T. Varema

Structural and optical properties of sprayed CuInS2 films

Abstract Polycrystalline CuInS 2 thin films have been prepared by spray pyrolysis of aqueous solution of copper chloride, indium chloride and thiourea onto heated glass substrates. It is shown that the parameters critical to structural and optical properties of sprayed CuInS 2 films are growth temperature and ion ratio of Cu/In in spraying solution. Excess of copper in starting solution promotes the recrystallization and growth of crystallites in the film. The X-ray diffraction patterns confirm that the use of copper-rich solutions reduces the temperature required for single-phase composition of CuInS 2 films from 380°C ( Cu/In =1 ) to 290°C ( Cu/In =1.25 ). The formation of CuInS 2 in spray process and chemical nature of additional phases in the films are discussed. Growth temperatures in the range of 320–380°C at Cu/In >1 are determined as optimal for depositing orientated in the (112) direction CuInS 2 thin films with chalcopyrite structure. Sprayed chalcopyrite CuInS 2 films have absorption coefficient ∼10 5 cm −1 in visible and red region of spectra and optical band gap 1.45 eV.

Composition and structure of CuInS2 films prepared by spray pyrolysis

Abstract CuInS2 thin films have been prepared by spray pyrolysis of an aqueous solution of CuCl2, InCl3 and thiourea onto heated glass substrates. The effect of the composition of starting solutions on the phase and chemical composition and the structure of sprayed films is studied. The Cu/In ratio in the films is higher than that in the solution. In-rich and S-rich solutions lead to the multiphase films with poor crystallinity according to X-ray diffraction. It was determined that organic phases as products of the thermal decomposition of thiourea ligand are present in the films at all used solution compositions. The formed organic secondary phases could be responsible for the formation of molten phase which together with copper sulphide act as recrystallisation agent during the growth of copper-rich films. The CuInS2 films, strongly orientated in the (112) direction with crystallites up to 300 nm in the matrix and with large crystals of some microns in agglomerated areas, are formed while the secondary phases segregated on the thin film surface.

Monograin layer solar cells

The paper presents results of studies directed towards the production of monograin powders of CuInSe for possible use in 2 solar cells preparation.The results concern the tailoring of chemical and defect composition of materials, development of the technology of manufacturing monograin layers (MGL) on the base of developed materials and the cleaning of open surfaces of the grains in the MGL by different mechanical, chemical and electrochemical methods.It is shown that up to now the low efficiency of MGL solar cells is associated with the chemical and defect composition of the monograin powder materials and with difficulties in cleaning the surfaces of the crystals in the MGL before depositing active contacts. 2003 Elsevier Science B.V. All rights reserved.

Cu2ZnSnSe4 Monograin Powders for Solar Cell Application

Cu₂ZnSnSe₄ monograin powders with different Zn/Sn concentration ratios were synthesized from binaries and elemental selenium, p-type CCu₂ZnSnSe₄ monograins of stannite structure had tetragonal shape with rounded edges. PL spectra showed one symmetrical band with peak position at 0.81 eV. Monograin layer solar cell structures graphite/Cu₂ZnSnSe₄/CdS/ZnO had open circuit voltage over 400 mV, short circuit current 15.5 mA/cm² and FF41 %

CZTS Monograin Powders and Thin Films

This paper reviews results of studies on different materials and technologies for polycrystalline solar cells conducted at Tallinn University of Technology. Structural properties and defect structure of kesterite CZTS compounds (Cu2ZnSnSe4, Cu2ZnSn(SSe)4) were studied. Influence of selenization parameters of a Zn-Cu-Sn stacked layer on the CZTS layer growth and on the morphology, distribution of elements was analyzed. All the results obtained have been used to optimize the technology of producing solar cell structures in different designs. Cu2ZnSnSe4 and, Cu2ZnSn(SSe)4 based monograin layer solar cells were developed.

Monograin layers and membranes for photovoltaics

The possibilities of producing monograin powders of different A2B6 and chalcopyrite materials; different technologies of manufacturing monograin layers and cleaning of open surfaces of grains in the monograin layer by mechanical, chemical and electrochemical methods were studied. It was shown that up to now too low efficiency of solar cells by the monograin layer is associated with the insufficient cleaning of surfaces of crystals in the monograin layer before depositing the active contacts.

Monograin powders and layers for photovoltaic application

The recrystallization of CdTe powders in different fluxes was studied and the possibility to grow p- and n-type CdTe monograin powders of high conductivity was shown. It was determined that very important technological factors for creating p-type conductivity in CdTe are the cooling rate and the presence of Te in flux. NaCl content in CdCl{sub 2} flux was found to inhibit the crystal growth and to allow for doping CdTe with Na that acts as an effective acceptor impurity in CdTe. The region of most effective added Na concentrations was found.

ZnO grown by chemical solution deposition

Zinc oxide (ZnO) films were grown on different substrates by the chemical solution deposition method from an ammonium zincate complex solution. The structure, morphology, composition, and optical properties of the films were studied by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and UV-VIS spectroscopy. Results reveal that the deposited layers consist of ZnO nanorods with strong c-axis orientation. The observed Raman spectra are in excellent agreement with those reported in literature. The E2(high) mode at 437 cm−1, having the strongest intensity of all samples, is characteristic of the ZnO hexagonal wurzite structure. The diameter of the rods is about 90 nm and the thickness of the ZnO layer is between 140 nm to 2,3 µm. Different deposition parameters such as the concentration of the zincate complex and the temperature of the bath, the deposition time and dipping process effects are discussed.

Growth of CuInSe 2 monograin powders with different compositions

CuInSe 2 monograin powders (MGP) were synthesized from Cu-In alloys of different Cu/In concentration ratios and elemental Se in liquid phase of flux material in evacuated quartz ampoules. The surface morphology, phase structure, and composition of the powder crystals were analyzed by scanning electron microscopy, X-ray diffraction, and energy-dispersive X-ray analysis respectively. Bulk composition was analyzed polarographically. Photoluminescence spectra were measured at 9 K. It was found that the composition of MGP material (Cu/In concentration ratio) can be controlled by the concentration ratio of precursor Cu-In alloys. Single phase CuInSe 2 growth is realisable between 0.7 2 . Samples with high In content exhibited two broad bands with peak positions at 0.86 and 0.93 eV.

Formation and properties of chemically sprayed ZnO films

ZnO thin films with electrical resistance as low as 10-3 (Omega) (DOT)cm and transparency of 90 - 95% have been prepared by spray pyrolysis of zinc acetate solutions. However, a careful optimization of all deposition conditions is needed in order to obtain films with these values. Thermal decomposition of precursor, the influence of preparation conditions on the film properties and on the film formation mechanism are studied. The stepwise thermal destruction of Zn(CH3CO2)2(DOT)2H2O is recorded and it is shown, that single solid phase ZnO is the only decomposition product at temperatures higher than 590 K in air. Three characteristic temperature regions were distinguished in the film growth process exhibiting each a different mechanism of crystal growth and leading to different thin film qualities. The optical, structural, morphological and electrical properties of sprayed films are mainly determined by growth temperature and are in dependence on the concentration of dopants. This can be explained on the basis of changes in the growth mechanisms and chemical purity of the films.