Zhencai Cao

Efficient polymer solar cells based on terpolymers with a broad absorption range of 300–900 nm

Three novel random conjugated terpolymers were designed and synthesized by copolymerizing a benzo[1,2-b:4,5-b′]dithiophene (BDT) donor with an electron-deficient diketopyrrolo[3,4-c]pyrrole (DPP) unit and a thiophene-vinylene-dithienyl-benzothiadiazole (TVDTBT) side group into a polymer backbone. By tuning the ratio of DPP and TVDTBT in the terpolymers, the optical properties and energy levels of these random terpolymers could be rationally controlled. As a result, the terpolymers exhibited a very broad absorption range of 300–900 nm with a high absorption coefficient and deep HOMO energy levels. Bulk heterojunction polymer solar cells fabricated from P3 and PC61BM exhibited a promising power conversion efficiency of 5.29% without any processing additives.

Synthesis and photovoltaic performances of conjugated copolymers with 4,7-dithien-5-yl-2,1,3-benzothiadiazole and di(p-tolyl)phenylamine side groups

Three new copolymers (PT-TPA, PT-DTBT and PT-DTBTTPA) based on benzo[1,2-b:4,5-b]dithiophene (BDT) and thiophene with different conjugated side chains (di(p-tolyl)phenylamine (TPA), 4,7-dithien-5-yl-2,1,3-benzothiadiazole (DTBT) and DTBT-TPA) were synthesized via Stille coupling polymerization. The TPA and the DTBT were introduced to improve the hole-transport ability and broaden the absorption spectrum. The effects of different conjugated side groups on thermal, optical, electrochemical, hole-transporting and photovoltaic properties of these copolymers were investigated. Field effect results show that the copolymer PT-DTBTTPA containing TPA and DTBT in the side chain showed the highest hole mobility. The three copolymers exhibit deep-lying HOMO energy levels, which were effectively tuned by changing the side groups. Photovoltaic cells were fabricated with the synthesized copolymers as electron donors and [6,6]-phenyl-C-butyric acid methyl ester (PCBM) as the electron acceptor. Bulk heterojunction polymer solar cells based on PT-DTBT and PT-DTBTTPA showed promising power conversion efficiencies of 5.50% and 5.16%, respectively.

Improved photovoltaic properties of terpolymers containing diketopyrrolopyrrole and an isoindigo side chain

A series of donor–acceptor (D–A) conjugated random terpolymers were synthesized by copolymerizing electron-rich alkylthienyl-substituted benzodithiophene (BDTT) and two electron-deficient units, a diketopyrrolopyrrole (DPP) moiety and isoindigo (TID) based side chain. The effects of the DPP and TID units on the thermal, photophysical and electrochemical properties of the polymers were investigated using thermogravimetric analysis, UV-vis-NIR absorption spectra, and cyclic voltammetry. Compared with the parent polymers (P1 and P2), the optical properties of the random terpolymers (P3–P6) were controlled successfully by tuning the ratio of DPP and TID. The increase in TID content induced an increased absorption between 450 and 600 nm and a lower highest occupied molecular orbital (HOMO) energy level, while higher DPP content resulted in stronger absorption between 600 and 900 nm. Bulk heterojunction solar cells based on the as-synthesized polymers as electron donors and (6,6)-phenyl-C-butyric acid methyl ester (PCBM) as the acceptor were fabricated. The best power conversion efficiency (PCE) of 5.62% was obtained from P5 (DPP : TID = 1 : 1) due to its high short-circuit current density (Jsc higher than 15 mA cm−2), mainly arising from the broadened light absorption. The results have demonstrated that the random terpolymers, with complementary light-absorption, have a great potential for increasing the photocurrent and PCE in polymer solar cells.