Synthesis of a Broadly Absorbing Modified PCBM and Application As Electron Acceptor with Poly(3-Hexylthiophene) As Electron Donor in Efficient Bulk Heterojunction Solar Cells

A simple and effective modification of PCBM was carried out by a two-step reaction. The modified PCBM i.e., A was obtained as a mixture of monoadduct and multiadducts arising from the Prato reaction and was soluble in THF. It showed broader and stronger absorption in the visible region of the solar spectrum than PCBM due to the presence of a cyanovinylene 4-nitrophenyl segment. The lowest occupied molecular energy level (LUMO) of A was measured and found that it is higher than that of PCBM by 0.15 eV. The polymer solar cell (PSC) based on the P3HT as the electron donor with A as the electron acceptor shows an open circuit voltage (V-oc) of 0.80 V, a short circuit current (J(sc)) of 9.0 mA/cm(2), and a power conversion efficiency (PCE) of 3.88%, which are higher than those for the PSC based on P3HT PCBM. The higher PCE for the PSC based P3HT A accounts for the increase in both J(sc) and V-oc, due to the increased absorption of A in the visible region and its higher LUMO level. The PCE of the PSC has been further increased to 4.50% and 5.32% when the P3HT A blend film is cast from mixed solvents and subsequent thermal annealing respectively. The improvement in PCE is mainly due to the increase in J(sc), which has been attributed to the increase in the crystallinity of the film leading to an increase in the hole mobility, which causes a balanced charge transport in the devices.