Transparent passivating contact for crystalline silicon solar cells
| Autor: | Köhler, Malte |
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| Jazyk: | angličtina |
| Rok vydání: | 2020 |
| Předmět: | |
| DOI: | 10.18154/rwth-2021-07000 |
| Popis: | Dissertation, RWTH Aachen University, 2020; Jülich : Forschungszentrum Jülich GmbH, Zentralbibliothek, Verlag, Schriften des Forschungszentrums Jülich. Reihe Energie & Umwelt = Energy & environment 538, 1 Online-Ressource (186 Seiten) : Illustrationen, Diagramme (2020). = Dissertation, RWTH Aachen University, 2020 The goal of this work is to develop a transparent, passivating and conductive contact for the light facing side of crystalline silicon solar cells. State of the art passivating contacts show a very high passivation quality of the silicon surface as well as a high electrical conductivity. However, due to their restricted transparency and comparably high parasitic absorption for the incoming sunlight these contacts are not ideal for the use on the sun facing side of the solar cells. With the aim of increasing the efficiency of crystalline silicon solar cells, the need for a transparent passivating contact arises. One material, which is suitable as a transparent passivating contact due to its high transparency and electrical conductivity, is n-type doped microcrystalline silicon carbide (mc-SiC:H(n)). It was shown in literature, that depositing mc-SiC:H(n) using hot-wire chemical vapor deposition (HWCVD) directly on the crystalline silicon surface leads to a deterioration of the passivation. Additionally it was shown, that using a thin silicon oxide (SiO2) in between the crystalline silicon and the mc-SiC:H(n) can prevent this deterioration of the passivation while showing a high transparency and high electrical conductivity. However, transferring these properties of the contact layer stack into a first working solar cell proved to be difficult. Despite the high passivation quality and the high conductivity of the material, neither the desired voltage nor a high fill factor could be achieved on solar cell level. The focus of this thesis is therefore on the systematic implementation of this layer stack in a silicon heterojunction solar cell. In the first part of this work, different wet-chemical oxidation processes for the formation of SiO2 are evaluated in order to enable a high passivation quality. It is shown that different oxidation processes have an impact on the properties of the SiO2 (thickness, stoichiometry, density). These properties then influence the passivation of the silicon surface after deposition of the mc-SiC:H(n). Furthermore, the results show that in addition to the oxidation process of the oxide, the wire temperature during the deposition of the mc-SiC:H(n) has a decisive influence on the passivation quality. The second part of this work includes an analysis of the electrical and optical properties of mc-SiC:H(n) as a function of the wire temperature used during the HWCVD process. Additionally, the influence of the wire temperature on the passivation quality and on the contact resistivity of the SiO2/mc-SiC:H(n) stack is investigated. It is shown that an increase in wire temperature increases the transparency and conductivity of the mc-SiC:H(n) and decreases the passivation quality of the contact. However, in order to achieve high power conversion efficiencies in the solar cell, the contact layers need to have high conductivity, which is reflected in low contact resistivity, and high passivation quality. To overcome this challenge, a double layer mc-SiC:H(n) was used. The layer in direct contact with the SiO2 was deposited at low wire temperature to ensure high passivation quality. The second layer was deposited at high wire temperature to provide low contact resistivity and high transparency. In the third part of this work, the insights from the first two parts are used for the preparation of solar cells. It is shown that using a mc-SiC:H(n) double layer stack instead of a single layer, mainly reduces the contact resistance and thus increases the fill factor of the solar cell (see Figure 0.1, green to yellow triangle). Furthermore, it is discussed that the deposition of the transparent conductive oxide (TCO), which is necessary for the lateral conductivity of the free charge carriers to the contact fingers of the solar cells, subsequently damages the passivation of the solar cell. Following an investigation of the mechanisms of sputter degradation, an optimization of the deposition conditions and the thermal post-treatment can significantly reduce the damage. This leads to an increase in the open circuit voltage of the solar cell (see figure 0.1, yellow to blue triangle). The additional use of a magnesium fluoride layer to reduce the reflection of the contact results in an increase of the short circuit current of the solar cell (see Figure 0.1, blue to red triangle). Due to this optimization of the contact, the highest efficiency (η) of this work is achieved with 24%. This means an increase in efficiency of 6.4%abs within this work. Published by Forschungszentrum Jülich GmbH, Zentralbibliothek, Verlag, Jülich |
| Databáze: | OpenAIRE |
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