Fabrication and characterization of elongate solar cells

Abstract

This thesis concerns fabrication of elongated silicon solar cells, together with development and application of supporting characterization techniques. Analysis of J-V curves can reveal considerable detail about recombination in a solar cell. In Chapter 2, the effects of the major recombination mechanisms on the J-V curve are described, taking into account surface, bulk and depletion region. The local ideality factor (m) is derived analytically as a function of operating voltage, and shown to have a value of 2.0, 0.67-0.76, and 1.0 for SRH, Auger and Radiative recombination in high injection respectively. The role of depletion region recombination is discussed. Elongate solar cells are cells that are long (cm), narrow (mm) and thin (tens of microns). The detailed process design and procedure for the fabrication of "plank" elongate silicon solar cells is presented in Chapter 3. Initial batches were tested to have poor open circuit voltage and fill factor which were determined to be caused by pinholes in masking dielectric layers, and dielectric damage caused by laser ablation. Chapter 4 is devoted to characterization of the difference between utilizing a Q-switch Nd:YLF laser or a spindle dicing saw for groove initiation in the formation of plank solar cells. Characterization of the damage was performed using etch rates of the cut, photoluminescence imaging and quasi-steady state photoconductance measurement. Damage recovery etch was also investigated, and it was demonstrated that it is possible to recover above 90% of effective lifetime with sufficient damage etch in a hydroxide etch solution. Due to the position of metal contacts on the edges of elongate solar cells, generated currents have to flow laterally to the edges to be collected. This causes the internal resistance of the device to vary significantly with operating conditions, making it more difficult to quantify accurately. In Chapter 5 the different methods of series resistance (Rs) measurement is reviewed with particular attention to the multi light method due to its ability to accurately measure Rs as a function of voltage. Rs measurements on elongate cells of various emitter sheet resistivity demonstrated that a high emitter sheet resistance causes large Rs variation with operating voltage, and can double between Rs(Voc) to RS(VMPP). It is demonstrated that an Rs-corrected J-V curve can be extracted, which is shown to have good correlation with the Jsc-Voc method. The method of Rs(V) extraction is shown to be generally applicable. In Chapter 6, the developed characterization techniques are applied to determine the causes of fill factor loss in elongate cells. Show more