Benjamin P. Lyons

The relative importance of domain size, domain purity and domain interfaces to the performance of bulk-heterojunction organic photovoltaics

The domain size, domain purity and interfacial width between domains for a bulk heterojunction are controllably altered through use of Cahn–Hilliard modeling and their relative effect on OPV performance is predicted using Monte Carlo modeling. It is found that locally sharp, well-connected domains of only 4 nm extent out perform morphologies with broadened interfaces and/or impure domains even when domain sizes were at the ‘optimum’ size of ∼10 nm. More generally, these data provide information on the most effective method to optimize the as-cast bulk heterojunction morphology depending upon initial domain purity and the nature of interfaces between domains. Further, it indicates why morphology optimization is more effective for some blends than others. It is shown that the quench depth of the blend can be used as a general technique to control the interfacial structure of the morphology and realize substantial increases in short circuit photocurrent.

New pyrimidine- and fluorene-containing oligo(arylene)s: synthesis, crystal structures, optoelectronic properties and a theoretical study

New pyrimidine containing oligo(arylene)s, notably the pyrimidine-fluorene hybrid systems 13-16, have been synthesised by Suzuki cross-coupling methodology. An efficient synthesis of the key reagent 9,9-dihexylfluorene-2,7-diboronic acid 10 from 2,7-dibromo-9,9-dihexylfluorene 9 is reported. Cross-coupling of 10 with two equivalents of 2-bromopyrimidine, 5-bromopyrimidine and 2,5-dibromopyrimidine gave 2,7-bis(2-pyrimidyl)-9,9-dihexylfluorene 13. 2,7-bis(5-pyrimidyl)-9,9-dihexylfluorene 14 and 2,7-bis(5-bromo-2-pyrimidyl)-9,9-dihexylfluorene 15 in 23-34% yields. A further two-fold Suzuki reaction of benzeneboronic acid with compound 15 gave 2,7-bis(5-phenyl-2-pyrimidyl)-9,9-dihexylfluorene 16 (35% yield). Ab initio calculations of the geometries and electronic structures at the Hartree Fock (HF) and density functional theory (DFT) levels of theory are reported for compounds 13, 14 and 16 (with ethyl substituents replacing hexyl) and for their dipyrazinyl and bistetraazenyl analogues, 17, 18, 20 and 21. The heterocyclic nitrogen atoms of 13 and 16 facilitate planarisation of the system, compared to 14, which is in agreement with X-ray structural data obtained for 5-bromo-2-phenylpyrimidine 6, 2,5-diphenylpyrimidine 7 and compound 15. Bistetrazenyl derivative 21 is calculated to be a fully planar system. The cyclic voltammogram (CV) of compound 16 in dichloromethane solution shows a quasi-reversible oxidation wave at E(1/2)0 = +1.36 V (vs. Ag/Ag+). Compound 13 is a poorer donor with an oxidation observed at Epa = +1.50 V which is in good agreement with the difference in the energies of their HOMO orbitals calculated at both HF and DFT levels of theory (0.11-0.12 eV). For compound 14 we were not able to measure an Eox potential which should lie at much more positive potentials. Compounds 15 and 16 are blue emitters in solution, with photoluminescence quantum yields (PLQY) of 25% and 85%, respectively. For thin films of 16 the PLQY is reduced to 21%. An OLED using compound 16 as the emissive layer has been fabricated in the configuration ITO/PEDOT/16/Ca/Al: blue-green light (lambda max 500 nm) most likely emanating primarily from excimer states is emitted at a high turn-on voltage.

Study of the energy transfer processes in polyfluorene doped with tetraphenyl porphyrin

The energy transfer processes between polyfluorene and tetraphenyl porphyrin have been investigated through steady state and time resolved measurements. Radiative transfer is found to be negligible and so the results are analyzed in terms of Forster resonance energy transfer. Rate equations are fitted to the experimental data to obtain values for the concentration dependent energy transfer rate. From this, we calculate a value of 38 A for the energy transfer distance. This compares well with the value for the Forster radius we calculate from the spectral overlap, even though Forster theory is not strictly valid for this system.

A comparison of the optical constants of aligned and unaligned thin polyfluorene films

The optical constants of aligned and unaligned thin polyfluorene films have been investigated using variable angle spectroscopic ellipsometry over the spectral range 250–1000nm. Films were prepared by spin coating, with alignment achieved by deposition on a rubbed polyimide substrate followed by annealing. The films could be well modeled by layers with uniaxial anisotropy. The unaligned film had its optical axis oriented normal to the film surface and was optically negative. There was large absorption and dispersion of the refractive index perpendicular to the optical axis and small absorption and dispersion of the refractive index parallel to it. The aligned film had its optical axis oriented in plane, normal to the rubbing direction, and was optically positive. There was large absorption and dispersion of the refractive index parallel to the optical axis and small absorption and dispersion of the refractive index perpendicular to it. These results will be useful in the design of polarized light-emitting...

Energy transfer processes in TPP-doped polyfluorene

The energy transfer processes between polyfluorene and tetraphenyl porphyrin have been investigated through steady state and time resolved measurements. Radiative transfer is found to be negligible and so the results are interpreted in terms of Forster resonance energy transfer. Rate equations are fitted to the results to obtain a concentration dependent transfer rate. From this we calculate a value for the Forster radius of 36A. This agrees well with the values we calculate from the spectral overlap, taking a wavelength dependent refractive index into account.

Multiple orientation and nematic-hexagonal transition in uniaxially aligned polyfluorene thin films

We present a structural study of poly(9,9-bis(2-ethylhexyl)-fluorene-2,7-diyl) (PF2/6) in aligned thin films on a rubbed polyimide substrate. PF2/6 forms 5/2-helical hairy-rodlike molecular structure which is self-organized in the hexagonal structure. In thin films, the aligned rigid polymer chains are parallel to the substrate in the direction corresponding to the molecular backbone, c axis. The cells are flattened in the direction of the surface normal and -in particular- reveal a multiple orientation where the greater proportion of the crystallites have one crystal axis a perpendicular to the substrate surface but a small proportion are aligned with the crystal axis a parallel to surface. We find further that by reducing the polymer chain length to approximately twenty repeat units the degree of axial alignment of PF2/6 is considerably improved while the local order changes from hexagonal packing towards nematic phase with no sign of multiple orientation.

Orientation of dopant molecules in thin polyfluorene films

Aligned and unaligned thin polyfluorene (PF) films doped with varying degrees of the dyes tetraphenyl porphyrin (TPP) and 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) have been investigated via polarised steady-state and time-resolved fluorescence anisotropy measurements. In the aligned films, the PF and DCM emission have dichroic ratios of approximately 10 and 4 respectively, while the TPP emission is completely depolarised. The polarised dopant emission in the case of the DCM is likely to arise from its linear shape and possible orientation parallel to the polymer chains, whereas the TPP is planar and exhibits no preferential orientation. These experiments show the possibility of polarised, red-shifted luminescence from polymer films.