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Optimization of fabrication steps for n-type c-Si solar cells

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2019
Orhan, Efe
Crystalline silicon (c-Si) solar cells fabricated on p-type wafers are still dominating the photovoltaic (PV) industry due to advantages in device processing and early focus on p-type cells in the development phase of the industry. Over the years, studies on n-type Czochralski (CZ) substrates have shown that they can be more desirable for the terrestrial applications due to superior material and process advantages such as higher minority carrier lifetime, easier passivation of the surface, absence of light induced degradation (LID) and low sensibility to metallic impurities compared to p-type substrates. With these advantages, n-type CZ based c-Si solar cells with ultimate energy conversion efficiency hold a great potential in the future PV industry. The goal of this thesis is to optimize boron doping, passivation of boron emitter surface and metallization processes for fabrication of n-type c-Si solar cells. For boron doping, boron trichloride (BCl3) was used as a gas precursor with LYDOPTM system designed by SEMCO engineering. In order to obtain proper sheet resistance distribution for boron emitter during fabrication, uniformity of borosilicate glass (BSG) formation during high temperature boron diffusion was investigated. Also, most boron diffusion technologies result in the formation of an undesirable layer at the Si interface which is called as boron-rich layer (BRL). Three different methods were used for removing BRL, namely low temperature oxidation (LTO), chemical etching treatment (CET) and nitric acid oxidation of silicon (NAOS). To fabricate n-type c-Si solar cells, one side of the Si wafer should be protected by a proper masking during the diffusion process. Hence, masking property of silicon dioxide (SiO2) was investigated for boron diffusion. In addition, to increase the minority carrier lifetime by passivated boron doped surface, aluminum oxide (Al2O3) passivation and aluminum oxide/ silicon nitride (Al2O3/SiNx) stack passivation layers were deposited onto surface of the solar cells using atomic layer deposition (ALD) and plasma enhanced chemical vapor deposition (PECVD) techniques. Finally, aluminum was deposited on p+ emitter by thermal evaporation technique to obtained low contact resistance. This thesis provides useful information and contributes to research and development activities on the formation of boron emitter that is the most problematic step in the fabrication of high-efficiency n-type c-Si solar cells.