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...

Steady-state Stern−Volmer analysis is uniformly used to assess in solution the efficiency of a sensing molecule for a particular analyte. We use a combination of steady-state Stern−Volmer analysis and time-resolved photoluminescence (TRPL) to determine the underlying mechanisms by which fluorescent sensing materials comprised of fluorene-based chromophores sense nitro-based explosive analytes. The ability of two first-generation dendrimers comprised of bifluorene-containing chromophores to sense explosive analytes was compared with the chemically related polymer poly(9,9-di-n-octylfluoren-2,7-diyl). One dendrimer was planar with a single chromophore with the second having four chromophores tetrahedrally arranged around an adamantyl center. All the materials had high photoluminescence quantum yields of around 90% and were able to sense explosive analytes via quenching of their fluorescence. The three-dimensional dendrimer based upon the adamantyl core was found to have the highest Stern−Volmer constants for all the analytes tested with the planar dendrimer also proving to be on average superior to the polymer. The TRPL measurements showed that sensing occurred by a combination of collisional and static quenching with the proportion of collisional quenching being based on the number of aromatic units in the analyte. Steady-state fluorescence polarization anisotropy measurements of the three materials revealed that for the three-dimensional dendrimer an exciton can migrate between all of the chromophores, meaning that an exciton formed on one chromophore of the dendrimer can be quenched by an analyte interacting with a second chromophore. This gives rise to the potential for sensing response amplification and explains its superior performance to the planar dendrimer and polymer.

Understanding and controlling the morphology of donor/acceptor blends is critical for the development of solution processable organic solar cells. By crosslinking a poly(3‐n‐hexylthiophene‐2,5‐diyl) (P3HT) film we have been able to spin‐coat [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) onto the film to form a structure that is close to a bilayer, thus creating an ideal platform for investigating interdiffusion in this model system. Neutron reflectometry (NR) demonstrates that without any thermal treatment a smaller amount of PCBM percolates throughout the crosslinked P3HT when compared to a non‐crosslinked P3HT film. Using time‐resolved NR we also show thermal annealing increases the rate of diffusion, resulting in a near‐uniform distribution of PCBM throughout the polymer film. XPS measurements confirm the presence of both P3HT and PCBM at the annealed film's surface indicating that the two components are intermixed. Photovoltaic devices fabricated using this bilayer approach and suitable annealing conditions yielded comparable power conversion efficiencies to bulk heterojunction devices made from the same materials. The crosslinking procedure has also enabled the formation of patterned P3HT films by photolithography. Pillars with feature sizes down to 2 μm were produced and after subsequent deposition of PCBM and thermal annealing devices with efficiencies of up to 1.4% were produced.

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.

The production of MOFs at large scale in a sustainable way is key if these materials are to be exploited for their promised widespread application. Much of the published literature has focused on demonstrations of preparation routes using difficult or expensive methodologies to scale. One such MOF is nano-zeolitic imidazolate framework-8 (ZIF-8) – a material of interest for a range of possible applications. Work presented here shows how the synthesis of ZIF-8 can be tracked by a range of methods including X-ray diffraction, thermo gravimetric analysis and inelastic neutron scattering – which offer the prospect of in-line monitoring of the synthesis reaction. Herein we disclose how the production of nano-ZIF-8 can be conducted at scale using the intermediate phase ZIF-L. By understanding the economics and demonstrating the production of 1 kg of nano-ZIF-8 at pilot scale we have shown how this once difficult to make material can be produced to specification in a scalable and cost-efficient fashion.

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With a multitude of potential applications, poly­(phosphine–borane)­s are an interesting class of polymer comprising main-group elements within the inorganic polymer backbone. A new family of primary alkylphosphine–borane polymers was synthesized by a solvent-free rhodium-catalyzed dehydrocoupling reaction and characterized by conventional chemicophysical techniques. The thermal stability of the polymers is strongly affected by the size and shape of the alkyl side chain with longer substituents imparting greater stability. The polymers show substantial stability toward UV illumination and immersion in water; however, they undergo a loss of alkylphosphine units during thermal degradation. The polymers exhibit glass transition temperatures (T g) as low as −70 °C. A group interaction model (GIM) framework was developed to allow the semiquantitative prediction of T g values, and the properties of the materials in this study were used to validate the model.

Stable film morphology is critical for long‐term high performance organic light‐emitting diodes (OLEDs). Neutron reflectometry (NR) is used to study the out‐of‐plane structure of blended thin films and multilayer structures comprising evaporated small molecules. It is found that as‐prepared blended films of fac‐tris(2‐phenylpyridyl)iridium(III) [Ir(ppy)3] in 4,4′‐bis(N‐carbazolyl)biphenyl (CBP) are uniformly mixed, but the occurrence of phase separation upon thermal annealing is dependent on the blend ratio. Films comprised of the ratio of 6 wt% of Ir(ppy)3 in CBP typically used in OLEDs are found to phase separate with moderate heating while a higher weight percent mixture (12 wt%) is found to be stable. Furthermore, it is found that thermal annealing of a multilayer film comprised of typical layers found in efficient devices ([tris(4‐carbazoyl‐9‐ylphenyl)amine (TCTA)/Ir(ppy)3:CBP/bathocuproine (BCP)]) causes the BCP layer to become mixed with the emissive blend layer, whereas the TCTA interface remains unchanged. This significant structural change causes no appreciable difference in the photo­luminescence of the stack although such a change would have a dramatic effect on the charge transport through the device, leading to changes in performance. These results demonstrate the effect of thermal stress on the delicate interplay between the chemical composition and morphology of OLED films.