Films of cationic dye thiocyanates adsorbed on copper based substrates form organized structures by a process of templated self-assembly. The growth process is initiated by strong S-bonding of ...thiocyanate ions to Cu sites, followed by repetitive and alternate bonding of dye cations and thiocyanate anions by electrostatic and dipolar forces. Rhodamine B thiocyanate and several other cationic dye thiocyanates solid films deposited this way exhibit J-aggregation behavior with large red-shifts in the absorption spectrum. Photoelectrochemical experiments with Rhodamine B thiocyanate deposited CuSCN cathodes indicated exciton diffusion lengths of order of 100 nm and space charge saturation current measurements suggested nearly three orders of magnitude enhancement of the electron mobility in the J-aggregated film. The photovoltaic properties of heterojunctions sandwiching J-aggregated dye films between n- and p-type semiconductor surfaces are investigated for potential application as solar cells and photon detectors.
A GaAs/AlGaAs heterojunction is used as a spin-split-off band IR detector operating at or around room temperature. This detector structure followed a similar layer architecture to the quantum well IR ...photo detectors (QWIP) and Heterojunction Interfacial Work function Internal Photoemission (HEIWIP) detectors. Compared to QWIPs, the emitter layer thickness is increased to avoid confinement. Unlike either the QWIPs or HEIWIPs, these detectors will have two energy gaps (barriers) to obtain the wavelength threshold which could be used to design detectors either for optimum operating temperature or optimum responsivity. The free carrier energy gap is determined by the Al fraction and the spin-split-off transition energy provides another handle on controlling the effective threshold of the detector. Unlike QWIPs, these will also detect normal incidence radiation. A preliminary detector showed a peak responsivity of 0.29
mA/W at 2.5
μm at room temperature.
All electronic devices are plagued with 1/f noise originating from many causes. The most important factors contributing to 1/f noise in a semiconductor is believed to be recombination of carriers and ...their trapping at defects and impurity sites. Adsorption of moisture and electron acceptor molecules enhances the intensity of 1/f noise. Amazingly, some molecular species that strongly chelate to the semiconductor surface, suppress 1/f noise owing to passivation of the recombination sites. Thus in addition to sensitization, the dye adsorbed on the nanocrystallites plays a key role in mitigation of recombinations. For this reason dye-sensitized heterojunctions could also find application as low noise NIR photon detectors. Experiments conducted with oxide semiconductors (TiO2, ZnO, SnO2) indicate that the mode of binding of dyes at specific sites determines the extent to which the recombination and 1/f noise are suppressed. The transport of electrons in a nanocrystalline matrix is diffusive with a diffusion coefficient D depending on the trapping and detrapping processes. Thus passivation of trapping sites by the adsorbed dye is expected to increase the response time which can be expressed as τ∼L2/D, where L=thickness of the nanocrystalline film. Measurement techniques and construction of a dye-sensitized NIR photon detector will be discussed.
Results are reported on dual-band detectors based on a GaN/AlGaN structure operating in both the ultraviolet–midinfrared (UV–MIR) and ultraviolet–farinfrared (UV–FIR) regions. The UV detection is due ...to an interband process, while the MIR/FIR detection is from free carrier absorption in the emitter/contact followed by internal photoemission over the barrier at the GaN/AlGaN interface. The UV detection, which was observed from 300K to 4.2K, has a threshold of 360nm with a peak responsivity of 0.6mA/W at 300K. The detector shows a free carrier IR response in the 3–7μm range up to 120K, and an impurity response around 54μm up to 30K. A response in the range 7–13μm, which is tentatively assigned to transitions from C impurities and N vacancies in the barrier region, was also observed. It should also be possible to develop a detector operating in the UV–visible–IR regions by choosing the appropriate material system. A dual-band detector design, which allows not only to measure the two components of the photocurrent generated by UV and IR radiation simultaneously but also to optimize the UV and IR responses independently, is proposed.
Why Gratzel’s cell works so well Jayaweera, P.V.V.; Perera, A.G.U.; Tennakone, K.
Inorganica Chimica Acta,
02/2008, Letnik:
361, Številka:
3
Journal Article
Recenzirano
Diffusive transport of an electron injected via dye-sensitization, the dye passivates the surface facilitating the transport.
In Gratzel’s cell, the electrons injected by the photo-excitation of dye ...molecules, anchored to a mesoporous TiO
2 film, efficiently diffuse to the back contact achieving solar energy conversion at efficiencies exceeding 10%. The mesoporous TiO
2 surface constituted of randomly arranged nanocrystallites with a roughness factor of the order 1000 is heavily populated with traps, defects and adsorbed species which act as recombination centers. Nevertheless, the cell functions, mitigating recombination expected to occur via the interaction electrons at the surface. Evidence based mainly on 1/
f noise measurements is presented to show that dye bonded to the TiO
2 surface passivates recombination centers. Furthermore the suppression of trapping–detrapping events at the surface increases the diffusion coefficient of the electrons through the nanocrystalline matrix facilitating electron transport to the back contact. The Gratzel cell is also unique, none of the high bandgap oxide materials other than TiO
2 yield energy conversion and quantum efficiencies as high as that of the cells based on TiO
2. 1/
f noise measurements also reveal a distinct difference between TiO
2 and ZnO mesoporous films suggesting that the films made from the latter material are more intensely populated with surface states that mediate recombination.
Reflectance measurements from p-type GaSb:Zn epitaxial films with different hole concentrations (1017–1018 cm-3) have been investigated over the frequency region of 100–1000 cm-1. A minimum ...broadening feature corresponding to the hole plasmon was observed in the reflectance spectra. The experimental infrared spectra were well fitted using a Lorentz-Drude dispersion model. The real part ε1 of the dielectric function decreases with increasing hole concentration. However, the imaginary part ε2 increases with hole concentration in the far-infrared region. This indicates that the acoustic- and optic-phonons mainly participate in the free carrier absorption processes. The hole mobility obtained from Hall-effect measurements is slightly larger than that derived from optical measurements and the average ratio of mobilities is estimated to be 1.33. Owing to overdamping effects, the upper branch of longitudinal-optical phonon plasmon (LPP) coupled modes was observed. The upper LPP+ frequency increases with hole concentration and it shows a transition from phonon-like to plasmon-like behavior. A theoretical analysis with solutions in the complex frequency plane describes these experimental results.
•Light-intensity causes zero VB offsets in low-barrier IR detectors at T>50K.•PL shows band gap increase in undoped GaAs with illumination at T=25K.•Undoped GaAs and p-AlGaAs are reluctant to BGN ...caused by incident light intensity.•Undoped GaAs/p-doped Al0.01Ga0.99As combination suitable for FIR/THz detectors.
Band gaps of semiconductor materials are reduced due to band gap narrowing (BGN). Photoluminescence measurements on GaAs and AlGaAs thin films revealed a dependency of incident light intensity, and temperature in BGN in addition to the doping density. As a result, the valence band offset of p-doped GaAs/AlGaAs heterojunctions were reduced under illumination and high temperatures. We present evidence of incident-light-intensity causing barrier reduction at temperature >50K causing zero valence band offsets in low-barrier heterostructures such as p-GaAs/Al0.01Ga0.99As, in addition to the dark-current increase by thermal excitations, causing the device failure at high temperatures.
GaAs/AlGaAs multilayer heterojunction structures with different aluminum (Al) fractions and emitter doping densities were tested to identify optimum parameters for high temperature coefficient of ...resistance (TCR). Higher Al fractions and lower doping densities showed higher TCR. Additionally, p-doped heterojunction structures showed a higher TCR compared to an n-doped one with similar parameters. A p-doped multilayer superlattice heterojunction structure with 30 periods of GaAs/Al
0.57Ga
0.43As junctions, operating at room temperature showed a TCR of ∼4% and bolometer like infrared (IR) response up to 20
μm with a
D* of 1.7
×
10
6
Jones. This TCR is higher than that of VO
x
or α-Si bolometers. At low temperatures (50
K) some of these devices have shown TCR values of over 30%.
The variation in spectral shape around the threshold frequencies between model and experimental responsivity spectra in heterojunction interfacial workfunction internal photoemission (HEIWIP) ...infrared detectors was investigated. This is attributed to the loss of photoexcited carriers, within the escape cone, prior to photoemission. The energy dependent transmission of excited carriers is incorporated to the existing photoemission model to show that emission around the threshold frequency is reduced considerably by quantum mechanical reflection of photoexcited carriers as observed in experimental results. In the new model, the photoemission of carriers become bias dependent.
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