Post-transit time-of-flight currents as a probe of the density of states in hydrogenated amorphous silicon

Abstract

We use the post-transit photocurrent in a time-of-Night experiment for spectroscopic purposes. It is shown, within the multiple-trapping framework, that the post-transit current is the Laplace transform of the density of states. A simple inversion procedure is suggested and is shown to be quite adequate provided the gap-state distribution does not vary too strongly with energy. The method has been applied to hydrogenated amorphous silicon. Experimental evidence shows that the posttransit photocurrent truly rejects the release out of deeper-lying traps and is not a consequence of nonuniform, time-dependent fields, contact-related properties, or injection phenomena. The measured po~d are compatible with the deconvoluted density of states. A comparative discussion of our results with other published data obtained with conventional methods like the field-eff'ect, spacecharge-limited current, and deep-level transient spectroscopy techniques is given.

Modeling of the transient mobility in disordered organic semiconductors with a Gaussian density of states

“…6 Wellknown consequences are the anomalous transit-time dispersion observed in time-of-flight (TOF) mobility measurements [7][8][9][10][11] or in transient photocurrent measurements, 12,13 an anomalous low-frequency electrical admittance, [14][15][16] and hysteresis or even long-time memory effects. 17 Transient effects in energetically disordered semiconductors have been studied by Monte Carlo modeling 1,18 and using the multiple-trapping (MT) model, 6,14,[19][20][21][22][23][24][25][26][27] which is better suitable for long time scales or at low frequencies. Within the MT model, the transport is assumed to be due to the fraction of carriers in a high-energy conduction level (E c ), while the remaining carriers reside in a distribution of low-lying localized states in which they are immobile.…”

“…Measurements of the post-transit-time photocurrent response in such experiments may even be used to determine the shape of the deep part of the DOS, i.e., the part of the DOS within which no thermal equilibrium occupation has been established yet at the transit time. 23 Again, also dispersive transport manifests itself in qualitatively different ways for the cases of an exponential DOS and a Gaussian DOS, viz. as a continuing process and a process which ends after a time…”

Integration of Expanding Thermal Plasma deposited Hydrogenated Amorphous Silicon in Solar Cells

“…The drift mobility of both electrons and holes, and the density of states (DOS) distribution have been studied in ~4 µm thick ETP a-Si:H films by time-of-flight (TOF) experiments by Adriaenssens et al [63] and by posttransit photo-current analysis (PTPA) [64], as a function of the temperature and applied electric field [65]. Electron-hole pairs are created by illumination with a short light pulse (~530 nm for 3 ns).…”

Optical and Electrical Properties of Undoped Microcrystalline Silicon Deposited by the VHF-GD with Different Dilutions of Silane in Hydrogen

“…Thus, at this stage it is not possible to discriminate the dominant transport process in our µc-Si:H material and more extensive measurements will be necessary here. Whereas short-time TOF monitors the transit of the photogenerated carrier package through the layer, the extension of the TOF experiment to a much longer time scale (post-transient TOF [11], [12]) gives information about the density of states (DOS) in the gap. Thereby, electrons and holes which were deep-trapped during transit are thermally reemitted at longer times and their collection becomes a direct fingerprint of the DOS.…”