How photoluminescence can predict the efficiency of solar cells

Abstract

The efficiency of solar cells depends on the photocurrent, on the open circuit voltage and on the fill factor, which in turn depends on the diode factor. We review how photoluminescence (PL) measurements on the absorber, without finishing the solar cell, reveal the maximum open circuit voltage and the best diode factor, that can be reached in the finished device. We discuss two different ways to extract the quasi Fermi level splitting (QFLS) from absolute calibrated PL spectra, measured at a one sun excitation. The absorption spectrum of the solar cell absorber can be extracted from the PL spectra and allows the reliable determination of tail states. Tail states are responsible for radiative and non-radiative losses in the QFLS. In the ideal case the open circuit voltage is given by the QFLS. However, recombination at the interface can reduce the open circuit voltage severely. We discuss various electronic structures at the interface that lead to a reduction of the open circuit voltage. The excitation dependence of the PL allows the determination of the diode factor of the absorber alone, which would be 1 in the ideal case. Metastable defects can increase the diode factor, even for recombination in the neutral zone and for low excitation. This effect decreases the fill factor of the solar cell.