Microscopic origins of radiative performance losses in thin-film solar cells at the example of (Ag,Cu)(In,Ga)Se2 devices

The present work provides an overview of radiative performance losses in thin-film solar cells, focusing on those related to the open-circuit voltage, using (Ag,Cu)(In,Ga)Se(2 )devices as examples. The microscopic origins of these losses are outlined, highlighting the presence of compositional variations, strain, and inhomogeneously distributed point defects on various length scales as contributors to band-gap and electrostatic potential fluctuations, which both contribute to the broadening of the absorption edge in the absorptance or quantum efficiency spectra of the semiconductor absorber layer or the completed solar-cell device. The relationship between this broadening and Urbach tails is discussed. It is shown that the photovoltaic band-gap energy as well as the broadening can be reliably determined from the arithmetic mean and standard deviation extracted from Gaussian fits to the first derivative of the absorptance or quantum efficiency spectra around the absorption edge. The more enhanced the broadening, the more the local maximum in the luminescence spectrum shifts to smaller energies with respect to the band-gap energy of the absorber layer, as verified for about 30 (Ag,Cu)(In,Ga)Se-2 solar cells.