Improving the performance of organic solar cell via tuning the interfacial n-doping of cathode-modifying layer

Abstract

Organic solar cells have been fabricated using thermally evaporated bathocuproine (BCP) and ytterbium n-doped BCP (BCP:Yb) to modify the interfaces of active layer and cathode. The device with 10 nm BCP presents open-circuit voltage of 0.791 V and fill factor of 0.670, greater than those (0.785 V and 0.636) of the device with 10 nm BCP:Yb, mostly due to that the BCP:Yb causes severe nonradiative recombination in active layer. The device with 1 nm BCP/9 nm BCP:Yb offers open-circuit voltage of 0.793 V, almost same as that of the device with 10 nm BCP, due to the two following reasons. Firstly, the interlayer of 1 nm BCP is able to separate active layer from BCP:Yb, thereby preventing the nonradiative recombination. Secondly, the BCP interlayer is so thin that the interfacial n-doping of Yb in it raises the Fermi level close to its lowest unoccupied molecular orbital level, whereby it forms ohmic contacts with active layer and BCP:Yb. Despite showing decreased fill factors, the devices with 10 nm BCP:Yb and 1 nm BCP/9 nm BCP:Yb give short-circuit current densities of 15.43 and 15.62 mA cm⁻², respectively, larger than that (14.47 mA cm⁻²) of the device with 10 nm BCP. The power conversion efficiency based on 1 nm BCP/9 nm BCP:Yb is 8.11 %, higher than those based on 10 nm BCP (7.67 %) and 10 nm BCP:Yb (7.71 %). The current research indicates that tuning the interfacial n-doping of cathode-modifying layers is a facial and effective method to improve the performance of organic solar cells.