Photoconductive Behavior Research Articles

Electrical and fotoconducting characterization of Al/coumarin:ZnO/Al novel organic-inorganic hybrid photodiodes

Coumarin doped ZnO/p-Si photodiodes were prepared by using the drop casting technique. The effect of coumarin contribution on the photoresponse properties of fabricated Al/Coumarin:ZnO/p-Si/Al organic-inorganic hybrid devices was investigated. Electrical characterization of Al/Coumarin:ZnO/p-Si/Al photodiodes was carried out in the dark and under various illumination intensities with current-voltage (I–V) and capacity/conductivity-voltage (C-G-V) characteristics. The photocurrent of the diodes were found to be higher under illuminated conditions than the dark situation. It was determined that the prepared photodiodes exhibit photovoltaic behavior. Also, the photosensitivity properties of photodiodes were investigated. The highest photosensitivity expressed as Iphoto/Idark ratio was found as 894.73 for the specimen having 3% coumarin weight ratio under the conditions of 100 mW/cm2 and 5 V. Transient photocurrent measurements confirmed that the photocurrent is sensitive to light intensity. In addition, capacitance/conductivity-voltage measurements were accomplished in frequencies between 10 kHz and 1 MHz. As a result of the continuous distribution of interface states, the capacity decreased with increasing frequency. It was determined that all photodiodes produced exhibit a linear photoconductivity behavior. The calculated ideality factors for Al/Coumarin:ZnO/p-Si/Al photodiodes prepared with various coumarin weight ratios exhibited that the non-ideal behaviors due to surface states and oxide layer. In addition, it was understood that the values of the barrier height and the interfacial density of the diodes could be changed with the doped of coumarin. As a result, it is concluded that the produced organic-inorganic devices can be used as optical sensors for optoelectronic applications.

Laser-induced electrical signal filtering by multilayer reduced graphene oxide decorated with Au nanoparticles.

Nanoscale plasmonic particles represent a crucial transformation on optical and electronic properties exhibited by advanced materials. Herein are reported remarkable interferometric optical effects with dependence on polarization for filtering or modulating electronic signals in multilayer nanostructures. Metallic nanoparticles were incorporated in randomly distributed networks of reduced graphene oxide by an in-situ vapor-phase deposition method. The polarization-selectable nonlinear optical absorption contribution on the photoconductivity of reduced graphene oxide decorated with gold nanoparticles was analyzed. Nanosecond pulses at 532 nm wavelength were employed in a two-wave mixing experiment to study photoconduction and nonlinear optical absorption in this nanohybrid material. The ablation threshold of the sample was measured in 0.4 J/cm2. Electrochemical impedance spectroscopy measurements revealed a capacitive response that can be enhanced by gold decoration in carbon nanostructures. A strong two-photon absorption process characterized by 5 × 10-7 m/W was identified as a physical mechanism responsible for the nonlinear photoconductive behavior of the nanostructures. Experimental shift of 1 MHz for the cutoff frequency associated with an electrical filter function performed by the sample in film form was demonstrated. Moreover, amplitude modulation of electronic signals controlled by the polarization of a two-wave mixing experiment was proposed. All-optical and optoelectronic nanosystems controlled by multi-photonic interactions in carbon-based materials were discussed. The key role of the vectorial nature of light in two-wave mixing experiments is a fascinating tool for the exploration of low-dimensional systems.

Photoconductive Behavior of the PPV/RGO Composites: Insights of Charge Transfer Process

The paper deals with a study of composites based on poly(p‐phenylenevinylene) (PPV) and reduced graphene oxide (RGO) in terms of photoconductivity and photocurrent (PC) dynamics in charge‐discharge cyclic processes. The explanation for the photoconductive behavior is built with the support of DeVore and Onsager theories. Scanning samples in both directions involves charge transport, to and from, available energy states called defect centers. The existence of these centers is confirmed by a decrease in the composite bandgap caused by the RGO localized states which are situated slightly above the first HOMO level in the PPV bandgap. The contribution of RGO to the photoconductive properties of PPV is revealed through a photocurrent value with two orders of magnitude higher than for PPV.

Photonic crystallization of two-dimensional MoS2 for stretchable photodetectors.

Low temperature synthesis of high quality two-dimensional (2D) materials directly on flexible substrates remains a fundamental limitation towards scalable realization of robust flexible electronics possessing the unique physical properties of atomically thin structures. Herein, we describe room temperature sputtering of uniform, stoichiometric amorphous MoS2 and subsequent large area (>6.25 cm2) photonic crystallization of 5 nm 2H-MoS2 films in air to enable direct, scalable fabrication of ultrathin 2D photodetectors on stretchable polydimethylsiloxane (PDMS) substrates. The lateral photodetector devices demonstrate an average responsivity of 2.52 μW A-1 and a minimum response time of 120 ms under 515.6 nm illumination. Additionally, the surface wrinkled, or buckled, PDMS substrate with conformal MoS2 retained the photoconductive behavior at tensile strains as high as 5.72% and over 1000 stretching cycles. The results indicate that the photonic crystallization method provides a significant advancement in incorporating high quality semiconducting 2D materials applied directly on polymer substrates for wearable and flexible electronic systems.

A Correlative Investigation of Photoconductive Behaviour of Bare and Al-Doped ZnO Thin Films

Zinc Oxide (ZnO) nanoparticles exhibit a wide range of applications owing to their unique properties such as low cost, non-toxicity, abundance in nature and suitability for doping. In the present study, ZnO and Aluminium doped Zinc Oxide (Al-ZnO) nanoparticle thin films were prepared on FTO glass substrate by sol-gel and spin coating methods. The structural and morphological characteristics of the undoped and Al-doped ZnO nanoparticles were examined by X-ray Diffraction (XRD) and High Scanning Electron Microscope (HR-SEM) analysis, respectively. The XRD studies revealed the hexagonal wurtzite structure of the as-synthesized ZnO samples and the crystallite size was estimated using Scherrer’s formula to be 28 and 30 nm respectively, in the case of the undoped and doped ZnO nanoparticles. The average particle sizes in nanoscale were analysed using FESEM and found to be 38 nm and 42 nm for the bare and doped samples. The surface area of the as-synthesised particles were correspondingly estimated as 46 m2/g and 25 m2/g and their relative pore volume and diameter were determined using BET analysis. The field dependent photoconductivity of Al-ZnO shows a significant increase as against the bare ZnO.

A Relative Investigation of the independent effects of Al-plasmonization and growth procedure on TiO2/ZnO Photoanodes

Nanoscience and nanotechnology open the door for creation of various nanostructured materials for application in Dye-Sensitized Solar Cells (DSSCs). The aim of the present work is to investigate the as-synthesized TiO2/ZnO and TiO2/ZnO encapsulated Al Thin film by doctor blade and spin coating methods and the samples hereby designated as TZ and TAZ and the methods as DB and SC. A detailed investigation into the microstructure, morphology, composition, surface area and optical studies of the as-synthesized samples has been carried out. XRD analysis suggested that the crystallite size of TAZ decreases in comparison with the TZ in both DB and SC. The HRSEM and TEM reports reveal the spherical morphology and reduction in particle size in TAZ which is in good agreement with the XRD results. EDAX analysis confirms the presence of elemental composition of the as-synthesized sample. The presence of the functional groups of the as-synthesized samples was affirmed from FT-IR studies. From the UV-DRS studies a slight modification in band gap was observed possibly due to the lattice disorder arises due to the addition of plasmonic nanoparticles Al. BET analysis confirmed that TAZ shows a significant increase in surface area which would increase the dye absorption, a key factor for enhancing the performance of the DSSCs. Electro-optical studies of TAZ thin film indicated an enhanced photoconductive behavior in the case of hence presenting it as a promising photo anode material for DSSCs

Silicon based photodetector with Ru(II) complexes organic interlayer

In present work, the electrical properties and illumination effects were investigated for the silicon based photodetector with organic Ru(II) complexes interfacial layer. The current-voltage (I-V) and capacitance/conductance-voltage (C/G-V) measurements were analyzed to determine electrical and photoelectrical properties under dark and different solar light intensity conditions. The reverse bias current values under light conditions were higher than dark conditions, and this situation demonstrates that the fabricated device displays a photoconducting behavior. Besides, some crucial electrical parameters such as series resistance, barrier height and ideality factor values of prepared device were calculated by using current-voltage measurements. The ideality factor and barrier height values of fabricated device were calculated as 9.42 and 0.59 for dark condition. Besides to these experiments, transient photocurrent and photo-capacitance/conductance were also investigated under different light conditions. It was determined from transient measurements that the fabricated device has a high sensitivity to light. The photoresponse of the diode was determined to be around 4479 ± 1.9 under 100 mW/cm2 illumination. The examined C/G-V characteristics of the fabricated device strongly depend on voltage and frequency. The analyzed results suggest that the fabricated Al/Ru(II) complexes/p-Si/Al device can be used in rapidly developing optoelectronic applications, especially for the organic materials-based photodetector technology.

Engineering Interface-Dependent Photoconductivity in Ge2Sb2Te5 Nanoscale Devices.

Phase-change materials are increasingly being explored for photonics applications, ranging from high-resolution displays to artificial retinas. Surprisingly, our understanding of the underlying mechanism of light-matter interaction in these materials has been limited to photothermal crystallization because of its relevance in applications such as rewritable optical discs. Here, we report a photoconductivity study of nanoscale thin films of phase-change materials. We identify strong photoconductive behavior in phase-change materials, which we show to be a complex interplay of three independent mechanisms: photoconductive, photoinduced crystallization, and photoinduced thermoelectric effects. We find that these effects also congruously contribute to a substantial photovoltaic effect, even in notionally symmetric devices. Notably, we show that device engineering plays a decisive role in determining the dominant mechanism; the contribution of the photothermal effects to the extractable photocurrent can be reduced to <0.4% by varying the electrodes and device geometry. We then show that the contribution of these individual effects to the photoresponse is phase-dependent with the amorphous state being more photoactive than the crystalline state and that a reversible change occurs in the charge transport from thermionic to tunnelling during phase transformation. Finally, we demonstrate photodetectors with an order of magnitude tunability in photodetection responsivity and bandwidth using these materials. Our results provide insights to the photophysics of phase-change materials and highlight their potential in future optoelectronics.

Atomic-level insights through spectroscopic and transport measurements into the large-area synthesis of MoS2 thin films

Abstract

ZnO-Based Schottky and Oxide Multilayer Devices for Visibly Transparent Photovoltaic Applications

Autonomous smart windows may be integrated with a stack of active components, such as electrochromic devices, to modulate the opacity/transparency by an applied voltage. This voltage may be generated with a visibly transparent photovoltaic device. This paper describes the processing and performance of zinc oxide (ZnO) films for integration with electrochromic stacks. Sputtered ZnO (2% Mn) films on indium–tin oxide with transparency in the visible range were used to fabricate metal–semiconductor (MS), metal–insulator–semiconductor, and p-i-n heterojunction devices, and their photovoltaic conversion under ultraviolet (UV) illumination was evaluated with and without oxygen plasma-treated surface electrodes (Au, Ag, Al, and Ti/Ag). The MS Schottky parameters were fit against the generalized Bardeen model to obtain the density of interface states ( ${D}_{\textsf {it}} \approx \textsf {8.0}\times \textsf {10}^{\textsf {11}}$ eV−1cm−2) and neutral level ( ${E}_{o} \approx -\textsf {5.2}$ eV). These devices have exhibited a photoconductive behavior at $\lambda = \textsf {365}$ nm, and low-noise Ag-ZnO detectors have exhibited the responsivity ( ${R}$ ) and photoconductive gain ( ${G}$ ) of $\textsf {1.93}\times \textsf {10}^{-\textsf {4}}$ A/W and $\textsf {6.57}\times \textsf {10}^{-\textsf {4}}$ , respectively. Confirmed via matched-pair analysis, postmetallization oxygen plasma treatment of Ag and Ti/Ag electrodes has resulted in increased Schottky barrier heights, which maximized with a 2-nm SiO2 electron blocking layer, coupled with the suppression of recombination at the MS interface and blocking of majority carriers. For interdigitated devices under monochromatic UV-C illumination, the open-circuit voltage ( ${V}_{\textsf {oc}}$ ) was 1.2 V and short-circuit current density ( ${J}_{\textsf {sc}}$ ), due to minority carrier tunneling, was 0.68 mA/cm2.

Determination of valence and conduction band offsets in Zn0.98Fe0.02O/ZnO hetero-junction thin films grown in oxygen environment by pulsed laser deposition technique: A study of efficient UV photodetectors

Here we present ZnO based thin films deposited in oxygen environment by pulsed laser deposition technique for UV photodetection applications. The valence and conduction band alignments are the key parameter required in understanding the type of charge carrier participating in conduction process. The valence and conduction band offset measurements have been carried out using the X-ray photoelectron spectroscopy (XPS) on the pristine ZnO, Fe (2 wt%) -doped ZnO (Zn0.98Fe0.02O) and the hetero-junction of Zn0.98Fe0.02O with ZnO (Zn0.98Fe0.02O/ZnO). XPS studies revealed that the valence band is upward shifted with 0.37 eV, whereas, there is a downward shifting of −0.393 eV in the conduction band. Structural analysis of the deposited films has been done using X-ray diffraction studies, which revealed the polycrystalline nature of all the films having wurtzite crystal phase. AFM studies suggest that the size of the grains decreases with Fe-doping in ZnO network due to different ionic radii of Zn and Fe. UV–Vis studies suggest the decrease in optical band gap with Fe-doping and hetero-junction formation. The optical band gap energy has also been obtained from the fitting of experimental data of absorption spectra with Mott -Davis's model and Hydrogenic excitonic model. Both the models are well corroborated with each other. The photoconduction behavior of the deposited films has been analyzed under UV light. The pristine ZnO film has a photoresponse of about 25% which is found to increase in Zn0.98Fe0.02O (∼36%) film. The responsivity values are found to be 1.56, 2.17 and 1.86 mA/W for the pristine ZnO, Zn0.98Fe0.02O and Zn0.98Fe0.02O/ZnO thin films, respectively.

Photoelectrochemical cell performances of Cu2ZnSnSe4 thin films deposited on various conductive substrates

This work studies the influence of various conductive substrates such as Al, Cu and Ag on the optical and photoconductive behavior of Cu2ZnSnSe4 (CZTSe) thin films prepared using thermal evaporation method. The XRD analysis indicates that the deposited CZTSe films are tetragonal structure for all substrates, however, the compound prepared using glass substrates shows some secondary phases. The SEM images show spherical particles for all the films. The films show dense and uniform surface and no voids are observed. EDX analysis witnesses for the deposition compound that exhibits near stoichiometric values. CZTSe thin films show the absorption coefficient about 104 cm−1 and the band-gap is found to be about 1.15 eV, 1.32 eV, 1.4 eV and 1.19 eV for CZTSe films deposited on glass, Al, Cu and Ag substrates respectively. From, the Mott-Schottky Plot, the CZTSe thin films confirms p-type conductivity and the J-V plot of CZTSe thin films shows the photoactivity. The CZTSe films deposited on Al substrates show higher PCE.

Triiodide-Induced Band-Edge Reconstruction of a Lead-Free Perovskite-Derivative Hybrid for Strong Light Absorption

Bismuth-based hybrid perovskites have recently attracted great attention for their environmentally friendly processing, chemical stability, and photoresponsive properties. However, most of the known lead-free hybrids show wide band gaps (Eg > 1.9 eV) that afford weak visible-light absorption. Here, we show a newly conceptual design strategy of intercalating triiodide to decrease the prototypic band gap by ca. ∼0.44 eV. A new hybrid semiconducting material of (4-methylpiperidinium)4·I3·BiI6 (MP-T-BiI6), adopting the zero-dimensional (0D) perovskite-like framework, was reconstructed from its prototype of (4-methylpiperidinium)3Bi2I9 (MP-Bi2I9). It is noteworthy that MP-T-BiI6 has a narrow band gap of 1.58 eV, almost comparable to that of CH3NH3PbI3 (∼1.5 eV), which reveals its potential as the highly efficient light absorber. Importantly, its semiconducting properties were solidly confirmed by notable hole mobility (∼12.8 cm2 V–1 s–1), charge-trap density (∼1.13 × 1010 cm–3), and photoconductive behaviors. ...

A phase transformable ultrastable titanium-carboxylate framework for photoconduction

Porous titanium oxide materials are attractive for energy-related applications. However, many suffer from poor stability and crystallinity. Here we present a robust nanoporous metal–organic framework (MOF), comprising a Ti12O15 oxocluster and a tetracarboxylate ligand, achieved through a scalable synthesis. This material undergoes an unusual irreversible thermally induced phase transformation that generates a highly crystalline porous product with an infinite inorganic moiety of a very high condensation degree. Preliminary photophysical experiments indicate that the product after phase transformation exhibits photoconductive behavior, highlighting the impact of inorganic unit dimensionality on the alteration of physical properties. Introduction of a conductive polymer into its pores leads to a significant increase of the charge separation lifetime under irradiation. Additionally, the inorganic unit of this Ti-MOF can be easily modified via doping with other metal elements. The combined advantages of this compound make it a promising functional scaffold for practical applications.

Dielectric, ferroelectric properties and photoconductivity effect of sol-gel grown SrTiO3/BaTiO3 thin film heterostructure

SrTiO3/BaTiO3 (ST/BT) thin-film heterostructure was deposited on Pt/Ti/SiO2/Si(100) substrate by spin-coating. X-ray diffraction pattern shows that the heterostructure is a perovskite structure composed of ST and BT without any impurity peaks. The dielectric constant and loss of the heterostructure at 10 kHz are 704 and 0.024, respectively. The electric-field dependence of dielectric response was investigated and the tunability of the sample under 200 kV/cm applied field is 21.5%. Compared with pure ST thin film, the polarization-electric field loops of ST/BT heterostructure display high polarization and good symmetry, which could be attributed to the introduction of BT with high dielectric constant and inhibition of the potential pitfalls movement for high barrier at ST/BT interface. Moreover, the heterostructure demonstrates photoconductivity behavior with 405 nm light illumination. The results indicate that ST/BT heterostructure is a potential material in application as multifunctional photoelectric devices.

Creatininium 4-nitrobenzoate – A phase matchable organic nonlinear optical material with large laser-induced damage threshold value for optoelectronics applications

A promising organic nonlinear optical (NLO) material of creatininium 4-nitrobenzoate (C4NB) was grown as single crystals using slow evaporation solution technique. The grown crystal was crystallized in orthorhombic system of acentric space group, P212121. The crystalline nature was analyzed by powder X-ray diffraction study. The molecular structure was evidently established by using 1H and 13C NMR spectral analysis. The UV-vis-NIR spectrum showed high optical transparency of C4NB crystal in Vis-NIR region with lower cut-off wavelength at 353 nm. Thermal analysis revealed that C4NB crystal was thermally stable up to 225 °C. The dielectric behavior was studied at different temperatures as a function of frequency varies from 50 Hz to 5 MHz. The mechanical nature and photoconductivity behaviour were carried out effectively for grown crystal. The NLO phase matching and laser-induced damage threshold (LDT) characteristics of C4NB crystal was done by Nd:YAG laser and compared with other organic crystals. All these characterizations and their results recognized that grown crystal may be exploited for optoelectronics applications in near future.

Chaotic signatures of photoconductive Cu2ZnSnS4 nanostructures explored by Lorenz attractors

Photoconductive and third-order nonlinear optical properties exhibited by Cu2ZnSnS4 nanostructures are presented. The samples were synthetized in thin film form by a spray pyrolysis processing route. Distinctions in the photoconductive behavior throughout the samples were clearly noted by modulating their optoelectronic response dependent on electrical frequency. Vectorial two-wave mixing experiments were carried out at a 532 nm wavelength provided by a Nd:YAG laser system to study the optical nonlinearities in the samples. An induced transparency effect was observed during nanosecond single-beam experiments in the nanostructures reported. Quantum and thermal processes were considered to be the main physical mechanism responsible for the photo-electrical phenomena and nonlinear refraction in the nanostructures. Potential applications for developing nanophotonic and nanoelectronic instrumentation systems can be contemplated.

X-ray photoelectron spectroscopy investigations of band offsets in Ga0.02Zn0.98O/ZnO heterojunction for UV photodetectors

Here, we report the valence and conduction band offset measurements in pure ZnO and the Ga0.02Zn0.98O/ZnO heterojunction by X-Ray photoelectron spectroscopy studies for UV photodetector applications. For detailed investigations on the band offsets and UV photodetection behavior of Ga0.02Zn0.98O/ZnO heterostructures, thin films of pristine ZnO, Ga-doped ZnO (Ga0.02Zn0.98O), and heterostructures of Ga-doped ZnO with ZnO (Ga0.02Zn0.98O/ZnO) were deposited using a pulsed laser deposition technique. The deposited thin films were characterized by X-ray diffraction, atomic force microscopy, and UV-Vis spectroscopy. X-ray photoelectron spectroscopy studies were carried out on all the thin films for the investigation of valence and conduction band offsets. The valence band was found to be shifted by 0.28 eV, while the conduction band has a shifting of −0.272 eV in the Ga0.02Zn0.98O/ZnO heterojunction as compared to pristine ZnO thin films. All the three samples were analyzed for photoconduction behavior under UVA light of the intensity of 3.3 mW/cm2, and it was observed that the photoresponse of pristine ZnO (19.75%) was found to increase with 2 wt. % doping of Ga (22.62%) and heterostructured thin films (29.10%). The mechanism of UV photodetection in the deposited samples has been discussed in detail, and the interaction of chemisorbed oxygen on the ZnO surface with holes generated by UV light exposure has been the observed mechanism for the change in electrical conductivity responsible for UV photoresponse on the present deposited ZnO films.

Investigations on structural, optical, electrical, mechanical and third-order nonlinear behaviour of 3-aminopyridinium 2,4-dinitrophenolate single crystal

The single crystals of 3-aminopyridinium 2,4-dinitrophenolate (APDP) have been synthesized and grown by slow evaporation technique at room temperature. The crystal system was identified and lattice dimensions were measured from the single-crystal X-ray diffraction (SXRD) analysis. UV–visible absorption and transmittance spectra have been recorded in the region between 250 and 1100 nm. The different vibrational modes of the molecule were studied by Fourier transform infrared (FTIR) spectroscopic analysis. The decreasing tendency of dielectric constant with increasing frequency was analysed in dielectric study. The polarizability value calculated using Penn analysis well agrees with the value calculated using Clausius–Mossotti equation. The photoconductivity and photoluminescence behaviour were also studied on grown APDP crystal. The mechanical strength of the crystal has been studied using a Vickers’ microhardness test. The stiffness constant and yield strength of the crystal were also calculated from the microhardness test. The third-order nonlinear optical parameters such as refractive index, absorption coefficient and third-order susceptibility were estimated by Z-scan studies.

Organic-inorganic hybrid nanomaterials for advanced light dependent resistors

We report the photoconduction behaviour of hybrid organic (polypyrrole) - inorganic (ZnO) nanostructures, synthesized by the surfactant mediated solution based chemical method at room temperature, for the energy saving light dependent resistor (LDR) applications. The formation of organic-inorganic hybrid nanostructures is confirmed by the combination of high resolution transmission electron microscopy and scanning electron microscopy, which revealed creation of cubical shaped wurtzite ZnO nanoparticles with different preferential orientations and transformation of clip-like nanofibres form of pure polypyrrole (PPy) into worm-like nanoribbons having embedded ZnO nanoparticles in it. Given p-type nature of PPy and n-type nature of ZnO, the embedment of ZnO nanoparticles in PPy is expected to create several localized discrete p-n junctions, which is why Raman spectroscopic results reveal the change in molecular structure of PPy due to its nanoscale interaction with ZnO. The photoresponse of PPy is found to continuously enhance from ∼4.5 to 10.7% with increase in ZnO concentration from 0 to 10% at the constant illumination intensity of 100 mW/cm2. The improved photoresponse can be attributed to the formation of localized p-n junctions (p-type PPy/n-type ZnO) in the structure. The photoresponse of PPy and its nanocomposites also increases with increase in illumination intensity from 30 to 100 mW/cm2, which is attributed to the increased number of photo-generated electron-hole pairs.