Organic photovoltaic (OPV) cells have attracted substantial commercial and scientifi c interest as they provide a pathway for a renewable energy source that is portable and potentially inexpensive. [ 1,2 ] Current state-of-the-art solution-processed OPV cells are based on the so-called bulk heterojunction (BHJ) architecture, consisting of an active layer that is a "blend" between an electron donor and acceptor forming an interpenetrating network structure due to self-organized phase separation. [3][4][5] Compared to a simpler planar heterojunction (i.e., a bilayer architecture), the BHJ geometry provides a greater total surface area for charge separation and reduced recombination, and these advantages should theoretically translate to dramatically enhanced power conversion effi ciency. [ 6,7 ] BHJ cells based on a donoracceptor pair of poly(3-n -hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C61-butyric-acid-methyl-ester (PCBM) have shown effi ciencies approaching 5%, [ 1 ] while effi ciencies of 6.77% have been reported with "low-bandgap" polymer donors. [ 8 ] Given that blending and morphology are critical for device performance, signifi cant research effort has focused on studying the phase separation mechanism and optimum conditions to permit effi cient splitting of excitons into charge carriers. [9][10][11][12] This is widely acknowledged as a delicate balance between forming the interpenetrating network and excessive phase segregation causing carrier traps. [ 13 ] Additionally, due to the short exciton diffusion length in organic semiconductors (approximately 10 nm), [ 14 ] the nanoscale morphology presents characterization and processing challenges. [15][16][17] In contrast, sequential processing to form the active bilayer is conceptually more straightforward. Additionally, as discussed in Ayzner et al. [ 18 ] sequential processing allows the use of organic electron acceptors that may not survive thermal evaporation and in principle allows each layer to be controlled and optimized independently. Such a strategy also lends itself more readily to multiple junction devices which are widely believed to be the pathway for OPV cells to be competitive with inorganic systems (conversion effi ciencies > 10%). However, until very recently, no solution processed bilayer device has performed anywhere near as well as its BHJ counterpart. [ 19 ] In this regard, Ayzner et al. [ 18 ] have reignited interest (and much debate) by reporting all-solution-processed P3HT/PCBM bilayer cells with performance approaching that of a BHJ. This performance was somewhat surprising given the individual active layer thicknesses were larger than the expected exciton diffusion lengths in either component and the planar heterojunction yields a reduced interfacial area for exciton separation. Motivated by these potentially very signifi cant, yet somewhat controversial observations, we have undertaken a detailed study of the morphology of solution processed P3HT/ PCBM devices formed by sequential layer deposition using a very similar approa...

We study the excited states of two iridium(III) complexes with potential applications in organic light-emitting diodes: fac-tris(2-phenylpyridyl)iridium(III) [Ir(ppy)(3)] and fac-tris(1-methyl-5-phenyl-3-n-propyl-[1,2,4]triazolyl)iridium(III) [Ir(ptz)(3)]. Herein we report calculations of the excited states of these complexes from time-dependent density functional theory (TDDFT) with the zeroth-order regular approximation (ZORA). We show that results from the one-component formulation of ZORA, with spin-orbit coupling included perturbatively, accurately reproduce both the results of the two-component calculations and previously published experimental absorption spectra of the complexes. We are able to trace the effects of both scalar relativistic correction and spin-orbit coupling on the low-energy excitations and radiative lifetimes of these complexes. In particular, we show that there is an indirect relativistic stabilisation of the metal-to-ligand charge transfer (MLCT) states. This is important because it means that indirect relativistic effects increase the degree to which SOC can hybridise singlet and triplet states and hence plays an important role in determining the optical properties of these complexes. We find that these two compounds are remarkably similar in these respects, despite Ir(ppy)(3) and Ir(ptz)(3) emitting green and blue light respectively. However, we predict that these two complexes will show marked differences in their magnetic circular dichroism (MCD) spectra.

We use a combination of low temperature, high field magnetic circular dichroism, absorption, and emission spectroscopy with relativistic time-dependent density functional calculations to reveal a subtle interplay between the effects of chemical substitution and spin-orbit coupling (SOC) in a family of iridium(III) complexes. Fluorination at the ortho and para positions of the phenyl group of fac-tris(1-methyl-5-phenyl-3-n-propyl-[1,2,4]triazolyl)iridium(III) cause changes that are independent of whether the other position is fluorinated or protonated. This is demonstrated by a simple linear relationship found for a range of measured and calculated properties of these complexes. Further, we show that the phosphorescent radiative rate, k(r), is determined by the degree to which SOC is able to hybridize T(1) to S(3) and that k(r) is proportional to the inverse fourth power of the energy gap between these excitations. We show that fluorination in the para position leads to a much larger increase of the energy gap than fluorination at the ortho position. Theory is used to trace this back to the fact that fluorination at the para position increases the difference in electron density between the phenyl and triazolyl groups, which distorts the complex further from octahedral symmetry, and increases the energy separation between the highest occupied molecular orbital (HOMO) and the HOMO-1. This provides a new design criterion for phosphorescent iridium(III) complexes for organic optoelectronic applications. In contrast, the nonradiative rate is greatly enhanced by fluorination at the ortho position. This may be connected to a significant redistribution of spectral weight. We also show that the lowest energy excitation, 1A, has almost no oscillator strength; therefore, the second lowest excitation, 2E, is the dominant emissive state at room temperature. Nevertheless the mirror image rule between absorption and emission is obeyed, as 2E is responsible for both absorption and emission at all but very low (<10 K) temperatures.

Determining how analytes are sequestered into thin films is important for solid-state sensors that detect the presence of the analyte by oxidative luminescence quenching. We show that thin (230 +/- 30 A) and thick (750 +/- 50 A) films of a first-generation dendrimer comprised of 2-ethylhexyloxy surface groups, biphenyl-based dendrons, and a 9,9,9',9'-tetra-n-propyl-2,2'-bifluorene core, can rapidly and reversibly detect p-nitrotoluene by oxidative luminescence quenching. For both the thin and thick films the photoluminescence (PL) is quenched by p-nitrotoluene by approximately 90% in 4 s, which is much faster than that reported for luminescent polymer films. Combined PL and neutron reflectometry measurements on pristine and analyte-saturated films gave important insight into the analyte adsorption process. It was found that during the adsorption process the films swelled, being on average 4% thicker for both the thin and thick dendrimer films. At the same time the PL was completely quenched. On removal of the analyte the films returned to their original thickness and scattering length density, and the PL was restored, showing that the sensing process was fully reversible.

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