In this study, we report bulk heterojunction inverted polymer solar cells (iPSCs) with high efficiency and long-term stability. The devices were fabricated on the basis of Thieno[3,4b]thiophene-alt-benzodithio phene (PTB7) and [6,6]-phenyl C71 butyric acid methyl ester (PC70BM). An ultra-thin film of titanium oxide (TiOx) was used as electron transport layer (ETL). The effects of TiOx on the performance parameters and stability of iPSCs were compared to the effects on conventional PSCs and iPSCs using poly [(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) as ETL. The power conversion efficiency (PCE) of TiOx-iPSCs was 5% and 11% higher than that of PFN-iPSCs and conventional PSCs, respectively. In this study the time the PCE takes to reduce to 80% is hereafter referred to as the cell's lifetime. In nitrogen, the lifetime of 6048 h of TiOx-iPSCs was found to be five times longer than that of conventional PCE (1200 h) and PFN-iPSC (1300 h) cells. The main objective of this study was to consider ambient conditions in the dark as a more realistic exposure scenario for non-encapsulated TiOx-iPSCs. Under these conditions, the lifetime was found to be of several hours. This reduction was significantly faster than under nitrogen and can be attributed to the degradation of the contact interfaces driven by ambient oxygen and water. Electron mobility was also reduced by 75% within 24 h. An additional encapsulation served to stabilize the TiOx-iPSCs under ambient conditions and lifetime was extended by up to 120 h with no observed loss in electron mobility. Impedance spectroscopy was used to investigate the degradation causes of active layer and electrodes interlayers. (c) 2017 Elsevier Ltd. All rights reserved.