Studies of the optimal structure and photovoltaic performance of thin film hydrogenated amorphous silicon (a-Si : H) solar cells
Creators: name: Zhu, Furong
Publisher: name: Charles Darwin Univeristy
Year: 1993

description: A thin film hydrogenated amorphous silicon (a-Si:H) solar cell is basically a multilayer thin film semiconductor device. The characteristic of such a multilayered thin film device is related directly to its configuration and optical an electronic properties of its individual layers. Usually a-Si:H thin film solar cells are produced using semi-empirical methods to determine the thickness of individual layers. In order to design low cost, stable and efficient thin film aSi:H solar cells, however,it is necessary to understand the optimal structure, i.e. the optimised thickness of each layer, and the role which each layer plays in the photovoltaic performance of the cell. The purpose of the present work is to study the optical and electronic properties, and optimal structure of a-Si:H solar cells. The theoretical techniques developed in this project can be expected to provide in advance the leading design information of a thin film solar cell with an optimised structure for a possible desired high conversion efficiency. A comprehensive optical admittance method is applied to study the optical properties of amorphous silicon thin film solar cells. The method is applied to Schottky barrier solar cells of the type TCO/Au/a-Si:H/rear contact, and p-i-n type solar cells of the type TCO/p/i/n/rear contact considering both of them as multilayer systems composed of absorbing and non-absorbing layers. The numerical technique uses experimental results of refractive index n(k) and extinction coefficient k(k) to calculate the optical absorbance and reflectance of such cells. The interference absorbing peaks thus obtained in the absorbance of a multi-layered cell are found to agree well with experimental results. Our results reveal that a high reflecting rear metal contact increases the spectral absorbance in the wavelength region 0.60 - 0.70 /*m, and peak positions in the absorption spectrum depend on the cell thickness. The optimised TCO parameters are found to be almost independent of the thickness of a-Si:H layer. The optimisation of the thickness of p layer in a p-i-n type solar cell is also discussed. This method can be applied for calculating the optical properties of any thin film solar cell. It is expected that, using the present method, one can design a solar cell of an optimal efficiency. It is found that by maximising the integrated absorbance in an optimal cell structure of a cell, one can increase the short circuit current density by 5.3%. We have further extended the optical admittance analysis method to design the structure of multi-junction a-Si:H thin film solar cells. The optimised thicknesses of the individual layers in any tandem as well as single junction thin film a-Si:H solar cells can be obtained. By minimising the reflectance using an optimal anti-reflection coating layer and selecting a suitable rear contact material, a significant increase in the photon collection efficiency can be achieved at long wavelength region. The effect of the variation in the thickness of different layers on the performance characteristics of a cell is discussed. The calculated results and analyses show that the present theoretical approach can be used directly to design any thin film solar cell with an optimised structure for a desired high efficiency. In addition, a detailed study of the relationship between band tail widths and photovoltaic performance of a p-i-n a-Si:H solar cell is presented. It is quantitatively found that the primary effect of the bandgap states is to increase the forward current of the cell, and reduce the voltage output. The influence of the bandgap states and cell thickness on the efficiency of an a-Si:H cell is also studied. Finally, using experimental plasmon loss energy of a-Si the number of valence electrons per unit volume in amorphous silicon thin films is determined. The characteristics of hydrogen incorporation in silicon network is studied by a quantitative model assuming that the structure of a good quality a-Si:H thin film dominantly consists of Si-H and Si-Si bonds only. Using the concept of Penn gap and bond polarizability, then we have derived an expression for the optical energy gap as a function of hydrogen concentration for a-Si:H thin films. The calculated results thus obtained agree very well with the experimental results. The aim of this part of the project is to find a relation between the hydrogen concentration with the resulting optical energy gap and stability of the a-Si:H thus produced.