Volume 74, Issue 8 - Special Issue: Trends and Perspectives in Energy Research \ Issue Editors: Martin Holthaus and Sascha Schäfer

With the spread of photovoltaics (PV) and increasing diversity in PV panel technology, quantitative comparison of the modules is highly desirable for consistent on-site quality assessment. Electroluminescence imaging reveals many defects, such as macroscopic crystal or electrical contact defects, but quantitative comparison outside the laboratory without controlled environment is still difficult, especially for different detector technologies. Here, we show how this problem can be addressed by adding reference spots in the module area: One passive dark spot and an active bright spot composed of a high-power back-contacted silicon PV cell. Those reference spots are used to evaluate the module’s electroluminescence signal under different environmental conditions and to establish comparable results. Additionally, the comparison of images acquired with different camera technology detectors, such as silicon and InGaAs, is realised for signal levels.

The preparation of compact and pinhole-free absorber layers is a major step towards device reproducibility and high performance for planar organic-inorganic perovskite solar cells. It is well known that the sequential deposition method exhibits some advantages over the common one-pot synthesis in terms of controlling the surface coverage. However, it still miscarries to produce pinhole-free layers from solution, mainly due to the occurrence of dissolution and recrystallisation processes. We show that by a careful choice of the permittivity of the alcoholic solvent in the conversion step the surface morphology can be finely modified, thereby yielding pinhole-free and compact absorber films comparable to that from vapour-assisted solution techniques. It is observed that the permittivity controls the intensity of the Ostwald ripening effect and that a low value of the former enables an in situ intercalation of precursor materials into the lead halide framework. We successfully prepared smooth and mirror-like perovskite surfaces that demonstrate enhanced optoelectronic properties and photovoltaic performance compared to films from the native two-step deposition in isopropanol. This strategy provides a facile approach for obtaining high-quality layers in the planar architecture by simple solution processing.

The organic and hydrophobic polymer poly[ N , N′ -bis(4-butilphenyl)- N , N′ -bis(phenyl)-benzidine] (polyTPD) represents a promising hole transport layer (HTL) for perovskite photovoltaics due to its suitable energy levels, whereby its highest occupied molecular orbital level matches well with the valence band level of methylammonium lead triiodide (CH 3 NH 3 PbI 3 , MAPbI 3 ) perovskite. However, processing a perovskite layer from the solution on the surface of this organic material, is found to be difficult due to the surface properties of the latter. In this study, we evaluate efficient p-i-n type MAPbI 3 perovskite solar cells employing differently processed polyTPD layers. We found that the surface coverage of the MAPbI 3 perovskite layer strongly depends on the preparation method of the underlying polyTPD layer. By varying the solvents for the polyTPD precursor, its concentration, and by applying an optimised two-step perovskite deposition technique we increased both the surface coverage of the perovskite layer as well as the power conversion efficiency (PCE) of the corresponding solar cell devices. Our simple solvent-engineering approach demonstrates that no further interface modifications are needed for a successful preparation of efficient planar photovoltaic devices with PCEs in the range of 15 %–16 %.

In this manuscript, we give an overview of the main insights into our growth procedure for kesterite solar cells and show the possibilities that are provided by this approach. The importance of using Cu–Sn alloy instead of elemental Sn and Cu in the precursor is shown. We discuss how the alloy approach stabilises the composition and helps guide the process along a preferred reaction pathway. A summary of our previously reported findings in the context of our latest results on kesterite solar cells prepared from Cu–Sn alloyed precursors is drawn. The positive impact of an alloy precursor configuration on the formation pathway, process control, and process resilience is demonstrated. Furthermore, a new optimisation strategy for kesterite, based on the reported pathway, is discussed, including a smooth phase transition from Cu-rich to Cu-poor kesterite. Finally, we demonstrate results on buffer optimisation and the application of a promising hybrid buffer configuration of CdS/Zn(O,S), which can reduce the optical losses in the solar cell structure.

In the future, many modern buildings may rely on solar windows for energy production. Large buildings often have glass facades that have the potential to convert sunlight to electrical power. The standard photovoltaic materials used today are bulky and not transparent, making them poor candidates for solar windows. Transition metal dichalcogenides (TMDCs) and other two-dimensional absorbers are a good alternative because of their unique properties and high transparency at the monolayer and few-layer regime. This work shows the potential for TMDC-based solar windows by simulating the transmission, quantum efficiency, current density, and colour appearance of different solar cell configurations. Different contacts were investigated, along with the influence of contact thickness, to demonstrate colour-neutral solar cells. In addition, four TMDC materials were compared: MoS 2 , MoSe 2 , WS 2 , and WSe 2 . Colour-neutral solar cells with transparencies of 35 % to 55 % are presented, where a current density of 8.33 mA/cm 2 was calculated for a solar cell with a 5-nm absorbing layer of MoSe 2 . While there are still challenges to overcome in terms of production, our simulations show that it is possible to use TMDCs for colour-neutral solar windows and act as a guideline for further research.

In the close vicinity of a hot body, at distances smaller than the thermal wavelength, a high electromagnetic energy density exists due to the presence of evanescent fields radiated by the partial charges in thermal motion around its surface. This energy density can surpass the energy density in vacuum by several orders of magnitude. By approaching a photovoltaic (PV) cell with a band gap in the infrared frequency range, this nonradiative energy can be transferred to it by photon tunnelling and surface mode coupling. Here we review the basic ideas and recent progress in near-field energy harvesting.

Rutile TiO 2 is an important model system for understanding the adsorption and conversion of molecules on transition metal oxide catalysts. In the last decades, point defects, such as oxygen vacancies and Ti 3+ interstitials, exhibited an important influence on the reaction of oxygen and oxygen-containing molecules on titania surfaces. In brief, partially reduced TiO 2 containing a significant amount of Ti 3+ is often more active for the conversion of such molecules. In this study, we investigate an even higher reduced surface prepared by argon ion bombardment of a rutile TiO 2 (110) single crystal. By X-ray photoelectron spectroscopy we show that, besides Ti 4+ , this surface is almost equally dominated by Ti 3+ and Ti 2+ . To probe the reactivity of these highly reduced surfaces, we have adsorbed two different classes of oxygen-containing molecules and utilized temperature programmed reaction spectroscopy to investigate the conversion. While alcohols (in this case methanol) already show a defect-dependent partial conversion in a deoxygenation reaction on the (stochiometric or slightly reduced) rutile TiO 2 (110) surface, ketones (e.g. acetone) are usually not converted on the rutile TiO 2 (110) surface independent on the bulk defect density. Here, we present a nearly full conversion for both molecules via deoxygenation reactions and reductive C–C coupling, forming different hydrocarbons at different temperatures between 375 K and 640 K on the sputtered Ti 2+ rich surface.

Transition metal phosphides are interesting candidates for application as catalysts in electrochemical water splitting or heteroatom removal reactions, energy storage, and magnetic refrigeration. Metal phosphides can adopt a wide range of stoichiometries, and have metallic or semiconducting properties. The electronic and magnetic characteristics of metal phosphides can be further controlled by changing the size and the shape of the crystallites and by combining two different metals. In this review, the wet-chemical methods to synthesize such ternary Fe, Co, and Ni phosphides are summarised. First, an overview of the general synthetic strategies and the crystallographic structure of the related materials is given. Then, examples of the synthesis of several ternary phosphides are described, and the motivation for synthesising the specific materials is discussed.

The role of molecular vibrations for the persistence of quantum coherences, recently observed in photoinduced charge transfer reactions in both biological and artificial energy conversion systems at room temperature, is currently being intensely discussed. Experiments using two-dimensional electronic spectroscopy (2DES) suggest that vibrational motion – and its coupling to electronic degrees of freedom – may play a key role for such coherent dynamics and potentially even for device function. In organic photovoltaics materials, strong coupling of electronic and vibrational motion is predicted, especially for ubiquitous C=C stretching vibrations. The signatures of such strong vibronic couplings in 2DES are, however, debated. Here we analyse the effect of strong vibronic coupling in model simulations of 2DES spectra and dynamics for an electronic dimer coupled to a single high-frequency vibrational mode. This system represents the simplest conceivable model for a prototypical donor–acceptor interface in the active layer of organic solar cells. The vibrational mode is chosen to mimic C=C stretching vibrations with typical large vibronic couplings predicted in organic photovoltaics materials. Our results show that the decisive signatures of strong vibronic coupling mediating coherent charge transfer between donor and acceptor are not only temporally oscillating cross-peaks, but also most importantly characteristic peak splittings in the 2DES spectra. The 2DES pattern thus directly reflects the new eigenstates of the system that are formed by strong mixing of electronic states and vibrational mode.