M.C. Lux-Steiner

Tunable optical transition in polymeric carbon nitrides synthesized via bulk thermal condensation

Polymeric derivatives of dicyandiamide were synthesized via a bulk thermal condensation method, using a range of process temperatures between 400 and 610 °C. The obtained carbon nitride powders exhibit an optical transition in the UV-green range that has been assigned to the direct bandgap of a semiconductor-like material. Within this context, the apparent bandgap is linearly tunable with increasing process temperatures, showing a temperature coefficient of - 1.7(1) meV K(-1) between 2.5 and 3.0 eV. The obtained results show a predominant optical transition within the tri-s-triazine unit of the polymer, with a bathochromic shift originating from a gradually increasing degree of polymerization.

Zn(O,OH) layers in chalcopyrite thin-film solar cells: Valence-band maximum versus composition

Zn(O,OH) layers deposited by the ion layer gas reaction (ILGAR) technique have the potential to replace the conventionally used CdS buffer layer in Cu(In(1−X)GaX)(SYSe(1−Y))2-based thin-film solar cells. To avoid stability issues, the fraction of metastable Zn(OH)2 should be reduced in the final buffer layer. However, hydroxide-poor or -free ZnO “buffers” result in noncompetitive devices. We have therefore investigated the impact of different oxide/hydroxide ratios on the electronic band alignment at the absorber/buffer heterointerface. The surface composition as well as the position of the valence-band maximum (VBM) of respective ILGAR-Zn(O,OH) samples was determined by photoelectron spectroscopy. The position of the conduction-band minimum (CBM) was estimated using optical band gaps determined from optical reflection/transmission measurements. From the comparison of these VBM and CBM values with the respective values of the absorber surface, predictions are made in terms of valence- and conduction-band ...

ZnO layers deposited by the ion layer gas reaction on Cu(In,Ga)(S,Se)2 thin film solar cell absorbers: Morphology, growth mechanism, and composition

Cu(In,Ga)(S,Se)2 (CIGSSe) based solar cells with a ZnO window extension layer (WEL) deposited by the ion layer gas reaction (ILGAR) reach competitive efficiencies compared to corresponding references with CdS buffer and lead to a simplified device structure. The WEL replaces not only the CdS buffer, but also the undoped part of the usually applied rf-sputtered ZnO window bilayer. Since the performance of corresponding solar cell devices depends strongly on the ILGAR process parameters (number of deposition cycles and process temperature), respective ILGAR-ZnO∕CIGSSe test structures were investigated by means of scanning electron microscopy and x-ray photoelectron spectroscopy. Thereby, the growth mechanism of ILGAR-ZnO on CIGSSe absorbers and its morphology was investigated. In addition, the surface composition was determined, showing that ILGAR-ZnO layers contain a certain amount of metastable hydroxide. Due to the systematic variation of the ILGAR process parameters it could be demonstrated that it is p...

Cd2+∕NH3-treatment of Cu(In,Ga)(S,Se)2: Impact on the properties of ZnO layers deposited by the ion layer gas reaction method

Cu(In,Ga)(S,Se)2- (“CIGSSe”) based solar cells with a ZnO layer deposited by the ion layer gas reaction (ILGAR) method yield superior efficiencies (15.0%) than the references with a chemical bath-deposited CdS buffer (14.1%). However, this high performance is only reached if the absorber is pretreated in a Cd2+- and aqueous ammonia-containing bath prior to the ILGAR-ZnO deposition. The photovoltaic as well as the dark device parameters are strongly influenced by this treatment. Scanning and transmission electron microscopy (TEM) as well as x-ray diffraction measurements reveal a different morphology and structure of ILGAR-ZnO layers on top of Cd2+∕NH3-treated and on as-deposited absorbers, indicating a considerably modified absorber surface. By energy dispersive x-ray analysis in the TEM, Cd could only be identified at the ILGAR-ZnO∕Cd2+∕NH3-treated-CIGSSe interface of the respective cross sections, if the absorber was treated in a bath with an atypically high Cd2+-concentration. In this case a Cd-contain...

The chemical and electronic surface and interface structure of CuGaSe2 thin-film solar cell absorbers

The chemical and electronic surface and interface structure of CuGaSe2 thin films was investigated by photoelectron spectroscopy. With bulk [Ga]/[Cu] ratios increasing from 0.94 to 1.39 a transition of the Cu:Ga:Se surface composition from 1:1:2 to 1:3:5 and a downward shift of the valence band maximum with respect to the Fermi energy were observed. The comparison with the conduction band minimum (CBM) of CdS reveals that at the CdS/CuGaSe2 interface the recombination barrier height simultaneously increases and a “clifflike” offset is formed to the CBM of CuGaSe2.

Up-scaling ILGAR In 2 S 3 buffer layers production for chalcopyrite solar modules

Indium sulfide buffer layers produced by the spray ion-layer-gas-reaction (ILGAR) have already been shown to be a suitable replacement for the standard CdS layer in chalcopyrite based thin film solar cells. However, to date, all of the results have been shown on small area solar cells. Here we demonstrate both the up-scaling of the spray ILGAR process for the first time and show results for mini-modules based on Cu(In,Ga)(Se,S)2 (CIGSSe) absorbers from the AVANCIS pilot line. The mini-module efficiencies reach 12.4% and damp heat stability is comparable to CdS buffered references. Analysis of the modules by thermography shows a strong correlation between homogeneity and module performance. This preliminary up-scaled 10 × 10 cm deposition chamber paved the way for further large scale development. This includes a tape coater, built in-house, a 30 × 30 cm machine under construction with STANGL semiconductor equipment AG and a roll-to-roll ILGAR machine, which is currently part of the pilot production line operating at CIS-Solartechnik in Hamburg.

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.

Cd/sup 2+//NH/sub 3/-treatment of high-gap CuGaSe/sub 2/ thin film solar cell absorbers

For chalcopyrite-based solar cells with conventional CdS buffers (prepared in a chemical bath), the Cd/sup 2+//NH/sub 3/-treatment can be used as a tool to visualize the processes during the initial stages of interface formation in the chemical bath. For high-gap CuGaSe/sub 2/ (CGSe) - an absorber material which has not yet shown its full potential in terms of solar cell performance - a CdSe surface compound was recently identified at the CGSe absorber surface after this kind of simple surface conditioning. In this contribution, we will clarify whether the [Ga]/[Cu]-ratio of the CGSe absorber has an impact on the formation of this non-abrupt interface. In addition, the lateral variation of the observed absorber surface modification will be investigated. Thus, as-deposited and Cd/sup 2+//NH/sub 3/-treated CGSe surfaces were characterized by X-ray excited Auger and photoelectron spectroscopy using Mg K/spl alpha/ and/or tuneable monochromatized synchrotron radiation.