| Nowadays,the solar photovoltaic(PV)has become an important path to achieve the global "carbon peak,carbon neutral" strategy,and is also the preferred technology to meet the sustainable development of human energy needs in the future.Since the first modern silicon solar cell was invented in the middle of the last century,continuous technical innovations and capital investments have promoted the vigorous development of the PV business.From an industry perspective,the improvement of the PV conversion efficiency of crystalline silicon solar cells is becoming a focus of the industry due to being beneficial to leverage cost reductions across the whole production chain.From the perspective of the scientific research,a crystalline silicon solar cell is a semiconducting photoelectric device with a planar p-n junction.Taking a solar cell as a "bucket",the optical and electrical properties of the substrate,front and back side of the solar cell are the three "boards"determine the efficiency of the "bucket".However,the traditional structure of combining texturing and anti-reflection layer is difficult to trap light efficiently and becomes a short board for further improving the cell efficiency.In recent years,the metal-catalyzed chemical etching(MCCE)technology studied in our group has made up for the major shortcoming of light trapping on the front surface of multicrystalline silicon(mc-Si)solar cells,and has been widely applied in mass production.With the application of Passivated Emitter and Rear Cell(PERC),Tunnel Oxide Passivated Contacts(TOPCon),HeteroJunction with intrinsic Thinlayer(HJT)and other new solar cell technologies,the surface and interface light trapping technologies of solar cells still face some challenges,such as:how to effectively coordinate light trapping properties of the front and rear sides of the bifacial solar cells,how to optimize the rear dielectric layers to reduce the rear metal parasitic absorption,and how to balance the light trapping and passivation performance of the front and rear surfaces.This dissertation focuses on the light trapping requirements of high-efficiency crystalline silicon solar cells.Combining simulations and experiments,several technologies were studied to construct front and rear coordinative light trapping structures on the surface and interface.Moreover,the effectiveness of the above technologies,structures and materials was tested in large-sized commercial solar cells and corresponding modules.The main results are summarized as follows:(1)OPAL 2 and SunSolve software based on the ray tracing method were used to simulate and analyze the light trapping performance of front and rear microstructures on cells.The sizes of these microstructures were usually larger than the wavelengths of incident light.An optical model of dielectric films of the front and rear interface in the cell was built,the classical vector method and transfer matrix method were applied to optical simulations,and new film designs that can enhance the light trapping performance of cells were effectively guided by simulation results.(2)In order to solve the problem that the acid texturing technology performed in the Diamond Wire Sawing(DWS)mc-Si wafer being accompanied with the trait of high reflectance,a laser texturing technology was applied to DWS mc-Si wafers to form the unique pits in craters(PIC)structures.Based on wafers with different defect ratios,back surface field solar cells were fabricated with different PIC structures and their optical and electrical properties were studied.Compared with the round-shaped structures produced with the conventional acid texturing,an approximately 2.4%reflectance reduction and the less reflectance variation dependence on incident angles were achieved in the optimized PIC structures.The negative effect that the carrier recombination being increased by applying the laser texturing was also reduced by optimizing the PIC structures.The short circuit current(Isc)of the solar cells with optimized PIC structures was increased by 127 mA,the open circuit voltage(Voc)and fill factor(FF)were decreased slightly,and the efficiency was increased by 0.16%.After being encapsulated into modules,compared with the cells with round-shaped structures,the short circuit current density of the cells with PIC structures showed less improvement,but the absolute value was still higer.Relative work has been published in Sol.Energy Mater Sol.Cells.2020.(3)An optical model of PERC solar cells was built,and with the guidance of simulation results,the front and rear dielectric multi-layers of cells were designed to optimize light trapping properties.The SiNx/SiNx/SiOxNy layers acted as the front anti-reflection coating(ARC)were first deposited on the front side of the cells,resulting in Isc and efficiency of the cells increased by 50 mA and 0.1%respectively,and the corresponding module power increased by 0.71 W.With the deposition of SiNx/SiOxNy/SiNx layers as capping layers of the Al2O3 passivation layer on the rear side,a 24 mA Isc gain and a 0.07%efficiency gain were obtained.Finally,the optimized front SiNx/SiNx/SiOxNy ARC and rear SiNx/SiOxNy/SiNx capping layer were integrated into the champion group,compared with the baseline with two-layer SiNx films of high/low refractive index as the front ARC and a single SiNxfilm as the rear capping layer,the champion group showed enhanced front antireflection in the short wavelength range and increased rear reflection in the long wavelength range;an 80 mA Isc gain and a 0.18%efficiency increment of the cells were achieved;the module power was observed with the increase of 1.75 W.Relative work has been published in Prog Photovolt Res Appl.2021.(4)To find suitable rear surface morphologies to improve light trapping in bifacial PERC solar cells under front and rear illumination,traditional acid polishing technologies,the optimized acid polishing technology and an environment-friendly alkaline polishing technology were studied.Based on the HF/HNO3/deionized water(DIW)acid polishing system and the KOH/additive/DIW alkaline polishing system,rear surfaces with slope angles of polished pyramids varying from 50.2° to 0° were realized in bifacial PERC solar cells.Under front illumination,attributed to the best balance of light trapping and rear surface recombination properties,cells with a slope angle of 11.2° showed the highest Isc of 10.606 A and the highest front efficiency of 22.86%;under rear illumination,benefitting from the lowest rear reflectance,cells with a slope angle of 50.2° exhibited the highest rear efficiency;even at 40%albedo,the 11.2° samples maintained the highest equivalent bifacial efficiency of 29.82%.The optical properties of single-cell double-glass modules were also investigated and simulated:under front illumination,the light trapping abilities of samples were enhanced by the introduction of encapsulation materials,and the original light trapping differences between cells with different rear morphologies were also narrowed in modules;under rear illumination,due to the higher internal reflection ratio at the interface of encapsulation materials and air,more obvious rear light trapping increases were observed in the 29.5° and 37.4° samples.This work has been published in ACSApplied Energy Materials.2022. |
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