p/n Switching of Ambipolar Bithiazole-Benzothiadiazole-Based Polymers in Photovoltaic Cells

Two new alternating copolymers P1 and P2, of bithiazole (BT) and benzothiadiazoles (BTZ), differing in their side chain positioning at the thiophene units which sandwich the BT unit, were designed and synthesized. Both polymers exhibited broad absorption ranging from 300 to 700 nm with a narrow optical bandgap in the film state. Control over structural ordering of polymer chains was achieved in P1 by treating with a small amount of additive (1,8-octanedithiol, ODT) as evident by a large red shift of absorption peak and also from the XRD measurements. In contrast, no such effects were observed in the case of P2 in the presence of additive. Flash-photolysis time-resolved microwave conductivity (FP-TRMC) experiments revealed that the transient photoconductivity of P1 is far superior to that of P2, which is further increased when processed with ODT. The charge carrier mobility, as determined by the space-charge-limited current (SCLC) technique, indicates that P1 exhibits both electron and hole mobilities with a clear dominance of the latter. The charge carrier mobilities become higher and more balanced for ODT-modified P1 films compared to that of P1 alone. TRMC analysis revealed that the photoconductivity of P1 reduced when blended with PCBM in the absence of additive, whereas significant enhancement was obtained in presence of additive. The blend with P3HT exhibited an increase in photoconductivity in both the presence and absence of additive. In complete accordance with the TRMC results, in the absence of additive, P1 acted as an n-type material (P3HT as donor), whereas in presence of additive, it exhibited ambipolar nature acting as both n-type and p-type (P3HT as donor and PCBM as acceptor, respectively) material. Switching of the major charge carrier species was demonstrated simply by the presence of additive for P1 in the present paper.