Peking University, May 18, 2017: Recently, Professors Zhu Rui and Gong Qihuang at School of Physics, Peking University with the collaborators have made great efforts in highly efficient inverted planar heterojunction perovskite solar cells. Eventually, they have achieved power conversion efficiencies exceeding 20% by using a simple antisolvent-free one-step solution process. This result has demonstrated that the inverted-structure perovskite solar cells have great potential in the research field of photovoltaics.

Owing to the global energy crisis and environmental issue, clean and renewable solar energy has attracted wide attention both from academia and industry. Among them, studies of high-efficiency and low-cost photovoltaic technologies are current “research hot spots”. Perovskite solar cells (PerSCs) have developed rapidly due to their easy fabrication, low cost and high efficiency. Presently, the device efficiency has reached over 22% through the intense efforts of researchers from all over the world. Consequently, the PerSCs have great potential and advantages in the next-generation photovoltaics in comparison to the other photovoltaic technologies.

Normally, the thin-film PerSCs have two kinds of device structures: regular structure (n-i-p structure) and inverted structure (p-i-n structure), in which the bottom transparent electrode (e.g., ITO or FTO) is believed to collect either electrons or holes generated in perovskite photoactive layer. For the regular-structure PerSCs, TiO₂ is the most commonly used electron selective contact layer sandwiched between the transparent electrode and perovskite photoactive layer, but it is usually required to be sintered over 450 °C. Although the efficiency of TiO₂-based regular device has already reached over 21%, relatively high temperature for the device fabrication process is not compatible with cost effectiveness.

Recently, perovskite solar cells with the inverted planar heterojunction (p-i-n structure) have been becoming more and more attractive, owing to their easy fabrication, cost effectiveness and suppressed hysteresis characteristics. Some recent advances in the device performance and stability have suggested their promising future. However, the device efficiency is still relatively lower than those of the regular-structure PerSCs so far.

To address the challenge of the low efficiency for the inverted planar heterojunction PerSCs, Professors Zhu and Gong have devoted their great efforts in morphology control, interface modulation and composition engineering, and achieving series of remarkable results published in some top journals. They have used trace amounts of a methylammonium bromide (MABr) as an additive in the Pb(Ac)₂-based precursor solution, which is shown to improve the perovskite film morphology, crystallinity and optoelectronic properties, leading to enhanced device performance. A champion power conversion efficiency of 18.32% with a stabilized output efficiency of 17.60% has been achieved in the inverted planar heterojunction PerSCs. This work was published in Advanced Functional Materials and selected as inside back cover. This paper was also the most accessed paper in the journal in April and May, 2016 (Adv. Funct. Mater., 2016, 26, 3508. Lichen Zhao and Deying Luo contributed equally to this work). Subsequently, they have proposed the concept of charge-carrier balance transport in the inverted planar heterojunction PerSCs based on interface modulation, and the device efficiency has been improved to ~19%. This work was published in Advanced Materials (Adv. Mater., 2016, 28, 10718, Ke Chen, Qin Hu and Tanghao Liu contributed equally to this work.). Recently, a dual-source precursor approach has been developed to incorporate the formamidinium (FA) cation into the methylammonium-based perovskite, which could expand light absorption of perovskites to near infrared region. Combining with the optimization of the hole transporting layer, the improved open circuit voltage (V_oc) in inverted planar heterojunction PerSCs based on mixed-cation perovskite has been achieved without sacrificing fill factor (FF) and short circuit current density (J_sc). With these efforts, they have obtained the inverted planar heterojunction PerSCs with efficiencies exceeding 20%. This work was published in Advanced Materials (Adv. Mater., 2017, DOI: 10.1002/adma.201604758, Deying Luo and Lichen Zhao contributed equally to this work). Moreover, a review of “inverted perovskite solar cells: progresses and perspectives” has published in Advanced Energy Materials (Adv. Energy Mater., 2016, 6, 1600457, Tanghao Liu is first author).

These works were supported by the Ministry of Science and Technology of China, the National Natural Science Foundation of China, State Key Laboratory for Artificial Microstructure and Mesoscopic Physics, 2011 Collaborative Innovation Center of Quantum Matter, China, Collaborative Innovation Center of Extreme Optics, 1000 Talents Program for Young Scientists of China, the Advanced Light Source Doctoral Fellowship in Residence at the Lawrence Berkeley National Laboratory.