Importance of Optimal Composition in Random Terpolymer-Based Polymer Solar Cells

A new series of donor-acceptor (D-A) conjugated random terpolymers (PBDTT-DPP-TPD) were synthesized from electron-rich thienyl-substituted benzo[1,2-b:4,S-b']dithiophene (BDTT), in conjugation with two electron-deficient units, pyrrolo[3,4-c]pyrrole-1,4-dione (DPP) and thieno[3,4-c]pyrrole-4,6-dione (TPD), of different electron-withdrawing strengths. The optical properties of these random terpolymers can be easily controlled by tuning the ratio between DPP and TPD; an increase in TPD induced increased absorption between, 400 and 650 nm and a lower highest occupied molecular orbital energy level, while higher DPP contents resulted in stronger absorption between 600 and 900 nm. The best power conversion efficiency (PCE) of 6.33% was obtained from PBDTT-DPP75-TPD25 with [6,6]-phenyl-C-71-butyric acid methyl ester (PC71BM) due to the improved light absorption and thus a short-circuit current density (J(SC)) higher than 16 mA/cm(2). Interestingly, the trend observed in the PCE values differed from that of optical behavior of the PBDTT-DPP-TPD in terms of the DPP to TPD ratio, showing nonlinear compositional dependence from 2 to 6%. Density functional theory calculations showed that the small portions of strong electron-withdrawing DPP in PBDTT-DPP25-TPD75 and PBDTT-DPP10-TPD90 could provide trap sites, which suppress efficient charge transfer. In contrast, for PBDTT-DPP90-TPD10 and PBDTT-DPP75-TPD25, the effect of minor portions of TPD on electron density distribution was found to be minimal. In addition, the polymer packing and nanomorphology were investigated by grazing-incidence X-ray scattering and atomic force microscopy. The findings suggested that controlling the ratio of electron-deficient units in the random terpolymers is critical for optimizing their performance in polymer solar cells because it affects the polymer packing structure, the optical and electrical properties, and the electron distribution in the polymers.