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

  • M. W. M. Van Cleef (a1), F. A. Rubinelli (a1) and R. E. I. Schropp (a1)

Abstract

We 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.

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