Efficient bulk heterojunction solar cells using an alternating phenylenevinylene copolymer with dithenyl(thienothiadiazole) segments as donor and PCBM or modified PCBM as acceptor

A novel low band gap alternating phenylenevinylene copolymer, P, with dithenyl (thienothiadiazole) segments was synthesized by Heck coupling. It was soluble in common organic solvents, showed broad absorption curve with long-wavelength absorption maximum at 580–598 nm and optical band gap of 1.74 eV, which is comparable with the electrochemical band gap of about 1.80 eV. P (electron donor) was blended with PCBM or modified PCBM i.e. F (electron acceptor) to fabricate bulk heterojunction (BHJ) solar cells. The power conversion efficiency (PCE) of the devices based on P:PCBM and P:F cast from 1.2-dichlorobenzene (DCB) was found to be 1.40% and 2.32%, respectively. The higher value of PCE for the device with P:F as compared to P:PCBM is attributed to the increase in both short circuit current (Jsc) and open circuit voltage (Voc). The increase in the Jsc is due to the stronger light absorption of F in visible region as compared to PCBM, i.e. more exciton generation in the blend. On the other hand, the higher difference between the LUMO levels of P and F, as compared to P and PCBM is responsible for the enhancement in the Voc. A maximum overall PCE of 4.20% was obtained for the BHJ polymer cell based on the active layer (P:F) deposited from mixed solvents 1-chloronapthalene/1,2-dichlorobenzene (CN/DCB) and subsequent thermal annealing at 120 °C. This improvement in the PCE has been attributed to the enhanced crystallinity of the blend and more balanced charge transport in the device due to the thermal treatment.