Abstract
Organometal halide perovskite, CH3NH3PbI3, has been used as light harvester in solar cell since 2009. Perovskite solar cell has been developed diverse structures so far, and the one with solid hole transport material (HTM) as electrolyte has been achieved 19.3% power conversion efficiency. In contrast, liquid electrolyte type perovskite solar cell faces the hurdle of the degradation caused from dissolution of CH3NH3PbI3 by I-/I3--contained electrolyte, even though it possesses several advantages in terms of easy fabrication, simple sealing and no need of expensive vacuum evaporation. Thereby, its highest power conversion efficiency remains only 6.7%. In this study, we use a commomly-used HTM, Spiro-OMeTAD, as the protecting layer between CH3NH3PbI3 and electrolyte for the purpose of isolating the physical contact of liquid electrolyte and perovskite as well as not interfering the electron transfer from electrolyte to CH3NH3PbI3. This arrangement did enhance device stability significantly but it is also found the corrosion still exists. It not only indicates the coating of protecting layer requires more engineering study but also shows the importance of digging out the root cause for corrosion. By analyzing the results of XRD, SEM, EDS and UV-Vis spectrum, some important information is concluded. First and foremost, I3- does not involve in the corrosion reaction, while I- is the main origin of corrosion of CH3NH3PbI3 in LiI/I2 (I-/I3-)-contained electrolyte. Besides, dissolution and redeposition of CH3NH3PbI3 take place simutaneously in the device and result in the appearance of giant CH3NH3PbI3 crystals within few micrometers on the mesoporous layer. Last but not least, the common additive, tBP, in electrolyte is found to stabilize CH3NH3PbI3 degradation by filling itself at the defects on crystal, although tBP itself corrodes CH3NH3PbI3. The information discovered in this study is beneficial to understand the fundamentals of corrosion and to further improve the performance of liquid electrolyte type perovskite solar cell.