Research On The Properties Of Boron And Phosphorus Compensated Silicon For Photovoltaic Applications

Solar photovoltaic (PV) power is one of the promising renewable energies. As the main products in the current PV market, crystalline silicon solar cells occupy more than90%of the share, and will continue to dominate for a long time. However, the goal of the industry and academia is to reduce the cost of crystalline silicon solar cells. One of the main strategies is to use low-cost polycrystalline silicon feedstock, such as upgraded metallurgical grade silicon (UMG-Si). However, this material usually contains certain amounts of boron (B) and phosphorus (P), which are not easily removed via metallurgical routes. The most serious issue for UMG-Si is the compensation of electrical dopants.The impact of dopant compensation on the properties of silicon wafers and solar cells is investigated in this thesis. The results are practically of significance for the UMG-Si application in the fabrication of low-cost and high-efficiency solar cells. Below are the innovative results of this thesis.(1) The impact of dopant compensation on the properties of silicon wafers is studied. It is found that dopant compensation will not have a strong influence on the segregation coefficients of B and P. A series of donor-acceptor (D-A) pair luminescence peaks are clearly observed in compensated silicon. The B and P ionization energies in compensated silicon are determined for the first time from the temperature-dependent D-A pair luminescence. With the reduction of the net doping concentration, the carrier lifetime in both p-type and n-type compensated silicon could get improved. Due to the greater ionized impurities scattering, dopant compensation could lead to the reduction on both majority and minority carrier mobilities.(2) The impact of dopant compensation on the electrical properties of crystalline silicon solar cells is systematically studied. Compared to the conventional cells, the compensated silicon solar cells show lower short-circuit current (Jsc), and higher open-circuit voltage (Voc) as well as higher fill factor (FF). As a result, the compensated silicon solar cells could have the same efficiency as the conventional ones. The compensated silicon solar cells suffer from the light induced degradation (LID) more seriously than the conventional ones. As for highly-doped and compensated silicon solar cells, the loss of cell efficiency is up to3-4%. By a following illumination at elevated temperatures, the LID effect in compensated silicon solar cells can be permanently deactivated, and all the electrical parameters are recovered to the original level and will be quite stable under the following illumination at room temperature. The deactivation rate and activation energy depend inversely on the total B concentration, which strongly suggests that B is directly involved in the deactivation of boron-oxygen (B-O) complexes at the solar cell level. At low irradiance intensity, the compensated silicon solar cells generate less electricity than the conventional ones, owing to the strong injection dependence of the carrier lifetime on high concentration of B-O complexes in compensated silicon. At high temperature, the compensated silicon solar cells generate more electricity than the conventional ones, which mainly originates from the lower temperature-variation of the minority electron mobility in compensated silicon.(3) The influence of the compensation level on the properties of silicon wafers and solar cells is demonstrated. As for the same B concentration in compensated silicon, with the compensation level increasing, the hole mobility is significantly reduced while the electron mobility almost keeps constant. The carrier lifetime in compensated silicon limited by both dopants and metal impurities could get improved. Correspondingly, the minority carrier diffusion length increases, causing a higher Jsc. However, the Voc first decreases and then increases, owing to the competition effect of the reduced net doping concentration and increased excess carrier concentration. All these factors could finally result in a higher efficiency solar cell. As for the same net doping concentration in compensated silicon, with the compensation level increasing, both hole and electron mobilities are significantly reduced. Correspondingly, the minority carrier diffusion length decreases, causing a lower Jsc. However, the Voc could get improved, owing to the reduced saturation current of p-n junction. Therefore, with the compensation level increasing, the cell efficiency slightly decreases. Besides, in the case of the same net doping concentration, the higher compensation level solar cell will have a lower temperature-coefficient.(4) The performance of n-type compensated silicon solar cells with the Al-alloyed rear-emitter is studied. It is found that the Voc is strongly influenced by the Al-alloyed emitter depth and the shallow depth might cause shunt paths. The Jsc is mainly limited by the moderate minority carrier diffusion length in n-type compensated silicon, which originates from the large net doping concentration and the great amounts of total dopants. Combined with the experimental information, the simulation suggests that the Jsc can be strongly improved by using thinner wafers and therefore the cell efficiency could get significantly improved.