  A detailed investigation of the functionality of inverted organic
photovoltaics (OPVs) using bare Ag contacts as top electrode is presented. The
inverted OPVs without 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 obtain fully operational
solar cell devices without HTL. Moreover, accelerated stability tests under
damp heat conditions (RH=85% and T=65oC) performed to non-encapsulated 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.
