Controlling the structures of organic semiconductor–quantum dot nanocomposites through ligand shell chemistry

Nanocrystal quantum dots (QD) functionalised with active organic ligands hold significant promise as solar energy conversion materials, capable of multiexcitonic processes that could improve the efficiencies of single-junction photovoltaic devices. Small-angle X-ray and neutron scattering (SAXS and SANS) were used to characterize the structure of lead sulphide QDs post ligand-exchange with model acene-carboxylic acid ligands (benzoic acid, hydrocinnamic acid and naphthoic acid). Results demonstrate that hydrocinnamic acid and naphthoic acid ligated QDs form monolayer ligand shells, whilst benzoic acid ligated QDs possess ligand shells thicker than a monolayer. Further, the formation of a range of nanocomposite materials through the self-assembly of such acene-ligated QDs with an organic small-molecule semiconductor [5,12-bis((triisopropylsilyl)ethynyl)tetracene (TIPS-Tc)] is investigated. These materials are representative of a wider set of functional solar energy materials; here the focus is on structural studies, and their optoelectronic function is not investigated. As TIPS-Tc concentrations are increased, approaching the solubility limit, SANS data show that QD fractal-like features form, with structures possibly consistent with a diffusion limited aggregation mechanism. These, it is likely, act as heterogeneous nucleation agents for TIPS-Tc crystallization, generating agglomerates containing both QDs and TIPS-Tc. Within the TIPS-Tc crystals there seem to be three distinct QD morphologies: (i) at the crystallite centre (fractal-like QD aggregates acting as nucleating agents), (ii) trapped within the growing crystallite (giving rise to QD features ordered as sticky hard spheres), and (iii) a population of aggregate QDs at the periphery of the crystalline interface that were expelled from the growing TIPS-Tc crystal. Exposure of the QD:TIPS-Tc crystals to DMF vapour, a solvent known to be able to strip ligands from QDs, alters the spacing between PbS–hydrocinnamic acid and PbS–naphthoic acid ligated QD aggregate features. In contrast, for PbS–benzoic acid ligated QDs, DMF vapour exposure promotes the formation of ordered QD colloidal crystal type phases. This work thus demonstrates how different QD ligand chemistries control the interactions between QDs and an organic small molecule, leading to widely differing self-assembly processes. It highlights the unique capabilities of multiscale X-ray and neutron scattering in characterising such composite materials

Mesoscopic modelling of 2-CN-PPV/PPV polymer LED

Although optoelectronic devices made of polymers are very attractive ones (low cost, easy to make), problems related to charge transport, exciton quenching, among others, can be an obstacle for their performance. The use of heterojunctions made of two polymers can be a strategy for improving the efficiency of polymer light emitting diodes (PLEDs) at low bias. Here we present a theoretical study of the influence of bilayer structure in a PLED made of PPV and 2-CN-PPV, by adopting a mesoscopic approach. Our results show that the presence of the polymer/polymer interface improves charge injection and leads to a confinement of charges near it, which will increase the number recombination events in the middle of the device compared to the equivalent single-layer PLEDs.Fundação para a Ciência e a Tecnologia (FCT) Programa Operacional “Ciência , Tecnologia, Inovação” POCTI/CTM/41574/2001, CONC-REEQ/443/EEI/2001 e SFRH/BD/22143/200

Mixed small-molecule matrices improve nanoparticle dispersibility in organic semiconductor-nanoparticle films

Controlling the dispersibility of nanocrystalline inorganic quantum dots (QDs) within organic semiconductor (OSC):QD nanocomposite films is critical for a wide range of optoelectronic devices. This work demonstrates how small changes to the OSC host molecule can have a dramatic detrimental effect on QD dispersibility within the host organic semiconductor matrix as quantified by grazing incidence X-ray scattering. It is commonplace to modify QD surface chemistry to enhance QD dispersibility within an OSC host. Here, an alternative route toward optimizing QD dispersibilities is demonstrated, which dramatically improves QD dispersibilities through blending two different OSCs to form a fully mixed OSC matrix phase

Quantum dots coordinated with conjugated organic ligands: new nanomaterials with novel photophysics

CdSe quantum dots functionalized with oligo-(phenylene vinylene) (OPV) ligands (CdSe-OPV nanostructures) represent a new class of composite nanomaterials with significantly modified photophysics relative to bulk blends or isolated components. Single-molecule spectroscopy on these species have revealed novel photophysics such as enhanced energy transfer, spectral stability, and strongly modified excited state lifetimes and blinking statistics. Here, we review the role of ligands in quantum dot applications and summarize some of our recent efforts probing energy and charge transfer in hybrid CdSe-OPV composite nanostructures

Two-dimensional electron-hole capture in a disordered hopping system

We model the two-dimensional recombination of electrons and holes in a system where the mean free path is short compared with the thermal capture radius. This recombination mechanism is relevant to the operation of bilayer organic light-emitting diodes (LED's), where electrons and holes accumulate on either side of the internal heterojunction. The electron-hole recombination rate can be limited by the time taken for these charge carriers to drift and diffuse to positions where electrons and holes are directly opposite to each other on either side of the interface, at which point rapid formation of an emissive neutral state can occur. In this paper, we use analytical and numerical techniques to find the rate of this two-dimensional electron-hole capture process. Where one species of carrier is significantly less mobile than the other, we find that the recombination rate depends superlinearly on the density of the less mobile carrier. Numerical simulations allow the effects of disorder to be taken into account in a microscopic hopping model. Direct solution of the master equation for hopping provides more efficient solutions than Monte Carlo simulations. The rate constants extracted from our model are consistent with efficient emission from bilayer LED's without requiring independent hopping of electrons and holes over the internal barrier at the heterojunctio

Monte Carlo modeling of geminate recombination in polymer-polymer photovoltaic devices

A Monte Carlo model is used to examine geminate pair dissociation in polymer-polymer photovoltaic devices. It is found that increasing one or both carrier mobilities aids geminate separation yield ηGS particularly at low fields. This, in turn, leads to improved maximum power output from polymer-polymer blend photovoltaics, even when carrier mobilities are unbalanced by a factor of 10. The dynamic behaviors of geminate charges that eventually separate and recombine are examined for the first time. It is shown that geminate pairs in a bilayer become effectively free when separated by ∼4nm, which is far smaller than the thermal capture radius of 16nm here. This may lead one to expect that ηGS would not be limited by the separation allowed by the morphology once the domain size has increased above 4nm. In fact it is found that ηGS in a blend improves continuously as the average domain size increases from 4to16nm. We show that although a small degree of separation may be available in a blend, the limited number of possible routes to further separation makes charge pairs in blends more susceptible to recombination than charge pairs in a bilayer