2,2-Dicyanovinyl-end-capped oligothiophenes as electron acceptor in solution processed bulk-heterojunction organic solar cells

Abstract

Three 2,2-dicyanovinyl (DCV) end-capped A-π-D-π-A type oligothiophenes (DCV-OTs) containing dithieno[3,2-b:2′,3′-d]silole (DTSi), cyclopenta[1,2-b:3,4-b′]dithiophene (DTCP) or dithieno[3,2-b:2′,3′-d]pyrrole (DTPy) unit as the central donor part, mono-thiophene as the π-conjugation bridge were synthesized. The absorption spectroscopies, cyclic voltammetry of these compounds were characterized. Results showed that all these compounds have intensive absorption band over 500–680nm with a LUMO energy level around −3.80eV, which is slightly higher than that of [6,6]phenyl-C61-butyric acid methyl ester (PC61BM, ELUMO=−4.01eV), but lower than that of poly(3-hexylthiophene) (P3HT, ELUMO=−2.91eV). Solution processed bulk heterojunction “all-thiophene” solar cells using P3HT as electron donor and the above mentioned oligothiophenes as electron acceptor were fabricated and tested. The highest power conversion efficiency (PCE) of 1.31% was achieved for DTSi-cored compound DTSi(THDCV)2, whereas PTB7:DTSi(THDCV)2 based device showed slightly higher PCE of 1.56%. Electron mobilities of these three compounds were measured to be around 10−5cm2V−1s−1 by space charge limited current method, which is much lower than that of PC61BM, and was considered as one of the reason for the low photovoltaic performance.