Study On Light Induced Degradation And Related Passivation Of PERC Crystalline Silicon Solar Cells

High efficicency and low cost are the two main targets for the crystalline silicon solar cells.Passivated emitter and rear contact solar cell(PERC)technology,well passivated on the rear side by the ALD-Al2O3 film,is one of the most promising solar cell technologies owing to the high efficiency.However,it is comfirmed that light induced degradation in PERC solar cells is very severe,which seriously impedes the improvement of the efficiency of PERC solar cells.So researches concentrated on the passivation of the soalr cells as well as the surpression of LID are of great significance for the development of high efficiency PERC solar cells?This manuscript has a good study of the suppression of LID in PERC solar cells by applying large forward current on the solar cells.It is demonstrated that the large forward current at elevated temperature can trigger the regeneration process of theCz-PERC solar cells,which demonstrate a good stability in the subsequent illumination tests.In addition,a batch of 300 PERC solar cells are treated by the large forwar current,which also showed good stability in the subsequent illumination.Moreover,the large current treatment also works on the mc-PERC solar cells,which are reported to suffer from serious LeTID(Light and Elevated Temperature Induced Degradation).And the LeTID was also demonstrated much smaller under illumination at elevated temperature after the annealing process.We also demonstrate that the surpression effect is better if the annealing treatement and large current treatment are combined together.The passivation of ALD-Al2O3 is demonstrated to increase steeply under illumination,but the improvement of the passivation is also found to decrease when the sample is kept in dark.It is shown that the improvement process of the passivation under illumination and the degradation process of the enhanced passivation in dark are thermal activated with a activation energy of 0.32 eV and 0.59 eV.Moreover,C-V measurements based on the Al/Al2O3/p-Si capacitor are carried out,identifying the negative charge increases under illumination.Based on these results,we propose the trapping and detrapping model that the photons induced eletrons in the p-Si substrate can tunnel through the ultrathin SiOx layer and been traped by the trap sites within the Al2O3 layer,resulting in the improvement of the passivation.In this manuscript,we find that hydrogen is able to passivate silicon surface defect states when both the silicon surfaces are clean and contaminated with copper.Nevertheless,the passivation efficacy of defects in clean silicon surface is much higher than that in copper contaminated silicon surface,which is attributed to that hydrogen can well passivate separated defects,but is not able to well passivate much closed distributed defect states in defect cluster that has a separation distance larger than the radius of hydrogen.It is also found that hydrogen is not able to change the properties of defect energy levels,for which it is still discrete energy levels in clean silicon surface and band-like energy region in copper contaminated silicon surface after hydrogenation that is consistent with the properties of silicon surface before hydrogenation as reported in literatures.Furthermore,the defect densities and capture cross sections are calculated before and after hydrogenation for both samples with clean and copper contaminated silicon surfaces.