ABSTRACT
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.
ABSTRACT
In this study, we focus on the induced degradation and spatial inhomogeneity of organic photovoltaic devices under different environmental conditions, uncoupled from the influence of any auxiliary hole-transport (HT) layer. During testing of the corresponding devices comprising the standard photoactive layer of poly(3-hexylthiophene) as donor, blended with phenyl-C61-butyric acid methyl ester as acceptor, a comparison was made between the nonencapsulated devices upon exposure to argon in the dark, dry air in the dark, dry air with illumination, and humid air in the dark. The impact on the active layer's photophysics is discussed, along with the device physics in terms of integral solar cell performance and spatially resolved photocurrent distribution with point-to-point analysis of the diode characteristics to determine the origin of the observed integrated organic photovoltaic device behavior. The results show that even without the widely used hygroscopic HT layer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), humidity is still a major factor in the short-term environmental degradation of organic solar cells with this architecture, and not only oxygen or light, as is often reported. Different from previous reports where water-induced device degradation was spatially homogeneous and formation of Al2O3 islands was only seen for oxygen permeation through pinholes in aluminum, we observed insulating islands merely after humidity exposure in the present study. Further, we demonstrated with laser beam induced current mapping and point-to-point diode analysis that the water-induced performance losses are a result of the exposed device area comprising regions with entirely unaltered high output and intact diode behavior and those with severe degradation showing detrimentally lowered output and voltage-independent charge blocking, which is essentially insulating behavior. It is suggested that this is caused by transport of water through pinholes to the organic/metal interface, where they form insulating oxide or hydroxide islands, while the organic active layer stays unharmed.
