Design Of Novel Silicon Heterojuction Solar Cell Structures

The rapidly growing needs of global energy in modern societypromote people to continuously chase for higher solar cell conversionefficiency. The silicon heterojunction (SHJ) solar cell, which has attractedboard attention around the world, serves as a potential candidate of thenew-generation solar cell to replace the crystalline-silicon (c-Si) solar cell.The SHJ solar cell simultaneously owns the advantages of the largebandgap of amorphous silicon (a-Si) and the stability of c-Si, and canavoid the light induced degradation within a-Si thin film solar cell. Thusthe SHJ solar cell presents predictable better open-circuit voltage,short-circuit current and conversion efficiency, compared to only a-Si orc-Si based solar cells.However, the performance of SHJ solar cell is strongly influencedby various factors, because it includes complex structures such as thec-Si/a-Si interface and ultra-thin intrinsic a-Si buffer layer. These diverseimpact factors limit the efficiency breakthrough of the SHJ solar cell. Among them the clean c-Si/a-Si heterojunction interface with low defectstates density is the first key point to ensure the satisfactory output of SHJsolar cell. The presence of defect states of high density at the interfacewould dominate the solar cell performance and lead to severe outputdegradation. In addition to the interfaces, the transparent conductiveoxides layer, doped a-Si emitter layer, intrinsic a-Si buffer layer, c-Sisubstrate layer and other layers in SHJ solar cell sensitively change theband diagram and carriers density distribution in structure, which alsoneed careful control and engineering. Besides, another importantapproach to improve SHJ solar cell efficiency is to introduce lighttrapping nanostructures. The widely adopted two-dimensionalvertically-oriented nanowires and nanopillars have excellentantireflection and absorption properties, but they also greatly enlarge thesurface area and thus surface recombination, which impedes the furtherapplication into SHJ solar cell. As a result, it is of great significance tostudy and analyze the mechanisms of the influencing factors and carrycorresponding structure optimization, in order to realize the efficiencybreakthrough of SHJ solar cell.In this dissertation we focus on presenting novel SHJ solar cellstructure to provide new methods for solving existed problems andrealizing high efficiency SHJ solar cell through modeling and calculation.Firstly, we establish an elaborate and accurate defect states distribution model of a-Si layer based on experimental data, and employ it tocalculate the influences of defect states density in a-Si doped emitterlayer and intrinsic buffer layer upon solar cell output. We have clearlystudied and demonstrated the mechanisms of trapping effect and give outthe allowed highest value of defect states density in a-Si layers to avoidthe trapping effect, which is very helpful to guild practical production.Furthermore, we propose to add a c-Si homojunction into SHJ solar cell,which can bring strong field-effect passivation to weaken the impacts ofinterface defect states upon band structure and solar cell performance. Asa result, the homohetero junctions solar cell can still achieve highefficiency with relatively high interface defect states density.Secondly, we present the one-dimensional horizontally-orientedc-Si/a-Si half-coaxial nanowire arrays (NWAs) for the light trappingstructure on SHJ solar cell. This NWAs effectively preserve the stronglight absorption enhancement within single nanowire (NW), thus canprovide extremely high absorption with only a small increment of surfacearea. By synthetically studying the effects of various structural factors,the optimized NWAs can absorb nearly all incident light with only10μmthick c-Si substrate. That is to say, the NWAs light tapping structure cancut the necessarily required thickness of c-Si substrate in SHJ solar cell toabout10μm, which certainly increase solar cell efficiency and reducecost. Finally, we expand the c-Si/a-Si half-coaxial structure to raise thedielectric/a-Si half-coaxial NWAs, for the extending application on thelight trapping structure of silicon thin film solar cell. The NWAs iscapable of reducing75%thickness of the absorbing layer, significantlyenhancing the fill factor of solar cell and cutting material cost. Besides,the NWAs can be easily realized by current standard etching anddepositing techniques on a relatively large area, and any semiconductorabsorbing layer can be applied to replace a-Si layer, which greatlybroaden the application field of NWAs.