Origin of thermal instability of CH3NH3PbI3−xClx films for photovoltaic devices

• The results reveal origin of thermal instability of perovskite films.
• It is helpful to produce solar cells fabricated directly on the ITO.
• The charge transfer generates between perovskite and conducting substrate.

The rapid improvements in perovskite solar cells have led to high power conversion efficiencies of over 20% in recent years. Despite this remarkable performance, the mechanism of thermal instability for those films is still one of the problems that we have to face. In this work, we systematically measure the thermal stability of CH3NH3PbI3−xClx films on substrates of ITO, PEDOT:PSS/ITO, TiO2/ITO, at various temperatures. Comparing their counterparts on Glass, PEDOT:PSS/Glass, TiO2/Glass, we found that the films on substrates with ITO are always less thermal stable, resulting from charge transfer between perovskite and ITO substrates, in particular at higher annealing temperatures. Therefore, it was believed that more thermal degradation can occur in CH3NH3PbI3−xClx film with less neutral electrical environment.