Morphology-Dependent Trap Formation in Bulk Heterojunction Photodiodes

We show that local structural variation affects the rate of aging in nanostructured polymer solar cells by comparing time resolved electrostatic force microscopy (trEFM) and conventional device measurements on model polymer blends. of the polyfluorene copolymers poly(9,9'-dioctylfluorene-co-bis-Specifically, we study photovoltaic devices made from 1:1 blends N,N'-(4-butylphenyl)-bis-N,N'-phenyl-1,4-phenylene-diamine) (PFB) and poly(9,9'-dioctylfluorene-co-benzothiadiazole) (F8BT). We photooxidize these films in situ using 365, 405, and 455 nm illumination under ambient conditions, with the wavelengths chosen to preferentially excite the different components. During photooxidation, we observe a faster loss of photocurrent generation from F8BT-rich domains, leaving the PFB-rich phases to show higher photoresponse even at wavelengths absorbed predominantly by F8BT. We propose that this effect is due to the more rapid degradation of PFB hole. transport pathways in the F8BT-rich regions, resulting in a loss of percolation pathways for hole transport in the F8BT-rich phase.