Marian Nanu

CuInS2–TiO2 heterojunctions solar cells obtained by atomic layer deposition

Abstract Chalcopyrites are being studied widely as a promising absorber material for high-efficiency, low-cost, thin-film solar cells. The present paper deals with the growth of CuInS 2 thin films by atomic layer deposition. CuInS 2 films are grown on glass, F-doped SnO 2 coated glass, and TiO 2 thin films at a pressure of 2 mbar and in the temperature range of 350–500 °C using CuCl, InCl 3 and H 2 S as precursors. The influence of the process conditions on the structural and the electrical properties is examined. The growth temperature, the purge time and the vapor pressure of the precursors are found to be the decisive parameters. The composition of the thin films is investigated with X-ray diffraction and Raman spectroscopy. Depending on the process conditions single phase CuInS 2 or a mix of Cu x S, CuInS 2 and CuIn 5 S 8 are formed. The effect of annealing the CuInS 2 films in an H 2 S or O 2 atmosphere is studied as well.

A time-resolved microwave conductivity study of the optoelectronic processes in TiO2?In2S3?CuInS2 heterojunctions

Photoinduced interfacial charge carrier generation, separation, trapping, and recombination in TiO2?In2S3?CuInS2 cells have been studied with time-resolved microwave conductivity (TRMC). Single layer, double layer, and complete triple layer configurations have been studied. Selective electronic excitation in one of the components is accomplished by using monochromatic pulsed laser excitation. In bare CuInS2 films and in TiO2?CuInS2 double layers, photoinduced charge carriers recombine on a subnanosecond time scale. This fast recombination slows down significantly when an In2S3 buffer layer is applied between TiO2 and CuInS2. In that case, the charge separation lifetime increases by more than one order of magnitude. A superlinear dependence of the TRMC signals on the incident laser intensity is observed for the triple layer configuration, which indicates saturation of electron traps in In2S3 or hole traps in CuInS2. Furthermore, TRMC signals from TiO2?In2S3?CuInS2 triple junctions and those from In2S3?CuInS2 double layers are identical, which shows that charge carrier separation exclusively takes place at the In2S3?CuInS2 interface.

Deep-level transient spectroscopy of TiO2∕CuInS2 heterojunctions

Deep-level transient spectroscopy (DLTS) has been used to measure the concentration and energy position of deep electronic states in CuInS2. Flat TiO2?CuInS2 heterojunctions as well as TiO2-CuInS2 nanocomposites have been investigated. Subband-gap electronic states in CuInS2 films are mostly due to antisite point defects and vacancies. Substitution of indium with copper, CuInII, leads to an acceptor state 0.15 eV above the valence band, while copper vacancies, VCuI, are acceptor states at 0.1 eV. Furthermore, indium on a copper position, InCu?, yields a donor state at 0.07 eV below the conduction band, while sulphur vacancies are donor states at 0.0 = eV. With DLTS, these states are indeed found. For flat configurations, VCuI are the dominant acceptors with a concentration of 1.83×1017?cm?3. In contrast for nanocomposites CuInII are the dominant acceptors having a concentration of 6.7×1017?cm?3. We conclude that the concentration of antisite defects in nanocomposite CuInS2 is significantly higher than that in flat films of CuInS2.

NANO-STRUCTURED MATERIALS FOR THE CONVERSION AND STORAGE OF SUSTAINABLE ENERGY

The paper is an overview of several recent studies on nano-structured materials, with emphasis on novel solar cells, Photo-Electrochemical Cells (PECs), and on hydrogen storage, for the conversion and storage of solar energy. Nano-structured metal oxide catalysts improve the kinetics of hydrogen sorption in metal hydrides. The presently-achieved conversion efficiency of 5% shows that this type of solar cell is a promising concept. It is anticipated that the 3D solar cells will reach efficiencies of over 8% in the coming years, and will start to replace silicon-based solar cells.