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Abstract
Global photovoltaic (PV) market harvested rapid growth annually during the past two decades. As the primary part of this rapid growth, most of Silicon (Si) wafer-based solar cells employ screen printing(SP) technology for metallization on both polarities of P-type CZ Si wafers and the solar cell efficiency is around 18%. This efficiency is significantly lower than that of high efficiency solar cells developed in research laboratory. The fabrication of high efficiency solar cells in laboratory usually involves multiple expensive processes and high quality silicon material which are not suitable for production. Compared to the complicated and expensive laboratory process for high efficiency solar cell, SP technology is a simple and robust process. However, such technology also has some disadvantages such as poor blue response, high shading issue, low aspect ratio of metal fingers and insufficient rear surface passivation.Selective emitter (SE) technology is one possible way to further increase solar cell efficiency by harvesting more blue light. SE technology is of high interest to the PV industry in recent past. One of the cost effective method of achieving SE is by using laser doping. The laser doped selective emitter (LDSE) solar cell developed at the University of New South Wales (UNSW) combines laser doping and self-aligned light induced plating (LIP) technologies which make it one of the most feasible solutions for industrial selective emitter solar cell structure. The main advantage of LDSE technology is the simultaneously creation of dielectric layer patterning and localized heavy doping without an extra high temperature process or any other masking processes which are required by other technologies. In this thesis, LDSE technology was employed to improve the conversion efficiency of solar cells using a commercial available continuous wave (CW) green laser. The basic laser theory was reviewed and laser induced defects by CW laser were studied. The impact of different laser parameters and dielectric layer combinations on the morphology of laser scanned region was investigated. The process optimization of standard LDSE solar cells was presented. The focus of optimization was given to two key processes: laser doping and light induce plating in the LDSE solar cell process. Wide ranges of parameters were investigated in detail for each process, such that systematic improvements were demonstrated. The influences of different parameters on final solar cell devices were studied through the use of techniques such as light I-V curve, spectral response, scanning electron microscope, focus ion beam etc. As a result of optimizations, a final solar cell device with efficiency >19% was achieved on p-type 1 Ω cm Czochralski (CZ) silicon wafer.The other very important aspect of improving solar cell efficiency is the rear surface design. The aluminum (Al) back surface field (BSF) used in SP solar cells only has moderate passivation effect. Such Al BSF was identified as a limiting factor for standard LDSE solar cells. To overcome this problem, a stack dielectric layer of silicon dioxide (SiO2) and silicon oxynitride (SiOxNy) was developed. Such stack layer demonstrated excellent surface and bulk passivation ability on commercial grade p-type 1 Ω cm CZ Si wafer with carrier lifetime of 670µs and implied open circuit voltage (iVoc) of 735 mV. The significance of rear surface passivation has been realized by researchers worldwide as a key approach for high efficiency solar cell designs and several dielectric layers were successfully developed to passivate Si surface and get high carrier lifetime. However, challenge lies in making local contact opening on the dielectric layers or forming local BSF (LBSF) without massive jeopardize passivation ability using industry suitable approach rather than expensive photolithograph which is normally employed in laboratory. As a local heating process, laser doping could achieve dopants diffusion and dielectric layer patterning in step without causing massive degradation in the passivation ability of dielectric layer. In this thesis, CW laser system was used to create double sided laser doped (DSLD) solar cell by performing phosphorous and boron laser doping to front and rear surface of industry p-type CZ Si wafer passivated by SiNx/SiO2 stack layers on the front surface and SiO2/SiOxNy stack layers on the rear surface. By optimizing the thermal stability of the dielectric layers, emitter sheet resistance and the laser doping parameters, over 700mV iVoc was achieved on CZ Si samples after laser process, prior to metallization. As proof-of-concept, DSLD solar cells were made on CZ samples. Open circuit voltage (Voc) in the range of 660mV was achieved with high short circuit current density. However, at this stage, the solar cells efficiency is limited by low fill factor (FF). The origin of low FF was investigated and possible causes and solutions were discussed. At the end of this thesis, DSLD solar cells with Voc of 680mV was achieved on p-type 1 Ω cm CZ silicon wafers by optimizing rear surface metallization process.