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Despite their outstanding photovoltaic performance, organic-inorganic perovskite solar cells still face severe stability issues and limitations in their device dimension. Further development of perovskite solar cells therefore requires a deeper understanding of loss mechanisms, in particular, concerning the origin and impact of trap states. Here, different surface properties of submicrometer sized CH3NH3PbI3 particles are studied as a model system by photoluminescence spectroscopy to investigate the impact of the perovskite crystal surface on photoexcited states. Comparison of single crystals with either isolating or electron-rich surface passivation indicates the presence of positively charged surface trap states that can be passivated in case of the latter. These surface trap states cause enhanced nonradiative recombination at room temperature, which is a loss mechanism for solar cell performance. In the orthorhombic phase, the origin of multiple emission peaks is identified as the recombination of free and bound excitons, whose population ratio critically depends on trap state properties. The dynamics of exciton trapping at 50 K are observed on a time-scale of tens of picoseconds by a simultaneous population decrease and increase of free and bound excitons, respectively. These results emphasize the potential of surface passivation to further improve the performance of perovskite solar cells. [ABSTRACT FROM AUTHOR]

The supreme aim of nanoparticle-based materials is to achieve new properties extending over the features of individual constituents. The emergence of cooperativity necessitates precise positioning and orientation of nanoparticle ensembles. Thus, it is important to understand and learn how to control self-assembly processes of nanoparticles. Besides shape, the structural uniformity plays a key role for ordering in superstructures. Therefore, it is challenging to synthesize nanorods with narrow polydispersity. An analysis of the systematic variation of aspect ratio and polydispersity is missing. A series of zinc oxide nanorods is presented and it is shown that their formation resembles step-polymerization with an amorphous precursor state as a monomer and polar ZnO particles as entities capable of growing. The width of nanorods is kept constant (15 nm) and the length is varied between 20 and 100 nm, as well as improving the polydispersity of the nanorod length from 36% to 10%. Best samples have been achieved by post-preparative treatment using gradient centrifugation. A method has been developed for semiquantitative evaluation of orientational order. Ordering in structures formed by quasispherical particles is always low despite low polydispersity. For rod-like nanoparticles with increasing aspect ratio, superstructure order depends on the occurrence of different defects, which correlate differently to nanoparticle polydispersity. [ABSTRACT FROM AUTHOR]

Signifikante Fortschritte in der Solarzellforschung werden aktuell von der Substanzklasse der metallorganischen Perowskite (MOPs) dominiert. Ursächlich dafür sind herausragende Eigenschaften dieser Materialien, wie hohe Absorptionskoeffzienten und die Fähigkeit zur effektiven Trennung photogenerierter Ladungsträger. In Analogie zu etablierteren Halbleitern kann erwartet werden, dass die Materialeigenschaften auch der MOPs durch eine definierte Nanostrukturierung stark verbessert werden können. Ihre chemische Anfälligkeit (z. B. gegen Hydrolyse) erschwert aber die direkte Übertragung gängiger Verfahren zur Herstellung von Nanomaterialien. Wir berichten über einen originären Ansatz zur Synthese verschiedener (CH3NH3)PbI3 ‐ Nanostrukturen unter Verwendung einer neuartigen Single ‐ Source ‐ Vorstufe. Nanoporöse MOP ‐ Einkristalle können durch eine Kristall ‐ Kristall ‐ Umwandlung erhalten werden, die mit einer spinodalen Mikrophasenseparation der Triethylenglykol ‐ haltigen Vorstufenverbindung einhergeht. Die Steuerung der Nanopartikelform gelingt durch Einsatz eines selektiv bindenden Kristallisationsinhibitors. Eine Kristall ‐ Kristall ‐ Umwandlung lieferte aus einer kettenförmigen Vorstufe den Perowskit ‐ Halbleiter CH3NH3PbI3 (siehe Bild). Der dabei auftretende Mechanismus der spinodalen Mikrophasentrennung kann genutzt werden, um zusätzlich Porosität zu erzeugen. Die Form der Perowskit ‐ Nanokristalle ließ sich mit einem Kristallisationsinhibitor steuern. [ABSTRACT FROM AUTHOR]

Significant progress in solar-cell research is currently made by the development of metal-organic perovskites (MOPs) owing to their superior properties, such as high absorption coefficients and effective transport of photogenerated charges. As for other semiconductors, it is expected that the properties of MOPs may be significantly improved by a defined nanostructure. However, their chemical sensitivity (e.g., towards hydrolysis) prohibits the application of methods already known for the synthesis of other nanomaterials. A new and general method for the synthesis of various (CH3NH3)PbI3 nanostructures from a novel single-source precursor is presented. Nanoporous MOP single crystals are obtained by a crystal-to-crystal transformation that is accompanied by spinodal demixing of the triethylene glycol containing precursor structure. Selective binding of a capping agent can be used to tune the particle shape of the MOP nanocrystals. [ABSTRACT FROM AUTHOR]

Significant progress in solar-cell research is currently made by the development of metal-organic perovskites (MOPs) owing to their superior properties, such as high absorption coefficients and effective transport of photogenerated charges. As for other semiconductors, it is expected that the properties of MOPs may be significantly improved by a defined nanostructure. However, their chemical sensitivity (e.g., towards hydrolysis) prohibits the application of methods already known for the synthesis of other nanomaterials. A new and general method for the synthesis of various (CH3NH3)PbI3 nanostructures from a novel single-source precursor is presented. Nanoporous MOP single crystals are obtained by a crystal-to-crystal transformation that is accompanied by spinodal demixing of the triethylene glycol containing precursor structure. Selective binding of a capping agent can be used to tune the particle shape of the MOP nanocrystals., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)