Contactless Transient Photoconductivity Measurements Research Articles

Analysis of the Surface Passivation Mechanisms of p-Doped Crystalline Silicon by Two Different Al<sub>2</sub>O<sub>3</sub> Coatings via Transient Photo-Conductance Measurements

In this paper, contactless transient photo-conductance measurements are applied to p-doped mono-crystalline Silicon (p-Si) coated by two different kinds of aluminum oxide (Al2O3) layers in order to study alternative routes to the standard atomic layer deposition (ALD). The aluminum oxides layers were deposited by either spin coating or ion layer gas reaction (ILGAR?). For both coatings an increase of the charge carrier life time is observed indicating a passivation of the p-Si surface. This study shows alternative deposition methods and the potential of transient photocon- ductance measurements for the elucidation of the origin of the passivation. We show that the passivation induced by coating deposited via ILGAR is at least partially due to charge carrier trapping and storage at the interface. It was also surprisingly found that for those coatings, annealing at 425℃ leads to a decrease of the life time. This points to temperature instability for both coatings.

Dye Regeneration Dynamics by Electron Donors on Mesoscopic TiO2 Films

The influence of different redox mediators and electrolyte constituents on charge transfer processes in dye-sensitized solar cells (DSC) was studied by contactless transient photoconductance (TPC) measurements. Device subsets without FTO (Fluorine-doped tin oxide) contacts were prepared to increase the reliability and sensitivity of the TPC measurements. Results revealed a decrease of electron decay rate with increasing concentration of the redox couple ([Co(III/II)(bpy)3](PF6)3/2) which was attributed to an improvement of the reduction of the dye cation (dye regeneration). Furthermore, the presence of the electrolyte additives 4-tert-butylpyridine (TBP) and chenodeoxycholic acid was found to decrease the electron decay rate, which was attributed to blocking of interfacial charge recombination. The use of other redox couples (quinone/hydroquinone and ferrocene/ferrocenium) with these electrolyte additives displayed a significant improvement for ferrocene/ferrocenium, while the electron lifetime for quinone/hydroquinone appeared unaffected. Prospects for a further optimization of DSC’s redox couples are discussed.

Charge carrier kinetics in MnOx, Mn2O3 and Mn3O4 films for water oxidation

Manganese oxide films on conductive substrates were prepared by electrochemical deposition. Charge carrier kinetics in these films was studied by contactless transient photoconductivity measurements. These measurements revealed the generation of mobile charge carriers with a nonneglectable lifetime. The transport from charge carriers generated in the oxides to the substrate was observed.

Influence of dye-adsorption on charge carrier kinetics in TiO2 colloidal systems

Charge carrier kinetics in TiO2 powders and films was analyzed by contactless transient photoconductance (TPC) measurements in the microwave frequency regime. The difference between the behavior of powders and films was relatively small and in both cases an extended decay behavior was observed. A decrease in decay rate of the TPC signal (due to excess electrons) upon dye-sensitization after band-to-band excitation with laser pulses at 355nm was detected, which was attributed to a reduced recombination rate due to the trapping of excess holes by adsorbed dye molecules. Furthermore, it was found that adsorbed dye molecules are very efficient hole traps.

Charge carrier kinetics in ZnO films and nanorods

The kinetics of charge carriers in ZnO films and nanorods was analyzed by contactless transient photoconductance (TPC) measurements. The use of two excitation wavelengths allowed to identify the decay processes. The TPC signal displayed a significantly accelerated decay in ZnO films after sensitization with a ruthenium bipyridyl dye which was attributed to a higher electron–hole recombination whereas the electron–dye cation recombination was relatively slow. ZnO nanorods displayed no such destructive influence of the dye adsorption. Recombination processes took place over a large time range because of the heterogeneity of the samples.

Analysis of Charge Carrier Kinetics in Nanoporous Systems by Time Resolved Photoconductance Measurements

A quasi-solid-state dye-sensitized solar cell is presented, where an electrolyte film replaces the conventional liquid electrolyte, reaching a solar light-to-current conversion efficiency of 3%. This proves the importance of surface conduction of the positive charge, e.g., via the Grotthuss mechanism. Contactless transient photoconductance measurements were performed on operating cells, revealing decay behavior of photoinduced charge carriers, dependent on external applied potential conditions. The measurements show that the decay is controlled by the injection of electrons into the front contact, hindered or enhanced by the field in the space charge region. Furthermore, the influence of redox molecules on the decay of photogenerated charge carriers in sensitized TiO2 films was analyzed and found to be dependent on the redox pair concentration. We ascribe this effect to the regeneration of the oxidized dye by iodide, in this way screening the positive charge from possible recombination with injected electrons and leading to a prolonged decay.

Photocatalytic Activity of BulkTiO2Anatase and Rutile Single Crystals Using Infrared Absorption Spectroscopy

A systematic study on the photocatalytic activity of well-defined, macroscopic bulk single-crystal TiO(2) anatase and rutile samples has been carried out, which allows us to link photoreactions at surfaces of well-defined oxide semiconductors to an important bulk property with regard to photochemistry, the life time of e-h pairs generated in the bulk of the oxides by photon absorption. The anatase (101) surface shows a substantially higher activity, by an order of magnitude, for CO photo-oxidation to CO(2) than the rutile (110) surface. This surprisingly large difference in activity tracks the bulk e-h pair lifetime difference for the two TiO(2) modifications as determined by contactless transient photoconductance measurements on the corresponding bulk materials.

Charge Carrier Transport in the Bulk and at the Surface of Nanoparticles: A Quasi-Solid-State Dye-Sensitized Solar Cell

AbstractA quasi-solid-state dye-sensitized solar cell is presented, where the conventional liquid electrolyte is replaced by an electrolyte film, reaching a solar light-to-current conversion efficiency of 3%. Contactless transient photoconductance measurements were performed, revealing decay behavior of photoinduced charge carriers, dependent on external applied potential conditions. The measurements show that the decay is controlled by the injection of electrons into the front contact, hindered or enhanced by the field in the space charge region.

Electronic transport in semiconductor nanoparticles for photocatalytic and photovoltaic applications

It is shown by contactless transient photoconductance measurements that electron transport in TiO2 is highly dispersive and is strongly influenced by surface treatments. A fast recombination process is observed after band to band excitation. Oxalic acid absorbed at the surface slows the decay appreciably, as does alcohol. The electron transport in TiO2 films and powder of the same composition is nearly identical pointing to an only minor importance of inter grain transport for the present measurements (up to 100 μs). Dye sensitization of TiO2 leads not only to charge carrier generation by light in the visible range but also seems also to improve the electron transport properties.

Optoelectronic properties of SnO2 / TiO2 junctions

Abstract Transparent, conductive SnO 2 (FTO) films and colloidal TiO 2 films deposited on these FTO films are investigated by means of contactless transient photoconductance measurements in the microwave frequency range (TRMC measurements). For the bare FTO–glass films excitation with 355 nm light pulses leads to appreciable TRMC signals indicating the semiconductor properties of the material. For TiO 2 films on FTO–glass films, injection of excess electrons from TiO 2 into FTO is observed.

Characterization of InP and GaAs films by contactless transient photoconductivity measurements

The characterization of III–V semiconductor films by contactless and non-invasive conductance and transient photoconductance measurements in the microwave frequency range were investigated. It was shown that the conductivity (of a GaSb film) and the majority carrier mobility (of GaAs and InP films) can be determined. Besides, information on charge carrier transport, in particular at the surface is obtained.

Structural influence on charge-carrier lifetimes in TiO2 powders studied by microwave absorption

A set-up for contactless transient photoconductivity measurements on powders by the Time Resolved Microwave Photoconductivity (TRMC) method has been realized. These measurements have been used as a tool for the investigation of excess charge-carrier lifetimes in TiO2 (anatase and rutile) powders. The influence of laser wavelength and intensity on TRMC decay shows the importance of the fast-recombination processes. The presence of a long time tail in the TRMC signals of the anatase modification of TiO2 is attributed to quenching of this recombination by hole-trapping at the surface. The influence of surface treatment by ethanol and water on TRMC decay evidences that dominant effects are bulk recombination in rutile and surface trapping in anatase. The influence of doping in rutile shows that increasing the doping rate accelerates the decay whatever the doping type is. The doping element acts as an impurity favoring recombination by creation of structural defects. The influence of thermal treatment in anatase shows that increasing the thermal treatment temperature increases the lifetimes. High crystalline quality leads to long charge-carrier lifetimes. The results are discussed in view of their relevance for photocatalysis.

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%) cm2 V−1 s−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.

Contactless characterization of a-Si:H films on crystalline silicon substrates

The passivating influence of a-Si:H films on the c-Si surface is shown by contactless transient photoconductivity measurements in the microwave frequency range. A relation between the signal and the performance of a-Si:H/c-Si heterojunctions for solar energy conversion was found. The injection of excess charge carriers from the a-Si:H film into the c-Si substrate was observed and shown to yield information on charge carrier transport in the a-Si:H film and in the space charge region.

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.

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.

Optoelectronic characterization of microcrystalline silicon films

The optoelectronic properties of μc-Si films produced in different ways are studied by contactless transient photoconductivity measurements in the microwave frequency range. A strong charge carrier trapping is observed in most μc-Si films. The effective mobility determined from these measurements appears to be an appropriate parameter to characterize these films. The films show a high absorption at 1064 nm, tentatively attributed mainly to defect absorption.

Passivation of silicon by silicon nitride films

It is shown by contactless transient photoconductivity measurements in the microwave frequency range that Si 3N 4 films are an outstanding passivation of the n-type c-Si surface. Si 3N 4 on n-type Si forms an accumulation layer, which acts as an ideally reflecting potential barrier for minority carriers (holes). Due to the small space charge layer capacitance, minority carrier storage at this interface is very limited. In contrast to the latter measurements on p-type Si wafers covered with Si 3N 4 are characterized by storage of excess charge carriers in the surface depletion layer. The stored charge carriers decay slowly. The minority carriers (electrons) collected at the surface show a reduced mobility.

Recombination at high charge carrier concentrations in a-Si:H films

The recombination at high excess charge concentrations (higher than 1018 cm−3) in a-Si:H has been studied by comparison of the excess electron concentration obtained by contactless transient photoconductivity measurements with numerical calculations. The first stage of the recombination process is described satisfactorily by a recombination between mobile excess electrons and all excess holes. In later stages this simple model predicts a too high recombination rate. A better description has been obtained if the recombination rate parameter depends on the energy of the hole.

The characterization of silicon nitride films by contactless transient photoconductivity measurements

It is shown by non-invasive photoconductivity measurements that the coating of Si wafers by silicon nitride films leads to the increase of number of excess charge carriers generated in the silicon substrate by visible light due to the antireflection properties of the films. The silicon surface is electrically passivated by the deposition of these films on top of it.