  Electric coupling between subcells of a monolithically grown multijunction
solar cell in short circuit allows their simultaneous and independent
characterization by means of photo- and electroreflectance. The photovoltage
generated by selective absorption of the pump beam in a given subcell during
photoreflectance measurements results in reverse biasing the complementary
subunits at the modulation frequency set on the pump illumination. Such voltage
bias modulation acts then as external perturbation on the complementary
subcells. The spectral separation of the different subcell absorption ranges
permits the probe beam to record in a single spectrum the response of the
complete device as a combination of photo- and electroreflectance, thereby
providing access for diagnosis of subcells on an individual basis. This form of
modulation spectroscopy is demonstrated on a GaInP/GaAs tandem solar cell.
