<p>Light ideality factor, the slope of open-circuit voltage versus the logarithm of the illumination intensity, is an important research factor in solar cells, which is widely accepted to determine whether the recombination mechanism such as Shockley–Reed–Hall (SRH) or Langevin recombination dominates in semiconductor materials. However, based on the basic theoretical framework of semiconductor physics, this paper deduces the relationship between these two recombination mechanisms and the ideality factor. It uses the drift–diffusion theory to construct a numerical model. Both theoretical derivation and numerical analysis proved that the electron and hole concentrations in semiconductor materials were in most cases not equal, so the SRH recombination mechanism could not make the light ideality factor reach 2. In this paper, the Einstein coefficient is introduced to weaken the influence of Boltzmann statistics on Fermi systems, and the results show that the high ideality factor is rooted in fermions, rather than in the widely accepted SRH recombination mechanism. This discovery proves that the currently widely accepted relationship between SRH recombination and ideality factor is wrong, and that, since electrons are fermions, only Fermi statistics can accurately study the properties of semiconductor materials.</p>