2, Korea Research Institute of Chemical Technology, Daejeon, , Korea (the Republic of)
The unprecedented progress in power conversion efficiency (PCE) of halide perovskite solar cells (HPSCs) have attracted a great attention. The PCE has reached to 22.7 % which is above that of conventional thin film solar cells such as CIGS and CdTe cells. It is notable that the high PCE can be achieved by solution process at low temperature under 200 oC because it shows great potential for low-cost, flexible, and light weight in photovoltaic devices. The progress in PCE is attributed to developments in terms of device architecture, halide material, and fabrication process based on material engineering. In initial stage, control of morphology of halide thin film was a key factor for improvement of PCE. Strategy to introduce a mediator during formation of halide crystal film successfully fabricated a compact halide thin film with several hundred nanometers, resulting in breakthrough of PCE. In particular, intramolecular exchanging process using PbI2(DMSO) intermediator thin film was developed for high quality formamidinium lead iodide (FAPbI3) film. Recently, we also reported key finding for the breakthrough in the PCE that is to reduce deep level trap density of the halide film. The trap is a main non-radiative recombination site which causes to lower open circuit voltage in photovoltaic device. Triiodide ions (I3-) are introduced to reduce the deep level traps formed during growing halide perovskite film. We found that the addition of the triiodide ion solution into organic halide solution during formation of halide perovskite film leads to reduction in deep level trap density which is markedly different from the addition of iodine solution prior the conversion to the triiodide ion. Furthermore, the photovoltaic performance of HPSC is not only related to halide materials themselves but also is dependent on n(p)-type oxide and organic semiconducting layers. Therefore, design of the heterojunction between halides and n(p)-type layers will be also discussed for further improvement of PCE in HPSCs.