Influence Of Trap States Research Articles

Photoconductive terahertz generation in semi-insulating GaAs and InP under the extremes of bias field and pump fluence.

This Letter analyzes photoconductive (PC) terahertz (THz) emitters based on the semi-insulating (SI) forms of GaAs and InP. The dependencies of the emitters are studied under the extremes of the bias field and pump fluence to reveal the underlying physics of charge carrier photoexcitation, transport, and emission. The bias field dependence shows that SI-GaAs PC THz emitters are preferentially subject to space-charge-limited current, under the influence of trap states, while SI-InP PC THz emitters are preferentially subject to sustained current, due to a prolonged charge carrier lifetime and the ensuing joule heating. The pump fluence dependence shows space-charge and near-field screening for all emitters, with SI-GaAs predisposed to near-field screening (under the influence of transient mobility) and SI-InP predisposed to space-charge screening. Such findings can support a deeper understanding of the underlying physics and optimal performance of SI-GaAs and SI-InP PC THz emitters.

An analytical modeling approach to the electrical behavior of the bottom-contact organic thin-film transistors in presence of the trap states

In this paper, a complete analytical model of the conduction behavior of organic thin-film transistors (OTFTs) within the framework of trap states has been presented. The aim of the work is to develop modeling equations which can be evolved into compact models generic to OTFTs rather than relying on the MOSFET-based models for the modeling and simulation of organic-based devices. The exponential density of trap states and the trap decay rate has been included in the modeling equations of the bottom-contact OTFTs with pentacene as the active layer. In this work, we propose for the first time a modeling approach to mobility. The mobility model takes into account the low-field mobility enhancement as well as high-field mobility degradation because of the presence of defect states. The current–voltage equations in the linear and saturation regimes have been modified to include the effect of trap states. The influence of trap states on the device capacitance has been modeled, and it has been found that the high density of trap states leads to deviation in the capacitance characteristics of OTFTs. Results obtained from the modeling equations of surface potential, total charge density, mobility and current–voltage relations show performance degradation with the increase in the density of trap states. The results predicted from the analytical modeling equations are in excellent agreement with the reported electrical behavior of the OTFTs, thus proving the feasibility of the proposed model. The proposed analytical model is valid for OTFTs in the linear and saturation regimes even for short-channel length transistors.

Trap State Effects in PbS Colloidal Quantum Dot Exciton Kinetics Using Photocarrier Radiometry Intensity and Temperature Measurements

Colloidal quantum dots (CQDs) have attracted significant interest for applications in electronic and optoelectronic devices such as photodetectors, light-emitting diodes, and solar cells. However, a poor understanding of charge transport in these nanocrystalline films hinders their practical applications. The photocarrier radiometry (PCR) technique, a frequency-domain photoluminescence method spectrally gated for monitoring radiative recombination photon emissions while excluding thermal infrared photons due to non-radiative recombination, has been applied to PbS CQD thin films for the analysis of charge transport properties. Linear excitation intensity responses of PCR signals were found in the reported experimental conditions. The type and influence of trap states in the coupled PbS CQD thin film were analyzed with PCR temperature- and time-dependent results.

The nature and role of trap states in a dendrimer-based organic field-effect transistor explosive sensor

We report the fabrication and charge transport characterization of carbazole dendrimer-based organic field-effect transistors (OFETs) for the sensing of explosive vapors. After exposure to para-nitrotoluene (pNT) vapor, the OFET channel carrier mobility decreases due to trapping induced by the absorbed pNT. The influence of trap states on transport in devices before and after exposure to pNT vapor has been determined using temperature-dependent measurements of the field-effect mobility. These data clearly show that the absorption of pNT vapor into the dendrimer active layer results in the formation of additional trap states. Such states inhibit charge transport by decreasing the density of conducting states.

Study of the influence of the Cd:Se precursor ratio during the synthesis of CdSe nanocrystals on the performance of CdSe/P3HT hybrid solar cells

AbstractThe influence of the Cd‐to‐Se precursor ratio during the synthesis of CdSe nanoparticles (nc‐CdSe) on the efficiency of solar cells made from semiconducting polymer/nanocrystal‐blends (hybrid solar cells) was investigated. In these hybrid solar cells regioregular poly‐(3‐hexylthiophene 2,5 diyl) (P3HT) was used as the electron donor material while the acceptor was established by CdSe nanocrystals prepared via colloidal synthesis. Furthermore, the influence of the nc‐CdSe‐to‐P3HT ratio in the semiconductor blend on the solar cell efficiency was investigated. CdSe:P3HT ratios of 8:1 to 10:1 were found to give the best results concerning the overall device performance as derived from current–voltage characterization. These findings were correlated with structural investigations of the active layer by means of atomic force microscopy (tapping mode AFM). Furthermore, the external quantum efficiency (EQE) of the hybrid solar cells was determined and also used to estimate the short circuit current density Jsc under standardized solar irradiation. The Jsc values from the EQE measurements were compared to the values obtained from the IV curves. Differences in these values could be explained by an intensity‐dependent influence of trap states.

Tin Oxide Nanowires: The Influence of Trap States on Ultrafast Carrier Relaxation.

We have studied the optical properties and carrier dynamics in SnO2nanowires (NWs) with an average radius of 50 nm that were grown via the vapor–liquid solid method. Transient differential absorption measurements have been employed to investigate the ultrafast relaxation dynamics of photogenerated carriers in the SnO2NWs. Steady state transmission measurements revealed that the band gap of these NWs is 3.77 eV and contains two broad absorption bands. The first is located below the band edge (shallow traps) and the second near the center of the band gap (deep traps). Both of these absorption bands seem to play a crucial role in the relaxation of the photogenerated carriers. Time resolved measurements suggest that the photogenerated carriers take a few picoseconds to move into the shallow trap states whereas they take ~70 ps to move from the shallow to the deep trap states. Furthermore the recombination process of electrons in these trap states with holes in the valence band takes ~2 ns. Auger recombination appears to be important at the highest fluence used in this study (500 μJ/cm2); however, it has negligible effect for fluences below 50 μJ/cm2. The Auger coefficient for the SnO2NWs was estimated to be 7.5 ± 2.5 × 10−31 cm6/s.

Influence of trap states on dynamic properties of single grain silicon thin film transistors

The transient properties of single grain–thin film transistors (SG-TFTs) with high electron mobility have been studied. Overshoot current induced by trap states has been observed in most of the devices. A method of ac measurements has been used to investigate the trap processes. Both transient and ac measurements show that the response of some SG-TFTs with high field effect mobility is dominated by a single trap level. Bias stressing on SG-TFT can induce more trap states and thus change the ac response of the device.

Influence of Carrier Mobility and Interface Trap States on the Transfer and Output Characteristics of Organic Thin Film Transistors

We have used two dimensional drift diffusion simulations to calculate the electrical properties of bottom contact pentacene based organic thin film transistor, taking into account field-dependent mobility and interface or bulk trap states. In order to derive from basic principles the transport properties of the organic semiconductor we have developed a Monte Carlo simulator to calculate the field dependent mobility. We analyzed the presence of trap states at the interface between organic material and gate insulator and we show the influence of trap states combined with the effect of field-dependent mobility on the transfer characteristic. Numerical calculations are also reported for the output characteristics of the device showing a very good agreement with the available experimental results.