Photoconductivity Of Thin Films Research Articles

Study of Photoconductivity of Thin Films of Cadmium and Selenium Obtained by Chemical Deposition

In this work, the photoconductivity (PC) spectrum of thin CdSe films was studied. In the course of studies on glass substrates, thin films of cadmium and selenium with a thickness of h = 200 nm and h = 400 nm were selected. The thickness of the samples obtained by chemical deposition was determined by the gravimetric method. Since CdSe crystal is a light-sensitive semiconductor material, the photoconductivity of thin films has been studied. The spectra obtained during studies carried out at a wavelength λ = 600-1100 nm were analyzed. It has been established that the spectrum is chaotic, since in the h = 200 nm layers the phase is not completely formed. In the layers h = 400 nm, a maximum centered at the wavelength λ = 710 nm was recorded.

An Indium Synthetic Etioporphyrin for Organic Electronics: Aggregation and Photoconductivity in Thin Films.

A new complex of indium(III)chloride with etioporphyrin-I was synthesized and characterized. As with naturally occurring extraligated etioporphyrins, the InCl-EtioP-I spectrum in solution has a very strong B-band and a more than an order of magnitude weaker Q-band, but this difference diminishes in solid films of InCl-EtioP-I obtained by thermal evaporation in vacuum. In a solid, molecules have a tight convex-convex arrangement in a 2D double layered structure with interplane distance of 3.066 Å. The conductivity of films can easily be activated by the action of temperature or light. In the cells with symmetrical lateral contacts the photocurrent exceeds the dark current by about three orders of magnitude, with the contribution of photons in the Q-band range being greater than expected from the experimental or calculated absorption spectrum. The Q-bands contribute significantly to the photovoltaic effect in the ITO/InCl-EtioP-I/Al sandwich cells. Such cells show an untypically strong signal in the photodiode regime, which yields the spectral detectivity of 10^12 Jones.

Photoconductivity and electronic processes in PCDTBT polymer composite with embedded CdSe nanoplatelets

The photoconductivity in thin films of a new composite based on CdSe nanoplatelets (NPLs) and PCDTBT polymer has been studied using photoconductivity temperature dependences, current-voltage and lux-ampere measurements at various temperatures and applied bias. The incorporation of 7 vol% of NPLs into the polymer blend led to an increase in photoconductivity by an order of magnitude compared to the pristine PCDTBT film in the temperature range of 310–360 K and the electric field strength above 200 V cm−1. An analysis of current-voltage characteristics measured at various temperatures demonstrated that the observed increase in the photoconductivity of the composite is due to a change in the transport mechanism of nonequilibrium charge carriers with an increase in their concentration. The observed saturation of photoconductivity with increasing temperature is supposed to occur due to the increase in exciton diffusion length with temperature increasing and limited effective exciton generation area.

Photoconductivity of Thin Films Obtained from a New Type of Polyindole

The optoelectronic properties of a new poly(2-ethyl-3-methylindole) (MPIn) are discussed in this paper. The absorption and photoluminescence spectra were studied. The electronic spectrum of MPIn showed a single absorption maximum at 269 nm that is characteristic of the entire series of polyindoles. The fluorescence spectra show that the emission peaks of the test sample are centered around 520 nm. The photoconductivity of thin film samples of MPIn polyindole was studied by measuring the current-voltage characteristics under ultraviolet radiation with a wavelength of 350 nm. Samples of phototransistors were obtained, where thin films of MPIn polyindole were used as a transport layer, and their characteristics were measured and analyzed. The value of the quantum efficiency and the values of the mobility of charge carriers in thin polyindole films were estimated.

Increasing the scope and precision of the steady-state photocarrier grating technique by measuring the photocurrents at several voltages

The steady-state photocarrier grating (SSPG) experiment is a popular technique for extracting the minority carrier diffusion length of photoconductive thin films in coplanar configuration. The diffusion length is basically obtained from the measurement of the steady-state photocurrent produced by a low applied voltage while the material is illuminated by two monochromatic laser beams of different intensities that interfere between the electrical contacts of the sample. Despite its simplicity and popularity, it is well known that the technique can overestimate the minority carrier diffusion length in some samples. In this paper, we show that the precision of the technique can be substantially increased by performing the same experiment at different voltages. Additionally, we show how to estimate fundamental material parameters from the experiment, such as the density of states at the majority carrier quasi-Fermi energy and the ratio between the recombination states’ capture coefficient and mobility of majority carriers. First, we show that the procedures found in the literature for correcting the overestimation produced by the standard technique do not work properly due to an oversimplification in the modeling. Then, we use a numerical simulation of an unintentionally-doped hydrogenated-amorphous-silicon-like material to evaluate the precision of the new formulas and procedures presented. We clarify the conditions under which the standard SSPG technique produces large overestimations. In these cases, we show that the precision of the new procedure can be more than ten times higher. Finally, we use the standard and the new method to characterize a hydrogenated amorphous (a-Si:H) and a hydrogenated polymorphous (pm-Si:H) silicon sample at different temperatures. We observe that the overestimations produced by the standard technique increase with the ratio between the majority and minority carrier diffusion lengths and the ratio between the recombination states’ capture coefficient and mobility of majority carriers.

Spintronics phenomena induced by THz radiation in narrow-gap HgCdTe thin films in an external constant electric field

The responses of uncooled (T = 300 K) and cooled to T = 78 K antenna-coupled Hg1–xCdxTe-based narrow-gap thin-film photoconductors having large spin-orbit coupling and irradiated by the terahertz (THz) radiation (linearly or circularly polarized) have been investigated. Powerful THz radiation excitation causes photocurrents, which signs and magnitudes are controlled by orientation of antenna axes, an external constant electric field direction and orientation of the polarized (circular or linear) radiation electric field falling onto photoconductors. The observed effects seem to be caused by the spin currents observed in devices where spintronic effects are revealed. spintronic phenomena, photoconductors, THz radiation, HgCdTe.

Photoconductive Thin Films Composed of Environmentally Benign AgBiS2 Nanocrystal Inks Obtained through a Rapid Phase Transfer Process

We demonstrate a fast solution phase ligand exchange process to generate AgBiS2 nanocrystal inks using a cinnamic acid derivative as an additive to accelerate the phase transfer to polar solvents. ...

Atomic Layer Deposition of Photoconductive Cu2O Thin Films.

Herein, we report an atomic layer deposition (ALD) process for Cu2O thin films using copper(II) acetate [Cu(OAc)2] and water vapor as precursors. This precursor combination enables the deposition of phase-pure, polycrystalline, and impurity-free Cu2O thin films at temperatures of 180–220 °C. The deposition of Cu(I) oxide films from a Cu(II) precursor without the use of a reducing agent is explained by the thermally induced reduction of Cu(OAc)2 to the volatile copper(I) acetate, CuOAc. In addition to the optimization of ALD process parameters and characterization of film properties, we studied the Cu2O films in the fabrication of photoconductor devices. Our proof-of-concept devices show that approximately 20 nm thick Cu2O films can be used for photodetection in the visible wavelength range and that the thin film photoconductors exhibit improved device characteristics in comparison to bulk Cu2O crystals.

A Novel Family of Polyiodo-Bromoantimonate(III) Complexes: Cation-Driven Self-Assembly of Photoconductive Metal-Polyhalide Frameworks.

In the presence of different cations, reactions of [SbBr6 ]3- and I2 result in a new family of diverse supramolecular 1D polyiodide-bromoantimonate networks. The coordination number of Sb, as well as geometry of assembling {Ix }n- polyhalide units, can vary, resulting in unprecedented structural types. The nature of I⋅⋅⋅Br interactions was studied by DFT calculations; estimated energy values are 1.6-6.9 kcal mol-1 . Some of the compounds showed strong photoconductivity in thin films, suggesting multiple feasible applications in optoelectronics and solar energy conversion.

PbSe mid-IR photoconductive thin films (part I): Phase analysis of the functional layer

Photoconductive thin films on the basis of PbSe have been used as functional materials for infrared (IR) detectors. High quality PbSe thin films were grown on glass substrates by chemical bath deposition (CBD) yielding a peak detectivity of 2.8 × 1010 cm Hz1/2 W−1 at room temperature. The photoconductive sensor function is established by a sensitization process by annealing PbSe films in oxygen and iodine rich atmospheres. By studying such sensitized PbSe layers we discovered that new, quaternary phases are formed at the top of the layered structure.We could show that after annealing the following layer structure had formed: a bottom layer of PbSe (1 μm) and a top layer of Pb-Se-O-I (400 nm), containing (i) a poly-crystalline and (ii) a nano-crystalline phase. The poly-crystalline phase was found to be a solid solution of Se and I and yielded an average Pb/I mole fraction ratio of 3.1 and Pb/Se of 3.9, respectively. It contained the largest iodine mole fraction of ∼20 at.% while the nano-crystalline phase yielded less iodine (<10 at.%). This proves the great efficiency of the reactive annealing for functionalizing the polycrystalline PbSe films. The d-spacings, the chemical composition and the plasmon energies of the phases were measured and bar code these phases. The quaternary Pb-Se-O-I phases in the top layer have large unit cells and d spacings exceeding 0.8 nm. The poly-crystalline phase yielded two sharp plasmon peaks at 17.2 eV and 23.8 eV, respectively. Therefore, it can be clearly distinguished from PbSe, which yields a plasmon energy of 15 eV and d spacings smaller than 0.353 nm.

High-performance transparent ultraviolet photodetector based on thermally reduced graphene oxide and ZnO thin films

A transparent planar electrode from thermally reduced graphene oxide (TRGO) incorporated with a photoconductive thin film of zinc oxide (ZnO) was successfully employed in a visible-blind ultraviolet (UV) photodetector. The TRGO layer was obtained from thermal reduction of drop-cast GO layer at the temperature of 400 °C after which the electrical resistance of layer decreased four orders of magnitude to ~100 Ω. Then a pure and uniform ZnO thin film with the thickness of ~80 nm was sputtered on the TRGO layer. High UV absorption and visible transparency with the average transmittance of 92% in the range of 400–800 nm (T400−800) were obtained for the ZnO thin film. Moreover, the TRGO layer was highly capable in collecting photogenerated electrons from ZnO thin film and efficient transport of charge carriers to the contact electrodes. The ZnO/TRGO device showed an ideal Ohmic behavior and high visible-light transparency with T400−800 = 80%. Furthermore, the results of current–time measurements with cyclic exposure of the device to UV light indicated stable ON/OFF switching states and highly repeatable photoresponse.

Multi-layer of Al-doped ZnO thin film photo conductor: fabrication, dark-photo current characteristics, temperature dependent conductivity and photo response studies

Aluminium doped ZnO (AZO) thin films of 6–24 multilayer were deposited on glass substrates by sol–gel spin coating method using zinc acetate and aluminium nitrate as precursors. The aluminium (Al) doping concentration of 1.5 wt% was used. The effect of multilayer on the microstructure, surface morphology, optical properties, dark-photo characteristics, photo response and temperature dependent conductivity were investigated. GIXRD of the prepared films showed poly-crystalline nature with maximum orientation along (002) plane. The FE-SEM images showed spherical shaped grains of agglomerated particles up to 10 layered films. However, agglomerations did not occur after 10 layers, and clear distinct grains, formation of micro pores was observed in the 14–24 layered films. The optical absorption was dominant in the UV region, and increased with increase in the number of layers and excitonic nature, vanishing for the 24 layered film. The average transmittance in the visible region decreased, with an increase in the number of layers and a red shift was observed, indicating band gap narrowing. The dark current increased as the number of layers increased and reached a maximum for the 14 layer film and thereafter showed a decreasing trend. The presence of fewer micro pores and lesser, randomness of crystallites, could be attributed for the maximum dark current in the 14 layered film. The photo current, increased with increase in the number of layers, with 18 layer film showing maximum value and a sharp decrease for the 24 layered film. The thermal activation energy in the temperature range of 300–400 K decreased with increasing number of layers and increased for the 24 layered film. The least activation energy of 0.31 eV was obtained for the 18 layer film.

Direct X-ray photoconversion in flexible organic thin film devices operated below 1 V.

The application of organic electronic materials for the detection of ionizing radiations is very appealing thanks to their mechanical flexibility, low-cost and simple processing in comparison to their inorganic counterpart. In this work we investigate the direct X-ray photoconversion process in organic thin film photoconductors. The devices are realized by drop casting solution-processed bis-(triisopropylsilylethynyl)pentacene (TIPS-pentacene) onto flexible plastic substrates patterned with metal electrodes; they exhibit a strong sensitivity to X-rays despite the low X-ray photon absorption typical of low-Z organic materials. We propose a model, based on the accumulation of photogenerated charges and photoconductive gain, able to describe the magnitude as well as the dynamics of the X-ray-induced photocurrent. This finding allows us to fabricate and test a flexible 2 × 2 pixelated X-ray detector operating at 0.2 V, with gain and sensitivity up to 4.7 × 104 and 77,000 nC mGy−1 cm−3, respectively.

Photoconductive thin films of AgSbS2 with cubic crystalline structure in solar cells

Polycrystalline thin films of AgSbS2 of thickness 140 nm and crystalline structure matching that of the mineral cuboargyrite (a = 0.5652 nm) are obtained by heating chemically deposited thin films at 150–320 °C. These films are deposited at 40 °C from a solution mixture containing antimony‐thiosulfate complex and Ag‐nitrate at a mole‐fraction Ag/(Ag+Sb) of 0.31. The as‐deposited films are progressively transformed to cubic‐AgSbS2, with the fraction of crystalline matter at 3.7% after heating at 150 °C and 98% at 280 °C. The crystalline grain diameter is limited to 8–11 nm. The amorphous film has an optical band gap (Eg) of 2.03 eV and the film heated at 320 °C has an Eg of 1.79 eV. The crystalline films are photoconductive; with electrical conductivity of 10−5 Ω−1 cm−1 under an intensity of illumination, 1000 W m−2 (tungsten‐halogen). The mobility–lifetime product of the material is 7.5 × 10−7 cm2 V−1. A solar cell, SnO2:F/CdS/AgSbS2(420 nm)/C, has open circuit voltage, 0.615 V; short‐circuit current density, 1.67 mA cm−2; and conversion efficiency, 0.57% for air‐mass 1.5 global solar radiation. Estimate for the maximum photo‐generated current density for cubic‐AgSbS2 is 16 mA cm−2; hence a supplementary absorber is recommended for future development of the solar cell.

Flexible, transparent, high dielectric and photoconductive thin films using ZnO nanosheets-multi-walled carbon nanotube-polymer nanocomposites

ZnO nanosheet-Multi-Walled Carbon Nanotube (ZnO-MWCNT) nanostructures have been grown by a sol–gel chemical process. ZnO-MWCNT nanostructures were dispersed in polyvinyl alcohol (PVA) polymer solutions and spin coated on indium tin oxide (ITO) coated polyethylene terephthalate (PET) substrates to fabricate a flexible, transparent, high dielectric and photoconductive thin film device. ZnO-MWCNT-PVA thin films demonstrated superior dielectric permittivity compare to ZnO-PVA and MWCNT-PVA nanocomposites. The dielectric permittivity and ac conductivity measurements of ITO/ZnO-MWCNT-PVA/Al devices have been carried out with different volume fraction of ZnO-MWCNT nanostructures. The photoconductive nature of ZnO-MWCNT-PVA composites has been investigated under illumination of broad band light source of intensity 80 mW/cm2. The flexibility study of the nanocomposite devices have been performed at different bending angles.

Plasmon-Enhanced Photoconductivity in Amorphous Silicon Thin Films by Use of Thermally Stable Silica-Coated Gold Nanorods

Enhancement of solar photovoltaics by integrating cells with metal nanoparticles is of potential interest to reduce the usage of semiconductor material. Appropriately shaped metal particles to optimize spectral response have inadequate thermal stability to withstand standard semiconductor processing. We synthesize silica-capped gold nanorods that maintain nonspherical shapes to over 600 °C and show that they can increase photoconductivity in thin films of amorphous silicon by much more than a factor of 2 across the entire visible spectrum. We also report mechanistic studies of this phenomenon that show that much of this effect is primarily due to strong near-field light concentration rather than scattering as has often been assumed.

Electropolymerized highly photoconductive thin films of cyclopalladated and cycloplatinated complexes.

The complete characterization of novel electropolymerizable organometallic complexes is presented. These are newly synthesized cyclometalated complexes of general formula (PPy)M(O ∧ N)(n) (H(PPy) = 2-phenylpyridine, M = Pd(II) or Pt(II), H(O ∧ N)(n) = Schiff base). Polymeric thin films have been obtained from these complexes by electropolymerization of the triphenylamino group grafted onto the H(O ∧ N)(n) ancillary ligand. The redox behavior and the photoconductivity of both of the monomers (PPy)M(O ∧ N)(n) and the electropolymerized species have been investigated. The polymeric films of (PPy)M(O ∧ N)(n) have shown a very significant enhancement of photoconductivity when compared to their monomeric amorphous counterparts. The high stability of the obtained films strongly suggests that electropolymerization of cyclometalated complexes represents a viable deposition technique of quality thin films with improved photoconduction properties, hence opening the door to a new class of materials with suitable properties for optoelectronic applications.

Improving the photocurrent of a PBDTTT-CF and PCBM based organic thin film photoconductor by forming a bilayer structure

By controlling and adjusting the fabrication process, all-solution-processed bilayer OTFPs exhibits a faster carrier transport which greatly enhanced the photocurrent.

High infrared photoconductivity in films of arsenic-sulfide-encapsulated lead-sulfide nanocrystals.

Highly photoconductive thin films of inorganic-capped PbS nanocrystal quantum dots (QDs) are reported. Stable colloidal dispersions of (NH4)3AsS3-capped PbS QDs were processed by a conventional dip-coating technique into a thin homogeneous film of electronically coupled PbS QDs. Upon drying at 130 °C, (NH4)3AsS3 capping ligands were converted into a thin layer of As2S3, acting as an infrared-transparent semiconducting glue. Photodetectors obtained by depositing such films onto glass substrates with interdigitate electrode structures feature extremely high light responsivity and detectivity with values of more than 200 A/W and 1.2 × 1013 Jones, respectively, at infrared wavelengths up to 1400 nm. Importantly, these devices were fabricated and tested under ambient atmosphere. Using a set of time-resolved optoelectronic experiments, the important role played by the carrier trap states, presumably localized on the arsenic-sulfide surface coating, has been elucidated. Foremost, these traps enable a very high photoconductive gain of at least 200. The trap state density as a function of energy has been plotted from the frequency dependence of the photoinduced absorption (PIA), whereas the distribution of lifetimes of these traps was recovered from PIA and photoconductivity (PC) phase spectra. These trap states also have an important impact on carrier dynamics, which led us to propose a kinetic model for trap state filling that consistently describes the experimental photoconductivity transients at various intensities of excitation light. This model also provides realistic values for the photoconductive gain and thus may serve as a useful tool to describe photoconductivity in nanocrystal-based solids.

Strain-induced photoconductivity in thin films of Co doped amorphous carbon.

Traditionally, strain effect was mainly considered in the materials with periodic lattice structure, and was thought to be very weak in amorphous semiconductors. Here, we investigate the effects of strain in films of cobalt-doped amorphous carbon (Co-C) grown on 0.7PbMg1/3Nb2/3O3-0.3PbTiO3 (PMN-PT) substrates. The electric transport properties of the Co-C films were effectively modulated by the piezoelectric substrates. Moreover, we observed, for the first time, strain-induced photoconductivity in such an amorphous semiconductor. Without strain, no photoconductivity was observed. When subjected to strain, the Co-C films exhibited significant photoconductivity under illumination by a 532-nm monochromatic light. A strain-modified photoconductivity theory was developed to elucidate the possible mechanism of this remarkable phenomenon. The good agreement between the theoretical and experimental results indicates that strain-induced photoconductivity may derive from modulation of the band structure via the strain effect.