Reducing ultraviolet-induced open-circuit voltage loss in inverted organic solar cells by maintaining charge selectivity of the electron collection contact using polyethylenimine

The effect of using Polyethylenimine (PEI) instead of ZnO in the electron extraction layers (EELs) of inverted organic solar cells (OSCs) on their photo-stability behavior is investigated. Studies on inverted OSCs based on two different donor/acceptor bulk heterojunction material systems show that replacing ZnO by PEI substantially reduces the open-circuit voltage (V-oc) loss in OSCs that is often observed under prolonged UV irradiation. Analysis of current density-voltage characteristics of the solar cells as well as of hole-only devices shows that the higher V-oc stability attained upon using PEI instead of ZnO is mainly due to the ability of the ITO/PEI contact to maintain a higher degree of electron selectivity and hole blocking properties under UV exposure in comparsion to the ITO/ZnO contact. Using V-oc vs light intensity analysis, a UV-induced transition in the dominant recombination mechanism from a trap-assisted bulk recombination mechanism to a surface recombination one at the contacts is found to occur in solar cells with ZnO EELs. Similar tests on PEI show that it suppresses that increase in surface recombination and confirms its ability to maintain the hole blocking properties of the contact upon UV exposure. This study provides a direct observation of increased surface recombination in OSCs due to UV irradiation and direct evidence of the importance of maintaining charge selectivity of contacts to suppress surface recombination and maintain high V-oc photo-stability.