Research And Application Of Inverted Rectangular Pyramid On Silicon Solar Cells

The photovoltaic?PV?industry is always in pursuit of new techniques for the mass production of silicon?Si?solar cells,requiring not only high efficiency but also low cost.Up to now,Si?multi-crystalline?mc-Si?and single-crystalline?c-Si??solar cells still account for the most share of the PV market.The pyramid texture for c-Si is generally based on alkaline solutions by the anisotropic etching with about 12%surface reflectance.The inverted pyramid texture,outperformed the pyramid texture because of their superior light-trapping and structure characteristics,was typically used in high efficiency solar cells.Due to the complexity and high cost of the photolithographic based patterning processes,such technique is not compatible with industrial solar cell manufacture.Our team discovered a new technical solution to fabricate micrometer sized inverted pyramidal structure by maskless Cu assisted chemical etching?CACE?which takes a shorter etching time in a lower etching temperature during the texturing process compared with the industrial pyramidal structure texturization.In this thesis,the different behaviors of anisotropic etching of Si by alkaline and Cu based acid etchant had been systematically investigated and the formation mechanism of pyramid and inverted pyramid had been studied.We developed a novel texture method by one-step Cu/Ag co-catalyzed chemical etching to solve the texture issue induced by the diamond wire sawing?DWS?of mc-Si.The obtained structure has the characteristics of low reflectivity and easy passivation,along with little color differences,which has huge application in photovoltaic cell mass production due to its simple and low cost process.In this thesis,the different behaviors of anisotropic etching of Si by alkaline solution and Cu based acid etchant have been systematically studied.The anisotropic behavior of alkali is due to small differences of the energy levels of the back bond surface states as a function of the crystal orientation.The anisotropic etching behavior and the whole surface etching process lead to the formation of upright pyramid texture.As for Cu based acid solutions,due to the lower reduction potential of Cu²⁺/Cu and different electronic properties of different Si planes,the deposition of Cu nanoparticles demonstrates orientation-dependent,which leads to the anisotropic etching of Si substrate.The formation of inverted pyramids results from the coexistence of anisotropic etching and localized etching process.The obtained structure by CACE is bounded by Si{111}planes,no matter what orientation of Si substrate is.Different from alkali etching,the value of anisotropic factor of CACE is almost the same value as that of the ratio between the free bond density of Si?100?and?111?planes.Much faster etching rate?0.54?m/min?on Si?111?plane had been realized in Cu based acid solutions compared to the etching rate which is almost zero in alkaline solutions.By thorough understanding the etching mechanism of CACE,the morphology,size and surface roughness of the etched inverted pyramids are under control.We fabricated the PERC?Passivated Emitter and Rear Cell?based on the inverted pyramid structures.Due to the superior light trapping ability and structural characteristic of inverted pyramids,much higher I_sc?short-circuit current?and FF?Fill Factor?had been obtained,and and the conversion efficiency had been improved ultimately.We developed a novel texture method by one-step Cu/Ag co-catalyzed chemical etching.The etched inverted rectangular pyramid structure has the characteristics of low reflectivity and easy passivation,which not only solves the texturization problem of DWS mc-Si,but also has little color differences among different grains.The etching mechanism of Cu/Ag co-catalyzed chemical etching is the cooperation of Ag catalyzed vertical etching and Cu catalyzed lateral etching,that is,during the etching process,the digging of the holes by Ag catalyzed etching and enlarging of the holes by Cu catalyzed etching are completed at the same step.What's more,by balancing the vertical etching of Ag catalyzed and lateral etching of Cu catalyzed chemical etching,the aspect ratio of inverted rectangular pyramid can be adjusted.The fabricating process of solar cells had been investigated on 156 mm×156 mm DWS mc-Si based on inverted rectangular pyramid by one-step Cu/Ag co-catalyzed chemical etching.Benefitting from the excellent light trapping and passivation effect of inverted rectangular pyramid structure,a super high efficiency of 19.49%had been obtained on an industrial production line,which is 0.56%and 1.5%absolutely higher than the acid textured solar cells which were based on slurry wire sawing mc-Si and DWS mc-Si respectively.Last but not the least,the low cost,one-step texturing method on DWS mc-Si without pre or post-treatment has huge application in mass production of photovoltaic cell.