M. Z. Kabir

Dark current in multilayer amorphous selenium x-ray imaging detectors

A theoretical model for describing the bias-dependent transient behavior of dark current in multilayer (n-i-p) amorphous selenium (a-Se) detectors has been developed. The transient dark currents in these detectors are measured and are compared to the proposed dark current model. It has been found that the dark current is mainly controlled by Schottky emission of holes from the metal/a-Se contact. The initial and steady state dark currents are mainly controlled by the barrier height and the trap centers in the n layer, respectively.

Effects of charge carrier trapping on polycrystalline PbO x-ray imaging detectors

The effects of charge carrier trapping on the x-ray sensitivity, resolution, and detective quantum efficiency (DQE) in polycrystalline lead oxide (PbO) x-ray imaging detectors are theoretically analyzed. The theoretical models for calculating carrier trapping-limited x-ray sensitivity and modulation transfer function (MTF), and DQE incorporating polyenergetic x-ray spectrum are described. A cascaded linear system model is developed for calculating the spatial frequency f dependent DQE of PbO detectors by incorporating the effects of bulk charge carrier trapping on the MTF and the x-ray interaction depth dependent charge collection efficiency and noise. The theoretical calculations are compared with the published experimental data and show a very good agreement. From the fittings of the sensitivity and MTF curves, the electron and hole ranges in polycrystalline PbO are found to be 3.5×10−7 and ∼10−8 cm2/V, respectively. The f dependent noise power spectrum NPS(f) and DQE(f) performances are analyzed as a f...

An analytical model for analyzing the current-voltage characteristics of bulk heterojunction organic solar cells

An analytical model for analyzing the current-voltage (J-V) characteristics of bulk heterojunction (BHJ) organic solar cells is developed by incorporating exponential photon absorption, dissociation efficiency of bound electron-hole pairs (EHPs), carrier trapping, and carrier drift and diffusion in the photon absorption layer. Modified Brauns model is used to compute the electric field-dependent dissociation efficiency of the bound EHPs. The charge carrier concentrations and hence the photocurrent are calculated by solving the carrier continuity equation for both holes and electrons in the organic layer. The overall load current is calculated considering the actual solar spectrum and voltage dependent forward dark current. The model is verified by published experimental results. The efficiency of the P3HT:PCBM based solar cells critically depends on the dissociation of bound EHPs. On the other hand, cells made of a blend of the conjugated polymer (PCDTBT) with the soluble fullerene derivative (PCBM) show...

Dependence of x-ray sensitivity of direct conversion x-ray detectors on x-ray exposure and exposure history

The dependence of the x-ray sensitivity of a-Se based x-ray image detectors on repeated x-ray exposures and exposure history is studied by considering deep trapping of charge carriers, trapped charges due to previous exposures, bimolecular recombination, space charge effects and electric field dependent electron-hole pair creation energy. We numerically solve the continuity equations of both holes and electrons, trapping rate equations, and the Poison equation across the photoconductor for long pulse x-ray exposures. The electric field distribution across the photoconductor and the relative x-ray sensitivity as a function of cumulated x-ray exposure have been studied for both mammographic and chest radiographic applications. The electric field distribution across the photoconductor has been found to vary widely for high exposures. The relative x-ray sensitivity decreases with increasing cumulated x-ray exposure and tents to saturate. The sensitivity reduction at negative bias is more pronounced than at positive bias.

Investigation of Ghosting Recovery Mechanisms in Selenium X-ray Detector Structures for Mammography

The ghosting recovery mechanisms in multilayer selenium X-ray detector structures for mammography are experimentally and theoretically investigated. The experiments have been carried out under low positive applied electric field . A ghost removal technique is investigated by reversing the bias polarity during the natural recovery process. The theoretical model considers accumulated trapped charges and their effects (trap filling, recombination, detrapping, structural relaxation and electric field dependent electron-hole pair creation), and effects of charge injection from the metal contacts. Carrier trapping in both charged and neutral defect states has been considered in the model. It has been found that the X-ray induced deep trap centers are charged defects. A faster sensitivity recovery is found by reversing the bias during the natural recovery process. During the reverse bias, a huge number of carriers are injected from the metal contacts, and fill the existing trap centers. This results in an abrupt recovery of the relative sensitivity. However, the relative sensitivity slightly decreases with time after this abrupt recovery due to the release of the trapped electrons as well as the long recovery time of the induced trap centers. The theoretical model shows a very good agreement with the experimental results.

Time and exposure dependent x-ray sensitivity in multilayer amorphous selenium detectors

A numerical model is developed to study the time and exposure dependent x-ray sensitivity of multilayer a-Se x-ray imaging detectors on repeated x-ray exposures by considering accumulated trapped charges and their effects (trap filling, recombination, electric field profile, electric field dependent electron–hole pair creation), the carrier transport in the blocking layers, x-ray induced metastable deep trap centre generations and the effects of charge injection. The time-dependent carrier detrapping and structural relaxation (recovery of metastable trap centres) are also considered. The continuity equations for both holes and electrons, trapping rate equations, and Poissons equation across the photoconductor for a step x-ray exposure are simultaneously solved by the backward Euler finite difference method. The electric field distribution across the multilayer detector and the dark current density under repeated exposures are also estimated. The sensitivity in a rested sample is recovered mainly by the carrier detrapping and the recombination of the injected carriers with the existing trapped carriers. The sensitivity is expected to recover almost fully by resting the sample longer than the recovery time constant of the metastable trap centres (the structural relaxation time constant), which is in the range of several hours. The simulation result is fitted with the experimental data. The proposed theoretical model shows very good agreement with the experimental relative sensitivity versus time and accumulative x-ray exposure characteristics.

Temperature and field dependent effective hole mobility and impact ionization at extremely high fields in amorphous selenium

An analytical model for the electric field and temperature dependent effective drift mobility of holes in amorphous selenium (a-Se) has been developed by considering density of states distribution near the valence band, field enhancement release rate from the shallow traps, and carrier heating. The models for the field-dependent microscopic mobility and momentum relaxation mean free path considering carrier heating are also proposed. The models are fitted with the published experimental results on effective hole mobility and impact ionization with wide variations of applied electric fields and temperatures. The fittings of the model with the published experimental data on the effective hole mobility reveal that, while the effective hole drift mobility increases with increasing temperature and field, the microscopic mobility and momentum relaxation mean free path in a-Se decreases with increasing electric field. A better fitting considering thermally activated tunneling for the field-enhancement release ra...

Modeling of the effects of charge transport on voltage-dependent photocurrent in ultrathin CdTe solar cells

An analytical model is developed to study the current–voltage characteristics of CdTe thin film solar cells by incorporating exponential photon absorption, carrier trapping, carrier drift, and diffusion in the photon absorber layer. An analytical expression for the external voltage-dependent photocurrent is derived, considering partial depletion of the absorber layer at operating terminal voltage. The overall load current is calculated considering the actual solar spectrum and dark current components. The analytical model is verified by Taurus Medici simulation and published experimental results for different absorber layer thicknesses. A good agreement of the analytical and numerical models with experimental data is ensured by reducing carrier lifetimes in the thinner absorber layers. The hole lifetime is reduced drastically by decreasing the width of the absorber layer from 1.1 to 0.5 μm, and the cell efficiency depends critically on the transport properties of the holes. The results of this paper indic...

Dark current mechanisms in amorphous selenium-based photoconductive detectors: an overview and re-examination

The transient and steady-state dark current behaviors in various amorphous selenium (a-Se) detectors are analyzed by developing mathematical models considering all possible mechanisms (e.g., carrier depletion, thermal generation, and carrier injection from electrodes) and charge carrier transport properties of a-Se. The theoretical models are validated by comparing them with recently published measured transient and steady-state dark currents in various a-Se detectors. The fittings of the models with the experimental results reveal various important material and device properties such as the optimum trap depth and concentrations in the blocking layers for faster stabilization and lower dark current, and the effective barrier height between the metal/a-Se contacts. The thermal generation current is significantly higher in avalanche detectors (at extremely high fields) than that in conventional detectors. The effective blocking layers for both holes and electrons are necessary for the minimum level of the dark current. The minimum dark current is determined by the thermal generation current. The thermal generation current can be lowered by reducing the mid gap defect states.

Transient and steady-state dark current mechanisms in amorphous selenium avalanche radiation detectors

A theoretical model for describing bias-dependent transient and steady-state behaviors of dark current in amorphous selenium (a-Se) avalanche detector structures has been developed. The analytical model considers bulk thermal generation current from mid-gap sates, transient carrier depletion, and carrier injection from the electrodes incorporating avalanche multiplication. The proposed physics-based dark current model is compared with the published experimental results on three potential a-Se avalanche detector structures. The steady-state dark current is the minimum for the structures that have effective blocking layers for both holes and electrons. The transient decay time to reach a plateau decreases considerably with increasing electric field.