Perovskite solar cell is a kind of solar cell with great application prospects that has arisen in recent years. It has the outstanding advantages of high photoelectric conversion efficiency, low cost and simple preparation process. After several years of rapid development, the perovskite solar cell efficiency has rapidly increased from the initial 3.8% to 22.1%, which has approached or exceeded the traditional high-efficiency thin-film solar cells (such as CIGS or cadmium telluride, etc.) to further develop , Comparable to silicon and gallium arsenide single crystal solar cells.
At present, highly efficient perovskite batteries use low temperature dense mesoporous TiO2 and high temperature porous TiO2 mesoporous structure as the electron transport layer, but the mesoporous layer preparation temperature is higher, the process is more complex, is not conducive to mass production. In recent years, it has been found that the organic / inorganic hybrid perovskite material has a carrier diffusion length of several micrometers or even up to hundreds of micrometers, which is much higher than the diffusion length of the traditional organic semiconductor by several tens of nanometers. In view of its excellent charge transport ability of perovskite itself, it is not necessary to use high-temperature sintering of TiO2 mesoporous layer to assist the electron transport to prepare a simple planar heterojunction perovskite solar cell.
The planar structure perovskite battery has the advantages of simple preparation process, low temperature preparation and compatible with flexible device preparation process. Perovskite cells based on TiO2 dense layers, which have been extensively studied, suffer from serious "electrical hysteresis" and obtain higher efficiency in reverse measurements (from open circuit voltage sweep to short circuit current), but forward measurements (from short circuit Current sweep to the open circuit voltage), the efficiency is low, so that people can not correctly judge the photoelectric conversion efficiency of such batteries. The phenomenon of "electrical hysteresis" has plagued planar perovskite cells. Possible reasons for this phenomenon include: the barrier formed by the movement of ions in the perovskite layer, the difference in the interface defects or energy levels of the perovskite / electron transport layer , And the low electron mobility of TiO2 hinders the effective transport of electrons from the perovskite to the electron transport layer. Recently, the use of fullerenes with ion passivation and fast charge-transfer ability to modify the surface of TiO2 dense layers has enhanced the extraction of electrons and effectively reduced the phenomenon of "electrical hysteresis". However, fullerenes are expensive and unstable in the air. Therefore, it is important to develop a low-cost, stable charge transport layer with strong electron extraction capability.
Recently, researcher of the Key Laboratory of Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, You Jingbi and Zhang Xingwang replaced the traditional TiO 2 with the use of inorganic transition metal oxide such as SnO2, Layer reduces the potential barrier between the perovskite and the electron transport layer, accelerates the transfer of electrons from the perovskite to the electron transport layer, and reduces the interfacial charge accumulation. They developed planar heterojunction perovskite cells based on the SnO2 electron transport layer (Fig. 1a). Experiments have shown that SnO2 can significantly increase the electron transfer rate from the perovskite absorber to the electron transport layer (Fig. 1b) The prepared battery basically no "electrical hysteresis" phenomenon (Figure 1c). In addition, the planar heterojunction perovskite cell with photoelectric conversion efficiency as high as 19.9 ± 0.6% was obtained with the aid of a slight overheating during the formation of the perovskite layer to generate a small amount of PbI2 passivating perovskite grain boundaries, thereby reducing the recombination process. And has been the United States photovoltaic demonstration authority Newport's certification authority. These results show that it is feasible to obtain a highly efficient flat-surface heterojunction perovskite cell without "electric hysteresis", which provides a new direction and idea for the preparation of high-efficiency non-hysteresis perovskite solar cell and promotes the perovskite solar energy Battery further development. Relevant results have been published in the recently published issue of Nature - Energy (Q. Jangetal., Nature Energy, 1, 16117 (2016)).
PhD student Jiang Qi is the first author of this article. You Jingbi and Zhang Xingwang are the authors of this paper. This work has been funded by the Youth Group of the Central Organization Department, the State Key Research and Development Program, the Beijing Municipal Science and Technology Commission and the National Natural Science Foundation of China.
Figure 1: (a) Scanning electron microscopy cross-section of a highly efficient planar heterojunction perovskite cell, (b) Steady-state fluorescence spectra of perovskites at different interfaces, and (c) Calcium using SnO2 nanoparticles as the electron transport layer Current-voltage curves of titanium cells under forward and reverse measurements.
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