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A series of poly(phenylene-vinylene)-based copolymers are synthesized using the Gilch method incorporating monomers with sterically bulky sidechains. The photochemical upconversion performance of these polymers as emitters are investigated using a palladium tetraphenyltetrabenzoporphyrin triplet sensitizer and MEH-PPV as reference. Increased incorporation of sterically bulky monomers leads to a reduction in the upconversion efficiency despite improved photoluminescence quantum yield. A phosphorescence quenching study indicates issues with the energy transfer process between the triplet sensitizer and the copolymers. The best performance with 0.18% upconversion quantum yield is obtained for the copolymer containing 10% monomer with bulky sidechains.

A series of poly(phenylene-vinylene)s (PPV) was synthesized with two monomer building blocks containing methoxy and 2-ethylhexyloxy (MEH) sidechains and sterically bulky fluorenyloxy sidechains. Variations in monomer incorporation were achieved by adjusting the ratio of the monomer precursors in the synthesis of the PPVs using the Gilch polymerization method. The increased incorporation of the bulky monomer led to expected variation in photophysical properties, in particular, the enhancement of photoluminescence quantum yield. Using palladium(II) tetraphenyltetrabenzoporphyrin (PdTPTBP) as the triplet sensitizer, all polymers displayed upconverted emission in solution with the comonomer ratio for the fluorenyloxy and MEH units at 1 : 10 showing the best performance with upconversion efficiency of 0.135%. This efficiency was up to three times higher than that of two commercially available PPVs, MEH-PPV and Super yellow PPV, under the same experimental conditions. Further analysis of the efficiency of the photochemical processes involved revealed several limitations to the performance of these conjugated polymer emitters. While increasing steric bulk of the sidechains improved the photoluminescence quantum yield, it also caused variation in excited state energies of the polymers through the series. This led to poor alignment with the triplet energy of our chosen sensitizer for some of the polymers. Even when the triplet–triplet energy transfer efficiency was high, the contact triplet pair formation efficiency was found to be very low. We reasoned that the structural conformation of the polymers hindered the chromophore orbital overlap during triplet collisions required for triplet–triplet annihilation.

The excellent performance and stability of perovskite solar cells (PSCs) based on quasi-2D Ruddlesden–Popper perovskites (RPPs) holds promise for their commercialization. Further improvement in the performance of 2D PSCs requires a detailed understanding of the microstructure of the quasi-2D perovskite films. Based on scanning transmission electron microscopy (STEM), time-resolved photoluminescence, and transient absorption measurements, a new sandwich-like structural model is proposed to describe the phase distribution of RPPs. In contrast to the conventional gradient distribution, it is found that small-n RPPs are sandwiched between large-n RPP phase layers at the front and back sides owing to crystallization initiated from both interfaces during film formation. This sandwich-like distribution profile facilitates excitons funneling from the film interior to both surfaces for dissociation while free carriers transport via large-n channels that permeate the film to ensure efficient charge collection by the corresponding electrodes, which is favorable for high-performance photovoltaics. This discovery provides a new fundamental understanding of the operating principles of 2D PSCs and has valuable implications for the design and optimization strategies of optoelectronic devices based on quasi-2D RPPs films.

Ligand-free Cs4PbBr6 particles of sub-micron dimensions were fabricated by a simple tip-sonication method showing 85.1% photoluminescence quantum yield (PLQY). Cs4PbBr6 microcrystals can be grown under saturated vapor pressure with the particles as nucleation units. Surface traps are the main defects in the Cs4PbBr6 particles with smaller size, whereas bulk defects dominate in the larger microcrystals. The intensity of the low energy PL peak increases with excitation intensity at low temperature, which can be assigned to band filling and charge transfer effects. The exciton binding energy of Cs4PbBr6 particles is 180 meV which is almost twice that of the Cs4PbBr6 microcrystals. Such tight binding of the excitons facilitates exciton radiative recombination. In addition, the long excited-state lifetime of the Cs4PbBr6 particles contributes to the higher PLQY compared to that of comparable microcrystals. Time-resolved emission microscopy images clearly indicate different recombination kinetics in the ordered and defect regions, which is hidden in ensemble measurements. This strongly suggests that surface trap states are the main reason for the high-efficiency photoluminescence of the intrinsic Cs4PbBr6 particles investigated here, although both surface and bulk traps can emit light.

The morphology of organic semiconductors is critical to their function in optoelectronic devices and is particularly crucial in the donor-acceptor mixture that comprises the bulk heterojunction of organic solar cells. Here, energy landscapes can play integral roles in charge photogeneration, and recently have been shown to drive the accumulation of charge carriers away from the interface, resulting in the buildup of large nanoscale electric fields, much like a capacitor. In this work we combine morphological and spectroscopic data to outline the requirements for this interdomain charge accumulation, finding that this effect is driven by a three-phase morphology that creates an energetic cascade for charge carriers. By adjusting annealing conditions, we show that domain purity, but not size, is critical for an electro-absorption feature to grow-in. This demonstrates that the energy landscape around the interface shapes the movement of charges and that pure domains are required for charge carrier buildup that results in reduced recombination and large interdomain nanoscale electric fields.

Introducing layered quasi-2D perovskite phases into a conventional 3D perovskite light-absorbing matrix is a promising strategy for overcoming the limited environmental stability of 3D perovskite solar cells. Here, we present a simple drop-casting method for preparing hybrid perovskite films comprising both quasi-2D and quasi-3D phases, formed using phenylethylammonium or iso-butylammonium as spacer cations. The film morphology, phase purity, and crystal orientation of the hybrid quasi-2D/3D perovskite films are improved significantly by applying a simple N2 blow-drying step, together with inclusion of methylammonium chloride as an additive. An enhanced power conversion efficiency of 16.0% is achieved using an iso-butylammonium-based quasi-2D/3D perovskite layer which, to our knowledge, is the highest recorded to date for a quasi-2D/3D perovskite solar cells containing a non-spin-cast perovskite layer prepared under ambient laboratory conditions. Perovskite solar cells have substantial potential for solar conversion, but developing simple and scalable fabrication processes is challenging. Here, a drop-casting process compatible with roll-to-roll production of quasi-2D/3D perovskite layers is developed, with a conversion efficiency of up to 16%.

Layered perovskite films, composed of two-dimensional (2D) Ruddlesden-Popper perovskites (RPPs), show improved stability compared to their conventional three-dimensional (3D) counterparts in perovskite solar cells (PSCs). However, 2D PSCs exhibit a lower power conversion efficiency (PCE), which has been attributed to compositional inhomogeneity and nonuniform alignment of the 2D perovskite phases. Methylammonium chloride (MACl) has been adopted as an additive to improve the PCE and the operational stability of 2D PSCs, although the role of MACl in performance enhancement is unclear. In this work, time- and spatially resolved fluorescence and absorption techniques have been applied to study the composition and charge carrier dynamics in MACl-doped BA2MA4Pb5I16 (⟨n⟩ = 5) layered perovskite films. The inhomogeneous phase orientation distribution in the direction orthogonal to the substrate for undoped layered perovskite films undergoes reorganization upon MACl doping. Based on structural and crystallographic analyses, it is revealed that MACl can facilitate the crystallization of small-n 2D perovskite phases at the cost of consuming an increased amount of BA cations. Consequently, an increase in the thickness of large-n 2D perovskite phases accompanies their enhanced perpendicular alignment ([101] crystalline orientation) to the substrate, which facilitates charge carrier transport and collection by electrodes. The defect passivation of the MACl-doped layered perovskite film provided by the small-n phase is also beneficial to the photovoltaic performance of the PSC device. A maximum PCE (∼14.3%) was achieved at 6 mol % MACl doping, with this optimum level influenced by the increased interfacial roughness of the layered perovskite film caused by the edges of small-n perovskite flakes emerging on the front surface.

Developing photostable compounds that undergo quantitative SF is a key challenge. As singlet fission necessitates electron transfer between neighboring molecules, the SF rate is highly sensitive to...

Organic photovoltaic (OPV) efficiencies continue to rise, raising their prospects for solar energy conversion. However, researchers have long considered how to suppress the loss of free carriers by recombination-poor diffusion and significant Coulombic attraction can cause electrons and holes to encounter each other at interfaces close to where they were photogenerated. Using femtosecond transient spectroscopies, we report the nanosecond grow-in of a large transient Stark effect, caused by nanoscale electric fields of ∼487 kV/cm between photogenerated free carriers in the device active layer. We find that particular morphologies of the active layer lead to an energetic cascade for charge carriers, suppressing pathways to recombination, which is ∼2000 times less than predicted by Langevin theory. This in turn leads to the buildup of electric charge in donor and acceptor domains-away from the interface-resistant to bimolecular recombination. Interestingly, this signal is only experimentally obvious in thick films due to the different scaling of electroabsorption and photoinduced absorption signals in transient absorption spectroscopy. Rather than inhibiting device performance, we show that devices up to 600 nm thick maintain efficiencies of >8% because domains can afford much higher carrier densities. These observations suggest that with particular nanoscale morphologies the bulk heterojunction can go beyond its established role in charge photogeneration and can act as a capacitor, where adjacent free charges are held away from the interface and can be protected from bimolecular recombination.

Zero-dimensional (0D) quantum confinement can be achieved in perovskite materials by the confinement of electron and hole states to single PbX64− perovskite octahedra. In this work, 0D perovskite (Cs4PbBr6) micro-crystals were prepared by a simple thermally-assisted solution method and thoroughly characterized. The micro-crystals show a high level of crystallinity and a high photoluminescence quantum yield of 45%. The radiative recombination coefficient of the 0D perovskite micro-crystals, 1.5 × 10−8 s−1 cm3, is two orders of magnitude higher than that of typical three-dimensional perovskite and is likely a strong contributing factor to the high emission efficiency of 0D perovskite materials. Temperature dependent luminescence measurements provide insight into the role of thermally-activated trap states. Spatially resolved measurements on single 0D perovskite micro-crystals reveal uniform photoluminescence intensity and emission decay behaviour suggesting the solution-based fabrication method yields a high-quality and homogenous single-crystal material. Such uniform emission reflects the intrinsic 0D nature of the material, which may be beneficial to device applications.

The photoluminescence quantum yield (ϕPL) of perylene diimide derivatives (PDIs) is often limited by aggregation caused quenching (ACQ) at high concentration or in the neat solid-state. Energy transfer in high dye concentration systems is also a key factor in determining ϕPL as a result of energy funneling to trap sites in the sample. By tuning the substituents, we present two classes of PDIs with aggregation and insulation of the PDI core. By combining these fluorophores in a polymer film, we demonstrate highly emissive samples (85% ϕPL) at high concentration (140 mM or 20% w/w). Experimental and theoretical studies provide insight into why such a combination is necessary to achieve high ϕPL. While insulated fluorophores maintain respectable ϕPL at high concentration, an improved ϕPL can be achieved in the presence of appropriately oriented fluorophore aggregates as emissive traps. The theoretical calculations show that the relative orientation of aggregated monomers can result in energetic separation of localized states from the charge-transfer and bi-excitonic states thereby enabling high ϕPL.

Summary Well-defined crystalline condensed superstructures of conjugated molecules are required for light-emitting optoelectronic devices. Similar to symmetry breaking in nature, stereoisomerism can programmatically and extraordinarily control molecular crystallization and provide access to unexpected optoelectronic behaviors. Here, we demonstrated supramolecular chiral oligofluorenols (2O8-DPFOH-SFX) with a hierarchical ordered structure and ultrastable crystalline-enhanced emission by controlling the molecular stereoisomerism. The raceme displays a highly ordered and uniform structure with molecular-level orientation, but no transition was observed for the mesomer. This well-defined architecture shows unique robust and ultrastable crystalline-enhanced emission with PLQE 90% higher than that of the pristine one. Finally, the nanosheet laser with deep-blue emission was manufactured, attributed to the 0–1 vibronic band of the quasi-planar conformation. The stereoisomerism-property relationship of conjugated aromatic molecules can serve as a model, offering a general guideline for optimizing the morphology and alignment of organic semiconductors for optoelectronic applications.

The ultralow band gap small-molecule IEICO-4F has been employed as a secondary acceptor in both fullerene-based (PTB7-Th:PC71BM) and nonfullerene-based (PBDB-T:ITIC) ternary organic solar cells (OSCs). Structural characterization methods combined with ultrafast spectroscopy have been applied to resolve the mechanisms, leading to the observed improvement in device efficiency upon addition of IEICO-4F. It is shown that IEICO-4F forms ternary mixed domains in the host systems and improves the device efficiency by broadening the absorption spectral range and enhancing both charge separation and charge transport. The enhanced crystallinity of the semiconductor polymer electron donors in the presence of the EIECO-4 provides additional channels for ultrafast charge transfer and transport compared to binary systems. The optimum ternary blend formulations required to improve device efficiencies are reported. This work provides new insights into the fabrication of high-performance ternary OSCs.

A high-performance large-area luminescent solar concentrator (LSC) is demonstrated, fabricated via careful choice of the fluorophore system, waveguide material, and coupled photovoltaic (PV) cells....

UV-illumination of phase-separated surfactant films prepared from mixtures of photopolymerizable 10,12-pentacosadiynoic acid and perfluorotetradecanoic acid results in the formation of fluorescent polydiacetylene fibers and aggregates. In this work, the orientation of polymer strands that comprise the resulting photopolymer structures has been probed using fluorescence anisotropy imaging in combination with defocused single-molecule fluorescence imaging. Imaging experiments indicate the presence of significant fiber-to-fiber heterogeneity, as well as anisotropy within each fiber (or aggregate), with both of these properties changing as a function of film preparation conditions. This anisotropy can be attributed to various alignments of the constituent polymer strands that comprise the larger fibers and aggregates. Intriguingly, when using defocused imaging, fiber images consisted of a series of discrete “doughnut” fluorescence emission patterns, which exhibited intermittent on–off blinking behavior; both of these properties are characteristic of individual emission transition dipoles (single molecules). Further, all of the individual emission transition dipoles had a uniform orientation with respect to the axis of the fiber, indicating a common orientation of discrete emitters in the larger polymer fiber. The implications of these results for future studies of the electronic properties of conjugated polymers in larger macroscopic systems are noted.

Vertically aligning fluorophores to the surface of a waveguide is known to be an effective approach to improve the optical quantum efficiency (OQE) of luminescent solar concentrators (LSCs). While the chromophore alignment assists waveguiding of the emitted photons to the LSC edges, it also significantly reduces the light-harvesting properties of the LSC. We report here a fluorophore pair consisting of a sphere-shaped energy donor and a rod-shaped emitter that was incorporated in LSCs to provide selective fluorophore alignment to address the reduced incident-light absorption issue. A liquid-crystal polymer matrix was used to perpendicularly align the rod-shaped acceptors to a favorable orientation for light guiding, while the sphere-shaped donor was randomly oriented to maintain its light-absorbing properties. The OQE of LSC devices with this selectively aligned donor–acceptor fluorophore system is 78% without significant loss of light-harvesting capability.

Diketopyrrolopyrrole (DPP) derivatives have been proposed for both singlet fission and energy upconversion as they meet the energetic requirements and exhibit superior photostability compared to many other chromophores. In this study, both time-resolved electronic and IR spectroscopy have been applied to investigate excited state relaxation processes competing with fission in dimers of DPP derivatives with varying linker structures. A charge-separated (CS) state is shown to be an important intermediate with dynamics that are both solvent and linker dependent. The CS state is found for a subset of the total population of excited molecules and it is proposed that CS state formation requires suitably aligned dimers within a broader distribution of conformations available in solution. No long-lived triplet signatures indicative of singlet fission were detected, with the CS state likely acting as an alternative relaxation pathway for the excitation energy. This study provides insight into the role of molecular conformation in determining excited state relaxation pathways in DPP dimer systems.

A zinc porphyrin-pendant norbornene polymer with a rigid backbone characterized by a 2:1 E/Z isomeric structure ratio has been synthesized, and its spectroscopic and photophysical properties are examined. Zinc tetraphenylporphyrin, the porphyrin-substituted norbornene monomer, and a previously reported zinc porphyrin-pendant polymer with a flexible polymethylene backbone have been used as comparators. Unlike its flexible counterpart, the rigid norbornene polymer exhibits clear exciton splitting of its Soret band, much more rapid relaxation rates of its excited singlet states, and a very small yield of an unusually short-lived triplet state. Unlike the flexible pendant polymer, which exhibits excimeric S2 fluorescence as a result of chromophore rotation, anti-Kasha emission from the norbornene polymer originates primarily from the unperturbed porphyrin E region. The low triplet yield in the polymer is attributed to greatly increased rates of competing internal conversion within the singlet manifold. Nevertheless, upconverted delayed fluorescence that is quenched by oxygen is observed upon intense steady-state Q-band excitation of degassed polymer solutions, signaling direct triplet involvement. Consistent with the polymer's rigid structure, this biexcitonic process is assigned to ultrafast singlet exciton migration and triplet-triplet annihilation following absorption of a second photon by the small steady-state concentration of polymer triplets.

Summary The realization of robust luminescence in crystalline and anisotropic nanowires of wide bandgap light-emitting conjugated polymers (LCPs) is challenging. Compared to the periodic arrangements in conventional crystalline nanostructure, hierarchical uniform structures will provide an electronic landscape to obtain excellent optoelectronic behaviors, which are desirable for polymer light-emitting diodes (PLEDs) and polymer lasers. Here, we construct a range of robust hierarchical uniform crystalline nanowires of wide bandgap polydiarylfluorenes for light-emitting optoelectronic applications. Steric hindrance interactions of the bulky groups enable these nanowires to show a robust deep-blue emission, due to the suppression of interchain aggregation. Therefore, efficient deep-blue nanowire-based PLEDs with an intrachain excitonic behavior are fabricated. Moreover, our β-conformational hierarchical uniform nanowires present a low-threshold deep-blue lasing behavior. To the best of our knowledge, this is the first deep-blue organic nanolaser based on LCPs self-assembled nanowires by constructing hierarchical uniform crystalline structure.

Several new polymers with rotatable zinc porphyrin pendants have been synthesized and their optical spectroscopic and photophysical properties, including upconversion efficiencies, determined in both fluid solution and thin films. Comparisons made with the β-substituted zinc tetraphenylporphyrin monomers and ZnTPP itself reveal that the yield of triplets resulting from either Q-band or Soret-band excitation of the polymers is surprisingly small. A detailed kinetic analysis of the fluorescence decays and transient triplet absorptions of the substituted monomers and their corresponding polymers reveals that this phenomenon is due to two parallel internal singlet quenching processes assigned to transient intrachain excimer formation. Consequently, the yields of upconverted S2 fluorescence resulting from Q-band excitation in the degassed polymers are significantly diminished in both fluid solution and thin films. Implications of these results for the design of polymer upconverting systems are discussed.

The electronic spectroscopy and photophysics of 1,2-benzazulene (BzAz) have been examined in solution and in thin solid films, with the objective of comparing its intramolecular and intermolecular excited state decay processes with those of azulene. Unlike azulene, the S2 – S0 absorption and fluorescence spectra exhibit a clear mirror image relationship dominated by a single strong Franck-Condon active progression. Picosecond transient absorption spectra and non-linear S2 fluorescence upconversion experiments reveal lifetimes that follow a well-established energy gap law correlation, indicative of a dominant S2 – S1 decay route. Mechanistic interpretations, including the possibility of S2 singlet fission in aggregates, are discussed.

A facile method is proposed to obtain modified shorn graphene oxide (DDAB-sGO) with improved dispersion in organic solvents. Didodecyl dimethylammonium bromide (DDAB)-sGO, which exhibits good dispersibility in the nonpolar solvent o-dichlorobenzene, was obtained via the sono-Fenton reaction and DDAB ionic functionalization. DDAB-sGO was used in the preparation of conjugated polymer:fullerene blend composites. UV–visible absorption spectra, steady-state photoluminescence spectra, fluorescence decay, and grazing incidence X-ray scattering measurements were applied to characterize morphologies, structural features, and charge-transport characteristics of the composites. Doped into poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7):[6,6]-phenyl C71 butyric acid methyl ester (PC71BM) conjugated polymer blends, DDAB-sGO is shown to facilitate increased crystallinity and phase separation of PTB7 and PC71BM to achieve a m...

The light-harvesting efficiency of luminescent solar concentrators (LSCs) is encumbered by reabsorption of emission and concentration quenching. Energy transfer from a high-concentration donor to a low-concentration energy trap can reduce reabsorption losses while maintaining efficient light collection. Emissive aggregates enable this approach by reducing the impact of concentration quenching, which is detrimental to the entire energy transfer process. Here we describe a LSC that utilizes emissive aggregates as energy-transfer pairs for downconversion. We characterize the photophysics of a benzothiadiazole-based emissive aggregate, PITBT-TPE, that complements a highly emissive donor, DPATPAN, and functions as a highly emissive energy-transfer acceptor even at high concentrations in excess of 180 mM in the PMMA matrix. Monte Carlo simulations of LSCs that leverage these emissive aggregates as energy-transfer pairs predicted notable optical efficiencies at large concentrator dimensions. We demonstrate for the first time a LSC that utilizes donor and acceptor AIE chromophores to reduce reabsorption.

Perovskite microplates have important implications in the fields of functional electronics and optoelectronics. We report a facile strategy, antisolvent treatment for the growth of perovskite microplates. The morphology and crystalline quality of the microplates could be controlled by the amount of the chlorobenzene antisolvent used. An appropriate amount of antisolvent facilitates the formation of high-quality perovskite microplates with no residual precursor remaining. Spatially and temporally resolved fluorescence measurements demonstrate the heterogeneity of defect-state density and recombination processes in various perovskite microplate regions. The body center shows higher defect state density when compared with that at the edge or the corner of the microplate. Excessive antisolvent degrades the microplates into smaller particles. The results of this study reveal the factors that influence the crystallization process and photophysical dynamics of perovskite microplates.

A series of four perylene diimide (PDI) chromophores were prepared with increasing steric bulk on the imide substituents with the aim of retarding the effect of concentration quenching on photoluminescence, commonly observed with these dyes. Spectroscopic investigations of the compounds in dilute solution confirmed that the photophysical properties of the PDI core chromophore were not perturbed by the bulky substituents. Solid film samples containing the PDI compounds at various concentrations dispersed in a poly(methyl methacrylate) (PMMA) matrix were examined and compared to amorphous neat films as well as crystalline samples. The PDI compounds containing di-tert-butylphenyl (bPDI-3) and trityl (bPDI-4) substituents showed near unity photoluminescence quantum yield (PLQY) up to 20 mM in PMMA compared to 10% PLQY for the reference compound (bPDI-1) without molecular insulation. Surprisingly, high concentrations (>40 mM) of a phenyl-substituted PDI compound (bPDI-2) with moderate molecular insulation form...

An effective method for preparing poly(3-hexylthiophene) (P3HT) coated graphene oxide (GO), (P-GO), based on an ethanol mediated mixing and solvent evaporation method is described. P-GO exhibits good dispersibility in the non-polar solvent o-dichlorobenzene (DCB) allowing the preparation of polymer blend composites. P-GO was doped into P3HT: PCBM blends by solution mixing and shown to facilitate phase separation of P3HT and PCBM in P3HT: PCBM blend films to achieve a more optimum morphology for polymer photovoltaic cells. Bulk heterojunction P3HT: PCBM solar cells exhibit ∼18% power conversion efficiency enhancement in the presence of P-GO.

2-Chlorophenyl isocyanate (CI) reacts with didodecyl dimethyl ammonium bromide (DDAB) functionalized graphene oxide (DDAB-GO) dispersed in ortho-dichlorobenzene under mild conditions. The CI treatment complements DDAB functionalization to further decrease the hydrophilicity of pristine GO sheets. The resulting CI–DDAB-GO exhibits improved compatibility with the conjugated polymer poly(3-hexylthiophene) (P3HT), compared to DDAB-GO. CI–DDAB-GO sheets can be homogeneously dispersed when blended with P3HT, resulting in an improved morphology compared to P3HT:DDAB-GO composites. The incorporation of CI–DDAB-GO can effectively reduce the dark current of photo-diodes based on P3HT composites, showing potential to enhance the performance of electronic devices based on conjugated polymer composites.

Spectroscopic, photophysical and computational studies designed to expose and explain the differences in the efficiencies of non-coherent photon upconversion (NCPU) by triplet-triplet annihilation (TTA) have been carried out for a new series of alkyl-substituted diphenyl and tetraphenyl zinc porphyrins, both in fluid solution and in solid films. Systematic variations in the alkyl-substitution of the phenyl groups in both the di- and tetraphenyl porphyrins introduces small, but well-understood changes in their spectroscopic and photophysical properties and in their TTA efficiencies. In degassed toluene solution TTA occurs for all derivatives and produces the fluorescent S2 product states in all cases. In PVA matrices, however, none of the di-phenylporphyrins exhibit measurable NCPU whereas all the tetraphenyl-substituted compounds remain upconversion-active. In PVA the NCPU efficiencies of the zinc tetraphenylporphyrins vary significantly with their steric characteristics; the most sterically crowded tetraphenyl derivative exhibits the greatest efficiency. DFT-D computations have been undertaken and help reveal the sources of these differences.

Nanoscale morphology and spin can have a significant impact on charge generation and short time scale recombination in organic photovoltaic materials. We reveal multiple efficient charge separation pathways and the suppression of triplet loss channels in a high-performing nematic liquid crystalline electron donor, benzodithiophene terthiophene rhodanine (BTR). BTR:PC71BM bulk heterojunction photovoltaic devices have been shown to exhibit charge generation quantum yields of ∼90% and power conversion efficiencies >9.5%, even in thick devices. Solvent vapor annealing increases device efficiency, delivering performance almost twice as high as that of untreated blend films, despite reduced exciton quenching. Broadband femtosecond transient absorption spectroscopy reveals both efficient hole and electron transfer on different time scales in the bulk heterojunction blends. BTR triplet excitons are formed due to subnanosecond bimolecular recombination in untreated blend films, though their formation is significan...

Maintaining high incident light absorption while minimizing luminescence reabsorption is a key challenge for organic luminescent solar concentrators (LSCs). Energy migration and trapping using light-harvesting donors and a low-energy highly emitting acceptor is one strategy to reduce the reabsorption issue. However, concentration quenching and the potential formation of quenching aggregates is a limiting factor in realizing efficient devices. We describe the synthesis of a novel molecularly insulated perylene diimide that can resist luminescence quenching at concentrations in excess of 50 mM. Photophysical measurements show the insulated perylene diimide has an excitation energy migration diffusion length of 230 ± 10 A at 60 mM in poly(methyl methacrylate). LSC devices using a mixture of the insulated perylene diimide light absorber and perylene red (LR305) as the low-energy trap emitter exhibit reduced reabsorption and a better current output than LR305 only devices. The results demonstrate that appropri...

We report the effect of the processing solvent on the nanoscale morphology and photophysical dynamics of poly(3-butyl-thiophene) nanowires (P3BT-nw). P3BT-nw assembled in ortho-dichlorobenzene (ODCB) show higher crystallization and a longer conjugation length with increased exciton delocalization compared with those assembled in chlorobenzene (CB). It is proposed that this solvent effect is associated with the higher ordered structures formed from ODCB solution state. Charge-transfer dynamics and phase separation for P3BT-nw:PCBM blends were investigated by ultrafast fluorescence techniques. The more efficient fluorescence quenching observed in P3BT-nw:PCBM blend films processed from ODCB suggests that there is intimate contact between P3BT-nw and PCBM that facilitates charge transfer. The superior performance of organic photovoltaic devices based on P3BT-nw:PCBM bulk heterojunctions processed using ODCB is attributed to the higher crystallization of P3BT-nw, optimized phase separation, and more efficient charge transfer from P3BT-nw to PCBM.

Calix[4]azulene is a non-alternant aromatic calixarene composed of four azulene chromophores linked by methylene groups. Its photochemical stability, photophysical properties and an analysis of its electronic spectra are reported using monomeric azulene as a known reference standard. The molecule is stable when excited in its visible and near uv absorptions and, unlike azulene, produces no measurable “anti-Kasha” fluorescence when excited to its second excited singlet state, S2. This lack of fluorescence places the lifetime of the initially excited, photochemically stable S2 species at less than 1 picosecond. A significant, bathochromic shift of the S2 absorption band system in the calixarene, and the appearance of an additional weak, broad absorption immediately to the red signals significant intramolecular chromophore interaction. Femtosecond transient absorption spectroscopy using excitation in this red-shifted tail of the S2 band system reveals a very weak transient signal most of which decays within one ps, but with suggestions of a slightly longer-lived underlying component. No longer-lived T1 triplet transient is observed. A complete analysis of the data using monomeric azulene as a reference suggests, following elimination of several alternate mechanisms, that the initially excited S2 species may be relaxing via a novel singlet-singlet fission process.

A Monte-Carlo ray tracing simulator with a graphical user interface (MCRTS-GUI) has been developed to provide a quantitative description, performance evaluation and photon loss analysis of luminescent solar concentrators (LSCs). The algorithm is applied to several practical LSC device structures including multiple dyes in the same waveguiding layer, and structures where a dye layer is sandwiched between clear substrates. The effect of the host matrix absorption and the influence of the neighboring layers are investigated. Validations demonstrate that the MCRTS-GUI developed provides a reliable and accurate description of LSC performance. Code for the mixed-dye single layer configuration is converted into a ray-tracing package with a user-friendly interface and is made available as open source software.

We report the observation of singlet fission in aqueous suspensions of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-Pn) nanoparticles (NPs) synthesized using the reprecipitation method. By altering the synthesis conditions we are able to fabricate NPs which evolve from a system of poorly coupled to highly coupled chromophores. This morphological evolution can also be suppressed for a period of several months. Absorption spectra confirm that the particles evolve over time, displaying increased intermolecular interaction, if the initial reaction conditions seeded a polycrystalline sample. We correlate these differences in morphologies to different rates of singlet state decay, where higher intermolecular interaction drives a more rapid rate of decay. Ultrafast time-resolved photoluminescence spectroscopy confirms a short first excited singlet state lifetime (

The successful synthesis is described for a donor–acceptor rod–coil block copolymer comprising blocks of poly[2,7-(9,9-dihexylfluorene)-alt-bithiophene] (F6T2) and polystyrene functionalized with fullerene (PS(C60)) (F6T2-b-PS(C60)). This new material was obtained by combining Suzuki polycondensation with radical addition fragmentation chain transfer. The block copolymer was characterized by nuclear magnetic resonance, gel permeation chromatography, and optical spectroscopy methods. Photophysical data for (F6T2-b-PS(C60)) and a related block copolymer (F6T2-b-PS(PCBM)) (PCBM, phenyl-C61-butyric acid methyl ester) are reported and their performance as compatibilizers in bulk heterojunction organic solar cells is assessed. It is demonstrated that the addition of the rod–coil block copolymers to the active layer extends the operational stability of organic photovoltaic devices. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 888–903

Organic semiconductor nanoparticles provide a potentially scalable approach for photovoltaics that can be processed from aqueous media. Particles of poly(3-hexylthiophene) (P3HT):phenyl-C61-butyric acid methyl ester (PCBM) were prepared using two techniques; those produced by a mini-emulsion method contained greater amounts of crystalline P3HT domains with charge generation resembling phase-separated annealed solvent-cast films.

Kunal Dhoke, Martin Zanni, Upendra Harbola, Ravi Kumar Venkatraman, Elangannan Arunan, King-Chuen Lin, Artur Nenov, Jonathan Skelton, R J Dwayne Miller, Jonathan D. Hirst, Vincenzo Aquilanti, John R. Helliwell, Srihari Keshavamurthy, Sai Ramesh, Mike Ashfold, Anuradha Pallipurath, Priyadarshi Roy Chowdhury, Sanghamitra Mukhopadhyay, Jemmis E D, Himani Medhi, Debabrata Goswami, Prasenjit Halder, Wolfgang Junge, Mahesh Hariharan, Santosh Kumar Singh, Siva Umapathy, Adithya Lakshmannam, Martin Meedom Nielsen, Sankarampadi Aravamudhan, Volker Deckert, Kenneth Ghiggino, Keisuke Tominaga and Alison Edwards

The energy levels of graphene oxide (GO) and reduced graphene oxide (rGO) were estimated in this work. We demonstrate that the energies of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of GO become closer to each other upon reduction. GO and rGO were blended with poly(3-hexylthiophene) (P3HT) to form composite films and their optoelectronic properties were evaluated. It was found that both electrons and holes from photoexcited P3HT can transfer to rGO leading mostly to recombination while only electrons transfer to GO.

Martin Zanni, E. D. Jemmis, Sankarampadi Aravamudhan, Elangannan Arunan, Neil Hunt, Sanghamitra Mukhopadhyay, Helena J. Shepherd, Srihari Keshavamurthy, Arend G. Dijkstra, Mike Ashfold, Himangshu Prabal Goswami, Artur Nenov, Himani Medhi, Kenneth Ghiggino, Kiran Moirangthem, R. J. Dwayne Miller, Debabrata Goswami, Siva Umapathy, John R. Helliwell, Jonathan D. Hirst, Stephen Meech, Shaul Mukamel, Biman Bagchi and Marylise Buron-Le Cointe

Non-coherent photon upconversion by rapid intramolecular triplet–triplet annihilation has been observed following nanosecond laser excitation of Ru(dmb) 3 and sequential two-fold triplet energy sensitization of a polymer containing 30 pendant diphenylanthracene groups. A global kinetic analysis of the system has been performed; the intramolecular annihilation step occurs on a sub-nanosecond time scale.

Time-resolved fluorescence anisotropy measurements (TRAMs) are widely used to probe the dynamics of the various processes that can lead to the depolarisation of emission following photoselection by polarised excitation. The most commonly investigated of these emission depolarising phenomena is molecular rotational motion, but TRAMs are very useful for determining the kinetics of a host of other processes. In this paper we review several examples for which we have observed in our laboratories initially unexpectedly complex temporal behaviour of the time-resolved fluorescence anisotropy signal from relatively 'simple' chemical systems. In certain circumstances the anisotropy (i) decays on timescales when superficially it might be thought it should remain constant, (ii) shows marked 'dip and rise' behaviour in its intensity, or (iii) can change sign as the anisotropy evolves in time. Fundamentally simple processes, including molecular rotational motion, energy migration and excited state photophysics, can cause such behaviour.

The photophysics of a conjugated triblock copolymer comprising poly(9,9-dioctylfluorene-co-bis-N,N'-(4-methylphenyl)-bis-N,N'-phenyl-1,4-phenylenediamine) (PFM) electron donor and poly(4-(9,9-dioctyl-9H-fluoren-2-yl)benzo[c][1,2,5]-thiadiazole) (F8BT) electron acceptor blocks has been studied in solution, in films, and as single chains. While an additional long-wavelength emission apparent in neat films of the copolymer is attributed to interchain exciplex formation, no such long-wavelength emission is apparent in solution or from single molecules. However, in these cases, time-resolved fluorescence measurements indicate the presence of a delayed fluorescence. The kinetics of the delayed emission can be interpreted in terms of an equilibrium between a locally excited and a charge-transfer state at the interface of the copolymer block components. Rate constants and thermodynamic quantities associated with these processes have been evaluated. The single-molecule results allow the assignment of an intramolecular charge-transfer state in an isolated conjugated block copolymer chain.