Hans-Gerd Boyen

Perovskite‐Based Hybrid Solar Cells Exceeding 10% Efficiency with High Reproducibility Using a Thin Film Sandwich Approach

Organometal halide perovskites have tremendous potential as light absorbers for photovoltaic applications. In this work we demonstrate hybrid solar cells based on the mixed perovskite CH3 NH3 PbI2 Cl in a thin film sandwich structure, with unprecedented reproducibility and generating efficiencies up to 10.8%. The successfulness of our approach is corroborated by the experimental electronic structure determination of this perovskite.

Perovskite-perovskite tandem photovoltaics with optimized band gaps

Tandem perovskite cells The ready processability of organic-inorganic perovskite materials for solar cells should enable the fabrication of tandem solar cells, in which the top layer is tuned to absorb shorter wavelengths and the lower layer to absorb the remaining longer-wavelength light. The difficulty in making an all-perovskite cell is finding a material that absorbs the red end of the spectrum. Eperon et al. developed an infrared-absorbing mixed tin-lead material that can deliver 14.8% efficiency on its own and 20.3% efficiency in a four-terminal tandem cell. Science, this issue p. 861 A mixed tin-lead perovskite material with a narrow band gap enables efficient tandem solar cells. We demonstrate four- and two-terminal perovskite-perovskite tandem solar cells with ideally matched band gaps. We develop an infrared-absorbing 1.2–electron volt band-gap perovskite, FA0.75Cs0.25Sn0.5Pb0.5I3, that can deliver 14.8% efficiency. By combining this material with a wider–band gap FA0.83Cs0.17Pb(I0.5Br0.5)3 material, we achieve monolithic two-terminal tandem efficiencies of 17.0% with >1.65-volt open-circuit voltage. We also make mechanically stacked four-terminal tandem cells and obtain 20.3% efficiency. Notably, we find that our infrared-absorbing perovskite cells exhibit excellent thermal and atmospheric stability, not previously achieved for Sn-based perovskites. This device architecture and materials set will enable “all-perovskite” thin-film solar cells to reach the highest efficiencies in the long term at the lowest costs.

Towards Efficient Hybrid Solar Cells Based on Fully Polymer Infiltrated ZnO Nanorod Arrays

L.B. and B.C. contributed equally to this work. The authors thank R. Rieke from Rieke Metals for useful suggestions. This work was financially supported by BOF, UHasselt, the Flemish Odysseus program, and the Interreg project Organext. A. H. is a postdoctoral research fellow of the Research Foundation-Flanders (FWO Vlaanderen).

Assessing the toxicity of Pb- and Sn-based perovskite solar cells in model organism Danio rerio.

Intensive development of organometal halide perovskite solar cells has lead to a dramatic surge in power conversion efficiency up to 20%. Unfortunately, the most efficient perovskite solar cells all contain lead (Pb), which is an unsettling flaw that leads to severe environmental concerns and is therefore a stumbling block envisioning their large-scale application. Aiming for the retention of favorable electro-optical properties, tin (Sn) has been considered the most likely substitute. Preliminary studies have however shown that Sn-based perovskites are highly unstable and, moreover, Sn is also enlisted as a harmful chemical, with similar concerns regarding environment and health. To bring more clarity into the appropriateness of both metals in perovskite solar cells, we provide a case study with systematic comparison regarding the environmental impact of Pb- and Sn-based perovskites, using zebrafish (Danio Rerio) as model organism. Uncovering an unexpected route of intoxication in the form of acidification, it is shown that Sn based perovskite may not be the ideal Pb surrogate.

Sequential ion-induced stress relaxation and growth: A way to prepare stress-relieved thick films of cubic boron nitride

It is shown that the bombardment of high quality cubic (c-) BN films with 300 keV Ar+ ions leads to a strong relaxation of their compressive stresses without destroying the cubic phase if the total ion fluence is kept below an upper limit. In addition, it was found that on top of such a stress-relieved film a further pure c-BN layer can be grown, but it builds up compressive stress again. Based on both results, a procedure is developed to grow thick (>1 μm) c-BN films (>80% c-BN) exhibiting low residual stress and long term stability under ambient conditions.

Lowering of the L10 ordering temperature of FePt nanoparticles by He+ ion irradiation

Arrays of FePt particles (diameter 7nm) with mean interparticle distances of 60nm are prepared by a micellar technique on Si substrates. The phase transition of these magnetic particles towards the chemically ordered L10 phase is tracked for 350kV He+ ion irradiated samples and compared to a nonirradiated reference. Due to the large separation of the magnetically decoupled particles the array can be safely annealed without any agglomeration as usually observed for more densely packed colloidal FePt nanoparticles. The He+ ion exposure yields a significant reduction of the ordering temperature by more than 100K.

An electron beam evaporated TiO2 layer for high efficiency planar perovskite solar cells on flexible polyethylene terephthalate substrates

The TiO2 layer made by electron beam (e-beam) induced evaporation is demonstrated as an electron transport layer (ETL) in high efficiency planar junction perovskite solar cells. The temperature of the substrate and the thickness of the TiO2 layer can be easily controlled with this e-beam induced evaporation method, which enables the usage of different types of substrates. Here, perovskite solar cells based on CH3NH3PbI3−xClx achieve power conversion efficiencies of 14.6% on glass and 13.5% on flexible plastic substrates. The relationship between the TiO2 layer thickness and the perovskite morphology is studied by scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). Our results indicate that the pinholes in the thin TiO2 layer lead to pinholes in the perovskite layer. By optimizing the TiO2 thickness, perovskite layers with substantially increased surface coverage and reduced pinhole areas are fabricated, increasing overall device performance.

Chemically Induced Metal-to-Insulator Transition in Au55 Clusters

The cluster compound Au55(PPh3)12Cl6 has been reanalyzed by photoelectron spectroscopy giving direct evidence for a nonmetallic behavior of the individual Au clusters as long as their ligand shell remains intact. The exposure to x-rays during the measurements is found to partly decompose the shell by removal of the chlorine atoms, resulting in a metallic behavior of the clusters as demonstrated by a steplike intensity at the Fermi energy. These observations resolve a long-standing controversy about the metallic behavior of ligated Au clusters emphasizing, in addition, the influence of the local environment on the electronic properties of nanoscaled materials.

A Universal Deposition Protocol for Planar Heterojunction Solar Cells with High Efficiency Based on Hybrid Lead Halide Perovskite Families

A robust and expedient gas quenching method is developed for the solution deposition of hybrid perovskite thin films. The method offers a reliable standard practice for the fabrication of a non-exhaustive variety of perovskites exhibiting excellent film morphology and commensurate high performance in both regular and inverted structured solar cell architectures.

A Molecular Toolkit for the Functionalization of Titanium‐Based Biomaterials That Selectively Control Integrin‐Mediated Cell Adhesion

We present a click chemistry-based molecular toolkit for the biofunctionalization of materials to selectively control integrin-mediated cell adhesion. To this end, α5β1-selective RGD peptidomimetics were covalently immobilized on Ti-based materials, and the capacity to promote the selective binding of α5β1 was evaluated using a solid-phase integrin binding assay. This functionalization strategy yielded surfaces with a nine-fold increased affinity for α5β1, in comparison to control samples, and total selectivity against the binding of the closely related integrin αvβ3. Moreover, our methodology allowed the screening of several phosphonic acid containing anchoring units to find the best spacer-anchor moiety required for establishing an efficient binding to titanium and to promote selective integrin binding. The integrin subtype specificity of these biofunctionalized surfaces was further examined in vitro by inducing selective adhesion of genetically modified fibroblasts, which express exclusively the α5β1 integrin. The versatility of our molecular toolkit was proven by shifting the cellular specificity of the materials from α5β1- to αvβ3-expressing fibroblasts by using an αvβ3-selective peptidomimetic as coating molecule. The results shown here represent the first functionalization of Ti-based materials with α5β1- or αvβ3-selective peptidomimetics that allow an unprecedented control to discriminate between α5β1- and αvβ3-mediated adhesions. The role of these two integrins in different biological events is still a matter of debate and is frequently discussed in literature. Thus, such bioactive titanium surfaces will be of great relevance for the study of integrin-mediated cell adhesion and the development of new biomaterials targeting specific cell types.