Thin film a-Si/c-Si1−xGex/c-Si heterojunction solar cells with Ge content up to 56%

Thin film a-Si(n
⁺
)/c-Si
1−x
Ge
x
(p)/c-Si(p
⁺
) heterojunction solar cells are fabricated with Ge content up to 56 atomic percent. Solar cells with junction layers consisting of Si, Si
0.75
Ge
0.25
, Si
0.59
Ge
0.41
, and Si
0.44
Ge
0.56
are compared to study the effect of increasing Ge concentration. The measured short-circuit current (J
sc
) increases from ∼14 mA/cm
²
for Si cells to 21 mA/cm
²
for the Si
0.44
Ge
0.56
cells, for one light pass and a 2 µm-thick SiGe layer. The results show an open-circuit voltage (V
oc
) of 0.61 V for Si cells, dropping to 0.32 V for Si
0.44
Ge
0.56
, consistent with the reduction in band-gap. Quantum efficiency measurements highlight the improved spectral response for higher Ge percentages. Physics based TCAD simulations combined with the experimental results are used to extract lifetime and interface velocity.