T. Raadik

The role of structural properties on deep defect states in Cu2ZnSnS4 studied by photoluminescence spectroscopy

In this study, we investigated the photoluminescence (PL) properties of Cu2ZnSnS4 polycrystals. Two PL bands at 1.27 eV and 1.35 eV at T = 10 K were detected. Similar behaviour with temperature and excitation power was found for both PL bands and attributed to the band-to-impurity recombination. Interestingly, the thermal activation energies determined from the temperature dependence of the PL bands coincide. With the support of the Raman results, we propose that the observed PL bands arise from the band-to-impurity-recombination process involving the same deep acceptor defect with ionization energy of around 280 meV but different Cu2ZnSnS4 phase with different bandgap energy.

Photoluminescence of spray pyrolysis deposited ZnO nanorods

Photoluminescence of highly structured ZnO layers comprising well-shaped hexagonal rods is presented. The ZnO rods (length 500-1,000 nm, diameter 100-300 nm) were grown in air onto a preheated soda-lime glass (SGL) or ITO/SGL substrate by low-cost chemical spray pyrolysis method using zinc chloride precursor solutions and growth temperatures in the range of 450-550°C. We report the effect of the variation in deposition parameters (substrate type, growth temperature, spray rate, solvent type) on the photoluminescence properties of the spray-deposited ZnO nanorods. A dominant near band edge (NBE) emission is observed at 300 K and at 10 K. High-resolution photoluminescence measurements at 10 K reveal fine structure of the NBE band with the dominant peaks related to the bound exciton transitions. It is found that all studied technological parameters affect the excitonic photoluminescence in ZnO nanorods.PACS: 78.55.Et, 81.15.Rs, 61.46.Km

Temperature dependent photoreflectance study of Cu2SnS3 thin films produced by pulsed laser deposition

The energy band structure of Cu2SnS3 (CTS) thin films fabricated by pulsed laser deposition was studied by photoreflectance spectroscopy (PR). The temperature-dependent PR spectra were measured in the range of T = 10–150 K. According to the Raman scattering analysis, the monoclinic crystal structure (C1c1) prevails in the studied CTS thin film; however, a weak contribution from cubic CTS (F-43m) was also detected. The PR spectra revealed the valence band splitting of CTS. Optical transitions at EA = 0.92 eV, EB = 1.04 eV, and EC = 1.08 eV were found for monoclinic CTS at low-temperature (T = 10 K). Additional optical transition was detected at EAC = 0.94 eV, and it was attributed to the low-temperature band gap of cubic CTS. All the identified optical transition energies showed a blueshift with increasing temperature, and the temperature coefficient dE/dT was about 0.1 meV/K.

Optical study of local strain related disordering in CVD-grown MoSe2 monolayers

We present temperature dependent micro-photoluminescence and room temperature photoreflectance spectroscopy studies on aged MoSe2 monolayers with high surface roughness. A0 and B0 exciton bands were detected at 1.512 eV and 1.72 eV, respectively, which are 50–70 meV lower than those commonly reported for high-quality samples. It is shown that the difference can be accounted for using a model of localized excitons for disordered MoSe2 monolayers where the optical band gap energy fluctuations could be caused by random distribution of local tensile strain due to surface roughness. The density of localized exciton states is found to follow the Lorentzian shape, where the peak of this distribution is about 70 meV from the energy of delocalized states.

The performance of thin film solar cells employing photovoltaic ZnSe/CdTe, CdS/CdTe and ZnTe/CdTe heterojunctions

This paper focuses on the photovoltaic parameters of ZnSe/CdTe, CdS/CdTe, and ZnTe/CdTe thin film heterojunction solar cells. ZnSe/CdTe, CdS/CdTe, and ZnTe/CdTe thin film heterojunction solar cells were fabricated by Close Space Sublimation (CSS) on TCO-coated glass substrates. All types of solar cells were fabricated in a superstrate configuration. The thickness of ZnSe and ZnTe layers was varied in order to adjust the solar cell performance. A similar cadmium chloride solution for the treatment of a CdTe layer with an elevated temperature air annealing of the completed devices before the back contact deposition was applied to ZnSe/CdTe and CdS/CdTe thin film heterojunctions solar cells with exception of ZnTe/CdTe. All cells were characterized through light and dark current density-voltage (J-V) measurements and quantum efficiency (QE) measurements. The saturation current, ideality factor and photovoltaic parameters for all thin film heterojunction solar cells are presented. The investigation at the room temperature under illumination of 100 mW/cm2 through the wide gap components of ZnSe/CdTe, CdS/CdTe, and ZnTe/CdTe heterojunctions showed a value of conversion efficiency (η) of solar energy to electric energy about 4.7%, 9.9%, and 1.3%, respectively. The incorporation of Zn at the ZnSe and CdTe interface doubles the short circuit current density and improves the performance of ZnSe/CdTe thin film heterojunction solar cells.

Chlorine Doping of Cadmium Sulfide on the Example of CBD CdS

The role of thermal annealing and of CdCl 2 as a main source of electrically active but vaporizable chlorine doping in chemical bath deposited CdS thin films is studied. The films were deposited on glass substrates from aqueous solution of either CdCl 2, NH 4Cl, NH 4OH, and thiourea, or CdSO 4, (NH 4)2SO 4, NH 4OH, and thiourea. Films deposited in the presence of CdCl 2 and annealed in H 2 atmosphere at 310 and 420 °C show a resistivity lower than 10 � ·cm, one order of magnitude less than for identically annealed films deposited in absence of CdCl 2. Annealing at 420 °C in closed ampoules, where a counter pressure of CdCl 2 builds up, leads to a lower resistivity on the order of 10 -1 � ·cm, confirming the key role of chlorine on the electronic properties. However, further characterization via photoluminescence raises new questions about chlorine-related defects and their role in the mechanisms that govern film resistivity.

Investigation of Morphology Changes on Nanocrystalline Diamond Film Surfaces during Reciprocating Sliding against Si3N4 Balls

This paper investigates the morphological modifications of the nanocrystalline diamond (NCD) film surface under reciprocating sliding test conditions. The surface morphology was characterized by atomic force microscopy (AFM). We observed longitudinal grooves and transverse ripples which were formed during the sliding tests on the NCD film surface. The primary goal of the study was to understand the influence of frequency, sliding distance and load variations on the formation of ripple patterns on the wear scars surface. The morphological alteration from continuous to broken ripple shapes was observed. Our study suggests that the geometrical shape of ripples is affected by the formation of the periodic array of grooves.

Wear Rate of Nanocrystalline Diamond Coating under High Temperature Sliding Conditions

The present study deals with the tribological behavior of nanocrystalline diamond (NCD) coatings at high temperature sliding conditions. The NCD coatings were grown by plasma enhanced chemical vapor deposition (PECVD) method on the hard metal (WC-Co) substrates. The friction and wear tests were performed on ball-on-disc tribometer using a high-temperature chamber with rotary drive. The tests were carried out at room temperature, 300, 450 and 600 °C. The scanning electron microscopy (SEM), optical microscopy, mechanical profilometry and Raman spectrometry were used for investigation of the morphology and chemical composition of the wear scars and pristine surface. The depth and width of the wear scars measured after the high temperature sliding tests are larger in comparison with room temperature tests. It was observed that the coefficient of friction (COF) increased with increasing temperature. The wear rate of NCD coatings tested at 300-450° C was about 10 times higher than that at room temperature. The mechanisms involved for these variations are discussed.

PVD of N-CuIn3Se5 Photoabsorber Films

Thin films of Cu-In-Se (CISe) photoabsorber with overall composition of CuIn3Se5 were deposited onto glass/ITO substrates by using physical vapour deposition (PVD) technique. Thermal conditions for the substrates during deposition process and following thermal annealing were selected with the purpose to prepare polycrystalline n-CuIn3Se5 photoabsorber layers for the hybrid photovoltaic structures based on inorganic photoabsorber and conductive polymer functional layers. It was found, that the CISe layers deposited at the temperature of substrate of 200 °C and annealed at the temperature range of 450-500 oC in vacuum and double annealed in argon and vacuum at 500 oC demonstrate high photosensitivity and photoconductivity under white light illumination of 100 mW/cm2 intensity. Obtained results show the chalcopyrite structure of prepared photoabsorber films with good adhesion to the glass/ITO substrate.

Growth of Cu-Rich/Poor CuInS 2 thin films by the sequential modulated flux deposition technique

CuInS 2 has emerged during recent years as a good candidate to substitute CuInSe 2 as polycrystalline absorber in thin film solar cells, mainly due to its direct band gap energy of 1.5 eV. In this study, absorber layers of both Cu-rich and Cu-poor types have been grown on soda-lime glass substrates by proper selection of the deposition parameters. The morphology and the optical properties of the resulting CuInS 2 films were studied in dependence of the deposition order of the elemental constituents: alternate evaporation of the precursors, simultaneous deposition of the three constituents and sequential modulation of the evaporation fluxes.