[Show abstract][Hide abstract] ABSTRACT: Temperature-dependent current-voltage measurements are used to determine the dominant recombination mechanism in thin-film heterojunction solar cells based on Cu(In,Ga)(S,Se)2 absorbers with chemical bath deposited ZnS buffer layer. The measurements are carried out in the dark and under illumination in the temperature range 200-320 K. The activation energy of the recombination under illumination follows the absorber band gap energy Eg=1.07eV of bulk Cu(In,Ga)(S,Se)2. The thermal dependence of the diode ideality factor is described by classical Shockley-Read-Hall (SRH) recombination via an exponential distribution of trap states in the bulk of the absorber. In the dark, the current flow is dominated by tunnelling enhanced bulk recombination with a tunnelling energy E00=18meV. Two activation energies higher than Eg, namely 1.21 and 1.40eV, have been found. These results may be explained by dominant recombination in a region close to the surface of the Cu(In,Ga)(S,Se)2 absorber with an enlarged band gap. Thus, the electronic loss in the ZnO/Zn(S,OH)/Cu(In,Ga)(S,Se)2 solar cell takes place mainly in the absorber and is determined by tunnelling enhanced bulk recombination with a tunnelling energy E00 influenced by illumination.
Full-text · Article · Sep 2003 · Journal of Physics and Chemistry of Solids
[Show abstract][Hide abstract] ABSTRACT: Chemical bath deposited Zn-compound buffer layers have been applied as an alternative to the CdS buffer layer in the development of Cu(In,Ga)(S,Se)2 (CIGSSe) thin film solar cells. We used CIGSSe absorbers developed by Shell Solar for large-scale production. When ZnO is sputtered directly on such absorbers, very poor performances are obtained. In contrast, when the CIGSSe films are treated in electrolyte containing Zn-ions before sputtering, device efficiency of 12% is achieved. Including a sulfur or selenium source, we have developed a process to fabricate Cd-free CIGSSe devices with over 14% efficiency, certified at NREL. The structure and composition of the CBD-ZnSe on CIGSSe surface were investigated. The growth mechanism of chemical bath deposited ZnSe and ZnS on CIGSSe are discussed.
[Show abstract][Hide abstract] ABSTRACT: Structural and compositional properties of Zn(Se,OH)/Zn(OH)2 buffer layers deposited by chemical bath deposition(CBD) on Cu(In,Ga)(S,Se)2 (CIGSS) absorbers are investigated. Due to the aqueous nature of the CBD process, oxygen and hydrogen were incorporated into the ‘ZnSe’ buffer layer mainly in the form of Zn(OH)2 as is shown by X-ray photoelectron spectroscopy and nuclear reaction analysis (NRA) measurements leading to the nomenclature ‘Zn(Se,OH)’. Prior to the deposition of Zn(Se,OH), a zinc treatment of the absorber was performed. During that treatment a layer mainly consisting of Zn(OH)2 grew to a thickness of several nanometer. The whole buffer layer therefore consists of a Zn(Se,OH)/Zn(OH)2 structure on CIGSS. Part of the Zn(OH)2 in both layers (i.e. the Zn(Se,OH) and the Zn(OH)2 layer) might be converted into ZnO during measurements or storage. Scanning electron microscopy pictures showed that a complete coverage of the absorber with the buffer layer was achieved. Transmission electron microscopy revealed the different regions of the buffer layer: An amorphous area (possibly Zn(OH)2) and a partly nanocrystalline area, where lattice planes of ZnSe could be identified. Solar cell efficiencies of ZnO/Zn(Se,OH)/Zn(OH)2/CIGSS devices exceed 14% (total area).
No preview · Article · Jan 2003 · Solar Energy Materials and Solar Cells
[Show abstract][Hide abstract] ABSTRACT: A Zn(Se,OH) buffer layer deposited by chemical bath process on
Cu(In,Ga)(S,Se)<sub>2</sub> (CIGSS) absorbers is investigated. Before
deposition of the actual ZnSe layer, the absorber is immersed in a
Zn-containing solution (Zn treatment). Transmission electron microscopy
(TEM) pictures of the buffer layer exhibit two different areas: a dark
structured layer containing small crystals of ZnSe and a bright
amorphous layer, which is thought to consist of Zn(OH)<sub>2</sub>
predominantly. We suggest that already during the Zn treatment an
amorphous buffer layer is growing. X-ray photoelectron spectroscopy
(XPS) measurements of the Cu<sub>2p</sub> and O<sub>1s</sub> peak of
Zn-treated absorbers show a coverage with an OH component, attributed to
Zn(OH)<sub>2</sub>. Nuclear reaction analysis (NRA) measurements reveal
a peak hydrogen concentration of 8×10<sup>21</sup>
H/cm<sup>3</sup> in the buffer layer. Micro-Raman spectra show a shift
of the LO branch of ZnSe from 250 cm<sup>-1</sup> to 258 cm<sup>-1</sup>