Delineation of degradation patterns of C₆₀-based organic solar cells under different environments

The photovoltaic (PV) parameters of a planar heterojunction solar cell with 1,1-bis-(4-bis(4-methyl- phenyl)-amino-phenyl)-cyclohexane as small molecular donor (D) and C₆₀ as archetypical acceptor (A) have been analyzed, after exposure of the photoactive layer (PAL) to different gaseous ambiences such as N₂, O₂, H₂O, and air. We find that for the same exposure time, even minor O₂ amounts lead to more detrimental results compared to H₂O exposure. For devices stressed closer to the D/A interface, we argue that the photo-conversion efficiency (PCE) decreases due to creation of recombination centers at the interface, which induce exciton diffusion and charge generation losses. For devices stressed further from it, traps at the outer C₆₀ layer affect the free carrier transport efficiency. We also show the relationship between the degradation of PV parameters and C₆₀ conductivity loss. We find a direct connection between gas pressure and PCE loss. In-situ XPS measurements of gas-exposed PALs are in agreement with the results obtained through electrical measurements.