Experimental and simulation studies of interface properties of crystalline germanium heterojunction solar cells

Numerical simulation of crystalline germanium (c-Ge) heterojunction solar cell was performed using simulation software AFORS-HET. We confirmed that a difference of the minority carrier band offset of n-type hydrogenated amorphous silicon (a-Si:H(n))/c-Ge(p) and a-Si:H(p)/c-Ge(n) structure has great impacts for solar cell performance. The a-Si:H(p)/c-Ge(n) valence band offset of 0.96 eV induces larger band bending in the c-Ge(n) absorber compared with the a-Si:H(n)/c-Ge(p) whose conduction band offset is 0.1 eV. The large band bending can reduce interface recombination due to the reduction of the majority carrier density near the interface. However the offset of 0.96 eV is too large and impedes photogenerated carrier collection. We applied crystalline-Si(p) whose band gap is lower than a-Si:H(p) as the emitter and confirmed that the current limitation was avoided keeping large band bending. On the other hand, although the band bending of the a-Si:H(n)/c-Ge(p) is too small to reduce the interface recombination, the donor doping to the interface layer, whose cell performance improvement effect was experimentally demonstrated, induces large band bending and improves cell performance.