Search NASA⌕ Search

SEARCH · Search NASA

Results for “PHOTOCONDUCTIVITY”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Preparations of methoxynitrophenazines and their photoconductivities

Eight methoxynitrophenazines with a methoxy group at the 1 or 2 position and a nitro group at the 6-, 7-, 8- or 9-position of the phenazine ring 1,6, 1,7. 1,8, 1,9, 2,6, 2,7, 2,8 and 2,9 were prepared and their photocurrents were measured by illuminating the surface-type cell with white light from a W lamp under a N atmosphere at room temp. The photocurrents of 1-nitrophenazine 6 and 2-nitrophenazine 7 are compared with those of the methoxynitrophenazines. The photocurrent (i sub p) increases with increasing light intensity (I), thus satisfying log i varies as n log I. The n values are 0.5-1.0. When the nitro group is located at the beta-position of the phenazine ring, the photocurrent becomes larger owing to the electron withdrawing property of the nitro group. On the other hand, the photocurrents of phenazines with the nitro group at the Alpha-position are extremely small. The photoconductivities of methoxynitrophenazines are lower in air.

Sugimoto, A.↗
NASA NTRS1988

Transient photoconductivity in the ferromagnetic semiconductor CdCr2Se4

The transient photoconductive response time of the ferromagnetic semiconductor CdCr2Se4 was measured to be smaller than 90 ps, the response time of the scope. The measurements were performed at room temperature (300 K) for 0.53 and 1.06 microns excitations using a mode locked frequency-doubled YAG laser.

Walser, A. D.↗
NASA NTRS1989

Phase and amplitude characteristics of InP:Fe modified interdigitated gap photoconductive microwave switches

The transmission amplitude and phase characteristics of InP:Fe modified interdigitated gap photoconductive microwave switches are reported. Measurements in the 0.1-1 GHz frequency range show that the phase is shifted about 90 deg and that the corresponding change in amplitude ranges from 43 dB at 0.1 GHz to 23 dB at 1 GHz when the illumination-induced conductance is switched from a low OFF-state value (aboit 4 x 10 to the -8 S) to a high ON-state value (5 mS). The observed characteristics can be described by a phase-shifting region and an amplitude modulation region. In the phase-shifting region the phase strongly depends on the conductance, and the amplitude is almost constant. In the amplitude modulation region both the amplitude and the phase depend on the conductance but the effect on the amplitude is much more pronounced. A lumped-element model describing the device performance is presented. It is concluded that these optoelectronic microwave switching devices are suitable for high-speed amplitude modulation but are not promising as high-speed phase shifters.

Andersson, Ingmar L.↗
NASA NTRS1989

Microwave response of a HEMT photoconductive detector

Interdigitated photoconductive detectors with 5-micron geometry have been fabricated on HEMT material and their optical response characteristics at 820 nm have been examined at dc and in the frequency range of 2-8 GHz. These have been compared with characteristics of similar 1-micron devices on MESFET material. The shapes of the frequency responses were found to differ, but the useful bandwidth of both types of devices was found to be similar.

Claspy, P. C.↗
NASA NTRS1995

Two Carrier Analysis of Persistent Photoconductivity in Modulation-Doped Structures

A simultaneous fit of Hall and conductivity data gives quantitative results on the carrier concentration and mobility in both the quantum well and the parallel conduction channel. In this study this method was applied to reveal several new findings on the effect of persistent photoconductivity (PPC) on free-carrier concentrations and mobilities. The increase in the two-dimensional electron-gas (2DEG) concentration is significantly smaller than the apparent one derived from single carrier analysis of the Hall coefficient. In the two types of structures investigated, delta doped and continuously doped barrier, the apparent concentration almost doubles following illumination, while analysis reveals an increase of about 20% in the 2DEG. The effect of PPC on mobility depends on the structure. For the sample with a continuously doped barrier the mobility in the quantum well more than doubles. This increase is attributed to the effective screening of the ionized donors by the large electron concentration in the barrier. In the delta doped barrier sample the mobility is reduced by almost a factor of 2. This decrease is probably caused by strong coupling between the two wells, as is demonstrated by self-consistent analysis.

Schacham, S. E.↗
DOE OSTI2024

Sensitivity Enhancement in Photoconductive Light Field Sampling

Abstract The emerging field of lightwave electronics is driving optoelectronic information processing beyond current classical limitations and has the potential to eventually reach petahertz frequencies. One of the major obstacles in reaching not only higher switching frequencies but also higher repetition rates with low‐power light sources is the efficiency of related devices. A device is presented based on a multilayered material that shows a 13‐fold enhancement in terms of converting light to electric current compared to bulk solids. Furthermore, the device exhibits an almost flat intensity response within its working range. The outstanding properties of engineered multilayered devices promise to push technology for lightwave electronics applications.

47 OTHER INSTRUMENTATION↗
DOE OSTI2021

Ultrafast Charge Transfer Dynamics at the Origin of Photoconductivity in Doped Organic Solids

In spite of their growing importance for optoelectronic devices, the fundamental properties and photophysics of molecularly doped organic solids remain poorly understood. Such doping typically leads to a small fraction of free conductive charges, with most electronic carriers remaining Coulombically bound to the ionized dopant. Recently, we have reported photocurrent for devices containing vacuum-deposited TAPC (1,1-bis(4-bis(4- methylphenyl)aminophenyl)cyclohexane) doped with MoO 3 , showing that photoexcitation of charged TAPC molecules increases the concentration of free holes that contribute to conduction. Here, we elucidate the excited-state dynamics of such doped TAPC films to unravel the key mechanisms responsible for this effect. We demonstrate that excitation of different electronic transitions in charged and neutral TAPC molecules allows bound holes to overcome the Coulombic attraction to their MoO 3 counterions, resulting in an enhanced yield of long-lived free carriers. This is caused by ultrafast back-and-forth shuffling of charges and excitation energy between adjacent cations and neutral molecules, competing with relatively slow nonradiative decay from higher excited states of TAPC •+ . The light-induced generation of conductive carriers requires the coexistence of cationic and neutral TAPC, a favorable energy level alignment, and intermolecular interactions in the solid state.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Electrolyte Immersion Increases Photoconductivity in a Model Polymer Photocathode

Immersing polymer solar cells in aqueous electrolyte for photoelectrochemical (PEC) hydrogen production is likely to cause photophysical changes that could present both challenges and opportunities for engineering functional and durable devices. Herein we study the bulk heterojunction blend poly(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl)-alt-(2-(((2-ethylhexyl)oxy)carbonyl)-3-fluorothieno[3,4-b]thiophene-4,6-diyl):poly(N,N'-di(2-octyldodecyl)naphthalene-1,8:4,5-bis(dicarboximide)-2,6-diyl)-alt-(2,2-bithiophene-5,5'-diyl) (PTB7-Th:N2200) excited-state dynamics in electrolyte from femtosecond to millisecond time scales using pump-probe microwave conductivity and absorption spectroscopy. While the blend swells very little, electrolyte exposure increases the microwave-frequency mobility and possibly the yield of photogenerated charges while also decreasing crystallinity. These results indicate an enhancement in key performance metrics, implying that any limitations on the performance of PEC test devices do not arise from active layer-electrolyte interactions. For the PTB7-Th:N2200 blend or similar photocathode systems, our results indicate that improving the interfacial kinetics and/or the carrier lifetime should be prioritized, not protecting the active layer from the electrolyte. Since this observation may not be universal to all polymer systems, future research should focus on identifying their limiting photophysical processes. electrolyte.

14 SOLAR ENERGY↗
DOE OSTI2020

High broadband photoconductivity of few-layered MoS 2 field-effect transistors measured using multi-terminal methods: effects of contact resistance

Among the layered two dimensional semiconductors, molybdenum disulfide (MoS 2 ) is considered to be an excellent candidate for applications in optoelectronics and integrated circuits due to its layer-dependent tunable bandgap in the visible region, high ON/OFF current ratio in field-effect transistors (FET) and strong light–matter interaction properties. In this study, using multi-terminal measurements, we report high broadband photocurrent response ( R ) and external quantum efficiency (EQE) of few-atomic layered MoS 2 phototransistors fabricated on a SiO 2 dielectric substrate and encapsulated with a thin transparent polymer film of Cytop. The photocurrent response was measured using a white light source as well as a monochromatic light of wavelength λ = 400 nm–900 nm. We measured responsivity using a 2-terminal configuration as high as R = 1 × 10 3 A W -1 under white light illumination with an optical power P opt = 0.02 nW. The R value increased to 3.5 × 10 3 A W -1 when measured using a 4-terminal configuration. Using monochromatic light on the same device, the measured values of R were 10 3 and 6 × 10 3 A W -1 under illumination of λ = 400 nm when measured using 2- and 4-terminal methods, respectively. The highest EQE values obtained using λ = 400 nm were 10 5 % and 10 6 % measured using 2- and 4-terminal configurations, respectively. The wavelength dependent responsivity decreased from 400 nm to the near-IR region at 900 nm. The observed photoresponse, photocurrent–dark current ratio (PDCR), detectivity as a function of applied gate voltage, optical power, contact resistances and wavelength were measured and are discussed in detail. The observed responsivity is also thoroughly studied as a function of contact resistance of the device.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Enhanced upconversion and photoconductive nanocomposites of lanthanide-doped nanoparticles functionalized with low-vibrational-energy inorganic ligands

Upconverting nanoparticles (UCNPs) convert near-infrared (IR) light into higher-energy visible light, allowing them to be used in applications such as biological imaging, nano-thermometry, and photodetection. It is well known that the upconversion luminescent efficiency of UCNPs can be enhanced by using a host material with low phonon energies, but the use of low-vibrational-energy inorganic ligands and non-epitaxial shells has been relatively underexplored. Here, we investigate the functionalization of lanthanide-doped NaYF4 UCNPs with low-vibrational-energy Sn2S64- ligands. Raman spectroscopy and elemental mapping are employed to confirm the binding of Sn2S64- ligands to UCNPs. This binding enhances upconversion efficiencies up to a factor of 16, consistent with an increase in the luminescent lifetimes of the lanthanide ions. Annealing Sn2S64--capped UCNPs results in the formation of a nanocomposite comprised of UCNPs embedded within an interconnected matrix of SnS2, enabling each UCNP to be electrically accessible through the semiconducting SnS2 matrix. This facilitates the integration of UCNPs into electronic devices, which we demonstrate through the fabrication of a UCNP-SnS2 photodetector that detects UV and near-IR light. Our findings show the promise of using inorganic capping agents to enhance the properties of UCNPs while facilitating their integration into optoelectronic devices.

Pan, Jia-Ahn↗