Studies On Effects Of Photo-thermal Annealing Of ZnO Cathode Buffer Layer On The Performance Of Polymer Solar Cells

In recent years, polymer solar cell（PSC）, especially inverted polymer solar cell（IPSC）, has become one of research focuses in the field of photovoltaic technology. Sol-gel deposited ZnO film has been widely used as cathode buffer layer in IPSCs due to its advantages such as easy-fabrication, high transparency and long term ambient stability. A post treatment of thermal annealing is usually used after ZnO film deposition, however, the annealing temperature is hard to achieve above 300℃ due to the poor thermal tolerance of ITO glass or plastic substrate which is commonly used in PSC devices. The ZnO film annealed at low temperature has a poor polycrystalline quality and is defect-dominated which bring negative effects on the extraction and transportation of electron and power conversion efficiency of IPSC. Although improvement of ZnO film polycrystalline quality by using rapid thermal annealing（RTP） has been reported, RTP device is relatively complicated to operate and not suitable to the production of large area PSCs due to the small chamber of RTP devices.In this thesis the sol-gel derived ZnO film was annealed by simply irradiating the sample with an array of halogen tungsten lamps, which can increase the annealing temperature to 500℃ while leaving the substrate unaffected. Results show that the photo thermal annealing（PTA） treated ZnO film has larger grain size, rougher surface and greatly decreased trap state density compared to that by conventional thermal annealing（CTA） and the performance of IPSCs. The details are as follows:Firstly, ZnO film sol-gel deposited on ITO glass was PTA treated and IPSC with the structure ITO/ZnO/P3HT:PC61BM/MoO3/Ag was prepared. The processing parameters were optimized and the annealing temperature and time was set to be 500℃ and 10 s respectively. Compared to that of CTA device, the power conversion efficiency（PCE） was increased by 21.4% and enhanced to 3.97%. SEM, PL and AFM results show that the ZnO film treated by PTA（PTA-ZnO） has larger grain size, decreased trap-state density and rougher surface. The rougher surface can increases the scattering of incident light and has “light trapping” effect which is similar to the crystal silicon with a texture structure. It can be seen from UV-vis spectra that the active layer exhibits an increased light absorption around 500 nm.Secondly, we use PTB7:PC₇₁BM blends as the materials for active layer and PTA-ZnO as cathode buffer layer, prepared IPSC with the structure ITO/PTA-ZnO/ PTB7:PC₇₁BM/MoO3/Ag. Similarly, the active layer on PTA-ZnO showed an increased absorption at the wavelength range 600-700 nm which can be attributed to the increased grain size and surface roughness induced by PTA treatment. The PCE of the IPSC with PTA-ZnO is 3.54, 8.3% greater than that based on CTA-ZnO cathode buffer layer.In summary, ZnO cathode buffer layer was thermal annealed by irradiation with an array of halogen tungsten lamps. The crystalline quality of the film was increased and the internal and surface trap states was greatly decreased by using the post treatment proposed. The rough surface of PTA-ZnO increased the reflection and refraction at the surface and increased the light absorption of active layer. The PTA technique proposed is easy processing and suitable for large area mass production of IPSC devices.