Photoinjection Of Holes Research Articles

New Photovoltaic Devices Based on the Sensitization of p-type Semiconductors: Challenges and Opportunities

Because solar energy is the most abundant renewable energy resource, the clear connection between human activity and global warming has strengthened the interest in photovoltaic science. Dye-sensitized solar cells (DSSCs) provide a promising low-cost technology for harnessing this energy source. Until recently, much of the research surrounding DSSCs had been focused on the sensitization of n-type semiconductors, such as titanium dioxide (Gratzel cells). In an n-type dye-sensitized solar cell (n-DSSC), an electron is injected into the conduction band of an n-type semiconductor (n-SC) from the excited state of the sensitizer. Comparatively few studies have examined the sensitization of wide bandgap p-type semiconductors. In a p-type DSSC (p-DSSC), the photoexcited sensitizer is reductively quenched by hole injection into the valence band of a p-type semiconductor (p-SC). The study of p-DSSCs is important both to understand the factors that control the rate of hole photoinjection and to aid the rational design of efficient p-DSSCs. In theory, p-DSSCs should be able to work as efficiently as n-DSSCs. In addition, this research provides a method for preparing tandem DSSCs consisting of a TiO(2)-photosensitized anode and a photosensitized p-type SC as a cathode. Tandem DSSCs are particularly important because they represent low-cost photovoltaic devices whose photoconversion efficiencies could exceed 15%. This Account describes recent research results on p-DSSCs. Because these photoelectrochemical devices are the mirror images of conventional n-DSSCs, they share some structural similarities, but they use different materials and have different charge transfer kinetics. In this technology, nickel oxide is the predominant p-SC material used, but much higher photoconversion efficiencies could be achieved with new p-SCs materials with deeper valence band potential. Currently, iodide/triiodide is the main redox mediator of electron transport within these devices, but we expect that this material could be advantageously replaced with more efficient redox couples. We also discuss valuable information obtained by ultrafast transient absorption spectroscopy, which sheds some light on the factors that govern the efficiency of the cell. Notably, we demonstrate that ultrafast hole injection generally occurs between the sensitizer and the SC, but the resulting charge-separated state (e.g. electron on the sensitizer and hole in the VB) is short-lived and recombines quickly. So far, the only effective strategy for slowing the back recombination reaction relies on a bimolecular system consisting of the sensitizer linked to an electron acceptor, which increases the separation distance between the charges. A photoconversion efficiency of 0.41% under AM 1.5 was recently measured with a p-type DSSC using this strategy.

Sensitized Hole Injection of Phosphorus Porphyrin into NiO: Toward New Photovoltaic Devices

This paper describes the preparation and the characterization of a photovoltaic cell based on the sensitization of a wide band gap p-type semiconductor (NiO) with a phosphorus porphyrin. A photophysical study with femtosecond transient absorption spectroscopy showed that light excitation of the phosphorus porphyrin chemisorbed on NiO particles induces a very rapid interfacial hole injection into the valence band of NiO, occurring mainly on the 2-20 ps time scale. This is followed by a recombination in which ca. 80% of the ground-state reactants are regenerated within 1 ns. A photoelectrochemical device, prepared with a nanocrystalline NiO electrode coated with the phosphorus porphyrin, yields a cathodic photocurrent indicating that electrons indeed flow from the NiO electrode toward the solution. The low incident-to-photocurrent efficiency (IPCE) can be rationalized by the rapid back recombination reaction between the reduced sensitizer and the injected hole which prevents an efficient regeneration of the sensitizer ground state from the iodide/triiodide redox mediator. To the best of our knowledge, this work represents the first example of a photovoltaic cell in which a mechanism of hole photoinjection has been characterized.

Electrode effect on photohole generation in smectic phenylnaphthalene liquid crystalline photoconductor

The photohole generation process in the smectic phases of a liquid crystalline photoconductor, 2-(4′-octylpheny)-6-dodecyoxylnaphthalene (8-PNP-O12) with different illuminated contact electrodes was investigated through steady-state and transient photocurrent measurements. It was revealed that the photogeneration of holes was governed by two different processes according to the electrode materials: the Onsager type of photocarrier generation in the bulk and the electrode-enhanced hole photoinjection with a delay of μs when Al, and either Pt or In2O3–SnO2 electrodes are applied, respectively. In the latter process, the photogeneration yield was one order of magnitude larger than that in the bulk generation process, but decreased as the molecular ordering degraded from the smectic B phase to the smectic A phase, then to the isotropic phase. According to the spectral response and the results of additional experiments on the effect of chemical doping with electron acceptors, the photohole injection was concluded to be attributed to the exciton decay at the interface of the electrode and liquid crystal, i.e., the charge transfer from photoexcited 8-PNP-O12 molecules to the electrode. The experimental results were analyzed according to a one-dimensional Onsager model and the diffusion length of excitons in the SmB phase was determined to be 30±10 nm.

Geminate pair dissociation in random organic systems

The temperature dependences of (i) intrinsic photogeneration, (ii) photoinjection of holes from an indium tin oxide (ITO) electrode via excited state dissociation at the interface and (iii) dark injection from an ITO anode have been studied for a variety of random organic systems. They approach activated behavior in the high-temperature limit while at lower temperature, typically T < 200 K, the yield becomes temperature-independent and, concomitantly, exceeds the yield expected for energetically ordered systems. The effect is in accord with recent simulation work on geminate pair dissociation in energetically disordered hopping systems.

Silicon‐Diamond Interface Band Structure

The properties of Si‐chemical vapor deposited (CVD) diamond interface were investigated by a measuring of CVD diamond films charging under illumination (in a photon energy region from 1.0 to 5.8 eV) as a result of photoexcited electrons and holes injection from silicon substrate and their trapping in a diamond layer. The photocharging was estimated from a change in the dark values of the contact potential difference which was obtained by a dynamic capacitor (Kelvin) method. Three characteristic energy thresholds for electron and hole photoinjection were obtained: 2.8, 3.8, and 5.4 eV. The band model of silicon‐diamond interface was proposed on the basis of the experimental data.

Photo-induced changes of hydrogen bonding in semi-insulating iron-doped indium phosphide

After illumination with 1–1.3 eV photons during cooling-down, metastable PH modes are observed by IR absorption at 5 K in semi-insulating InP:Fe. They correlate with the photo-injection of holes, but not with a change of the charge state of the H-related centres present at equilibrium. They are explained by a change of the bonding of H, induced by hole trapping, from IR-inactive centres to PH-containing centres, stable only below 80 K. One metastable centre has well-defined geometrical parameters and the other one could be located in a region near from the interface with (Fe,P) precipitates.

Evidence for Photoinduced Hole Coupling in BaTiO3:Co

AbstractA possible photoinduced correlation mechanism in BaTiO3:Co is described. The underlying microscopic process is a local hole photoinjection at a Co3+ site close to a preexisting metastable hole. The Jahn‐Teller field at the Co2+ site transfers this pair of holes in the next cell along the rhombic direction [111] before normal polaronic or band conduction. The spatial extension of this correlation depends on a characteristic „delocalization”︁ length. This unstable conductive state appears to be possibly frozen in a permanent way below Tc.

Generation of delocalized E′δ defects in buried Si oxide by hole injection

The generation of delocalized E′δ defects through hole injection in buried SiO2 (BOX) layers of standard separation-by-implantation-of-oxygen structures has been studied by electron spin resonance. Selective photoinjection of holes was carried out under positive biasing of deposited semitransparent electrodes. This leads to the production of numerous delocalized E′δ centers (density≊1.8×1013 cm−2) in addition to a large density (∼9×1013 cm−2) of the common E′γ defect (O3≡Si⋅+Si≡O3)—a result qualitatively similar to previous glow discharge damage data. Comparison of damaging agents applied to BOX indicates that E′γ and E′δ defects are generally produced in an approximately fixed relative ratio. The comparison further bears out that, while energetic photons (x and γ) and ion bombardment activate similar E′δ defects of axial symmetry, hole injection generates a more isotropic type, termed E′δ2. It reveals the tendency that as the damaging species gets ‘‘softer,’’ the g matrix anisotropy fades together with the disorder-induced g spread.

Photoinduced holes coupling in Co: BaTiO3

Abstract : Authors describe a possible photo induced mechanism in Co: BaTiO3 based on a local hole photoinjection at a Co3+ site close to a preexisting metastable hole. The Jahn-Teller field at the Co2+ site transfers the pair of holes over 4A° in the rhombic direction [111].

Photocharging Technique for Barrier Determination on Semiconductor-Insulator Interfaces

The charge accumulation in the insulator of a MIS structure under photoinjection is used to determine the interfacial barrier energy. Phenomenological model of the small-signal photocharging is presented and its experimental application is discussed. For the electron photoinjection from the silicon valence band into the thermal-oxide conduction band the photocharging technique is found to provide coinciding results with those of the conventional photocurrent spectroscopy. The energy barriers for hole photoinjection from Si into SiO2 are determined and the oxide bandgap width near the silicon surface is found to be (8.9 ± 0.1) eV. This bandgap width is close to the oxide bulk one. In high electric fields the electron states distributed within 1 eV above the oxide valence band are involved in the hole injection. [Russian Text Ignored].

Photoinjection of holes and electrons into Phenazine: TCNQ

We have measured in Phenazine: TCNQ single crystals the transport of holes and electrons photoinjected by illumination with a cw He-Ne laser. The analysis of the results is carried out in the framework of the theory of Lampert and Mark. We found that for both hole and electron transport the behavior of the current as a function of voltage is linear and, in the case of electrons, we saw the transition from the ohmic to SCLC with shallow traps behavior.

Effects of oxide thickness on charge trapping in metal-nitride-oxide- semiconductor structures

Charge trapping in chemically vapor-deposited Si3N4 thin films of metal-nitride-oxide- semiconductor (MNOS) structures has been studied using the internal photoelectric-effect technique in combination with high-frequency capacitance-voltage measurements. The trapped charge density in the Si3N4 film was investigated as a function of the experimental parameters of the internal photoelectric-effect technique and the oxide thickness (300–20 Å) of the MNOS structure. The optimum trapped electron density in the Si3N4 film was measured to be 1.5×1018/ cm3 using 4.14-eV photon energy, 3.0-mW/cm2 light intensity, and −20-V applied gate voltage bias for the MNOS structures whose oxide thicknesses were greater than 70 Å. The photoinjection of holes from Si into Si3N4 was inhibited in thick-oxide (300–43 Å) MNOS structures due to the large barrier height at the Si-SiO2 interface. This eliminated simultaneous trapping of holes and electrons in the Si3N4 film. As the oxide thickness of the MNOS structure was reduced below the critical thickness of 43 Å, the photoinjection of holes from Si into Si3N4 was enhanced substantially with subsequent dominant hole trapping in the Si3N4 film.

Photoconduction and vacuum ultraviolet photoelectron spectroscopy of poly(p-xylylene)

Photoconduction and vacuum ultraviolet photoelectron spectroscopy (UPS) of poly(p-xylylene) (PPX) were studied in an attempt to understand its electronic structure. The analysis of photoconduction revealed two mechanisms of carrier generation in PPX, i.e. electron-hole pair electron-hole pair generation through an exciton excitation and the photoinjection of holes from metal electrodes. A detailed study of the photoinjection of holes determined the interfacial barrier height (Au-PPX-Au) and the photoelectron emission threshold as 2.26 and 6.84 eV respectively. These results coincided well with the results of UPS, which gave 6.9 eV for the photoelectron emission threshold and revealed the structure of the valence band of PPX.

Photoinjection Barriers at Chalcogenide/Se Interfaces

The photoinjection of holes from As-Se-Te alloys into Se has been studied in Se/chalcogenide/Se/poly-N-vinylcarbazole (PVK) multilayered samples. The field dependence of the photoinjection currents was found to obey the Richardson-Schottky emission law. The Schottky coefficients and the activation energies of injection were determined, keeping the field dependence of the hole transport in the PVK layer separate from that of the photoinjection currents. The former agreed approximately with the theoretical values. The latter were calculated to be 0.1 eV, 0.3 eV and 0.4 eV at ∼105 V/cm field, for Se/PVK, As2Se2Te/Se, and As2Se1.5Te1.5/Se interfaces; respectively.

Photoinjection of holes and electrons into sulfur single crystals

Photoinjection of holes and electrons into sulfur single crystals

Photoinjection of holes into poly(2-chloro-p-xylylene (PCPX). II

For pt.I see ibid., vol.11, p.L139 (1978). Photoinjection of holes into poly(2-chloro-p-xylylene) (PCPX) has been studied by using Au, Ag, Cu, Sn, Pb and Al electrodes. The threshold energy shifted to lower energies with the higher work-function of metals and also the applied field. It was estimated to be 2.24 eV for Au with no fields applied. The ionisation potential measured from the valence band top to the vacuum level was calculated to be 6.84+or-0.25 eV. The barrier height as well as the photocurrent in itself were affected by the presence of oxygen, which suggested that oxygen formed acceptor-like surface levels.

Photoconductivity Mechanism in Beta-Carotene Glass

Photoconductivity spectra are measured on beta-carotene glass consisting of a mixture of isomers using a sandwich type cell and a surface type cell. In the sandwich cell, the different spectra are obtained by the polarity of the illuminating electrode. When the illuminating electrode is positive, it is concluded that the origin of the photoconductivity spectra in the wavelength below 400 nm is hole photoinjection from the electrode. The photoconductivity band is found in the wavelength 600∼800 nm, and the origin of the band is suggested to be intrinsic.

Hole Photoinjection from Gold into Poly (N-Vinylcarbazole)

Photoinjection currents from gold into poly (N-vinylcarbazole) were measured. The electric field and temperature dependence of the injection current are analyzed on the basis of Blossey's photoinjection model developed for metal-low mobility insulator systems, taking the mean free path x 0 of the injected hot holes as a parameter. The barrier height is estimated to be 1.10 eV from the linear extrapolation of the plot of x 0 2 as a function of photon energy.

Photoinjection of holes from amorphous selenium into a molecularly doped photoconductor

The photoconductive response of a sensitized molecularly doped photoconductor is reported. The photoconductor consists of triphenylamine dispersed in a bisphenol-A-polycarbonate polymer, sensitized by a thin layer of vacuum-deposited amorphous selenium. The wavelength and field dependence of the photoinjection efficiency has been measured using photoinduced discharge techniques. An efficiency of 0.7 was measured at a field of 6.4×105 V/cm.

Photo- and electro-chemical processes at organic crystal electrodes. III. Hole photo-injection into anthracene single crystals

Adsorption to an organic substrate by a suitable reducible solute influences the magnitude of the hole saturation photo-current and hence the charge-carrier injection efficiency. Efficiencies up to 0.65 have been observed with organic photo-injectors, and 0.31 for a hexacyanoferrate(111) hole photo-injector in the singlet region of anthracene (λ 395 nm). About 5 � 4% of the holes formed in the initial photo-injection do not reach the external circuit, presumably because they react at the injecting surface.