Martha Ch. Lux-Steiner

Determination of the band gap depth profile of the penternary Cu(In(1-X)GaX)(SYSe(1-Y))2 chalcopyrite from its composition gradient

A simple model is introduced which determines the optical band-gap energy Eg for penternary Cu(In(1−X)GaX)(SYSe(1−Y))2 (CIGSSe) alloys from its Ga∕(Ga+In) ratio as well as from its S∕(S+Se) ratio. In order to verify the model the depth dependent composition of a CIGSSe sample was revealed by elastic recoil detection analysis. Applying the model, the concentration profiles were transferred in an Eg profile. Finally, these values were compared with optical band-gap energies, which were obtained directly by independent characterization methods.

Effect of internal surface area on the performance of ZnO∕In2S3∕CuSCN solar cells with extremely thin absorber

Solar cells with an extremely thin light absorber were realized by wet chemical preparation on arrays of ZnO nanorods. The absorber consisted of an In2S3 layer (∼20nm thickness) and its interface region with a transparent CuSCN hole conductor. By changing the length of the nanorods (0–3.3μm) and keeping the In2S3 layer thickness constant at ∼20nm, the short circuit current increased from about 2–10mA∕cm2. A marked increase of the external quantum efficiency at longer wavelengths is attributed to light scattering and a solar energy conversion efficiency of 2.5% has been demonstrated.

CdS and Cd(OH)2 formation during Cd treatments of Cu(In,Ga)(S,Se)2 thin-film solar cell absorbers

The surface modifications induced by treating Cu(In,Ga)(S,Se)2 films in an aqueous ammonia hydroxide-based solution of Cd2+ ions—as used in record Cu(In,Ga)(S,Se)2 solar cells without a CdS buffer layer—have been investigated for different Cd2+ concentrations. Employing a combination of x-ray photoelectron spectroscopy, Auger electron spectroscopy, and x-ray emission spectroscopy, it is possible to distinguish two different surface modifications. For Cd2+ concentrations below 4.5 mM in the solution we observe the formation of a CdS monolayer, while higher Cd2+ concentrations lead to the additional deposition of a cadmium hydroxide film on the CdS/Cu(In,Ga)(S,Se)2 surface.

A novel deposition technique for compound semiconductors on highly porous substrates: ILGAR

Abstract ILGAR (ion layer gas reaction), a novel low-cost technology for the preparation of sulfidic thin layers is described, which can be analogously applied for other chalcogenides. The process consists of three steps: (1) application of a precursor solution on a substrate by dipping or spraying, (2) drying in an inert gas stream, (3) sulfurization of the solid precursor (e.g. a metal halide) by hydrogen sulfide gas. This cycle is repeated until the desired layer thickness is obtained. Not only on smooth, but also on structured and porous substrates the method allows the deposition of homogenous thin films following the microscopic structure, where other methods often have problems with shading. Once the film is closed, the growth per dip cycle is constant and reproducible during the process. The binary compounds CdS, Cu 2 S, In 2 S 3 and also the ternary CuInS 2 have been prepared by ILGAR on glass and on porous TiO 2 or SiO 2 . The layers were characterised by XRD, SEM and EDX.

Highly efficient Cu(Ga,In)(S,Se)2 thin film solar cells with zinc-compound buffer layers

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.

Formation of a ZnS∕Zn(S,O) bilayer buffer on CuInS2 thin film solar cell absorbers by chemical bath deposition

The application of Zn compounds as buffer layers was recently extended to wide-gap CuInS2 (CIS) based thin film solar cells. Using an alternative chemical deposition route for the buffer preparation aiming at the deposition of a single-layer, nominal ZnS buffer without the need for any toxic reactants such as hydrazine has helped us to achieve a similar efficiency as respective CdS-buffered reference devices. In order to shed light on the differences of other Zn-compound buffers deposited in conventional chemical baths [chemical bath deposition (CBD)] compared to the buffer layers deposited by this alternative CBD process, the composition of the deposited buffers was investigated by x-ray excited Auger electron and x-ray photoelectron spectroscopy to potentially clarify their superiority in terms of device performance. We have found that in the early stages of this alternative CBD process a thin ZnS layer is formed on the CIS, whereas in the second half of the CBD the growth rate is greatly increased and Zn(S,O) with a ZnS∕(ZnS+ZnO) ratio of ∼80% is deposited. Thus, a ZnS∕Zn(S,O) bilayer buffer is deposited on the CIS thin film solar cell absorbers by the alternative chemical deposition route used in this investigation. No major changes of these findings after a postannealing of the buffer/CIS sample series and recharacterization could be identified.The application of Zn compounds as buffer layers was recently extended to wide-gap CuInS2 (CIS) based thin film solar cells. Using an alternative chemical deposition route for the buffer preparation aiming at the deposition of a single-layer, nominal ZnS buffer without the need for any toxic reactants such as hydrazine has helped us to achieve a similar efficiency as respective CdS-buffered reference devices. In order to shed light on the differences of other Zn-compound buffers deposited in conventional chemical baths [chemical bath deposition (CBD)] compared to the buffer layers deposited by this alternative CBD process, the composition of the deposited buffers was investigated by x-ray excited Auger electron and x-ray photoelectron spectroscopy to potentially clarify their superiority in terms of device performance. We have found that in the early stages of this alternative CBD process a thin ZnS layer is formed on the CIS, whereas in the second half of the CBD the growth rate is greatly increased and ...

XPS, TEM and NRA investigations of Zn(Se, OH)/Zn(OH)2 films on Cu(In, Ga)(S, Se)2 substrates for highly efficient solar cells

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).

Experimental indication for band gap widening of chalcopyrite solar cell absorbers after potassium fluoride treatment

The implementation of potassium fluoride treatments as a doping and surface modification procedure in chalcopyrite absorber preparation has recently gained much interest since it led to new record efficiencies for this kind of solar cells. In the present work, Cu(In,Ga)Se2 absorbers have been evaporated on alkali containing Mo/soda-lime glass substrates. We report on compositional and electronic changes of the Cu(In,Ga)Se2 absorber surface as a result of a post deposition treatment with KF (KF PDT). In particular, by comparing standard X-ray photoelectron spectroscopy and synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES), we are able to confirm a strong Cu depletion in the absorbers after the KF PDT which is limited to the very near surface region. As a result of the Cu depletion, we find a change of the valence band structure and a shift of the valence band onset by approximately 0.4 eV to lower binding energies which is tentatively explained by a band gap widening as expected for Cu defic...

Current-voltage characteristics and transport mechanism of solar cells based on ZnO nanorods/In2S3∕CuSCN

Temperature dependent current-voltage characteristics in the dark and under illumination have been analyzed on up to 3.2% efficient solar cells with extremely thin absorber based on ZnO nanorods/In2S3∕CuSCN structures. The diode ideality factor and the open circuit voltage are strongly influenced on a thermal activation process. Significant enhancement of the devices efficiency by annealing at moderate temperatures has been demonstrated. After this annealing, the activation energy of the saturation current increased from 1.00to1.46eV (in the dark). Transport mechanisms at the In2S3∕CuSCN interface region are discussed.

Spatially distributed p-n heterojunction based on nanoporous TiO2 and CuSCN

Directed semiconductor growth in nanoporous ceramic films is reported. A p-n heterojunction with an interface that is spatially distributed across the complete thickness of the ceramic film is established. The interface area is estimated to be several 100 times larger than its geometric projection. The p-n junction shows excellent rectification and may serve as the basic building block for photovoltaic devices.