Enhancement Of The Performance And Stability Of Polymer Photovoltaic Cells

Organic photovoltaic has considerable technological potentials as an alternative, clean, renewable source of energy, and offers the combined attraction of light-weight, flexibility and low cost to manufacture by large-area roll-to-roll and free vacuum coating processes. However, the relatively low power conversion efficiency (PCE), poor stability and lifetime severely are considered to be the major obstacle for commercialization. Besides, how to simplify the fabrication process and reduce the production cost is also important. With the purpose of improving the PCE and stability of organic photovoltaic device, we focus our research on the optimization of performance by using lithium benzoate as cathode buffer layer and on the morphological stabilization of P3HT: PCBM blend by additive, as following the three aspects:1. The organic photovoltaic cells with lithium benzoate as cathode buffer layer prepared by vacuum thermal evaporation were studied. Effect of the C₆H₅COOLi thickness on the performance and PCE variation with storeage time were investigated. The device performance was greatly optimized at the C₆H₅COOLi thickness of 1.0 nm with PCE of (3.41+0.07) %. The improved performance may attribute to the dissociation of semi-conducting C₆H₅COOLi upon deposition to liberate Li with a low work function, which reduces the interface resistance between the active layer and the cathode and enhances the interior electric field that may result in efficient charge transportion. In addition, the C₆H₅COOLi layer may serve as an effective oxygen and moisture diffusion barrier for the organic solar cells.2. The organic photovoltaic cells with lithium benzoate as cathode buffer layer prepared by spin-coating were studied. The effect of different concentration of C₆H₅COOLi alcohol solution on the device performance was investigated. Spin-coated C₆H₅COOLi buffer layer can improve the device performance in a concentration range from 0.2 to 0.8 mg/ml, especially for the improvement of fill factor (FF). And the maximum PCE of (3.84+0.16) % was obtained. The result shows that spin-coated C₆H₅COOLi buffer layer not only reduces the interface resistance between the active layer and the cathode, but also simplifies the fabrication process of organic photovoltaic devices and lowers the cost. Therefore, C₆H₅COOLi is a promising candidate as an interlayer to enhance the PCE of organic photovoltaic devices.3. Ditert butyl peroxide (DTBP) was introduced as an additive of the P3HT: PCBM active layer, and the influence of different DTBP content and different thermal annealing duration on performance of photovoltaic cells were investigated. Compared to the devices without DTBP, the best optimized device with 0.5 wt % DTBP exhibits enhanced performance with Jsc of (10.44+0.13) mA/cm², FF of (57.61+1.56) %, and PCE of (3.82+0.11) %, increased by 10 %, 26 % and 33%, respectively. The enhanced performance is attributed to thermal crosslinking occurred between P3HT alkyl side chain, which leads to the rough morphology of active layer such that more effective interface area is formed between the P3HT and PCBM regions. FET measurement revealed that DTBP additive increased the mobilities of carriers. Furthermore, the polymeric network in the blend will limit the diffusion pathway of PCBM molecules and restrict PCBM from aggregating into larger clusters. Therefore, the large-scale phase separation between P3HT and PCBM in the active layer can be greatly suppressed, and the stabilization of the film morphology can be improved.