Alexander Steigert

Solar hydrogen evolution using metal-free photocatalytic polymeric carbon nitride/CuInS2 composites as photocathodes

Polymeric carbon nitride (g-C3N4) films were synthesized on polycrystalline semiconductor CuInS2 chalcopyrite thin film electrodes by thermal polycondensation and were investigated as photocathodes for the hydrogen evolution reaction (HER) under photoelectrochemical conditions. The composite photocathode materials were compared to g-C3N4 powders and were characterized with grazing incidence X-ray diffraction and X-ray photoemission spectroscopy as well as Fourier transform infrared and Raman spectroscopies. Surface modification of polycrystalline CuInS2 semiconducting thin films with photocatalytically active g-C3N4 films revealed structural and chemical properties corresponding to the properties of g-C3N4 powders. The g-C3N4/CuInS2 composite photocathode material generates a cathodic photocurrent at potentials up to +0.36 V vs. RHE in 0.1 M H2SO4 aqueous solution (pH 1), which corresponds to a +0.15 V higher onset potential of cathodic photocurrent than the unmodified CuInS2 semiconducting thin film photocathodes. The cathodic photocurrent for the modified composite photocathode materials was reduced by almost 60% at the hydrogen redox potential. However, the photocurrent generated from the g-C3N4/CuInS2 composite electrode was stable for 22 h. Therefore, the presence of the polymeric g-C3N4 films composed of a network of nanoporous crystallites strongly protects the CuInS2 semiconducting substrate from degradation and photocorrosion under acidic conditions. Conversion of visible light to hydrogen by photoelectrochemical water splitting can thus be successfully achieved by g-C3N4 films synthesized on polycrystalline CuInS2 chalcopyrite electrodes.

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...

A Molecular Approach to Self‐Supported Cobalt‐Substituted ZnO Materials as Remarkably Stable Electrocatalysts for Water Oxidation

In regard to earth-abundant cobalt water oxidation catalysts, very recent findings show the reorganization of the materials to amorphous active phases under catalytic conditions. To further understand this concept, a unique cobalt-substituted crystalline zinc oxide (Co:ZnO) precatalyst has been synthesized by low-temperature solvolysis of molecular heterobimetallic Co(4-x)Zn(x) O4 (x = 1-3) precursors in benzylamine. Its electrophoretic deposition onto fluorinated tin oxide electrodes leads after oxidative conditioning to an amorphous self-supported water-oxidation electrocatalyst, which was observed by HR-TEM on FIB lamellas of the EPD layers. The Co-rich hydroxide-oxidic electrocatalyst performs at very low overpotentials (512 mV at pH 7; 330 mV at pH 12), while chronoamperometry shows a stable catalytic current over several hours.

Role of chlorine in In2S3 for band alignment at nanoporous-TiO2/In2S3 interfaces

Coatings of Cl-free and Cl-containing In2S3 (In2S3:Cl) layers were prepared by spray ion layer gas reaction on nanoporous TiO2 (np-TiO2). Charge separation in FTO/np-TiO2/In2S3:Cl, FTO/In2S3:Cl, FTO/np-TiO2/In2S3:acac, and FTO/In2S3:acac systems has been investigated by modulated surface photovoltage spectroscopy (SPV). The exponential defect tails below the band gap of In2S3 were reduced for In2S3:Cl. The analysis of the SPV spectra allowed to distinguish different mechanisms of charge separation and to deduce the conduction band offsets (about 0.3 eV and 0.05 eV for np-TiO2/In2S3:Cl and np-TiO2/In2S3:acac, respectively) in correlation with photoelectron measurements.

Enhanced absorption in tandem solar cells by applying hydrogenated In2O3 as electrode

To realize the high efficiency potential of perovskite/chalcopyrite tandem solar cells in modules, hydrogenated In2O3 (IO:H) as electrode is investigated. IO:H with an electron mobility of 100 cm2 V−1 s−1 is demonstrated. Compared to the conventional Sn doped In2O3 (ITO), IO:H exhibits a decreased electron concentration and leads to almost no sub-bandgap absorption up to the wavelength of 1200 nm. Without a trade-off between transparency and lateral resistance in the IO:H electrode, the tandem cell keeps increasing in efficiency as the IO:H thickness increases and efficiencies above 22% are calculated. In contrast, the cells with ITO as electrode perform much worse due to the severe parasitic absorption in ITO. This indicates that IO:H has the potential to lead to high efficiencies, which is otherwise constrained by the parasitic absorption in conventional transparent conductive oxide electrode for tandem solar cells in modules.

Enhancement of photocurrent in an ultra-thin perovskite solar cell by Ag nanoparticles deposited at low temperature

Ultra-thin perovskite absorber layers have attracted increasing interest since they are suitable for application in semi-transparent perovskite and tandem solar cells. In this study, size and density controlled plasmonic silver nanoparticles are successfully incorporated into ultra-thin perovskite solar cells through a low temperature spray chemical vapor deposition method. Incorporation of Ag nanoparticles leads to a significant enhancement of 22.2% for the average short-circuit current density. This resulted in a relative improvement of 22.5% for the average power conversion efficiency. Characterization by surface photovoltage and photoluminescence provides evidence that the implemented silver nanoparticles can enhance the charge separation and the trapping of electrons into the TiO2 layer at the CH3NH3PbI3/TiO2 interface. The application of these silver nanoparticles therefore has promise to enhance the ultra-thin perovskite solar cells.

Active and Stable Nickel-Based Electrocatalysts Based on the ZnO:Ni System for Water Oxidation in Alkaline Media

The synthesis of monodisperse, surfactant‐free, Ni‐substituted ZnO nanocrystallites (ZnO:Ni) by the mild solvolysis of heterobimetallic Ni4−xZnxO4 cubane‐like precursors (x=1–3) in benzylamine is reported. Ni4−xZnxO4 was grafted by electrophoretic deposition onto fluorine‐doped tin oxide glass substrates and used as an active and stable working electrode for water oxidation. Upon the application of a voltage at the electrodes, the ZnO:Ni precatalyst leads to an active composite material that can oxidize water (>15 h) with an increasing catalytic current. In contrast, the performance of homometallic NiO reference materials decreases rapidly over time and is surpassed by the composite from the ZnO:Ni precatalyst in terms of both stability and activity. Extensive characterization of the as‐prepared and activated ZnO:Ni precatalyst by using hard X‐ray photoelectron spectroscopy revealed that the excellent performance of the electrode material is because of the formation of a unique self‐supported turbostratically disordered mixture of γ‐NiOOH/α‐Ni(OH)2‐like phases from the rapid dissolution of ZnII in the ZnO:Ni precatalyst into the electrolyte during activation.

Design, preparation and performance of Cu(In, Ga)(S, Se) 2 /Zn(O, S)/ZnO:Al solar cells

Junction formation by chemical bath deposition of CdS is a well established and robust process. To avoid the well known drawbacks of this approach, we propose to omit the CdS buffer layer and to directly sputter a modified window layer where Zn(O, S) is used instead of ZnO to improve the band line-up. This could result in completely dry in-line manufacturing of Cd-free modules using only proven deposition technologies. Key requisites for a new module structure are that the efficiency is not adversely affected and that the process is stable with a wide process window. For a first assessment, tests were carried out using absorbers from industrial production.

The chemical structure of the ZnO/SiC heterointerface as revealed by electron spectroscopies

ZnO layers were deposited on 6H-SiC single crystalline wafers by radio frequency magnetron sputtering. The chemical structure of the ZnO/SiC interface was studied by x-ray photoelectron and x-ray excited Auger electron spectroscopy. A complex chemical structure, involving not only silicon–carbon and zinc–oxygen bonds but also silicon–oxygen and zinc–silicon–oxygen bonds was revealed to form at the ZnO/SiC interface. Based on the comparison with the presumably inert (i.e. chemically abrupt) ZnO/Mo interface, it was concluded that a willemite-like zinc silicate (i.e. Zn2SiO4) interface species develops between ZnO and SiC. The presence of this species at the ZnO/SiC interface will affect the electronic structure of the heterojunction and thus needs to be considered for device optimization.

Optimization of buffer layer/i-layer band alignment

We investigate how conduction band offset between the buffer and the i-layer of Cu(InGa)Se2 solar cells affects their performance. Employing SCAPS simulations, we explore two specific cases with either a CdS buffer or an alternative Zn(OS) buffer of a higher conduction band energy. These studies are supported by experimental results using (ZnMg)O as a model i-layer material. We find that pure ZnO is the suitable i-layer for chalcogenide-based solar cells only if combined with the CdS buffer. However, another i-layer material with a higher conduction band edge is needed if CdS is replaced by an alternative buffer of a wider band gap, such as Zn(OS).