ABSTRACT
A detailed investigation of the functionality of inverted organic photovoltaics (OPVs) using bare Ag contacts as the top electrode is presented. The inverted OPVs without a hole-transporting layer (HTL) exhibit a significant gain in hole-carrier selectivity and power-conversion efficiency (PCE) after exposure in ambient conditions. Inverted OPVs comprised of ITO-ZnO-poly(3-hexylthiophene-2,5-diyl)/phenyl-C61-butyric acid methyl ester (P3HT/PCBM)-Ag demonstrate over 3.5% power conversion efficiency only if the devices are exposed in air for over 4 days. As concluded through a series of measurements, the oxygen presence is essential to obtaining fully operational solar cell devices without HTL. Moreover, accelerated stability tests under damp heat conditions (RH = 85% and T = 65 °C) performed to nonencapsulated OPVs demonstrate that HTL-free inverted OPVs exhibit comparable stability to the reference inverted OPVs. Importantly, it is shown that bare Ag top electrodes can be efficiently used in inverted OPVs using various high-performance polymer-fullerene bulk heterojunction material systems demonstrating 6.5% power-conversion efficiencies.
