High resolution Schottky barrier detectors for alpha particles have been fabricated on 20 μm n-type 4H-SiC epitaxial layers. Schottky barrier contact structure was accomplished by deposition of 10 nm nickel on the Si face of the epilayers. The detectors were characterized for structural, electrical, and spectroscopic properties. Scanning electron microscopy and Nomarski optical microscopy revealed a micropipe density lower than 1 cm-2. The current-voltage (I-V) characteristics of the device exhibited very low leakage current of the order of 6.5 pA at an operating bias of 90 V. C-V measurements revealed a typical effective doping concentrations of 2.4 × 1014 cm-3 in these epilayers. The detectors were evaluated for alpha particles detection using a 241Am source. An energy resolution of ∼0.98% for 5.48 MeV alpha particles was observed. The separate contribution of charge carrier drift and diffusion to the total charge collection efficiency has been calculated in these detectors following a drift-diffusion model. Detailed electronic noise analysis in terms of equivalent noise charge (ENC) was carried out to study the effect of various noise components that contribute to the total electronic noise in the detection system. Effect of shaping time, presence of source and bias on the ENC has been studied in details.

Large-area Cu2ZnSnS4 (CZTS) thin films were deposited by low-cost spray pyrolysis technique on Mo-coated soda-lime glass (SLG) substrates at varied substrate temperatures of 563-703°K. Deposition conditions were optimized to obtain best quality films and effect of post deposition thermal processing of the as-deposited films under H2S ambient were investigated. Structural, morphological, and compositional characterization of as-deposited and H2S treated CZTS absorber layers were carried out by x-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM) and energy dispersive x-ray analysis (EDX). Optical and electrical properties were measured by UV-Vis spectroscopy, van der Pauw, and Hall-effect measurements. Films grown at ∼360°C substrate temperature showed superior optoelectronic properties, improved stoichiometry and smoother morphology compared to films grown at much higher or lower temperatures. Film properties were significantly improved after the H2S processing. Our results show that large area high quality CZTS films can be fabricated by low-cost spray pyrolysis technique for high throughput commercial production of CZTS based heterojunction solar cells.

Cd0.9Zn0.1Te (CZT) single crystal has been grown using a tellurium solvent method. Two CZT crystals have been chosen from two different locations of the grown ingot. The two crystals were characterized using infrared transmission (IR) imaging and radiation detectors in planar geometry were fabricated on them. Current-voltage characteristics (I-V) revealed a resistivity of ∼8.6×1010 Ω−cm for detector A (6.9×6.9×4.8 mm3) and 6.7×1010 Ω−cm for detector B (11.5×11.7×2.6 mm3). IR imaging showed a lesser concentration of Te inclusions/precipitates in detector A. The transport properties viz., electron drift-mobility and electron mobility-lifetime product were measured using alpha spectroscopy in these detectors in a planar configuration. Detector A showed better charge transport properties compared to detector B. The superior transport properties of crystal A were reflected in the spectroscopic properties of the detectors. Gamma pulse height measurements using a 241Am isotope revealed an energy resolution of ∼5 % for detector A and ∼7 % for detector B. A digital spectrometer and a biparametric correction scheme was incorporated to recover the pulse height spectrum of high energy gamma rays (137Cs source) from the effect of poor hole movement.

CdS host nanocrystals with 4.2-5.5 nm in diameter have been synthesized from air stable precursors via a synthetic chemical route and doped with rare earth (RE) terbium (Tb3+) and ytterbium (Yb3+) ions. RE3+-doped CdS cores were shelled by ZnS layers of different thicknesses. The resulting core/shell nanocrystals show a complete broadband absorption below 400-460 nm to the deep UV region depending on the size of the cores. RE3+-doped CdS nanocrystals showed a red shift in the emission as observed under irradiation of 302 nm UV light and was confirmed by room temperature photoluminescence (PL) measurements. The nanocrystals were further characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), and energy dispersive x-ray (EDX) analysis. The results show that these RE3+-doped nanocrystals can be used as solar spectral matching downconversion material to enhance photovoltaic efficiency of existing solar cells.

Optimum quality polycrystalline AgGaSe2 thin films were deposited on H-terminated n-Si substrates by controlled thermal evaporation method. The film deposition conditions were varied to optimize the structure and optoelectronic properties of AgGaSe2 thin films. X-ray diffraction (XRD) studies showed that all AgGaSe2 films were of chalcopyrite structure and while the films deposited at room temperature (300 K) had random grain orientation, the films deposited at higher substrate temperature (≥ 450K) showed preferred (112) orientation. The composition of the films were analyzed by electron probe microanalysis (EPMA) deposited at different substrate temperatures. The ultraviolet-visible (UV-Vis) spectra showed the optical bandgap of 1.80 eV, with sharper band edge for the films deposited at higher temperature. The films were p-type and the resistivities of the as deposited films at 300 and 650K were ~5×103 and ~200 Ω.cm respectively. p-AgGaSe2/n-Si heterojunction solar cells, having an active area of 0.18 cm2 without any antireflection coating were designed and fabricated. It was observed that the films deposited at 650K produced heterojunctions with significantly improved photovoltaic properties. The evidence of the barrier height modifications have been provided by C-V measurements. Under solar simulator AM1 illumination, the improved junction exhibited an efficiency of 5.2%, whereas the AgGaSe2 films deposited at 300K showed a lower efficiency of 2.1%.

Cd0.9Zn0.1Te (CZT) detector grade crystals were grown from zone refined Cd, Zn, and Te (7N) precursor materials, using the tellurium solvent method. These crystals were grown using a high temperature vertical furnace designed and installed in our laboratory. The furnace is capable of growing up to 8” diameter crystals, and custom pulling and ampoule rotation functions using custom electronics were furnished for this setup. CZT crystals were grown using excess Te as a solvent with growth temperatures lower than the melting temperatures of CZT (1092°C). Tellurium inclusions were characterized through IR transmittance maps for the grown CZT ingots. The crystals from the grown ingots were processed and characterized using I-V measurements for electrical resistivity, thermally stimulated current (TSC), and electron beam induced current (EBIC). Pulse height spectra (PHS) measurements were carried out using a 241Am (59.6 keV) radiation source, and an energy resolution of ~4.2% FWHM was obtained. Our investigation demonstrates high quality detector grade CZT crystals growth using this low temperature solvent method.

GaTe and GaTe:In single crystals were grown from high purity Ga (7N) and zone refined Te (>7N) precursor materials. InSe thin films were deposited by thermal evaporation onto the sulfur passivated GaTe:In substrates at various substrate temperatures from 450K-550K to fabricate p-GaTe:In/n-InSe heterojunction solar cells. Scanning electron microscopy (SEM), X-ray diffraction (XRD), electron probe microanalysis (EPMA), and X-ray photoelectron spectroscopy (XPS) were used to characterize GaTe:In crystals and InSe thin film surfaces. The current-voltage characteristics of p-GaTe:In/n-InSe solar cells were measured under dark and under illumination of 75mW/cm2. Dark J-V measurements showed that the reverse saturation current density (J0) decreased from 3.8 x 10-6 A/cm2 to 1.5 x 10-9 A/cm2 and the ideality factor was reduced from 2.04 to 1.15 as a result of surface passivation. Under illumination of 75 mW/cm2, the open-circuit voltage (Voc) increased from 0.54V to 0.68V and short-circuit current density (Jsc) increased from 7.19 mA/cm2 to 8.65 mA/cm2 for solar cells with surface passivated GaTe:In substrates, leading to an increased solar cell efficiency of 5.03%. EPMA measurements revealed that the InSe thin films deposited at 550 K on GaTe:In substrates were near stoichiometric with enhanced grain size contributing also to better solar cell performance.

Ab initio electronic structure calculations within density functional theory have been carried out in pure GaSe and GaSe doped with substitutional impurities (Cd, In and Sn) at the Ga site in order to understand the nature of the defect states and how they depend on the nominal valence of these three impurities. We find that Cd impurity introduces a defect state located between 0.1 – 0.18 eV above the valence band, in good agreement with photoluminescence peaks seen at 0.13 eV and 0.18 eV. Using both experimental and theoretical effective mass parameters we show that effective mass model fails to describe these acceptor states. Sn changes the single particle density of states (DOS) near the bottom of the conduction band, and gives rise to resonant states deep in the valence band. In, on the other hand, behaves like Ga, it does not make noticeable change in the DOS of the host GaSe crystal.

The single crystal growth of layered semiconductors GaSe and GaTe by vertical Bridgman technique using zone refined selenium (Se), tellurium (Te) and high purity (HP) gallium (Ga) have been described. The grown crystals (2.5 cm diameter and ∼10 cm long) have demonstrated efficient broadband tunable THz emission and as sensitive THz detectors. The crystals have shown promising characteristics with good optical quality, high dark resistivity, wide band gap (GaSe-2.01 eV and GaTe-1.66 eV at 300 K), good anisotropic (parallel, p & perpendicular, pa) electrical properties (σ∥ vs σ⊥ and μ∥ vs σ⊥) and long term stability. Different steps involved in processing GaSe and GaTe crystals as THz sources and sensors are described.

The dielectric function and momentum relaxation time of carriers for a single-crystal GaSe were investigated using terahertz time-domain spectroscopy over the frequency range from 0.4 to 2.4 THz. The key parameters determined from THz data using the Drude model are: the plasma frequency ωp = 6.1 ± 0.5 THz, the average momentum relaxation time <τ> = 51 +± 6 fs for electrons. The THz absorption spectrum showed resonance structures attributed to the difference frequency combinations associated with acoustical and optical phonons. This paper has already been published, see B. L. Yu, F. Zeng, V. Kartazayev, and R. R. Alfano, “Terahertz studies of the dielectric response of and second order phonons in a GaSe crystal,” Appl. Phys. Lett. 86, 182104 (2005).

Dye sensitized nanocrystalline TiO2 solar cells have been reported with over 11% efficiency and are extremely promising as very low cost and lightweight photovoltaic sources. However, most reports are for cells of low area fabricated on glass, which withstands processing temperatures of ∼450°C. In this paper, we describe the fabrication and performance of cells made on flexible ITO-coated polyethylene terephthalate (PET) substrates with 6” × 3” dimensions. To improve the efficiency in the cells, we enhanced the ITO current collection efficiency with metallization fingers. The fingers resulted in a >10 fold increase in short-circuit current under normal solar illumination compared to cells without metallization. Further improvements were realized by passivating the metallization fingers at the metal/polymer electrolyte interface.

High quality polycrystalline AgGaTe2 (AGT) thin films were deposited on H-terminated n-Si substrates by controlled thermal evaporation method at various substrate temperatures (300–500 K). X-ray diffraction (XRD) studies showed that all films were of chalcopyrite structure and while the films were deposited at 300 K had random grain orientation, the films deposited at higher substrate temperature (500 K) showed preferred (112) orientation. The composition of the films was thoroughly analyzed by energy dispersive x-ray analysis (EDAX) and by x-ray photoelectron spectroscopy (XPS) with and without argon ion etching. The ultraviolet-visible (UV-Vis) spectra showed the optical bandgap of 1.16 eV, with sharper band edge for the films deposited at higher temperature. The films were p-type and the resistivities of the as deposited at 300 and 500 K were 2.8 × 104 and 1.2 × 103 Ω. cm respectively. p-AgGaTe2/n-Si heterojunction solar cells, having an active area of 0.12 cm2 and without any antireflection coating, were fabricated. It was observed that the films deposited at 500 K produced junctions with improved photovoltaic properties. Under solar simulator AM1 illumination, the improved junctions exhibited an efficiency of 4.8% whereas the films deposited at 300 K showed an efficiency of 2.1%.

This paper describes our recent research on the development of fully flexible and low-cost dye-sensitized nanocrystalline TiO2 solar cells (DSSC). At EIC Laboratories, we have developed various large-area solid-state versions, which include both ionically, and hole-conducting allpolymer DSSCs made by continuous coating and lamination process. Various critical steps involve ∼10 micron thick nanocrystalline TiO2 film deposition on conducting PET plastics by various methods such as spraying, spin coating, and screen printing, a low temperature sintering (∼150°C) process, and polymer laminated electrolyte have been demonstrated as valuable active components. The photovoltaic (PV) performance presented in this paper show the fabricated solar cells of ∼1.2 cm2 area with reproducible AM1.5 (Air Mass 1.5) efficiencies of 4.7%. A prototype solar cell with a new hole conducting star polymer with AM 1.5 efficiency ∼2.12% has been thoroughly characterized and discussed.