L. Stolt

ZnO/CdS/CuInSe2 thin‐film solar cells with improved performance

An important milestone in the development of photovoltaic thin‐film solar cells is the achievement of 15% conversion efficiency. This letter describes the highest efficiency single junction thin‐film cell reported to date. An active area efficiency of 14.8% is obtained with the cell structure n‐ZnO/n‐CdS/p‐CuInSe2 deposited on a soda‐lime glass substrate. The current achievements are due to improved properties of the CuInSe2 layer and the heterojunctions compared to previously reported results. The rate and substrate temperature profiles used during the coevaporation process yield a relatively large‐grained material with very strong 〈112〉 orientation and low porosity. This results in reduced recombination rates, hence higher open circuit voltage and fill factor.

Structure and chemistry of CuInSe2 for solar cell technology: current understanding and recommendations

The results of a workshop on the structure and chemistry of CuInSe2 (CIS), held in Jackson Hole, WY (USA), are presented. The participants are the authors of this paper. Issues in CIS properties and device performance were divided into those dominating common CIS and those relevant to the best CIS. Common CIS in non-equilibrium material containing numerous lattice defects and second phases surrounding the grains or segregated to the surface. Common CIS is capable of producing solar cells yielding in excess of 10% conversion efficiency provided that there is no CuxSe present. The best physical vapor deposited (PVD) CIS is formed at high temperatures, where rapid segregation should minimize the incorporation of dissolved point defects. This CIS has relatively large, strongly (112)-oriented, nearly perfect grains. Growth of the majority of the film under strongly Cu-rich conditions provides the best results, although the final film must be free of CuxSe phases. The differences between common CIS and the best CIS are relatively process-independent (but process-variable-dependent) and tend to limit the open circuit voltage and fill factor of devices. The workshop consensus recommendations were as follows. The selenization reaction must be studied to determine how desirable properties, reproducibly demonstrated in PVD CIS, can be reliably achieved. Controlled experiments should be carried out to determine the effect of extrinsic doping and grain boundaries on minority carrier lifetimes. Finally, a mechanism is needed for analysis of films to show how materials produced by different methods compare.

Conductance noise and percolation in YBa2Cu3O7 thin films

Abstract Experimental results of the conductivity noise in the superconducting transition region of YBCO thin films prepared by co-evaporation are presented. In the case of ex situ fabricated samples, Cooper-pair number fluctuations (induced by electron trapping) have been identified in the high-temperature part of the transition. Classical percolation noise was found in the low-temperature part of the transition. In the case of samples made by the in situ method, the noise is smaller by several orders of magnitude in the upper part of the transition. This indicates a much more ordered microstructure in these samples. Electron mobility fluctuations, shunted by the conductance of Cooper-pairs, were identified in this temperature range. In the low-temperature part of the transition, a new type of fluctuation has been discovered; the fluctuation of the volume fraction of the superconducting phase. This implies new scaling exponents very different from the exponents of classical percolation models. This effect is a consequence of intergrain critical current fluctuations and can be caused by, for instance, magnetic flux motion, defect motion or trapping of electrons in the barriers between grains. Moreover, in the in situ fabricated samples, a dimensional crossover, 3D→2D, of the percolating network has been observed. From this effect, the length scale of the microscopic disorder can be estimated.

Spontaneous conductivity fluctuations in YBaCuO thin films: Scaling of fluctuations, experimental evidence of percolation at the superconducting transition

Abstract Spontaneous conductivity fluctuations in high- T c superconducting films on sapphire and SrTiO 3 substrates have been measured through the superconducting transition. It is found that the normalized spectrum for the sapphire substrate film is two orders of magnitude larger than for the SrTiO 3 substrate film, due to a stronger disorder. At sufficiently low temperatures, in the superconducting transition region, the normalized spectrum scales with the conductivity. This is experimental evidence of a temperature controlled percolating superconducting network in high- T c superconductors.

Coevaporated thin films of Y-Ba-Cu-O utilizing quadrupole mass-spectrometer rate control

Abstract A study of coevaporation of Y, BaF 2 and Cu using a quadrupole mass-spectrometer as an evaporation rate control has been performed. Actual rate values have been determined by RBS analysis and compared with mass-spectrometer settings for different source combinations and partial pressures of oxygen during deposition. It is shown that Y-Ba-Cu films of precise composition can be codeposited by using a mass-spectrometer controlled coevaporation process. The method has been utilized for a deposition on seven different substrates (LiNbO 3 , SrTiO 3 , LaAlO 3 , CaNdAlO 4 , LaGaO 3 , MgO and Al 2 O 3 ) simultaneously. After a post-heat treatment in flowing wet oxygen the films have been analysed regarding resistivity and critical current density.

YBaCuO thin films on sapphire substrates

Abstract Thin films of YBaCuO have been fabricated by co-evaporation of Y, Cu, and BaF 2 -powder in an oxygen atmosphere. The effect of various post-deposition annealing procedures are reported. Results from resistivity and critical current measurements as well as SIMS and Auger analysis are presented.

Studies of YBa2Cu3O7−x formation in coevaporated YBaF2CuO thin films on sapphire substrates

Abstract Degradation mechanisms when films of YBa2Cu3O7−x (YBCO) were formed on sapphire substrates have been studied. The films were synthesized by post-deposition annealings in a wet oxygen atmosphere of coevaporated layers of YBaF2CuO. X-ray diffraction revealed the existence of impurity phases in the annealed samples, the most significant being BaAl2O4, Y2BaCuO5, CuO and Y2O3. Redistribution of the constituents as a function of time was examined by secondary ion mass spectrometry and Rutherford backscattering spectrometry, giving evidence of a Ba redistribution against the interfacial region and an agglomeration of Cu within the film. While zero-resistivity temperatures of 1 μm thick films typically were between 80 and 88 K, magnetic characterization indicated weak coupling between the superconducting grains, resulting in low transport critical currents ⩽ 103 A cm−2 at 77 K. The film properties of YBCO/sapphire were compared with those of YBCO processed in parallel on SrTiO3 or LaAlO3.

Magnetic measurements on coevaporated YBa2Cu3O7 thin films

Abstract The field an time dependence of the magnetic susceptibility of coevaporated YBa 2 Cu 3 O 7 thin films on SrTiO 3 and sapphire substrates have been investigated. Different relaxation and field dependent behaviour for the two films is reported.

Y-Ba-Cu-O thin films deposited by mass-spectrometer controlled coevaporation

This paper describes a coevaporation system where the stoichiometry of Y-Ba-Cu-layers is controlled by utilizing a quadrupole mass-spectrometer rate control system. Results from analysis of superconducting thin film YBa 2 Cu 3 O 7−x are also reported.