Quasi-Fermi level splitting and identification of recombination losses from room temperature luminescence in Cu(In1−xGax)Se2 thin films versus optical band gap

Photoluminescence (PL) from high-quality Cu(In,Ga)Se2 films has been analyzed at room temperature and under excitation of AM1.5-equivalent photon fluxes. CIGS films deposited on glass and on Mo were front-side passivated with a 50-nm-thick CdS-window layer. From spectral luminescence of these films and from their absorption, we have extracted the Bose-term in Planck's generalized law describing the emission of radiation from matter, and thus we determine the splitting of the quasi-Fermi levels which corresponds to the upper limit for the maximum achievable open-circuit voltage of final devices. The spectral absorption of CIGS analyzed by transmission and reflection (integrating sphere) shows for each Ga content non-negligible subgap absorption indicating a substantial combined density of states in the gap at energies almost independent of Ga content. At variance with the shift of the optical gap, the shift of the low-energy onset of the luminescence towards higher photon energies and the rise of the PL yield is comparatively weak and, accordingly, the increase in Fermi level separation versus rise in band gap is small. The absorption at low photon energies and the spectral “pinning” of the luminescence signalizes a substantial density of states in the gap at energy irrespective of the degree of Ga alloying. The experimentally detected departure of the Bose-term at subgap energies points towards local inhomogeneities of Cu(In,Ga)Se2 in terms of material composition, metallurgical phases, and consequently of electronic and optical properties.