The effect of side-chain length on regioregular poly[3-(4-n-alkyl) phenylthiophene]/PCBM and ICBA polymer solar cells

The photovoltaic, charge transport, light absorption and morphology properties of a series of poly[3-(4-n-alkyl) phenylthiophene] (PAPT):acceptor (phenyl-C-61-butyric acid methyl ester (PCBM) or indene-C-60 bisadduct (ICBA)) blend films are reported as a function of alkyl side-chain length. Regioregular poly[3-(4-n-butyl) phenylthiophene] (PBPT), poly[3-(4-n-hexyl) phenylthiophene] (PHPT), poly[3-(4-n-octyl) phenylthiophene] (POPT), and poly[3-(4-n-decyl) phenylthiophene] (PDPT) were successfully synthesized by a modified Grignard metathesis (GRIM) polymerization method. The effects of the alkyl side-chain length on the optical, electrochemical and structural properties of the polymers were carefully investigated to establish a relationship between the molecular structure and device function. Bulk heterojunction solar cells made of PAPTs blended with either PCBM or ICBA showed a strong photovoltaic property dependence on the alkyl side-chain length. Among all PAPTs used in this study, the best performance was observed in PHPT-based devices. This is the first report of the use of PHPT in polymer solar cells. In addition, PAPT devices blended with ICBA generally showed higher open-circuit voltages (V-oc) and power conversion efficiencies than PCBM-based devices. For example, a PHPT: ICBA photovoltaic device showed a V-oc of 0.79 V and a power conversion efficiency of 3.7%, which is the highest performance reported thus far using PAPT polymers. While the optical properties of PAPT/electron acceptor films exhibit the strongest effect on the short-circuit current of the devices, a balance between the optical, electrical and morphological properties of blended films caused by the alkyl side-chain length determines the overall photovoltaic performance of these devices. Therefore, our work provides a fundamental understanding of the molecular structure-device function relationship, especially with respect to changes in the alkyl side-chain length in conjugated polymers.