Effects of Band Offsets on a-Sic:H/c-Si Heterojunction Solar Cell Performance

Abstract

ABSTRACTWe used the internal photoemission technique to determine the exact valence and conduction band offsets at the a-SiC:H/c-Si interface and investigated with numerical simulations their effects on the photocarrier collection in p+a-SiC:H/n c-Si heterojunction solar cells. The valence and conduction band offsets were found to be 0.60 eV and 0.55 eV, respectively. Simulation results show that a high valence band offset increases the open circuit voltage (higher built-in potential) but on the other hand can decrease the fill factor (by blocking the collection of photogenerated holes at the front contact). Interestingly, despite having a large barrier inside the valence band (ΔEv = 0.6 eV), our highly doped p+a-SiC:H/n c-Si heterojunction solar cells show no collection problems (FF= 0.73). Both IPE measurements and simulation results indicate that tunneling of holes through this barrier in the valence band can explain this effect. For thin highly doped (Eact = 0.33 eV) p+a-SiC:H layers, the tunnel barrier becomes very narrow (< 70 Å) and the tunneling probability is strongly enhanced.