Photocurrent generation in organic photovoltaics (OPVs) relies on the dissociation of excitons into free electrons and holes at donor/acceptor heterointerfaces. The low dielectric constant of organic semiconductors leads to strong Coulomb interactions between electron-hole pairs that should in principle oppose the generation of free charges. The exact mechanism by which electrons and holes overcome this Coulomb trapping is still unsolved, but increasing evidence points to the critical role of hot charge-transfer (CT) excitons in assisting this process. Here we provide a real-time view of hot CT exciton formation and relaxation using femtosecond nonlinear optical spectroscopies and non-adiabatic mixed quantum mechanics/molecular mechanics simulations in the phthalocyanine-fullerene model OPV system. For initial excitation on phthalocyanine, hot CT excitons are formed in 10(-13) s, followed by relaxation to lower energies and shorter electron-hole distances on a 10(-12) s timescale. This hot CT exciton cooling process and collapse of charge separation sets the fundamental time limit for competitive charge separation channels that lead to efficient photocurrent generation.

Hydrogen bonding can be used to significantly enforce the intra‐columnar stacking order in discotic mesogens. The ordered hexagonal columnar mesophase of a HAT‐CONHR derivative is characterized by the smallest inter‐disk distance ever found in columnar liquid crystals (3.18–3.20 Å). This additional attractive interaction between the disks in the column results in a regular disc stacking and thus in a high charge‐carrier mobility over the whole investigated temperature range (from room temperature up to 200 °C).

Hexaazatrinaphthylene (HATNA) derivatives with six alkylsulfanyl chains of different length (hexyl, octyl, decyl and dodecyl) have been designed to obtain new potential electron-carrier materials. The electron-deficient nature of these compounds has been demonstrated by cyclic voltammetry. Their thermotropic behaviour has been studied by means of differential scanning calorimetry and polarised optical microscopy. The supramolecular organisation of these discotic molecules has been explored by temperature-dependent X-ray diffraction on powders and oriented samples. In addition to various liquid crystalline columnar phases (Col(hd), Col(rd)), an anisotropic plastic crystal phase is demonstrated to exist. The charge-carrier mobilities have been measured with the pulse-radiolysis time-resolved microwave-conductivity technique. They are found to be higher in the crystalline than in the liquid crystalline phases, with maximum values of approximately 0.9 and 0.3 cm(2) V(-1) s(-1), respectively, for the decylsulfanyl derivative. Mobilities strongly depend on the nature of the side chains.

The formation of solid thin films from colloidal semiconductor quantum dots (QDs) is often accompanied by red shifts in excitonic transitions, but the mechanisms responsible for the red shifts are under debate. We quantitatively address this issue using optical absorption spectroscopy of two-dimensional (2D) and three-dimensional (3D) arrays of PbSe QDs with controlled inter-QD distance, which was determined by the length of alkanedithiol linking molecules. With decreasing inter-QD distance, the first and second exciton absorption peaks show increasing red shifts. Using thin films consisting of large and isolated QDs embedded in a matrix of small QDs, we determine that a dominant contribution to the observed red shift is due to changes in polarization of the dielectric environment surrounding each QD (∼88%), while electronic or transition dipole coupling plays a lesser role. However, the observed red shifts are more than 1 order of magnitude larger than theoretical predictions based on the dielectric polarization effect for spherical QDs. We attribute this anomalously large polarization effect to deviations of the exciton wave functions from eigenfunctions of the idealized spherical quantum well model.

While the effect of the insoluble block length on micelle properties is well understood, the effect of the soluble block is still controversial. We, therefore, have investigated the effect of the molecular weight of the soluble block on the critical micelle concentration (CMC), aggregation number, and hydrodynamic radius of spherical polymer micelles. Spherical micelles were formed from polystyrene-b-polyisoprene (PS-b-PI) in heptane, which was a good solvent for PI and a poor solvent for PS. Measurements were performed on two series of PS-b-PI with a constant PS block (19 and 39 kDa, respectively) and PI blocks varying from 10 to 100 kDa. For samples with large PI blocks, the experimental data were found to be in agreement with the commonly used star-like model. However, the experimental data for samples with short PI blocks deviated from the crew-cut micelle model. To correctly capture the crossover between the crew-cut and star-like regimes, it was found necessary to use recently developed scaling theory which explicitly considers all contributions to the free energy of the micelle. In agreement with theory, the aggregation number decreased while hydrodynamic radius and CMC increased with the molecular weight of the PI block. An interesting finding of these experiments is that the micelles of the 19 kDa series are in equilibrium at 25°C, whereas the 39 kDa samples with the longer PS core block are "frozen" at room temperature. This was confirmed by SAXS measurements of core expansion upon heating which revealed a glass transition temperature of the 39 kDa samples at 28 ( 1°C. The temperature value is consistent with 10% swelling of the PS core with heptane as determined by SAXS and SLS. IntroductionWhen diblock polymers are dissolved in a selective solvent (good solvent for one block and poor solvent for the other) above a certain concentration, called the critical micelle concentration (CMC), the diblocks will associate to form micelles with a core of the insoluble block and a corona of the soluble block. [1][2][3][4] The free energy of the micelle is the sum of the free energies of the core, corona, and interface between them. By forming micelles, the diblocks are able to lower their free energy, since the insoluble blocks aggregate and thus reduce their interface with the solvent. However, formation of micelles results in extension of the core and corona blocks raising their elastic free energy. As a result, changing the size of the two blocks changes the balance of the free energy and in turn changes the aggregation number, the micelle hydrodynamic radius, and the CMC.There has been a significant amount of work on the micelles structure for different types of neutral 5-11 and charged 12-14 block copolymers. Since molecular weight of the insoluble block has a stronger effect on the micellar properties than the soluble block does, most of these studies have focused on the effect of the insoluble core-block weight. The few that did investigate the effect of the soluble block used complicated systems containin...

Columnar liquid crystals (LCs) are reported to align spontaneously homeotropically—that is, orthogonally to the surface (see figure and inside cover)—on a glass surface covered with a layer of poly(tetrafluoroethylene) transferred by friction (rubbing). This strategy for producing macroscopic monodomains of homeotropically aligned LCs may find important applications in the fabrication of LC‐based organic solar cells.

Interface dipole determines the electronic energy alignment in donor/acceptor interfaces and plays an important role in organic photovoltaics. Here we present a study combining first principles density functional theory (DFT) with ultraviolet photoemission spectroscopy (UPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) to investigate the interface dipole, energy level alignment, and structural properties at the interface between CuPc and C60. DFT finds a sizable interface dipole for the face-on orientation, in quantitative agreement with the UPS measurement, and rules out charge transfer as the origin of the interface dipole. Using TOF-SIMS, we show that the interfacial morphology for the bilayer CuPc/C60 film is characterized by molecular intermixing, containing both the face-on and the edge-on orientation. The complementary experimental and theoretical results provide both insight into the origin of the interface dipole and direct evidence for the effect of interfacial morphology on the interface dipole.

It has been proposed that interface morphology affects the recombination rate for electrons and holes at donor-acceptor heterojunctions in thin film organic photovoltaic cells. The optimal morphology is one where there is disorder at the heterointerface and order in the bulk of the thin films, maximizing both the short circuit current and open circuit voltage. We show that an amorphous, buried functionalized molecular squaraine donor layer can undergo an "inverted" quasi-epitaxial growth during postdeposition processing, whereby crystallization is seeded by a subsequently deposited self-assembled nanocrystalline acceptor C60 cap layer. We call this apparently unprecedented growth process from a buried interface "inverse quasi-epitaxy" where the crystallites of these "soft" van der Waals bonded materials are only approximately aligned to those of the cap. The resulting crystalline interface hastens charge recombination, thereby reducing the open circuit voltage in an organic photovoltaic cell. The lattice registration also facilitates interdiffusion of the squaraine donor and C60 acceptor, which dramatically improves the short circuit current. By controlling the extent to which this crystallization occurs, the voltage losses can be minimized, resulting in power conversion efficiencies of ηP = 5.4 ± 0.3% for single-junction and ηP = 8.3 ± 0.4% for tandem small-molecule photovoltaics. This is a general phenomenon with implications for all organic donor-acceptor junctions. That is, epitaxial relationships typically result in a reduction in open circuit voltage that must be avoided in both bilayer and bulk heterojunction organic photovoltaic cells.