Advanced Laser Processing and Photoluminescence Characterisation of High Efficiency Silicon Solar Cells

Abstract

Many current technologies used in solar cell fabrication have been successfully adapted from the integrated circuits industry. The success of laser processing applications in this industry indicates that such techniques should be considered to reduce manufacturing costs and to improve electrical efficiency of solar cells. This thesis examines the application of advanced laser processing to improve the electrical performance and reduce manufacturing costs of solar cells. It focuses on several different aspects of laser processing; (1) understanding and characterising the effect of laser pulses on silicon, (2) developing new fabrication technologies and (3) integrating laser processes with working solar cell devices. The thesis begins with an overview of existing solar cell structures that incorporate laser processing. A study is then presented that explores the detrimental effects of laser processing, how it can be avoided and how to characterise its influence on solar cell electrical properties. Experimental results combine Yang defect etching, photoconductance decay measurements and a new technique of photoluminescence imaging to isolate the influence of laser ablation and laser melting on silicon wafers. This understanding is used in the development of several laser processes. A laser texturing technique is developed to texture the surface of multicrystalline wafers that cannot be effectively textured with the alkaline etches used on single crystal material. Three advanced laser contacting schemes; laser micro contacts, laser defined aluminium electrodes and laser doping, are assessed as techniques to improve cell efficiency and to reduce fabrication costs. In the final chapter the integration of laser processing with solar cell devices is demonstrated through the fabrication and characterisation of n-type double-sided solar cells with laser doped contacts. Efficiencies of up to 17.4% with an open circuit voltage of 672 mV are reported. This thesis also presents the application of a new characterisation technique, based on photoluminescence, to aid in improving both new and existing fabrication technologies. The work presented in this thesis demonstrates the applicability of advanced laser processing to solar cell fabrication and shows how laser processes can be used in a variety of ways to improve the electrical performance and reduce the fabrication complexity of solar cell devices.