Laser Annealed Back Contacts for CdTe Solar Cells

Abstract

The CdTe solar cell back contact interface gets activated by means of thermal annealing. Depending on the back contact (BC) material the annealing time can vary between 20 - 60 minutes. In this study fast contact anneal times - $< 90$ sees - are investigated using a 60 Watt dual diode 808nm laser. Two types of back contacts are used: (a) Cu-Graphite, and (b) MoN/Mo-Cu. Laser power density (LPD) and Cu concentration were varied to study their effect on device performance. Capacitance-voltage characteristics revealed a correlation between LPD and the net doping concentration in CdTe. Higher Cu concentration was found to be detrimental to the device performance, possibly due to excess Cu reaching the CdTe/CdS interface as indicated by secondary ion mass spectroscopy (SIMS) measurements. Solar cells exhibited open-circuit voltage (Voc), short-circuit current (Jsc), and fill-factor (FF) similar to the baseline thermally anneal devices. To-date the LPD was optimized with best cell parameters being: $\mathrm{V_{oc}} ={830} \mathrm{mV}, \mathrm{J_{sc}} ={22} \mathrm{mA/cm}^{2}$, and $\mathrm{FF}=70\%$, for laser anneal time of 90 seconds.