Publicação
Laser shallow melting of p-type silicon wafers as substrates for tunnel junctions in tandem solar cells
| Resumo: | The fabrication of tandem solar cells requires a specific connectivity between layers, with specific characteristics such as the case of a tunnel junction that is made of two heavily doped narrow layers of different doping types (e.g. Phosphorus and Boron). When a certain amount of voltage is applied to the edges of the tunnel junction, the result is the alignment of both valence and conduction bands of the different subregions, enabling the electron flux from one cell to another without the need to energy variation of the electron. The Gas Immersion Laser Doping (GILD) technique uses a laser as the source of radiation and a dopant gas. A silicon wafer is inserted inside a compartment with a controlled atmosphere saturated with the doping gas, which receives energy from the laser as pulsed beams, melting and solidifying into a very fast process. When the melting occurs, the dopants penetrate the wafer and remain there upon solidification. When a high concentration of the dopant is confined inside an ultra-shallow depth of the wafer, the probability of forming a tunnel junction is increased. Therefore, before implementing this process, it is necessary to study the interaction between the laser and silicon samples. The purpose of this dissertation is to test and pre-select sample areas that were submitted to an infrared laser irradiation, varying on different sets of parameters, such has laser scan speed, number of scans, infrared transparent window type, among others. Samples will be evaluated regarding their topography, in terms of surface and in-depth melt homogeneity. These samples will be used as tests for the objective of the project in which this dissertation is inserted. The final goal of the project is thus to develop a process of tunnel junction formation with recurse to the GILD technique, using POCl3 as the dopant source and to create abrupt n++/p+ doping profiles (with 10 nm wide). The approach should lead to a scalable and low-cost industrial process for forming tunnel junctions directly integrated in tandem solar cells. Optical analysis of laser interaction on the silicon surface can be notable in slower laser scan speed for more than one successive scattering process. The results show an effective laser processed areas in a p-type Cz-Si wafer rather than in an emitter p++ on an p+ Cz-Si wafer. The chamber and the window that confines the sample in a controlled atmosphere affects positively the melting process when using a Quartz material rather than the Polycarbonate window. |
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| Autores principais: | Souza, Yan Ribeiro Cardoso de |
| Assunto: | Interação entre Laser e Silício Células Solares de Silício Junções de túnel Fusão de Silício Teses de mestrado - 2023 |
| Ano: | 2023 |
| País: | Portugal |
| Tipo de documento: | dissertação de mestrado |
| Tipo de acesso: | acesso aberto |
| Instituição associada: | Universidade de Lisboa |
| Idioma: | inglês |
| Origem: | Repositório da Universidade de Lisboa |
| Resumo: | The fabrication of tandem solar cells requires a specific connectivity between layers, with specific characteristics such as the case of a tunnel junction that is made of two heavily doped narrow layers of different doping types (e.g. Phosphorus and Boron). When a certain amount of voltage is applied to the edges of the tunnel junction, the result is the alignment of both valence and conduction bands of the different subregions, enabling the electron flux from one cell to another without the need to energy variation of the electron. The Gas Immersion Laser Doping (GILD) technique uses a laser as the source of radiation and a dopant gas. A silicon wafer is inserted inside a compartment with a controlled atmosphere saturated with the doping gas, which receives energy from the laser as pulsed beams, melting and solidifying into a very fast process. When the melting occurs, the dopants penetrate the wafer and remain there upon solidification. When a high concentration of the dopant is confined inside an ultra-shallow depth of the wafer, the probability of forming a tunnel junction is increased. Therefore, before implementing this process, it is necessary to study the interaction between the laser and silicon samples. The purpose of this dissertation is to test and pre-select sample areas that were submitted to an infrared laser irradiation, varying on different sets of parameters, such has laser scan speed, number of scans, infrared transparent window type, among others. Samples will be evaluated regarding their topography, in terms of surface and in-depth melt homogeneity. These samples will be used as tests for the objective of the project in which this dissertation is inserted. The final goal of the project is thus to develop a process of tunnel junction formation with recurse to the GILD technique, using POCl3 as the dopant source and to create abrupt n++/p+ doping profiles (with 10 nm wide). The approach should lead to a scalable and low-cost industrial process for forming tunnel junctions directly integrated in tandem solar cells. Optical analysis of laser interaction on the silicon surface can be notable in slower laser scan speed for more than one successive scattering process. The results show an effective laser processed areas in a p-type Cz-Si wafer rather than in an emitter p++ on an p+ Cz-Si wafer. The chamber and the window that confines the sample in a controlled atmosphere affects positively the melting process when using a Quartz material rather than the Polycarbonate window. |
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