The effect of fluorination on the low and high frequency dielectric constants of non-polymeric organic semiconductors – towards homojunction solar cells

Bulk heterojunction organic solar cells have undergone significant improvements in power conversion efficiency and stability over the past decade. However, translation from laboratory-scale devices to commercial panels is complicated by the sensitivity of the efficiency to morphological changes, which requires delicate control of the processing conditions. A homojunction architecture, with only a single chromophoric material in the light absorbing layer, could provide a pathway to overcoming problems associated with controlling complex blend morphologies. However, to achieve that goal the organic semiconductor would need to have a dielectric constant of ≈10 such that free charge carriers are formed directly upon photoexcitation. In this study we explore the effect of fluorination on the low and high frequency dielectric constants of two sets of materials with monomeric (A′–A–D) or dimeric (A′–A–D–D–A–A′) structures, where A′ is an aldehyde, A is a protonated or fluorinated benzothiadiazole, and D is a glycolated 4H-cyclopenta[2,1-b:3,4-b′]dithiophene (CPDT) donor unit. Fluorination was found to have only a small effect on the film absorption properties (small blue shift and decreased coefficient), but led to films with increased density relative to the protonated material (1.44 g cm−3versus 1.32 g cm−3), and dispersive unbalanced electron (≈10−6 cm2 V−1 s−1) and hole transport (≈10−5 cm2 V−1 s−1). Critically, while the low frequency dielectric constant of the fluorinated dimeric material was larger than that of the protonated (8.3 versus 7.3), the optical frequency dielectric constant that is thought to be critical for exciton dissociation was smaller (3.3 versus 3.6).