Effect of Impurities and Defects on Performance and Degradation of Solar Cells

Abstract

The solar photovoltaic (PV) industry continues to be one of the world’s fastest growing industries. The global industry ended 2008 with over 8 GW of PV modules manufactured. Any mature solar cell technology seems likely to evolve to the stage where costs are dominated by those of the constituent materials. First generation solar cells had high production costs with moderate efficiency. Second generation cells offer much lower overall production cost, but efficiencies are even lower. Third generation cells aim at high efficiency and slightly higher production costs. Therefore, different materials and technologies in PV manufacturing have been used in an attempt to reduce manufacturing costs. Therefore, research in the field of solar cells is continuously increasing, in order to fabricate high efficiency cells and using inexpensive materials and technologies. In this dissertation, defect’s investigation has been performed in solar cells. For this purpose, I report on research on different kind of solar cells by the means of characterization, analysis and simulation. In the third generation solar cells, impurity photovoltaic effect (IPV) was suggested some time ago as a new approach to improve solar cell performance, especially the short-circuit current, by using sub-band gap photons and extending the infrared response. In this thesis, the IPV effect in silicon solar cells with indium impurity level was reinvestigated, new results are given using the software scaps, which was extended for this purpose in a joint work between the two laboratories, ELIS (Gent, Belgium) and LPDS (Béchar, Algeria). Also, for the first time to our knowledge, IPV effect was investigated in wide band gap material (GaAs) with more than one impurity level in the band gap of the semiconductor. New results have been obtained and published, showing the possibilities and also the limitations of the IPV effect in solar cells. Furthermore, some selected problems have been analyzed during my research by use of numerical simulation. GaAs concentrator solar cells offer the possibility of reducing the panel cost by using higher power incident on the cell. However, the use of concentrator increases the temperature of the solar cell, which reduces the cell efficiency if no special measures are taken for cooling. In this perspective, the effect of concentration and temperature on GaAs solar cell performance has been analyzed by simulation. Micro thin film solar cells are good candidates for built-in power sources for a new generation of MEMS (Micro-Electro-Mechanical-Systems). They have the advantage of being small and light weight, and above all, a high output voltage can be obtained with cell sizes on the order of micrometers. For that reason, a number of single solar cells are connected in series to produce the required high voltage. However, in this kind of cells (small area mesa diode), the current due to perimeter recombination is very important and can reduce the cell output. In this thesis, the perimeter recombination effect was investigated in GaAs-based micro-solar cells. The calculations obtained by scaps software were modified to take into account the perimeter recombination effect which is a two dimensional effect. The use of GaAs pin diodes in photovoltaic application has been recently the focus of a lot of investigations, some of them being in multiple quantum well solar cells. GaAs solar cells with different intrinsic layer thicknesses, were fabricated at the Institute of electronic structure and lasers of the University of Heraklion (Greece), and then sent to our University for electrical characterization. Measurement numerical analyses have been carried out on these cells in order to find a correlation between the quality and the thickness of the intrinsic layer and the device performance. Thin film solar cells are attractive because they could produce electricity cheaper than conventional silicon solar cells. Absorber materials based on amorphous and microcrystalline silicon (a-Si, μc-Si), on copper indiumgallium diselenide (CIGS), and on cadmium telluride have been considered as the prime candidates for thin film solar cells. The challenge is to reliably and cheaply produce thin film solar cells at large scale and with a decent efficiency performance. Some of these materials show excellent promise in term of efficiency (this is the case for CIGS), whilst others excel in the ease of large scale producibility (the case of a-Si and CdTe) . For example, CIGS solar cells have shown the highest efficiency of any thin film cell with 20 % efficiency on a glass substrate, thereby largely exceeding the efficiency of present day commercial silicon solar panels (η = 15 – 16 %). But, it remains difficult until now to reach this level of efficiency in mass-produced cells. Flexible solar cells offer distinct advantages over cells on rigid glass substrate, in terms of manufacturability and application possibilities, and this has motivated quite a lot of studies of these cells. However, the substitution of a glass substrate by an alternative material results in reducing the device performance, for both the low-temperature process on polyimide and the high-temperature process on metal substrates. Metallic foil substrates offer some attractive features when compared to plastic foil substrates, the most important being that they can withstand the high processing temperatures needed to optimize the CIGS materials quality. However, unwanted diffusion of impurities from the metal substrate to the CIGS cell should be avoided or be under control. Increasing activities to develop and to fabricate flexible CIGS-based devices have been observed over the past years. PVFlex Solar GmbH is a German company specializing in development and manufacturing of flexible light weight solar panels. In the framework of a European project, we received several flexible CIGS thin film solar cells grown on different metallic foils with and without barrier layers. The substrate structures were completed to CIGS solar cells by the Helmholtz Zentrum Berlin (HZB; formerly called Hahn Meitner Institute, HMI). Measurements and simulation have been carried out on these cells in order to find a correlation between the properties of the CIGS devices and the substrate treatment. Besides the electrical characterization, additional measurements have been performed on these cells such as XPS and DLTS characterization to investigate the nature of defects existing in these cells, and limiting or not their performance.