-2- Time-of-Flight experiments were carried out on the following Se based amorphous xerographic photoreceptors:​ (i) pure Se;​ (ii) Se + y ppm Cℓ (y ≲ 90 at.);​ (iii) Se_1-xAs_x + y ppm Cℓ (x ≲ 0.005, y ≲ 60 at.);​ (iv) Se_1-xTe_x + y ppm Cℓ (x ≲ 0.08, y ≲ 90 at.). The drift mobility u and the mean drift lifetime t of the charge carriers were measured, where possible, as a function of applied electric field, temperature and composition.

Mobility-temperature data at various applied fields on these systems have been discussed within the framework of shallow trap controlled transport mechanisms. Drift data indicate that in undoped amorphous Se (a-Se), hole transport is controlled by a discrete set of traps at ~0.29 eV above E_v whereas electron Ev transport is limited by the localized tail states below E_c and/or a discrete set of traps at ~0.35 eV below E_c.

Doping of a-Se with Cℓ introduces an additional set of shallow hole traps;​ at ~0.45 eV above E_c. Alloying a-Se with As affects the electron transport by increasing the extent of the localized tail states below E_c. As addition seems to neutralize the Cℓ induced hole traps at ~0.45 eV above E_v

Addition of Te to a-Se introduces shallow hole traps at ~0.43 eV above _v, and electron traps at ~0.5 eV below E_c. These are likely to be Te^-₁ and Te⁺₃ defect centres respectively.

The microscopic mobility in these systems is probably determined by nearly diffusive motion in extended states, μ₀ ≈ 0.1 - 0.3 cm² v⁻¹ s⁻¹, both for holes and electrons.

Hole lifetime τ_h in a-Se depends on the origin of Se as well as on preparation conditions. Its behaviour with temperature and Te content in a-Se_1-xTex indicates that τ_h is inversely proportional to the hole drift mobility Ph

The hole lifetime in the system Se_1-xAs_x + y ppm Cℓ can be effectively controlled by simultaneous doping with As and Cℓ. Alloying with As reduces the lifetime whereas C addition enhances it.

Electron drift parameters, particularly the lifetime τ_e, have been found to be sensitive to the origin of Se and impurities. There was no detectable electron transport signal when a-Se and a-Se/Te alloys were doped with Cℓ.