Solution processing of thin films for solar cell applications: CuIn(S,Se)2, Cu(In,Ga)(S,Se)2 and ZnO:Al
2016-11-21T11:01:59Z (GMT) by
Cu(In,Ga)(Se,S)2 (CIGS) solar cells have attracted a lot of attention due to their high performance and the prospect for lower manufacturing costs over conventional crystalline silicon solar cells. All recent record efficiency CIGS absorbers have been deposited using vacuum processing which introduces high manufacturing costs. CIGS can also be compatible with low cost, atmospheric processing which can significantly reduce manufacturing costs. Recently, there has been some progress in developing atmospheric solution-based processes for CIGS. Among different solution approaches, deposition of molecular precursors can be advantageous in terms of simplicity and straightforward compositional control. Nonetheless, the developed methodologies involve highly toxic reagents or large impurity content in the device, limiting the potential for commercialisation. This thesis describes the development of a novel solution-based approach for the deposition of CIGS absorber layers. Metal chalcogenides are used as the starting precursors, which are free from detrimental impurities. These compounds contain strong covalent bonds and, consequently, they are insoluble in common solvents. Until recently, hydrazine, which is highly toxic and explosive, was the only solvent to effectively dissolve these types of precursors, limiting the feasibility of this approach for industrial applications. In this work, metal chalcogenides are dissolved in a safer solvent combination of 1,2-ethanedithiol and 1,2-ethylenediamine, completely eliminating hydrazine from the process. By using this solvent system, optically transparent solutions are formed which exhibit long-term stability. The precursor solutions are decomposed cleanly and they are converted to single phase CIGS upon selenisation. CuIn(S,Se)2 solar cells with power conversion efficiencies up to 8.0% were successfully fabricated by spray depositing the precursor solution, followed by a selenisation step. This progress has been made by continuously optimising the deposition, drying, and especially the selenisation configuration. Among other parameters, the working pressure during selenisation was found to have a dramatic effect on the material crystalline quality. Rapid thermal processing was also explored as an alternative selenisation configuration to tube furnace annealing and it was shown to improve the back contact/absorber interface. It has been demonstrated that Ga can easily be incorporated in the absorber for band-gap tuning and, consequently, for VOC enhancement of the solar cells. The structural properties of the films were investigated with Ga content, as well as the opto-electronic characteristics of the corresponding solar cells. The band-gap of the material was conveniently varied by simply adjusting the precursor ratio, allowing for fine compositional control. By using this technique, Cu(In,Ga)(Se,S)2 solar cells with conversion efficiencies of up to 9.8% were obtained. The solar cell performance in this work is limited by the porosity of the absorber and the back contact quality. Despite a significant improvement during the course of this work, the remaining porosity of the absorber causes selenium to diffuse towards the back forming a thick MoSe2 layer and causing a high series resistance in the device. A low cost, solution-based technique was also developed for the deposition of aluminium-doped zinc oxide films that can be used as the transparent conductive oxide layer in thin film solar cells. This methodology involves the use of an ultrasonic spray pyrolysis system, which is a very versatile and easily controlled deposition technique. Although the presence of oxygen makes the film closer to stoichiometric (fewer oxygen vacancies) good electronic and optical properties have been obtained by process optimisation. Films deposited with optimum conditions exhibited a sheet resistance of 23 Ω/sq, which can be further reduced by increasing the thickness with minimal transmittance losses. The simplicity, low toxicity and straightforward control make the proposed methodologies extremely potential for low cost and scalable deposition of thin film solar cells.