High charge-separation efficiency combined with the reduced fabrication costs associated with solution processing and the potential for implementation on flexible substrates make 'plastic' solar ...cells a compelling option for tomorrow's photovoltaics. Attempts to control the donor/acceptor morphology in bulk heterojunction materials as required for achieving high power-conversion efficiency have, however, met with limited success. By incorporating a few volume per cent of alkanedithiols in the solution used to spin-cast films comprising a low-bandgap polymer and a fullerene derivative, the power-conversion efficiency of photovoltaic cells (air-mass 1.5 global conditions) is increased from 2.8% to 5.5% through altering the bulk heterojunction morphology. This discovery can potentially enable morphological control in bulk heterojunction materials where thermal annealing is either undesirable or ineffective.
The influence of protonation reactions between poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and a thiadiazolo3,4‐cpyridine small‐molecule donor are reported; these result in ...poor solar‐cell performance due to a barrier for charge extraction. The use of a NiOx contact eliminates such deleterious chemical interactions and results in substantial improvements in open‐circuit voltage, fill factor, and an increased power conversion efficiency from 2.3% to 5.1%.
White‐light‐emitting polymer diodes containing hole‐injection/transport and electron‐injection/transport layers have been fabricated by solution processing. Illumination‐quality light is obtained ...from these multilayer, high‐performance devices, with stable Commission Internationale d'Eclairage coordinates, stable color temperatures, and stable, high color‐rendering indices, which are all close to those of “pure” white light indicated by the dotted oval in the Figure.
Interchain interactions modulate the frequency of emission from a cationic water-soluble conjugated polymer. The polymer, PFPB, is obtained by a Suzuki copolymerization of p-phenylenebisboronic acid ...with a 95:5 mixture of 2,7-dibromo-9,9-bis(6'-bromohexyl)fluorene and 4,7-dibromo-2,1,3-benzothiadiazole, followed by quarternization of the pendant groups by addition of NMe3. The structure of PFPB contains 5% of the 2,1,3-benzothiadiazole (BT) chromophore within a cationic poly(fluorene-co-phenylene) polymer chain. The emission of PFPB is blue under dilute conditions (<1 x 10-6 M in repeat units) and green at higher concentrations. Energy transfer to dye-labeled ss-DNA is more efficient, relative to the parent polymer poly(9,9-bis(6'-N,N,N,-trimethylammonium)hexyl)fluorene-co-alt-1,4-phenylene) dibromide (PFP), as a result of improved spectral overlap. By using a peptide nucleic acid (PNA-C*) labeled with a red-emitting chromophore one can obtain three different emission colors, depending on the nature of the substrate under interrogation. If no ss-DNA is present, the solution emits blue. With a ss-DNA that is noncomplementary to PNA-C*, green emission is observed. Red emission occurs upon addition of ss-DNA complementary to the PNA sequence.
Understanding the charge-separation mechanism in organic photovoltaic cells (OPVs) could facilitate optimization of their overall efficiency. Here we report the time dependence of the separation of ...photogenerated electron hole pairs across the donor-acceptor heterojunction in OPV model systems. By tracking the modulation of the optical absorption due to the electric field generated between the charges, we measure ∼200 millielectron volts of electrostatic energy arising from electron-hole separation within 40 femtoseconds of excitation, corresponding to a charge separation distance of at least 4 nanometers. At this separation, the residual Coulomb attraction between charges is at or below thermal energies, so that electron and hole separate freely. This early time behavior is consistent with charge separation through access to delocalized π-electron states in ordered regions of the fullerene acceptor material.
Multilayer polymer light‐emitting diodes are made using semiconducting polymers. An emissive polymer layer is cast from solution in an organic solvent, and a water‐soluble (or methanol‐ soluble) ...conjugated copolymer is used for the electron transport layer (ETL) in the following device configuration: (indium tin oxide)/poly(3,4‐ethylenedioxythiophene)/emissive polymer/ETL/Ba/Al. Devices with an ETL have significantly lower turn‐on voltages, higher brightness values, and improved luminous efficiencies.
The light-harvesting properties of cationic conjugated polymers are used to sensitize the emission of a dye on a specific peptide nucleic acid (PNA) sequence for the purpose of homogeneous, ..."real-time" DNA detection. Signal transduction is controlled by hybridization of the neutral PNA probe and the negative DNA target. Electrostatic interactions bring the hybrid complex and cationic polymer within distances required for Förster energy transfer. Conjugated polymer excitation provides fluorescein emission >25 times higher than that obtained by exciting the dye, allowing detection of target DNA at concentrations of 10 pM with a standard fluorometer. A simple and highly sensitive assay with optical amplification that uses the improved hybridization behavior of PNA/DNA complexes is thus demonstrated.
The electronic delocalization between chromophores in the solid is an important parameter to optimize when designing organic materials for optoelectronic applications. The 2.2paracyclophane framework ...allows for the synthesis of well-defined, nonfluxional molecules that bring together two chromophores into close proximity. From the photophysical properties of these molecules we can examine how the chromophore conjugation length, their relative orientation, and the regiochemistry of contact affects the electronic delocalization between the two subunits.
A new small molecule, p‐DTS(FBTTh2)2, is designed for incorporation into solution‐fabricated high‐efficiency organic solar cells. Of primary importance is the incorporation of electron poor ...heterocycles that are not prone to protonation and thereby enable the incorporation of commonly used interlayers between the organic semiconductor and the charge collecting electrodes. These features have led to the creation of p‐DTS(FBTTh2)2/PC71BM solar cells with power conversion efficiencies of up to 7%.
White emission from polymer light‐emitting diodes (PLEDs) is demonstrated using semiconducting polymers blended with organometallic emitters and spin‐cast from solution. Because a single ...semiconducting polymer was used as the common host for red, green, and blue emission, the color coordinates, color temperatures, and color rendering indices of the white emission are insensitive to the brightness, applied voltage, and applied current density. The PLEDs have luminous efficiency of 4.3 cd A–1 and luminance of 223 cd m–2 at current density of 5.2 mA cm–2 (V = 14 V); at 17 V, L = 1.2 × 104 cd m–2.
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