Modulated Photocurrent Technique Research Articles

New insight into the modulated photocurrent technique using 2D full numerical simulations

AbstractThe modulated photocurrent technique has been used for a few decades now to provide information on the localized density of states (DOS) in semiconductor thin films. In previous studies, a simplified analytical treatment has been performed in 1D to derive equations that one has been using to reconstruct the DOS located near the majority carrier band. In these former studies, all the physical quantities were considered as constant within the sample depth, however, we know that it is not the case especially in the presence of a strong gradient of photogeneration in some materials. In this paper, for the first time, we show results of 2D simulations of the modulated photocurrent (MPC) experiment in order to study the effect of vertical photogeneration inhomogeneity on the MPC and to discuss the validity of the 1D assumption in more realistic cases. Thanks to these 2D simulations, new insight is gained in the understanding of the MPC experiment. Indeed, we show that we can explain and reproduce some of the experimental behaviors that the 1D theory failed to predict. Hydrogenated amorphous silicon (a‐Si:H) is taken as a reference material for demonstration purposes, but this 2D approach can also be extended to other semiconductor materials and other types of inhomogeneities.

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.

Radiation-assisted Frenkel-Poole transport in single-crystal diamond

The measurement of the density of occupied states as a function of the applied electric field, performed on single-crystal chemical vapour deposition diamond by x-ray modulated photocurrent technique, is reported. Two regimes of non-linear charge transport were observed: a classical Frenkel-Poole (FP) process at high electric fields (>6800 V/cm), and a radiation-assisted transport mechanism at intermediate electric fields (2000 to 6800 V/cm), consisting of a double-step process in which the direct re-emission into the extended band occurs following multiple photo-induced FP-like hopping transitions.

Charge carriers' trapping states in pentacene films studied by modulated photocurrent

The modulated photocurrent (MPC) technique is employed to study the charge carriers' trapping states of pentacene films. The characteristics of the experimental MPC spectra were found to be compatible with trapping-detrapping process of holes in gap states in which their occupancy can be modified by the bias illumination. A demarcation energy level separating empty from partially occupied traps was deduced from the MPC spectra, which can be used to monitor bias-light induced changes in the quasi Fermi level. An exponential trap distribution from structural disorder and a deep metastable gaussian trap distribution from adsorbed environmental impurities were extracted by means of the MPC spectroscopy. An attempt to escape frequency of the order of 1010s−1 was deduced for the gap sates. The derived trap distributions agree with those found before by means of other techniques. The present results indicate that the MPC technique can be used as a valuable tool for pentacene films characterization since it can be also applied to field effect samples.

On the density of states of germanium telluride

Germanium telluride (GeTe) is one of the most studied phase change materials. Surprisingly, only little is known about the density of states (DOS) in its band gap. In this paper, the DOS of amorphous GeTe films is investigated both experimentally and theoretically. We propose a model for this DOS as well as estimates of some of the transport parameters of this material. Thin films of amorphous GeTe have been deposited by sputtering. Their dark and photoconductivity have been measured as a function of temperature. By means of the modulated photocurrent technique their DOS was probed, while their absorption was investigated by photothermal deflection spectroscopy at room temperature. Numerical calculations were employed to reproduce the experimental results with a proper set of transport parameters and choice of DOS. These data constitute a good basis for further study on the influence of the DOS on the aging of the sample resistance (“resistance drift”).

Light-Emitting Characteristics of Organic Light-Emitting Diodes with Ba/Al Cathode and Effect of Ba Thickness by Measuring their Built-in Potential

The electronic nature of metal–organic semiconductor contacts is a fundamental issue in the field of organic semiconductor device physics, because these contacts control the charge injection. The built-in potential in organic light-emitting diodes (OLEDs) with a Ba/Al cathode was investigated by using the modulated photocurrent technique. To measure the built-in potential, a device with a glass/tin-doped indium oxide (ITO)/tris(8-quinolinolato)aluminum (III) (Alq3, 150 nm)/Ba (x nm, x=3, 2, 1, and 0)/Al (150 nm) structure was fabricated and encapsulated in a nitrogen atmosphere. The device with Ba/Al cathode showed a higher built-in potential, compared with the Al-only device, which reduced the barrier height for electron injection from the Ba/Al cathode to Alq3. For the device with a Ba thickness of 3 nm, the barrier height for electron injection showed a low value of 0.1 eV. On the basis of the built-in potential data, the device with the ITO/4,4',4''-tris(2-naphthylphenyl-1-phenylamino)triphenylamine (2-TNATA, 30 nm)/4,4'-bis(N-(1-napthyl)-N-phenyl-amino)-biphenyl (NPB, 18 nm)/Alq3 (62 nm)/Ba (3 nm)/Al (100 nm) structure showed the best characteristics with the highest luminance of 54,000 cd/m2 and the highest efficiency of 2.7 lm/W, as compared to the other devices with Ba thicknesses of less than 3 nm.

Solution-processed conjugated polymer organic p-i-n light-emitting diodes with high built-in potential by solution- and solid-state doping

Polymer p-i-n homojunction light-emitting diodes (LEDs) comprising p-doped poly(dioctylfluorene-alt-benzothiadiazole) (F8BT) hole-injection, intrinsic F8BT emitter, and n-doped F8BT electron-injection layers have been demonstrated. A thin F8BT film was photocrosslinked and bulk p-doped by nitronium oxidation, then overcoated with an F8BT layer which was then surface n-doped by contact printing with naphthalenide on an elastomeric stamp. These LEDs exhibit high built-in potential (Vbi=2.2 V), efficient bipolar injection, and greatly improved external electroluminescence efficiency compared to control devices without the p-i-n structure. A modulated photocurrent technique was used to measure this Vbi, which systematically improves with diode structure.

Built-in Voltage in Alq3 Based Organic Light-Emitting Diodes Incorporating PEDOT:PSS and LiF Layer

Built-in voltage was measured in Alq3 based organic light-emitting diodes incorporating a PEDOT:PSS and LiF layer by using a modulated photocurrent technique at ambient conditions. A device was made with a structure of anode/Alq3/cathode. Either an ITO or an ITO/PEDOT:PSS was used as an anode, and a LiF/Al was used as a cathode. The built-in voltage of the device is determined from a bias voltage-dependent photocurrent. The applied bias voltage when the magnitude of modulated photocurrent is zero corresponds to a built-in voltage. It was found that an incorporation of PEDOT:PSS layer between the ITO and Alq3 increases a built-in voltage by about 0.4 V. This is consistent to a difference of a highest occupied energy states of ITO and PEDOT:PSS. This implies that a use of PEDOT:PSS layer in anode lowers a hole barrier height. With a use of bilayer cathode system LiF/Al, it was found that a built-in voltage increases as the LiF layer thickness increases in the thickness range of 0 ∼ 1 nm. It indicates that a very thin alkaline metal compound LiF lowers an electron barrier height. These results could be related to an improvement of device efficiency brought about by the insertion of PEDOT:PSS and LiF layer.

Top-emitting organic light-emitting diodes with Ba∕Ag/indium tin oxide cathode and built-in potential analyses in these devices

Top-emitting organic light-emitting diodes (TEOLEDs) with a thin semitransparent conducting cathode (STCC) of Ba∕Ag/indium tin oxide (ITO) were fabricated and their electric/optical characteristics were investigated. At the wavelength of 520nm, optical properties of STCC of the Ba(3nm)∕Ag(15nm)/ITO (100nm) structure showed the transmittance of 63% and the reflectance of 37%. The light out-coupling properties of the TEOLED, which is composed of glass/Ag(150nm)/ITO (130nm)∕4,4′,4″-tris[2-naphthylphenyl-1-phenylamino]triphenylamine (2-TNATA, 30nm)/4,4′-bis[N-(1-napthyl)-N-phenyl-amino]-biphenyl (18nm)/tris(8-quinolinolato)aluminum (III) (62nm)∕Ba (xnm,x=3,2,and1nm)∕Ag(15nm)/ITO (100nm), was increased as the deposition thickness of Ba is increased. This driving performance of the devices could be interpreted on the base of carrier injection barrier by measuring built-in voltage as well as both the optical properties and electric properties of the cathode. The optical properties (e.g., transmittance and reflectance) and electric properties of all STCCs (Ba∕Ag/ITO) used in this study were nearly equal. However, built-in voltage studied using modulated photocurrent technique was increased as the thickness of barium composing of STCC was increased. This rising of a built-in voltage means a lowering of barrier height for electron injection in the devices.

Alg3두께 변화에 따른 유기 발광 소자의 내장 전압

Built-in voltage in ITO/<TEX>$Alq_3$</TEX>/ Al organic light-emitting diodes was studied by varying a thickness of <TEX>$Alq_3$</TEX> layer using modulated photocurrent technique at ambient condition. A thickness of the <TEX>$Alq_3$</TEX> layer was varied from 100 to 250 nm. From the bias voltage-dependent photocurrent, built-in voltage of the device was able to be determined. The obtained built-in voltage is about 0.8 V irrespective of the <TEX>$Alq_3$</TEX> layer thickness in the device. This value of built-in voltage confirms that the built-in voltage is generated due to a difference of work function of the anode and cathode. The <TEX>$Alq_3$</TEX> layer thickness independent built-in voltage indicates that the built-in electric field in the device is uniform across the organic layer.

Photoconductivity in molecularly doped poly(methylmethacrylate) sandwich cells

Poly(methyl)methacrylate was made photoconducting by molecular doping and the photoconductivity was investigated using modulated photocurrent technique. Low-temperature current-voltage measurements showed that the transport mechanism was thermally activated hopping. An experimental investigation of the photoconductivity action spectrum along with theoretical calculation enabled an estimation of the diffusion coefficient of the material. The presence of states with a distribution of lifetimes could be understood from the frequency response of the photocurrent. The photocurrent was due to the field-assisted dissociation of these states.

Modulated photoconductivity method for investigation of band gap states distribution in silicon-based thin films

It has been demonstrated that the well known modulated photocurrent technique can be modified to escape imperfect data in intrinsic parameters of amorphous and microcrystalline silicon films and to simplify measurements of the density of localized states distribution in the band gaps of these semiconductors. The information on the density-of-states distribution can be extracted from temperature dependence measurements of the constant and modulated components of the photoconductivity in the film illuminated by the light modulated with some selected frequencies. The modified method has been applied to microcrystalline hydrogenated silicon films with n- and p-type conduction. The study has demonstrated that the tail of the density-of-states distribution near the valence band of microcrystalline hydrogenated silicon is less steep than that near the conduction band.

Investigation of bandgap states using the modulated photocurrent technique in both high and low frequency regimes

The modulated photocurrent (MPC) technique is reviewed, emphasizing both the widely used high frequency regime and the newly developed low frequency regime in order to characterize localized bandgap states. The matching of the density of states reconstructed from both regimes in principle allows a good determination of the ratio of the capture coefficient to the extended states mobility of the type of carriers giving the predominant contribution to the MPC. Experimental discrepancies are discussed in the framework of non-homogeneous generation rates and the existence of states having quite different capture coefficients.

Post-growth annealing effects in compensated μc-Si:H samples

In this work we study the effect of low temperature annealing on the transport properties of μc-Si:H. We performed post-growth annealing on a series of micro-doped μc-Si:H samples having different degrees of compensation, extending from n-type to p-type character. The samples were isothermally annealed at 423 K, 10 K below the deposition temperature. An increase of up to four orders of magnitude was observed in the dark conductivity ( σ dk) for the low temperature region. Simultaneously, the activation energy ( E a) of the dark conductivity changes depending on the doping level and the annealing time. Absorption coefficient and density of states measurements have been done for different annealing stages by using the constant photocurrent method and the modulated photocurrent technique, respectively. The density of defects in the midgap region increases during the annealing while the band tails remain unaffected. The σ dk and E a behavior can be interpreted in terms of a shift of the Fermi level towards the valence band. A model based in doping activation is proposed in order to explain this behavior.

Modulated photocurrent to characterize the density of states of thin film semiconductors in the recombination regime

It has been demonstrated that the Modulated Photo Current (MPC) technique is a very good tool to probe the density of states (DOS) in the band gap of semiconductors. In this technique two regimes have been underlined: the low frequency or recombination regime and the high frequency or trapping-and-release regime. It is the latter case that has been mainly used up to now. In this paper we concentrate on the recombination regime. Calculations are made for a single species of trapping states characterized by the respective capture coefficients of electrons and holes. From the general continuity equations, a very simple relation is found between the density of states around the pseudo Fermi level and the slope of the tangent of the phase shift as function of the angular frequency of the excitation. A simulation illustrates the validity of this relation by comparing the DOS reconstructed from both the high frequency and the low frequency regimes of the MPC technique with that introduced in the simulation. The approximations used to obtain this very simple relation are underlined. Finally, it is suggested that by combining the results of the MPC technique obtained in the low and high frequency regimes, good orders of magnitude of the capture coefficient of the majority carriers and of their mobility can be obtained.

The modulated photocurrent technique: a powerful tool to investigate band gap states in silicon based thin films

The Modulated PhotoCurrent (MPC) technique is a very simple experiment that provides insight into localized gap states in semiconductors. It is particularly well suited to disordered silicon based thin films. We here give a review of the technique, emphasizing the effect of experimental parameters (frequency, temperature, dc generation rate) and the reconstruction of the density of states (DOS) in the two regimes: the widely used high frequency regime and the recently developed low frequency regime. We discuss the effect of different trap species and propose a new way to determine capture cross sections from the matching of the reconstructed DOS in both regimes. Illustrations are given theoretically with the help of a numerical modelling, and experimentally from data obtained on polymorphous silicon-germanium alloys. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Photocarrier transport in undoped microcrystalline silicon studied by the modulated photocurrent technique

A detailed investigation of photocarrier transport and localized states in undoped hydrogenated microcrystalline silicon (μc-Si:H) has been performed using the modulated photocurrent (MPC) technique. The MPC-derived drift mobility shows a systematic variation with crystalline grain size and volume fraction, as well as a distinct anisotropy. The physical implications of these observations are discussed in accordance with models that consider connectivity of crystalline grains and geometry of individual grains. The frequency spectra of MPC mobility and their dependence on excitation intensity are also analyzed in comparison with those observed for hydrogenated amorphous silicon (a-Si:H), leading to a common property of localized states that can explain the experimental results for these materials.

Gap states of hydrogenated amorphous silicon near and above the threshold of microcrystallinity with subtle boron compensation

The effects of hydrogen dilution, subtle boron compensation, and light-soaking on the gap states of hydrogenated amorphous silicon films (a-Si:H) near and above the threshold of microcrystallinity have been investigated in detail by the constant photocurrent method and the improved phase-shift analysis of modulated photocurrent technique. It is shown that high hydrogen dilution near the threshold of microcrystallinity leads to a more ordered network structure and to the redistribution of gap states; it gives rise to a small peak at about 0.55 eV and a shoulder at about 1.2 eV below the conduction band edge, which are associated with the formation of microcrystallites embedded in the amorphous silicon host matrix. A concurrent subtle boron compensation is demonstrated to prevent excessive formation of microcrystallinity, and to help promote the growth of the ordered regions and reduce the density of gap defect states, particularly those associated with microcrystallites. Hydrogen-diluted and appropriately boron-compensated a-Si:H films deposited near the threshold of microcrystallinity show the lowest density of the defects in both the annealed and light-soaked states, and hence, the highest performance and stability.

Electronic properties of CVD polycrystalline diamond films

Electronic properties of polycrystalline diamond films have been analysed using steady-state photoconductivity under UV (λ=190 nm) and red (λ=633 nm) illumination. We look at the effect of several post-deposition treatments: annealing in vacuum at 400°C, successive annealing under air and methane or the reverse, and X-ray exposure. These are shown to affect the photoresponses in different manners. Globally, the mobility–lifetime products of samples are found in the range 10 −7–10 −6 cm 2 V −1 under UV illumination, and ratios of photoresponses in UV and red are in the range 10 3–10 5. First results obtained using the modulated photocurrent technique are also presented.

New results on the modulated photocarrier grating technique

A modulated photocarrier grating technique (MPG) has been used to measure some properties of amorphous silicon samples. The intermediate region where neither the lifetime, τ, nor the dielectric relaxation time, τ d, are dominant and local charge neutrality may not be maintained has been investigated. Light intensity dependence of the small-signal lifetime estimated from a modulated photocurrent technique (MPC) was also studied. A qualitative agreement has been found between the data and the MPG theory which shows that this technique measures the response time/relaxation time ratio. Based on the results we suggest an overestimate of the ambipolar diffusion length obtained by the steady-state photocarrier grating technique (SSPG) in these samples which is an indication that local charge neutrality is not preserved.