Modulated Photocurrent Measurements Research Articles

Photo-induced carrier dynamics in nano-porous TiO2 electrochemically doped with cuprous oxide

In the present paper, photo-induced carrier dynamics and charge transfer processes on porous anodic TiO2, modified with Cu2O via pulse electrodeposition, are studied with steady-state and modulated photocurrent measurements in a wide range of experimental conditions. Surface composition was estimated using XPS. The obtained oxides are p-type semiconductors that effectively absorb visible light, and rates of charge transfer and recombination are strongly dependent on synthesis conditions. A tentative mechanism of photo-induced hydrogen evolution on such materials is proposed using recent simulation results from the literature and the dependence of formal charge transfer rate constant on potential.

The effect of luminescent coupling on modulated photocurrent measurements in multijunction solar cells.

Luminescent coupling in multijunction solar cells has a major impact on device response, and its impact on current-voltage and quantum efficiency measurements is well established. However, the role of luminescent coupling in more advanced characterization techniques such as modulated photocurrent spectroscopy is virtually unknown. Here we present measurements of the frequency-dependent photocurrent of a triple junction solar cell with significant coupling between adjacent junctions. We develop an equivalent circuit model that includes luminescent coupling which shows good agreement with the measured frequency response. The model also shows how the system response can elucidate the type of charge carrier recombination in these III-V semiconductor materials.

Determination of trapping–detrapping events, recombination processes and gap-state parameters by modulated photocurrent measurements on amorphous silicon

Analysis of the out-of-phase modulated photocurrent (MPC) signal, the so-called Y signal, is proposed for determining the trapping–detrapping events, recombination processes and gap-state parameters in amorphous silicon. This is demonstrated by analysing experimental Y spectra obtained on this material from different laboratories including our own. Model simulations are also employed in which the amphoteric nature of the dangling bonds and their distribution according to the defect-pool model are taken into account. From the reconstruction of the Y signal, phase shift and MPC amplitude spectra, several contributions resolved from the frequency dependence of the experimental Y spectra are identified. Two electron trapping–detrapping processes are resolved. These are attributed to hydrogen-related positive defects and to transitions involving the D+/0 level of the normal dangling bonds from the defect-pool distribution. At lower frequencies a residual contribution is resolved that is attributed to a term related to recombination through the D+/0 and D0/− levels. Between 300 and 150 K the above recombination contribution is essentially from the D0/− and dominates the Y signal at lower frequencies. In this region a characteristic phase lead appears, which is attributed to the existence of safe hole traps in the valence band tail. Around 150 K, trapping–detrapping events in the conduction band tail dominate.

Dye adsorption on TiO2 electrodes studied using modulated photocurrent measurements

An in-situ modulated photocurrent technique to study kinetics of dye adsorption and to determine binding constants of dye molecules on the TiO2 surface is presented. An AC-photocurrent signal is measured, when a TiO2 electrode is in contact with the dye bath, containing an inert salt for increased conductivity, upon excitation with on/off modulated light. This method provides relatively rapid results and is very useful for comparative studies. Organic dyes (D149, D5, D35) and ruthenium complexes (N719, Z907, black dye) were investigated and the effect of a coadsorbant in the dye bath (deoxycholic acid) was also analyzed.

Space charge capacitance spectroscopy in amorphous silicon Schottky diodes: Theory, modeling, and experiments

Numerical modeling of capacitance spectroscopy of hydrogenated amorphous silicon Schottky diodes is carried out. We test the accuracy of the determination of the density of states at the Fermi level, g(EF), from an analytical treatment of the temperature (T) and frequency (f) dependence of the capacitance (C). Assessment of the position of the Fermi level and the attempt-to-escape frequency of states at EF is also addressed. It is shown that the precision and reliability of the determination of g(EF) is strongly dependent on the position of the Fermi level and the shape of the DOS and that the attempt-to-escape frequency is overestimated. Numerical calculations are then used to fit experimental capacitance data. Material parameters that provide the best fits are found in quite good agreement with independent modulated photocurrent and constant photocurrent measurements. Again, the attempt-to-escape frequency deduced from the simplified analytical treatment of capacitance data is found to be overestimated.

The interaction of holes with the gap states of a-As2Se3 in annealed and light exposed states

Modulated photocurrent measurements of a-As2Se3 in annealed and light exposed states are analyzed to determine the trapping and recombination centers of holes. Under weak bias illumination a single kind of neutral trapping centers was observed. Their capture radius was found to reversibly increase after strong light exposure. Under moderate bias illumination additional gap states were observed which act as recombination centers and dominate recombination of holes.

Photovoltaic properties of polymer p–n junctions made with P3OT/BBL bilayers

The photovoltaic and interfacial properties of heteorojunction polymer device consisting of p-type poly(3-octyl thiophene) and n-type poly(benzimidazobenzophenanthroline) are investigated. The photocurrent efficiency is enhanced at least by a factor of 100 in these bilayers compared to their respective single layer structures. The interfacial properties are studied using current–voltage characteristics, transient photocurrent measurement and modulated photocurrent spectroscopy. The open circuit photovoltage corresponds to the estimated fermi level difference of the polymers. The dispersive transport and interfacial charging in these bilayers are studied using transient and modulated photocurrent measurements.

Density of states in the gap of amorphous semiconductors determined from modulated photocurrent measurements in the recombination regime

An experimental technique to study the energy profile of localized states in the gap of amorphous semiconductors is proposed. The method is based on the relationship between the recombination lifetime and the density of states (DOS) at the quasi-Fermi level for trapped carriers. We use the modulated photocurrent experiment in the recombination-limited regime as a convenient method to measure the recombination lifetime. Measurements performed as a function of temperature allow the DOS above the Fermi energy to be determined. The accuracy and limitations of the method are studied by means of computer simulations. The experimental technique is applied to obtain the density of defect states of a hydrogenated amorphous silicon sample.

Various defect states ina-Si:H studied by modulated photoconductivity spectroscopy

Various kinds of defect state distributions dominating electron trapping and recombination in a-Si:H films from different laboratories are studied by analyzing modulated photocurrent measurements obtained by varying the frequency or the bias light level. The characteristic features of the experimental modulated photocurrent spectra are compared with those predicted theoretically and a clear physical meaning is obtained. Two kinds of defect states are extracted directly from the spectra without prior assumptions about the form of the density of states. The energetic distributions of these defect states at the quasi-Fermi level and above it are calculated and their magnitude was found to differ by about an order of magnitude, while their capture coefficients differ by as much as 2--3 orders of magnitude. We assign the two kinds of defects to silicon dangling bonds with three backbonded silicon atoms and silicon dangling bonds where one backbond is substituted by hydrogen.

Modulated photocurrent measurements on an Al–Pd–Re icosahedral quasicrystal

Modulated photocurrent measurements, originally developed for analyzing the electronic states of semiconductors have been performed on an Al–Pd–Re quasicrystal having high electrical resistivity using two kinds of lasers with different excitation energies. No noticeable difference has been found in the phase shift between the two excitation energies. The data of the amplitude and phase shift of modulated photocurrent can be explained well by a simple model in which only the two processes, carrier generation and recombination, are involved. The recombination time is by about six orders larger than those reported for conventional semiconductors. The results obtained are discussed in terms of electron density of states and energy dependence of carrier mobility.

Modulated Photocurrent Measurements on Pure and V-Doped β-Rhombohedral Boron

The modulated photocurrent method has been applied to pure and vanadium (V)-doped β-rhombohedral boron (β-B) with the goal of investigating the difference in the distribution of electronic states in the band gap between them. Excitation light intensity dependence of the amplitude and phase shift of photocurrent shows that V-doped β-B has a much larger trapping states density for photoexcited carriers than pure β-B. With increasing temperature, the amplitude increases and decreases for pure and V-doped β-B, respectively, indicating that the conduction mechanism for photoexcited carrier is completely different between the two samples. The unusual negative temperature dependence for V-doped β-B is similar to that for Al–Pd–Re quasicrystal and the change of dependence from positive to negative is consistent with the approach to aluminum-based icosahedral quasicrystals in atomic structure and in transport properties by V-doping to β-B. The modulated frequency dependence of the amplitude and phase shift cannot be explained by the usual photoconduction processes, which are indicating that the gap states distribution and photoconduction processes in these materials are complicated.

Defect bands in a-Si–Ge:H alloys with low Ge content

We have studied a series of optimized glow discharge a-Si 1− x Ge x :H films with Ge content 0.02⩽ x⩽0.2. The drive level capacitance profiling method indicated defect densities < 5×10 15 cm −3 in these samples. Modulated photocurrent measurements detected two defect bands at 0.68±0.05 and 0.79±0.05 eV, compared to the single band normally observed in pure a-Si:H. The magnitudes of these bands vary with Ge content and with the state of light-induced degradation. Based on electron spin resonance (ESR) measurements on matched samples we have identified these defect bands as Si and Ge neutral dangling bonds. The Ge dangling bond concentration was found to be larger in the annealed state for all samples, even at 2 at.% Ge. However, in the light-induced degraded state the density of Si dangling bonds was larger for all the alloys in this series.

Modulated Photocurrent Measurements on Icosahedral Cluster Solids: Boron-Rich Solids and Aluminum-Based Quasicrystals

Modulated photocurrent measurements were performed on amorphous boron,β-rhombohedral boron, and Al–Pd–Re quasicrystal. For the quasicrystal, this is the first report on photoconductivity. The data obtained for amorphous boron are analyzed by a typical model which assumes interband photocarrier generation and trap-limited conduction. Those forβ-rhombohedral boron and the quasicrystal are analyzed by a modified model which takes account of two excitation processes for carriers: (i) photoexcitation directly to conduction (or valence) band and (ii) that to localized states followed by thermal excitation into conduction (or valence) band. The results indicate that the localized states significantly contribute to photoconduction. The existence of large densities of localized states is suggested both forβ-rhombohedral boron and quasicrystals.

Density of states in CuIn(SSe)2 thin films from modulated photocurrent measurements

Results of modulated photocurrent measurements in the temperature range between 180 and 300 K on polycrystalline CuIn(S,Se)2 thin films are presented. Modeling of the obtained phase shifts implies a continuous energetic distribution of traps in the band gap. A superposition of an exponential energetic distribution of traps with a characteristic energy of 60 meV and a Gaussian peak with a depth of 250 meV can explain the obtained phase shift and amplitudes between 250 and 300 K. For lower temperatures a change of the transport path, reducing the effective depth of the peak in the density of states and reducing the ‘‘attempt to escape frequency’’ is likely. At low temperatures the phase shift depends on the photon flux due to the separation of the demarcation levels whereas in the case of the higher temperatures no significant dependence on the light intensity could be detected.

Relaxation of the D center in amorphous silicon and how this accounts for the observed energy distributions of deep defects within the mobility gap

Transient capacitance measurements disclose that the mobility gap energy of the D defect in a-Si:H relaxes following a change in its charge state. Using the relaxation parameters thus determined, we solve the resulting dynamical equation to obtain the steady-state energy distributions of both the D − and D 0 sub-bands. These results agree with the experimental distributions determined by modulated photocurrent and steady-state junction capacitance measurements.

Defect Relaxation Dynamics in Amorphous Silicon

Using transient capacitance and transient spin techniques, we have the determined the manner in which the mobility gap energy of the D defect is altered following a change in its charge state. This relaxation process gives rise to a power law rather an exponential thermal release of defect electrons with time and also causes the charge emission and spin transients to obey a scaling law. We also deduce that the D°/D+ transition rate depends on the tenure of the proceeding D-/D° transition. This last aspect of the D defect emission behavior implies that it must be treated as a non-Markovian process. Such relaxation dynamics have profound consequences for the steady state distribution of D defect energies. Using the relaxation parameters determined by the transient measurements we have been able to solve a set of coupled differential equations under steady-state conditions to provide the energy distributions of both the D° and DD+ defect sub-bands. The results of these calculations agree remarkably well with the experimental distributions determined by modulated photocurrent and steady-state capacitance measurements. This implies that the statistical variations in the occupation history of the defect may be the dominant factor determining both distributions.

Defect structure and stability of a-Si:H by modulated photocurrent studies

In undoped a-Si:H metastable changes of gap state profiles with light-soaking and reverse bias annealing have been investigated by modulated photocurrent measurements. The gap state distribution above midgap shows a correlation with the Fermi level position, with a minimum at E f and in addition to the midgap defect band a peak above E f. Light-soaking increases the overall defect density whereas after reverse bias annealing only the band above E f is increased and the DOS below E f is quenched. The experimental results are inconsistent with the idea of two defect bands produced by one set of defects in different charge states.

Transient photocurrent study of the dangling bond centre in undoped amorphous silicon

Abstract The spectra of all four cross-sections for optical transitions associated with a correlated dangling bond centre in undoped amorphous silicon have been derived by combining transient and modulated photocurrent measurements. Analysis of the spectral shape of these cross-sections yields the energies of transitions between extended band states and the dangling bond centres with single and double occupancy, providing a unique energy placement of the dangling bond states. The results show that the singly and doubly occupied dangling bond states are located about 0.88 and 0.62 eV below the conduction-band edge, respectively, and the effective correlation energy is around 0.26 eV. Lattice relaxation energies are found to be between 0.09 and 0.13 eV. This implies a negligible role for lattice relaxation in the dangling bond transitions.