Colloidal nanocrystals : electrifying quantum dots for lasers

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Nanoimprint lithography as a route to nanoscale back-contact perovskite solar cells

Funding: Engineering and Physical Sciences Research Council - EP/T01119X/1.Back-contact perovskite solar cells are of great interest because they could achieve higher performance than conventional designs while also eliminating the need for transparent conductors. Current research in this field has focused on making electrode structures with reduced widths to collect charges more efficiently, but current lift-off-based fabrication techniques have struggled to achieve electrode widths smaller than 1000 nm and are difficult to implement on large areas. We demonstrate nanoimprint lithography in an etch-down procedure as a simple and easily scalable method to produce honeycomb-shaped, quasi-interdigitated electrode structures with widths as small as 230 nm. We then use electrodeposition to selectively deposit conformal coatings of a range of different hole-selective layers and explore how the efficiency of back-contact perovskite solar cells changes as the feature sizes are pushed into the nanoscale. We find that the efficiency of the resulting devices remains almost unchanged as the electrode width is varied from 230 to 2000 nm, which differs from reported device simulations. Our results suggest that reducing recombination and improving the quality of the charge transport layers, rather than reducing the minimum feature size, are likely to be the best pathway to maximizing the performance of back-contact perovskite solar cells.Publisher PDFPeer reviewe

Absorption cross-sections of hole polarons in glassy and beta-phase polyfluorene

F. Montilla acknowledges financial support provided by Ministerio de Economía y Competitividad (Ramón y Cajal RYC1463/06 and José Castillejo JC2009-00 227 Programs). The authors also acknowledge the Engineering and Physical Sciences Research Council for financial support.Absorption induced by electrochemically injected holes is studied in poly-9,9-dioctylfluorene (PFO) films. Injected charges form positive polarons which are delocalised over four fluorene units in the glassy phase and about seven fluorene units in its beta-phase. Polaron absorption cross-sections at the 640 nm peak are similar to the published values of chemically reduced oligofluorenes in solution. The absorption cross-section of polaron in the beta-phase at 470 nm is about eight times smaller than the stimulated emission cross-section derived from published data. This indicates that beta-phase-rich PFO is an attractive candidate for a lightemitting layer in double-heterostructure organic laser diodes.Publisher PDFPeer reviewe

Light harvesting for organic photovoltaics

The authors are grateful to the Engineering and Physical Sciences Research Council of the UK (grants EP/J009016/1 and EP/L017008/1) and the European Research Council (grant number 321305) for financial support. IDWS also acknowledges a Royal Society Wolfson Research Merit Award.The field of organic photovoltaics has developed rapidly over the last 2 decades, and small solar cells with power conversion efficiencies of 13% have been demonstrated. Light absorbed in the organic layers forms tightly bound excitons that are split into free electrons and holes using heterojunctions of electron donor and acceptor materials, which are then extracted at electrodes to give useful electrical power. This review gives a concise description of the fundamental processes in photovoltaic devices, with the main emphasis on the characterization of energy transfer and its role in dictating device architecture, including multilayer planar heterojunctions, and on the factors that impact free carrier generation from dissociated excitons. We briefly discuss harvesting of triplet excitons, which now attracts substantial interest when used in conjunction with singlet fission. Finally, we introduce the techniques used by researchers for characterization and engineering of bulk heterojunctions to realize large photocurrents, and examine the formed morphology in three prototypical blends.Publisher PDFPeer reviewe

Enhancing exciton diffusion length provides new opportunities for organic photovoltaics

Authors acknowledge support from the European Research Council (grant 321305) And are also grateful to EPSRC for support from grants (EP/L017008/1) and (EP/M025330/1).Organic semiconductors can potentially revolutionize solar cell technology by offering very thin, lightweight, and flexible modules for outdoor and indoor power generation. Light absorption in organic semiconductors generates a bound electron-hole pair (exciton), which needs to travel to the interface between electron donor and acceptor materials to dissociate into charge carriers. Because the exciton diffusion length in organic semiconductors is typically much shorter than the light absorption depth (∼100 nm), planar donor-acceptor heterojunctions are inefficient, and most effort has been dedicated to optimization of bulk heterojunctions with nanoscale phase separation. In this Perspective, we review recent findings and new approaches to increase the exciton diffusion length and discuss how these improvements can benefit environmentally friendly production of solar modules using organic nanoparticles or graded heterojunctions obtained by sequential deposition of electron donor and acceptor.PostprintPeer reviewe

Effect of a high boiling point additive on the morphology of solution-processed P3HT-fullerene blends

Funding: UK Engineering and Physical Sciences Research Council (EPSRC) EP/L505079/1, EP/G03673X/1, EP/J009016/1The use of high boiling point additives in solution processing has been widely employed to control the active layer morphology in bulk heterojunction organic solar cells. The morphology of the heterojunction is crucial in controlling charge separation and extraction by the electrodes, and therefore the power conversion efficiency (PCE) of the device. This paper presents a study of time-resolved fluorescence quenching in blends of P3HT containing varying concentrations of the fullerenes PC61BM or PC71BM. The relationship between the fluorescence quenching rate and fullerene concentration indicates that the fullerene molecules are dispersed within the P3HT film for up to 5% by mass of fullerene. For higher fullerene concentrations, the additional fullerene molecules aggregate and form fullerene domains. The high degree of phase segregation observed in these blends is beneficial for solar cell performance because the segregated fullerene phase provides electron percolation pathways through the blend. The addition of 1,8-diiodooctane (DIO) to the solutions for spin coating into films changes the scale of fullerene segregation when the ratio by mass of fullerene exceeds 20%. At high fullerene concentrations the rate of fluorescence quenching decreases in P3HT:PC61BM blends when prepared with DIO indicating a larger scale phase separation. The effect of DIO on the morphology of P3HT:PC71BM blends is the opposite in that it causes faster quenching in the blends. Overall the results show that DIO can be used to control the morphology of photovoltaic blends of P3HT with fullerenes.Publisher PDFPeer reviewe

A figure of merit for efficiency roll-off in TADF-based organic LEDs

Funding: We are grateful to the Engineering and Physical Sciences Research Council of the UK for financial support through grants EP/R035164/1 and EP/P010482/1.Organic light-emitting diodes (OLEDs) are a revolutionary light-emitting display technology that has been successfully commercialized in mobile phones and TVs1,2. The injected charges form both singlet and triplet excitons, and for high efficiency it is important to enable triplets as well as singlets to emit light. Currently materials that harvest triplets by thermally activated delayed fluorescence (TADF) are a very active field of research as an alternative to phosphorescent emitters which usually employ heavy metal atoms3,4. Whilst excellent progress has been made, there is a severe decrease of efficiency as the drive current is increased, i.e. efficiency roll-off, in most TADF OLEDs. At present much of the literature suggests that efficiency roll-off should be reduced by minimising the energy difference between singlet and triplet excited states (EST) in order to maximise the rate of conversion of triplets to singlets via reverse intersystem crossing (kRISC) 5-20. We analyse the efficiency roll-off in a wide range of TADF OLEDs and find that neither of these parameters fully accounts for the reported efficiency roll-off. By considering the dynamic equilibrium between singlets and triplets in TADF materials, we propose a figure of merit (FOM) for materials design to reduce efficiency roll-off and discuss its correlation with reported data of TADF OLEDs. Our new FOM will guide the design and development of TADF materials that can reduce efficiency roll-off. It will help improve the efficiency of TADF OLEDs at realistic display operating conditions and expand the use of TADF materials to applications that require high brightness, such as lighting, augmented reality, and lasing.Peer reviewe

Charge pair dissociation and recombination dynamics in a P3HT–PC60BM bulk heterojunction

The authors thank the EPSRC, SUPA, the Condensed Matter Doctoral Training Centre and the European Union Seventh Framework programme (grant agreement 321305) for financial support.The mechanism by which Coulombically bound charge pairs dissociate into free carriers in photovoltaic donor–acceptor blends is of great interest. Here, we use polarization-sensitive transient absorption (TA) to study the diffusion of photogenerated holes in a polythiophene (P3HT)–fullerene (PC60BM) blend. We observe an initial anisotropy value of 0.4 for the absorption of photogenerated holes, indicating that holes generated on a 100 fs time scale are localized on the same polymer chain as their precursor excitons. Depolarization dynamics indicate fast initial hole motion on a 0.3 ps time scale and slower migration up to 100 ps. Charge pair recombination is found to occur on a much longer time scale of 10–1000 ns via a purely bimolecular process independent of excess energy. Our results show that nearly all charge pairs get separated by at least 7 nm in the absence of an external field and indicate that high charge mobility is crucial for charge separation.PostprintPeer reviewe

Organic light-emitting diodes for optogenetic stimulation of Drosophila larvae

We are grateful for financial support from the Scottish Funding Council (through SUPA), Human Frontier Science Program (RGY0074/2013), Wellcome Trust Institutional Strategic Support Fund St Andrews, the RS Macdonald Charitable Trust, and EPSRC via grant EP/J01771X/1. CM acknowledges funding by the European Commission through a Marie Skłodowska Curie individual fellowship (703387).Optogenetics is an emerging method in biology that enables controlling neurons with light. We use organic light-emitting diodes to stimulate neurons in Drosophila larvae and investigate subsequent behavioral changes at different light intensities.Postprin

Green perovskite distributed feedback lasers

This work was supported by the Engineering and Physical Sciences Research Council (EPSRC) of the UK Grants; EP/K503162/1, EP/M506631/1, EP/M025330/1 and EP/L017008/1. IDWS acknowledges funding from a Royal Society Wolfson research merit award.A visible perovskite distributed feedback laser is fabricated for the first time. Through the use of nanocrystal pinning, highly luminescent methylammonium lead bromide films are used to produce stable lasers emitting at 550 nm, with a low threshold of 6 µJcm−2. The lasers were able to support multiple polarisations, and could be switched between transverse magnetic and transverse electric mode operation through simple tuning of the distributed feedback grating period.Publisher PDFPeer reviewe