Chen Weijie

The latest research advancements of all‐inorganic perovskite solar cells (PSCs) are summarized systematically and discussed deeply from the perspective of phase stability, effective inorganic charge transport materials, device structures, and interfacial engineering.

The large‐scale commercial application of organic–inorganic hybrid perovskite solar cells (PSCs) based on organic hole transport material (HTM) is still hindered by poor long‐term operational stability, although a certified record power conversion efficiency (PCE) as high as 25.2% can be achieved. In the recent several years, all‐inorganic PSCs have received tremendous attention due to their superb thermal and moisture stability and considerable progresses have been witnessed. Herein, the recent advancements of all‐inorganic PSCs are reviewed comprehensively. First, the recent progresses of the strategies for stabilizing the black phase of inorganic perovskites through either increasing tolerance factor or enhancing the energy barrier of phase transition from black to yellow phase are summarized and discussed. Second, the deposition and growth techniques of inorganic perovskite films are discussed. Third, the effective inorganic HTMs in normal all‐inorganic PSCs are described. Fourth, HTM‐free normal all‐inorganic PSCs are discussed. Afterward, the effective inorganic electron transport materials in inverted all‐inorganic PSCs are discussed. Subsequently, the advancements of interface engineering for increasing the PCE and stability of all‐inorganic PSCs are reviewed. Finally, a brief summary and outlook are presented to push up the PCE of all‐inorganic PSCs to over 20% in the near future.

Publication date: 14 October 2020

Source: Joule, Volume 4, Issue 10

Author(s): Li Nian, Yuanyuan Kan, Ke Gao, Ming Zhang, Na Li, Guanqing Zhou, Sae Byeok Jo, Xueliang Shi, Francis Lin, Qikun Rong, Feng Liu, Guofu Zhou, Alex K.-Y. Jen

Sodium dodecyl benzene sulfonate (SDBS) is used as a multifunctional chemical additive for efficient and stable planar fully air‐processed perovskite solar cells (PSCs). The introduction of SDBS can promote the preferential growth of crystal orientation, reduce defects, inhibit the migration of iodide ions, enhance the built‐in potential, and improve the water resistance of perovskite films.

The device performance of organic–inorganic hybrid halide perovskite solar cells (PSCs) is highly dependent on the quality of perovskite layer. Herein, a multifunctional chemical additive strategy is reported to simultaneously improve the efficiency and stability of fully air‐processed PSCs. The planner methylammonium lead trihalide (MAPbI₃)‐based PSCs incorporating sodium dodecyl benzene sulfonate (SDBS) exhibit a champion power conversion efficiency (PCE) of 19.20% and negligible hysteresis, which is one of the top efficiencies of MAPbI₃‐based PSCs made in air. The increased efficiency is due to the reduction of defects and inhibition of ion migration in the perovskite films. Furthermore, the enhancement of device performance and stability can also be ascribed to highly preferred and efficient perovskite crystals protecting the perovskite films from humidity. The corresponding unencapsulated device retains 92.34% of its initial efficiency after 90 days (>2100 h) storage in air and maintains 85.20% of its original PCE after being exposed to 85 °C for 27 h. The results indicate that SDBS is a promising chemical additive to enhance the performance of air‐processed PSCs for future applications.

Pinhole‐free perovskite films with large grains are fabricated in ambient air by a spinning–bathing–spinning method. The effects of moisture on the formation of I‐dominant grain and Cl‐enriched boundaries and surfaces in the perovskite films are revealed, which enable the air‐processed perovskite solar cells with a high efficiency of more than 20%.

Metallic halide perovskite films are usually fabricated in inert environment due to their high sensitivity to moisture and oxygen. However, the fabrication process in the strictly controlled environment is not economical for mass production. Therefore, the fabrication of high‐quality perovskite films in ambient air is more practical for optoelectronic devices. Herein, a spinning–bathing–spinning (SBS) method is demonstrated to deposit pinhole‐free perovskite films with large grains in ambient air for solar cells. The effect of moisture on the rapid crystallization and grain coarsening can be suppressed using this SBS method. Furthermore, the moisture is found to encourage the halogen separation in the perovskite films when using PbI₂–PbCl₂ as the lead halide precursor, resulting in the formation of I‐dominant perovskite grains and Cl‐enriched boundaries and surface in the films. The Cl‐enriched grain boundaries and film surface, which mainly originate from the confined methylammonium chloride (MACl), can passivate defects and prevent further damage from moisture and oxygen. This spontaneous inner‐to‐outside passivation enables the air‐processed perovskite solar cells with the high power conversion efficiencies of more than 20% and improved stability.

Different concentrations of ester‐substituted thiophene are introduced into the conjugated backbone of the polymer acceptor to rationally adjust the aggregation behaviors, absorption properties, and energy levels, and finally improve the photovoltaic performance of the PYEx‐based all‐polymer solar cells. Among them, blends of PYE2 with polymer donor PBDB‐T are achieved with a maximum power conversion efficiency (PCE) of 13.57%.

Finding effective molecular design strategies and fine tuning the molar ratios of donor/acceptor (D/A) random copolymers to optimize the blend microstructure of the photoactive layer is one of the main long‐standing challenges in developing and fabricating highly efficient all‐polymer solar cells (all‐PSCs). Herein, a random ternary copolymerization strategy to develop four random copolymer acceptors PYEx (x = 10, 20, 30, 40) is used by polymerizing a fused‐ring A–D–A‐type acceptor unit modified from Y5 with a thiophene‐connecting unit and a controlled amount of an ester‐substituted thiophene (EST) unit. Compared with PYT (PYE0) of only Y5‐like units and thiophene units, the ternary copolymers PYEx show slightly down‐shifted lowest unoccupied molecular orbital (LUMO) energy levels, reduced absorption coefficients, and decreased electron mobilities. However, it is also demonstrated that this design approach rationally modifies the molecular aggregations of polymer acceptors, effectively fine tuning the blend morphology and physical mechanisms, and enhances the device performance of the PYEx‐based all‐PSCs. Among them, blends of PYE20 with donor polymer PBDB‐T combine 13.6% power conversion efficiency (PCE). Of particular note is that all of the PYEx‐based devices exhibit the best PCEs of over 13%, indicating the high tolerance on molar ratios.

Nonfullerene all‐small‐molecule organic solar cells (NF‐ASM OSCs) have become a hot research topic due to the advantages of no batch effect, better phase purity, and better crystallinity. Herein, the structure–property relationships and the perspectives for future development of NF‐ASM OSCs are discussed. Meanwhile, related mechanism research content is also included.

Organic solar cells (OSCs) have great potential to completely change the development trend of entire industries and become the next generation of commercial solar cells. Compared with polymer solar cells, the nonfullerene all‐small‐molecule organic solar cells (NF‐ASM OSCs) system has the advantages of easy purification, no batch effect, etc. Moreover, their power conversion efficiency (PCE) exceeds the commercialization threshold of 10%. Recently, the diversity of small‐molecule materials and the steady increase in PCE values suggest a bright future for NF‐ASM OSCs. Herein, small‐molecule donor and acceptor materials that have been developed in recent years to produce higher device efficiency are introduced.

Halide perovskites have undergone remarkable developments as highly efficient optoelectronic materials for a variety of applications. Several studies indicated the critical role of defects on the performance of perovskite devices. However, the parameters of defects and their interplay with free charge carriers remain unclear. In this study, we explored the dynamics of free holes in methylammonium lead tribromide (MAPbBr₃) single crystals using the time-of-flight (ToF) current spectroscopy. By combining ToF spectroscopy and Monte Carlo simulation, three energy states were detected in the bandgap of MAPbBr₃. In addition, we found the trapping and detrapping rates of free holes ranging from a few microseconds to hundreds of microseconds. Contrary to previous studies, we revealed a strong detrapping activity of traps. We showed that these traps substantially affect the transport properties of MAPbBr₃, including mobility and mobility-lifetime product. Our results provide an insight on charge transport properties of perovskite semiconductors.

Efficient ternary blend nonfullerene organic solar cells based on two polymer donors and one fused‐ring electron acceptor are fabricated. The fine‐tuning blend morphology and reduced nonradiative energy loss in this ternary system enable achievement of a power conversion efficiency of over 16%, which is among the highest values for ternary organic solar cells with two polymer donors.

High‐performance nonfullerene ternary organic solar cells (OSCs) with two polymer donors are less frequently reported because of the limited numbers of efficient polymer donors with good compatibility. Herein, a wide‐bandgap polymer P1 with a deep‐lying highest occupied molecular orbital (HOMO) level is incorporated as the third component into the benchmark PM6:Y6 binary system to fabricate ternary OSCs. The introduction of P1 not only leads to extended absorption coverage and forms a cascade‐like energy level alignment but also shows excellent compatibility with PM6, resulting in a favorable morphology in the ternary blend. More importantly, P1 possesses a deeper HOMO level (−5.6 eV) than most well‐known donor polymers, which enables resulting ternary OSCs with an improved open‐circuit voltage. As a result, the optimized ternary OSCs with 40 wt% P1 in donors achieve a power conversion efficiency (PCE) of 16.2% with a small nonradiative recombination loss of 0.23 eV, which is among the highest values of ternary OSCs based on two polymer donors. In addition, the ternary OSCs show a broad composition tolerance with a high PCE of over 14% throughout the whole blend ratios. These results provide an effective approach to fabricate efficient ternary OSCs by synergizing two wide‐bandgap polymer donors.

The novel use of cheap copper‐based corrole as hole transporting material in perovskite solar cells is shown by improving the device thermal stability of n–i–p mesoscopic architecture under prolonged 85 °C stress conditions. Corrole‐based devices show a remarkable power conversion efficiency above 16% by retaining more than 65% of the initial power conversion efficiency after 1000 h of thermal stress.

Abstract

Perovskite solar cells (PSCs) represent nowadays a promising starting point to develop a new efficient and low‐cost photovoltaic technology due to the demonstrated power conversion efficiency (PCE) exceeding 25% on small area devices. However, best reported devices suffer from stability issue under real working conditions thus slowing down the race for the commercialization. In particular, the hole transporting material commonly employed in mesoscopic n–i–p PSCs (nip‐mPSCs), namely spiro‐OMeTAD, is strongly corrupted when subjected to temperatures above 70 °C due to intrinsic thermal instability and because of the dopant employed to improve the hole mobility. In this work, the novel use of a copper‐based corrole as HTM is proposed to improve the device thermal stability of nip‐mPSCs under prolonged 85 °C stress conditions. Corrole‐based devices show remarkable PCE above 16% by retaining more than 65% of the initial PCE after 1000 h of thermal stress, while spiro‐OMeTAD cells abruptly lose more than 60% after the first 40 h. Once scaled‐up to large area modules, the proposed device structure can truly represent a possible way to pass thermal stress tests proposed by IEC‐61646 standards and, not less importantly, the high temperature required by the lamination process for panel production.

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.

Solution‐processed fullerene derivative, [6,6]‐phenyl‐C61 butyric acid n‐hexyl ester, is reported as an effective electron transport material in perovskite solar cells. It allows smooth capping of the perovskite surface, resulting in high efficiencies, reaching 18.4% for large‐area, flexible devices. Furthermore, compared to other fullerenes, it shows reduced recombination losses at the interface with perovskite and facile scalability with the ink‐jet printing technique.

Abstract

Metal halide perovskites have raised huge excitement in the field of emerging photovoltaic technologies. The possibility of fabricating perovskite solar cells (PSCs) on lightweight, flexible substrates, with facile processing methods, provides very attractive commercial possibilities. Nevertheless, efficiency values for flexible devices reported in the literature typically fall short in comparison to rigid, glass‐based architectures. Here, a solution‐processable fullerene derivative, [6,6]‐phenyl‐C61 butyric acid n‐hexyl ester (PCBC6), is reported as a highly efficient alternative to the commonly used n‐type materials in perovskite solar cells. The cells with the PCBC6 layer deliver a power conversion efficiency of 18.4%, fabricated on a polymer foil, with an active area of 1 cm². Compared to the phenyl‐C61‐butyric acid methyl ester benchmark, significantly enhanced photovoltaic performance is obtained, which is primarily attributed to the improved layer morphology. It results in a better charge extraction and reduced nonradiative recombination at the perovskite/electron transporting material interface. Solution‐processed PCBC6 films are uniform, smooth and displayed conformal capping of perovskite layer. Additionally, a scalable processing of PCBC6 layers is demonstrated with an ink‐jet printing technique, producing flexible PSCs with efficiencies exceeding 17%, which highlights the prospects of using this material in an industrial process.

Metal halide perovskite (MHP)‐based tandem solar cells, including MHP/silicon, MHP/CuInGa, MHP/organic photovoltaic, MHP/quantum dot, and all‐perovskite tandem cells, which are boosting the development of cost‐effective and high‐performance, next‐generation solar cells than can compete with fossil fuels, are reviewed.

Abstract

Metal halide perovskite (MHP)‐based tandem solar cells are a promising candidate for use in cost‐effective and high‐performance solar cells that can compete with fossil fuels. To understand the research trends for MHP‐based tandem solar cells, a general introduction to single‐junction and multiple‐junction MHP solar cells and the configuration of tandem devices is provided, along with an overview of the recent progress regarding various MHP‐based tandem cells, including MHP/crystalline silicon, MHP/CuInGaS, MHP/organic photovoltaic, MHP/quantum dot, and all‐perovskite tandem cell. Future research directions for MHP‐based tandem solar cells are also discussed.

Nature Communications, Published online: 09 September 2020; doi:10.1038/s41467-020-18373-0

An N‐type semiconductor material, (CH₃)₂Sn(COOH)₂ (CSCO), is prepared for the first time as an electron transport layer for n‐i‐p planar perovskite solar cells, which leads to one of the highest power conversion efficiencies of 22.21%, and to remarkable stability, retaining over 83% of its initial power conversion efficiency without encapsulation after 130 days of storage in ambient conditions.

Abstract

The electron transport layer (ETL) has an important influence on the power conversion efficiency (PCE) and stability of n‐i‐p planar perovskite solar cells (PSCs). This paper presents an N‐type semiconductor material, (CH₃)₂Sn(COOH)₂ (abbreviated as CSCO) that is synthesized and prepared for the first time as an ETL for n‐i‐p planar PSCs, which leads to a high PCE of 22.21% after KCl treatment, one of the highest PCEs of n‐i‐p planar PSCs to date. Further analysis reveals that the high PCE is attributed to the excellent conductivity of CSCO because of its more delocalized electron cloud distribution due to its unique −O=C−O− group, and to the defect passivation of the Cs_0.05(FA_0.85MA_0.15)_0.95Pb(I_0.85Br_0.15)₃ (denoted as CsFAMA) perovskite through the interaction between the O (Sn) atoms of CSCO and the Pb (halogen) atoms of CsFAMA at CSCO/CsFAMA interface, while the traditional ETL materials such as SnO₂ film lack this function. In addition to the high PCE, the optimal PSCs using CSCO as ETL show remarkable stability, retaining over 83% of its initial PCE without encapsulation after 130 days of storage in ambient conditions (≈25 °C at ≈40% humidity), much better than the traditional SnO₂‐based n‐i‐p PSCs.

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.

Publication date: November 2020

Source: Nano Energy, Volume 77

Author(s): Seojun Lee, Janghyuk Moon, Jun Ryu, Bhaskar Parida, Saemon Yoon, Dong-Gun Lee, Jung Sang Cho, Shuzi Hayase, Dong-Won Kang