Interpretation of quasi-Fermi level splitting in Cu(Ga,In)Se 2-absorbers by confocally recorded spectral luminescence and numerical modeling

Abstract

Spectral room temperature photoluminescence (pl) of polycrystalline Cu(In,Ga)Se 2 films (CIGSe) is evaluated with respect to optoelectronic properties and in particular for the determination of the splitting of quasi-Fermi levels ( E Fn − E Fp). For lateral resolution of ≤ 1 µm a confocal pl-setup is used. The depth profile of the excess carrier densities determining the rates of radiative transitions strongly govern the spectral pl-shape which has been numerically modeled with a matrix transfer formalism. In this optical approach we discriminate for wave propagation and attenuation in a multilayer system between a plane-wave ansatz and a 3D-spherical formalism, depending on excitation area large or small/similar compared to the thickness of the absorber. In both cases re-absorption of photons in energetic regimes with absorption approaches unity, from which the splitting of the quasi-Fermi levels is preferentially deduced, substantially influence the spectral luminescence signal. For heterojunctions usually located at the light entrance side of the device our evaluation with good agreement reflects ( E Fn − E Fp) in the vicinity of the barrier and thus indicates the maximum achievable open circuit voltage of the finally processed diode. Departures of the spectral pl from the idealized Bose-term signalize unfavorable carrier profiles and a depth dependence of optoelectronic absorber properties.