Improving the Quantum Yields of Perylene Diimide Aggregates by Increasing Molecular Hydrophobicity in Polar Media.

Here we report the quantum yield of four aggregated perylene diimide (PDI) species that vary by the length of the branched side chains attached at the N,N' imide positions. The PDI molecules were dissolved in binary water:methanol solvents as a means to vary the solvent polarity and control the degree of aggregation in solution. By performing spectroscopy, kinetics, and light scattering experiments, the nature of the molecular interactions in the solutions was determined. The maximum quantum yield of the aggregated molecules increased from 0.04 for the shortest chain molecule (B2) to 0.20 for the largest chain molecule (B13). The higher quantum yield of B13 compared with B2 correlates well with an increase in the fluorescence lifetime. The monomer emission lifetime was 4.8 ns whereas a lifetime as high as 21.2 ns was measured for the B13 aggregate fluorescence. A shorter sub-nanosecond lifetime was also measured for suspended colloids in B5, B9, and B13. The enhanced quantum yield is attributed to an increase of disorder in the B13 aggregates. As the polarity of the solution increases, the hydrophobic effect further enhances the disorder, and, therefore, the quantum yields in these particles.

Enhanced Davydov Splitting in Crystals of a Perylene Diimide Derivative.

We report the polarized absorption spectra of high-quality, thin crystals of a perylene diimide (PDI) species with branched side chains (B2). The absorption spectrum shows exemplary polarization-dependent H-like and J-like aggregate behavior upon orthogonal excitation, with a sizable Davydov splitting (DS) of 1230 cm-1 and peak to peak splitting of 3040 cm-1. The experimental results are compared to theoretical calculations with remarkable agreement. The theoretical analysis of the polarized absorption spectra shows evidence of a high degree of intermolecular charge transfer, which, along with Coulombic coupling, conspires to create the unprecedented DS for this family of dye molecules. The large polarization dependence of the electronic spectra is attributed to the unique twisted crystal structure, in which a substantial rotational displacement exists between neighboring chromophores within a π-stack. These results highlight the strong sensitivity of the Davydov splitting to intermolecular geometry in PDI systems.

Lock-arm supramolecular ordering: a molecular construction set for cocrystallizing organic charge transfer complexes.

Organic charge transfer cocrystals are inexpensive, modular, and solution-processable materials that are able, in some instances, to exhibit properties such as optical nonlinearity, (semi)conductivity, ferroelectricity, and magnetism. Although the properties of these cocrystals have been investigated for decades, the principal challenge that researchers face currently is to devise an efficient approach which allows for the growth of high-quality crystalline materials, in anticipation of a host of different technological applications. The research reported here introduces an innovative design, termed LASO-lock-arm supramolecular ordering-in the form of a modular approach for the development of responsive organic cocrystals. The strategy relies on the use of aromatic electronic donor and acceptor building blocks, carrying complementary rigid and flexible arms, capable of forming hydrogen bonds to amplify the cocrystallization processes. The cooperativity of charge transfer and hydrogen-bonding interactions between the building blocks leads to binary cocrystals that have alternating donors and acceptors extending in one and two dimensions sustained by an intricate network of hydrogen bonds. A variety of air-stable, mechanically robust, centimeter-long, organic charge transfer cocrystals have been grown by liquid-liquid diffusion under ambient conditions inside 72 h. These cocrystals are of considerable interest because of their remarkable size and stability and the promise they hold when it comes to fabricating the next generation of innovative electronic and photonic devices.

Effects of Exciton Polarity in Charge-Transfer Polymer/PCBM Bulk Heterojunction Films.

Charge-transfer copolymers with local electron density gradients, systematically modified by quantity and position of fluorination, result in widely variable (2-8%) power conversion efficiencies (PCEs). Ultrafast, near-infrared, transient absorption spectroscopy on the corresponding films reveals the influence of exciton polarity on ultrafast populations and decay dynamics for the charge-separated and charge-transfer states as well as their strong correlation to device PCEs. By using an excitation energy-dependent, dynamic red shift in the transient absorption signal for the polymer cation, the exciton polarity induced by push-pull interactions within each polymer fragment is shown to enhance charge dissociation on time scales of tens to hundreds of picoseconds after excitation. These results suggest the important role played by the local electronic structure not only for exciton dissociation but also for device performance.

Room-temperature ferroelectricity in supramolecular networks of charge-transfer complexes.

Materials exhibiting a spontaneous electrical polarization that can be switched easily between antiparallel orientations are of potential value for sensors, photonics and energy-efficient memories. In this context, organic ferroelectrics are of particular interest because they promise to be lightweight, inexpensive and easily processed into devices. A recently identified family of organic ferroelectric structures is based on intermolecular charge transfer, where donor and acceptor molecules co-crystallize in an alternating fashion known as a mixed stack: in the crystalline lattice, a collective transfer of electrons from donor to acceptor molecules results in the formation of dipoles that can be realigned by an external field as molecules switch partners in the mixed stack. Although mixed stacks have been investigated extensively, only three systems are known to show ferroelectric switching, all below 71 kelvin. Here we describe supramolecular charge-transfer networks that undergo ferroelectric polarization switching with a ferroelectric Curie temperature above room temperature. These polar and switchable systems utilize a structural synergy between a hydrogen-bonded network and charge-transfer complexation of donor and acceptor molecules in a mixed stack. This supramolecular motif could help guide the development of other functional organic systems that can switch polarization under the influence of electric fields at ambient temperatures.

Ultrafast intramolecular exciton splitting dynamics in isolated low-band-gap polymers and their implications in photovoltaic materials design.

Record-setting organic photovoltaic cells with PTB polymers have recently achieved ~8% power conversion efficiencies (PCE). A subset of these polymers, the PTBF series, has a common conjugated backbone with alternating thieno[3,4-b]thiophene and benzodithiophene moieties but differs by the number and position of pendant fluorine atoms attached to the backbone. These electron-withdrawing pendant fluorine atoms fine tune the energetics of the polymers and result in device PCE variations of 2-8%. Using near-IR, ultrafast optical transient absorption (TA) spectroscopy combined with steady-state electrochemical methods we were able to obtain TA signatures not only for the exciton and charge-separated states but also for an intramolecular ("pseudo") charge-transfer state in isolated PTBF polymers in solution, in the absence of the acceptor phenyl-C(61)-butyric acid methyl ester (PCBM) molecules. This led to the discovery of branched pathways for intramolecular, ultrafast exciton splitting to populate (a) the charge-separated states or (b) the intramolecular charge-transfer states on the subpicosecond time scale. Depending on the number and position of the fluorine pendant atoms, the charge-separation/transfer kinetics and their branching ratios vary according to the trend for the electron density distribution in favor of the local charge-separation direction. More importantly, a linear correlation is found between the branching ratio of intramolecular charge transfer and the charge separation of hole-electron pairs in isolated polymers versus the device fill factor and PCE. The origin of this correlation and its implications in materials design and device performance are discussed.

Strong steric hindrance effect on excited state structural dynamics of Cu(I) diimine complexes.

The metal-to-ligand-charge-transfer (MLCT) excited state of Cu(I) diimine complexes is known to undergo structural reorganization, transforming from a pseudotetrahedral D(2d) symmetry in the ground state to a flattened D(2) symmetry in the MLCT state, which allows ligation with a solvent molecule, forming an exciplex intermediate. Therefore, the structural factors that influence the coordination geometry change and the solvent accessibility to the copper center in the MLCT state could be used to control the excited state properties. In this study, we investigated an extreme case of the steric hindrance caused by attaching bulky tert-butyl groups in bis(2,9-di-tert-butyl-1,10-phenanthroline)copper(I), [Cu(I)(dtbp)(2)](+). The two bulky tert-butyl groups on the dtbp ligand lock the MLCT state into the pseudotetrahedral coordination geometry and completely block the solvent access to the copper center in the MLCT state of [Cu(I)(dtbp)(2)](+). Using ultrafast transient absorption spectroscopy and time-resolved emission spectroscopy, we investigated the MLCT state property changes due to the steric hindrance and demonstrated that [Cu(I)(dtbp)(2)](+) exhibited a long-lived emission but no subpicosecond component that was previously assigned as the flattening of the pseudotetrahedral coordination geometry. This suggests the retention of its pseudotetrahedral D(2d) symmetry and the blockage of the solvent accessibility. We made a comparison between the excited state dynamics of [Cu(I)(dtbp)(2)](+) with its mono-tert-butyl counterpart, bis(2-tert-butyl-1,10-phenanthroline)copper(I) [Cu(I)(tbp)(2)](+). The subpicosecond component assigned to the flattening of the D(2d) coordination geometry in the MLCT excited state was again present in the latter because the absence of a tert-butyl on the phenanthroline allows flattening to the pseudotetrahedral coordination geometry. Unlike the [Cu(I)(dtbp)(2)](+), [Cu(I)(tbp)(2)](+) exhibited no detectable emission at room temperature in solution. These results provide new insights into the manipulation of various excited state properties in Cu diimine complexes by certain key structural factors, enabling optimization of these systems for solar energy conversion applications.

Effects of additives on the morphology of solution phase aggregates formed by active layer components of high-efficiency organic solar cells.

Processing additives are used in organic photovoltaic systems to optimize the active layer film morphology. However, the actual mechanism is not well understood. Using X-ray scattering techniques, we analyze the effects of an additive diiodooctane (DIO) on the aggregation of a high-efficiency donor polymer PTB7 and an acceptor molecule PC(71)BM under solar cell processing conditions. We conclude that DIO selectively dissolves PC(71)BM aggregates, allowing their intercalation into PTB7 domains, thereby optimizing both the domain size and the PTB7-PC(71)BM interface.

Examining the effect of the dipole moment on charge separation in donor-acceptor polymers for organic photovoltaic applications.

A new low band gap copolymer PBB3 containing [6,6']bi[thieno[3,4-b]thiophenyl]-2,2'-dicarboxylic acid bis-(2-butyloctyl) ester (BTT) and 4,8-bis(2-butyloctyl)benzo[1,2-b:4,5-b']dithiophene (BDT) units was synthesized and tested for solar cell efficiency. PBB3 showed a broad absorbance in the near-IR region with a substantially red-shifted (by more than 100 nm) λ(max) at 790 nm as compared to the PTB series of polymers, which have been previously reported. The PBB3 polymer also showed both a favorable energy level match with PCBM (with a LUMO energy level of -3.29 eV) and a favorable film domain morphology as evidenced by TEM images. Despite these seemingly optimal parameters, a bulk heterojunction (BHJ) photovoltaic device fabricated from a blend of PBB3 and PC(71)BM showed an overall power conversion efficiency (PCE) of only 2.04% under AM 1.5G/100 mW cm(-2). The transient absorption spectra of PBB3 showed the absence of cationic and pseudo charge transfer states that were observed previously in the PTB series polymers, which were also composed of alternating thienothiophene (TT) and BDT units. We compared the spectral features and electronic density distribution of PBB3 with those of PTB2, PTB7, and PTBF2. While PTB2 and PTB7 have substantial charge transfer characteristics and also relatively large local internal dipoles through BDT to TT moieties, PTBF2 and PBB3 have minimized internal dipole moments due to the presence of two adjacent TT units (or two opposing fluorine atoms in PTBF2) with opposite orientations or internal dipoles. PBB3 showed a long-lived excitonic state and the slowest electron transfer dynamics of the series of polymers, as well as the fastest recombination rate of the charge-separated (CS) species, indicating that electrons and holes are more tightly bound in these species. Consequently, substantially lower degrees of charge separation were observed in both PBB3 and PTBF2. These results show that not only the energetics but also the internal dipole moment along the polymer chain may be critical in maintaining the pseudocharge transfer characteristics of these systems, which were shown to be partially responsible for the high PCE device made from the PTB series of low band gap copolymers.

Nanoscale structure, dynamics and power conversion efficiency correlations in small molecule and oligomer-based photovoltaic devices.

Photovoltaic functions in organic materials are intimately connected to interfacial morphologies of molecular packing in films on the nanometer scale and molecular levels. This review will focus on current studies on correlations of nanoscale morphologies in organic photovoltaic (OPV) materials with fundamental processes relevant to photovoltaic functions, such as light harvesting, exciton splitting, exciton diffusion, and charge separation (CS) and diffusion. Small molecule photovoltaic materials will be discussed here. The donor and acceptor materials in small molecule OPV devices can be fabricated in vacuum-deposited, multilayer, crystalline thin films, or spin-coated together to form blended bulk heterojunction (BHJ) films. These two methods result in very different morphologies of the solar cell active layers. There is still a formidable debate regarding which morphology is favored for OPV optimization. The morphology of the conducting films has been systematically altered; using variations of the techniques above, the whole spectrum of film qualities can be fabricated. It is possible to form a highly crystalline material, one which is completely amorphous, or an intermediate morphology. In this review, we will summarize the past key findings that have driven organic solar cell research and the current state-of-the-art of small molecule and conducting oligomer materials. We will also discuss the merits and drawbacks of these devices. Finally, we will highlight some works that directly compare the spectra and morphology of systematically elongated oligothiophene derivatives and compare these oligomers to their polymer counterparts. We hope this review will shed some new light on the morphology differences of these two systems.

Electronic processes in conjugated diblock oligomers mimicking low band-gap polymers: experimental and theoretical spectral analysis.

Conjugated oligomers containing a common central thienothiophene unit symmetrically connected to two identical thiophene oligomers were studied as model systems for a series of low bandgap organic diblock copolymers. The oligothiophene side chain fragments were varied in length as a means to tune the electronic coupling between the thienothiophene and oligothiophene moieties. The fragment length dependence of both the ground- and excited-state electronic and structural properties of a series of diblock oligomers were investigated in detail. The charge transfer character in these diblock oligomers, revealed by their optical absorption and fluorescence spectra, is responsible for their low band gap and energy gap tunability compared with their homooligomer counterparts. The electronic spectra and theoretical analysis indicate a partially localized central charge in the first excited state. Using experimental results and comparing them with theoretical calculations, we estimate that the electronic effects from a single thienothiophene unit spreads over seven to nine adjacent units through π-conjugation along the oligomers.

Room temperature light emission from the low-dimensional semiconductors AZrPS6 (A = K, Rb, Cs).

The new semiconducting thiophosphate compounds KZrPS(6), RbZrPS(6), and CsZrPS(6) exhibit red light emission at room temperature. The materials have longer photoluminescence lifetimes than most of the inorganic chalcogenide semiconductors. They can be solution processed into thin films for potential device fabrication.

Dye-sensitized solar cells: sensitizer-dependent injection into ZnO nanotube electrodes.

Two porphyrin-based dyes with carboxylic acid tethers of differing acidity in both protonated and deprotonated forms were examined on ZnO nanotube electrodes. All of the dyes have similar surface coverage, but only the more acidic dye in the acid form injects electrons well; this dye is the only one that corrodes the ZnO. In control experiments on TiO(2) nanoparticle electrodes, both dyes load and inject in protonated and deprotonated forms. These results are consistent with a requirement that the dye must partially corrode the ZnO surface in order for efficient injection to occur. Alternatively, it may possibly point to a coupling of electron injection to proton uptake.

Zinc oxide nanorod growth on gold islands prepared by microsphere lithography on silicon and quartz.

Gold islands, vapor deposited on silicon and quartz by microsphere lithography patterning, are used to nucleate arrays of ZnO nanorods. ZnO is grown on approximately 0.32 microm2 Au islands by carbothermal reduction in a tube furnace. Scanning electron microscopy (SEM) and energy dispersive atomic X-ray spectroscopy (EDS) confirm that the gold effectively controls the sites of nucleation of ZnO. Atomic force microscopy (AFM) shows that approximately 30 nm diameter nanorods grow horizontally, along the surface. Alloy droplets that are characteristic of the vapor-liquid-solid (VLS) mechanism are observed at the tips of the nanorods. The spatial growth direction of VLS catalyzed ZnO nanorods is along the substrate when they nucleate from gold islands on silicon and quartz. The energy of adhesion of the VLS droplet to the surface can account for the horizontal growth.

Ultrafast wavelength-dependent lasing-time dynamics in single ZnO nanotetrapod and nanowire lasers.

The ultrafast lasing dynamics of single zinc oxide nanotetrapods and nanowires are investigated by two-color femtosecond excitation/optical injection spectroscopy. The transient spectral gain induced by time-delayed optical injection pulses (400 nm) is used to investigate the spectrally and temporally resolved lasing properties in a single tetrapod or nanowire laser excited by 267-nm pulses. The lasing output pulse exhibits a faster lasing decay time than the carrier decays due to the superlinear dependence of the lasing on the carrier density. Lasing at the low-energy side of the gain bandwidth (392 nm) has a full width at half maximum (fwhm) for stimulated emission of 1.7 ps. Lasing at 390 nm, the high-energy side of the gain bandwidth, has a fwhm of 2.1 ps for a single example nanowire. The change in lasing dynamics as a function of wavelength is affected by band gap renormalization, since lasing in the electron-hole plasma regime depends not only on the carrier density but also on the band gap shift with carrier density.