The Impact of the Presence of Aromatic Rings in the Substituent on the Performance of C60/C70 Fullerene-Based Acceptor Materials in Photovoltaic Cells

Abstract

In this work, a series of C60/C70 fullerene derivatives bearing non-aromatic and mono-, bi-, tri- and tetracyclic aryl-substituents was synthesized according to the modified Bingel method and characterized using spectral methods: ESI-MS, 1H NMR, 13C NMR, UV–Vis, FT-IR and cyclic voltammetry (CV). HOMO and LUMO energy levels and the band gaps for optimized structures of reported fullerene derivatives were determined according to the DFT functionals, B3LYP 6-31G(d) and PBE/6-311G(d,p). Results obtained from CV and UV–Vis measurements, which showed very good agreement, were compared to calculated theoretical values also revealing satisfactory level of compliance of the obtained results. We have studied the impact of presence and number of aromatic rings in malonate substituent of C60/C70 fullerene derivatives employed as acceptor materials on performance of BHJ solar cells prepared using PTB7-Th as donor material. We successfully prepared solar cells based on all the synthesized compounds, and the highest performance of the obtained photovoltaic devices was observed for fullerene derivatives bearing monocyclic and bicyclic aromatic moieties. The obtained voltage was about 0.8 V and current density was above 10 mA/cm2. Optical studies showed absorption edges at 1.9 eV and 1.8 eV for C60 and C70 derivatives, respectively, with absorption coefficients comparable to C60/C70 PCBM samples. Photocurrent spectroscopy showed 20–40% quantum efficiency. Long-term ageing measurements showed T80 time between 105 days and 115 days for derivatives with phenyl and naphthalene substituents. Taking into account that synthesis of reported fullerene derivatives is very convenient, the reported compounds are very promising materials for construction of BHJ solar cells.