Photoconductivity Measurements Research Articles

Diffusivity Reveals Three Distinct Phases of Interlayer Excitons in MoSe_{2}/WSe_{2} Heterobilayers.

Charge separated interlayer excitons in transition metal dichalcogenide heterobilayers are being explored for moiré exciton lattices and exciton condensates. The presence of permanent dipole moments and the poorly screened Coulomb interaction make many-body interactions particularly strong for interlayer excitons. Here we reveal two distinct phase transitions for interlayer excitons in the MoSe_{2}/WSe_{2} heterobilayer using time and spatially resolved photoluminescence imaging: from trapped excitons in the moiré potential to the modestly mobile exciton gas as exciton density increases to n_{ex}∼10^{11} cm^{-2} and from the exciton gas to the highly mobile charge separated electron-hole plasma for n_{ex}>10^{12} cm^{-2}. The latter is the Mott transition and is confirmed in photoconductivity measurements. These findings set fundamental limits for achieving quantum states of interlayer excitons.

Low temperature carrier transport mechanism and photo-conductivity of WSe2

This work reports the electrical-transport and temperature-dependent photoconductivity in tungsten diselenide (WSe2) thin films. Temperature-dependent electrical conductivity has been demonstrated using different models. At lower temperatures (< 190 K), carriers become localized to small regions in the film due to Mott’s hopping mechanism. The middle-temperature region (190–273 K) follows Seto’s parameters and obtained low barrier height (0.0873 eV) may be responsible for the improved carrier mobility. At higher temperature (> 273 K) region, thermally activated conduction is dominated with two activation energies of ~138 meV and 98 meV. The peaks obtained in photoluminescent analysis attributes to the presence of mid-bandgap states or defect states which play an important role in the photoconductivity of WSe2. The transient photoconductivity measurements show consistent temperature-dependent behaviour. The effect of light intensity and wavelength variation on the photoconductivity of WSe2 thin films is also discussed. The photo-current is 1.19 * 10−5 A at 125 K, while at 350 K was observed to be 3.12 * 10−4 A. The light-on/off current cycles show that the current can recover to its initial state after various cycles, which points to the WSe2 thin-film device's stable and reversible properties that can be used in optoelectronic applications.

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.

Express Characterization of the HgCdTe/CdHgTe Quantum Well Waveguide Heterostructures with the Quasi-Relativistic Carrier Dispersion Law by Room-Temperature Photoluminescence Spectroscopy

An express technique for characterizing narrow-gap waveguide heterostructures with the HgCdTe/CdHgTe quantum wells is proposed, which is based on an analysis of their room-temperature luminescence response. Advantages and constraints of this characterization technique are discussed in comparison with methods for more detailed diagnostics that entail photoluminescence and photoconductivity measurements in a wide temperature range.

Highly Mobile Large Polarons in Black Phase CsPbI3

We report the band-like transport of photogenerated charge carriers within all-inorganic black γ-phase CsPbI3 (γ-CsPbI3) thin films, with local mobilities up to 270 ± 44 cm2V–1s–1 recorded using terahertz (THz) spectroscopy at room temperature. Temperature-dependent, high-frequency photoconductivity measurements indicate that large polaron formation governs charge carrier transport, following the Feynman polaron model. The mobility values derived using THz spectroscopy are nearly 1 order of magnitude higher than that reported for hybrid organic–inorganic lead halide perovskites and approach the theoretical limit for polarons scattering from longitudinal optical (LO) phonons. Our results identify γ-CsPbI3 as a fascinating all-inorganic perovskite semiconductor with high charge carrier mobility for optoelectronics and reveal the effect of polaron formation on charge transport properties.

Photoconductance Determination of Carrier Capture Cross Sections of Slow Traps in Silicon Through Variable Pulse Filling

The capture cross sections are a key metric to characterize carrier traps in semiconductors. In this article, photoconductance measurements are used to estimate the capture cross sections of traps that are induced by thermal donors in Czochralski-grown silicon wafers. Here, we use a pulse-filling technique with light-emitting diode excitation to measure the capture rate. The measured capture rates are shown to be similar to the values that were measured previously by deep-level transient spectroscopy. Our method allows the contactless measurements of the capture cross sections. The minority carrier cross sections for the two distinct trap states that are studied here are of the order of 10−18 cm2 and the majority carrier cross sections are orders of magnitude smaller. Therefore, it is concluded that the slow traps that are originating from the thermal donors are dominated by the minority carrier trapping.

Solution processed transparent anatase TiO2 nanoparticles/MoO3 nanostructures heterojunction: high performance self-powered UV detector for low-power and low-light applications.

Ultraviolet (UV) photodetectors are considered as the major players in energy saving technology of the future. Efforts are needed to further develop such devices, which are capable of operating efficiently at low driving potential as well as with weak illumination. Herein, we report an all-oxide, highly transparent TiO2/MoO3 bilayer film, with nanoparticulate anatase TiO2 as the platform, fabricated by a simple solution based method and demonstrate its use in UV photodetection. Photoconductivity measurement with 352 nm light reveals the self-powered UV detection capability of the device due to the built-in potential at the bilayer interface. The device exhibits a high photoresponsivity (46.05 A W−1), detectivity (2.84 × 1012 Jones) and EQE (16 223%) even with a weak illumination of 76 μW cm−2, at a low bias of only −1 V. The self-powered performance of the bilayer device is comparable to that of commercial Si photodetectors as well as other such UV detectors reported based on metal oxide heterojunctions. The improved and faster photoresponse shown by the device is due to the formation of an effective heterojunction, as evidenced by XPS, electrochemical and I–V studies. It can be further attributed to the better charge transport through the densely aligned nanostructures, reduced recombination and the better mobility of anatase TiO2 nanoparticles. The performance is best-in-class and proves the potential of the transparent heterojunction to be used in highly responsive, self-powered UV detectors for low bias, low light applications.

Synthesis and Controllable Growth Dynamics of Third Order Nonlinear Optical Material of 2-Amino-4-Methylpyridinium 2-Chloro 4-Nitro Benzoate Electro-Mechanical Investigation and Device Fabrication

An organic nonlinear optical material 2-Amino-4-methylpyridinium 2-chloro 4-nitro benzoate (AMPCNB) was synthesized and large size single crystals were grown by slow evaporation solution growth method. Single crystal X-ray diffraction study showed that the title compound belongs to monoclinic crystal system with P21/n space group. A powder XRD study was performed to ascertain its phase. The UV–VIS-NIR study was performed to investigate the transparency window and lower cutoff wavelength of the compound, no absorption has been observed between 318 and 1100 nm. The third harmonic efficiency of the title compound has been studied using the Z-scan technique using continuous-wave Nd:YAG laser to confirm its saturable absorption and self-defocusing effect. The photo conducting and charge transport properties are analyzed using photoconductivity and dielectric measurements. Theoretical calculation of molecular polarizability, which is helpful in device fabrication, was carried out from Penn gap, Clausius-Mosotti equations and the obtained results were compared. Mechanical properties of the grown crystal were investigated by Vicker’s microhardness study.

Growth and characterization of organic material 2-bromobenzyl alcohol single crystal by vertical Bridgman technique

The organic material 2-bromobenzyl alcohol (C7H7BrO) single crystal was grown by using vertical Bridgman technique. The lattice parameters of the 2-bromobenzyl alcohol crystal were determined by single-crystal X-ray diffraction technique. The Fourier Transform Infrared spectroscopy is used to identify the functional groups of the grown 2-bromobenzyl alcohol. Thermal analyses were carried out to analyze the melting and decomposition points of the grown crystal. The UV–Vis–NIR and Fluorescence studies assessed the optical properties of the grown crystals. The second-harmonic generation study analyzed non-linear properties. The laser damage threshold value for the grown 2-bromobenzyl alcohol single crystal was measured by using Q-switched Nd:YAG laser. The photoconductivity measurements of the grown crystal were estimated by KEITHLEY 6487 instrument. The dielectric permittivity was analyzed at various temperatures with different frequencies from 1 to 5 MHz. The mechanical strength of the grown crystal was analyzed by using Vickers microhardness analysis.

In situ photoconductivity measurements of imidazole in optical fiber break-junctions.

We developed a method based on the mechanically controllable break junction technique to investigate the electron transport properties of single molecular junctions upon fiber waveguided light. In our strategy, a metal-coated tapered optical fiber is fixed on a flexible substrate, and this tapered fiber serves as both the optical waveguide and metal electrodes after it breaks. For an imidazole bridged single-molecule junction, two probable conductance values below 1G0 are observed. The higher value shows an approximately 40% enhancement under illumination, while the lower one does not show distinguishable difference under illumination. Theoretical calculations reveal these two conductance values resulting from the imidazole monomer junction and the imidazole dimer junction linked via a hydrogen bond, respectively. In imidazole monomer junctions, the absorption of a single photon strongly shifts the transmission function resulting in optical-induced conductance enhancement. In contrast, the transmission function of imidazole dimer junctions remains at the same level in the bias window despite the light illumination. This work provides a robust experimental framework for studying the underlying mechanisms of photoconductivity in single-molecule junctions and offers tools for tuning the optoelectronic performance of single-molecule devices in situ.

Effect of Covalent Surface Functionalization of Si on the Activity of Trifluoromethanesulfonic Anhydride for Suppressing Surface Recombination.

An examination of the efficacy of combining physisorbed and chemisorbed passivation strategies on crystalline Si has been performed. This report compares the influence of a linear alkyl adsorbate tethered by either a Si-C or Si-Si linkage, prepared by reaction of Si(111) with organometallic Grignard reagents or organosilanes, respectively. These modified surfaces are first analyzed and compared by IR and X-ray photoelectron spectroscopies. Their behavior toward a known potent physisorbate, trifluoromethanesulfonic anhydride (Tf2O), is then examined. Microwave photoconductivity measurements were obtained which indicate that, while Tf2O shows a beneficial lowering of surface recombination on both surface types initially, only surfaces featuring Si-C linkages exhibit long-lasting suppressed surface recombination. The data for Grignard-treated Si after exposure to Tf2O in fact represent the longest known report of surface recombination suppression by a physisorbate. Conversely, the data for the Si surfaces prepared by dehydrogenative coupling suggest that these passivating groups themselves introduce defect states that cannot be ameliorated by Tf2O physisorption.

Evaluating the ratio of electron and hole mobilities from a single bulk sample using Photo-Seebeck effect

When a semiconductor is under photoexcitation, the voltage response to a temperature gradient is the photo-Seebeck effect. Here we study this effect, focusing on the contribution from transport of photo-excited carriers. We demonstrate that by combining photo-Seebeck with photoconductivity measurements, one can determine the ratio between electron and hole mobilities, and hence both of them when one is known. This is found for the case of defect-free samples, where no detail on the absorbance, carrier lifetime or recombination is necessary. Our method reported here does not require chemical doping, which could introduce defects and is often not feasible. It applies to both thin film and bulk samples. Experiment-wise, photo-Seebeck effect is relatively easy to implement, or added to existing systems. In a broader context, for semiconductors with significant influence from defects, our result suggests that the photo-Seebeck behavior can still be understood. In this case another photo-transport property is necessary, in order to identify the mobilities of carriers and information regarding the defects. This framework integrates the information from photoexcitation and thermal gradients to provide a general method to determine fundamental electronic properties of materials.

Optical–Microwave Pump–Probe Studies of Electronic Properties in Novel Materials

Combined microwave–optical pump–probe methods are emerging to study the quantum state of spin qubit centers and the charge dynamics in semiconductors. A major hindrance is the limited bandwidth of microwave irradiation/detection circuitry which can be overcome with the use of broadband coplanar waveguides (CPWs). The development and performance characterization of two spectrometers is presented as follows: an optically detected magnetic resonance spectrometer (ODMR) and a microwave‐detected photoconductivity measurement. In the first method, light serves as detection and microwaves excite the investigated medium, whereas in the second, the roles are interchanged. The performance is demonstrated by measuring ODMR maps on the nitrogen‐vacancy (NV) center in diamond and time‐resolved photoconductivity in p‐doped silicon. The results demonstrate both an efficient coupling of the microwave irradiation to the samples as well as an excellent sensitivity for minute changes in sample conductivity.

Quantitative detection system for immunostrips in 180nm standard CMOS technology

In this work, a CMOS based optical read-out system for biomarker on immunostrips detection is presented. For the proposed system, a CMOS integrated circuit containing an on-chip photodiode is designed in standard 180 nm UMC CMOS Technology. The system also contains cost-effective 3D Printed structures for holding both IC and the sample immunostrip together. The proposed system can be operated in two modes (1) light reflectance and (2) light transmittance. In the system, a laser with a wavelength of 637 nm is applied to the CMOS IC through immunostrip. Photovoltaic and photoconductive measurements are carried out for each mode on a custom Gluten biomarker immunostrip. Sensing operation of the biomarker is successfully realized with optical powers from 5 mW to 8 mW. Biomaterial density on the immunostrip is sensed and images of the biomarker with varying intensities are constructed from the measurements. Feasibility of the system for low power biomarker sensing applications is demonstrated.

Near‐Infrared Light‐Responsive Cu‐Doped Cs2AgBiBr6

AbstractLead‐free halide double perovskites (A2BIBIIIX6) with attractive optical and electronic features are considered to be a promising candidate to overcome the toxicity and stability issues of lead halide perovskites (APbX3). However, their poor absorption profiles limit device performance. Here the absorption band edge of Cs2AgBiBr6 double perovskite to the near‐infrared range is significantly broadened by developing doped double perovskites, Cs2(Ag:Cu)BiBr6. The partial replacement of Ag ions by Cu ions in the crystal lattice is confirmed by the X‐ray photoelectron spectroscopy (XPS) and solid‐state nuclear magnetic resonance (ssNMR) measurements. Cu doping barely affects the bandgap of Cs2AgBiBr6; instead it introduces subbandgap states with strong absorption to the near‐infrared range. More interestingly, the near‐infrared absorption can generate band carriers upon excitation, as indicated by the photoconductivity measurement. This work sheds new light on the absorption modulation of halide double perovskites for future efficient optoelectronic devices.

Heteroleptic Tin-Antimony Sulfoiodide for Stable and Lead-free Solar Cells

Summary The quaternary chalcogenide halides of group IV and V elements have attracted much attention due to their interesting semiconducting properties as well as a suitable band gap for solar cells. Here, for the first time, we report on solar cells using tin-antimony sulfoiodide (Sn2SbS2I3). Sn2SbS2I3 solar cells were fabricated using a chemical single-step deposition process with a solution containing a SbCl3-thiourea complex and SnI2 with the configuration of TiO2 and poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-enzothiadiazole)] as the electron- and hole-transporting layers, respectively. The best-performing cell exhibits a power conversion efficiency of 4.04% under the illumination of standard AM 1.5G conditions (100 mW cm−2). These unencapsulated cells exhibited good stabilities at 80% relative humidity, 85°C in air, and under illumination, respectively. These results provide guidelines for fabrication of lead-free heteroleptic perovskite solar cells by hosting divalent or combinations of monovalent and trivalent metal cations.

Annealing temperature mediated enhanced photosensitivity in bicone-like and flower-like structures of ZnO synthesized using Co-precipitation method

Abstract Samples of two ZnO structures - bicone-like as well as flower-like structures - have been synthesized by co-precipitation method using diethanolamine and diethanolamine with polyethylene glycol as starting materials. Further, the synthesized ZnO samples have been annealed at 400 °C, 600 °C and 800 °C separately. All synthesized samples as well as annealed samples have been characterized using UV–visible spectroscopy, photoluminescence (PL) spectroscopy, field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD) technique, high-resolution transmission electron microscopy (HR-TEM), photoconductivity measurement technique and fourier transform infrared spectroscopy (FTIR). UV–visible spectra show absorption edges around 306–311 nm and 374–380 nm. PL spectra of prepared ZnO samples show a sharp and intense UV emission due to high concentration of zinc interstitial defects. XRD patterns confirm that all prepared ZnO samples possess hexagonal wurtzite structure. FE-SEM and HRTEM images show that the as-synthesized samples possess both bicone and flower-like structures, whereas annealed samples possess mostly flower-like structures. Photoconductivity property has been investigated under UV–vis illumination in air and vacuum. The samples annealed at 800 °C show highest photosensitivity as compared to as-synthesized samples and samples annealed at 400 °C and 600 °C. The photosensitivity gets increased with increase in annealing temperature as annealing results in increased concentration of zinc interstitial defects. In air medium, the annealed polymer-mediated ZnO samples show rise and decay curves with sharp rise and decay as compared to non-polymer-mediated ZnO counterparts. The photosensitivity of ZnO samples is improved in vacuum as compared to that in air, which may due to occurrence of carbo-optical reactions as a result of presence of hydrocarbons.

Electrical, optical and photoconductive properties of Sn-doped indium sulfofluoride thin films

This work reports on undoped and Sn-doped indium sulfofluoride thin-films deposited by radio-frequency plasma-enhanced reactive thermal evaporation. The deposition was performed evaporating pure indium or indium-tin alloy in SF6 plasma at substrate temperatures ranging from 373 to 423 K. Rutherford backscattering analysis and secondary-ion mass spectrometry were used to determine the chemical composition of the films. The film characterization includes electrical, optical, and photoconductivity measurements. The resistivity of undoped material varies in a wide range of 1 GΩ-cm to 2 TΩ-cm depending on deposition conditions. Sn doping leads to a decrease in the resistance down to 8 MΩ-cm. The films are highly transparent in the visible-infrared region due to an indirect bandgap of 2.7–3 eV. Moreover, the doped material is highly photosensitive in the blue - UV region. Photoconductivity kinetics under various excitation conditions was also studied. The synthesized material is a promising candidate for a buffer layer in chalcogenide-based solar cells.

Influence of phase transformation on structure–property relationship in quaternary In10Sb10Ag10Se70 chalcogenide films

This work reports the effect of thermal annealing (353–413 K) on structure and physical properties of thermally evaporated In10Ag10Sb10Se70 chalcogenide films. The samples were characterized by X-ray diffraction, transmission electron microscopy, TEM, Raman spectroscopy, optical absorption and photoconductivity measurements. Thermal annealing favours amorphous to polycrystalline structure transition with the growth of Se, AgInSe2 and AgSbSe2 phases. A significant change in position/shape of Raman bands upon annealing has been observed. The indirect optical gap decreases from 1.44 to 0.68 eV while tailing parameter increases with annealing. Photosensitivity decreases from 29.3 to 14.2 with annealing temperature. The decay of photocurrents is well fitted to stretched exponential function; the value of decay time constant increases while local minimum value of dispersion parameter at 353 K has been observed. These results are explained on the basis of amorphous crystalline phase transformation in chalcogenides and are important for developing new materials for emerging technologies.

Phototransport Properties of CuInSe2 Thin Films: The Influence of Na and Planar Defects

The presence of in-gap states, especially deep ones, that act as recombination centers in solar-cell absorbers influence the phototransport properties of the films, and thus, strongly affect the performance of the corresponding solar cells. The type and density of such defects is influenced by the deposition process. In particular, for $\mathrm{Cu}({\mathrm{In}}_{1\text{\ensuremath{-}}x}{\mathrm{Ga}}_{x}){\mathrm{Se}}_{2}$, it is well established that the presence of a small amount of alkali metal atoms (e.g., $\mathrm{Na}$) improves the performance of the solar cell. Furthermore, in a coevaporation process, a $\mathrm{Cu}$-rich [[$\mathrm{Cu}$]/([$\mathrm{In}$] + [$\mathrm{Ga}$]) g 1] intermediate step decreases the density of extended structural defects, such as stacking faults. Here, we apply temperature-dependent and intensity-dependent photoconductivity measurements, along with Shockley-Read-Hall model calculations, to study the phototransport properties of $\mathrm{Cu}({\mathrm{In}}_{1\text{\ensuremath{-}}x}{\mathrm{Ga}}_{x}){\mathrm{Se}}_{2}$ films with and without a $\mathrm{Cu}$-rich process step, as well as with and without $\mathrm{Na}$. Our experimental and theoretical results indicate a correlation between the presence of planar defects and the formation of a shallow recombination level. Our results further suggest that $\mathrm{Na}$ eliminates deep recombination centers, and thus, increases the lifetime of the photogenerated charge carriers.