Silicon-based solar cells Characteristics and production processes ...

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Silicon-based solar cells – characteristics and production processes I. Chemical etching and surface texturization The mc-Si and Cz-Si silicon plates obtained from the producer have a defected surface, down to the depth of ~ 5 µm, formed as a result of diamond or wire saw cutting. The two-sided elimination of the defected layer is performed in the process of chemical etching in a KOH solution. In order to reduce the reflection coefficient R ref , the surface of the plate is covered with texture. In the case of Cz-Si, the surface texturization is performed in the chemical process at the temperature of 80 o C for the time of 30 min. In a KOH:IPA:H 2 O solution, the texturization of the mc-Si plates is conducted on the basis of the HF:HNO 3 :H 2 O acid solutions. In the next stage, which aims at the elimination of organic and metallic impurities, as well as natural oxides, the Si plates are placed in baths of H 2 SO 4, HCl and HF solutions, respectively. Between each stage, the plates are rinsed in deionized water of resistivity not lower than 5 MΩcm. The reagents used in these processes must be in the chemical purity class of ”analytically pure”. Because of the applied types of chemical solutions, the plates are placed in teflon holders. Fig. 12. Chemically prepared silicon pates placed in teflon holders (left) and chemical etching process (right). The chemical processes performed before producing the p-n junction aim at the elimination the defected layer, a formation of a surface texture which reduces the coefficient of reflection from the silicon surface, as well as the cleaning of the surface of all impurities and oxides. Thus prepared Si wafer is placed in the reactor of a diffusion furnace. 29
Silicon-based solar cells – characteristics and production processes Fig. 13. Mc-Si silicon wafer, 25 cm 2 in surface area, after texturization in KOH solution, microphotograph of the area between grains, microphotograph of mc-Si after texturization in HF:HNO 3 :H 2 O, Cz-Si plate with crystallographic orientation (100) obtained from the producer, with a part of the surface texturized in KOH, microphotograph of texture (from left to bottom). II. Diffusion of phosphorus impurity and p-n junction production The p-n junction in the base silicon with the p-type conductivity is produced through diffusion of phosphorus atoms in the Si surface, the former originating from phosphorus oxychloride POCl 3 . This is one of the most important stages of the cell production, determining the impurity concentration profile, which directly affects the process of generation and recombination of the charge carriers and the series resistance of the front electrode contact to silicon. The Si plates in the quartz holder are placed in a heated quartz reactor which is provided with protective gas N 2 and reactive gases POCl 3 and O 2. At the temperature above 800 o C, in the presence of oxygen, a dissolution of the POCl 3 occurs and, at the same time, phosphosilicate glass xSiO 2 ∙yP 2 O 5 (PSG) is formed on the Si surface, which further constitutes a local source of phosphorus diffusing in silicon. This process takes place according to the following dependences [55]: 4POCl 3 + 3O 2 → 2P 2 O 5 + 6Cl 2 (18) 2P 2 O 5 + 5Si → 5SiO 2 + 4P (19) 30
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Silicon-based solar cells Characteristics and production processes ...

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