A Study Of The Enhancement Of N-type Crystalline Silicon Solar Cells By Post-treatment

Crystalline silicon solar cells dominate the photovoltaic industry.With the energy conversion efficiency of p-type PERC（passivated-emitter and rear cell）solar cells gradually approaching its theoretical limit,in recent years,n-type crystalline silicon solar cells,mainly silicon-based heterojunction（SHJ）solar cells and tunnel oxide passivated contact（TOPCon）solar cells,have been in a rapid development period with the advantages of the higher theoretical limit of energy conversion efficiency,higher mass-production conversion efficiency,and lower temperature coefficient,and the n-type crystalline silicon solar cells can also be enhanced by low-cost post-treatment.It has also demonstrated the obvious gain of power generation per watt relative to p-type solar cells in the photovoltaic module application process.To study the post-treatment for improving the energy conversion efficiency of n-type crystalline silicon solar cells,the following work has been carried out,including the experimental research and theoretical analysis on the phenomenon and mechanism of light-thermal post-treatment,electro-thermal post-treatment,and microwave post-treatment of SHJ solar cells,and the preliminary experimental research on the light-thermal and electro-thermal efficiency enhancing post-treatment technology of TOPCon solar cells,and the main results of the research are as follows:（1）Increased temperature and light intensity during light-thermal treatment can accelerate the improvement of interfacial passivation quality of SHJ solar cell materials and increase the effective minority lifetime of silicon wafers(τ_eff)by more than one times higher,theτ_eff decreases at temperatures up to 220°C.After optimization of the light-thermal post-treatment process,the solar cells with an average energy conversion efficiency of 23.80%can be improved by an average of 0.77%_abs in 60 s,with a maximum enhancement of 0.87%_abs.It is shown that the efficiency gains are attributed to the enhancement of the open-circuit voltage（Voc）and the fill factor（FF）,while the short-circuit current density（Jsc）remains almost unchanged.The Voc enhancement is a result of the passivation improvement of the a-Si:H/c-Si hetero-interface,and the FF enhancement is due to the reduction of R_S by lowering the thin film contact resistance and improving the doped layer and TCO conductivity,in addition to the passivation enhancement.The level of efficiency gain is generally related to the initial efficiency of the cells in the following relationship:“the lower the initial efficiency,the higher the efficiency gain”,but this rule is not obvious in the batch of the cells with the difference of the initial efficiency in the range of±0.25%_abs.In addition,it is found that the solar cells are metastable after light-thermal treatment,and they will degrade after resting or low-temperature heat treatment,this process is related to the passivation degradation on the silicon surface,which is recovered by repeating the light-thermal treatment again.Through the semi-shading experiments of the cell and the design of light-thermal experiments with monochromatic LEDs of different wavelengths,it is found that the mechanism of light-thermal enhancement is directly related to the injected carriers,while the light plays only an indirect role.（2）Based on the study of the mechanism of light-thermal enhancement,electro-thermal post-treatment of SHJ solar cells was investigated at high electrical injection levels of 10-80 A.The solar cells with an efficiency of 22.50%can be improved by0.60%_abs on average.The conversion efficiency improvement is due to the Voc and FF enhancement,while the improvement of the passivation properties at the a-Si:H/c-Si interface is the main endogenous factor for the enhancement,and only～40%of the FF enhancement is due to the reduction of R_S.Increasing the injected current and increasing the temperature can effectively accelerate the enhancement,and the efficiency gain is in a stretched exponential function with temperature.There is a strong correlation between the electro-thermal treatment and light-thermal treatment.The electro-thermal treatment of the stacked solar cells resulted in an average improved efficiency of 0.50%_abs and no metal electrode damage by photoluminescence testing.The solar cells after electro-thermal treatment were rested for 100 h with an average degradation of 0.01%_abs,which shows good stability.（3）Theτ_eff of double-sided a-Si:H thin film passivation wafer increased almost double after 5 s microwave post-treatment.The microstructure factor（R*）of intrinsic a-Si:H films decreases with the processing time,Si-H₂ cracking releases hydrogen atoms faster,and excessive hydrogen loss results in poor passivation.The conversion efficiency of SHJ solar cells fabricated from microwave-treated i/c-Si/i precursors was improved by 1.61%_abs compared to the untreated reference solar cell,mainly due to the improvement of heterogeneous interface passivation.The Si-H polar bonds are more likely to break under varying magnetic and electric fields,releasing hydrogen atoms,and the activated hydrogen atoms passivate the dangling bonds on the silicon surface under the thermal effect.（4）The above three post-treatment enhancement methods for n-type crystalline silicon solar cells are all based on activating hydrogen atoms（carrier injection,microwave treatment on polar Si-H bonds）and moving with thermal effect to passivate the silicon dangling bonds at the c-Si surface and in the amorphous silicon thin-film material to enhance the performance of the solar cells.The TOPCon solar cells with an initial conversion efficiency of 22.25%can achieve an efficiency improvement of0.60%_abs after a light-thermal treatment of 8 k W/m² at 320°C for 5 min.The electro-thermal post-treatment can also enhance the solar cells.It is found that high-temperature annealing can effectively activate the hydrogen atoms and enhance the TOPCon solar cells prepared by high-temperature processes,which is the same as that of light-thermal treatment.（5）A light-thermal post-treatment pilot line was developed based on the research on light-thermal post-treatment enhancement of n-type crystalline silicon solar cells in this paper.Batch processing of 187 pcs SHJ solar cells with an average efficiency of24.09%obtained an average efficiency gain of 0.47%_abs.