Rodrigo Sáez-Araoz

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 ...

Recombination mechanisms in highly efficient thin film Zn(S,O)/Cu(In,Ga)S2 based solar cells

Progress in fabricating Cu(In,Ga)S2 based solar cells with Zn(S,O) buffer is presented. An efficiency of 12.9% was achieved. Using spectral response, current-voltage and temperature dependent current-voltage measurements, current transport in this junction was studied and compared to that of a highly efficient CdS/Cu(In,Ga)S2 solar cell with a special focus on recombination mechanisms. Independently of the buffer type and despite the difference in band alignment of the two junctions, interface recombination is found to be the main recombination channel in both cases. This was unexpected since it is generally assumed that a cliff facilitates interface recombination while a spike suppresses it.

Intermixing at the heterointerface between ZnS∕Zn(S,O) bilayer buffer and CuInS2 thin film solar cell absorber

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. After identifying the deposited Zn compound, as ZnS∕Zn(S,O) bilayer buffer in former investigations [M. Bar et al., J. Appl. Phys. 99, 123503 (2006)], this time the focus lies on potential diffusion/intermixing processes at the buffer/absorber interface possibly, clarifying the effect of the heat treatment, which drastically enhances the device performance of respective final solar cells. The interface formation was investigated by x-ray photoelectron and x-ray excited Auger electron spectroscopy. In addition, photoelectron spectroscopy (PES) measurements were also conducted using tunable monochromat...

Current transport in Cu(In,Ga)S2 based solar cells with high open circuit voltage-bulk vs. interface

Cu(In,Ga)S 2 thin films prepared by rapid thermal sulfurization of metallic precursors yielded solar cells with efficiencies reaching 12.9% [1]. A good short circuit current density was observed together with open circuit voltages up to 850 mV. However, the fill factor was close to, but typically did not exceed 70%. In this contribution we report on the role of junction formation by chemical bath deposition on these parameters. Concentrations in the bath and deposition times were varied. A comparison is made between CdS and Zn(S,O) buffer layers. The influence of the incorporated gallium on surface properties was investigated by ultraviolet photoelectron spectroscopy (UPS) for the valence band edge and near edge X-ray absorption fine structure (NEXAFS) for the conduction band edge. Even in our best cell (13.1%) the activation energy of the saturation current is found to be still smaller than the band gap. High diode ideality factors and voltage dependent current collection prevent higher fill factors.

Quality CuInSe2 and Cu(In,Ga)Se2 thin films processed by single-step electrochemical deposition techniques

Ternary CuInSe2 and quaternary Cu(In,Ga)Se2 chalcopyrite semiconductor films with potential applications as solar absorbers were deposited by single-step electrochemical deposition (ECD) on molybdenum coated glass substrates. The films have been structurally characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM) combined with energy dispersive x-ray analysis (EDAX), x-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Chalcopyrite phase formation was confirmed already in as-deposited films. The crystal structure of the films was further improved by thermal treatment. Element interdiffusion at the chalcopyrite/Mo/glass interface has been prevented by retaining moderate temperatures of deposition (70 °C) and subsequent annealing (300 °C). The SEM/EDAX analysis revealed the presence of CuxSe secondary phases on the surface of ternary films and almost stoichiometric growth of quaternary deposited on top of ternary films. The XRD and Raman analysis confirmed the high quality assessment of the films being almost equal to that of chalcopyrite selenide layers grown by physical vapor deposition at high temperatures (550–750 °C). The surface sensitive XPS analysis confirmed the absence of other impurities in the ECD processed films except from oxygen and carbon adsorbents by sample exposure to atmospheric air.

Chemical composition and electronic properties of CuInS2/Zn(S,O) interfaces

We report on the chemical deposition and electronic properties of CuInS2/Zn(S,O) interfaces. The Zn(S,O) buffer was grown by a new chemical bath deposition (CBD) process that allows the tailoring of the S/O ratio in the films. Resulting Zn(S,O) films exhibit transparencies above 80% (for λ>390 nm) and an optical energy band gap of 3.9 eV which decreases to 3.6 eV after annealing in air at 200°C. Production line CuInS2 (CIS) absorbers provided by Sulfurcell Solartechnik GmbH are used as substrates for the investigation of the CIS/Zn(S,O) interface and the chemical composition of Zn(S,O). A ZnS/(ZnS+ZnO) ratio of 0.5 is found by X-ray photoelectron spectroscopy and X-ray excited Auger electron spectroscopy (XPS and XAES). The valence band offset between the heterojunction partners (ΔEV =1.8 ± 0.2 eV) has been determined by means of XPS and ultraviolet photoelectron spectroscopy (UPS). Considering the energy band gap of the CIS absorber and the measured band gap of Zn(S,O), the conduction band offset (ΔEC) is calculated as: ΔEC=Eg-Eg-ΔEV resulting in a spike of 0.5±0.3 eV in the conduction band at the heterojunction before annealing. After the heat treatment, the valence band offset is reduced to 1.5±0.2 eV and the calculated conduction band offset remains at 0.5±0.3 eV.

Preparation of an amorphous optically transparent and hydrophobic Al 2 O 3 top-protective layer for first-surface CSP reflectors

In Concentrated Solar Power (CSP) application, top coatings are of tremendous importance as a way to protect the reflective layers form degradation and conserve the efficiency and durability of the mirrors. The choice of Al<inf>2</inf>O<inf>3</inf> as a top-protective coating for CSP reflectors is based on its high stability, hardness and transparency. In this study, high quality and stable CSP first-surface silvered thick glass mirrors were prepared with a transparent and hydrophobic amorphous Al<inf>2</inf>O<inf>3</inf> top-protective layer with different thicknesses (1µm − 4µm) in order to investigate the effect of the film-thickness on the optical properties and surface morphology of the samples. The spectrophotometric measurements were conducted using a Perkin Elmer Lambda 950 UV/VIS/NIR Spectrophotometer and showed no significant change in the optical properties of the amorphous Al<inf>2</inf>O<inf>3</inf> layers with different thicknesses. The surface morphology was characterized using scanning electron microscopy (SEM), and atomic force microscopy (AFM). The results show that increasing the thickness of the Al<inf>2</inf>O<inf>3</inf> layer up to 3 µm increased the surface hydrophobicity of the mirrors whereas a decrease in the water contact angle was noticed with 4 µm thickness. The measured water contact angles (WCA) were 94°, 98°, 102° and 95° for 1 µm, 2 µm, 3 µm and 4 µm, respectively. A decrease in the water contact angle (WCA=33°) was noticed by achieving a phase transformation from amorphous to crystalline (γ-Al2O3) using annealing at 800°C for 2 hours, exhibiting a hydrophilic behavior.