Stability Of Efficient Crystalline Silicon Dopant-free Heterojunction Solar Cells

Crystalline silicon solar cells continue to dominate the PV market due to their well-established industrial system,high conversion efficiency,cost performance,and stability.Various new technologies,such as passivated emitter and rear contact(PERC),amorphous silicon/crystalline silicon heterojunction(SHJ),tunnel oxide passivation contact(TOPCon),and interdigitated back contact(IBC)technology,have been developed and applied through extensive research on crystalline silicon cells.These advancements have resulted in crystalline silicon solar cells achieving laboratory efficiency of 26.81%and industrial efficiency exceeding 24%.However,the challenge lies in reducing manufacturing costs while maintaining high conversion efficiencies for crystalline silicon cells.In this thesis,the research topic chosen is the dopant-free silicon heterojunction,primarily because of its potential advantages in terms of preparation cost and efficiency.The objective of this study is twofold:first,to investigate the factors contributing to the degradation of electron selective contact(ESC)stability in dopant-free heterojunction solar cells;and second,to propose solutions to address these issues.Concurrently,the study aims to achieve highly efficient and stable dopant-free heterojunction solar cells by continuously optimizing the IBC technology.In this study,we investigated the common i-a-Si:H/Li F/Al ESC,focusing on the degradation of its passivation and contact properties following annealing at various temperatures.Our interfacial characterization revealed that the primary cause of ESC degradation is the internal diffusion of Al atoms.To address this diffusion issue and enhance ESC stability,we introduced thermally deposited Ti as a protective layer between Li F and Al.This protective layer effectively prevented the diffusion of Al into the i-a-Si:H layer.Consequently,the constructed i-a-Si:H/Li F/Ti/Al ESC maintained a chemically stable interface even after annealing at 200℃.Leveraging the excellent passivation properties and robust process stability of these ESCs,we successfully fabricated a dopant-free heterojunction solar cell with an impressive PCE of 22.12%.Remarkably,this solar cell demonstrated outstanding stability,retaining an initial efficiency of over 94%after annealing at 200℃.Based on a study demonstrating that a thermal evaporated Ti layer can effectively block the diffusion of Al atoms,we introduced a series of metal layers(including Yb,Mg,Ti,and Ni)between Li F and Al.This was done to evaluate various protective layer materials and enhance the thermal stability of ESC.The insertion of these metal layers resulted in modified ESCs with distinct energy band arrangements and contact behaviors,emphasizing the direct link between the work function of the protective layer and the contact properties of ESCs.Furthermore,the contact and passivation characteristics of these ESCs displayed varying degrees of stability following annealing,a phenomenon closely tied to the interfacial reactions taking place within the introduced metal layers.Consequently,we draw the conclusion that an ideal interlayer for ESC should possess the following attributes:firstly,it must have a low work function to facilitate efficient electron transfer at the interface.Additionally,to prevent device degradation under practical application conditions due to interfacial reactions,it is crucial to minimize the chemical reactivity between the overcoat material and other functional layers.To further investigate the factors contributing to the degradation of the contacting properties of i-a-Si:H/Li F/Ti/Al ESC during the annealing process,we explored the stability of ESCs with varying thicknesses of the XF film(X:Li and Cs).The results indicated a clear correlation between the thickness of the XF film and the stability of the ESC contact performance:the thicker the XF film,the less stable the ESC became.However,we ruled out the interfacial diffusion of Al atoms as a cause of the ESC property degradation,as confirmed by interfacial elemental characterization.Instead,when considering the changes in the surface roughness of the Cs F films observed during the annealing process,we proposed a model involving agglomerate shrinkage of the XF films as a potential factor contributing to the deterioration of ESC contact properties.We have implemented several optimizations in the manufacturing processes of high-efficiency crystalline silicon dopant-free heterojunction IBC cells.Firstly,to address the thermal stability issue in the dopant-free contact structure,we improved the electrode template,enabling simultaneous deposition of busbars and fingers and eliminating the need for a post-annealing process.Secondly,to restore the passivation properties of the i-a-Si:H layer,we conducted a pre-annealing process at appropriate temperatures,successfully repair the layer’s passivation properties.Additionally,we improved passivation conditions in the gap region of the IBC cell by preserving the Si N_X:H film through photolithography,which effectively reduced edge recombination in the devices.By combining these optimized processes,we achieved the fabrication of crystalline silicon dopant-free heterojunction IBC cells with an impressive efficiency of 23.73%.