B. Marsen

Cliff-like conduction band offset and KCN-induced recombination barrier enhancement at the CdS/Cu2ZnSnS4 thin-film solar cell heterojunction

The electronic structure of the CdS/Cu2ZnSnS4 (CZTS) heterojunction was investigated by direct and inverse photoemission. The effects of a KCN etch of the CZTS absorber prior to CdS deposition on the band alignment at the respective interface were studied. We find a “cliff”-like conduction band offset at the CdS/CZTS interface independent of absorber pretreatment and a significant etch-induced enhancement of the energetic barrier for charge carrier recombination across the CdS/CZTS interface.

Impact of KCN etching on the chemical and electronic surface structure of Cu2ZnSnS4 thin-film solar cell absorbers

The chemical and electronic surface structure of Cu2ZnSnS4thin-filmsolar cell absorbers has been investigated by direct and inverse photoemission. Particular emphasis was placed on the impact of KCN etching, which significantly alters the surface composition and is best explained by a preferred etching of Cu and, to a lesser degree, Sn. As a consequence the surfaceband gap increased from (1.53 ± 0.15) eV, which agrees with optically derived bulk band gap values, to (1.91 ± 0.15) eV.

Native oxidation and Cu-poor surface structure of thin film Cu2ZnSnS4 solar cell absorbers

Air-exposed Cu2ZnSnS4 (“CZTS”) thin-film solar cell absorbers have been investigated by surface-sensitive x-ray photoelectron and x-ray-excited Auger electron spectroscopy, as well as by bulk-sensitive energy dispersive x-ray spectroscopy. We find a native surface oxidation of (mainly) tin, but also (to a lesser extent) of zinc and sulfur as well as evidence for a Cu-poor region at the surface of the absorber, best described by a Cu-free Zn-Sn-S surface layer.

Improved current collection in WO 3 :Mo/WO 3 bilayer photoelectrodes

We report on the incorporation of molybdenum into tungsten oxide by co-sputtering and its effect on solar-powered photoelectrochemical (PEC) water splitting. Our study shows that Mo incorporation in the bulk of the film (WO 3 :Mo) results in poor PEC performance when compared with pure WO 3 , most likely due to defects that trap photo-generated charge carriers. However, when a WO 3 :Mo/WO 3 bilayer electrode is used, a 20% increase of the photocurrent density at 1.6 V versus saturated calomel reference electrode is observed compared with pure WO 3 . Morphological and microstructural analysis of the WO 3 :Mo/WO 3 bilayer structure reveals that it is formed by coherent growth of the WO 3 :Mo top layer on the WO 3 bottom layer. This effect allows an optimization of the electronic surface structure of the electrode while maintaining good crystallographic properties in the bulk.

IBPOWER: Intermediate band materials and solar cells for photovoltaics with high efficiency and reduced cost

IBPOWER is a Project awarded under the 7th European Framework Programme that aims to advance research on intermediate band solar cells (IBSCs). These are solar cells conceived to absorb below bandgap energy photons by means of an electronic energy band that is located within the semiconductor bandgap, whilst producing photocurrent with output voltage still limited by the total semiconductor bandgap. IBPOWER employs two basic strategies for implementing the IBSC concept. The first is based on the use of quantum dots, the IB arising from the confined energy levels of the electrons in the dots. Quantum dots have led to devices that demonstrate the physical operation principles of the IB concept and have allowed identification of the problems to be solved to achieve actual high efficiencies. The second approach is based on the creation of bulk intermediate band materials by the insertion of an appropriate impurity into a bulk semiconductor. Under this approach it is expected that, when inserted at high densities, these impurities will find it difficult to capture electrons by producing a breathing mode and will cease behaving as non-radiative recombination centres. Towards this end the following systems are being investigated: a) Mn: In1-xGaxN; b) transition metals in GaAs and c) thin films.

Correlation between composition and photovoltaic properties of Cu 2 ZnSnS 4 thin film solar cells

The influence of composition on the quaternary Cu2ZnSnS4 (CZTS) absorber material on the secondary phase content, recombination characteristics and solar cell performance is investigated. Best solar efficiencies are found in a very narrow Cu-poor, Zn-rich composition range. Photoluminescence measurements performed at 12 K indicate strong compensation for Cu-poor composition, similar to what is observed in chalcopyrite-type compounds. We find that strongly copper-deficient CZTS contains significant amounts of ZnS, which degrades the solar cell performance.

Cu 2 ZnSnS 4 thin-film solar cell absorber composition revealed by energy-dispersive and soft x-ray emission spectroscopy

Different Cu2ZnSnS4 (CTZS) thin-film solar cell absorbers have been investigated by bulk-sensitive energy-dispersive spectroscopy (EDS) and surface-near bulk-sensitive soft x-ray emission spectroscopy (XES). While we find a good agreement between the computed Zn/Sn composition ratio based on the EDS and XES data, the XES determined Cu/(Zn+Sn) composition ratio significantly deviates from that based on EDS measurements for some samples. While the first can be explained by a homogenous Zn/Sn composition throughout the CTZS samples, the latter is interpreted as a variation of the Cu depth profile in the respective thin-film solar cell absorbers.