Annealing Process Optimization And Optical Simulation Of Cathode Buffer Layers For Polymer Solar Cells

Polymer solar cells(PSCs) have received considerable attentions due to their advantages such as simple processing, low cost and mechanical flexibility. In the inverted polymer solar cell device, the Zinc oxide thin film is prepared by a sol-gel method as the cathode buffer layer(CBL). Traditionally, sol-gel drived Zn O films need a preheating at 200-500oCfor 5min and a postheating at 500-800oC for 1 hour to get the Zn O films with high crystallinity. However, the PSC’s flexible substrate or glass substrate is poor in heat resistance, which cannot withstand prolonged high temperature heat treatment. In additions, there will be an increase in resistance of the ITO when at high temperature for a long time. Taking into account the above factors, the annealing condition of zinc oxide thin film is adjusted to 300oC 5min or 200oC 1h in the production of the device. In this annealing conditions, the Zn O thin film with poor crystallization and low conductivity is not conducive to improve device performance.For these problems, the rapid thermal annealing(RTA) process is applied to improve the annealing quality of the cathode buffer layer. Cesium-doped zinc oxide is applied as the cathode buffer layer to improve the electron-transporting property. The device structure is ITO/CZO/P3HT:PC61BM/Mo O3/Ag. Experiments have explored the optimal condition of the rapid thermal annealing and the optimum doping concentration of Cs2CO3. The result shows that when the annealing condition is RTA 500oC 15 s, Cs2CO3 doping concentration is 0.005 M, the device has a highest energy conversion efficiency of 3.88%, which is increased by 12% compared with the conventional thermal annealing optimum condition.Moreover, in order to obtain the optimal thickness of the CZO cathode buffer layer, the transfer matrix method is used to simulate the effect of the thickness of CZO thin film on the optical electric field distribution in the active layer and the short-circuit current density of the device. The device structure is ITO/CZO/P3HT:PC61BM/Mo O3/Ag. The simulation results show that when the thickness of CZO is 80 nm, the short-circuit current density of the device is the maximum, the maximum is 10.62 m A/cm2, which has 17% improvement.Finally, this paper produced several solar cell devices with CZO films of different thicknesses. Experimental results show that the trends of the short-circuit current density’s experimental are consistent with the simulation results, when CZO’s thickness is 80 nm, the device has a highest energy conversion efficiency of 4.25%. The experimental results demonstrate that the optical simulation which is based on the transfer matrix method has a guiding role on the optimization of the buffer layer’s thickness.