Gettering of multicrystalline silicon for high-efficiency solar cells
The effect of phosphorus diffusion gettering was investigated for p-type multicrystalline silicon. Time and temperature of the diffusion process were varied between 860-900°C and one to five hours, but no significant difference was observed for any of the investigated combination. To develop a microscopic model of the gettering process, spatially resolved measurements of minority carrier lifetime and dislocation density were taken. After phosphorus diffusion at 880°C for 90 min an increase of the minority carrier lifetime by about a factor of three was measured in lowly dislocated regions whereas in highly dislocated areas no gettering efficiency was observed. To test whether the phosphorus diffusion was successful in upgrading multicrystalline silicon for high-efficiency silicon solar cells, five different multicrystalline silicon materials from four manufacturers were gettered. Base resistivity varied between 0.5 and 5 ohm cm for boron- and gallium-doped p-type wafers. A high-efficiency solar cell structure with passivated rear and local laser-fired contacts was used to fabricate numerous solar cells with aperture areas of 1 cm2 and 4 cm2. Efficiencies in the 20 % range were achieved for all materials with an average value of 18%. Best efficiencies for 1 cm2 (20.3%) and 4 cm2 (19.8%) cells were achieved on 0.6 ohm cm and 1.5 ohm cm, respectively. This proves that multicrystalline silicon of very different material specification can yield in very high efficiencies if an appropriate phosphorus gettering process is included.