Perovskite solar cells are a type of solar cell with great application prospects that have emerged in recent years. They have outstanding advantages such as high photoelectric conversion efficiency, low cost, and simple preparation process. After several years of rapid development, the efficiency of perovskite solar cells has rapidly increased from the initial 3.8% to 22.1%, which has approached or surpassed the traditional high-efficiency thin-film solar cells (such as copper indium gallium selenide or cadmium telluride, etc.) and further developed. It is comparable to single crystal solar cells such as silicon and gallium arsenide.
At present, high-performance perovskite cells adopt the mesoporous structure with a combination of low-temperature dense TiO2 and high-temperature porous TiO2 as the electron transport layer, but the preparation temperature of the mesoporous layer is high and the process is complicated, which is not conducive to large-scale production. In recent years, it has been found that organic/inorganic hybrid perovskite materials have a carrier diffusion length of several micrometers or even up to hundreds of micrometers, which is much higher than the diffusion length of several tens of nanometers of conventional organic semiconductors. In view of perovskite's own excellent charge transport capability, it is completely possible to prepare a simple planar heterojunction perovskite solar cell without the aid of high temperature sintering of a TiO2 mesoporous layer to assist electron transport.
The planar structure perovskite battery has the advantages of simple preparation process, low temperature preparation and compatibility with a flexible device preparation process. However, the widely studied TiO2 dense layer-based perovskite batteries have a serious “electric lag†phenomenon, and can achieve higher efficiency in reverse measurement (from open circuit voltage sweep to short circuit current), but forward measurement (from short circuit The efficiency is lower when the current is swept to the open circuit voltage, making it impossible for people to correctly judge the photoelectric conversion efficiency of such batteries. "Electrical hysteresis" phenomenon has been plagued with the study of planar perovskite cells. Possible causes of this phenomenon include: barriers formed by ion mobility in the perovskite layer, TiO2 interfacial defects in the perovskite/electron transport layer, or energy level differences. And the low electron mobility of TiO2 inhibits the efficient transport of electrons from the perovskite to the electron transport layer. Recently, people have used the fullerene material with ion passivation and rapid charge transfer ability to modify the surface of the dense layer of TiO2, which enhances the extraction of electrons and effectively reduces the "electric hysteresis" phenomenon. However, fullerenes have the disadvantages of being expensive and unstable in the air. Therefore, it is very important to develop a low-cost, stable charge transport layer with strong electron extraction capability.
Recently, researchers at the Key Laboratory of Materials Science at the Institute of Semiconductors, Chinese Academy of Sciences, You Jingbi and Zhang Xingwang have used the inorganic stable metal oxide SnO2 with a higher energy level and higher mobility than the TiO2 conduction band to replace the traditional TiO2 as the electron transport of perovskite cells. The 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 interface charge accumulation. They developed a planar heterojunction perovskite cell based on the SnO2 electron transport layer (Fig. 1a). The experiment confirmed that SnO2 can significantly increase the electron transfer rate from the perovskite absorber to the electron transport layer (Fig. 1b). The prepared battery basically had no "electric lag" phenomenon (Fig. 1c). In addition, the formation of a small amount of PbI2 passivated perovskite grain boundary during the annealing process of perovskite layer formation reduces the recombination process, and a planar heterojunction perovskite cell with photoelectric conversion efficiency as high as 19.9±0.6% is obtained. And got the United States photovoltaic demonstration agency Newport's authoritative certification. These results demonstrate that it is entirely feasible to obtain a highly efficient "electric hysteresis" planar heterojunction perovskite cell, which provides new directions and ideas for the preparation of highly efficient electrohydraulic perovskite solar cells, and strongly promotes perovskite solar energy. The further development of the battery. The related results were published in the recently published Nature-Energy Journal (Q. Jiang et al., Nature Energy, 1, 16117 (2016)).
The doctoral student Jiang Qi is the first author of the article. You Jingbi and Zhang Xingwang are the authors of the paper. This work was supported by the funding of the China Youth Ministry’s 1000-member youth plan, the national key R&D program, the Beijing Science and Technology Commission, and the National Natural Science Foundation of China.
Figure 1: (a) Scanning electron microscope cross-section of a high-efficiency planar heterojunction perovskite cell, (b) Steady-state fluorescence spectra of perovskites at different interfaces, and (c) Calcium using SnO2 nanoparticles as electron transport layer Current-voltage curves for titanium ore cells under forward and reverse measurement conditions.
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