A new theoretical expression is derived for the internal quantum efficiency of solar cells homoepitaxially grown on highly doped monocrystalline substrates. This expression is used to characterize our thin‐layer silicon cells grown by chemical vapor deposition. These cells reach a confirmed efficiency of 17.3% although the active layer thickness is only 48 &mgr;m. The internal quantum efficiency analysis demonstrates that the open circuit voltage is limited by carrier injection into the highly doped substrate. Carrier generation in the substrates accounts for 0.7% of the short circuit current. ©1995 American Institute of Physics.