Designing amorphous silicon solar cells for their optimal photovoltaic performance
Creators: name: Stulik, Pavel
Publisher: name: Charles Darwin University
Year: 1998

description: The technology of fabricating amorphous silicon (a-Si:H) solar cell has ad-vanced significantly in the last two decades. Now even a single junction solar cell of efficiency more than 13% has been produced in the laboratory. However, the degradation problem in a-Si:H photovoltaic devices is yet to be resolved. In this project the admittance analysis method is used to model both single junction and tandem structure a-Si:H solar cells to optimise their photovoltaic performance and also to study the degradation problem. The optical admittance analysis method allows one to calculate the optical properties of any multilayer structures, such as thin film solar cells, which consist of a few thin layers. In order to enhance the photovoltaic performance of a multilayer structure device, one has to increase the absorbance in some layers and minimise it in others. Accordingly in a-Si:H solar cells we want to maximise absorbance in the i-layer and minimise it in the other layers. One can calculate reflectance, transmittance and absorbance of any multilayer system using the admittance analysis method, which requires the complex refractive indexes of all its layers as input parameters. As the complex refractive index depends on the wavelength (λ) of the incident radiation, the reflectance, transmittance and absorbance calculated in this way also depend on λ. In addition, these optical properties depend on the thicknesses of all the layers in a structure as well. One can then vary the thicknesses of various layers to study the influence on the absorbance in any particular layer. The admittance method is extended here to study the influence of a second transparent conducting oxide (TCO) on the photovoltaic performance of a single junction cell with the structure glass/TCO/p-i-n/TCO/rear metal. Using this design, the thicknesses of both the top and bottom TCO layers are optimised for two types of rear metal electrodes; silver and aluminium. The introduction of the bottom TCO shows some effect on the performance of a-Si:H solar cells with aluminium (Al) as rear metal contact, but no significant change is found for solar cells with silver (Ag) as rear metal contact. It is also found that the use of glass of higher refractive index can make slight improvement in the performance of a-Si:H solar cells. Using the admittance analysis method, the optimal design of a single junction a-Si:H solar cell is suggested and its photovoltaic parameters are calculated. The technique is then extended to design a tandem structure of two cells stacked one on the top of the other and connected in series. The top cell is considered to be made of a-Si:H and bottom of a-SiGe:H alloy and the condition of current matching is applied to determine the tandem's optimal design. In this design it is assumed that all photogenerated charge carriers contribute to the photogenerated current, which means that the collection efficiency, X, is assumed to be unity. The theoretical efficiency of the single junction cell with the optimal design thus obtained is 13.1% and that of the tandem cell with the perfect current matching is 20.8%. An alternative approach of calculating the photogenerated current in a tandem cell is developed and applied. The photogenerated current is expressed as light current minus dark current and the light current is derived by solving the continuity and Poisson equations. The admittance analysis method is used here to calculate the absorbance in the i-layers and the optically limited current. The condition of current matching is fulfilled by varying the thicknesses of the i-layers in the top and bottom cells, which enables one to design a tandem of two cells for its optimal photovoltaic performance. Instead of assuming the collection efficiency to be unity, an analytical cx-pression for the collection efficiency incorporating the effect of recombination at dangling bonds is also derived as a function of the defect density and the thickness of an a-Si:H thin film. It is assumed in the derivation of the collection efficiency that the generation rate of charge carriers is constant through the whole thickness of the i-layer. The dangling bonds are considered to be in three charged states D0, D- and D+ representing neutral, negatively charged and positively charged, respectively. The technique is then applied to calculate the short circuit current (Jsc) in a single junction a-Si:H solar cell using collection efficiency thus obtained. Our method enables us to calculate the optimal thickness (the thickness at maximum Jsc) of the i-layer at a given defect density. The technique is then extended with the incorporation of the current matching condition to design a tandem structure of two cells. Finally, the absorption profile of the i-layer is calculated using a modified admittance analysis method. The absorption profile produces a more realistic generation rate for the charge carriers in the i-layer and hence one does not have to use the constant generation rate, for calculating the collection efficiency as clone in chapter 7. The collection efficiency, thus calculated is expected to be more accurate in comparison with the previous ones calculated either using a constant generation rate or an empirical exponential function for the generation of charge carriers throughout the i-layer. The collection efficiency is then used to obtain the short circuit current as a function of the thickness of the i-layer as done earlier. The influence of light induced defects on the short circuit current is also studied with a view to minimise the effect of degradation in a-Si:H solar cells. It is expected that the results obtained in this thesis will help designing both single junction as well as tandem structure a-Si:H solar cells for higher efficiency and stability. Therefore the outcome of this project is going to provide valuable information prior to the fabrication of a-Si:H solar cells.