LED pumped polymer laser sensor for explosives

The authors would like to acknowledge financial support for this research from the EPSRC HYPIX project (grant number EP/ F059922/1 and EP/F05999X/1), and the TIRAMISU project, funded by the European Commission's Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 284747.A very compact explosive vapor sensor is demonstrated based on a distributed feedback polymer laser pumped by a commercial InGaN light-emitting diode. The laser shows a two-stage turn on of the laser emission, for pulsed drive currents above 15.7 A. The 'double-threshold' phenomenon is attributed to the slow rise of the ∼30 ns duration LED pump pulses. The laser emits a 533 nm pulsed output beam of ∼10 ns duration perpendicular to the polymer film. When exposed to nitroaromatic model explosive vapors at ∼8 ppb concentration, the laser shows a 46% change in the surface-emitted output under optimized LED excitation. A very compact explosive vapor sensor is demonstrated based on a distributed feedback polymer laser pumped by a commercial InGaN light-emitting diode. The laser shows a two-stage turn on of the laser emission, for pulsed drive currents above 15.7 A. The 'double-threshold' phenomenon is attributed to the slow rise of the ∼30 ns duration LED pump pulses. The laser emits a 533 nm pulsed output beam of ∼10 ns duration perpendicular to the polymer film. When exposed to nitroaromatic model explosive vapors at ∼8 ppb concentration, the laser shows a 46% change in the surface-emitted output under optimized LED excitation.Publisher PDFPeer reviewe

Efficient eco-friendly inverted quantum dot sensitized solar cells

Recent progress in quantum dot (QD) sensitized solar cells has demonstrated the possibility of low-cost and efficient photovoltaics. However, the standard device structure based on n-type materials often suffers from slow hole injection rate, which may lead to unbalanced charge transport. We have fabricated efficient p-type (inverted) QD sensitized cells, which combine the advantages of conventional QD cells with p-type dye sensitized configurations. Moreover, p-type QD sensitized cells can be used in highly promising tandem configurations with n-type ones. QDs without toxic Cd and Pb elements and with improved absorption and stability were successfully deposited onto mesoporous NiO electrode showing good coverage and penetration according to morphological analysis. Detailed photophysical charge transfer studies showed that high hole injection rates (108 s−1) observed in such systems are comparable with electron injection in conventional n-type QD assemblies. Inverted solar cells fabricated with various QDs demonstrate excellent power conversion efficiencies of up to 1.25%, which is 4 times higher than the best values for previous inverted QD sensitized cells. Attempts to passivate the surface of the QDs show that traditional methods of reduction of recombination in the QD sensitized cells are not applicable to the inverted architectures

Light-emitting poly(dendrimers)

Organic light-emitting diodes (OLEDs) have great potential for displays and lighting applications. For large area displays the ideal materials would be both phosphorescent and solution processible. These requirements mean that the materials need to be able to be patterned and the most advanced method for forming pixelated displays is inkjet printing. Light-emitting phosphorescent dendrimers have given high efficiency monochrome displays with the emitting layer deposited by spin-coating. However, the viscosity of the dendrimer solutions is insufficient for inkjet printing. We report the development of a new class of light-emitting materials, namely poly(dendrimers) in which a green emissive phosphorescent dendrimer is attached to a poly(styrene) backbone. Free radical polymerization of a dendrimer-styrene monomer gave a poly(dendrimer) with a weight average molecular weight of 24000 and a polydispersity of 3.6. A dilute Solution of the dendrimer had a viscosity 15% higher than the neat solvent. Comparison of the photophysical studies of the poly(dendrimer) versus a model monomer dendrimer showed that the PL spectrum was broader and red-shifted, and the PL quantum yield around 50% lower. This was attributed to intermolecular interactions of the emissive dendrimers, which are held closely together oil the polymer backbone

Effect of fullerene acceptor on the performance of solar cells based on PffBT4T-2OD

We have studied bulk-heterojunction (BHJ) solar cells composed of the polymer PffBT4T-2OD as electron donor and three different electron accepting fullerenes, namely PC71BM, PC61BM and indene-C60-bis-adduct (ICBA) in order to understand the impact of different fullerenes on the morphology and efficiency of the corresponding photovoltaic devices. Despite PffBT4T-2OD:ICBA devices being characterised by higher values of Voc, they display the lowest power conversion efficiency (PCE) due to their lower Jsc and FF values. We find that although all blend films have similar morphologies, X-ray scattering indicates a reduced degree of order within the fullerene domains in the ICBA-based film. Due to the high LUMO level of ICBA, the corresponding blends are characterised by a lower initial exciton dissociation and this associated with the reduced ordering within the ICBA domains results in increased geminate recombination of the photogenerated electrons in the fullerene-rich domains and a consequently reduced PCE of the corresponding devices

Correlating phase behavior with photophysical properties in mixed‐cation mixed‐halide perovskite thin films

Mixed cation perovskites currently achieve very promising efficiency and operational stability when used as the active semiconductor in thin‐film photovoltaic devices. However, an in‐depth understanding of the structural and photophysical properties that drive this enhanced performance is still lacking. Here the prototypical mixed‐cation mixed‐halide perovskite (FAPbI3)0.85(MAPbBr3)0.15 is explored, and temperature‐dependent X‐ray diffraction measurements that are correlated with steady state and time‐resolved photoluminescence data are presented. The measurements indicate that this material adopts a pseudocubic perovskite α phase at room temperature, with a transition to a pseudotetragonal β phase occurring at ≈260 K. It is found that the temperature dependence of the radiative recombination rates correlates with temperature‐dependent changes in the structural configuration, and observed phase transitions also mark changes in the gradient of the optical bandgap. The work illustrates that temperature‐dependent changes in the perovskite crystal structure alter the charge carrier recombination processes and photoluminescence properties within such hybrid organic–inorganic materials. The findings have significant implications for photovoltaic performance at different operating temperatures, as well as providing new insight on the effect of alloying cations and halides on the phase behavior of hybrid perovskite materials

Intermolecular States in Organic Dye Dispersions: Excimers vs Aggregates

Rapid excited-state quenching in the solid state is a widespread limitation for organic chromophores. Even when molecules are dispersed in neutral host matrices, photoluminescence quantum yields decrease sharply with increased concentration, pointing to efficient intermolecular non-radiative decay pathways that remain poorly understood. Here we study the nature of the intermolecular states formed in dispersions of the prototypical BODIPY dyes. Using temperature-dependent and time-resolved photoluminescence measurements, we describe the processes of energy transfer into excimer states and, in materials with suitable chemical structure, excitonically coupled dimers. These dimer states exhibit remarkable near-unity quantum yield

Tuning the emission of cyclometalated iridium complexes by simple ligand modification

The photo-excitation and relaxation of a new cyclometalated complex, Ir(ppy)(2)fppy, is investigated through the measurement of absorption and photoluminescence spectra. Comparison of Ir(ppy)(2)fppy with Ir(ppy)(3) reveals that the lowest energy excited state of Ir(ppy)(2)fppy is localised on the aldehyde-substituted ligand, thus giving the complex its characteristic orange emission (lambda(max) approximate to 600 nm). OLEDs containing Ir(ppy)(2)fppy doped into poly(vinylcarbazole) (PVK) exhibit the same characteristic emission as found in solution, implying efficient energy transfer between the PVK and the iridium complex. Our results provide the potential for the synthesis of iridium complexes with different emission wavelengths through the modification of only one ligand

Solution-processed boron subphthalocyanine derivatives as acceptors for organic bulk-heterojunction solar cells

GC and IDWS thank the EPSRC for funding (Grant EP/I00243X). NBAP thanks Directorate General of Higher Education (DGHE) of Republic Indonesia for PhD Scholarship Grant (948/E4.4/K/2013). IDWS also acknowledges a Royal Society Wolfson Research Merit Award.We report the fabrication of solution-processed bulk heterojunction devices from subphthalocyanine (SubPc) units as the acceptor component and conventional polymeric donor materials such as MEH-PPV, P3HT and PTB7. The high solubility of the SubPc derivatives facilitated the formation of efficient donor/acceptor networks and provided power conversion efficiencies of 0.4% with MEH-PPV, 1.1% with P3HT and 3.5% with PTB7. A clear contribution of photon harvesting by the acceptor was identified from the external quantum efficiency spectra. Analysis of the current-voltage characteristics and photoluminescence quenching revealed trap-assisted and geminate recombination as a loss mechanism. Our results show that solution-processable SubPcs are a promising alternative to fullerenes for polymer solar cells.Publisher PDFPeer reviewe

Nanoscale mobility mapping in semiconducting polymer films

© 2020 Elsevier B.V. Local electrical properties of thin films of the polymer PTB7 are studied by conductive atomic force microscopy (C-AFM). Non-uniform nanoscale current distribution in the neat PTB7 film is revealed and connected with the existence of ordered PTB7 crystallites. The shape of local I-V curves is explained by the presence of space charge limited current. We modify an existing semi-empirical model for estimation of the nanoscale hole mobility from our experimental C-AFM measurements. The procedure of nanoscale charge mobility estimation was described and applied to the PTB7 films. The calculated average C-AFM hole mobility is in good agreement with macroscopic values reported for this material. Mapping of nanoscale hole mobility was achieved using the described procedure. Local mobility values, influenced by nanoscale structure, vary more than two times in value and have a root-mean-square value 0.22 × 10−8 m2/(Vs), which is almost 20% from average hole mobility