Steady-state Photocurrent Research Articles

Defects in Irradiated ZnO Thin Films Studied by Photoluminescence and Photoconductivity

AbstractPulsed-laser-deposited ZnO thin films were exposed to a 1.5 MeV helium ion beam to study the changes in radiative and non-radiative recombination. We first measured photoluminescence (PL) spectra at 4.2 K excited with the 325 nm line of a HeCd laser. The as-deposited films showed a donor-bound exciton peak at 3.3567 eV attributed to Zn interstitials. After irradiation the donor-bound-exciton dominated PL spectra shifted to acceptor-bound behaviour with a signal at 3.3519 eV, tentatively attributed to Li or Na acceptors. In contrast to the approximately 30 % decrease of the PL signal near the band edge, we observed a strong concomitant enhancement of the green/orange PL band, located between 2.1 eV and 2.8 eV, by a factor of over 4. Candidates for those transitions are Li impurities and/or O vacancies. For comparison, the steady-state photocurrent decreased strongly in the irradiated region, which can also be attributed to increased non-radiative recombination through oxygen-related defects.

Photoelectrochemical Characterization of a Robust TiO2/BDD Heterojunction Electrode for Sensing Application in Aqueous Solutions

Titanium dioxide (TiO(2)) and boron-doped diamond (BDD) are two of the most popular functional materials in recent years. In this work, TiO(2) nanoparticles were immobilized onto the BDD electrodes by a dip-coating technique. Continuous and uniform mixed-phase (anatase and rutile) and pure-anatase TiO(2)/BDD electrodes were obtained after calcination processes at 700 and 450 degrees C, respectively. The particle sizes of both types of TiO(2) film range from 20 to 30 nm. In comparison with a TiO(2)/indium tin oxide (ITO) electrode, the TiO(2)/BDD electrode demonstrates a higher photoelectrocatalytic activity toward the oxidation of organic compounds, such as glucose and potassium hydrogen phthalate. Among all the tested TiO(2) electrodes, the mixed-phase TiO(2)/BDD electrode demonstrated the highest photoelectrocatalytic activity, which can be attributed to the formation of the p-n heterojunction between TiO(2) and BDD. The electrode was subsequently used to detect a wide spectrum of organic compounds in aqueous solution using a steady-state current method. An excellent linear relationship between the steady-state photocurrents and equivalent organic concentrations was attained. The steady-state oxidation photocurrents of the mixed-phase TiO(2)/BDD electrode were insensitive to pH in the range of pH 2-10. Furthermore, the electrodes exhibited excellent robustness under strong acidic conditions that the TiO(2)/ITO electrodes cannot stand. These characteristics bestow the mixed-phase TiO(2)/BDD electrode to be a versatile material for the sensing of organic compounds.

Photoconductive Enhancement of Single-Layer Tin Oxide–Coated ZnO Nanowires

In this study, zinc oxide nanowires (ZnO NWs) coated with crystalline tin oxide by an ultrasonic spray pyrolysis were investigated. High resolution transmission electron microscopy analysis confirms that a continuous single layer of about 10 nm has been coated on the ZnO NWs. Photoconductivity measurements revealed that the ZnO NWs coated with the single uniform layer had a higher photoconductivity ratio by about a factor of 8 compared to the pristine ZnO NWs, attributed to the multi-photoexcited electrons. Under a steady radiation of UV light , a substantial increase in the steady-state photocurrent is noticeable, suggesting that extra photoinduced charges are transferred from to ZnO NWs.

Molecule-modulated photoconductivity and gain-amplified selective gas sensing in polar GaN nanowires

We report the strong molecular effects on the surface-dominant photoconductivity with high-gain transport in the polar GaN nanowires. Both the transient and steady-state photocurrents are sensitive and selective to the adsorptions of oxygen and hydrogen. The surface band bending of GaN nanowires is proposed to be effectively reduced or enhanced by oxygen or hydrogen, respectively, as a donorlike or acceptorlike surface state. The molecular effect, corroborated with the high-gain photoresponse nature of GaN nanowires is found to amplify the molecule-selective photocurrent signal by near three orders of magnitude higher than its counterpart in dark current. The molecule-tunable photoconductivity, as demonstrated here, would benefit a variety of applications, ranging from the high-gain optoelectronic devices, photoelectric energy transducer, as well as gas and chemical sensors.

Charge carrier photogeneration and hole transport properties of blends of a π-conjugated polymer and an organic-inorganic hybrid material

This study examined the charge carrier photogeneration and hole transport properties of blends of poly (9-vinylcarbazole) (PVK), a π-conjugated polymer, with different weight proportions (0~29.4 wt%) of (PEA)-VOPO4·H2O (PEA: phenethylammonium cation), a novel organic-inorganic hybrid material, using IR, UV-Vis, and energy dispersive spectroscopy (EDS), thermogravimetric analysis (TGA), steady state photocurrent (SSPC) measurement, and atomic force microscopy (AFM). The SSPC measurements showed that the photocurrent of PVK was reduced by approximately three orders of magnitude by the incorporation of a small amount (~12.5 wt%) of (PEA) VOPO4·H2O, suggesting that hole transport occurred through the PVK carbazole groups, whereas a reverse trend was observed at high proportions (>12.5 wt%) of (PEA)VOPO4·H2O, suggesting that transport occurred via (PEA)VOPO4·H2O molecules. The transition to a trap-controlled hopping mechanism was explained by the difference in ionization potential and electron affinity of the two compounds as well as the formation of charge percolation threshold pathways.

Drift-diffusion modeling of photocurrent transients in bulk heterojunction solar cells

We utilize a time-dependent drift-diffusion model incorporating electron trapping and field-dependent charge separation to explore the device physics of organic bulk-heterojunction solar cells based on blends of poly(3-hexylthiophene) (P3HT) with a red polyfluorene copolymer. The model is used to reproduce experimental photocurrent transients measured in response to a step-function excitation of light of varied intensity. The experimental photocurrent transients are characterized by (i) a fast rise of order 1 μs followed by (ii) a slow rise of order 10–100 μs that evolves into a transient peak at high intensity, (iii) a fast decay component after turn-off and (iv) a long-lived tail with magnitude that does not scale linearly with light intensity or steady-state photocurrent. The fast rise and decay components are explained by the transport of mobile carriers while the slow rise and decay components are explained by slower electron trapping and detrapping processes. The transient photocurrent peak at high intensities with subsequent decay to the steady-state value is explained by trap-mediated space-charge effects. The build-up of trapped electrons in the device produces reduction in the strength of the electric field near the transparent anode that increases the likelihood of bimolecular recombination, and lowers the overall efficiency of charge dissociation in the device. Notably the model demonstrates that a reduction in free charge generation rate by space-charge effects is as significant as bimolecular recombination in this device assuming Langevin-type bimolecular recombination. The model is also used to explore the dynamics of charge separation with an upper bound of 50 ns set for the lifetime of electron-hole pairs, and to provide an estimate of the trap density of 1.3×1022 m−3.

Evidence of trapping levels and photoelectric properties of Cu3BiS3 thin films

Abstract Voltage and Cu/Bi ratio mass dependence of photocurrent was studied in Cu 3 BiS 3 thin films prepared by co-evaporation technique. The intensity dependence of steady state photocurrent (I ph ) follows a power law with intensity (F), I ph ∝ F γ where the power γ is in the range between 0.5 and 1.0, which suggests monomolecular recombination process. Photocurrent signal was found to be decaying as a function of both applied voltage and intensity, initially decreasing at considerably fast rates and later at slower ones due to the continuos distribution of defect states. Additionally, the differential life time constants were calculated. Measurements of temperature effect on conductivity (from 100 K to 450 K) were carried out. It was found that at temperatures greater than 350 K, the conductivity is predominantly affected by transport of free carriers in extended states of the conduction band, whereas in the range of temperatures below 250 K, the conductivity is dominated by the VRH (Variable Range Hopping) transport mechanism.

Metastability of defects, potential fluctuations, and percolation transition in GaAs

Potential fluctuations due to donor–acceptor compensation have been used to observe localization–delocalization transition in semi-insulating GaAs. Photoinduced transients, resulting from relaxation of stored charges in potential valleys, have two components. The long-lived power law decay at low temperature signifies a microscopically inhomogeneous disordered phase, and single exponential decay at a higher temperature signifies a homogeneous ordered phase. Temperature dependence of steady state photocurrent and kinetics of photocurrent decay suggest percolation as a possible mechanism for photoinduced transition in semi-insulating GaAs.

Thin-film capacitor M/Pb(ZrTi)O3/M as a polarization-sensitive photocell

A steady-state short-circuit photocurrent of preliminarily polarized submicron capacitors with a polycrystalline Pb(ZrTi)O3 (PZT) film is investigated under irradiation by light with a wavelength λ > 0.4 μm. The structures with different M/PZT interfaces that differ in the leakage current by more than an order of magnitude are found to demonstrate virtually the same value of the photocurrent, which is always directed opposite to the ferroelectric polarization of the PZT film. Although the magnitude of photocurrent is determined by the degree of polarization of the film, the observed photocurrent is not a depolarization current of the ferroelectric film. Therefore, the M/PZT/M capacitor behaves like a polarization-sensitive photocell. Within the proposed theory of a heterophase medium, the dependence of the photocurrent on the magnitude of the preliminary polarization is calculated and proves to be in reasonable agreement with the experimental results.

PHOTOVOLTAIC EFFECT BASED ON POLARIZATION CHARGE IN POLYCRYSTALLINE Pb(ZrTi)O3 FILM

ABSTRACT A steady-state short-circuit photocurrent across polycrystalline Pb(ZrTi)O3(PZT) film is found to be directed opposite to ferroelectric polarization, defined by the magnitude of polarization, but it is not the depolarization current of the ferroelectric. To explain this, the authors assume that Pt/PZT/Ir capacitor consists of PZT grains and semiconductor PbO phase segregated on PZT grain boundaries and forming conducting channels. In such medium, uncompensated polarization charge can arise at the PZT/PbO interface and generate the electric field acting on carriers in PbO channels, thus producing the photocurrent across the film. The relation between the polarization charge and electric field is considered.

Enhanced Photoelectrochemical Response of a Composite Titania Thin Film with Single-Crystalline Rutile Nanorods Embedded in Anatase Aggregates

A composite titania thin film consisted of quasi-aligned single-crystalline rutile nanorods embedded in sol-gel-derived anatase aggregates was fabricated and its photoelectrochemical behavior was studied in detail. A monolayer of nearly single-crystalline rutile nanorods was first deposited on metallic Ti substrates through a controlled reaction between Ti and hydrogen peroxide. The gaps among the rutile nanorods were then filled with titania nanoparticles through a sol-gel dip-coating approach, which achieved a composite thin film of single-crystalline rutile nanorods embedded in anatase aggregates, after a subsequent thermal treatment at 723 K. The X-ray diffraction measurement revealed that the composite film with thickness of 180 nm contained anatase and rutile in a nearly 50:50 ratio. The UV-visible diffusive reflectance spectra estimated a band gap of 2.99 eV for the rutile monolayer and a slightly increased value of 3.03 eV for the composite film due to the embedding of the anatase nanoparticles. Photocurrent versus potential diagrams, photocurrent transient curves, and open-circuit potential measurement supported the fact that the anatase nanoparticles possessed better inherent photoelectrochemical properties than rutile. However, electrochemical impedance spectra characterization, together with photocurrent transient curves and open-circuit potential measurement, suggests that the single-crystalline rutile nanorods exhibited a higher electron-transfer rate. The combination of the two components in such an appropriate way enhanced significantly the charge separation arising from the anatase/rutile couple, which hence combined efficiently the crystal effect and the morphological effect. A steady-state photocurrent more than double the simple sum of those generated by the two components alone was detected for the current composite film, which can be attributed to the single-crystalline rutile nanorods that provide channels for rapid electron transfer to the conductive Ti substrates under an applied bias potential. As a result, this study concludes that the present nanostructure magnified significantly the well-established mixed crystal effect, that is, an enhanced charge separation arising from a mixture of anatase and rutile.

Charge transportation and photo generation process in polythiophene functionalized with tin (II) phthalocyanine (SnPc-PT) thin film

Charge conduction and photo-generation process in polythiophene functionalized with tin (II) phthalocyanine (SnPc-PT) is investigated. The dark current density–voltage ( J–V) characteristics of indium tin oxide (ITO)/SnPc-PT/aluminum(Al) device has been analyzed at different temperatures ranging from 250 to 350 K and shows rectification behavior originated from the formation of Schottky barrier with low work function aluminum (4.2 eV) and ohmic contact with high work function ITO (4.8 eV), respectively. Conduction mechanism is governed by holes wherein at low forward bias, the current density was found to be limited by the thermionic emission of holes while at high voltages, space charge-limited current mechanism predominates by the exponential distribution of traps. Impedance measurement on Schottky barrier diodes has been discussed in the frequency range 40 Hz–1 MHz. The device with configuration ITO/SnPc-PT/Al shows the Schottky behavior, which can be modelled by a simple equivalent circuit consisting two RC elements in series, representing a bulk and a junction region, respectively. Low-frequency device capacitance shows pronounced voltage-dependence behavior and from a detailed analysis, the ionized acceptor concentration, potential barrier height and width of depletion layer have been calculated and discussed in detail. Photogeneration process and photovoltaic effect of sandwich devices were investigated by measuring steady state photocurrent resulting from illuminating the device through the ITO and Al electrode as well. On comparing with pure SnPc characteristics it is found that after occupying the interstitials space in polythiophene network, a dramatic reduction in band gap ( E g) takes place to become SnPc-PT, which is also evidenced by the extension of absorption band towards low energy side.

Effect of the Presence of Iodide on the Electron Injection Dynamics of Dye-Sensitized TiO2-Based Solar Cells

The electron injection dynamics of dye-sensitized TiO_2-based solar cells have been investigated to determine the effects of replacing the I_3^−/I^− redox system by non-redox-active supporting electrolytes. TiO-2 films were sensitized with Ru(dcbpy)_2(NCS)_2, where dcbpy = 4,4′-dicarboxylic acid-2,2′-bipyridine (the “N3” dye), and placed in contact with either M(ClO_4) or M(I_3−/I−) solutions (M = Li^+ or (n-C_4H_9)_4N^+); cells that contained I_3−/I− were fully functional solar cells whose steady-state photocurrents were directly measured. In (n-C_4H_9)_4N^+-containing solutions, significant differences were observed between the measured kinetics when ClO_4^− was replaced by the redox-active I3^−/I^− system. In particular, a ps time scale loss of the metal-to-ligand charge-transfer excited-state of the N3 dye, associated with electron injection, that was observed in cells containing either LiClO_4 or [(n-C_4H_9)4N]ClO_4 was absent in fully functional solar cells that contained [(n-C_4H_9)_4N]I/I_2. These results underscore the importance of performing kinetics measurements on this class of solar cells under operational conditions if one is to obtain reliable correlations between the dynamics data and the steady-state performance metrics of the solar cell devices.

Photosensitivity of Pb1 − x Sn x Te:In films in the region of intrinsic absorption

The steady-state photocurrent in the fundamental absorption region of Pb1 − xSnx Te:In films is calculated with the field injection of electrons from the contact and their capture by traps in the bulk taken into account. The calculated and experimental current-voltage characteristics are compared at liquid-helium temperature. The represented experimental data on the dependence of the Hall effect on the injection level agree well with the considered model.

PHOTOCONDUCTIVE PROPERTIES OF PVK:Alq3 BLEND FILMS STUDIED BY STEADY-STATE AND TIME-RESOLVED TRANSIENT PHOTOCURRENT SPECTRA

The carrier transport properties of the blends of the hole transport material poly(N-vinylcarbazole) (PVK) and the electron transport material tris (8-hydroxyquinolinolato) aluminum III (Alq3) are investigated at room temperature using steady-state and time-resolved transient photocurrent measurements as a function of doping concentration of Alq3. Due to lower LUMO and higher HOMO energy level of Alq3 than those of PVK, Alq3 molecules may act as carrier trap states in PVK films at low concentration. However, at high concentration of Alq3, phase separation reduces trap states to some extent, which leads to the rise of photocurrent. It is concluded that strong excitation transfer from PVK to Alq3 does harm to photocurrent, because strong fluorescence effect of Alq3 inhibits photogeneration of charge carriers. In time-resolved transient photocurrent measurements, it is found that the decay time is obviously shortened because the recombination rate increases and the electron mobility of Alq3 is higher than the hole mobility of PVK.

Study of the factors affecting the photoelectrode characteristics of a perylene/phthalocyanine bilayer working in the water phase

Some types of phthalocyanines (MPc (M = H(2), Cu, or Zn), a p-type semiconductor) were used in combination with 3,4,9,10-perylenetetracarboxyl-bisbenzimidazole (PTCBI, an n-type semiconductor), with which those photoelectrode characteristics in the water phase were investigated in terms of kinetics. Each film of the PTCBI/MPc bilayer functioned as a photoanode, where the photoinduced oxidation of thiol occurs at the MPc/water interface along with the hole conduction through the MPc layer. The holes originate on account of the photophysical events in the p/n interior, involving the charge separation of excitons at the p/n interface. The typical photoelectrochemical characteristic in the PTCBI/MPc photoanodes involved a transient photocurrent occurring in the initial stage under illumination (under potentiostatic conditions): thereafter, it attained a steady state. Moreover, both the initial spiky photocurrents and the steady-state photocurrents exhibited saturation at higher concentrations. An analysis with photoelectrode kinetics was performed by assuming an adsorption step prior to a rate-limiting charge transfer step, where equations were applied to photocurrents based on the Langmuir adsorption equilibrium. The kinetic analyses evidently showed that the photoanodic reactions are kinetically dominated by the charge transfer between MPc and thiol, where the overall kinetics for thiol oxidation decreases in the following order: H(2)Pc > ZnPc > CuPc; that is, it appeared that H(2)Pc acts as the more efficient photofunctional interface capable of oxidation in the water phase when PTCBI was concurrently employed as an electron conductor. Considering that the photocurrent generated is proportional to the surface concentration of thiol (Gamma) at the MPc as well as the intrinsic oxidation rate (cf., ZnPc > H(2)Pc approximately CuPc), the higher efficiency in the output at the H(2)Pc surface was attributed to an exceptionally high Gamma (i.e., from the kinetic analyses, the Gamma value at the H(2)Pc surface was also inferred to be 2-3 times higher than that at the other MPcs). Through the present kinetic analysis, it also revealed that the activity for thiol oxidation taking place at Pc ring is comparable to that at the conventional active catalysts (i.e., polycarboxyphthalocyaninato Co(ii) and Fe(iii)) where a central metal is an active site.

Enhanced photoresponse in ZnO nanowires decorated with CdTe quantum dot

The photoresponse of ZnO nanowires irradiated with photons having energies below the band gap of ZnO (3.4eV) was studied before and after deposition of CdTe quantum dots via a pulsed electron-beam technique. The small amount of deposited CdTe did not increase the dark current of the samples. However, a substantial increase in the steady state photocurrent was observed after CdTe deposition suggesting a clear photosensitization effect. Results revealed that CdTe influences the photoconductivity transients of ZnO by minimizing its interaction with oxygen in air as well as providing additional traps that serve to increase the photocurrent time constant.

Steady-state and transient photocurrents in rubrene single crystal free-space dielectric transistors

We report results of photocurrent studies performed on rubrene single crystal transistors in the air-gap configuration under 405nm irradiation. The phototransistors show a strong photoresponse indicative of photoconduction. Under illumination, the phototransistors show an average threshold voltage shift of 22V and a maximum photosensistivity of 2.65×103. A small persistent photoconductivity effect is observed in the transistors tested under continuous illumination which is explained by delayed recombination aided by spatial separation of the photocarriers. Photocurrent transients measured by applying short pulses on the other hand show a complete recovery in the microsecond regime implying immediate recombination.

Origin of photoresponse in heterophase ferroelectric Pt∕Pb(ZrTi)O3∕Ir capacitors

A steady-state short-circuit photocurrent is measured in polycrystalline Pb(ZrTi)O3(PZT) films with Pb excess. It is found that although the photocurrent is directed opposite to ferroelectric polarization, it is not the depolarization current of the ferroelectric. To explain this, the authors assume that Pt/PZT/Ir capacitor consists of PZT grains and semiconductor PbO phase segregated on PZT grain boundaries during the PZT formation. In such medium, the current across the film flows through channels formed by PbO phase, where an electric field is generated by uncompensated polarization charge of PZT grains. Using this concept, the authors described the experimental dependence of photocurrent on polarization.

Photoelectrical properties of (Sb 15As 30Se 55) 100−xTe x (0 ⩽ x ⩽ 12.5 at.%) thin films

This paper reports photoelectrical properties of (As 30Sb 15Se 55) 100− x Te x amorphous chalcogenide films (0 ⩽ x ⩽ 12.5 at.%) through measurements of ‘steady state’ and ‘transient’ photocurrents. The composition dependence of the steady state photocurrent at room temperature shows that the photoconductivity increases while the photosensitivity decreases with increasing Te content. A study of photoconductivity of (As 30Sb 15Se 55) 100− x Te x at different levels of light intensity reveals that, the photoconductivity increases exponentially with increase in light intensity. The Photocurrent ( I ph) when plotted against light intensity ( G) follows a power law ( I ph = G γ ) the exponent γ for (As 30Sb 15Se 55) 100− x Te x films has been found nearly 0.5 suggesting bimolecular recombination. The transient photoconductivity shows that the lifetime of the carrier decreases with increasing the light intensity. This decrease suggests that the photoconductivity mechanism in our samples was controlled by the transition trapping processes. The increase of Te content results in a monotonic decrease in the band gap and the free carrier life time of (As 30Sb 15Se 55) 100− x Te x thin films. These results were interpreted on the basis of the chemical-bond approach.