In this article, the effect of hole transport layer (HTL) on the thermal stability of inverted polymer solar cells (PSCs) consisting of the blend of poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl-C60 butyric acid methyl ester fullerene derivative (PCBM) as active layer (AL) is investigated. The two conventional HTLs, poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) and MoO3, were used in this study to demonstrate the effect of HTLs on the thermal stability of PSCs. The inverted devices were heated at 80–110 °C as accelerated test for different time intervals to illustrate the temporal variation of performance. The different temporal behaviors during (1) metastable period and (2) thermally unstable period are described here. Moreover, the effect of photoactive film thickness on the thermal stability of devices based on the two HTLs was considered. This study shows that the spin- and spray-coated devices exhibit different characteristics of thermal stability for the PSCs with MoO3 and PEDOT:PSS as HTLs, respectively. The temporal behavior caused by the effect of HTL during the thermally-unstable period is quantitatively studied. This study could provide vital information required to develop high durability in commercial PSCs. Copyright 2016 Elsevier Ltd