Tuning the optoelectronic properties of vinylene-linked donor-acceptor copolymers for organic photovoltaics

Five new donor-acceptor copolymers containing the electron acceptor benzothiadiazole (BTZ) linked to the electron donors fluorene (FL) or cyclopentadithiophene (CPDT) via vinylene units were synthesized to study polymer structure-property relationships in organic photovoltaic devices. Both alternating (P) and random copolymers (P1-P4) were prepared via Suzuki and Stille polycondensations, respectively. The cyclopentadithiophene copolymers (P2 and P4) have smaller electrochemical band gaps (1.79 and 1.64 eV) compared to the fluorene-containing copolymers (2.08 and 1.95 eV for P1 and P3). However, the presence of CPDT raises the electrochemical HOMO energy levels (-4.83 and-4.91 eV for P2 and P4) compared to the FL copolymers (-5.06 and-5.15 eV for P1 and P3) leading to small open circuit voltages (V_oc) in solar cells. The primary solution and thin-film UV-vis absorption peaks of P3 and P4, which do not contain alkylated thiophenes appended to the BTZ unit, are at lower energy and have larger absorption coefficients than their P1 and P2 counterparts. Detailed theoretical analyses of the geometric structure, electronic structure, and excited-state vertical transitions using density functional theory provide direct insight into the interplay between the structural modifications and resulting electronic and optical changes. A high molecular weight (M_n = 25 kg/mol) polymer with a large degree of polymerization (DP_n = 21) was easily achieved for the random copolymer P1, leading to thin films with both a larger absorption coefficient and a larger hole mobility compared to the analogous alternating polymer P (M_n = 22 kg/mol, DP_n = 18). An improved short circuit current and a power conversion efficiency up to 1.42% (J_sc = 5.82 mA/cm², V_oc = 0.765 V, and FF = 0.32) were achieved in bulk heterojunction solar cells based on P1.