Pressure-induced delocalization of photoexcited states in a semiconducting polymer.

We present broadband transient absorption spectroscopy on the fluorescent copolymer poly(9,9-dioctylfluorene-co-benzothiadiazole) under hydrostatic pressure of up to 75 kbar. We observe a strong reduction of the stimulated emission intensity under pressure, coupled with slower decay kinetics and reduced fluorescence intensity. These observations indicate increased delocalization of photogenerated singlet excitons, facilitated by an increased dielectric constant at high pressure. Spin triplet excitons, generated via an iridium complex-F8BT oligomer, show reduced lifetimes under pressure

Forecasting with dimension switching VARs

This paper develops methods for VAR forecasting when the researcher is uncertain about which variables enter the VAR, and the dimension of the VAR may be changing over time. It considers the case where there are N variables which might potentially enter a VAR and the researcher is interested in forecasting N ∗ of them. Thus, the researcher is faced with 2 N − N ∗ potential VARs. If N is large, conventional Bayesian methods can be infeasible due to the computational burden of dealing with a huge model space. Allowing for the dimension of the VAR to change over time only increases this burden. In light of these considerations, this paper uses computationally practical approximations adapted from the dynamic model averaging literature in order to develop methods for dynamic dimension selection (DDS) in VARs. We then show the benefits of DDS in a macroeconomic forecasting application. In particular, DDS switches between different parsimonious VARs and forecasts appreciably better than various small and large dimensional VARs

Spin signatures of exchange-coupled triplet pairs formed by singlet fission

We study the effect of an exchange interaction on the magnetic-field-dependent photoluminescence in singlet fission materials. We show that, for strongly interacting triplet exciton pairs (intertriplet exchange interaction greater than the intratriplet spin-dipolar interaction), quantum beating and magnetic-field effects vanish apart from at specific magnetic fields where singlet and quintet levels are mixed by a level anticrossing. We characterize these effects and show that the absence of a magnetic-field effect or zero-field quantum beats does not necessarily mean that fission is inoperative. These results call for a reconsideration of the observations that are considered hallmarks of singlet fission and demonstrate how the spin coherence and exchange coupling of interacting triplet pairs can be measured through magneto-photoluminescence experiments.Engineering and Physical Sciences Research Council (Grant ID: EP/G060738/1)This is the author accepted manuscript. The final version is available from the American Physical Society via http://dx.doi.org/10.1103/PhysRevB.94.04520

New Strategies for Defect Passivation in High-Efficiency Perovskite Solar Cells

Lead halide perovskite solar cells now show excellent efficiencies and encouraging levels of stability. Further improvements in performance require better control of the trap states which are considered to be associated with vacancies and defects at crystallite surfaces. Herein, a reflection on the ways in which these traps can be mitigated is presented by improving the quality of the perovskite layer and interfaces in fully assembled device configurations. In this review, the most recent design strategies reported in the literature, which have been explored to tune grain orientation, to passivate defects, and to improve charge-carrier lifetimes, are presented. Specifically, the advances made with single-cation, mixed-cation and/or mixed-halide, and 3D/2D bilayer-based light absorbers are discussed. The interfacial, compositional, and band alignment engineering along with their consequent effects on the open-circuit voltage, power conversion efficiency, and stability are a particular focus

Electroluminescence from Organometallic Lead Halide Perovskite-Conjugated Polymer Diodes

Organometallic lead perovskite-based solar cells can be converted to light-emitting diodes by engineering the current density. Diodes are fabricated with adjacent perovskite and conjugated polymer layers using orthogonal solvents. Under forward bias, these devices show simultaneous emission from both the luminescent conjugated polymer and the perovskite, providing direct information on electron and hole recombination as a function of device architecture and bias voltage.We gratefully acknowledge funding from the Engineering and Physical Sciences Research Council (EPSRC). A.K. acknowledges NRF-Singapore for a scholarship.This is the final version of the article. It first appeared from Wiley via http://dx.doi.org/10.1002/aelm.20150000

A facile low temperature route to deposit a TiO<inf>2</inf> scattering layer for efficient dye-sensitized solar cells

Hydrolysis of TiCl4 at low temperature formed an efficient scattering layer in dye-sensitized solar cell architecture, which leads to an improvement in the light harvesting and a remarkable reduction of electronic disorder of mesoporous-TiO2.Nava Technology Limited, Iran Nanotechnology Initiative Council, Nyak Technology Limited, Engineering and Physical Sciences Research CouncilThis is the author accepted manuscript. The final version is available from Royal Society of Chemistry via http://dx.doi.org/10.1039/C6RA13273

Kinetic Control of Perovskite Thin-Film Morphology and Application in Printable Light-Emitting Diodes

Highly luminescent methylammonium lead bromide (CH₃NH₃PbBr₃) perovskite fibrous microstructure thin films have been fabricated using a perylene-3,4,9,10-tetracarboxylic dianhydride-containing antisolvent-mediated single-step fabrication method. Confocal microscopy of these thin films reveals homogeneous emission over the entire area, in contrast to the localized emission from the thin films fabricated by conventional methods. The antisolvent treatment produces pinhole-free, mazelike morphology with high photoluminescence yield. These films were incorporated as the emissive layer in thin-film light emitting diodes (LEDs), made using a low-temperature meltable and processable alloy as cathode. These LEDs gave luminescence efficiency of up to 50 Cd/m². The proposed LED structure highlights the prospects of using low-temperature roll-to-roll processing for manufacturing perovskite-based solar cells and LEDs.We acknowledge UK-India Solar Energy Project; Newton-APEX-II; Engineering and Physical Research Council, United Kingdom; and Department of Science and Technology, India for the funds. B.Z. acknowledges support from Cambridge Trust and China Scholarship Council. K.S.N. and P.K. acknowledge Dr. Subi Jacob George and Mr. Suman Kuila, NCU, JNCASR, Bangalore for helping with the TCSPC measurements

Impact of exciton delocalization on exciton-vibration interactions in organic semiconductors

Organic semiconductors exhibit properties of individual molecules and extended crystals simultaneously. The strongly bound excitons they host are typically described in the molecular limit, but excitons can delocalize over many molecules, raising the question of how important the extended crystalline nature is. Using accurate Green's function based methods for the electronic structure and non-perturbative finite difference methods for exciton-vibration coupling, we describe exciton interactions with molecular and crystal degrees of freedom concurrently. We find that the degree of exciton delocalization controls these interactions, with thermally activated crystal phonons predominantly coupling to delocalized states, and molecular quantum fluctuations predominantly coupling to localized states. Based on this picture, we quantitatively predict and interpret the temperature and pressure dependence of excitonic peaks in the acene series of organic semiconductors, which we confirm experimentally, and we develop a simple experimental protocol for probing exciton delocalization. Overall, we provide a unified picture of exciton delocalization and vibrational effects in organic semiconductors, reconciling the complementary views of finite molecular clusters and periodic molecular solids

Graphene-passivated nickel as an efficient hole-injecting electrode for large area organic semiconductor devices

Efficient injection of charge from metal electrodes into semiconductors is of paramount importance to obtain high performance optoelectronic devices. The quality of the interface between the electrode and the semiconductor must, therefore, be carefully controlled. The case of organic semiconductors presents specific problems: ambient deposition techniques, such as solution processing, restrict the choice of electrodes to those not prone to oxidation, limiting potential applications. Additionally, damage to the semiconductor in sputter coating or high temperature thermal evaporation poses an obstacle to the use of many device-relevant metals as top electrodes in vertical metal–semiconductor–metal structures, making it preferable to use them as bottom electrodes. Here, we propose a possible solution to these problems by implementing graphene-passivated nickel as an air stable bottom electrode in vertical devices comprising organic semiconductors. We use these passivated layers as hole-injecting bottom electrodes, and we show that efficient charge injection can be achieved into standard organic semiconducting polymers, owing to an oxide free nickel/graphene/polymer interface. Crucially, we fabricate our electrodes with low roughness, which, in turn, allows us to produce large area devices (of the order of millimeter squares) without electrical shorts occurring. Our results make these graphene-passivated ferromagnetic electrodes a promising approach for large area organic optoelectronic and spintronic devices.We acknowledge funding from EPSRC (EP/P005152/1, EP/M005143/1). R.M. and K.N. acknowledges funding from the EPSRC Cambridge NanoDTC (Grant No. EP/G037221/1). J.A.-W. acknowledges the support of his Research Fellowship from the Royal Commission for the Exhibition of 1851, and Royal Society Dorothy Hodgkin Research Fellowship. R. S. W. acknowledges support from a CAMS-UK fellowship