Marinus Kunst

Relaxation of Photogenerated Carriers in P3HT:PCBM Organic Blends

The after-pulse time-resolved microwave conductivity (TRMC) decays observed in P3HT:PCBM blends display a dependence on time close to t(-beta), independent of excitation intensity, in the 10 ns-1 micros range. This is explained in terms of the relaxation of carriers in a Gaussian density of states (DOS). The model is based on a demarcation level that moves with time by thermal release and retrapping of initially trapped carriers. The model shows that when the disorder is large the after-pulse decay of the type t(-beta) is obtained, while at low disorder and large temperature the carrier distribution becomes independent of time. In the measurements different beta values were observed depending on the solvent used for spin-coating: 0.4-0.6 for chlorobenzene and 0.3-0.4 for toluene. The model was applied to extract the shape of the DOS from the TRMC decays, giving a dispersion parameter of about 120 meV for blends with high P3HT content.

Theoretical and experimental study of charge carrier kinetics in crystalline silicon

A model simulating excess charge carrier kinetics in Si wafers was developed taking into account space charge fields and surface recombination. This model was applied to experimental data obtained by contactless transient and frequency resolved photoconductivity measurements of silicon wafers with accumulation or depletion layers at the surface. It is shown that a surface accumulation layer has only a minor influence and surface recombination can be analyzed with a surface recombination velocity weakly depending on the excess carrier concentration. At low injection level a surface depletion layer leads to a strongly nonlinear behavior of the photoconductivity due to excess charge carriers stored in the space charge region. The presence of these charge carriers is also revealed by the tail of the transient photoconductivity signal characterized by a decay time longer than the volume lifetime. At higher injection level the contribution of these charge carriers to the photoconductivity can be neglected and t...

Charge separation and recombination in radial ZnO/In2S3/CuSCN heterojunction structures

A ZnO-nanorod/In2S3/CuSCN radial hetero structure has recently shown promising photovoltaic conversion efficiencies. In this work, the charge separation and recombination in single ZnO/ In2S3 and In2S3/CuSCN interfaces as well as the complete ZnO/In2S3/CuSCN structure were studied by time resolved microwave photoconductivity. Photoconductivity transients were measured for different thicknesses of the In2S3 light absorbing layer, under variation of the exciting light flux and before and after annealing of the ZnO nanorods at 450°C. Upon excitation with 532nm light, a long lived (ms) charge separation at the In2S3/ZnO interface was found, whereas no charge separation was present at the In2S3/CuSCN interface. The presence of the CuSCN hole conductor increased the initial amplitude of the TRMC signal of the In2S3/ZnO interface by a factor of 8 for a 6nm thick In2S3 layer, but the enhancement in amplitude dropped strongly for thicker films. The measurements show that the primary charge separation is located at the In2S3/ZnO interface but the charge injection yield into ZnO depends critically on the presence of CuSCN.

Microwave photoconductivity techniques for the characterization of semiconductors

The use of microwave photoconductivity measurements as a tool for the non-destructive characterization of semiconductors is studied and the experimental equipment is described. Through some examples of experiments on silicon wafers covered with a silicon nitride coating the merits of different ways to perform these measurements are discussed.

Influence of a space charge region on charge carrier kinetics in silicon wafers

p-type Si wafers provided with silicon nitride films at the surface are investigated by contactless transient photoconductivity measurements. Comparing wafers of different resistivities and wafers at one side and at two sides provided with nitride films shows that separation and storage of excess charge carriers in the space charge region is the main kinetic process at low injection level. Fitting of the experimental data with a simple model leads to the conclusion that the electron mobility in the interface is strongly reduced, a value 250(−+20%) cm2u200aV−1u200as−1 is determined, independent of the wafer resistivity. This confirms the reduction of the electron mobility reported in the literature on the basis of completely different measurements.

Amorphous silicon/crystalline silicon heterojunctions for solar cells

Amorphous silicon/crystalline silicon (c-Si) heterojunctions are studied by contactless transient photoconductivity measurements at the microwave frequency range. Excellent passivation of the n-type crystalline silicon (n c-Si) surface by n-type, hydrogenated amorphous silicon (n a-Si:H) films is observed. The influence of band offsets is discussed. It is shown that energy converting junctions can be characterized accurately by these measurements. Charge carrier kinetics in the junctions are controlled by charge carrier separation determined by band bending and by interface recombination.

The influence of doping on charge carrier transport in a-Si : H

Abstract The influence of doping on the (opto)electronic properties of a-Sixa0:xa0H films is investigated by transient photoconductivity and photo-induced absorption measurements. The decay rate of the majority charge carrier decreases with doping due to fast minority carrier trapping already active at low doping levels. The transition from electron to hole conduction is observed at 3xa0ppm B2H6. At high doping levels the recombination rate becomes faster by recombination via doping-induced defects.

Measurement of the microwave Hall effect for the characterization of semiconductors

Abstract Microwave Hall experiments in a bimodal cavity are presented. The principle of the measurements is elucidated and the experimental set-up is considered. A quantitative evaluation of the measurement data is given. Precise cavity tuning with the help of a rotating sample holder is discussed. Results of measurements on crystalline silicon and pyrite (FeS 2 ) are shown.

Potential dependent transient microwave photoconductivity at the ZnO/electrolyte interface

Transient microwave photoconductivity measurements with pulsed laser excitation have been performed on a monocrystaline ZnO electrode in contact with aqueous electrolyte. The decay time is related to decay in the concentration of photoinduced charge carriers at the semiconductor - electrolyte interface. Measurements are carried out at different values of applied potential in the electrochemical cell, i.e. band bending in the semiconductor space charge layer (SCL), and different intensities of the laser pulse. At a lower pulse intensity, shows a pronounced maximum under low depletion conditions in the SCL. Such behaviour is not observed at higher intensity where only slightly decreases with the applied voltage. The maximum in the decay time is related to a maximum in the stationary microwave conductivity signal reported in previous papers, and reflects longer lifetime of minority carriers under low depletion conditions. Microwave absorption transient techniques can be used for fast kinetic studies in photoelectrochemistry, bypassing the typical time response constraints of other techniques.

Optoelectronic properties of microcrystalline silicon films

The optoelectronic properties of microcrystalline silicon (μc-Si) films were determined by contactless transient photoconductivity measurements in the microwave frequency range. High mobilities were observed in μc-Si produced by laser annealing although the material contains impurities diffused from the substrate and is doped. Hot wire and plasma enhanced chemical vapor deposited (PECVD) μc-Si films are characterized in the best case by mobilities in the order of magnitude of the mobility in a-Si:H.