Abstract
In this study, we mainly investigated the cell characteristics of Blend/Cs2CO3/Al devices, the electron-injection mechanism of the Cs2CO3/Al cathode, as well as the post-annealing effect on the device performance. We found that an insertion of Cs2CO3 improves the device performances including Voc, FF, and PCE. In the device of Blend/Cs2CO3/Al, a Voc of 0.57 V, a Jsc of 9.45 mA/cm2, a FF of 0.60, and a PCE of 3.05% were achieved. The result of XPS measurement revealed that a portion of electrons would be transferred into the active blend after inserting an interlayer of Cs2CO3, inducing the N-type dopping of the active layer. The N-type dopping process enhances the electron injection, being responsible for the reduced series resistance of the device. Meanwhile, the maximum Voc can be determined form the temperature dependence of Voc. The (Voc)max after inserting Cs2CO3 was found be 1.02 V which is extremely close to the corresponding value of the energy difference between the HOMO of donor and the LUMO of acceptor, suggesting that Cs2CO3/Al electrode can form a better ohmic contact. From the AFM images, the adhesion can be improved after the ITO/PEDOT: PPS/P3HT: PCBM/Cs2CO3/Al device was post-annealed. This prevents the forming of defects near the interface between the active layer and the cathode and therefore reduces leakage paths, resulting in the increase of the calculated shunt resistance. Under the optimum post-annealing condition (140 ˚C, 4 min), the power conversion efficiency of the device can be improved to 3.7%.