<p>Cu(In,Ga)Se₂ (CIGS) is a promising candidate for a bottom cell role in a tandem structure, having a tunable band gap covering the optimal band gap range values. Correct determination of the absorber band gaps in a tandem structure is very important, as small changes in band gap create high relative current differences. Shockley–Queisser theory predicts the optimal bottom cell band gap in relation to the top cell band gap, and the maximum expected performance, but it assumes a uniform band gap throughout the absorber. The best CIGS cells have spatial compositional variation, making the concept of assigning a single band gap value ambiguous and the band gap determination nontrivial. In this work, we look more closely at this ambiguity. In addition, using numerical simulations, we analyze optimal grading profiles for a bottom cell, illuminated only with low energy photons and compare them to the ones of a single cell, illuminated with a full AM1.5 spectrum.</p>