The radiative and nonradiative decay rates of lissamine dye molecules, chemically attached to differently sized gold nanoparticles, are investigated by means of time-resolved fluorescence experiments. A pronounced fluorescence quenching is observed already for the smallest nanoparticles of 1 nm radius. The quenching is caused not only by an increased nonradiative rate but, equally important, by a drastic decrease in the dye's radiative rate. Assuming resonant energy transfer to be responsible for the nonradiative decay channel, we compare our experimental findings with theoretical results derived from the Gersten-Nitzan model. DOI: 10.1103/PhysRevLett.89.203002 PACS numbers: 33.50.-j, 81.07.Pr Resonant energy transfer (RET) systems consisting of organic dye molecules and noble metal nanoparticles have recently gained considerable interest in biophotonics [1][2][3][4] as well as in materials science [5,6]. Closely related are donor-acceptor pairs of organic dye molecules forming Förster resonant energy transfer (FRET) systems. They have been theoretically modeled [7] and applied in biophysics extensively during the past decade (see, e.g., [8]). Yet these classical purely dye-based systems show disadvantages regarding quenching efficiency [4] and photostability [9].If the donor molecule is placed in the vicinity of a metal surface instead of an organic acceptor, not only resonant energy transfer takes place but also the radiative lifetime of the donor molecule changes. For metal films this has been investigated extensively [10 -13]. Much less is known about donor molecules in the vicinity of metal nanoparticles. Theoretical treatments of the moleculenanoparticle problem [14 -17] predict energy transfer rates and radiative decay rates that deviate substantially from what has been found for dye molecules in front of a metal film. Both radiative and nonradiative rates are expected to depend critically on size and shape of the nanoparticle, the distance between the dye molecule and the nanoparticle, the orientation of the molecular dipole with respect to the dye-nanoparticle axis, and the overlap of the molecule's emission with the nanoparticle's absorption spectrum. Recent experimental investigations deal with metal island films or rough surfaces only (see [18,19] and references in [20]), where the above mentioned parameters are undefined.Here we report results of time-resolved fluorescence experiments on a donor-acceptor system composed of lissamine molecules (donor) chemically attached to a gold nanoparticle (acceptor). The distance between the lissamine molecule and the surface of the nanoparticle is kept constant at 1 nm, whereas the nanoparticle radius is varied between 1 and 30 nm. We find time constants for the energy transfer on a picosecond time scale which turn out to decrease with increasing nanoparticle size. In addition, the dye's radiative rate is reduced by more than an order of magnitude. Both effects are responsible for the drastic quenching of the fluorescence yield as predicted by the so-called...

Control of the band-edge offsets at heterojunctions between organic semiconductors allows efficient operation of either photovoltaic or light-emitting diodes. We investigate systems where the exciton is marginally stable against charge separation and show via E-field-dependent time-resolved photoluminescence spectroscopy that excitons that have undergone charge separation at a heterojunction can be efficiently regenerated. This is because the charge transfer produces a geminate electron-hole pair (separation 2.2-3.1 nm) which may collapse into an exciplex and then endothermically (E(A)=100-200 meV) back transfer towards the exciton.

Organic semiconductors fabricated as thin-film light-emitting diodes, LEDs, now provide a promising new display technology.[1] Solution-processed semiconductor polymers make possible direct printing (using ink-jet deposition) and allow high-resolution full-color displays to be conveniently manufactured. [2] Multiple-layer deposition, used in vacuum-sublimed molecular semiconductor LEDs, is difficult to achieve by solution processing. We have instead fabricated distributed heterojunction' structures that are formed by de-mixing of two polymers co-deposited from common solution. We have used hole-accepting and electron-accepting derivatives of polyfluorene, and have optimized these structures to achieve high-efficiency diodes (above 19 lm W ±1 for green emission) that operate at very low voltages (100 cd m ±2 at 2.1 V for green emission). This very low voltage operation is achieved because electron±hole capture across the heterojunction is arranged to be a barrier-free process to form an interface state (termed an exciplex) that has significant charge-transfer character and is lower in energy than the charge-separated state. With respect to the bulk exciton, the exciplex is red-shifted (here between 140 and 360 meV) and its radiative lifetime is strongly increased (between 68 and 118 ns at low temperatures). The barrier for thermal excitation of the exciplex to allow it to move away from the heterojunction is small (100± 250 meV), and this process can give efficient bulk exciton emission at room temperature. The heterojunction formed between dissimilar organic semiconductors is generally found to be remarkably free of gap-states and other defects that would otherwise compromise semiconductor device operation. Heterojunction LEDs are designed so that the offsets between conduction and between valence band edges are type II' and electrons and holes accumulate on opposite sides of the heterojunction (Fig. 1). In a non-interacting electron scheme, type II heterojunctions would destabilize an exciton present in either semiconductor, since the exciton state would be higher in energy than the charge-separated state. However, organic semiconductors are low dielectric constant materials (typically having values less than 4) so that the coulomb interaction between electron and hole gives a substantial exciton binding energy (of order 0.5 eV). When this binding energy is larger than the band-edge offsets, excitons are stable at the interface. By selecting semiconductors with larger band-edge offsets, charge separation at the heterojunction can be readily achieved, giving efficient photovoltaic behavior. [3] LEDs made using molecular semiconductors are generally fabricated as multiple-layer heterojunction structures by successive vacuum sublimation steps.[1] However, with solutionprocessed polymers it is possible to make distributed heterojunction' diodes by de-mixing of two polymers spin-coated from common solution. [4] This is an obviously desirable structure for photovoltaic diodes, because it allows excitons photogenerated...

We report a compositional analysis of blended hole-accepting and electron-accepting polyfluorene related materials, poly(9,9‘-dioctylfluorene-co-bis-N,N ‘-(4-butylphenyl)-bis-N,N ‘-phenyl-1,4-phenylene-diamine) [PFB] and poly(9,9‘-dioctylfluorene-co-benzo-thiadiazole) [F8BT], in films and in photovoltaic devices. We find that photoluminescence quenching is insensitive to blend composition but the photovoltaic quantum yield is strongly composition dependent. This indicates that charge transport, and not charge generation, is the factor limiting device performance. We demonstrate that a meso-length scale phase separation optimizes charge transport properties.

We have examined the Coulombic interactions at the interface in a blend of two copolymers with intramolecular charge-transfer character and optimized band offsets for photoinduced charge generation. The combination of both time-resolved measurements of photoluminescence, and quantum-chemical modeling of the heterojunction allows us to show that relative orientation across the heterojunction can lead to either a repulsive barrier ( approximately 65 meV) or an attractive interaction which can enhance the charge-transfer processes. We conclude that polymer orientation at the heterojunction can be as important as energy-band offsets in determining the dynamics of charge separation and optical emission.

Optoelectronic devices made from semiconductor polymers often employ partially phase-separated binary polymer blends with "distributed heterojunctions" in the polymer film, and the migration of bulk excitons towards these heterojunctions crucially influences the device performance. Here, we investigate exciton migration in blend films of two polyfluorene derivatives. Localized exciplex states form in electron-hole capture at the heterojunction between the two polymers and these can be thermally excited to transfer to bulk excitons. Rapid radiative emission from these excitons can then allow efficient light-emitting diode operation. We show here that when these excitons migrate to another heterojunction site within their lifetime they are re-trapped at the interface and again form exciplex states or dissociate completely. We demonstrate that in polymer blend light-emitting diodes this can reduce the exciton population by more than 54% and can strongly influence the emission spectrum. We then analyze exciton re-trapping in detail using time-resolved photoluminescence spectroscopy on blends with different morphologies and find that for nanometer-scale phases exciton emission is completely suppressed. We show that the data agree well with a simple kinetic model which confirms the importance of the blend morphology for the exciton trapping efficiency.

Charge generation in organic solar cells proceeds via photogeneration of excitons in the bulk that form geminate electron–hole pairs at the heterojunction formed between electron donor and acceptors. It is shown that an externally applied electric field increases the number of free charges formed from the geminate pair, and quenches the luminescence from the relaxed exciplex with one‐to‐one correspondence.

We investigate the photo- and electroluminescence from bilayers of electron- and hole-transporting polyfluorene derivatives at different device temperatures. We show that barrier-free charge capture at the heterojunction is the sole capture mechanism at low driving voltages (below 2.4×105V∕cm2 at room temperature). In this mechanism, which we suggested recently [Morteani et al., Adv. Mater. 15, 1708 (2003)], charge capture produces an interfacial excited state (exciplex) directly and bulk exciton electroluminescence is only achieved through endothermic transfer (activation energy 200meV) from the exciplex. For high driving voltages (above 8.3×105V∕cm2 at 43K), however, we find that charges are injected over the heterojunction barriers and subsequent charge recombination occurs in the polymer bulk.