Structural order in organic solar cells is paramount: it reduces energetic disorder, boosts charge and exciton mobilities, and assists exciton splitting. Owing to spatial localization of electronic states, microscopic descriptions of photovoltaic processes tend to overlook the influence of structural features at the mesoscale. Long-range electrostatic interactions nevertheless probe this ordering, making local properties depend on the mesoscopic order. Using a technique developed to address spatially aperiodic excitations in thin films and in bulk, we show how inclusion of mesoscale order resolves the controversy between experimental and theoretical results for the energy-level profile and alignment in a variety of photovoltaic systems, with direct experimental validation. Optimal use of long-range ordering also rationalizes the acceptor-donor-acceptor paradigm for molecular design of donor dyes. We predict open-circuit voltages of planar heterojunction solar cells in excellent agreement with experimental data, based only on crystal structures and interfacial orientation.

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Solar cells based on conjugated polymer and fullerene blends have been developed as a low-cost alternative to silicon. For efficient solar cells, electron-hole pairs must separate into free mobile charges that can be extracted in high yield. We still lack good understanding of how, why and when carriers separate against the Coulomb attraction. Here we visualize the charge separation process in bulk heterojunction solar cells by directly measuring charge carrier drift in a polymer:fullerene blend with ultrafast time resolution. We show that initially only closely separated (o1 nm) charge pairs are created and they separate by several nanometres during the first several picoseconds. Charge pairs overcome Coulomb attraction and form free carriers on a subnanosecond time scale. Numerical simulations complementing the experimental data show that fast three-dimensional charge diffusion within an energetically disordered medium, increasing the entropy of the system, is sufficient to drive the charge separation process.

Herein, we focus on the principles of photoconduction in random semiconductors—the key processes being optical generation of charge carriers and their subsequent transport. This is not an overview of the current work in this area, but rather a highlight of elementary processes, their involvement in modern devices and a summary of recent developments and achievements. Experimental results and models are discussed briefly to visualize the mechanism of optical charge generation in pure and doped organic solids. We show current limits of models based on the Onsager theory of charge generation. After the introduction of experimental techniques to characterize charge transport, the hopping concept for transport in organic semiconductors is outlined. The peculiarities of the transport of excitons and charges in disorderd organic semiconductors are highlighted. Finally, a short discussion of ultrafast transport and single chain transport completes the review.

We demonstrate the alignment-preserving transfer of parallel graphene nanoribbons (GNRs) onto insulating substrates. The photophysics of such samples is characterized by polarized Raman and photoluminescence (PL) spectroscopies. The Raman scattered light and the PL are polarized along the GNR axis. The Raman cross section as a function of excitation energy has distinct excitonic peaks associated with transitions between the one-dimensional parabolic subbands. We find that the PL of GNRs is intrinsically low but can be strongly enhanced by blue laser irradiation in ambient conditions or hydrogenation in ultrahigh vacuum. These functionalization routes cause the formation of sp defects in GNRs. We demonstrate the laser writing of luminescent patterns in GNR films for maskless lithography by the controlled generation of defects. Our findings set the stage for further exploration of the optical properties of GNRs on insulating substrates and in device geometries.

Triplet emitters based on platinum(II) complexes have gained major attention in recent times.[1] They can form aggregates or excimers, causing shifts in the emitted wavelengths and affecting the photoluminescence quantum yields (PLQYs). [2] Even though this effect can be exploited for the construction of white organic light emitting diodes (WOLEDs), [3] it is disadvantageous for applications where color purity is desirable. Terpyridine ligands [4] and their N^C^N and N^N^C analogues [5] have been coordinated to platinum(II), leading to neutral, mono-, or doubly charged species, some of which display bright luminescence. They can form supramolecular structures, such as nanowires, nanosheets, and polymeric mesophases, with interesting optical properties.[6]For low-molecular-weight organo-or hydrogelators, [7] the operating mechanism of gelation has been recognized as a supramolecular effect, where the constituting fibers, usually of microscale lengths and nanoscale diameters, are formed in solution predominantly by unidirectional self-assembly.[8] The entanglement of filaments gives a network that entraps solvent molecules within the compartments. As supramolecular gels provide fibrous aggregates with long-range order, they could be of interest in the fields of optoelectronic devices and sensors. In this context, organometallic gelators can display metal-metal interactions that influence their properties.[9]Herein we present a straightforward one-pot synthesis of neutral, soluble platinum(II) coordination compounds bearing a dianionic tridentate terpyridine-like ligand. The coordination of an alkyl pyridine ancillary moiety to the 2,6-bis(tetrazolyl)pyridine complex allowed us to enhance the solubility and thus the processability. The synthetic approach involved mild reaction conditions that involved a nonnucleophilic base and an adequate inorganic platinum(II) precursor. Moisture-and oxygen exclusion were not required, and the product was easily purified by repeated precipitation (Scheme 1). The emission intensity of the complex attained a PLQY of up to 87 % in thin films, with concentrationindependent color and efficiency. We demonstrated its suitability as a dopant in solution-processed OLEDs. Furthermore, we discovered that this complex is also able to selfassemble into bright nanofibers, which can interlock to yield highly emissive gels (90 % PLQY), thus constituting a versatile building block for luminescent supramolecular architectures. Scheme 1. One-pot synthesis of platinum(II) complex 4 and a representation of the self-assembly process, going from luminescent aggregates to fibers and gels.

We report on measurements of phosphorescence (Ph) and delayed fluorescence (DF) in poly(2,7-(9,9-bis(2-ethylhexyl)fluorene)) (PF2/6) present as dilute solid solution, as well as in bulk films. From combined experimental investigations of the time and intensity dependence of DF and Ph, as well as the temperature dependence of DF, we are able to show that DF in PF2/6, both in solution and film, is dominated by triplet-triplet annihilation (TTA). As a consequence of the intrachain diffusion and, hence, relaxation of triplets, the kinetics of the bimolecular reaction show a turnover from dispersive to nondispersive regime as borne out by the DF decay. This is in accord with theoretical predictions of TTA in disordered solids. From the transition to equilibrium kinetics, we are able to estimate the width of the density of states of triplets to 40 meV in good agreement with the width of the inhomogeneous broadenend S0←T1 transition of PF2/6.

Intense research is currently directed towards polymer light-emitting diodes (PLEDs) because of their potential for application in flat-panel displays. [1,2] Based on their compatibility with solution processing, the exploration of their suitability for organic electronic devices has primarily been motivated by the need for low-cost production over large areas by utilizing spin-coating, ink-jet printing, or screenprinting technologies. According to simple statistics, the process of charge injection and recombination in PLEDs generates singlet excitons with a quantum efficiency of only 25 %, setting an upper limit to the efficiency of PLEDs based on fluorescent polymers. Even though the singlet±triplet ratio in PLEDs is still a topic of debate, [3±6] it is expected that the radiative decay of both singlet and triplet states would substantially increase the efficiency of PLEDs. Phosphorescent dyes have been used to overcome the efficiency limit imposed by the unavoidable formation of triplet excitons, and highly efficient phosphorescent light-emitting diodes based on low molecular weight materials have been demonstrated.[7±9]These high-efficiency devices typically consist of several layers including a hole-transporting layer, an emission layer doped with the phosphorescent dye, an exciton-/hole-blocking layer and an electron-injecting layer. In fact, a multilayer LED utilizing fac-tris(2-phenylpyridine)iridium (Ir(ppy) 3 ) as the emitting species exhibited a very high external quantum efficiency (EQE) of 19.2 % and a power conversion efficiency (PCE) of 72 lm W ±1 at 65 cd m ±2 .[9]Polymer phosphorescent light-emitting diodes (PPLEDs) usually utilize a phosphorescent dye doped into a chargetransporting polymer matrix.[10±17] One of the criteria for the selection of the polymer matrix is that the energy of the lowest lying triplet state (T 1 ) of the host is larger or at least comparable to that of the phosphorescent guest. In case of Ir(ppy) 3 with a triplet energy of about 2.4 eV, mostly non-conjugated polymers, such as poly(N-vinyl-carbazole) (PVK), have been used. [11±16] In order to optimize the balance of charge-carrier injection and transport and to confine the emissive triplet excitons within the emission layer, the structure of PPLEDs generally resembles that of low molecular weight material multilayer devices.[11±15] Yang and Tsutsui were the first to publish an efficient multilayer PPLED with a PVK:Ir(ppy) 3 emission layer and an evaporated low molecular weight electron-transporting layer.[11] These devices exhibited an external quantum efficiency of up to 7.5 %. A luminance of 100 cd m ±2 was reached at about 14 V and the power conversion efficiency was 5.8 lm W ±1 under these conditions.Lamansky et al. [14] achieved EQEs of 3.4 % in single-layer structures by adding the low molecular weight electron-transporting molecule 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) to the PVK host to facilitate electron transport. Vaeth and Tang [15] reported an EQE of 8.5 % and a PCE of 9.9 lm W ±1 ...