Effects of material properties of band-gap-graded Cu(In,Ga)Se-2 thin films on the onset of the quantum efficiency spectra of corresponding solar cells

Polycrystalline Cu(In,Ga)Se-2 (CIGSe) thin-film solar cells exhibit gradual onset in their external quantum efficiency (EQE) spectra whose shape can be affected by various CIGSe material properties. Apart from influences on the charge-carrier collection, a broadening of the EQE onset leads to enhanced radiative losses in open-circuit voltage (V-oc). In the present work, Gaussian broadening of parameters describing the EQE onset of thin-film solar cells, represented by the standard deviation, sigma(total), was evaluated to study the impacts of the effective band-gap energy, the electron diffusion length, and the Ga/In gradient in the CIGSe absorber. It is shown that sigma(total) can be disentangled into contributions of these material properties, in addition to a residual component sigma(residual). Effectively, sigma(total) depends only on a contribution related to the Ga/In gradient as well as on sigma(residual). The present work highlights the connection of this compositional gradient, the microstructure in the polycrystalline CIGSe absorber, and the luminescence emission with the residual component sigma(residual). It is demonstrated that a flat band-gap with no compositional gradient in the bulk of the CIGSe absorber is essential to obtain the lowest sigma(total) values and thus result in lower recombination losses and gains in V-oc.