AbstractPolycrystalline Cu(In,Ga)Se2 (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 (Voc). In the present work, Gaussian broadening of parameters describing the EQE onset of thin‐film solar cells, represented by the standard deviation, 𝜎total, 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 𝜎total can be disentangled into contributions of these material properties, in addition to a residual component 𝜎residual. Effectively, 𝜎total depends only on a contribution related to the Ga/In gradient as well as on 𝜎residual. 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 𝜎residual. 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 𝜎total values and thus result in lower recombination losses and gains in Voc.