Photoconductivity Of Thin Films Research Articles

PIN architecture for ultrasensitive organic thin film photoconductors.

Organic thin film photoconductors (OTFPs) are expected to have wide applications in the field of optical communications, artificial vision and biomedical sensing due to their great advantages of high flexibility and low-cost large-area fabrication. However, their performances are not satisfactory at present: the value of responsivity (R), the parameter that measures the sensitivity of a photoconductor to light, is below 1 AW−1. We believe such poor performance is resulted from an intrinsic self-limited effect of present bare blend based device structure. Here we designed a PIN architecture for OTFPs, the PIN device exhibits a significantly improved high R value of 96.5 AW−1. The PIN architecture and the performance the PIN device shows here should represent an important step in the development of OTFPs.

High-performance flexible ultraviolet photoconductors based on solution-processed ultrathin ZnO/Au nanoparticle composite films

Transparent ultraviolet (UV) ZnO thin film photoconductors are expected to have great applications in environmental monitoring, large-area displays, and optical communications, and they have drawn enormous interests in recent years. However, at present their performances are not satisfactory: the responsivity R (a parameter characterizing the sensitivity of the device to light) is not high (<1.0 × 103 AW−1), and the transparency T is not high either (<80%). Realizing high R and high T remains a big challenge today. In this paper, by employing solution-processed ultrathin ZnO/Au nanoparticle composite films, R as high as 1.51 × 105 AW−1 and T of over 90% are achieved. High values for detectivity D* and linear dynamic range LDR are also obtained, which are 2.05 × 1015 Jones and 60 dB, respectively. Moreover, such high-performance devices can be fabricated on flexible PET (polyethylene terephthalate) substrates.

Non-ultraviolet-based patterning of polymer structures by optically induced electrohydrodynamic instability

We report here an approach to rapidly construct organized formations of micron-scale pillars from a thin polydimethylsiloxane (PDMS) film by optically induced electrohydrodynamic instability (OEHI). In OEHI, a heterogeneous electric field is induced across two thin fluidic layers by stimulating a photoconductive thin film in a parallel-plate capacitor configuration with visible light. We demonstrated that this OEHI method could control nucleation sites of pillars formed by electrohydrodynamic instability. To investigate this phenomenon, a tangential electric force component is assumed to have arisen from the surface polarization charge and is introduced into the traditional perfect dielectric model for PDMS films. Numerical simulation results showed that this tangential electric force played an important role in OEHI.

Surface chemistry of arenethiolate-capped PbS quantum dots and application as colloidally stable photovoltaic ink

Suitable post-synthesis surface modification of lead-chalcogenide quantum dots (QDs) is crucial to enable their integration in photovoltaic devices. We have developed a solution-phase ligand exchange strategy that exploits arenethiolate anions to replace the pristine oleate ligands on PbS QDs, while preserving the long-term colloidal stability of QDs and allowing their solution-based processability into photoconductive thin-films. Complete QD surface modification is demonstrated by IR spectroscopy analysis, whereas UV–Vis–NIR Absorption Spectroscopy provides quantitative evaluation of stoichiometry and thermodynamic stability of the resulting system. Arenethiolate ligands permit to reduce the inter-particle distance in PbS QD solids, leading to a drastic improvement of the photoinduced charge transport properties. Therefore, smooth dense-packed thin-films of arenethiolate-capped PbS QDs obtained via a single solution-processing step are integrated in heterojunction solar cells: such devices generate remarkable photocurrent densities (14mAcm−2) and overall efficiencies (1.85%), which are outstanding for a single PbS QD layer. Solution-phase surface modification of QDs thus represents an effective intermediate step towards low-cost processing for all-inorganic and hybrid organic/inorganic QD-based photovoltaics.

Integrating Multiple Resistive Memory Devices on a Single Carbon Nanotube

AbstractNano‐objects would be of great interest for the development of new types of electronic circuits if one could combine their nanometer scale with original functionalities beyond the conventional transistor action. However, the associated circuit architectures will have to handle the increasing variability and defect rate intrinsic to the nanoscale. In this context, there is a very fast growing interest for memory devices, and in particular resistive memory devices, used as building blocks in reconfigurable circuits tolerant to defects and variability. It was recently shown that optically gated carbon nanotube field effect transistors (OG‐CNTFETs) based on large assemblies of nanotubes covered by an organic photoconductive thin film can be operated as programmable resistors and thus used as artificial synapses in circuits with function‐learning capabilities. Here, the potential of such approach is evaluated in terms of scalability by integrating and addressing several individually programmable resistances on a single carbon nanotube. In addition, the charge storage mechanism can be controlled at a length scale smaller than the device length allowing to also program the direction in which the current flows. It thus demonstrates that a single nanotube section can combine all‐in‐one the properties of an analog resistive memory and of a rectifying diode with tunable polarity.

Lithium Intercalation Reaction into Titanium Dioxide Thin Films under UV Irradiation

Two types of titanium dioxide thin films were prepared, and their lithium intercalation properties were examined under UV irradiation. Thin films of tetratitanate nanosheets were prepared via electrophoretic deposition. Thin films of TiO2 nanoparticles were obtained by spin-coating a suspension of the nanoparticles, and subsequent heating. Thin films of both tetratitanate nanosheets and TiO2 nanoparticles exhibited a larger capacity on charging under UV irradiation, versus no irradiation. We observed, for the first time, an enhancement in capacity by light irradiation in a lithium ion battery electrode. This enhancement was produced because of an irreversible oxidation reaction induced by UV irradiation in thin films of tetratitanate nanosheets, and as a result of photoconductivity in thin films of TiO2 nanoparticles.

Colloidal quantum dots for mid-IR applications

The use of colloidal material offers an interesting alternative to top down approaches for the realization of low cost infrared detectors. We demonstrate photoconduction in thin films of a colloidal material in the mid-infrared (up to 7μm), using HgTe colloidal quantum dots. Thin films of the colloidal quantum dots have a large absorption coefficient (>104cm−1), and the photoconductive response is dramatically improved by encapsulating the nanoparticle into an inorganic matrix of As2S3. Such devices show fast response and large detectivity (>1010jones) at temperatures above 200K.

High-responsivity solution-processed organic–inorganic hybrid bilayer thin film photoconductors

We designed and fabricated an inorganic–organic hybrid bilayer thin film photoconductor. The advantages of the organic material's strong light absorption ability and the inorganic material's high carrier mobility are combined; a high responsivity (R) value of 11 101 A W−1 is achieved, which is much higher than that of present pure inorganic and pure organic thin-film photoconductors.

Gated three-terminal device architecture to eliminate persistent photoconductivity in oxide semiconductor photosensor arrays

The composition of amorphous oxide semiconductors, which are well known for their optical transparency, can be tailored to enhance their absorption and induce photoconductivity for irradiation with green, and shorter wavelength light. In principle, amorphous oxide semiconductor-based thin-film photoconductors could hence be applied as photosensors. However, their photoconductivity persists for hours after illumination has been removed, which severely degrades the response time and the frame rate of oxide-based sensor arrays. We have solved the problem of persistent photoconductivity (PPC) by developing a gated amorphous oxide semiconductor photo thin-film transistor (photo-TFT) that can provide direct control over the position of the Fermi level in the active layer. Applying a short-duration (10 ns) voltage pulse to these devices induces electron accumulation and accelerates their recombination with ionized oxygen vacancy sites, which are thought to cause PPC. We have integrated these photo-TFTs in a transparent active-matrix photosensor array that can be operated at high frame rates and that has potential applications in contact-free interactive displays.

Spatially resolved photoconductivity of thin films formed by colloidal octapod-shaped CdSe/CdS nanocrystals

We studied the optical absorption and photoconductive properties of thin films consisting of core-shell octapod-shaped nanocrystals, which consisted of CdS pods that branch out from a CdSe core. The current-voltage characteristics were measured at room and cryogenic temperatures and agreed well with a phenomenological exponential fitting model, from which we could extract the sheet resistance and the average voltage barrier for the charge tunneling between the octapods. The temperature dependence of the photocurrent showed temperature activated behavior above 220 K and a non-Arrhenius exponential (T/T(0))(n) dispersion below 220 K. Furthermore, we mapped the photocurrent generation within the octapod film via scanning photocurrent microscopy, which revealed photocurrent enhancement near micron-size voids and spatial shifts of the photocurrent maxima with bias voltage.

Charging/discharging dynamics of CdS and CdSe films under photoillumination using dynamic x-ray photoelectron spectroscopy

Thin films of CdS and CdSe are deposited on HF-cleaned SiO2∕Si substrates containing ∼5nm thermally grown silicon oxide. x-ray photoelectron spectroscopy (XPS) data of these films are collected in a dynamic mode, which is based on recording the spectrum under modulation with an electrical signal in the form of ±10V square-wave pulses. Accordingly, all peaks are twined and shifted with respect to the grounded spectrum. The binding energy difference between the twinned peaks of a dielectric system has a strong dependence on the frequency of the electrical stimuli. Therefore, dynamic XPS provides a means to extract additional properties of dielectric materials, such as effective resistance and capacitance. In this work, the authors report a new advancement to the previous method, where they now probe a photodynamic process. For this reason, photoillumination is introduced as an additional form of stimulus and used to investigate the combined optical and electrical response of the photoconductive thin films of CdS and CdSe using dynamic XPS.

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

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

Comparison of photocurrent spectra measured by FTPS and CPM for amorphous silicon layers and solar cells

Fourier Transform Photocurrent Spectroscopy (FTPS) has been recently introduced as a fast and highly sensitive method for the evaluation of the optical absorption coefficient of photoconductive thin films such as microcrystalline silicon layers. This contribution represents the first study of FTPS utilization for amorphous silicon layers and cells. FTPS spectra are compared with results of Constant Photocurrent Method (CPM) and Dual Beam Photoconductivity (DBP) measured at different chopping frequencies. We will concentrate to highlight the appropriate measuring conditions and evaluation procedures for correct data interpretation. Moreover, we will present our novel approach for the interference free determination of absorption coefficients of thin films grown on transparent substrates, which is mainly important for very thin layers where broad interference fringes do not allow correct evaluation of parameters such as a slope of the Urbach tail and the defect density.

Compensation effect in thermally activated photoconduction in amorphous thin films of Se75In25− x Pb x alloys

In the present paper, we report the observation of the compensation effect (also known as Meyer–Neldel rule) in thermally activated photoconduction in thin films of Se75In25− x Pb x . The Meyer–Neldel rule was investigated via two different approaches. In the first, the light intensity was varied while measuring the photoconductivity in amorphous thin films of each sample instead of the changing composition of the glassy system. In the second approach, the composition itself was varied at a particular intensity. The observation of the correlation between pre-factor σ 00 and Meyer–Neldel energy can be described by multiple excitations stimulated by optical phonon energy as described by Yelon and Movaghar [Phys. Rev. Lett. 65 618 (1990)].

Fast quantum efficiency measurement of solar cells by Fourier transform photocurrent spectroscopy

Fourier Transform Photocurrent Spectroscopy (FTPS) was introduced four years ago as a method providing fast and highly sensitive evaluation of the spectral dependence of the photoconductive thin film optical absorption coefficient. Recently the method was also applied to the quality assessment of thin film silicon solar cells. In this contribution, we present the FTPS characterization of various thin film solar cells to provide a rapid quantum efficiency (QE) measurement. Furthermore, we study the possibility of the FTPS–QE measurement of a single cell incorporated in the solar module when only the module terminals can be used for connection. FTPS results are compared to lengthy standard QE measurements. The homogeneity study was done for a-Si:H single junction solar module (30×30cm) consisting of 27 series-connected subcells. The QE–FTPS measurement is proved to be very fast, enabling a quick verification of the light intensity dependence or the influence of the forward and reverse bias on QE results over the whole measured spectral range (400–1200nm).

Photoconductive Antimony Sulfide-Selenide Thin Films Produced by Heating a Chemically Deposited Se – Sb2S3 Layer

Photoconductive thin films of antimony sulfide-selenide have been obtained by heating thin films of , deposited sequentially by the chemical bath method. Heating these layers in nitrogen results in films of a composite of , , and for temperatures up to and in a complete solid solution of near . Heating the layer at for also leads to the formation of the solid solution. X-ray diffraction (XRD) studies have indicated the presence of the compounds and the solid solution in the respective films. X-ray fluorescence (XRF) analyses indicate loss of sulfur and selenium from the films, up to 25 and 45%, respectively, during the formation of the solid solution. For the solid solution formed at , is estimated to be 1.5 (XRF)-1.8 (XRD). The composite film formed at shows three distinctive bandgaps corresponding to , , and , while the solid solution formed at shows a bandgap of and an absorption coefficient in the visible region. The dark conductivity of the films is , and the photoconductivity (, tungsten-halogen) is 50–80 times higher in each case.

Ultraviolet excitation of photoconductivity in thin films of sol–gel SnO 2

Tin dioxide (SnO 2) thin film photoconductivity spectra were measured for a large temperature range using a deuterium source. The intensity of photocurrent spectra in the range 200–400 nm is temperature dependent, and the photocurrent increases in the ultraviolet even for illumination with photon energies much higher than the bandgap transition. This behavior is related to recombination of photogenerated electron–hole pairs with oxygen adsorbed at grain boundaries, which is consistent with nanoscopic crystallite size of sol–gel deposited films.

On the role of magnetic field spin effect in photoconductivity of composite films of MEH-PPV and nanosized particles of PbS

The photoconductivity of thin films consisting of a conjugated polymer poly[2-methoxy, 5-(2′-ethyl-hexyloxy)-p-phenylene-vinylene] (MEH-PPV) and dispersed nanosized particles of an inorganic semiconductor lead sulphide (PbS) was investigated using steady-state photoconductivity and magnetic field spin effect (MFSE) techniques. It was shown that the addition of small amount of PbS nanoparticles leads to the reduction of the photocurrent while higher concentration of PbS increases the photocurrent.

Mechatronic system of dielectric elastomer actuators addressed by thin film photoconductors on plastic

We demonstrate the first mechatronic device made of an electro-active polymer (EAP) based actuator driven by a thin film photoconductive high voltage switch of amorphous silicon fabricated on a plastic film. The actuator is made of an elastomer dielectric that deforms and expands under high voltage. Its geometry is designed to linearly translate a lightweight rectangular block by the electrostatic expansion of the elastomer. The power supply is connected to the EAP through a photoconductor switch which is addressed and closed by illumination. We describe the configuration of the EAP actuator and the fabrication of the thin film photoconductor, and the electrical parameters and responses of the EAP/photoconductor system.

Photogeneration in an Acceptor-Type Poly(Phenylenevinylene)

Stationary photoconduction in thin films of a cyano-phenylamine-substituted poly-phenylenevinylene, in both sandwich and gap arrangement, has been studied. In order to separate electrode from bulk effects a thin layer of SiOx could be inserted between the indium-tinoxide (ITO) electrode and the film. Photocurrents were measured as a function of electric field, temperature (100 to 300K), photon energy (2.5 to 5 eV) and polarity of the applied bias. The results indicate that for ho < 3.2 eV the photocurrent is a superposition of bulk ionization involving relaxed singlet excitons and, depending on the direction of the electric field, exciton induced injection of predominantly electrons from the ITO. For ho > 3.2 eV intrinsic photogeneration from hot singlet excitons takes over.