Activation Energy Of Photoconductivity Research Articles

Photoconduction in amorphous thin films of Se90Sb10-xAgx glassy alloys

ABSTRACTThe present paper reports the steady state photoconductivity and photosensitivity response of thermally evaporated amorphous thin films of Se90Sb10-xAgx(x = 2, 4, 6, 8, 10). Temperature dependence of dark conductivity is studied and activation energy is calculated for different samples. Temperature dependence of photoconductivity is also studied at different intensities. From temperature dependence of photoconductivity activation energy is computed at different intensities which are found to vary from 0.26 to 0.47 eV. Intensity dependence of photoconductivity has also been studied at different temperatures. These curves are plotted on logarithmic scale and found to be straight lines which show that photoconductivity follows a power law with intensity. Composition dependence of dark conductivity, activation energy of DC conduction and photosensitivity show that these parameters are highly. composition dependent and show a discontinuity at a particular composition when Ag concentration becomes 6 at. %. This is explained in terms of transition from floppy state to mechanically stabilized state at this composition.

Photoconduction in the Peierls conductor monoclinic TaS3

Photoconduction in the monoclinic phase of quasi-one-dimensional conductor TaS3 has been observed at T<70K. It was studied jointly with low-temperature ohmic and non-linear dark conduction. The strong sample quality dependence of both photoconduction and dark conduction at this temperature region has been observed. Together with a similarity of the main features of the photoconduction characteristic of both monoclinic (m-TaS3) and orthorhombic (o-TaS3) samples the following new peculiarities of photoconduction in m-TaS3 were found: (1) the dependence of the activation energy of photoconduction on temperature, T, (2) the change of the recombination mechanism from the linear type to the collisional one at low T with a sample quality growth, and (3) the existence of a fine structure of the electric-field dependence of photoconduction. Spectral study gives the Peierls energy gap value 2Δ⁎=0.18eV.

TRAP DISTRIBUTION NEAR THE ENERGY BAND GAP EDGE IN MDMO-PPV

Carrier transport and trapping was investigated in poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) layers by thermally stimulated currents (TSC) depending on the exciting light spectral range. The upper edge of the light spectra was varied from 1.77 eV up to 3.1 eV to assure selective excitation of the defect states. We had shown that material conductivity is affected by several thermally activated processes, i.e., carrier generation from trapping states and thermally stimulated mobility growth. If the below band gap excitation was used, the effective photoconductivity activation energy values of 0.13–0.15 eV were obtained. After the above band gap excitation, the effective photoconductivity activation energy values decreased to 0.05 eV. The energy distribution of the trapping state density was shown to follow the Gaussian distribution function. The deeper states with activation energies of 0.28–0.3 eV and 0.8–0.85 eV were identified too. The results are direct indication by photo-thermo-electrical methods of distributed in energy trapping and transport states with the standard deviation of the density of states of about 0.015 eV.

Effect of In additive on the photosensitivity of glassy Se80Te20 alloy

Photo-electrical measurements are done on thin films of Se80−x Te20In x (0 ≤ x ≤ 20) to see the effect of In on the photoconductive properties of binary Se80Te20 alloy. The photosensitivity (σph/σd) and activation energy of photoconduction (ΔE ph) are determined for this purpose. It has been found that σph/σd and ΔE ph are decreased with increasing concentration of the third element. The results are explained in terms of the increase in density of the defect states with the increase in concentration of the additive in thin films of Se80−x Te20In x .

Features of conductivity and photoconductivity of polymer composites containing heteropolynuclear M(II)/Cr(III) complexes

Electrical and photoelectrical properties of polymeric composites containing heteropolynuclear Zn(II)/Cr(III) or Co(II)/Cr(III) complexes were studied. Under excitation of d-d transitions of Cr3+, the internal photoelectric effect in composite films under study is stronger for the zinc complex than for the cobalt complex. The photoconductivity increases as the C60 acceptor is introduced into composites. The electroconductivity and photoconductivity activation energy is >1 eV and depends weakly on the external electric field. The photocurrent decreases under the external magnetic field, which occurs to a greater extent for composites containing the Co(II)/Cr(III) complex. The differences in the photoconductivity are associated with different magnetic properties of metal centers in heteropolynuclear complexes.

Effect of lead additive on photoconductive properties of Se–Te chalcogenide films

Steady state and transient characteristics of photoconductivity have been studied in amorphous thin films of Se85Te15−xPbx (x=0, 2, 4 and 6). The studies of temperature dependence of photoconductivity show that photoconduction is a thermally activated process. The value of activation energy of photoconduction is, however, smaller as compared to activation energy in dark. The results of intensity dependence of steady state photoconductivity indicate that bimolecular recombination is predominant in these materials. The Se85Te15−xPbx is found highly composition dependent as lead impurity drastically changes the conduction parameters. The incorporation of Pb impurity is found to affect the transient photoconductivity properties drastically. A spike is observed in the rise curve of photocurrent in undoped a-Se85Te15, which disappears in Pb doped samples. The lead incorporation is found to delay the onset time of recombination in the rise and decay of photocurrent which is attributed to the trapping of charge carriers in deep localized states produced by lead impurity. The photocurrent rise and decay is explained by the trap-controlled recombination model proposed by Iovu et al. and the dispersion parameter α of localized state energy distribution is determined from the experimental results.

Photoconductivity of polymer compositions with a high content of organic dyes

The photoconductivity of films of an amorphous molecular semiconductor based on poly-N-epoxypropylcarbazole with high contents of organic dye molecules and films of a polymer composition with organic dye aggregates is studied. It is found that the dependence of the photoconductivity activation energy on the external electric field disappears as one passes from polymer compositions of the former type to those of the latter. The experimental results are explained by assuming that carrier photogeneration and transport in polymer compositions with regions of quasi-ordered dye molecules, in contrast to amorphous molecular semiconductors, take place within such regions.

Photoconductivity in a-Se 90Ge 10− xIn x thin films

Temperature dependence of conductivity is studied in dark as well as in presence of light in amorphous thin films of Se 90Ge 10- x In x ( x=0, 2, 4, 6, 8 and 10) prepared by vacuum evaporation technique. The dark conductivity increases exponentially with temperature, between 293 and 373 K, indicating that the conductivity is a thermally activated process. Temperature dependence of conductivity in presence of light at different intensities indicate that photoconductivity is also thermally activated in the above temperature range in all the samples studied. The activation energy of photoconduction is found to decrease with the increase in the intensity which indicates the shift of the Fermi level on light shining due to splitting of Fermi level into quasi-Fermi levels. Transient photoconductivity measurements at different temperatures and intensities indicate that the decay of photoconductivity is quite slow. A persistent photocurrent is also observed which increases at higher temperatures. This is attributed to light-induced effects in these materials. Photoconductivity decay, even after subtraction of persistent photoconductivity, is found to be non-exponential in the present case indicating the presence of continuous distribution of defect states.

Excitonic effects in the photoconductivity of quantum-well GaxIn1−x As/InP structures

The photoconductivity spectra of quantum-well structures consisting of 50 alternating 7 to 12-mm-thick GaxIn1−x As (x=0.47) layers forming quantum wells and 10 to 15-nm-thick InP barriers have been investigated. Characteristic excitonic peaks 11H, 11L, 13H, 22H, and 22L were observed in the high-quality structures. A strong temperature dependence was found for the 11H exciton, while no such dependence was observed for any of the other excitons. This is explained by the fact that the 11H state of an exciton falls in the range of energies which are forbidden for free carriers, while the remaining states are resonance states and overlap with the continuous spectrum. The thermal activation energy of photoconductivity was found to be 150±30 meV, much greater than the exciton binding energy and close to the depth of the potential well for electrons. This shows that the photosensitivity is due to above-barrier charge-carrier transfer and that the tunneling transfer between the wells is negligible.

Electron transport in [formula omitted] superlattices

The electron transport in a- Si: H,F a- Si,Ge: H,F multilayers is analyzed. We use the dark conductivity activation energy E a,d, its preexponential factor σ 0, and the photo conductivity activation energy E a,ph and its exponent γ to highlight the asymmetry between transport parallel and perpendicular to the layers. The carrier confinement in the wells translates to small, quantitative differences with respect to bulk alloys for parallel transport, but to qualitatively new transport mechanisms for perpendicular transport.

Steady-State Photoconductivity in Undoped Amorphous Silicon

ABSTRACTAn analytical expression for the thermal activation energy of the steady-state photoconductivity is shown to agree with experimental data in a range of temperature and generation rate for undoped hydrogenated amorphous silicon (a-Si:H). This agreement supports our suggestion that the commonly observed small activation energy of the photoconductivity in undoped a-Si:H originates in the strong temperature dependence of the quasi-Fermi level for electrons.

Influence of an electric field on the spectral distribution of photoconductivity in thin films of chalcogenide glasses of the system (As 2Se 3) 1− x:(Tl 2Se) x

In this paper are reported the measurements of the effect of an electric field on the photoconductivity activation energy of vitreous As 2Se 3 and its compounds with Tl 2Se, in the thin film form. The electric field decreases the photoconductivity band gap energy of these semiconductors. A shift in the band gap energy of the order of 0.03 eV was noted. The threshold electric field for this shift was found to be composition dependent and seemed to vary inversely as the concentrations of the Tl 2Se molecules. It was the lowest for the semiconductor having the smallest energy gap. Also the magnitude of the shift was the highest for the glass having no concentration of Tl 2Se.

A study of traps for current carriers in organic solids The effect of crystallinity on ethyl 2,5-di(anilino)-terephthalate and its oligomer

The dark electric conductivity, photoconductivity, thermostimulated currents (TSC) and space-charge-limited currents (SCLC) were measured on amorphous, polycrystalline and spherulitic layers of ethyl 2,5-di(anilino)terephthalate and of its oligomer. The electric behaviour is considerably influenced by processes of trapping of charge carriers. The activation energy of photoconductivity is determined by the trap depth. The crystalline and spherulitic domains in the amorphous phase form new traps, ca. 0·6 eV deep.

Photo and dark conductivity of doped amorphous silicon

AbstractThe photo and dark conductivity of boron‐ and phosphorus‐doped amorphous silicon is measured in the temperature range from 100 to 400 K, for various doping levels. Increasing doping generally decreases the activation energy of dark conductivity, down to 0.2 eV, and also decreases the σ0‐value (extrapolation of dark conductivity for T → ∞). This lowering of σ0 is explained by a temperature shift of the Fermi level plus a change in the conduction mechanism. The activation energy of photoconductivity is also lowered, at least by phosphorus doping, which is understood by an influence of doping on the tailing of the bands.

Photoelectric properties of linear oligoaniline derivatives

Dark conductivity and photoconductivity were measured on polycrystalline layers and single crystals of the arylamine type compounds. The photoconductivity process is largely affected by the effects of trapping of the charge carriers followed by their thermal releasing from traps. The activation energy of photoconductivity is determined by the depth of the traps, similarly to the activation energy of the dark conductivity at lower temperatures. At laboratory temperatures and higher ones the activation energy of the dark conductivity is determined by the position of the Fermi level.

Semiconductivity and Photoconductivity of TCNQ Crystal

Abstract The results of a preliminary investigation on the semiconductive and photoconductive behaviors of the TCNQ crystal are described. The single crystal of TCNQ shows a resistivity of the order 1011 Ω·cm at room temperature with an activation energy of about 0.6 eV. The photoconductive sensitivity of a TCNQ crystal is comparatively high. There are two types of crystal with respect to sensitivity to visible light. In both cases, the low-energy tail is extended up to about 1.5 eV. It is shown that both the activation energy of photoconduction and the light-intensity dependence of photocurrent differ appreciably in visible and ultraviolet regions.