Book/Dissertation / PhD Thesis

FZJ-2020-02592

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Ab initio perspective on hydrogenated amorphous silicon for thin-film and heterojunction photovoltaics

Czaja, P. (Corresponding author)FZJ*

2020
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich
ISBN: 978-3-95806-474-4

Please use a persistent id in citations: http://hdl.handle.net/2128/25480  urn:nbn:de:0001-2020081215

Abstract: Hydrogenated amorphous silicon (a-Si:H) has applications in photovoltaics as an absorber material in thin-film solar cells and as a passivation material in silicon-heterojunction cells, where it forms an interface with the crystalline silicon (c-Si) absorber. The physical processes occurring at this interface have crucial impact on the characteristics of the entire photovoltaic device. The key to improving the solar cell performance lies therefore in the optimization of the interface, in particular with respect to its transport and recombination properties. This requires a profound understanding of the microscopic structure of a-Si:H and a-Si:H/c-Si interfaces, and of its effect on the macroscopic properties relevant for photovoltaics, such as absorption, optical and mobility gap, band offsets, and local density of gap states. In this thesis we present an ab initio study that seeks to provide insight into the atomic and electronic structure of bulk a-Si:H and a-Si:H/c-Si interfaces, extract the relevant electronic and optical properties, and explore the computational limitations that have to be overcome in order to arrive at a predictive ab initio simulation of the silicon hetero junction. In the first step bulk a-Si:H is investigated, for which we use atomic configurations of a-Si:Hwith 72 and 576 atoms, respectively. These were generated with ab initio molecular dynamics, where the larger structures are defect free, closely matching the experimental situation and enabling the comparison of the electronic and optical properties with experimental results. Density functional theory calculations are applied to both configurations in order to obtain the electronic wave functions. These are analyzed and characterized with respect to their localization and their contribution to the density of states, and are used for calculating abinitio absorption spectra of a-Si:H. The results show that both the size and the defect structure of the configurations affect the electronic and optical properties and in particular the values of the optical and mobility gap. These values can be improved by calculating quasi particle(QP) corrections to the single-particle spectra using the G$_{0}$W$_{0}$ method. Thereby we find that the QP corrections can be described by a set of scissors shift parameters, which can also be used in calculations of larger structures. The analysis of individual contributions to the absorption by evaluating the optical matrix elements indicates that strong localization enhances the optical coupling, but has little effect on the average transition probability, for which we find a dependence E$^{2}$+ const on the photon energy E, irrespective of the nature of the initial or final state. In the second step the previously analyzed defect-free a-Si:H structure is combined with c-Si to form a realistic a-Si:H/c-Si interface structure, which undergoes a high-temperature annealing in order to obtain a very low defect density. Throughout the annealing, we monitor the evolution of the structural and electronic properties. The analysis of the bonds by means of the electron localization function reveals that dangling bonds move toward the free a-Si:H surface, leaving the interface region itself completely defect free. The hydrogen follows thi smovement, which indicates that in the case under consideration, hydrogen passivation does not play a significant role at the interface. A configuration with a satisfactory low density of defect states is reached after annealing at 700 K. A detailed characterization of the electronic statesin this configuration in terms of their energy, localization, and location reveals that, despite the absence of dangling bonds near the interface, localized interface states still exist, lying mostly below the conduction band edge from where they seem to move deeper into the gap throughout the annealing. The quantitative description of electronic localization also allows for the determination of the a-Si:H mobility gap, which, together with the c-Si band gap, yields band offsets that are in qualitative agreement with experimental observations. We find, however, that the error in determining the band edges is too large for an accurate calculation of the band offsets, and can be decreased only by using larger configurations.

Note: Dissertation, Universität Mainz, 2019

---

Contributing Institute(s):

1. Photovoltaik (IEK-5)

Research Program(s):

1. 899 - ohne Topic (POF3-899) (POF3-899)

Appears in the scientific report 2020

---

Database coverage:
;

---