33 research outputs found

    Magnetic Field Modulation of Recombination Processes in Organic Photovoltaics

    Get PDF
    Polymer:fullerene photovoltaics have potential in small-scale power production but low open-circuit voltages limit their efficiency. Understanding the processes affecting the charge recombination rate is key to increasing device efficiency through optimizing open-circuit voltage. Most polymer-fullerene systems have an intramolecular triplet exciton state lower in energy than the interfacial charge-transfer state, and its formation can provide a terminal recombination pathway that may limit device performance. We used magnetic fields to modulate intersystem crossing in a prototypical system, and monitored the effect on the open-circuit voltage to infer changes in the steady-state carrier density and hence in the net recombination rate constant. We analyzed these effects using density-matrix modeling, and quantified the various recombination rate constants for a working device

    Formation of Long-Lived Color Centers for Broadband Visible Light Emission in Low-Dimensional Layered Perovskites.

    Get PDF
    We investigate the origin of the broadband visible emission in layered hybrid lead-halide perovskites and its connection with structural and photophysical properties. We study ⟨001⟩ oriented thin films of hexylammonium (HA) lead iodide, (C6H16N)2PbI4, and dodecylammonium (DA) lead iodide, (C12H28N)2PbI4, by combining first-principles simulations with time-resolved photoluminescence, steady-state absorption and X-ray diffraction measurements on cooling from 300 to 4 K. Ultrafast transient absorption and photoluminescence measurements are used to track the formation and recombination of emissive states. In addition to the excitonic photoluminescence near the absorption edge, we find a red-shifted, broadband (full-width at half-maximum of about 0.4 eV), emission band below 200 K, similar to emission from ⟨110⟩ oriented bromide 2D perovskites at room temperature. The lifetime of this sub-band-gap emission exceeds that of the excitonic transition by orders of magnitude. We use X-ray diffraction measurements to study the changes in crystal lattice with temperature. We report changes in the octahedral tilt and lattice spacing in both materials, together with a phase change around 200 K in DA2PbI4. DFT simulations of the HA2PbI4 crystal structure indicate that the low-energy emission is due to interstitial iodide and related Frenkel defects. Our results demonstrate that white-light emission is not limited to ⟨110⟩ oriented bromide 2D perovskites but a general property of this class of system, and highlight the importance of defect control for the formation of low-energy emissive sites, which can provide a pathway to design tailored white-light emitters

    On white-collar boxing and social class

    Get PDF
    This article is based on the first sociological research of white-collar boxing in the UK. Grounded in an ethnography of a boxing gym in the Midlands, the article argues that the term ‘white-collar boxing’ in this context is immediately misleading, and entails the term being used in a way with which sociologists are unaccustomed. Whereas white-collar boxing originated in the context of post-industrial New York City as a pastime only for the extremely wealthy, the situation in the UK is different. Participants actively reject this understanding of white-collar boxing. The term white-collar boxing does not signify the social class of participants, but refers to their novice status. Given that boxing is an example through which Bourdieu’s theory of distinction is discussed, and that white-collar boxing is a distinctly late-modern version of the sport containing an erroneous class signifier, this version of the sport is a site through which such discussions of consumption can be furthered. Whilst consumed by actors in various class positions, a logic of distinction is present in white-collar boxing, which becomes recognisable through analysis of the ‘plurality of consumption experiences’. This is proffered as a concept which can aid in the analysis of consumption beyond white-collar boxing

    Maximizing and stabilizing luminescence from halide perovskites with potassium passivation

    Get PDF
    Metal halide perovskites are of great interest for various high-performance optoelectronic applications. The ability to tune the perovskite bandgap continuously by modifying the chemical composition opens up applications for perovskites as coloured emitters, in building-integrated photovoltaics, and as components of tandem photovoltaics to increase the power conversion efficiency. Nevertheless, performance is limited by non-radiative losses, with luminescence yields in state-of-the-art perovskite solar cells still far from 100 per cent under standard solar illumination conditions. Furthermore, in mixed halide perovskite systems designed for continuous bandgap tunability2 (bandgaps of approximately 1.7 to 1.9 electronvolts), photoinduced ion segregation leads to bandgap instabilities. Here we demonstrate substantial mitigation of both non-radiative losses and photoinduced ion migration in perovskite films and interfaces by decorating the surfaces and grain boundaries with passivating potassium halide layers. We demonstrate external photoluminescence quantum yields of 66 per cent, which translate to internal yields that exceed 95 per cent. The high luminescence yields are achieved while maintaining high mobilities of more than 40 square centimetres per volt per second, providing the elusive combination of both high luminescence and excellent charge transport. When interfaced with electrodes in a solar cell device stack, the external luminescence yield—a quantity that must be maximized to obtain high efficiency—remains as high as 15 per cent, indicating very clean interfaces. We also demonstrate the inhibition of transient photoinduced ion-migration processes across a wide range of mixed halide perovskite bandgaps in materials that exhibit bandgap instabilities when unpassivated. We validate these results in fully operating solar cells. Our work represents an important advance in the construction of tunable metal halide perovskite films and interfaces that can approach the efficiency limits in tandem solar cells, coloured-light-emitting diodes and other optoelectronic applications.M.A.-J. thanks Nava Technology Limited and Nyak Technology Limited for their funding and technical support. Z.A.-G. acknowledges funding from a Winton Studentship, and ICON Studentship from the Lloyd’s Register Foundation. This project has received funding from the European Union’s Seventh Framework Programme (FP7/2007-2013) under REA grant agreement number PIOF-GA-2013-622630, the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement number 756962), and the Royal Society and Tata Group (UF150033). We thank the Engineering and Physical Sciences Research Council (EPSRC) for support. XMaS is a mid-range facility at the European Synchrotron Radiation Facility supported by the EPSRC and we are grateful to the XMaS beamline team staff for their support. We thank Diamond Light Source for access to beamline I09 and staff member T.-L. Lee as well as U. Cappel for assistance during the HAXPES measurements. S.C., C.D. and G.D. acknowledge funding from the ERC under grant number 25961976 PHOTO EM and financial support from the European Union under grant number 77 312483 ESTEEM2. M.A. thanks the president of the UAE’s Distinguished Student Scholarship Program, granted by the Ministry of Presidential Affairs. H.R. and B.P. acknowledge support from the Swedish research council (2014-6019) and the Swedish foundation for strategic research. E.M.H. and T.J.S. were supported by the Netherlands Organization for Scientific Research under the Echo grant number 712.014.007

    Magnetic field modulation of recombination processes in organic photovoltaics

    No full text
    Polymer-Fullerene photovoltaics have potential in small-scale power production, but low open-circuit voltages limit their efficiency. Understanding the processes affecting the charge recombination rate is key to increasing device efficiency through optimizing open-circuit voltage. Most of the polymer-fullerene systems have an intramolecular triplet exciton state lower in energy than the interfacial charge-transfer state, and its formation can provide a terminal recombination pathway that may limit device performance. We used magnetic fields to modulate intersystem crossing in a prototypical system and monitored the effect on the open-circuit voltage to infer changes in the steady-state carrier density and hence in the net recombination rate constant. We analyzed these effects using density matrix modeling and quantified the various recombination rate constants for a working device
    corecore