Polymer Heterojunction Solar Cells: The Research Of Polymer Structure,Active Layer Morphology And Cathode Interface Modification

Nowadays, conventional energies such as coal, oil, natural gases are confronted with depletion due to the economic development and population increasing, thus people has realized that the importation and urgency of seeking reliable renewable energies. As a green and environment-friendly form of energy, solar energy has received increased attention of academia and industry. People has proved that the inorganic single crystalline silicon solar cells can convert sunlight to electricity efficiently, however, the high manufacture cost limits them to be widely used. Bulk heterojunction solar cells based on polymer have many advantages such as low cost, flexibility and large-area fabrication etc., which attracts great attention in recent years.The photovoltaic performance of polymer bulk heterojunction(BHJ) solar cells could be influenced by many factors, such as the structure of donor and acceptor materials, the ratio of donor and acceptor, the morphology of active layer, the interface between active layer and electrode. In this work, we focus on how the side chain structure of materials, device fabrication process conditions and the function mechanism of interface material affect the photovoltaic performance of polymer bulk heterojunction solar cell. The details are listed below:1. First section illustrates the important steps during the OPV development, the device structure, performance characterization, related measuring instruments and device optimization methods of polymer BHJ solar cells, and we also reviewed the recent works, which focused onsynthesis of materials, device physics and related processing and so on.2. In the second chapter, we report a donor material for polymer solar cells(PDVF), and it can effectively improve the power conversion efficiency(PCE) of the device from 3.57% to4.56% by regulating the length of side chain of the polymer. Besides, we investigated the morphology variation of PDVF-8/10:PC71BM active layer by AFM/TEM, the vertical distribution change of donor and acceptor by XPS-depth profiling, and the relationship between morphology and photovoltaic properties by IS.3. In the third chapter, the methanol solvent additives and processing methods were adopted to optimize the device performance, which significantly improves the PCE of the device. As 3%vol 1-chloronaphthalene(CN) was added into the CF solvent, the PCE was raised to 4.26% from 0.71%, and PCE further increased to 4.69% after treatment with methanol.Herein, AFM, TEM, the projection electron microscope energy dispersive spectroscopy(TEM-EDS) and XPS depth analysis also have been used to investigate the mechanism of the improved performance based on solvent additives and methanol treatment(MT).4. In the fourth chapter, the UPS and ICT model were utilized to investigate the cathode energy level alignment of conventional and inverted solar cell devices based on PFN the cathode buffer layer(CBL). It is found that the PFN buffer layer can effectively reduce the electronic spare barrier(EEB) of conventional cathode(Ag/PFN/PC71BM) and inverted cathode(ITO/PFN/PC71BM), from 0.72 eV, 0.58 eV to 0.38 eV, 0.36 eV, respectively. In addition, there exists a mixed layer containing PFN and PC71 BM between PFN layer and PC71 BM layer, which can lead to a unique interface energy level alignment. Finally we created a universal model to describe the energy level alignment of conventional cathode and inverted cathode for alcohol-water soluble CBL.