Solar cells with 20% efficiency and lifetime evaluation of epitaxial wafers
We present n-type epitaxially grown wafers deposited in a reactor that allows a process transfer to inline high-throughput reactors. Those wafers exhibit an effective lifetime of up to 1720 ms locally for a phosphorous concentration of 2BL1015 cm-3 and a wafer thickness of about 100 mm. In these wafers the most detrimental defects are stacking faults with polycrystalline silicon inclusions. Comparing two samples with stacking faults densities differing by one order of magnitude revealed a difference in average effective minority carrier lifetime of also one order of magnitude (reduction from more than 1500 ms down to 111 ms for the defective sample). A solar cell fabricated from a 200 mm thick epitaxial wafer of low stacking fault density and a phosphorous concentration of 3BL1016 cm-3 reaches an independently confirmed efficiency of 20%, an open circuit voltage of up to 658 mV, a short circuit current density of up to 39.6 mA/cm2 and a fill factor of up to 76.9%. Differences between this cell and FZ references can be attributed to a reduced bulk lifetime caused by the high doping concentration and most probably additional recombination due to polycrystalline silicon inclusions in stacking faults, although their amount is comparably low. A second solar cell made of an epitaxial wafer with a high stacking fault density exhibits an efficiency reduction of 0.5% absolute compared to the cell made of the high quality epitaxial wafer. This result underlines the importance of minimizing the stacking fault density in epitaxial wafers, in particular the density of those with polysilicon inclusions.