Steady-state Photoconductivity Measurements Research Articles

Space-Charge-Limited Photocurrents and Transient Currents in (Cd,Zn)Te Radiation Detectors

We combined steady-state photoconductivity and laser-induced transient current measurements under above-band-gap illumination to study the space charge formation in CdZnTe. Analytical as well as numerical models describing space charge limited photocurrents were developed and an excellent agreement with measured data was obtained especially with the Drift-diffusion model. Linear rise of photocurrent at low bias was observed and ascribed to the trapping of injected holes at the region close to the cathode side. Influence of space charge formation, photoconductive gain, contribution of shallow and deep levels to photocurrent-voltage characteristics were numerically simulated. According to the measurements and calculations, recent principles used at the evaluation of detector properties, mainly the mobility-lifetime product, via the photoconductivity are critically assessed.

Can we use time-resolved measurements to get steady-state transport data for halide perovskites?

Time-resolved, pulsed excitation methods are widely used to deduce optoelectronic properties of semiconductors, including now also Halide Perovskites (HaPs), especially transport properties. However, as yet, no evaluation of their amenability and justification for the use of the results for the above-noted purposes has been reported. To check if we can learn from pulsed measurement results about steady-state phototransport properties, we show here that, although pulsed measurements can be useful to extract information on the recombination kinetics of HaPs, great care should be taken. One issue is that no changes in the material are induced during or as a result of the excitation, and another one concerns in how far pulsed excitation-derived data can be used to find relevant steady-state parameters. To answer the latter question, we revisited pulsed excitation and propose a novel way to compare between pulsed and steady state measurements at different excitation intensities. We performed steady-state photoconductivity and ambipolar diffusion length measurements, as well as pulsed time-resolved microwave conductivity and time-resolved photoluminescence measurements as a function of excitation intensity on the same samples of different MAPbI3 thin films, and found good quasi-quantitative agreement between the results, explaining them with a generalized single level recombination model that describes the basic physics of phototransport of HaP absorbers. Moreover, we find the first experimental manifestation of the boundaries between several effective recombination regimes that exist in HaPs, by analyzing their phototransport behavior as a function of excitation intensity.

Role of Oxygen for Boron Diffusion From Borosilicate Glass Layers Deposited by Atmospheric Pressure Chemical Vapor Deposition

We investigate the impact of different oxygen concentrations during boron diffusion at 950°C using borosilicate glass (BSG) layers deposited by atmospheric pressure chemical vapor deposition. Therefore, we vary the boron concentration in the BSG layer and the oxygen concentration in the gaseous ambient of the high-temperature step. Characterization includes sheet resistance as well as electrochemical capacitance voltage measurements for the determination of the resulting boron doping profile. In addition, we perform quasi steady-state photoconductance measurements to investigate the influence of the oxygen concentration on the recombination parameters of the boron-diffused surface. Low recombination current densities J 0 of J 0 = 38 fA/cm² at a sheet resistance of 54 Ω/sq are observed. Finally, we show that the oxygen concentration is a crucial parameter to prevent the formation of a boron-rich layer at the silicon surface.

Photoluminescence and Photoconductivity to Assess Maximum Open-Circuit Voltage and Carrier Transport in Hybrid Perovskites and Other Photovoltaic Materials.

Photovoltaic (PV) device development is much more expensive and time-consuming than the development of the absorber layer alone. This Perspective focuses on two methods that can be used to rapidly assess and develop PV absorber materials independent of device development. The absorber material properties of quasi-Fermi level splitting and carrier diffusion length under steady effective 1 Sun illumination are indicators of a material's ability to achieve high VOC and JSC. These two material properties can be rapidly and simultaneously assessed with steady-state absolute intensity photoluminescence and photoconductivity measurements. As a result, these methods are extremely useful for predicting the quality and stability of PV materials prior to PV device development. Here, we summarize the methods, discuss their strengths and weaknesses, and compare photoluminescence and photoconductivity results with device performance for four hybrid perovskite compositions of various bandgaps (1.35-1.82 eV), CISe, CIGSe, and CZTSe.

Obtainment of the density of states in the band tails of hydrogenated amorphous silicon

In this work, we present two new pairs of formulas to obtain a spectroscopy of the density of states (DOS) in each band tail of hydrogenated amorphous silicon (a-Si:H) from photoconductivity-based measurements. The formulas are based on the knowledge of the small-signal recombination lifetime τ′, the characteristic decay time of the concentration of trapped carriers generated in excess by the illumination, and that can be measured by methods like the Oscillating Photocarrier Grating (OPG) or Moving Grating Technique (MGT). First, we deduce the formulas and test their accuracy by numerical simulations using typical a-Si:H parameters. Next, we characterize an a-Si:H sample using well-known methods, like Fourier transform photocurrent spectroscopy to evaluate the valence band tail and modulated photoconductivity to measure the conduction band tail. We also performed measurements of steady-state photoconductivity, steady-state photocurrent grating and MGT, for a range of generation rates. From these measurements—and taking typical values for the capture coefficients, the extended states mobilities and the DOS at the band edges—we apply the new formulas to get the band tails. We find that the results obtained from the application of our formulas are in good agreement with those found with the traditional methods for both band tails. Moreover, we show that MGT/OPG measurement to get τ′ can be avoided if one of the band tails is measured by one of the traditional methods, since the known band tail can be used to evaluate τ′ with one pair of equations, and then the other pair can be applied to get the other band tail.

Effects of Low Temperature Anneal on the Interface Properties of Thermal Silicon Oxide for Silicon Surface Passivation.

High quality surface passivation has gained a significant importance in photovoltaic industry for reducing the surface recombination and hence fabricating low cost and high efficiency solar cells using thinner wafers. The formation of good-quality SiO2 films and SiO2/Si interfaces at low processing temperatures is a prerequisite for improving the conversion efficiency of industrial solar cells with better passivation. High-temperature annealing in inert ambient is promising to improve the SiO2/Si interface. However, annealing treatments could cause negative effects on SiO2/Si interfaces due to its chemical at high temperatures. Low temperature post oxidation annealing has been carried out to investigate the structural and interface properties of Si-SiO2 system. Quasi Steady State Photo Conductance measurements shows a promising effective carrier lifetime of 420 μs, surface recombination velocity of 22 cm/s and a low interface trap density (D(it)) of 4 x 10(11) states/cm2/eV after annealing. The fixed oxide charge density was reduced to 1 x 10(11)/cm2 due to the annealing at 500 degrees C. The FWHM and the Si-O peak wavenumber corresponding to the samples annealed at 500 degrees C reveals that the Si dangling bonds in the SiO2 films due to the oxygen defects was reduced by the low temperature post oxidation annealing.

Structural and photoconductivity properties of silicon carbon thin films

AbstractNanostructured films composed of silicon crystallites dispersed in an hydrogenated amorphous silicon carbon matrix have been deposited by plasma enhanced chemical vapour deposition from silane‐methane mixtures diluted in hydrogen by varying the rf power. Compositional, structural, optical and photoconductivity properties of the films have been investigated. The increase in rf power in the 40‐80 W range enhances the incorporation of carbon and hydrogen in the amorphous matrix and decreases the volume fraction and size of the silicon crystallites leading to an enlargement of the optical band gap from 2.07 to 2.20 eV. Steady state photoconductivity measurements, performed under monochromatic radiations in the visible range, have demonstrated the occurrence of monomolecular recombination kinetics in the samples under illumination and that the mobility‐lifetime product of the free electrons as a function of the optical gap decreases from 1.9 × 10–7 to 2.5 × 10–9 cm2V–1 as in the case of device quality films deposited by silane‐methane mixtures diluted in hydrogen. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electronic properties of undoped microcrystalline silicon oxide films

The electronic properties of undoped microcrystalline silicon oxide films have been investigated by transient photocurrent (TPC) density of states (DOS) spectroscopy, supported by dark conductivity, steady-state photoconductivity, and constant-photocurrent measurements (CPM). Film compositions span the range from amorphous to microcrystalline and contain up to 10% oxygen content, yielding optical bandgap values E04 (the photon energy at which the absorption depth equals one micrometre) between 1.85 and 2.11 eV. Carrier transport is consistent with multiple-trapping in a localised DOS, which depends upon film structure and oxygen content. TPC measurements indicate that both conduction band-tail energy and deep defect density increase with increasing oxygen content, accompanied by a reduction in majority carrier mobility-lifetime product. CPM measurements on amorphous films show a broadening of the Urbach tail with increasing oxygen content. Significantly higher oxygen incorporation without seriously compromising electronic quality appears possible in microcrystalline films. This suggests potential application as solar cell absorber layers offering increased optical bandgap and open-circuit voltage.

Defect states in amorphous Ge2Sb2Te5 phase change material

Steady-state photoconductivity measurements in the temperature range 100–300 K on amorphous Ge2Sb2Te5 thin film prepared by dc sputtering are analyzed. The dark conductivity is thermally activated with a single activation energy that allocates the position of the Fermi level approximately in the middle of the energy gap relative to the valance band edge. The temperature dependence of the photoconductivity ensures the presence of a maximum normally observed in chalcogenides with low- and high-temperature slopes, which predict the location of discrete sets of localized states (recombination levels) in the gap. The presence of these defect states close to the valence and conduction band edges leaves the quasi Fermi level shifts in a continuous distribution of gap states at high temperatures, as evidenced from the γ values of the lux–ampere characteristics.

Direct monitoring of minority carrier density during light induced degradation in Czochralski silicon by photoluminescence imaging

In this paper, we present a new method for studying the light induced degradation process, in which the minority carrier density is monitored directly during light soaking by photoluminescence imaging. We show experimentally that above a certain minority carrier concentration limit, Δnlim, the boron oxygen (B-O) defect generation rate is fully independent of the injected carrier concentration. By simulation, we determine Δnlim for a range of p-type Czochralski silicon samples with different boron concentrations. The normalized defect concentrations, Nt*, are determined for the same samples by time-resolved Quasi Steady State Photoconductance measurements. After 10 min of light degradation, no correlation between Δnlim, and Nt* is observed. These results indicate that the role of the excess carriers during the rapid decay is to first change the charge state of the defects by shifting the electron quasi-Fermi level across the energy level of the defect centre in its passive state (Elat = EV + (635 ± 18) meV) and that, subsequently, another rate-determining step proceeds before the defect centre becomes recombination active.

Validity of Calibrated Photoluminescence Lifetime Measurements of Crystalline Silicon Wafers for Arbitrary Lifetime and Injection Ranges

We investigate the validity of calibrated photoluminescence lifetime measurements of crystalline silicon wafers for arbitrary lifetime and injection ranges. Absolute lifetime images are obtained from steady-state photoluminescence measurements by relating the photoluminescence signal to the excess carrier density. Since the luminescence signal is expected to be related to the integral of the depth distribution of the excess carrier density, an adequate calibration of the luminescence signal requires a secondary method which yields the integral of the depth distribution of the excess carrier density in absolute units. In this paper, we investigate the applicability of steady-state photoconductance measurements for the calibration of the photoluminescence signal. We derive a generalized relation linking the photoluminescence signal with the excess carrier density, considering the impact of an inhomogeneous carrier concentration profile. We experimentally verify the impact of the carrier distribution on the photoluminescence calibration by investigating two silicon wafers with different electronic bulk properties. Finally, we propose an iterative correction procedure reducing the deviations due to an inhomogeneous carrier density profile of calibrated photoluminescence-based lifetime measurements significantly.

Non-linear injection level dependence of the excess carrier lifetime of p-type GaSb thin films: A two-layer model

The injection dependence of the excess carrier lifetime has been investigated for undoped and zinc doped p-type gallium antimonide thin films using steady-state photoconductivity measurements. Discrepancies between the observed lifetimes and the theoretical values are resolved using a two-layer generation/recombination model. Simulation of the room temperature experimental results yields values for the surface band bending, an upper limit to the surface recombination velocity and a lower limit to the Shockley-Read-Hall carrier lifetime.

Photoconduction spectroscopy of p-type GaSb films

Excess carrier lifetimes (77K) have been measured as function of the absorbed flux density in undoped p-type gallium antimonide films (GaSb/GaAs) using steady state photoconductivity measurements with the illumination wavelength of 1.1μm. Using the results from Hall effect measurements along with the relations describing the lifetimes of the excess minority carriers in the bulk of the films and at the surface, the theoretical values of the effective excess carrier lifetime in the materials were also calculated. Discrepancies between the experimental and theoretical results were described using a two-layer model, by considering the variation in the charge distribution within the layer due to the presence of surface states, as well as the band offset between the layer and the substrate. Theoretical modeling of the experimental result yields values of different parameters such as band bending at the surface, minimum value of Shockley–Read–Hall lifetime and maximum value of the surface recombination velocity.

Photocarrier relaxation through the manifold of localized states in a polymer-fullerene bulk heterojunction material

We investigate the transport and relaxation of photogenerated carriers in a polymer-fullerene bulk-heterojunction (BHJ) material by measuring the transient photoconductivity as a function of the sample temperature. Data in the nanosecond time regime indicate that following the initial trapping (within ∼100 ps), the photogenerated carriers relax into the manifold of disorder-induced localized states near the band edge, and the transport becomes thermally activated. By measuring the temporal dependence of the transport activation energy, we are able to monitor the relaxation of carriers into deeper traps, with a corresponding reduction in carrier mobility. A comparison of BHJs made from different molecular weight poly(3-hexylthiophene) (P3HT) (21 and 95 kDa), indicates that higher molecular weight polymer in the BHJ material yields a consistently higher activation energy. The reduction in the carrier mobility in BHJ materials made with higher molecular weight P3HT is confirmed by steady-state photoconductivity measurements. In contrast to disordered inorganic semiconductors (e.g., amorphous silicon), the activation energies associated with the density of states in the disorder-induced band tails are a small fraction (<10%) of the band gap energy of P3HT.

Light, annealing and plasma induced changes on the electrical properties of a-GaSe thin films

Thin films of GaSe were deposited at room temperature on glass substrate by the physical vapor deposition technique. These films were irradiated with light for 6 hrs at room temperature using heat filtered white light by using a tungsten lamp (1035 lux), annealed at 523 K for 1 hr and irradiated with argon plasma (p∼0.2 mbar, I=20 mA) for 1 hr. Dark conductivity measurements were made on as-deposited and irradiated GaSe thin films in the temperature range 100–370 K in order to identify the conduction mechanism and the effect of different treatments on its electrical properties. The obtained results revealed two distinct regions. The mechanisms of such regions were analyzed. At high temperatures dc conductivity (σd) obeys the law ln σ∝1/T, indicating conduction in extended states, and at low temperatures, obeys the law ln σ∝T−1/4, indicating variable-range hopping in localized states near the Fermi level. The density of localized states N(EF) and various other Mott’s parameters like the degree of disorder (To), hopping distance (R) and hopping energy (W) near the Fermi level is calculated before and after different treatments using dc conductivity measurements at low temperatures. Steady-state photo-conductivity measurements were done in the temperature range 303–373 K by using heat filtered white light.

Observation of Meyer-Neldel rule in thermally activated Poole-Frenkel photoconduction in a-Se70Te30−x Cd x thin films

In the present paper, we report the compensation effect on photoconductivity of thin films of Se70Te30−xCdx (x = 2, 4, 6) alloys for high field conduction. Steady state photoconductivity measurements have been made in thin films for different electric fields. The photoconduction was found to be ohmic at low fields and Poole-Frenkel type at high fields. Meyer-Neldel rule (MN rule) for the pre-exponential factor (σph)0 and activation energy (ΔEph) of photoconduction in thin films of Se70Te30−xCdx is observed in the present study.

Steady-state and transient photoconductivity in c-axis GaN nanowires grown by nitrogen-plasma-assisted molecular beam epitaxy

Analysis of steady-state and transient photoconductivity measurements at room temperature performed on c-axis oriented GaN nanowires yielded estimates of free carrier concentration, drift mobility, surface band bending, and surface capture coefficient for electrons. Samples grown (unintentionally n-type) by nitrogen-plasma-assisted molecular beam epitaxy primarily from two separate growth runs were examined. The results revealed carrier concentration in the range of (3–6)×1016 cm−3 for one growth run, roughly 5×1014–1×1015 cm−3 for the second, and drift mobility in the range of 500–700 cm2/(V s) for both. Nanowires were dispersed onto insulating substrates and contacted forming single-wire, two-terminal structures with typical electrode gaps of ≈3–5 μm. When biased at 1 V bias and illuminated at 360 nm (3.6 mW/cm2) the thinner (≈100 nm diameter) nanowires with the higher background doping showed an abrupt increase in photocurrent from 5 pA (noise level) to 0.1–1 μA. Under the same conditions, thicker (151–320 nm) nanowires showed roughly ten times more photocurrent, with dark currents ranging from 2 nA to 1 μA. With the light blocked, the dark current was restored in a few minutes for the thinner samples and an hour or more for the thicker ones. The samples with lower carrier concentration showed similar trends. Excitation in the 360–550 nm range produced substantially weaker photocurrent with comparable decay rates. Nanowire photoconductivity arises from a reduction in the depletion layer via photogenerated holes drifting to the surface and compensating ionized surface acceptors. Simulations yielded (dark) surface band bending in the vicinity of 0.2–0.3 V and capture coefficient in the range of 10−23–10−19 cm2. Atomic layer deposition (ALD) was used to conformally deposit ≈10 nm of Al2O3 on several devices. Photoconductivity, persistent photoconductivity, and subgap photoconductivity of the coated nanowires were increased in all cases. TaN ALD coatings showed a reduced effect compared to the Al2O3 coated samples.

Electrical and photovoltaic properties of cobalt doped zinc oxide nanofiber/n-silicon diode

Nanofiber Co-doped ZnO (ZnO:Co) film was deposited on n-type silicon substrate at room temperature by using sol–gel method. Scanning electron microscopy results indicate that cobalt doped zinc oxide film has a nanofiber structure. The electrical and photovoltaic properties of the Au/ZnO:Co/n-Si diode were investigated by current–voltage, capacitance–voltage, transient current and steady state photoconductivity measurements. The Au/ZnO:Co/n-Si diode exhibits a photovoltaic behavior with a maximum open circuit voltage V oc (0.195 V) and short-circuit current I sc (2.63 μA) values under 100 mW cm 2 illumination intensity and the photoconductivity mechanism of the diode is controlled by the presence of continuous distribution of traps. The interface state density for the diode was determined by conductance method under dark and illumination conditions. The interface state density D it and time constant τ values for the diode under dark and 100 mW/cm 2 conditions were found to be 1.26 × 10 10 eV −1 cm −2, 6.02 × 10 −5 s and 9.81 × 10 10 eV −1 cm −2, 7.35 × 10 −7 s, respectively. The obtained photovoltaic and photocapacitance results indicate that the diode can be used both as a photodiode and photocapacitive sensor for visible light sensor applications.

A hybrid p-Si/poly(1,4-diaminoanthraquinone) photoconductive diode for optical sensor applications

We report on the fabrication and characterization of a hybrid Si/poly(1,4-diaminoanthraquinone) photoconductive diode for optical sensor applications. The electrical and photoconductivity properties of the diode have been investigated by dark current–voltage, steady-state and transient photoconductivity measurements. At lower voltages, the current mechanism of the diode is controlled by thermionic emission theory, whereas at higher voltages, the current mechanism is controlled by space charge limited current due to the electrical conductivity of the poly(1,4-diaminoanthraquinone). The ideality factor, barrier height and series resistance values of the diode were found to be 1.72, 0.82 eV and 1.15 MΩ, respectively. The steady-state photoconductivity mechanism of the diode indicates the presence of continuous distribution of trap levels. The transient photoconductivity results indicate that the photocurrent of the diode was varied from 1.81 × 10 −12 to 8.16 × 10 −7 A. This suggests that the photocurrent under the illumination of 3500 lx is 4.50 × 10 5 times higher than the dark current. It is evaluated that the hybrid Si/poly(1,4-diaminoanthraquinone) device is a photoconductive diode with photovoltaic properties.

Photogeneration and charge transport mechanisms in thin films of organic electronacceptor 2,4,7-trinitro-9-fluorenone

The photogeneration process in thin films of vacuum-evaporated amorphous 2,4,7-trinitro-9-fluorenone(a-TNF) is investigated by steady-state photoconductivity measurements and by thephotoinduced discharge method. It was found that in the intrinsic spectral range of absorption(λ≤400 nm) the charge carrier photogeneration mechanism includes several steps. The final step,thermal dissociation of light generated electron–hole pairs, is realized according to theOnsager mechanism. The effect of trapping centers on the conductivity of thin filmsof a-TNF is investigated by space charge limited current (SCLC), steady-statephotoconductivity, and transient photoconductivity methods. It is suggested that thetransport of charge carriers in a-TNF is controlled by traps.