Study On The Performance Improvement Of Polymer Solar Cells Through Cathode Interfacial Modification

Due to the current energy crisis and environmental pollution problems, to search clean and sustainable energy is one of the critical problems for the future economic development of the entire society. Solar energy, which is clean and inexhaustible, stands out from the numerous new energy sources. Currently, technique for crystalline silicon solar cells have been well developed and dominate the photovoltaic market. However, silicon solar cells still have several problems, such as the limited source materials, the severe dependence on material quality of purity, and huge energy consumption process. Polymer solar cells have some advantages over the silicon solar cells, which have diverse source materials and easy processability. Due to the good flexibility of the polymers, polymer solar cells can be fabricated through roll-to-roll process, which can greatly reduce the cost of production. Although the power conversion efficiency of polymer solar cells has been rapidly improved in the past ten years, the efficiency is still not high enough for commercial applications. Moreover, environmental stability of polymer solar cell is another key parameters needed to be improved before their commercialization.The interface layers of polymer solar cells have multiple functions. First of all, it can reduce the energy barrier between an active layer and an electrode to form ohm contact for charge extraction. Second, they are able to select specific charge carriers, for example, an electron transport layer can be used as a hole blocking layer to reduce the charge recombination. Many interface materials, such as titanium oxide and cesium carbonate, can be used to modify the work function of electrodes and achieve polymer solar cells with inverted structure. Moreover, the interface layer can also change the optical field and protect the active layers for polymer solar cells. This article focused on cathode interface modification, searching for suitable cathode modification interfacial materials and optimizing the structure and processes, to improve the device performance.Conductive polymer PFN is a new kind of cathode modify material, which can form dipole layer between the ITO electrode and the photosensitive layer and reduce the work function of ITO. The ITO work function can match with the active layer, leading to improved electron extraction and device performance. However, PFN is a kind of insulation and has poor electron mobility, so that the PFN layer thickness has great influence on the performance of the device. A thick PFN layer leads to high series resistance, whereas a thin PFN has a poor surface coverage and bad hole-blocking effect. Zn O nanoparticles is an inorganic semiconductor materials, which has high electrical conductivity and lower work function and can be used as cathode modification material. However, their surface defects and the uniform spatial distribution affect the device performance. We used the combination of Zn O nanoparticles and PFN layer in polymer solar cells, which can have the benefits of both Zn O and PFN. Due to the high electron mobility and hole block effect of Zn O nanoparticles and the good electronic extraction of PFN layers, devices based on the PFN/Zn O bilayer structure have enhanced open circuit voltage Voc, short circuit current Jsc and fill factor FF, leading to a 10% enhancement in the power conversion efficiency(8.59%).Heat treatment is an important factor to improve the film quality. Rapid thermal process, which can heat a substrate to a high temperature in a very short time, can effectively inhibit impurities redistribution. Zn O conductivity can be highly improved using rapid thermal process, due to the improved crystallinity of Zn O after optimizing the heat temperature, atmosphere and time. At the same time, the heat treatment condition can reduce the surface defects of Zn O and structure defects, and prevent the exciton quenching and reduce carrier recombination. Combined with our bilayer structure, the Jsc and FF of polymer solar cells were significantly improved and a highest power conversion efficiency of 9.31% was achieved.The high electron mobility is one of the advantages of Zn O as the cathode modification material. Al doped Zn O is another potential cathode modification material, which has higher electron mobility, lower work function for better electron transport/extraction and better optical transparency. In this paper, a trilayer structure of Zn O/AZO/PFN was used as the cathode modification layers in polymer solar cells. The AZO layer can modify the energy structure for better electron extraction; the PFN layer can passivate the surface defects of AZO and reduce the carrier recombination; and the interfacial dipole can improve the built-in electric field and improves Jsc. Therefore, Zn O/AZO/PFN trilayer structure of cathode modification, can significantly improve Jsc and power conversion efficiency(9.17%).Moreover, PFN, an unstable organic material, can decrease the device stability when used as cathode interfacial layers. Zn O nanoparticles can highly improve the device stability. In our work, the devices based on the trilayer structure showed the better device stability than the devices based on the bilyer modification structure, both of which were much better than that of PFN-based devices.