From colossal magnetoresistance to solar cells: An overview on 66 years of research into perovskites

© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Perovskites are a huge family of compounds to which the natural titanium mineral CaTiO3 is the common ancestor. The cubic structure looks apparently simple, but the variety of metal ions and mixtures thereof that fit into a perovskite lattice is tremendous. Even in the case that the ionic radii do not allow for a perfect cubic ordering, there are various superstructures and orbital-ordering effects to cope elegantly with distortions. The compositional and structural flexibility offers a large toolbox to design and synthesize perovskites with tailored properties searched for by physicists, chemists, materials scientists and device engineers. These materials are equally of interest for fundamental studies and for applied research while both viewpoints cross-fertilize each other regularly. Our overview starts with the discovery of ferromagnetism in manganites in 1950 and ranges until 2016: Today, halide perovskites are fully in focus for their potential in photovoltaic applications. This is certainly not an endpoint, but another milestone in a long series of often-unexpected discoveries on an ‘evergreen material’. Ball-and-stick model of the ideal cubic perovskite structure. The cation at the central position or ‘B site’ (small black dot) defines the group name (e.g., titanates) and plays a key role for the physical properties of the material.status: publishe

Branched and linear A2–D–A1–D–A2 isoindigo-based solution-processable small molecules for organic field-effect transistors and solar cells

To establish a correlation between the molecular structure, physicochemical properties, thin film morphology, charge carrier mobility and photovoltaic performance of isoindigo-based electron donor type molecular semiconductors, a series of branched and linear A2–D–A1–D–A2 small molecules (A = acceptor, D = donor) are synthesized. The extended π-conjugated molecular chromophores have an electron-accepting isoindigo core, a bridging oligothiophene electron donor part and terminal octyl cyanoacrylate acceptor moieties. Their photophysical, thermal and electrochemical properties are analysed and the materials are applied in organic field-effect transistors and bulk heterojunction organic solar cells. Compared to an analogous benzothiadiazole-based small molecule, the isoindigo core deepens the HOMO energy level, enabling higher open-circuit voltages in organic solar cells. The linear isoindigo-based small molecule shows an enhanced hole mobility compared to the branched derivatives. The best power conversion efficiency of the investigated set is also obtained for the solar cell based on the linear (CA-3T-IID-3T-CA-l) donor molecule in combination with PC71BM

Tuning of PCDTBT:PC71BM blend nanoparticles for eco-friendly processing of polymer solar cells

© 2016 We report the controlled preparation of water processable nanoparticles (NPs) employing the push-pull polymer PCDTBT and the fullerene acceptor PC71BM in order to enable solar cell processing using eco-friendly solvent (i.e. water). The presented method provides the possibility to separate the formation of the active layer blend and the deposition of the active layer into two different processes. For the first time, the benefits of aqueous processability for the high-potential class of push-pull polymers, generally requiring high boiling solvents, are made accessible. With our method we demonstrate excellent control over the blend stoichiometry and efficient mixing. Furthermore, we provide visualization of the nanomorphology of the different NPs to obtain structural information down to ~2 nm resolution using advanced analytical electron microscopy. The imaging directly reveals very small compositional demixing in the PCDTBT:PC71BM blend NPs, in the size range of about <5 nm, indicating fine mixing at the molecular level. The suitability of the proposed methodology and materials towards the aspects of eco-friendly processing of organic solar cells is demonstrated through a processing of lab scale NPs solar cell prototypes reaching a power conversion efficiency of 1.9%.publisher: Elsevier articletitle: Tuning of PCDTBT:PC71BM blend nanoparticles for eco-friendly processing of polymer solar cells journaltitle: Solar Energy Materials and Solar Cells articlelink: http://dx.doi.org/10.1016/j.solmat.2016.09.008 content_type: article copyright: © 2016 Elsevier B.V. All rights reserved.status: publishe

Designing small molecule organic solar cells with high open-circuit voltage

Three extended 2,5-dithienylthiazolo[5,4-d]thiazole-based small molecule chromophores are prepared via a sustainable direct arylation approach and their physicochemical and opto-electrical material characteristics are analyzed toward integration in solution-processed bulk heterojunction organic photovoltaics. Efficient charge separation and high values of the charge transfer state energy are derived from sensitive ground and excited state absorption and photoluminescence measurements on blends of the thiazolo[5,4-d]thiazole-based electron donor components with the PC71BM fullerene acceptor. Upon implementation in organic solar cells, a maximum power conversion efficiency of 2.7% and particularly high open-circuit voltages (0.93−0.98 V) are observed, which are correlated to the charge transfer state energies as derived from photoluminescence, Fourier transform photocurrent spectroscopy and combined electrochemical and photophysical data. Furthermore, several loss processes at the origin of the modest short-circuit current densities and fill factors are elucidated

Low bandgap copolymers based on monofluorinated isoindigo towards efficient polymer solar cells

To explore the effectiveness of monofluorinated isoindigo as an electron-deficient building block in push-pull conjugated polymers for organic solar cell applications, four low bandgap copolymers are effectively synthesized and characterized. The effects of fluorine introduction, thiophene spacer length and polymer molar mass on the general electro-optical polymer characteristics, thin film blend microstructure and electronic performance are investigated. Isoindigo monofluorination effectively improves the power conversion efficiency from 2.8 up to 5.0% upon molar mass optimization, without using any processing additives or post-treatments

Improved efficiency of polymer-fullerene bulk heterojunction solar cells by the addition of Cu(II)-porphyrin-oligothiophene conjugates

Ternary organic photovoltaic devices were prepared through the addition of small amounts of metalloporphyin-terthiophenes to two established low bandgap polymer:fullerene blends. The methodology afforded a PCDTBT:PC71BM-based device that displayed an initial power conversion efficiency of 5.1%, an increase of 16% when compared to the binary blend. Among the range of metalloporphyrins considered in this study, the Cu(II)-porphyrin derivatives were found to result in the most significant efficiency improvements.clos