The drift length Ldrift=μτE within the i layer of a-Si:H solar cells is a crucial parameter for charge collection and efficiency. It is strongly reduced not only by light-induced reduction of μτ, but also by electric field deformation ΔE by charges near the p–i and i–n interfaces. Here, a simple model is presented to estimate contributions of free carriers, charges trapped in band tails and charged dangling bonds to ΔE. It is shown that the model reproduces correctly trends observed experimentally and by ASA simulations: charged dangling bonds contribute most to ΔE of meta-stable cells. Electrons trapped in the conduction band tail near the i–n interface lead to the strongest field deformation in the initial state, while positively charged dangling bonds near the p–i interface get more important with degradation under AM1.5g spectrum. The measurable parameter Vcoll is proposed as an indirect parameter to estimate the electric field, and an experimental technique is presented that could enable the distinction of defects near the p–i and the i–n interfaces.