Tuning zinc oxide/organic energy level alignment using mixed triethoxysilane monolayers

Interfacial energy level alignment influences several critical organic optoelectronic device characteristics including charge transfer barriers, turn-on voltage, and open circuit voltage (V sub(oc)). Introduction of dipolar molecular monolayers on metal oxide surfaces has allowed improvements in device performance as well as fundamental studies of energy level alignment in these devices. We demonstrate that dipolar mixed monolayers can be covalently bonded to zinc oxide (ZnO) through the triethoxysilane chemical attachment scheme, and that these monolayers tune the work function of the ZnO surface over 0.6 eV. We then employ mixed monolayer-treated ZnO surfaces as the electron-selective contact in inverted bulk heterojunction photovoltaic devices to determine the correlation between the V sub(oc) and the work function of the contact. We find the relationship of -0.14 V eV super(-1) between the V sub(oc) and contact work function is consistent with current results and theories of contact influence on V sub(oc).