Investigation On The Surface Treatment Technology Of Substrates Used In High Efficiency Solar Cells

Silicon hetero-junction solar cell with intrinsic thin layer (HIT), one of the new highefficiency solar cells, has been paid much more attention due to its advantages of low-costand high-efficiency. The light absorption efficiency of HIT solar cells with the texturedmonocrystalline silicon substrates can be increased. However, in the depositing process ofamorphous silicon layer on the silicon substrates, the epitaxial growth is created in thevalleys among the pyramids, which leads to increase defects at the interface betweenmonocrystalline silicon substrate and amorphous silicon layer and then reduce theperformance of the solar cells. Therefore, it is important to research monocrystalline siliconsubstrate surface treatment technologies and obtain the suitable substrate morphologyimproving HIT solar cell performance. The optimized cleaning and texturing technologiesof monocrystalline silicon substrates used in HIT solar cells were investigated in detail inthis paper.Firstly, the minority carrier lifetime of the substrate was improved obviously byoptimizing monocrystalline silicon substrate cleaning process. The influence ofpre-cleaning, isopropyl alcohol (IPA), NaOH, temperature on the texturing effect in theopen system was studied, and optimized texturing technology was obtained. In addition,optimized two-step texturing technology was gained through regulating the temperature andsolution concentration ratio.Secondly, the effects of IPA density, etching time, substrate type and the sodiumsilicate content on the performance of textured substrates were investigated and optimizedtexturing technology was obtained. The results showed that the reflectance of texturedn-type monocrystalline substrate at wavelength of1011nm reduced to9.14%by using theoptimized texturing technology. HIT solar cells based on this etched silicon substrateshowed higher spectral response and70%at wavelength of400nm.Finally, the modified processes of substrate surface were studied. Two methods,smooth etching and tetramethyl ammonium hydroxide (TMAH) texturing, were adopted tomodify the micro-morphology of textured monocrystalline silicon surface. It is indicatedthat the textured substrate smoothed by mixed acids had better light trapping and interfacequality, resulting in the open circuit voltage of HIT solar cell increase to609.4mV from 564.6mV before smoothing and conversion efficiency to12.4%from11.8%. And texturingof substrates by TMAH makes conversion efficiency of HIT solar cell increase to13.7%. Itis suggested that the substrates etched by TMAH can improve the performance of HIT solarcell, because there is no metal ions pollution and it is easy to get rounded pyramid imagesof textured substrate surface in TMAH etching.