  High power conversion efficiency (PCE) inverted organic photovoltaics (OPVs)
usually use thermally evaporated MoO3 as a hole transporting layer (HTL).
Despite the high PCE values reported, stability investigations are still
limited and the exact degradation mechanisms of inverted OPVs using thermally
evaporated MoO3 HTL remain unclear under different environmental stress
factors. In this study, we monitor the accelerated lifetime performance of
non-encapsulated inverted OPVs using thiophene-based active layer materials and
evaporated MoO3 under the ISOS D-2 protocol (heat conditions 65 {\deg}C). The
investigation of degradation mechanisms presented focus on optimized
P3HT:PC[70]BM-based inverted OPVs. Specifically, we present a systematic study
on the thermal stability of inverted P3HT:PC[70]BM OPVs using solution
processed PEDOT:PSS and evaporated MoO3 HTL. Using a series of measurements and
reverse engineering methods we report that the P3HT:PC[70]BM/MoO3 interface is
the main origin of failure of P3HT:PC[70]BM-based inverted OPVs under intense
heat conditions.
