Ambipolar diffusion length in a-Si:H(F) and a-Si,Ge:H,F measured with the steady-state photocarrier grating technique

The ambipolar diffusion lengths in 14 samples of a-Si:H, a-Si:H,F, and a-Si,Ge:H,F were measured using the steady-state photocarrier grating technique. The Tauc optical gap of these samples ranges from 1.73 eV to 1.13 eV. A diffusion length of 0.4 μm was observed in one a-Si:H,F sample. The diffusion length saturates at approximately 0.03 μm as the bandgap of the alloys decreases, probably due to the resolution limit of the technique. The diffusion length decreases as the bandgap decreases, as the bulk defect density increases, as the Urbach energy increases, and as the Fermi level increases. The observed correlations are explained qualitatively by existing models for carrier recombination.