Synthesis of a Modified PC_(70)BM and Its Application as an Electron Acceptor with Poly(3-hexylthiophene) as an Electron Donor for Efficient Bulk Heterojunction Solar Cells

摘要

A simple and effective modification of phenyl-C_(70)-butyric acid methyl ester (PC_(70)BM) is carried out in a single step after which the material is used as electron acceptor for bulk heterojunction polymer solar cells (PSCs). The modified-PC_(70)BM, namely CN-PC_(70)BM, showed broader and stronger absorption in the visible region (35O-S5O nm) of the solar spectrum than PC_(70)BM because of the presence of a cyanovinylene 4-nitrophenyl segment The lowest unoccupied molecular energy level (LUMO) of CN-PC_(70)BM is higher than that of PC_(70)BM by 0.15 eV. The PSC based on the blend (cast from tetrahydrofuran (THF) solution) consists of P3HT as the electron donor and CN-PC_(70)BM as the electron acceptor and shows a power conversion efficiency (PCE) of 4.88%, which is higher than that of devices based on PC_(70)BM as the electron acceptor (3.23%). The higher PCE of the solar cell based on P3HT:CN-PC_(70)BM is related to the increase in both the short circuit current (J_(sc)) and the open circuit voltage (V_(oc)). The increase in J_(sc) is related to the stronger light absorption of CN-PC_(70)BM in the visible region of the solar spectrum as compared to that of PC_(70)BM. In other words, more excitons are generated in the bulk heterojunction (BHJ) active layer. On the other hand, the higher difference between the LUMO of CN-PC_(70)BM and the HOMO of P3HT causes an enhancement in the V_(oc). The addition of 2% (v/v) 1-chloronapthalene (CN) to the THF solvent during film deposition results in an overall improvement of the PCE up to 5.83%. This improvement in PCE can be attributed to the enhanced crystallinity of the blend (particularly of P3HT) and more balanced charge transport in the device.