Analysis of the degradation mechanism of ITO-free organic solar cells under UV radiation

This work reports on the stability of encapsulated ITO-free bulk heterojunction organic solar cells (BHJ-OSC) under UV radiation in ambient air with the layer sequence Cr/Al/Cr/photoactive layer (PAL)/poly (3, 4-ethylenedioxythiophene) poly (styrenesulfonate) (PEDOT:PSS)/metal-grid. The PAL consisted of poly(3-hexylthiophene) (P3HT) as a donor and (6,6)-phenyl-C60 butyric acid methyl ester (PCBM) as an acceptor. BHJ-OSC with this setup showed remarkable stability under continuous illumination (1000 W/m2) with a low UV content. In contrast, the devices degraded significantly under UV radiation which was characterized by a reduction in fill factor and short-circuit current density. Additional experiments revealed an increase of the sheet resistance of the PEDOT:PSS layer which was interestingly much more pronounced in pure PEDOT:PSS samples as compared to samples where a PAL was deposited underneath. In addition, current extraction by linear increasing voltage (CELIV) measurements indicated a decrease of the effective charge carrier mobility of the PAL. Numerical simulations based on the experimentally determined parameters showed good agreement of the solar cell performance as a function of UV exposure duration. This suggests that the increase of the sheet resistance of the polymeric hole contact and to a lesser extent the change of the effective mobility of the PAL are the main factors governing the deterioration of the photovoltaic performance upon UV exposure. A comparison to devices with a setup ITO/ZnO/PAL/PEDOT:PSS and a full metallization showed clearly improved UV stability, although the absorption of UV in the PAL is very similar. This further supports our interpretation that the degradation of the PAL plays a very minor role. The issue with the degradation of the PEDOT:PSS can easily be solved by incorporating an UV-filter into the device or preferably the use of UV-stabilized PEDOT:PSS formulations.