Environmental Control of Triplet Emission in Donor-Bridge-Acceptor Organometallics

Carbene-metal-amides (CMAs) are a promising family of donor-bridge-acceptor molecular charge-transfer emitters for organic light-emitting diodes (OLEDs). Here a universal approach is introduced to tune the energy of their charge-transfer emission. A shift of up to 210 meV is achievable in the solid state via dilution in a polar host matrix. The origin of this shift has two components: constraint of thermally activated triplet diffusion, and electrostatic interactions between the guest molecules and the polar host. This allows the emission of mid-green CMA archetypes to be blue shifted without chemical modifications. Monte-Carlo simulations based on a Marcus-type transfer integral successfully reproduce the concentration- and temperature-dependent triplet diffusion process, and reveal a substantial shift in the ensemble density of states in polar hosts. In gold-bridged CMAs this substantial shift does not lead to a significant change in luminescence lifetime, thermal activation energy, reorganisation energy or intersystem crossing rate. These discoveries thus offer new experimental and theoretical insight in to the coupling between the singlet and triplet manifolds in these materials. Similar emission tuning can be achieved in related materials where chemical modification is used to modify the charge-transfer energy

Charge Dynamics in Solution-Processed Nanocrystalline CuInS2 Solar Cells.

We investigate charge dynamics in solar cells constructed using solution-processed layers of CuInS2 (CIS) nanocrystals (NCs) as the electron donor and CdS as the electron acceptor. By using time-resolved spectroscopic techniques, we are able to observe photoinduced absorptions that we attribute to the mobile hole carriers in the NC film. In combination with transient photocurrent and photovoltage measurements, we monitor charge dynamics on time scales from 300 fs to 1 ms. Carrier dynamics are investigated for devices with CIS layers composed of either colloidally synthesized 1,3-benzenedithiol-capped nanocrystals or in situ sol-gel synthesized thin films as the active layer. We find that deep trapping of holes in the colloidal NC cells is responsible for decreases in the open-circuit voltage and fill factor as compared to those of the sol-gel synthesized CIS/CdS cell.We gratefully acknowledge support by EPSRC (Grant No. EP/G060738/1), Cambridge Display Technology and the Worshipful Company of Armourers & Brasiers for a Gauntlet Trust award, as well as the MacDiarmid Institute for use of their EM facilities.This is the final version of the article. It first appeared from ACS via http://dx.doi.org/10.1021/acsnano.5b0043

Design of OsII-based Sensitizers for Dye-Sensitized Solar Cells:Influence of Heterocyclic Ancillaries

A series of OsII sensitizers (TFOS-x, in which x=1, 2, or 3) with a single 4,4′-dicarboxy-2,2′-dipyridine (H2dcbpy) anchor and two chelating 2-pyridyl (or 2-pyrimidyl) triazolate ancillaries was successfully prepared. Single-crystal X-ray structural analysis showed that the core geometry of the OsII-based sensitizers consisted of one H2dcbpy unit and two eclipsed cis-triazolate fragments; this was notably different from the RuII-based counterparts, in which the azolate (both pyrazolate and triazolate) fragments are located at the mutual trans-positions. The basic properties were extensively probed by using spectroscopic and electrochemical methods as well as time-dependent density functional theory (TD-DFT) calculations. Fabrication of dye-sensitized solar cells (DSCs) was then attempted by using the I−/I3−-based electrolyte solution. One such DSC device, which utilized TFOS-2 as the sensitizer, showed promising performance characteristics with a short-circuit current density (JSC) of 15.7 mA cm−2, an open-circuit voltage of 610 mV, a fill factor of 0.63, and a power conversion efficiency of 6.08 % under AM 1.5G simulated one-sun irradiation. Importantly, adequate incident photon-to-current conversion efficiency performances were observed for all TFOS derivatives over the wide spectral region of 450 to 950 nm, showing a panchromatic light harvesting capability that extended into the near-infrared regime. Our results underlined a feasible strategy for maximizing JSC and increasing the efficiency of DSCs

Highly photoluminescent copper carbene complexes based on prompt rather than delayed fluorescence

Linear two-coordinate copper complexes of cyclic (alkyl)(amino)-carbenes (CAAC)CuX (X = halide) show photoluminescence with solid-state quantum yields of up to 96%; in contrast to previously reported Cu photoemitters the emission is independent of temperature over the range T = 4 – 300 K and occurs very efficiently by prompt rather than delayed fluorescence, with lifetimes in the sub-nanosecond range

Enhanced performance in fluorene-free organometal halide perovskite light-emitting diodes using tunable, low electron affinity oxide electron injectors.

Fluorene-free perovskite light-emitting diodes (LEDs) with low turn-on voltages, higher luminance and sharp, color-pure electroluminescence are obtained by replacing the F8 electron injector with ZnO, which is directly deposited onto the CH3NH3PbBr3 perovskite using spatial atmospheric atomic layer deposition. The electron injection barrier can also be reduced by decreasing the ZnO electron affinity through Mg incorporation, leading to lower turn-on voltages.The authors would like to acknowledge funding from the Cambridge Commonwealth, European and International Trusts, Rutherford Foundation of New Zealand, A*STAR National Science Scholarship, Girton College Cambridge, Gates Cambridge Scholarship, EPSRC (Reference: EP/G060738/1), the ERC Advanced Investigator Grant, Novox, ERC-2009-adG 247276 and Cambridge Display Technology.This is the final version of the article. It was first published by Wiley at http://onlinelibrary.wiley.com/doi/10.1002/adma.201405044/abstract

Nano-scale lithography and microscopy of organic semiconductors

The development of organic electronic and photonic devices increasingly requires the development of micro- and nano-structured morphologies, which in turn require the development of both prototyping and scalable patterning methods. This thesis presents investigations which explore and develop unconventional patterning techniques for a variety of conjugated polymers and organic molecules, using scanning near-field optical lithography (SNOL), scanning thermal lithography (SThL) and molecular self-assembly. Optimised formation of organic nanostructures is demonstrated, at resolutions which equal or better the current state of the art, with patterning resolution for isolated structures below 60nm for SNOL and 30nm for SThL. SThL in particular is demonstrated as a technique which can achieve serial write-speeds of over 100 μm/s, with significant potential for up-scaling. Furthermore, arbitrarily defined two-dimensional large-area nanostructures up to 20 × 20 μm are demonstrated using SNOL while maintaining both high resolution and the integrity of the probe. The nanostructures fabricated in the course of this work, and others, are characterised using both optical and topographic techniques, primarily atomic force microscopy and near-field microscopy. The detailed formation mechanisms for structures fabricated using SNOL via an in-situ conversion route are systematically investigated and contrasted with other formation routes, resulting in a comprehensive account of the factors affecting structure morphology. In addition, the optimised nanostructures achieved in this work are shown, within this context, to be very close to best achievable with an apertured scanning near-field system

Vertical stratification and its impact on device performance in a polycarbazole based copolymer solar cells

Using neutron-reflectivity, we study vertical stratification and device performance in bulk hetero-junction organic photovoltaic (OPV) cells consisting of a blend of PC71BM with a carbazole-based donor–acceptor copolymer PCDTBT1. We find that when the blend is cast on a PEDOT:PSS/ITO anode, a PC71BM-depleted (polymer-rich) layer is formed at the PEDOT:PSS interface, whilst a PC71BM-depleted layer is instead located at the air-interface when the same blend is cast on a solution processed MoOx thin film. OPV device characterization measurements indicate that unfavourable vertical segregation can have a profound effect on OPV device characteristics via increased charge recombination

Encapsulation of methylammonium lead bromide perovskite in nanoporous GaN

Halide perovskites hold exceptional promise as cheap, low temperature solution-processed optoelectronic materials. Yet they are hindered by poor structural and chemical stability, rapidly degrading when exposed to moisture or air. We demonstrate a solution-phase method for infiltrating methylammonium lead bromide perovskite (CH3NH3PbBr3, or MAPbBr3) into nanoporous GaN which preserved the green photoluminescence of the perovskite after up to 1 year of storage under ambient conditions. Besides a protective effect, confinement within the porous GaN matrix also resulted in a blueshift of the perovskite emission with decreasing pore size, suggesting an additional templating effect of the pores on the size of the perovskite crystals within. We anticipate that our method may be generalised to related perovskite materials, offering a route to producing composites of interest for use in optoelectronic devices for various applications