Low band-gap phenylenevinylene and fluorenevinylene small molecules containing triphenylamine segments: Synthesis and application in bulk heterojunction solar cells

Starting from triphenylamine, two low band gap small molecules, PH and FL, based on phenylenevinylene and fluorenevinylene, respectively were synthesized. They were soluble in common organic solvents such as tetrahydrofuran, chloroform and dichloromethane. Their long-wavelength absorption maximum was at 605–643 nm with optical band gap of 1.64–1.66 eV. These small molecules showed a band gap lower than that of P3HT and also have deeper HOMO level, which is beneficial for improvement of the open circuit voltage. The photovoltaic properties have been investigated using the bulk heterojunction (BHJ) active layer of PH or FL with PCBM. The device based on FL:PCBM displayed higher power conversion efficiency (PCE) (1.42%) than the device based on PH:PCBM (1.02%), when the blend films were cast from chloroform solvent. Moreover, various casting solvents were used for the BHJ solar cells based on FL:PCBM blend and their effect on the photovoltaic performance was investigated. The results indicate that high boiling point solvents lead to an enhanced self-organization of FL in the active layer, which causes an increased charge transport. Finally, we have used a modified PCBM, i.e. F as electron acceptor along with FL as electron donor, to increase the light harvesting in the wavelength region below 500 nm and the PCE is about 4.38% when the BHJ (FL:F blend) device was spin casted from mixed 1-chloronaphthalene/o-dichlorobenzene solvents. The improved PCE has been attributed to the increased light absorption and higher hole mobility in the active layer, which resulted in more balanced charge transport.

Graphical abstractTwo new low band gap small molecules, PH and FL, were synthesized. Bulk heterojunction solar cells were fabricated using PH and FL as electron donors and PCBM and a modified PCBM i.e. F as electron acceptors. A power conversion efficiency of 4.38% was obtained for the device based on FL:F blend cast from mixed solvents.Figure optionsDownload full-size imageDownload as PowerPoint slideResearch highlights► Two low band gap small molecules based on phenylenevinylene and fluorenevinylene. ► PCBM and a modified PCBM as acceptors for bulk heterojunction solar cells. ► Influence of various solvents on the performance of the solar cells. ► Power conversion efficiency of 4.38%.