能好、稳定性高四大优点的辅助材料。在钙钛矿太阳能电池方面,离子液体的低功函数和高电子迁移率以及一些特殊性质如钝化反应、黏度效应等,都能够实现对电子萃取率、电荷转移电阻、钙钛矿结晶情况等方面的控制以满足实际设计要求,进而有助于钙钛矿太阳能电池的光电转换效率、填充因子等性能指标不同程度的提升。在有机光电探测器方面,引入的离子液体能促使在与之接触的界面处形成双电层,双电层的形成及离子液体的高导电率使得入射光不必照射有机光电探测器上下电极的重叠区域仍旧可以产生较大的光电流输出,从而可以有效摆脱有机光电探测器对电极材料透光性要求的局限性。同时双电层的形成还将促进有机光电探测器工作层中的电荷分离,进一步提高有机光电探测器的响应率。 本文主要从染料敏化太阳能电池、钙钛矿太阳能电池、有机光电探测器三个方面,综述了离子液体在有机光电转换器件中的国内外应用研究进展,就离子液体对提升有机光电转换效率及其实现器件新功能的工作机理进行了详细分析,并对其未来的应用研究方向进行了展望,为今后进一步设计出更适合有机光电转换领域应用的离子液体提供参考。     

Collaborative Passivation for Dual Charge Transporting Layers Based on 4-(chloromethyl)benzonitrile Additive toward Efficient and Stable Inverted Perovskite Solar Cells

Poor carrier transport capacity and numerous surface defects of charge transporting layers (CTLs), coupled with misalignment of energy levels between perovskites and CTLs, impact photoelectric conversion efficiency (PCE) of inverted perovskite solar cells (PSCs) profoundly. Herein, a collaborative passivation strategy is proposed based on 4-(chloromethyl) benzonitrile (CBN) as a solution additive for fabrication of both [6,6]-phenyl-C61-butyric acid methylester (PCBM) and poly(triarylamine) (PTAA) CTLs. This additive can improve wettability of PTAA and reduce the agglomeration of PCBM particles, which enhance the PCE and device stability of the PSCs. As a result, a PCE exceeding 20% with a remarkable short circuit current of 23.9 mA cm⁻², and an improved fill factor of 81% is obtained for the CBN- modified inverted PSCs. Devices maintain 80% and 70% of the initial PCE after storage under 30% and 85% humidity ambient conditions for 1000 h without encapsulation, as well as negligible light state PCE loss. This strategy demonstrates feasibility of the additive engineering to improve interfacial contact between the CTLs and perovskites for fabrication of efficient and stable inverted PSCs.     

Highly Efficient and Stable Perovskite Solar Cells: Competitive Crystallization Strategy and Synergistic Passivation

Defects of perovskite (PVK) films are one of the main obstacles to achieving high-performance perovskite solar cells (PSCs). Here, the authors fabricated highly efficient and stable PSCs by introducing prolinamide (ProA) into the PbI₂ precursor solution, which improves the performance of PSCs by the competitive crystallization and efficient defect passivation of perovskite. The theoretical and experimental results indicate that ProA forms an adduct with PbI₂, competes with free I⁻ to coordinate with Pb²⁺, leads to the increase of the energy barrier of crystallization, and slows down the crystallization rate. Furthermore, the dual-site synergistic passivation of ProA is revealed by density functional theory (DFT) calculations and experimental results. ProA effectively reduces non-radiative recombination in the resultant films to improve the photovoltaic performance of PSCs. Notably, ProA-assisted PSCs achieve 24.61% power conversion efficiency (PCE) for the champion device and the stability of PSCs devices under ambient and thermal environments is improved.     

纳米金属氧化物在钙钛矿电池中的应用研究进展

近几年来, 钙钛矿电池发展迅速, 其单电池效率从最初的3.8%迅速提升至目前20.1%, 接近硅基太阳能电池的光电转换效率。TiO₂、ZnO、Al₂O₃等诸多无机纳米金属氧化物材料作为重要的载流子输运材料与钙钛矿生长骨架也被广泛地应用于钙钛矿电池。依据钙钛矿电池功能结构的差异, 本文分别介绍了此类材料作为钙钛矿电池中的致密层及介孔层的制备方法, 并在此基础上介绍了基于表面修饰、掺杂、复合等氧化物的改性手段调节材料理化性能与氧化物/钙钛矿界面特性, 进而改进钙钛矿电池性能的方法。并阐述了进一步提高钙钛矿电池光电转换效率需要关注的重点问题及展望。     

Stable and High-Efficiency Methylammonium-Free Perovskite Solar Cells

Organic–inorganic metal halide perovskite solar cells (PSCs) have achieved certified power conversion efficiency (PCE) of 25.2% with complex compositional and bandgap engineering. However, the thermal instability of methylammonium (MA) cation can cause the degradation of the perovskite film, remaining a risk for the long-term stability of the devices. Herein, a unique method is demonstrated to fabricate highly phase-stable perovskite film without MA by introducing cesium chloride (CsCl) in the double cation (Cs, formamidinium) perovskite precursor. Moreover, due to the suboptimal bandgap of bromide (Br⁻), the amount of Br⁻ is regulated, leading to high power conversion efficiency. As a result, MA-free perovskite solar cells achieve remarkable long-term stability and a PCE of 20.50%, which is one of the best results for MA-free PSCs. Moreover, the unencapsulated device retains about 80% of the original efficiencies after a 1000 h aging study. These results provide a feasible approach to enhance solar cell stability and performance simultaneously, paving the way for commercializing PSCs.     

Understanding Low Bandgap Polymer PTB7 and Optimizing Polymer Solar Cells Based on It

Solution processed single junction polymer solar cells (PSCs) have been developed from less than 1% power conversion efficiency (PCE) to beyond 9% PCE in the last decade. The significant efficiency improvement comes from progress in both rational design of donor polymers and innovation of device architectures. Among all the novel high efficient donor polymers, PTB7 stands out as the most widely used one for solar cell studies. Herein the recent development of PTB7 solar cells is reviewed. Detailed discussion of basic property, structure property relationship, morphology study, interfacial engineering, and inorganic nanomaterials incorporation is provided. Possible future directions for further increasing the performance of PTB7 solar cells are discussed.     

To Be Higher and Stronger—Metal Oxide Electron Transport Materials for Perovskite Solar Cells

Organometallic mixed halide perovskite solar cells (PSCs) have emerged as a promising photovoltaic technology with increasingly improved device efficiency exceeding 24%. Charge transport layers, especially electron transport layers (ETLs), are verified to play a vital role in device performance and stability. Recently, metal oxides (MOs) have been widely studied as ETLs for high‐performance PSCs due to their excellent electronic properties, superb versatility, and great stability. This Review briefly discusses the development of PSCs' architecture and outlines the requirements for MO ETLs. Additionally, recent progress of MO ETLs from preparation to optimization for efficient PSCs is systematically summarized and highlighted to associate the versatility of MO ETLs with the performance of devices. Finally, a summary and prospectives for the future development of MO ETLs toward practical application of high‐performance PSCs are drawn.