Influence of Environmentally Affected Hole-Transport Layers on Spatial Homogeneity and Charge-Transport Dynamics of Organic Solar Cells

After organic photovoltaic (OPV) cells achieved efficiency of more than 10%, the control of stability and degradation mechanisms of solar cells became a prominent task. The increase of device efficiency due to incorporation of a hole-transport layer (HTL) in bulk-heterojunction solar cells has been extensively reported. However, the most widely used HTL material, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), is frequently suspected to be the dominating source for device instability under environmental conditions. Thereby, effects like photooxidation and electrode corrosion are often reported to shorten device lifetime. However, often in environmental device studies, the source of degradation, whether being from the HTL, the active layer, or the metal cathode is rather difficult to distinguish because the external diffusion of oxygen and water affects all components. In this study, different HTLs, namely, those prepared from traditional PEDOT:PSS and also two types of molybdenum trioxide (MoO₃) are exposed to different environments, such as oxygen, light, or humidity, prior to device finalization under inert conditions. This allows investigating any effects within the HTL and from reactions at its interface to the indium tin oxide electrode or the active layer. The surface and bulk chemistry of the exposed HTL has been monitored and discussed in context to the observed device physics, dynamic charge transport, and spatial performance homogeneity of the corresponding OPV device. The results show that merely humidity exposure of the HTL leads to decreased device performance for PEDOT:PSS, but also for one type of the tested MoO₃. The losses are related to the amount of absorbed water in the HTL, inducing loss of active area in terms of interfacial contact. The device with PEDOT:PSS HTL after humid air exposure showed seriously decreased photocurrent by microdelamination of swelling/shrinkage of the hygroscopic layer. The colloidal MoO₃ with water-based precursor solution presents slight decay of solar cell performance, also here caused by swelling/shrinking reaction, but by a combination of in-plane particle contact and resistance scaling with particle expansion. However, the device with quasi-continuous and alcohol-based MoO₃ showed unharmed stable electrical performance.