Steady-state Photoconductivity Research Articles

Modelling of transport and recombination of photocarriers in un-doped hydrogenated amorphous silicon (a-Si:H)

In this paper, we report on the simulation of steady state photoconductivity in un-doped a- Si:H at temperatures from 30 to 500 K. The model is based on recombination at dangling bond states and band tail states. It takes also into account the hopping transitions in the conduction hand tail states to describe the conduction in localized states at low temperatures. At high temperatures, the multiple trapping process is considered to describe the conduction in extended states. The density of states includes the exponential density of conduction band tail states and valence band tail slates and the density of dangling bond states. This later is determined by the Defect Pool Model ‘DPM’. The experimental features observed on the temperature dependence of the photoconductivity are generally the thermal quenching, the low activated region and the temperature independent photoconductivity at very low temperatures. All these observations are well reproduced by the model in un-doped a-Si:H. By the examination of the relative contributions of two processes of conduction: (i) the multiple trapping and (ii) the multiple trapping associated with the hopping, the model results show that the multiple trapping process of electrons where the conduction is assured by free carriers in the thermal quenching region above 140 K is important while the hopping process of electrons is negligible. At 140 K and below, the hopping transport of electrons in the conduction band tail states makes an important contribution in the photoconductivity. It explains successfully the low activated region and the temperature independent photoconductivity at very low temperatures.

Adequate method to study the surface passivation effectiveness in HEM multicristallin silicon wafers

In this work we have examined the effectiveness of surface passivation on ascut multicrystalline silicon (mc-Si) wafers using different techniques. The study is based on minority carrier lifetime measurements with quasi steady state photo-conductance, ‘QSSPC’. Effective minority carrier lifetime ()measured values of 12.4, 8.9, 4.9 and 3.1 are obtained respectively with four silicon surface passivation techniques: 1- Shallow phosphorous diffusion emitter (n+p), 2- Iodine-Ethanol (I-E), 3-Hydrofluoric acid (HF) emersion and 4-SiNx layer deposition. These results suggest that the shallow n+p emitter gives the eff close to the bulk lifetime due to the better surface passivation quality. Simulations made with Hornbeck-Haynes model indicate that the improvement can be correlated with the decrease of the surface recombination velocity (SRV) and the increment of bulk lifetime.

The native and metastable defects and their joint density of states in hydrogenated amorphous silicon obtained from the improved dual beam photoconductivity method

In this study, undoped hydrogenated amorphous silicon (a-Si:H) thin films deposited under moderate dilution ratios of silane by radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) have been investigated using steady-state photoconductivity and improved dual beam photoconductivity (DBP) methods to identify changes in multiple gap states in annealed and light-soaked states. Four different gap states were identified in annealed state named as A, B, C, and X states. The peak energy positions of these Gaussian distributions are consistent with those recently identified by Fourier transform photocurrent spectroscopy (FTPS). After in situ light soaking, their density increases with different rates as peak energy positions and half-widths remain unaffected. The electron-occupied A and B states located below the dark Fermi level and their density and ratios in the annealed and light-soaked states correlate well with those defects detected by time-domain pulsed electron paramagnetic resonance (EPR) experiments. The A, B, and X states located closer to the middle of the bandgap anneal out at room temperature in dark and define the “fast” states. However, the C states show no sign of room temperature annealing such that they must define the “slow” states in undoped a-Si:H. The results found in this study indicate that the anisotropic disordered network is a more appropriate model than previously proposed defect models based on the continuous random network to define the nanostructure of undoped a-Si:H, where multiple defects, D0 and non-D0 defects, can be identified by using the improved DBP method.

Characterization of some optical and physical properties of As11.2S48.0Sb28.8Te12.0 and As20.8S48.0Sb19.2Te12.0 nanostructured polycrystalline semiconductors

This work presents the characterization of As-S-Sb-Te nanostructured polycrystalline semiconductors by X-ray fluorescence (XRF), X-ray diffraction (XRD), electron microscopy (SEM), optical as well as photoelectric methods. The XRD patterns have been shown the presence of amorphous and nanocrystalline phases with the main structural units As2S3, Sb2S3 and Sb2Те3. The IR transmission spectra show a high transparence just with a single weak absorption band about ν=2340 cm-1 caused of the presence of H2S impurity. It was also observed that for the alloys of As11.2S48.0Sb28.8Te12.0 and As20.8S48.0Sb19.2Te12.0, with an increase in the number of As atoms, the transmission in the IR region also increase. The maximum of steady-state photoconductivity for both materials, measured in the linear portion of I-V characteristics, is situated around =0.96 m.

Hybrid perovskite films deposited by thermal evaporation from a single source

Hybrid perovskites, such as MAPbI3, are very promising materials to fabricate highly efficient and low-cost solar cells and photodetectors. In this work, we propose a new method of deposition of hybrid perovskite thin films through thermal evaporation of MAPbI3 powder from a single source for which was evaluated the effect of evaporation parameters (distance, time, substrate temperature, chamber pressure, and crucible heating rate) on the structural, optical, and morphological properties. It was found that it is necessary to control adequately the chamber partial pressure, substrate temperature, and the crucible heating rate in order to obtain single-phase MAPbI3 perovskite films. Also, it was studied the solvent annealing treatment as a method to increase the grain size of evaporated films. Finally, a study of transient and steady-state photoconductivity allowed verifying that the evaporated MAPbI3 films present fast band to band photogeneration and bimolecular recombination, and also that the grain size growth leads to an increase of the photoconductivity intensity due to the improvement of the diffusion length.

Solution-Processed Donor-Acceptor Poly(3-hexylthiophene):Phenyl-C61-butyric Acid Methyl Ester Diodes for Low-Voltage α Particle Detection.

Diodes fabricated using a blend of poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester (6-80 μm thick) as an organic semiconductor component achieved consistent 4 MeV α particle detection. Current-voltage characteristics and current-time measurements were obtained under α irradiation and in its absence. Steady-state and transient (time-of-flight) photoconduction measurements were additionally performed. Low-bias (<20 V) α particle detection gain-efficiency products of order 10-2 were measured. The α particle detection was achieved reproducibly, reversibly, and repeatably in different devices of varying organic semiconductor layer thicknesses using both the steady-state and time-dependent (dynamic) diode responses. Conductive gain, due to trapped electrons, increased the α particle gain-efficiency product in both forward and reverse bias conditions as well as increasing steady-state photoconduction. The device thickness was optimized to maximize the gain-efficiency product by matching the penetration depth of the α particle, obtained by modeling, to the organic semiconductor layer thickness. Very high confidence α particle detection was achieved (with signal-to-noise ratios exceeding 20) under optimized device dimensions and drive conditions. Hecht function fitting of the gain-efficiency product versus electric field data returns mobility-lifetime products of order 10-6-10-7 cm2 V-1. This work demonstrates that solution-processed organic semiconductor diodes are viable for low-voltage α particle detection.

Absorption and photoconductivity spectra of amorphous multilayer structures.

The experimental results regarding optical absorption and steady-state photoconductivity of amorphous single-layer structures (Al–As0.40S0.30Se0.30–Al, Al–Ge0.09As0.09Se0.82–Al, and Al–Ge0.30As0.04S0.66–Al) and of an amorphous heterostructure (Al–As0.40S0.30Se0.30/Ge0.09As0.09Se0.82/Ge0.30As0.04S0.66–Al) at different values of the voltage, with positive or negative polarity, applied to the illuminated top Al electrode are presented and discussed. The complex structure of the photocurrent spectra is attributed to the different values of the optical bandgap of the involved amorphous layers (Eg ≈ 2.0 eV for As0.40S0.30Se0.30 and Ge0.09As0.09Se0.82 and Eg ≈ 3.0 eV for Ge0.30As0.04S0.66). The obtained experimental results are discussed taking into account the light absorption depending on the nature and the thickness of each amorphous layer, on the wavelength, and on contact phenomena at the interfaces between different layers and between the amorphous layers and the metal electrodes with different work functions.

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.

Wet Chemical Oxidation to Improve Interfacial Properties of Al2O3/Si and Interface Analysis of Al2O3/SiOx/Si Structure Using Surface Carrier Lifetime Simulation and Capacitance–Voltage Measurement

A thin silicon oxide (SiOx) layer (thickness: 1.5–2.0 nm) formed at an Al2O3/Si interface can enhance the interface properties. However, it is challenging to control the characteristics of thin SiOx layers because SiOx forms naturally during Al2O3 deposition on Si substrates. In this study, a ~1.5 nm-thick SiOx layer was inserted between Al2O3 and Si substrates by wet chemical oxidation to improve the passivation properties. The acidic solutions used for wet chemical oxidation were HCl:H2O2:H2O, H2SO4:H2O2:H2O, and HNO3. The thicknesses of SiOx layers formed in the acidic solutions were ~1.48, ~1.32, and ~1.50 nm for SiOx-HCl, SiOx-H2SO4, and SiOx-HNO3, respectively. The leakage current characteristics of SiOx-HNO3 were better than those of the oxide layers formed in the other acidic solutions. After depositing a ~10 nm-thick Al2O3 on an SiOx-acidic/Si structure, we measured the effective carrier lifetime using quasi steady-state photoconductance and examined the interfacial properties of Al2O3/SiOx-acidic/Si using surface carrier lifetime simulation and capacitance–voltage measurement. The effective carrier lifetime of Al2O3/SiOx-HNO3/Si was relatively high (~400 μs), resulting from the low surface defect density (2.35–2.88 × 1010 cm−2eV−1). The oxide layer inserted between Al2O3 and Si substrates by wet chemical oxidation helped improve the Al2O3/Si interface properties.

The effects of air, oxygen and water exposure on the sub-bandgap absorption, the electronic conductivity and the ambipolar diffusion length in highly crystalline microcrystalline silicon films for photovoltaic applications

Reversible and irreversible changes due to long term air and short term de-ionized water (DIW) or pure oxygen exposure were investigated in about 1 µm thick hydrogenated microcrystalline silicon (µc-Si:H) films deposited on rough glass substrates, thereby comparing highly crystalline with compact material. Time and temperature dependent dark conductivity, steady-state photoconductivity, the steady-state photocarrier grating and dual-beam photoconductivity methods have been used to study the effects. Standard measurement procedures defined previously have been carefully applied to record the changes after different treatments using the steady-state methods under light. After long term air exposure of highly crystalline µc-Si:H films, a thermal annealing step leads to an increase in dark conductivity (σD) and steady-state photoconductivity (σph) as well as to a significant increase in the sub-bandgap absorption. These effects are likely due to a reversible recovery from surface adsorbents in a porous microstructure after air exposure resulting in surface charge and Fermi level shifts in agreement with earlier results. Compact µc-Si:H films showed only marginal effects upon an annealing after long term air exposure suggesting much reduced susceptibility to surface adsorbent induced by Fermi level shifts. Five hours exposure to de-ionized water at 80 °C caused more than an order of magnitude increase in σD and σph and a substantial decrease in the sub-bandgap absorption spectrum in highly crystalline as well as in compact µc-Si:H films. In addition, minority carrier diffusion lengths measured by the steady-state photocarrier grating method improved significantly. The changes after exposure to water were not reversible upon our standard annealing procedure. Exposure to high purity oxygen gas at 150 °C resulted in similar effects like the exposure to DIW. Also here the changes in material properties were not reversible upon annealing. Results are discussed in terms of adsorption and chemical reactions on surfaces in the porous highly crystalline material versus the materials with more compact structures. Results are compared to earlier observations and consequences for device application will be indicated.

Photochromic Materials: Photochromic Response and Steady‐State Photoconductivity of Fibrous‐Reticulated α‐MoO3 Thin Films Prepared by Modified Spray Pyrolysis Technique (Crystal Research and Technology 10/2019)

Crystal Research and TechnologyVolume 54, Issue 10 1970028 Cover PictureFree Access Photochromic Materials: Photochromic Response and Steady-State Photoconductivity of Fibrous-Reticulated α-MoO3 Thin Films Prepared by Modified Spray Pyrolysis Technique (Crystal Research and Technology 10/2019) First published: 07 October 2019 https://doi.org/10.1002/crat.201970028AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Graphical Abstract Molybdenum oxide thin films show considerable changes in their physical and chemical properties after UV exposure. In article number 1900122, Aly M. Badr and co-workers use a modified spray pyrolysis technique for preparing α-MoO3 thin films. The thickness effect on the structural, morphological, optical, electrical, and photo-electrical properties of the films is studied. The UV exposure time effect as well as the thickness effect on the photochromic response of the samples are investigated and discussed based on the particle size analysis. Additionally, the photosensitivity is elucidated, and the mechanism of the recombination processes in these films is determined. Volume54, Issue10October 20191970028 RelatedInformation

Temperature-dependent photoconductive properties of Ge-Sb-Te thin films

ABSTRACTThe present paper reports the study of photoconductive properties viz. steady state photoconductivity, dark conductivity , dark activation energy , transient photoconductivity on vacuum evaporated thin films of Ge20Te80-xSbx (x = 0, 2, 4, 6, 10 at. %). At higher concentration of antimony, dark activation energy is found to decrease. A similar trend in the optical band gap is also observed. Optical band gap also decreases with the addition of antimony in the system. This drop in activation energy points towards an increase in the density of defect states in the system. Dark conductivity values are found to increase, and this rise is due to an increase in polarizability of antimony and partial ionic behaviour of the system. Photosensitivity decreases with an increase in the antimony content in the matrix. Photocurrent versus light intensity follows the power law and predominance of bimolecular recombination is found. During transient measurements, the decay rate is found to be non-exponential. A differential lifetime is found to increase with the addition of antimony as a dopant. The study of the decay of photocurrent reveals relaxation processes in the present chalcogenide glassy matrix. This behaviour indicates a slow decay process due to the increase in the density of defect state in the mobility gap. Similar results are also confirmed with an increase in the carrier concentration.

Carrier transport in methylammonium lead iodide perovskite single crystals studied by dark current, steady state and transient photocurrent measurements

Electronic transport properties of methylammonium lead iodide perovskite single crystals prepared by inverse temperature crystallisation have been investigated using dark current, steady state and transient photocurrent measurements. Above 250 K the dark conductivity at electric fields below 100 V/cm is thermally activated, with single activation energy 0.7 eV and magnitude 4×10−8 S/cm at 300 K. At higher electric fields, conductivity increases as a power law suggesting space charge limitation. Steady state photoconductivity exhibits a roughly linear dependence on photon flux below 1015 cm−2s−1 and square root dependence above this, consistent with trap-limited and band-to-band recombination mechanisms respectively. Photocurrent overshoot and undershoot on a timescale of 10-100 s are observed on application and removal of a steady light source. Transient photocurrent measurements with temperature reveal a peak in the localised density of states 0.26 eV into the band gap, with an attempt to escape frequency of 1011 s−1. Deeper states are distributed exponentially with characteristic energy 54 meV, decreasing towards mid-gap.

The determination of carrier lifetimes and associated mobility magnitudes using photoconductivity recovery dynamics in thin-film amorphous semiconductors

The extraction of trap-limited mobility information using photoconductivity relaxation dynamics in thin-film amorphous semiconductors is evaluated. Simulated photoconductivity dynamics suggest that free carrier decay is characterised by a spectrum of recombination lifetimes, where the spectra are sensitive to the underlying distribution of shallow trap densities and the experimental photoconductivity generation conditions employed. Weighted averaging of the spectral lifetimes is found to return trap-limited lifetime estimates which agree to within 5% of the lifetime values expected from free-to-trapped carrier ratios established under steady-state photoconductivity conditions. The averaged spectral lifetimes may consequently be used with mobility-lifetime magnitudes to determine the associated trap-limited mobility. The analysis procedure is evaluated using photoconductivity data obtained from a set of a-Si:H films deposited over a range of substrate temperatures.

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.

Quantum Oscillations of Photoconductivity Relaxation in p–i–n GaAs/InAs/AlAs Heterodiodes

The photoconductivity and its relaxation characteristics in tunneling p–i–n GaAs/AlAs heterostructures under pulsed illumination is studied. Quantum oscillations in the photoconductivity are detected depending on the bias voltage with the period independent of the light wavelength, as well as an oscillating component of the relaxation curves caused by modulation of the recombination rate at the edge of a triangular quantum well in the undoped i layer, as in the case of photoconductivity oscillations. The common nature of oscillations of the steady-state photoconductivity and relaxation curves under pulsed illumination is directly confirmed by the lack of an oscillating component in both types of dependences of some studied p–i–n heterostructures. Simultaneous suppression of the observed oscillations of dependences of both types as the temperature increases to 80 K also confirms the proposed mechanism of their formation. The dependences of these oscillations on the magnetic field and light flux power are studied. Oscillation-amplitude suppression in a magnetic field of ~2 T perpendicular to the current is caused by the effect of the Lorentz force on the ballistic motion of carriers in the triangular-quantum-well region.

Estimation of dark and active dielectric constants in the sub-THz frequency domain of an optically tunable organic semiconductor blend of poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester

The dielectric properties of a 95% poly(3-hexylthiophene):5% phenyl-C61-butyric acid methyl ester blend are measured in the dark and under white light illumination by quasi-optical transmissometry and terahertz time-domain spectroscopy. The real part of the dielectric constant varies monotonically between 2.75 and 3.50, in agreement with the literature, and displays a reversible photoinduced drop of 0.05–0.55 at sub-THz. The imaginary part fluctuates between 0.1 and 1.5 in the dark and displays a reversible increase upon illumination of 0.10–0.52 at sub-THz. The corresponding charge carrier concentration under illumination (using transient and steady-state photoconduction) is 1014 to 1015 cm−3.

Квантовые осцилляции релаксации фотопроводимости в p-i-n-гетеродиодах GaAs/InAs/AlAs

AbstractThe photoconductivity and its relaxation characteristics in tunneling p – i – n GaAs/AlAs heterostructures under pulsed illumination is studied. Quantum oscillations in the photoconductivity are detected depending on the bias voltage with the period independent of the light wavelength, as well as an oscillating component of the relaxation curves caused by modulation of the recombination rate at the edge of a triangular quantum well in the undoped i layer, as in the case of photoconductivity oscillations. The common nature of oscillations of the steady-state photoconductivity and relaxation curves under pulsed illumination is directly confirmed by the lack of an oscillating component in both types of dependences of some studied p–i–n heterostructures. Simultaneous suppression of the observed oscillations of dependences of both types as the temperature increases to 80 K also confirms the proposed mechanism of their formation. The dependences of these oscillations on the magnetic field and light flux power are studied. Oscillation-amplitude suppression in a magnetic field of ~2 T perpendicular to the current is caused by the effect of the Lorentz force on the ballistic motion of carriers in the triangular-quantum-well region.