Organic semiconductors have become essential parts of thin-film electronic devices, particularly as hole transport layers (HTLs) in perovskite solar cells (PSCs) where they represent one of the major bottlenecks to further enhancements in both device stability and efficiency. Small molecule 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (spiro-OMeTAD) and polymer poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) are two of the first successful HTLs used in PSCs, and have remained at the forefront of developing high efficiency devices for almost a decade. Since their first application, many investigations into the properties of spiro-OMeTAD and PTAA have contributed to a growing understanding of the mechanisms that enable their success as HTLs. This review summarizes and discusses the key electronic and morphological properties, doping strategies and mechanisms, and degradation pathways of both spiro-OMeTAD and PTAA. A critical comparison between the two materials is provided, highlighting both the similarities which explain their enduring popularity as well as key differences in electrical and morphological properties. From this analysis emerges an improved understanding of the fundamental properties that enable the persistent success of HTL materials, which are found to include not only hole conductivity, band gap, and morphology, but also interactions with dopants, the perovskite, and environmental stressors. The knowledge about these properties, which are critically summarized in this review, is also applicable to the many other types of organic electronic devices now employing spiro-OMeTAD and PTAA. A detailed examination of the properties of materials reveals a clear set of guiding principles for the development of future generation HTLs. Applying these design strategies to produce more advanced HTLs will be essential to further improve the stability, efficiency, and commercialization of PSCs.

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Lessons learned from spiro-OMeTAD and PTAA in perovskite solar cells

Halide perovskites are a compelling candidate for the next generation of clean-energy-harvesting technologies owing to their low cost, facile fabrication and outstanding semiconductor properties. However, photovoltaic device efficiencies are still below practical limits and long-term stability challenges hinder their practical application. Current evidence suggests that strain in halide perovskites is a key factor in dictating device efficiency and stability. Here we outline the fundamentals of strain within halide perovskites relevant to photovoltaic applications and rationalize approaches to characterize the phenomenon. We examine recent breakthroughs in eliminating the adverse impacts of strain, enhancing both device efficiencies and operational stabilities. Finally, we discuss further challenges and outline future research directions for placing stress and strain studies at the forefront of halide perovskite research. An extensive understanding of strain in halide perovskites is needed, which would allow effective strain management and drive further enhancements in efficiencies and stabilities of perovskite photovoltaics.

Strain analysis and engineering in halide perovskite photovoltaics.

Perovskite solar cells (PSCs) based on the regular n–i–p device architecture have reached above 25% certified efficiency with continuously reported improvements in recent years. A key common factor for these recent breakthroughs is the development of SnO2 as an effective electron transport layer in these devices. In this article, the key advances in SnO2 development are reviewed, including various deposition approaches and surface treatment strategies, to enhance the bulk and interface properties of SnO2 for highly efficient and stable n–i–p PSCs. In addition, the general materials chemistry associated with SnO2 along with the corresponding materials challenges and improvement strategies are discussed, focusing on defects, intrinsic properties, and impact on device characteristics. Finally, some SnO2 implementations related to scalable processes and flexible devices are highlighted, and perspectives on the future development of efficient and stable large‐scale perovskite solar modules are also provided.

Advances in SnO2 for Efficient and Stable n–i–p Perovskite Solar Cells

The power conversion efficiency of organic-inorganic hybrid perovskite solar cells has increased rapidly, but the device stability remains a big challenge. Previous studies show the grain boundary (GB) can facilitate ion migration and initiate device degradation. Herein, methimazole (MMI) is employed for the first time to construct a surface "patch" by in situ converting residual PbI2 at GBs. The resultant MMI-PbI2 complex can effectively suppress ion migration and inhibit diffusion of the metal electrodes. The origin of the surface "patch" effect and their working mechanisms are investigated experimentally and theoretically at the microscopic level. It hence demonstrates a simple and effective method to prolong the device stability in the context of GB engineering, which could be extensively applied to perovskite-based optoelectronics.

Grain‐Boundary “Patches” by In Situ Conversion to Enhance Perovskite Solar Cells Stability

Despite the rapid progress in the power conversion efficiency (PCE) from 3.8% to 25.5% with ten years of effort, the real outdoor applications of perovskite solar cells (PSCs) are still significantly limited by their poor device stability. The factors determining the stability of PSCs can be divided into extrinsic and intrinsic factors. Advanced encapsulation techniques can be applied to only exclude the effect from extrinsic environmental factors such as moisture and oxygen in ambient air. The intrinsic stability issues of PSCs are difficult to thoroughly resolve by device engineering, determining the short life span of devices. Therefore, in this review, we only focus on the intrinsic stability of PSCs, especially the stability related to the properties of the perovskite active materials. Several degradation mechanisms of perovskite materials resulting from (1) ion dissociation and migration, (2) metal–perovskite reactions, and (3) residual strain are thoroughly analyzed and the corresponding possible strategies to improve the device stability are summarized. Finally, the perspective of research directions in the perovskite photovoltaic field for future large-scale deployment has been discussed. This review will draw tremendous attention from worldwide researchers to the intrinsic stability study of perovskites and provide useful guidance to further improve the operational stability of PSCs.

Pushing commercialization of perovskite solar cells by improving their intrinsic stability

The invention discloses a high-efficiency poly(3-hexylthiophene)-based organic solar cell. The high-efficiency poly(3-hexylthiophene)-based organic solar cell comprises a substrate, a cathode, a cathode modification layer, an active layer, an anode modification layer and an anode, wherein the active layer is a blend membrane of an electron donor and an electron acceptor. The electron donor is poly(3-hexylthiophene) (P3HT), and the electron acceptor is a non-fullerene acceptor DFPCBR. The large [Pi] conjugated system and the high donor-acceptor capacity of a center unit are employed, the DFPCBRhas a high HOMO energy level and LUMO energy level and is matched with the P3HT energy level to inhibit the energy loss and allow the solar cell to have a high open-circuit voltage. The intramolecular charge transfer (ICT) of the DFPCBR is enhanced, the band gap is shortened, and the absorption spectrum is broadened. Therefore, the organic solar cell prepared by the present invention achieves thehigh energy conversion efficiency (PCE) of up to 5.34%. At the same time, the organic solar cell prepared by the invention can maintain a high PCE even when the active layer is thick which is beneficial to practical applications in the future.

High-efficiency poly(3-hexylthiophene)-based organic solar cell

The invention discloses a thermally stable perovskite solar cell. The solar cell comprises a substrate, an anode, an anode modification layer, an active layer, a cathode modification layer and a cathode from bottom to top. The anode modification layer is a two-layer film formed through successively coating two polymers PEDOT: PSS and PTAA on an anode surface. Because a two-layer polymer film constructs stepped energy level arrangement which facilitates hole collection, the perovskite solar cell of the invention achieves high photoelectric energy conversion efficiency (18.7%). More importantly,chemical inertness and good mechanical flexibility of the two-layer polymer film are used to inhibit thermal decomposition of a perovskite active layer and crystal damages caused by a thermal stress.The perovskite solar cell has excellent thermal stability. Efficiency still maintains more than 80% of an initial value after heating at 100 DEG C in an inert atmosphere for 120 hours.

Thermally stable perovskite solar cell

Toward Highly Thermal Stable Perovskite Solar Cells by Rational Design of Interfacial Layer.

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High-efficiency poly(3-hexylthiophene)-based organic solar cell

Thermally stable perovskite solar cell

Toward Highly Thermal Stable Perovskite Solar Cells by Rational Design of Interfacial Layer.