Laser-doped selective emitter and local back surface field solar cells with rear passivation

摘要

This thesis examines two different ways to improve the performance of single sided laser-doped solar cells. The first is replacing the aluminum rear contact with localised contacts and a high-quality passivation layer. The second is optimising the laser processing to minimise any detrimental effects. It is demonstrated that annealing in the 600-820°C range significantly improves the passivation of different SiNx films on different silicon surfaces. Significant bulk lifetime enhancement is seen when SiNx-passivated CZ wafers are annealed. Using an optimal annealing condition, the implied Voc of CZ silicon substrates increased to a value comparable to that of FZ wafers - almost 720 mV.Laser-induced defects are investigated using a wide range of characterisation techniques. It is found that laser doping degrades the electrical performance of the device. This degradation is more pronounced when a dielectric layer is present during the laser process, possibly due to the thermal expansion mismatch between the silicon and the overlying dielectric layer. Methods to reduce defect density are discussed.The influence of laser parameters on the electrical performance of laser-doped solar cells is studied. It is demonstrated that a wide range of laser diode currents can be used to create a p-n junction by laser doping. Grooves formed through intermediate levels of ablation can be used to improve the adhesion between the silicon and metal without significantly degrading the cell performance. Electroless and photo-plating are compared; higher pseudo-FFs are achieved for photoplated laser-doped solar cells. If the photoplating technique is combined with well-optimised Ni sintering, the pseudo-FF is almost independent of the laser diode current.A new double sided laser-doped structure is developed. This structure is based on silicon nitride passivation of the rear surface and the formation of a selective emitter and local back-surface field by laser doping. One-sun implied Voc above 680 mV is achieved on commercial grade CZ p-type wafers when measured after laser doping and prior to metallisation. This is ~50 mV higher than the Voc obtained for the single-sided laser-doped cell at the same stage. This high Voc demonstrates the potential of this structure to achieve efficiencies exceeding 20%.