Generation Rate Of Charge Carriers Research Articles

Photocurrent in bulk heterojunction solar cells with similar electron and hole mean free path

We present photocurrents at several temperatures carried out in a bulk heterojunction photovoltaic device. To explain the results, we developed an analytical model assuming non-injecting contacts and equal mean free paths for electrons and holes. The fitting of the equation to the experimental data provided the temperature evolution of the charge transfer state dissociation probability (P) and μτ, where μ is the charge carrier mobility and τ is the charge carrier lifetime. The photocurrent expression tends towards a saturation value of eGPL for high electric fields, where GP is the generation rate of charge carriers.

Improved power conversion efficiency by insertion of RGO–TiO2 composite layer as optical spacer in polymer bulk heterojunction solar cells

We report that the power conversion efficiency (PCE) can be enhanced in polymer bulk heterojunction solar cells by inserting an interfacial electron transporting layer consisting of pristine TiO2 or reduced graphene oxide–TiO2 (RGO–TiO2) between the active layer and cathode Al electrode. The enhancement in the PCE has been analyzed through the optical absorption, current–voltage characteristics under illumination and estimation of photo-induced charge carrier generation rate. It was found that either TiO2 or RGO–TiO2 interfacial layers improve the light harvesting, as well as the charge extraction efficiency, acting as a blocking layer for holes, and also reducing charge recombination. The combined enhancement in light harvesting property and charge collection efficiency improves the PCE of the organic solar cell up to 4.18% and 5.33% for TiO2 and RGO–TiO2 interfacial layer, respectively, as compared to a value of 3.26% for the polymer solar cell without interfacial layer.

Annealing and thickness related performance and degradation of polymer solar cells

We investigated annealing and thickness related performance and degradation of bulk heterojunction solar cells based on regioregular poly(3-hexylthiophene) (RR-P3HT):[6,6]-phenyl C61-butyric acid methyl ester (PCBM). The devices were fabricated with slow drying for different active layer thicknesses (100–200nm thick), followed by an identical thermal annealing. Photoinduced charge carrier generation and dissociation as well as series resistance were extracted from the device current–voltage characteristics and correlated to the active layer absorbance and morphology to understand the behavior of the fabricated devices. It was observed that with slow drying method the thickest absorber device had the highest efficiency but upon the followed thermal annealing it degraded while the thinnest one became substantially improved in performance. For the degraded device the charge carrier generation rate was found nearly unreduced, while the dissociation probability at maximum power voltage and series resistance largely deteriorated with thermal annealing. This implies a likely hampered charge transport and increased recombination losses with a discontinuation of the bi-phase networks caused by a further phase separation in the heat treatment. Thus, morphology control for effective charge transport appears crucial to the device performance and stability, and taking into account the active layer thickness effect in annealing is important.

Three dimensional optical modeling of amorphous silicon thin film solar cells using the finite-difference time-domain method including real randomly surface topographies

In this paper, modeling of light propagation in silicon thin film solar cells without using any fitting parameter is presented. The aim is to create a realistic view of the light trapping effects and of the resulting optical generation rate in the absorbing semiconductor layers. The focus is on real three dimensional systems. Our software Sentaurus tcad, developed by Synopsys, has the ability to import real topography measurements and to model the light propagation using the finite-difference time-domain method. To verify the simulation, we compared the measured and simulated angular distribution functions of a glass/SnO2:F transparent conducting oxide system for different wavelengths. The optical generation rate of charge carriers in amorphous silicon thin film solar cells including rough interfaces is calculated. The distribution of the optical generation rate is correlated with the shape of the interface, and the external quantum efficiencies are calculated and compared to experimental data.

Enhanced charge carrier generation in dye sensitized solar cells by nanoparticle plasmons

An interesting possibility to improve the conversion and cost efficiencies of photovoltaic solar cells is to exploit the large optical cross sections of localized (nanoparticle) surface plasmon resonances (LSPRs). We have investigated this prospect for dye sensitized solar cells. Photoconductivity measurements were performed on flat TiO2 films, sensitized by a combination of dye molecules and arrays of nanofabricated elliptical gold disks. An enhanced dye charge carrier generation rate was found and shown to derive from the LSPR contribution by means of the polarization dependent resonance frequency in the anisotropic, aligned gold disks.

The x‐ray time of flight method for investigation of ghosting in amorphous selenium‐based flat panel medical x‐ray imagers

Amorphous selenium (a-Se) based real-time flat-panel imagers (FPIs) are finding their way into the digital radiology department because they offer the practical advantages of digital x-ray imaging combined with an image quality that equals or outperforms that of conventional systems. The temporal imaging characteristics of FPIs can be affected by ghosting (i.e., radiation-induced changes of sensitivity) when the dose to the detector is high (e.g., portal imaging and mammography) or the images are acquired at a high frame rate (e.g., fluoroscopy). In this paper, the x-ray time-of-flight (TOF) method is introduced as a tool for the investigation of ghosting in a-Se photoconductor layers. The method consists of irradiating layers of a-Se with short x-ray pulses. From the current generated in the a-Se layer, ghosting is quantified and the ghosting parameters (charge carrier generation rate and carrier lifetimes and mobilities) are assessed. The x-ray TOF method is novel in that (1) x-ray sensitivity (S) and ghosting parameters can be measured simultaneously, (2) the transport of both holes and electrons can be isolated, and (3) the method is applicable to the practical a-Se layer structure with blocking contacts used in FPIs. The x-ray TOF method was applied to an analysis of ghosting in a-Se photoconductor layers under portal imaging conditions, i.e., 1 mm thick a-Se layers, biased at 5 V/ microm, were irradiated using a 6 MV LINAC x-ray beam to a total dose (ghosting dose) of 30 Gy. The initial sensitivity (S0) of the a-Se layers was 63 +/- 2 nC cm(-2) cGy(-1). It was found that S decreases to 30% of S0 after a ghosting dose of 5 Gy and to 21% after 30 Gy at which point no further change in S occurs. At an x-ray intensity of 22 Gy/s (instantaneous dose rate during a LINAC x-ray pulse), the charge carrier generation rate was 1.25 +/- 0.1 x 10(22) ehp m(-3) s(-1) and, to a first approximation, independent of the ghosting dose. However, both hole and electron transport showed a strong dependence on the ghosting dose: hole transport decreased by 61%, electron transport by up to approximately 80%. Therefore, degradation of both hole and electron transport due to the recombination of mobile charge carriers with trapped carriers (of opposite polarity) were identified as the main cause of ghosting in this study.

Photocatalytic oxidation of ethanol on micrometer- and nanometer-sized semiconductor particles

The photocatalytic oxidation reaction of ethanol on small nanometer-sized and large micrometer-sized ZnS particles as well as on nanometer-sized SnO2 under stationary illumination has been investigated in order to study the particle size effect in this size regime. It has been demonstrated that the reaction mechanism of alcohol oxidation on micrometer- and nanometer-sized particles is quite different. Ethanol is selectively oxidized on μm-ZnS to acetaldehyde without side products by a “two-hole” process. This reaction required two absorbed photons in one particle which are transferred in a very short time interval (55ns) from the μm-ZnS particle to the ethanol molecule forming directly acetaldehyde. In the case of nm-ZnS, long-lived α-hydroxyethyl radicals are formed via a “one-hole” process due to the low generation rate of charge carriers (about 18e−/h+s−1) in one nm-ZnS particle. These radicals undergo secondary reactions, i.e. dimerization to 2,3-butanediol and disproportionation to acetaldehyde in the electrolyte. The electrochemical rate constant ket for the slowest partial reaction occurring on the illuminated nm- and μm-ZnS particles has been calculated. One obtains similar rate constants, namely ket=9.8×10−9 and 4.3×10−9cms−1 for μm-ZnS and nm-ZnS, respectively. In the case of illuminated nm-SnO2 particles, only the oxidation product acetaldehyde was observed. Here, the initially formed α-hydroxyethyl radical via a “one-hole” process is further oxidized to the corresponding aldehyde by electron injection from the radical into the conduction band of the same SnO2 particle by avoiding the formation of 2,3-butanediol.

Radiation effects on the current–voltage and capacitance–voltage characteristics of advanced p–n junction diodes surrounded by shallow trench isolation

This paper investigates the impact of 20 MeV proton irradiation on the current–voltage ( I– V) and capacitance-voltage ( C– V) characteristics of different geometry n +–p-well junction diodes surrounded by shallow trench isolation and processed in a 0.18 μm CMOS technology. From I– V characteristics, a higher current damage coefficient was found for the bulk than for the peripheral component. The radiation-induced boron de-activation resulted in a lowering of the p-well doping, which has been derived from high-frequency C– V measurements. This was confirmed by deep level transient spectroscopy (DLTS) analysis, revealing the presence of interstitial boron related radiation defects. As will be demonstrated for the bulk leakage-current damage coefficient, the electric field enhanced generation rate of charge carriers and the radiation-induced boron de-activation should be accounted for properly.

Effect of stress on the carrier generation rate in gold-doped silicon

The thermal generation rate of charge carriers in a depletion region in gold-doped silicon was measured as a function of strain applied in the [110] direction over a range of temperatures. Thermal generation rate was also measured as a function of [100] strain at a single temperature. Relaxation to equilibrium of a deeply depleted metal-oxide-semiconductor capacitor was used to measure generation rate. The generation rate was observed to increase nearly 100 per cent from zero applied strain to an applied strain of 5 × 10 −3, at −20°C. Experimental results were explained using a model based on Shockley-Read statistics and results from deformation-potential theory. Agreement between theory and experiment was obtained under the condition that the gold impurity energy level split with applied strain and the the center of gravity of the two levels shifted upward with a hydrostatic coefficient. The increase in generation rate was in excess of that caused by band extrema shifts only.