Improved Exciton Dissociation at Semiconducting Polymer:ZnO Donor:Acceptor Interfaces via Nitrogen Doping of ZnO.

Exciton dissociation at the zinc oxide/poly(3-hexylthiophene) (ZnO/P3HT) interface as a function of nitrogen doping of the zinc oxide, which decreases the electron concentration from approximately 1019 cm-3 to 1017 cm-3, is reported. Exciton dissociation and device photocurrent are strongly improved with nitrogen doping. This improved dissociation of excitons in the conjugated polymer is found to result from enhanced light-induced de-trapping of electrons from the surface of the nitrogen-doped ZnO. The ability to improve the surface properties of ZnO by introducing a simple nitrogen dopant has general applicability.

Nanosecond intersystem crossing times in fullerene acceptors: implications for organic photovoltaic diodes.

Triplet-exciton formation through intersystem crossing of photogenerated singlet excitons in fullerene acceptors can compete with charge generation in organic photovoltaic diodes. This article reports the intersystem crossing timescale (τISC ) of the most commonly used fullerene acceptors, PC60 BM and PC70 BM, in solutions and in spin-coated films. These times are on the nanosecond timescale, and are longer than the characteristic times for charge generation (τd ).

Efficiency limitations in a low band-gap diketopyrrolopyrrole-based polymer solar cell.

We report the performance and photophysics of a low band-gap diketopyrrolopyrrole-based copolymer used in bulk heterojunction devices in combination with PC71BM. We show that the short lifetime of photogenerated excitons in the polymer constitutes an obstacle towards device efficiency by limiting the diffusion range of the exciton to the donor-acceptor heterojunction. We employ ultrafast transient-probe and fluorescence spectroscopy techniques to examine the excited state loss channels inside the devices. We use the high boiling point solvent additive 1,8-diiodooctane (DIO) to study the photoexcited state losses in different blend morphologies. The solvent additive acts as a compatibiliser between the donor and the acceptor material and leads to smaller domain sizes, higher charge formation yields and increased device efficiency.

High-performance ambipolar diketopyrrolopyrrole-thieno[3,2-b]thiophene copolymer field-effect transistors with balanced hole and electron mobilities.

Ambipolar OFETs with balanced hole and electron field-effect mobilities both exceeding 1 cm(2) V(-1) s(-1) are achieved based on a single-solution-processed conjugated polymer, DPPT-TT, upon careful optimization of the device architecture, charge injection, and polymer processing. Such high-performance OFETs are promising for applications in ambipolar devices and integrated circuits, as well as model systems for fundamental studies.

Tuning the electronic coupling in a low-bandgap donor-acceptor copolymer via the placement of side-chains.

We present a spectroscopic and theoretical investigation of the effect of the presence and position of hexyl side-chains in the novel low-bandgap alternating donor-acceptor copolymer poly[bis-N,N-(4-octylphenyl)-bis-N,N-phenyl-1,4-phenylenediamine-alt-5,5'-4',7',-di-2-thienyl-2',1',3'-benzothiadiazole] (T8TBT). We use electronic absorption and Raman spectroscopic measurements supported by calculations of chain conformation, electronic transitions, and Raman modes. Using these tools, we find that sterically demanding side-chain configurations induce twisting in the electronic acceptor unit and reduce the electronic interaction with the donor. This leads to a blue-shifted and weakened (partial) charge-transfer absorption band together with a higher photoluminescence efficiency. On the other hand, sterically relaxed side-chain configurations promote coupling between donor and acceptor units and exhibit enhanced absorption at the expense of luminescence efficiency. The possibility of tuning the donor-acceptor character of conjugated polymers by varying the placement of side-chains has very important ramifications for light emitting diode, Laser, display, and photovoltaic device optimization.

Sequential energy and electron transfer in polyisocyanopeptide-based multichromophoric arrays.

We report on the synthesis and detailed photo-physical investigation of four model chromophore side chain polyisocyanopeptides: two homopolymers of platinum-porphyrin functionalized polyisocyanopeptides (Pt-porphyrin-PIC) and perylene-bis(dicarboximide) functionalized polyisocyanopeptides (PDI-PIC), and two statistical copolymers with different ratios of Pt-porphyrin and PDI molecules attached to a rigid, helical polyisocyanopeptide backbone. (1)H NMR and circular dichroism measurements confirm that our model compounds retain a chiral architecture in the presence of the chromophores. The combination of Pt-porphyrin and PDI chromophores allows charge- and/or energy transfer to happen. We observe the excitation and relaxation pathways for selective excitation of the Pt-porphyrin and PDI chromophores. Studies of photoluminescence and transient absorption on nanosecond and picosecond scales upon excitation of Pt-porphyrin chromophores in our multichromophoric assemblies show similar photophysical features to those of the Pt-porphyrin monomers. In contrast, excitation of perylene chromophores results in a series of energy and charge transfer processes with the Pt-porphyrin group and forms additional charge-transfer states, which behave as an intermediate state that facilitates electronic coupling in these multichromophoric systems.

Exciton fission and charge generation via triplet excitons in pentacene/C60 bilayers.

Organic photovoltaic devices are currently studied due to their potential suitability for flexible and large-area applications, though efficiencies are presently low. Here we study pentacene/C(60) bilayers using transient optical absorption spectroscopy; such structures exhibit anomalously high quantum efficiencies. We show that charge generation primarily occurs 2-10 ns after photoexcitation. This supports a model where charge is generated following the slow diffusion of triplet excitons to the heterojunction. These triplets are shown to be present from early times (<200 fs) and result from the fission of a spin-singlet exciton to form two spin-triplet excitons. These results elucidate exciton and charge generation dynamics in the pentacene/C(60) system and demonstrate that the tuning of the energetic levels of organic molecules to take advantages of singlet fission could lead to greatly enhanced photocurrent in future OPVs.

Multichromophoric phthalocyanine-(perylenediimide)(8) molecules: a photophysical study.

We describe the synthesis of a series of phthalocyanine (Pc)-perylenediimide (PDI)(8) "octad" molecules, in which eight PDI moieties are attached to a Pc core through alkyl-chain linkers. There is clear spectroscopic evidence that these octads can exist as non-aggregated "monomers" or form aggregates along the Pc cores, depending on the type of Pc and the solvent medium. In the low dielectric constant solvents, into which the octads are soluble, photoexcitation of the PDI units leads to rapid energy transfer to the Pc centre, rather than a charge separation between moieties. In octad monomers, the Pc singlet excited-state decays within tens of ps, whereas the excitons are stabilised in the aggregated form of the molecules, typically with lifetimes in the order of 1-10 ns. By contrast, in an octad design in which pi-pi interactions are suppressed by the steric hindrance of a corona of incompatible glycol tails around the molecule, a more straightforward photophysical interaction of Förster energy transfer between the PDI moieties and Pc core may be inferred. We consider these molecules as prototypical multichromophoric aggregates, giving delocalised states with considerable flexibility of design.

Subnanosecond geminate charge recombination in polymer-polymer photovoltaic devices.

We present direct spectroscopic evidence for substantial subnanosecond charge recombination in polymer-polymer blend photovoltaic devices. Early dynamics are dominated by exciton-charge interactions associated with high initial excitation densities. Independent of density, 30% of charges subsequently recombine geminately within just 2 nanoseconds, in contrast with fullerene blends. The remainder recombines with a half-life of approximately 200 ns. The morphological invariance of subnanosecond recombination suggests that its origin is inherent in the molecular structure at the polymer-polymer interface.

Effect of annealing on P3HT:PCBM charge transfer and nanoscale morphology probed by ultrafast spectroscopy.

We employ sub-picosecond TA spectroscopy on operating P3HT:PCBM devices to probe the effect of annealing on charge transfer dynamics and nanoscale morphology. Our measurement configuration allows us to remove the effect of high excitation densities that would otherwise dominate. Charge transfer in pristine P3HT:PCBM devices proceeds on a sub-picosecond time scale, implying molecular level intermixing and explaining the more localized character of excitons and charges. In annealed devices, annealing results in diffusion-limited charge generation with a half-life time of approximately 3 ps, complete only after 30 ps. This is the result of exclusion of PCBM molecules and ordering of P3HT domains and is correlated with improved photovoltaic efficiency. We are able to use the spectra and dynamics of optical excitations themselves to interpret blend morphologies on the appropriate time- and length scales of photoinduced charge generation.

Pressure-induced delocalization of photoexcited states in a semiconducting polymer.

We present broadband transient absorption spectroscopy on the fluorescent copolymer poly(9,9-dioctylfluorene-co-benzothiadiazole) under hydrostatic pressure of up to 75 kbar. We observe a strong reduction of the stimulated emission intensity under pressure, coupled with slower decay kinetics and reduced fluorescence intensity. These observations indicate increased delocalization of photogenerated singlet excitons, facilitated by an increased dielectric constant at high pressure. Spin triplet excitons, generated via an iridium complex-F8BT oligomer, show reduced lifetimes under pressure.

Ion-induced formation of charge-transfer states in conjugated polyelectrolytes.

We use time-resolved optical spectroscopy to demonstrate that the luminescence quenching observed when ions are incorporated in films of conjugated polymers can be explained by the formation of charge-transfer (CT) states that are stabilized by the Coulomb field of ions. Our investigation is focused on a conjugated polyelectrolyte (CPE) derived from F8BT (poly(9,9'-dioctylfluorene-alt-benzothiadiazole)). The statistical copolymer contains tetra-alkyl ammonium moieties and BF(4)(-) counteranions attached to a moderate (approximately 7%) density of polymer alkyl side chains, providing a film morphology comparable to F8BT but with ions distributed on the length scale of exciton diffusion. The ionic substituents have little influence over the polymers electronic absorption and emission properties in solution, however photoluminescence (PL) quantum efficiency (approximately 6%) is considerably lower for the polyelectrolyte compared with F8BT (approximately 60%) in thin films. Time-resolved PL spectroscopy reveals that the primary exciton lifetime is shortened in the polyelectrolyte and a red-shifted CT emission peak with a longer lifetime emerges. Transient absorption spectroscopy of thin films enables us to detect CT states that persist beyond the primary decay and are found to be immobile. The PL intensity of the partially ionic film is found to increase with decreasing temperature, consistent with thermally activated exciton hopping (E(act) = 28 meV) prior to formation of CT states at ionic regions. We suggest that ion-induced stabilization of CT states is a general phenomenon in CPEs, which raises the possibility that ions might be arranged to direct the flow of excitons toward charge-separating interfaces in polymer photovoltaic devices.