Conjugated polymers blended with soluble fullerene derivatives have demonstrated a remarkable potential for low-cost, large-area and flexible organic solar cells (OSCs) with exceeding 10% in power conversion efficiency [
1]. For commercialization, further improvement in power conversion efficiency and the cell reliability are desired [
2,
3]. To the power conversion efficiency, there still exists the room of improvement by synthesizing more efficient materials [
4], optimizing more rational device structure [
5] and even electrical doping for improving the light-harvesting efficiency in polymer solar cells [
6,
7]. Importantly, the issue of cell stability has to be paid more attentions after the power conversion efficiency approaching 10%. There are many degradation channels during cell operation and each contributes to the overall deterioration of photovoltaic (PV) performance that consequently shortens the device lifetime. Great efforts have been taken in the cell stability especially the materials stability in OSCs. According to each layer that usually constitutes an OSC, the materials stability can be subdivided: active layer [
8,
9], electrodes [
10,
11], electron transport layer [
12,
13], and hole transport layer [
14,
15] which contributes their own degradation mechanisms. In addition, the physical stabilities such as the cell structure [
16], the interfacial condition [
17,
18], and even the film processing means also play important roles in the whole cell degradation [
19]. The most significant contributor is believed to be a combined effect by above mentioned mechanisms due to the diffusion of atmosphere species into the cells. Electrical trap states, which are mainly induced by the impurities and the structural defects in the organic bulk films [
20,
21], can be easily evaluated by basis electrical and/or optical measurements [
22–
25].