Influence of Cathode Materials on the Efficiency of PTB7:PC70BM Bulk Heterojunction Solar Cells

Herein, studies on the influence of the cathode metal work function (W) on electric parameters of PEDOT:PSS/PTB7:PC70BM/metal solar cells, especially their power conversion efficiency (PCE) and open-circuit voltage (V-oc), are reported. Along with standard cathode materials, like Al and Ag, 12 more metals and metal bilayers, In, InAl, Al3004 alloy, CaAl, MgAl, ZnAl, Sn, Ti, Au, TiAu, and Ni, are checked. It is found that although the photo-voltage is generated mainly in the active layer, the W of cathode also affects V-oc and the overall efficiency. V-oc increases with W and the linear coefficient is about 0.33-0.39. A similar effect is observed for solar cells with P3HT donor material. The electrical and optical properties of the cells with different cathodes just after preparation are investigated and aging measurements are also conducted. The highest PCE is obtained for InAl cathode, 4.9%, which is better than for pure Al cathode. Moreover, long-time measurements show that the cells with InAl cathode degrade about two times slower than the cells with Al electrode. Photocurrent spectroscopy reveals a blue shift of the solar cell sensitivity spectrum with an increase of W which is probably due to state filling (Burstein-Moss effect).