Density of localized state distribution near the valence band in stabilized a‐Se using interrupted field time of flight measurements with long interruption times

Abstract

In the present paper, we have investigated hole transport in stabilized a‐Se films using interrupted‐field‐time‐of‐flight (IFTOF) experiments with interruption times up to 600 μs. A distinct advantage of IFTOF measurements is that one can monitor the average “free” hole concentration p(t) (= p(x,t) averaged over the thickness of the sample L) at a given location x1 in the sample inasmuch as the applied field is removed at a certain time t1 for an interruption period of ti. At time t1 + ti, the field is reapplied and the recovered photocurrent i2 at t = t1 + ti is measured with respect to the original photocurrent i1 at t = t1. The experimental results are interpreted by the comparison of experimental photocurrent transients with numerical and Monte‐Carlo simulations of multiple trapping hole transport for different DOS models; and their agreement with a featureless DOS distribution in the vicinity of valence band is confirmed. The examination of IFTOF experiments (i2/i1 as a function of ti) for very long interruption times puts the energy position of deep traps controlling the hole lifetime to be above 0.65 eV from Ev. The results are critically discussed in view of past experiments on the DOS in a‐Se and recent structural modeling work on defects in the structure. Further, the work in this article vindicates the use of shallow trap controlled mobility and a single deep trapping time in the modeling of hole transport in recently developed a‐Se based X‐ray detectors; that is we only need shallow and deep traps to model the detector performance.

Frequency- and time-resolved photocurrents in vacuum-deposited stabilised a-Se films: the role of valence alternation defects

“…In this work, we suggest that VAP defects, and particularly, the interconversion between the two stable VAP configurations can explain our experimental data without invoking any stable populations of the various neutral overcoordinated and hypervalent defects that have been proposed to exist [31]. Previous VAP-based interpretations of various electrical and optical properties of a-Se such as photoconductivity, post-transit time-of-flight (TOF) signals, photodarkening and interrupted field TOF have demonstrated the usefulness of VAP defect models [27,29,30,[32][33][34][35][36][37][38][39]. In fact a VAP defect-based model has been successfully used to explain the "ghosting" and the drop in sensitivity of a-Se photoconductors [40].…”

Determination of the Lifetime in As Doped a-Se from Transient Photocurrents using Laplace Transform Technique

“…The density of states studied in detail by Kasap et al [5,6] is a featureless, monotonically decreasing distribution in energy. According to Benkhedir et al [7], in pure a-Se above the valance band edge E v DOS consists of a shallow defect level at ∼ 0.25 eV above E v and a defect level at ∼ 0.45 eV.…”