Synergistic effects of solvent and polymer additives on solar cell performance and stability of small molecule bulk heterojunction solar cells

We developed p-DTS(FBTTh₂)₂:PC₇₁BM-based small molecule bulk heterojunction solar cells using 1,8-diiodooctane (DIO) and small amounts of PCDTBT polymer. In the film, PCDTBT effectively suppresses the over aggregation of the p-DTS(FBTTh₂)₂ donor phase and promotes formation of percolating networks between the donor and acceptor phases. Moreover, the portion of p-DTS(FBTTh₂)₂ crystallites with the face-on orientation in the blend film is significantly increased and phase separation is decreased, which enables efficient charge generation and transport. Consequently, these solar cells consistently exhibit high fill factors and photocurrent densities and high efficiencies in the range 6.68-8.13% regardless of the DIO content (0.4-3 v/v%). In contrast, a large variation was found in the performance of the solar cells with blend films processed with DIO alone, with efficiencies of 2.75-6.68% depending on the DIO content. More importantly, the PCDTBT-processed solar cells exhibit remarkably improved stability under heating and 65 °C/85% RH. Thus, processing photoactive layers utilizing a combination of DIO and PCDTBT is an effective way of preparing promising small molecule solar cells: DIO promotes the crystallinity of the donor phase, and the intermolecular interactions between the polymer and the push-pull moiety in p-DTS(FBTTh₂)₂ induce distribution of donor crystallites to form percolating networks by suppressing donor over-segregation.