The Rear Surface Morphology Investigation Of Bifacial Passivated Emitter And Rear Cells

Passivated emitter and rear cells（PERCs）have dominated the present photovoltaic market due to easy process steps,low cost and high conversion efficiency,which is the best choice for mass production.The process flow of bifacial PERCs is highly compatible with monofacial PERCs only by replacing full area aluminum back surface field with aluminum finger grids on the rear side.The rear grid design makes bifacial power generation become possible and has wide application prospect.The research of bifacial PERCs should focus on how to improve rear-side efficiency while maintaining the high front-side efficiency.In this disseration,we have systematically investigated the impact of different rear surface morphologies on the electrical performance of bifacial PERCs with both five-busbar（5BB）and nine-busbar（9BB）grid design.Firstly,the minority carrier lifetime test results demonstrate the smoother the rear surface structure,the superior the electrical performance.When light incident from front side,the most flattened rear surface owns the highest long-wavelength reflection,which is beneficial for the re-absorption of long-wavelength photons.Meanwhile,the roughest rear surface owns best light-trapping ability when light comes from rear side.For 5BB and 9BB bifacial PERCs,the electrical parameters as a function of rear surface roughness have similar trends.Due to the decrease of shading area and series resistance,the batch conversion efficiency of 9BB bifacial PERCs increases by0.2%–0.3%comparing to 5BB ones depending on the rear surface roughness.Consequently,a highest front-side average efficiency of 22.57%(V_oc=680.8 m V,J_sc=40.95 m A/cm²,FF=80.96%)has been achieved for 9BB bifacial p-Si PERCs with a nearly planar rear surface.A highest bifaciality of 78.7%is realized for both 5BB and 9BB bifacial PERCs with the roughest rear surface.The scanning electron microscope images of local Al-Si contact show that the depth of local aluminum back surface field is hardly affected by the rear surface morphology,all around 5μm.The influence of rear surface roughness on electrical losses have also been simulated.The results show that rougher rear surface leads to more serious electrical losses for both front side and rear side,and the rear-side electrical losses are higher than that of front-side.We have further conducted simulation of electric power generation when applying bifacial PERCs in practical application.When the ground albedo is lower than 30%,the cells with an A_F value of 1.068 can generate higher electric power.The cells with an A_F value of 1.621 are recommended if the ground albedo is higher than 40%.