Research On Mass Production And Silicon Solar Cell Of Inverted Pyramid Structures

The main driver of the photovoltaic?PV?industry is always in pursuit of high-efficient solar cells,and one method to improve the cell efficiency is reducing the optical loss.Surface texturing is the most effective and direct way to suppress the optical loss.For the mono-crystalline silicon?c-Si?solar cells,upright pyramid?UP?structure obtained through anisotropic alkali etching is used to reduce the reflectance,which can low the surface reflectance to 10-12%.However,it comes to the bottleneck for the UP to further suppress the reflectance to a much lower level,because the limit of one-step alkaline chemical texturing has been reached;on the other hand,it is very difficult for the UPs to find a tradeoff between the optical and electrical loss,because the sharp pyramidal peaks will deteriorate the passivation effect during the a-Si:H passivation in HJT solar cells,and developing the chemical polish treatment although can improve the passivation effect,but at the price of a corresponding decrease in the light-trapping ability.Therefore,new texturing structure balancing the passivation and light trapping effect must be established,and inverted pyramid?IP?would be a good candidate to promote the cell efficiency.It is known to all that the light-trapping ability of IP is excellent,where the light would go triple even four bounce before reflecting away.Besides,to a certain degree,since the shapes of IPs and the craters are alike,thus,the IP structures are free of sharp peaks,which is benefit for passivating.Till now,only-copper-catalyzed chemical etching method is the most possible technology which can be applied to batch the IPs.Unfortunately,the diamond wire saw?DWS?technology brings a big reform for the IP texturization,the reason for which is the saw grooves and the amorphous silicon mainly existing on the saw grooves which generated during the slicing process would act as an efficient mask,resulting in greatly hindering the Cu deposition.Here,Ag was incorporated as an efficient additive in the Cu etching system to thoroughly texture the c-Si with size-controlled inverted pyramids,which will help IPs-texturization be free of the diamond wire saw marks.More importantly,the mass production technology of IPs is studied for the first time,which would be the basic for realizing the industrialization of the UPs.Firstly,we study the orientation and the optical properties of the IPs on different grains via texturing the DWS multi-crystalline silicon?mc-Si?with IPs through only-copper-catalyzed chemical etching,which can effectively texture the surface because of the more defects existing on the mc-Si.It is found that there is a one-to-one correspondence between the orientation of IPs and grain orientations.According to the simplified model on the formation of final surface structure,a mathematical relationship is firstly established,which enables one to distinguish the grain orientations according to the final IP structures.In addition,the optical properties of IPs on different grains are investigated,which is greatly changed due to the difference of the surface structure on different grains;the reflectance for Si{100}with the standard IPs is the lowest,while the reflectance for Si{111}is the highest.Finally,the proximity-weighted average reflectance for the mc-Si wafer-scale with IPs is much lower than that for the DWS wafers textured with HF/HNO₃ acid etching.The cell efficiency for the DWS wafers textured with IPs is 0.5%absolute higher than for the wafers etched by HF/HNO₃ mixed solutions,showing that the IPs have a great advantage in promoting the cell efficiency.Even though the only-copper-catalyzed chemical etching can successfully texture the DWS mc-Si,but it is failure in efficiently texturing the DWS c-Si,because the c-Si has much fewer defects than mc-Si,which resulting in less nucleation points are generated to facilitate the Cu deposition.Ag is reincorporated as an efficient catalyst to assist Cu in thoroughly texturing DWS c-Si wafers;at the same time,varying the Ag concentration results in size-controlled inverted pyramids from nanometer to micrometer scale.The role of Ag in inverted pyramid texturing is that Ag can dramatically increase the nucleation points prior to Cu deposition due to its higher catalytic capability and vertical-etching ability;thus,to obtain the balance between increasing the nucleation points and vertical etching through adjusting the Ag concentration,the saw grooves disappear and size-controlled IPs are acquired.Besides,detailed comparisons of the optical properties and cell performances among different-sized inverted pyramids and upright pyramids are carried out to give a clear evaluation of how structure influences cell performance.The inverted pyramid sample with the largest structure size has the lowest reflectance and vice versa.Finally,the cell parameters based on the conventional Al-BSF mc-Si production line clearly reveals how the structure size influences the cell performance,where the largest size results in the best cell efficiency.Moreover,the average cell efficiency of the IPs with the largest structure size is as high as 19.955%which is 0.15%higher than that of the UP,confirming that IPs with larger sizes have great advantages in increasing cell efficiency.Additionally,the research on the mass production of IPs obtained on DWS c-Si by Ag-assisted Cu catalyzed chemical etching is executed for the first time,which is the foundation to the large-scale industrial application of the IPs.Process parameters about IPs texturing,such as mother liquor,reaction temperature,reaction time and process feeding rate,have been optimized to achieve the stable mass production of the DWS c-Si textured wafers based on IPs.So far,the pilot production on IP-textured wafers is in process,and batch production?100 batches?has been realized,where the IP-textured DWS c-Si wafers can suppress the reflectance to below 8%with a weight loss of about0.30g per wafer and an acceptable appearance.What is more,the cell performance of the batch IP-textured DWS c-Si wafers based on PERC cells without any production parameters optimization have been tracked,whose results shows that the performances of the IP-based PERC cells is uniform stability among batches,and the average cell efficiency is 0.10%higher than that for the PERC cells based on the UPs.It is worth mentioning that the texturing method to obtain IPs by Ag-assisted Cu catalyzed chemical etching is compatible with the production line,meaning the IP texturing method can be easily copied to the production line;and the IP-based textured wafers would have more space to further improve the cell efficiency.Therefore,we believe that the IPs would be in popularization in the near future.