Random Corrosion Structure Affecting Thin-film Silicon Solar Cells' Efficiency

Random Corrosion Structure Affecting Thin-film Silicon Solar Cells' Efficiency


Abstract: Solar cells were prepared by using the suede structure obtained by alkali etching of monocrystalline silicon as the substrate. The morphology of the pyramid suede was observed with a scanning electron microscope, and the reflectivity was tested with an ultraviolet-visible spectrophotometer. The effect of the mixed solution of 15% to 23% w. t. Na2CO3 plus 4% w. t. NaHCO3 at 95℃ for 20 min in reducing the reflectivity was compared. DC magnetron sputtering, thermal evaporation and n-100 monocrystalline silicon as the substrate were used to prepare Al/AZO/PH silicon solar cells. The effect of the suede structure on the short-circuit current and open circuit voltage of silicon hydride solar cells was preliminarily discussed in this article.
 
Aluminum-doped zinc oxide (AZO) is a typical semiconductor thin film with high transmittance and low resistivity in the visible light range. It is not easy to diffuse with hydrogen at high temperatures. It has good chemical stability, and is not easy to reduce the activity of solar cell materials. AZO has been widely used as the front electrode of silicon hydride (Si: H) solar cells. The preparation of high-efficiency Si: H solar cells requires effective light trapping and low reflectivity in the entire solar spectrum. The suede structure is an effective way to improve the photoelectric conversion efficiency. Texturing AZO and monocrystalline silicon not only can reduce the front surface reflection, but also achieve the purpose of a light trapping effect, increasing the battery's absorption for long-wavelength spectrum. In this article, chemical etching was used to texturize the AZO film and monocrystalline silicon. The alkaline solution has anisotropic characteristics for (100) crystal orientation corrosion of single crystal silicon, and can prepare randomly distributed pyramid suede structure on the surface. Q5% dilute hydrochloric acid is used for AZO to make the pit-like suede structure. These structures reduce the reflectivity of the solar cell surface to a certain extent, and improve the photoelectric conversion efficiency of the cell.
 
The effect of the pyramid suede structure produced by the corrosion of the Na2CO3-NaHCO3 mixture on the single crystal silicon in reducing the reflectivity of the solar cell was discussed in this article. Then, the textured n-type monocrystalline silicon was sequentially plated with p-type Si:H films, AZO, and aluminum electrode by a DC sputtering method, and the effect of the suede structure on the short-circuit current of the single-junction silicon hydride solar cell was discussed. The structure of the Si:H solar cell was n-type monocrystalline silicon substrate or p-type SiH/AZO/Al. Among them, the silicon p-n junction mainly generated photocurrent. The AZO film was used to collect current, and Al was the electrode. Used n-type (phosphorus doped) -100 crystal orientation single crystal silicon. The thickness was about 450μm, and the resistivity was 5.4 to 7 Q cm. The texturing processes were as follows: the silicon wafers were washed sequentially with alcohol, acetone, and they were deionized. They were dried with nitrogen, and placed in a mixed solution of HCl: H2O2: H2O=1:1:5 at 80℃ to react for 10 minutes; then used 1% HF solutions to react at 80℃ for 10 minutes to remove the intrinsic oxide layer (RCA). After that, the sample was placed in a mixed solution of 15% to 23% w. t NaCO3 plus 4% w. t NaHCO3 at 95℃for 20 minutes. The surface morphology was observed by SEM (Hitachi TM-1000); the surface reflectance of the suede was tested with a UV spectrophotometer (Varian Cary-500).
 
 
Figure 1 Corrosion of a mixed solution of 20% Na2CO3 plus 4% NaHCO3 at 95℃

The cell preparation of n-type (100) single crystal silicon used n-type single crystal silicon as the substrate 20 minutes later. The direct current sputtering method was used to sequentially deposit Si:H and AZO thin films. The gases used for sputtering Si:H films were argon, hydrogen, and borane. Figure 1 showed that a pyramid structure of uneven sizes appeared after the single crystal silicon was corroded. After further research, it is found that when the concentration of Na2CO3 was between 15% and 23%, the reflectivity of the monocrystalline silicon surface gradually decreased as the concentration increased. When the concentration was 20% or 23%, the reflectivity of the silicon wafer was very close. The reasons were that the solubility of Na2CO3 at room temperatures has reached saturation at about 20%, and the dissolution could only continue when the temperature increased and by rapid stirring. The Na2CO3 solution with concentration of 23% would crystallize during the experiment. In addition, when the concentration reached 20%, the surface pyramid was already very uniform, and when the concentration of the Na2CO3 solution was increased, the surface microstructure no longer changed significantly.
 
The preliminary results showed that when the sputtering power was 135W and substrate temperature was 150℃, the flow rate of borane (diluted with H2, 99.5% H2 and 0.5% BH3) was 10Scem, hydrogen 12 Scem, and argon At 5Sccm. The short-circuit current and open-circuit voltage of Si:H prepared on an untextured single crystal silicon substrate were 0.2mA and 0.32V. Using a mixed solution of 20% NaCO3 and 4% NaH-CO3 at 95℃for 20 minutes, the reflectivity of the Si:H cell prepared on the single crystal silicon with a textured structure used as the substrate dropped from 34.3% to 27.6%. The short-circuit current and open-circuit voltage were 0.4mA and 0.24V. It can be seen that texturing could effectively increase the short-circuit current of Si:H solar cells.
 
 
Figure 2 The influence of Na2CO3 with concentrations of 15%, 20%, 23% on the surface reflectivity of the silicon wafer (The corrosion temperature was 90℃, and the time was 20 minutes.)