Recent Progress in Organic Electron Transport Materials in Inverted Perovskite Solar Cells

Organic n‐type materials (e.g., fullerene derivatives, naphthalene diimides (NDIs), perylene diimides (PDIs), azaacene‐based molecules, and n‐type conjugated polymers) are demonstrated as promising electron transport layers (ETLs) in inverted perovskite solar cells (p–i–n PSCs), because these materials have several advantages such as easy synthesis and purification, tunable frontier molecular orbitals, decent electron mobility, low cost, good solubility in different organic solvents, and reasonable chemical/thermal stability. Considering these positive factors, approaches toward achieving effective p–i–n PSCs with these organic materials as ETLs are highlighted in this Review. Moreover, organic structures, electron transport properties, working function of electrodes caused by ETLs, and key relevant parameters (PCE and stability) of p–i–n PSCs are presented. Hopefully, this Review will provide fundamental guidance for future development of new organic n‐type materials as ETLs for more efficient p–i–n PSCs.     

Ion‐Migration Inhibition by the Cation–π Interaction in Perovskite Materials for Efficient and Stable Perovskite Solar Cells

Migration of ions can lead to photoinduced phase separation, degradation, and current–voltage hysteresis in perovskite solar cells (PSCs), and has become a serious drawback for the organic–inorganic hybrid perovskite materials (OIPs). Here, the inhibition of ion migration is realized by the supramolecular cation–π interaction between aromatic rubrene and organic cations in OIPs. The energy of the cation–π interaction between rubrene and perovskite is found to be as strong as 1.5 eV, which is enough to immobilize the organic cations in OIPs; this will thus will lead to the obvious reduction of defects in perovskite films and outstanding stability in devices. By employing the cation‐immobilized OIPs to fabricate perovskite solar cells (PSCs), a champion efficiency of 20.86% and certified efficiency of 20.80% with negligible hysteresis are acquired. In addition, the long‐term stability of cation‐immobilized PSCs is improved definitely (98% of the initial efficiency after 720 h operation), which is assigned to the inhibition of ionic diffusions in cation‐immobilized OIPs. This cation–π interaction between cations and the supramolecular π system enhances the stability and the performance of PSCs efficiently and would be a potential universal approach to get the more stable perovskite devices.     

Efficient and Stable Inverted Perovskite Solar Cells Incorporating Secondary Amines

Large‐bandgap perovskites offer a route to improve the efficiency of energy capture in photovoltaics when employed in the front cell of perovskite–silicon tandems. Implementing perovskites as the front cell requires an inverted (p–i–n) architecture; this architecture is particularly effective at harnessing high‐energy photons and is compatible with ionic‐dopant‐free transport layers. Here, a power conversion efficiency of 21.6% is reported, the highest among inverted perovskite solar cells (PSCs). Only by introducing a secondary amine into the perovskite structure to form MA_1?xDMA_xPbI₃ (MA is methylamine and DMA is dimethylamine) are defect density and carrier recombination suppressed to enable record performance. It is also found that the controlled inclusion of DMA increases the hydrophobicity and stability of films in ambient operating conditions: encapsulated devices maintain over 80% of their efficiency following 800 h of operation at the maximum power point, 30 times longer than reported in the best prior inverted PSCs. The unencapsulated devices show record operational stability in ambient air among PSCs.     

A Nontoxic Bifunctional (Anti)Solvent as Digestive-Ripening Agent for High-Performance Perovskite Solar Cells

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

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.     

Recent Advances in n‐Type Polymers for All‐Polymer Solar Cells

All‐polymer solar cells (all‐PSCs) based on n‐ and p‐type polymers have emerged as promising alternatives to fullerene‐based solar cells due to their unique advantages such as good chemical and electronic adjustability, and better thermal and photochemical stabilities. Rapid advances have been made in the development of n‐type polymers consisting of various electron acceptor units for all‐PSCs. So far, more than 200 n‐type polymer acceptors have been reported. In the last seven years, the power conversion efficiency (PCE) of all‐PSCs rapidly increased and has now surpassed 10%, meaning they are approaching the performance of state‐of‐the‐art solar cells using fullerene derivatives as acceptors. This review discusses the design criteria, synthesis, and structure–property relationships of n‐type polymers that have been used in all‐PSCs. Additionally, it highlights the recent progress toward photovoltaic performance enhancement of binary, ternary, and tandem all‐PSCs. Finally, the challenges and prospects for further development of all‐PSCs are briefly considered.     

Pb–Sn–Cu Ternary Organometallic Halide Perovskite Solar Cells

Exploiting organic/inorganic hybrid perovskite solar cells (PSCs) with reduced Pb content is very important for developing environment‐friendly photovoltaics. Utilizing of Pb–Sn alloying perovskite is considered as an efficient route to reduce the risk of ecosystem pollution. However, the trade‐off between device performance and Sn substitution ratio due to the instability of Sn²⁺ is a current dilemma. Here, for the first time, the highly efficient Pb–Sn–Cu ternary PSCs are reported by partial replacing of PbI₂ with SnI₂ and CuBr₂. Sn²⁺ substitution results in a redshift of the absorption onset, whereas worsens the film quality. Interestingly, Cu²⁺ introduction can passivate the trap sites at the crystal boundaries of Pb–Sn perovskites effectively. Consequently, a power conversion efficiency as high as 21.08% in inverted planar Pb–Sn–Cu ternary PSCs is approached. The finding opens a new route toward the fabrication of high efficiency Pb–Sn alloying perovskite solar cells by Cu²⁺ passivation.     

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.     

On the Current–Voltage Hysteresis in Perovskite Solar Cells: Dependence on Perovskite Composition and Methods to Remove Hysteresis

Current‐density–voltage (J–V) hysteresis in perovskite solar cells (PSCs) is a critical issue because it is related to power conversion efficiency and stability. Although parameters affecting the hysteresis have been already reported and reviewed, little investigation is reported on scan‐direction‐dependent J–V curves depending on perovskite composition. This review investigates J–V hysteric behaviors depending on perovskite composition in normal mesoscopic and planar structure. In addition, methodologies toward hysteresis‐free PSCs are proposed. There is a specific trend in hysteresis in terms of J–V curve shape depending on composition. Ion migration combined with nonradiative recombination near interfaces plays a critical role in generating hysteresis. Interfacial engineering is found to be an effective method to reduce the hysteresis; however, bulk defect engineering is the most promising method to remove the hysteresis. Among the studied methods, KI doping is proved to be a universal approach toward hysteresis‐free PSCs regardless of perovskite composition. It is proposed from the current studies that engineering of perovskite film near the electron transporting layer (ETL) and the hole transporting layer (HTL) is of vital importance for achieving hysteresis‐free PSCs and extremely high efficiency.     

11.2% All‐Polymer Tandem Solar Cells with Simultaneously Improved Efficiency and Stability

All‐polymer solar cells (all‐PSCs) that contain both p‐type and n‐type polymeric materials blended together as light‐absorption layers have attracted much attention, since the blend of a polymeric donor and acceptor should present superior photochemical, thermal, and mechanical stability to those of small molecular‐based organic solar cells. In this work, the interfacial stability is studied by using highly stable all‐polymer solar cell as a platform. It is found that the thermally deposited metal electrode atoms can diffuse into the active layer during device storage, which consequently greatly decreases the power conversion efficiency. Fortunately, the diffusion of metal atoms can be slowed down and even blocked by using thicker interlayer materials, high‐glass‐transition‐temperature interlayer materials, or a tandem device structure. Learning from this, homojunction tandem all‐PSCs are successfully developed that simultaneously exhibit a record power conversion efficiency over 11% and remarkable stability with efficiency retaining 93% of the initial value after thermally aging at 80 °C for 1000 h.     

Superfast Room‐Temperature Activation of SnO2 Thin Films via Atmospheric Plasma Oxidation and their Application in Planar Perovskite Photovoltaics

The power conversion efficiency (PCE) of perovskite solar cells (PSCs) has now exceeded 20%; thus, research focus has shifted to establishing the foundations for commercialization. One of the pivotal themes is to curtail the overall fabrication time, to reduce unit cost, and mass‐produce PSCs. Additionally, energy dissipation during the thermal annealing (TA) stage must be minimized by realizing a genuine low‐temperature (LT) process. Here, tin oxide (SnO₂) thin films (TFs) are formulated at extremely high speed, within 5 min, under an almost room‐temperature environment (<50 °C), using atmospheric Ar/O₂ plasma energy (P‐SnO₂) and are applied as an electron transport layer of a “n–i–p”‐type planar PSC. Compared with a thermally annealed SnO₂ TF (T‐SnO₂), the P‐SnO₂ TF yields a more even surface but also outstanding electrical conductivity with higher electron mobility and a lower number of charge trap sites, consequently achieving a superior PCE of 19.56% in P‐SnO₂‐based PSCs. These findings motivate the use of a plasma strategy to fabricate various metal oxide TFs using the sol–gel route.     

Tin and Mixed Lead–Tin Halide Perovskite Solar Cells: Progress and their Application in Tandem Solar Cells

Metal halide perovskites have recently attracted enormous attention for photovoltaic applications due to their superior optical and electrical properties. Lead (Pb) halide perovskites stand out among this material series, with a power conversion efficiency (PCE) over 25%. According to the Shockley–Queisser (SQ) limit, lead halide perovskites typically exhibit bandgaps that are not within the optimal range for single-junction solar cells. Partial or complete replacement of lead with tin (Sn) is gaining increasing research interest, due to the promise of further narrowing the bandgaps. This enables ideal solar utilization for single-junction solar cells as well as the construction of all-perovskite tandem solar cells. In addition, the usage of Sn provides a path to the fabrication of lead-free or Pb-reduced perovskite solar cells (PSCs). Recent progress in addressing the challenges of fabricating efficient Sn halide and mixed lead–tin (Pb–Sn) halide PSCs is summarized herein. Mixed Pb–Sn halide perovskites hold promise not only for higher efficiency and more stable single-junction solar cells but also for efficient all-perovskite monolithic tandem solar cells.     

Causes and Solutions of Recombination in Perovskite Solar Cells

Organic–inorganic hybrid perovskite materials are receiving increasing attention and becoming star materials on account of their unique and intriguing optical and electrical properties, such as high molar extinction coefficient, wide absorption spectrum, low excitonic binding energy, ambipolar carrier transport property, long carrier diffusion length, and high defects tolerance. Although a high power conversion efficiency (PCE) of up to 22.7% is certified for perovskite solar cells (PSCs), it is still far from the theoretical Shockley–Queisser limit efficiency (30.5%). Obviously, trap‐assisted nonradiative (also called Shockley–Read–Hall, SRH) recombination in perovskite films and interface recombination should be mainly responsible for the above efficiency distance. Here, recent research advancements in suppressing bulk SRH recombination and interface recombination are systematically investigated. For reducing SRH recombination in the films, engineering perovskite composition, additives, dimensionality, grain orientation, nonstoichiometric approach, precursor solution, and post‐treatment are explored. The focus herein is on the recombination at perovskite/electron‐transporting material and perovskite/hole‐transporting material interfaces in normal or inverted PSCs. Strategies for suppressing bulk and interface recombination are described. Additionally, the effect of trap‐assisted nonradiative recombination on hysteresis and stability of PSCs is discussed. Finally, possible solutions and reasonable prospects for suppressing recombination losses are presented.     

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.     

Advances in cesium lead iodide perovskite solar cells: Processing science matters

The prevailing perovskite solar cells (PSCs) employ hybrid organic–inorganic halide perovskites as light absorbers, but these materials exhibit relatively poor environmental stability, which potentially hinders the practical deployment of PSCs. One important strategy to address this issue is replacing the volatile and hygroscopic organic cations with inorganic cesium cations in the crystal structure, forming all-inorganic halide perovskites. In this context, CsPbI₃ perovskite is drawing phenomenal attention, primarily because it exhibits an ideal bandgap of 1.7 eV for the use in tandem solar cells, and it shows significantly enhanced thermal stability that is the key to the long-term device operation. Within only half a decade, the power conversion efficiency (PCE) of CsPbI₃ PSCs has ramped beyond 20%, which has been driven by inventions of numerous processing methods for high-quality CsPbI₃ perovskite thin films. These methods are broadly classified into three categories: vapor deposition, nanocrystals assembly, and solution deposition. Herein we present a systematic review on these methods and related materials sciences. In particular, we comprehensively discuss the dimethylammonium-additive-based solution deposition, which has resulted into the best-performing CsPbI₃ PSCs. We also present the challenges and prospects on future research towards the realization of the full potential of CsPbI₃ PSCs.     

无机电荷传输层在有机-无机杂化钙钛矿太阳能电池中的应用及研究进展

近年来,基于有机-无机杂化钙钛矿材料为光活性层构建的太阳能电池由于具有直接带隙、吸光系数高、激子束缚能低、激子和载流子扩散距离长,以及成本低、制备工艺简单、光电转换率高、易于实现大面积柔性器件等优点,而成为当今新型光伏技术中一颗耀眼的新星。在钙钛矿太阳能电池中,电荷传输层在提高光电转换效率、稳定性及寿命等方面扮演着非常重要的角色,其中无机电荷传输层因具有载流子迁移率高、稳定性好、制备工艺简单等优势越来越受到人们的关注。本文总结了无机电荷传输层在钙钛矿太阳能电池中的应用,详细介绍了各种无机电子/空穴传输层在钙钛矿太阳能电池中的研究进展,并对其发展趋势进行了展望。     

Efficient Inverted Perovskite Solar Cells by Employing N‐Type (D–A1–D–A2) Polymers as Electron Transporting Layer

It is highly desirable to employ n‐type polymers as electron transporting layers (ETLs) in inverted perovskite solar cells (PSCs) due to their good electron mobility, high hydrophobicity, and simplicity of film forming. In this research, the capability of three n‐type donor–acceptor₁–donor–acceptor₂ (D–A₁–D–A₂) conjugated polymers (pBTT, pBTTz, and pSNT) is first explored as ETLs because these polymers possess electron mobilities as high as 0.92, 0.46, and 4.87 cm² (Vs)^?1 in n‐channel organic transistors, respectively. The main structural difference among pBTT, pBTTz, and pSNT is the position of sp²‐nitrogen atoms (sp²‐N) in the polymer main chains. Therefore, the effect of different substitution positions on the PSC performances is comprehensively studied. The as‐fabricated p–i–n PSCs with pBTT, pBTTz, and pSNT as ETLs show the maximum photoconversion efficiencies of 12.8%, 14.4%, and 12.0%, respectively. To be highlighted, pBTTz‐based device can maintain 80% of its stability after ten days due to its good hydrophobicity, which is further confirmed by a contact angle technique. More importantly, the pBTTz‐based device shows a neglected hysteresis. This study reveals that the n‐type polymers can be promising candidates as ETLs to approach solution‐processed highly‐efficient inverted PSCs.     

A perspective on the recent progress in solution-processed methods for highly efficient perovskite solar cells

Perovskite solar cells (PSCs) were developed in 2009 and have led to a number of significant improvements in clean energy technology. The power conversion efficiency (PCE) of PSCs has increased exponentially and currently stands at 22%. PSCs are transforming photovoltaic (PV) technology, outpacing many established PV technologies through their versatility and roll-to-roll manufacturing compatibility. The viability of low-temperature and solution-processed manufacturing has further improved their viability. This article provides a brief overview of the stoichiometry of perovskite materials, the engineering behind various modes of manufacturing by solution processing methods, and recommendations for future research to achieve large-scale manufacturing of high efficiency PSCs.     

Fine Multi‐Phase Alignments in 2D Perovskite Solar Cells with Efficiency over 17% via Slow Post‐Annealing

Layered Ruddlesden–Popper (RP) phase (2D) halide perovskites have attracted tremendous attention due to the wide tunability on their optoelectronic properties and excellent robustness in photovoltaic devices. However, charge extraction/transport and ultimate power conversion efficiency (PCE) in 2D perovskite solar cells (PSCs) are still limited by the non‐eliminable quantum well effect. Here, a slow post‐annealing (SPA) process is proposed for BA₂MA₃Pb₄I₁₃ (n = 4) 2D PSCs by which a champion PCE of 17.26% is achieved with simultaneously enhanced open‐circuit voltage, short‐circuit current, and fill factor. Investigation with optical spectroscopy coupled with structural analyses indicates that enhanced crystal orientation and favorable alignment on the multiple perovskite phases (from the 2D phase near bottom to quasi‐3D phase near top regions) is obtained with SPA treatment, which promotes carrier transport/extraction and suppresses Shockley–Read–Hall charge recombination in the solar cell. As far as it is known, the reported PCE is so far the highest efficiency in RP phase 2D PSCs based on butylamine (BA) spacers (n = 4). The SPA‐processed devices exhibit a satisfactory stability with <4.5% degradation after 2000 h under N₂ environment without encapsulation. The demonstrated process strategy offers a promising route to push forward the performance in 2D PSCs toward realistic photovoltaic applications.     

Aromatic‐Diimide‐Based n‐Type Conjugated Polymers for All‐Polymer Solar Cell Applications

All‐polymer solar cells (all‐PSCs) have attracted immense attention in recent years due to their advantages of tunable absorption spectra and electronic energy levels for both donor and acceptor polymers, as well as their superior thermal and mechanical stability. The exploration of the novel n‐type conjugated polymers (CPs), especially based on aromatic diimide (ADI), plays a vital role in the further improvement of power conversion efficiency (PCE) of all‐PSCs. Here, recent progress in structure modification of ADIs including naphthalene diimide (NDI), perylene diimide (PDI), and corresponding derivatives is reviewed, and the structure–property relationships of ADI‐based CPs are revealed.