Correlating the Polymorphism of Titanyl Phthalocyanine Thin Films with Solar Cell Performance

The structure of titanyl phthalocyanine (TiOPc) thin films is correlated with photovoltaic properties of planar heterojunction solar cells by pairing different TiOPc polymorph donor layers with C₆₀ as an acceptor. Solvent annealing and the insertion of two different templating layers, namely 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) and CuI, prove to be effective methods to control the TiOPc thin film structure. The crystal phase of TiOPc thin films was identified by combining X-ray reflectivity (XRR) measurements with spectroscopic techniques, including absorption and micro-Raman measurements. Implementation of a donor layer with an absorption spectrum extending into the near-infrared (NIR) led to solar cells with external quantum efficiencies (EQEs) above 27% from λ = 600 – 890 nm, with the best device yielding a power conversion efficiency (PCE) of 2.6%. Our results highlight the need to understand the relationship between processing parameters and thin film structure, as these have important consequences on device performance

Mesoporous Oxide-Diluted Magnetic Semiconductors Prepared by Co Implantation in Nanocast 3D-Ordered In₂O_3–<i>y</i> Materials

Mesoporous In₂O_3–<i>y</i> materials have been implanted using Co ions to induce a moderate ferromagnetic response at room temperature, forming a “mesoporous oxide-diluted magnetic semiconductor” (MODMS). X-ray photoemission spectroscopy (XPS) reveals that implantation results in up to 1 at. % Co (for 6 × 10¹⁵ ions/cm² at 40 keV) and 15 at. % Co (for 1 × 10¹⁷ ions/cm² at 60 keV). This is in both cases accompanied by a pronounced increase in the amount of oxygen vacancies with respect to the pristine, nonimplanted, In₂O_3–<i>y</i>. Further increase in the ion fluence (up to 2 × 10¹⁷ ions/cm² at 60 keV) results in the collapse of the mesoporous structure, i.e., loss of the 3D-ordered porous configuration. XPS also reveals that virtually no metallic Co is formed at 40 keV, while a mixture of Co²⁺ and Co⁰ states is detected after implantation at 60 keV. Most of the Co²⁺ is incorporated in the bixbyite structure of the In₂O_3–<i>y</i> matrix. These results are consistent with previous models suggesting that the origin of the obtained ferromagnetic response in oxide-diluted magnetic semiconductors can be ascribed to ferromagnetic exchange interactions mediated by oxygen vacancies. This work constitutes the first report on MODMS prepared by nanocasting followed by implantation of transition metal ions

Correlation of Interface Impurities and Chemical Gradients with High Magnetoelectric Coupling Strength in Multiferroic BiFeO₃–BaTiO₃ Superlattices

The detailed understanding of magnetoelectric (ME) coupling in multiferroic oxide heterostructures is still a challenge. In particular, very little is known to date concerning the impact of the chemical interface structure and unwanted impurities that may be buried within short-period multiferroic BiFeO₃–BaTiO₃ superlattices during growth. Here, we demonstrate how trace impurities and elemental concentration gradients contribute to high ME voltage coefficients in thin-film superlattices, which are built from 15 double layers of BiFeO₃–BaTiO₃. Surprisingly, the highest ME voltage coefficient of 55 V cm^–1 Oe^–1 at 300 K was measured for a superlattice with a few atomic percent of Ba and Ti that diffused into the nominally 5 nm thin BiFeO₃ layers, according to analytical transmission electron microscopy. In addition, highly sensitive enhancements of the cation signals were observed in depth profiles by secondary ion mass spectrometry at the interfaces of BaTiO₃ and BiFeO₃. As these interface features correlate with the ME performance of the samples, they point to the importance of charge effects at the interfaces, that is, to a possible charge mediation of ME coupling in oxide superlattices. The challenge is to provide cleaner materials and processes, as well as a well-defined control of the chemical interface structure, to push forward the application of oxide superlattices in multiferroic ME devices

Improving the Magnetic Properties of Co–CoO Systems by Designed Oxygen Implantation Profiles

Oxygen implantation in ferromagnetic Co thin films is shown to be an advantageous route to improving the magnetic properties of Co–CoO systems by forming multiple nanoscaled ferromagnetic/antiferromagnetic interfaces homogeneously distributed throughout the layer. By properly designing the implantation conditions (energy and fluence) and the structure of the films (capping, buffer, and Co layer thickness), relatively uniform O profiles across the Co layer can be achieved using a single-energy ion implantation approach. This optimized configuration results in enhanced exchange bias loop shifts, improved loop homogeneity, increased blocking temperature, reduced relative training effects and increased retained remanence in the trained state with respect to both Co/CoO bilayers and O-implanted Co films with a Gaussian-like O depth profile. This underlines the great potential of ion implantation to tailor the magnetic properties by controllably modifying the local microstructure through tailored implantation profiles