Studies On Black Silicon Solar Cells And Aluminum-doped Back-junction Back-contact Solar Cells

In order to make solar energy become a leading source of energy in the coming years,the cost of solar cells must be reduced.Increasing the efficiencies of the solar cells while maintaining low costs via simple fabrication schemes is the best solution.The efficiency of solar cells depends critically on the optical losses caused by front-side reflection and metal shading.This thesis systematically study two different ways to reduce the optical losses of silicon solar cells.The first way is to replace the micro-pyramid with nano-structures as antireflection layers of solar cells.Nano-textured black silicon(b-Si)surface is supposed to enhance the efficiency of solar cells by significantly minimizing the front-side reflection.On the other hand,employing a back contact is a potent way to eliminate the shading losses due to the front contact grid.In this thesis,two types of p-type b-Si and the back-contact back-junction(IBC)solar cells were prepared,and related scientific issues were systematically studied.This thesis includes seven chapters and our main research work are given in chapter 3 to 6.The summary and prospection are performed in chapter 7.In Chapter 1,we introduce the background of this thesis and the developments of high efficiency silicon-based solar cells.In Chapter 2,we introduce not only the preliminary concepts and knowledge of solar cells but characterization methods that used in this thesis.In Chapter 3,the metal assisted chemical etching(MACE)technique are used to fabri-cate uniform silicon nanopores on 156×156mm2 wafers.The formation mechanism of silicon nanopores was studied.The influences of depositing time of Ag,the concentration of H2O2 and etching time on the morphology and reflectivity of samples were studied.In Chapter 4,MACE-prepared b-Si based solar cell were fabricated and studied.The fo-cus was put on the study of recombination mechanism of nanostructured silicon surfaces and the development technology in order to improve the efficiency of b-Si solar cell.Experimental results show that the loss in efficiency of b-Si solar cells is greatly dominated by the increased surface recombination and Auger recombination.In order to suppress the carrier recombina-tion on the b-Si surfaces,we developed a technique to modify the surface morphology and the doping concentration of the emitter.By adopting an optimized SiO2/SiNx passivation scheme,we obtained a compromise between low emitter recombination velocities and low reflectance.Remarkable gains of 3.7%on average efficiency,34mV on open circuit voltage,3.65mA/cm2 on short circuit current density,and 4.65%on FF were obtained,comparing with the b-Si solar cells fabricated by standard industrial process.A best solar cell of 18.5%efficiency was achieved.In Chapter 5,we developed a multi-scale surface textured solar cell.The multi-scale sur-face is achieved by superimposing nano-structures on micro-pyramidal surface texture.This multi-scale texture provides both ideal light scattering and light in-coupling,which presents a low reflectivity in the range of 350-1100nm.Through a further tetramethylammonium hydroxide(TMAH)treatment,the carrier recombination at and near the surface is great-ly suppressed,due to a lower surface dopant concentration after the surface modification.Large-area solar cells with an average conversion efficiency of 19.03%are achieved by using the TMAH treatment of 30s.This efficiency is 0.18%higher than that of standard silicon solar cells with pyramidal surfaces,and also a remarkable improvement compared with black silicon solar cells without TMAH modifications.In Chapter 6,the IBC solar cells were systematically investigated.The chapter focuses on the development of a low-cost and industrially feasible technology to fabricate IBC solar cells.Moreover,the details of our IBC solar cells were studied by performance analysis and numerical simulations(PC1D,TCAD and Quokka).For our IBC solar cells,the shunt path induced by the non-uniformity of screen-printed alluminum-alloyed emitter and the direct contact between alluminum metal with n region has been minimized by p+ emitter modifi-cation.By controlling the modification time,the performance of IBC can be significantly improved.The researches have shown that phosphorus paste(P-paste)can be used to form a heavily-doped n+ region on the back side of IBC solar cells.The sheet resistances optimized to form ohmic contact with Ag metal by adjusting the high temperature treating time.Fur-thermore,the 2D and 3D models of IBC solar cells were builded by combining with theories and experiments.The front side field(FSF),the back side field(BSF)and the emitter of IBC solar cells are systematically investigated based on these models.The studies on n+ FSF show that n+n high-low junction could provide surface passivation for solar cell through the hole shielding by reduction of hole concentration within n+ reigion and the decrease in the hole diffusion coefficient.The other function of n+ FSF is lateral current transport.Final-ly,all-screen-printed IBC solar cells with alluminum-alloyed emitter were fabricated without using photolithography and laser process,cell efficiency of 18.14%is reached.The summary and prospection are given in Chapter 7.