Elucidating the electronic properties of single-wall carbon nanohorns

Single-walled carbon nanohorns are an allotrope of carbon with promising properties for a variety of applications. Despite their promise, the majority carrier type (i.e. electrons or holes) that defines the electronic properties of this novel semiconductor is poorly understood and so far only indirect measurements have been employed to arrive at contradictory results. Here, we directly determine the majority carrier type in single-wall carbon nanohorns for the first time by means of thermopower measurements. Using this direct method, we show that SWCNH films exhibit a positive Seebeck coefficient indicating that SWCNHs behave as p-type semiconductors. This result is further corroborated by intentionally tuning the hole or electron concentrations of SWCNH layers via redox doping with molecular electron acceptors and donors, respectively. These results provide a framework for both measuring and chemically tuning the majority carrier type in this emerging nanocarbon semiconductor

A faux hawk fullerene with PCBM-like properties

Reaction of C60, C6F5CF2I, and SnH(n-Bu)3 produced, among other unidentified fullerene derivatives, the two new compounds 1,9-C60(CF2C6F5)H (1) and 1,9-C60(cyclo-CF2(2-C6F4)) (2). The highest isolated yield of 1 was 35% based on C60. Depending on the reaction conditions, the relative amounts of 1 and 2 generated in situ were as high as 85% and 71%, respectively, based on HPLC peak integration and summing over all fullerene species present other than unreacted C60. Compound 1 is thermally stable in 1,2-dichlorobenzene (oDCB) at 160 °C but was rapidly converted to 2 upon addition of Sn2(n-Bu)6 at this temperature. In contrast, complete conversion of 1 to 2 occurred within minutes, or hours, at 25 °C in 90/10 (v/v) PhCN/C6D6 by addition of stoichiometric, or sub-stoichiometric, amounts of proton sponge (PS) or cobaltocene (CoCp2). DFT calculations indicate that when 1 is deprotonated, the anion C60(CF2C6F5)− can undergo facile intramolecular SNAr annulation to form 2 with concomitant loss of F−. To our knowledge this is the first observation of a fullerene-cage carbanion acting as an SNAr nucleophile towards an aromatic C–F bond. The gas-phase electron affinity (EA) of 2 was determined to be 2.805(10) eV by low-temperature PES, higher by 0.12(1) eV than the EA of C60 and higher by 0.18(1) eV than the EA of phenyl-C61-butyric acid methyl ester (PCBM). In contrast, the relative E1/2(0/−) values of 2 and C60, −0.01(1) and 0.00(1) V, respectively, are virtually the same (on this scale, and under the same conditions, the E1/2(0/−) of PCBM is −0.09 V). Time-resolved microwave conductivity charge-carrier yield × mobility values for organic photovoltaic active-layer-type blends of 2 and poly-3-hexylthiophene (P3HT) were comparable to those for equimolar blends of PCBM and P3HT. The structure of solvent-free crystals of 2 was determined by single-crystal X-ray diffraction. The number of nearest-neighbor fullerene–fullerene interactions with centroid⋯centroid (⊙⋯⊙) distances of ≤10.34 Å is significantly greater, and the average ⊙⋯⊙ distance is shorter, for 2 (10 nearest neighbors; ave. ⊙⋯⊙ distance = 10.09 Å) than for solvent-free crystals of PCBM (7 nearest neighbors; ave. ⊙⋯⊙ distance = 10.17 Å). Finally, the thermal stability of 2 was found to be far greater than that of PCBM

The effect of ring expansion in thienobenzo[b]indacenodithiophene polymers for organic field-effect transistors

A fused donor, thienobenzo[b]indacenodithiophene (TBIDT), was designed and synthesized using a novel acid-promoted cas-cade ring closure strategy, and copolymerized with a benzothiadiazole (BT) monomer. The backbone of TBIDT is an expan-sion of the well-known indacenodithiophene (IDT) unit and was expected to enhance the charge carrier mobility, by improving backbone planarity and facilitating short-contacts between polymer chains. However, the optimized field-effect transistors demonstrated an average saturation hole mobility of 0.9 cm2 V−1s−1, lower than the performance of IDT-BT (~1.5 cm2 V−1s−1). Mobilities extracted from time-resolved microwave conductivity (TRMC) measurements were consistent with the trend in hole mobilities in OFET devices. Scanning Tunneling Microscopy (STM) measurements and computational modelling illustrated that TBIDT-BT exhibits a less ordered microstructure in comparison to IDT-BT. This reveals that a regular side chain pack-ing density, independent of conformational isomers, is critical to avoid local free volume due to irregular packing, which can host trapping impurities. DFT calculations indicated that TBIDT-BT, despite containing a larger, planar unit, showed less stabilization of planar backbone geometries, in comparison to IDT-BT. This is due to the reduced electrostatic stabilizing inter-actions between the peripheral thiophene of the fused core with the BT unit, resulting in a reduction of the barrier to rotation around the single bond. These insights provide a greater understanding of the general structure-property relationships required for semiconducting polymer repeat units to ensure optimal backbone planarization, as illustrated with IDT-type units, guiding the design of novel semiconducting polymers with extended fused backbones for high-performance field-effect transistors

Origin of line broadening in the electronic absorption spectra of conjugated polymers: Three-pulse-echo studies of MEH-PPV in toluene

Integrated three-pulse stimulated echo peak shift data are compared for N,N-bis-dimethylphenyl-1-2,4,6,8-perylenetetracarbonyl diamide and poly[2-(2'-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] (MEH-PPV) in toluene solvent. These two molecules represent a model probe of solvation dynamics and a prototypical soluble, electroluminescent conjugated polymer, respectively. The results indicate that it is inappropriate to describe the linear absorption spectrum of MEH-PPV as being primarily inhomogeneously broadened. Conformational disorder along the polymer backbone gives rise to an ensemble of polyene electronic oscillators that are strongly coupled to each other. As a consequence, fluctuations in the electronic energy gap on a time-scale of 50-fs derive primarily from bath-mediated exciton scattering. The data reported here provide an explanation for the broad, structureless electronic absorption of MEH-PPV. This interpretation provides a valuable insight into the nature of the initial photoexcited state, and the efficient population of the emissive state

Hybrid organic-inorganic solar cells

In this special section of the Journal of Photonics for Energy, there is a focus on some of the science and technology of a range of different hybrid organic-inorganic solar cells. Prior to 1991 there were many significant scientific research reports of hybrid organic-inorganic solar cells; finally, however, it wasn’t until the dye-sensitized solar cell entered the league table of certified research cell efficiencies that this area experienced an explosion of research activity

Photoinduced electron transfer in composites of conjugated polymers and dendrimers with branched colloidal nanoparticles

Charge generation and separation dynamics in donor:acceptor systems based on composites of branched CdSe nanoparticles with a phenyl-cored thiophene-containing dendrimer (4G1-3S), or a low-bandgap conjugated polymer (PCPDTBT) are reported upon exclusive excitation of the donor or the acceptor. Time-resolved microwave conductivity is used to study the dynamics of either transfer of holes from the nanoparticle to dendrimer, or conversely the transfer of electrons from the polymer to the nanoparticle. Higher photoconductance signals and longer decay-times are correlated with device efficiencies, where composites with higher nanoparticle concentration exhibit higher solar photovoltaic power conversion efficiencies and an increase in external quantum efficiencies. This work evaluates the contribution of both components to device performance, but specifically the role of photoexcited nanoparticles.15 page(s

Quantitative Transient Absorption Measurements of Polaron Yield and Absorption Coefficient in Neat Conjugated Polymers

Transient absorption methods are crucial for probing photogenerated polaron dynamics in conjugated polymers but are usually limited to qualitative studies because the polaron absorption coefficient is unknown. Herein, we quantify polaron absorption coefficients by exploiting the parasitic exciton–polaron quenching process, which appears in transient absorption experiments as a decrease in polaron yield at high fluence. We modulate the charge density in neat polymer films and measure the exciton–polaron quenching rate constant and dopant density via time-resolved photoluminescence. Using these parameters, we fit relative yield–fluence curves obtained from transient absorption, quantifying the yield and absorption coefficient of the polarons. We use time-resolved microwave conductivity as the transient probe and present results for the GHz mobility and polaron yield in films of three common conjugated polymers that are consistent with previous reports where they exist. These experiments demonstrate a new, generally accessible spectroscopic method for quantitative study of polaron dynamics in conjugated polymers

Resonance Energy Transfer Enables Efficient Planar Heterojunction Organic Solar Cells

Poor energy transport in disordered organic materials is one of the key problems that must be overcome to produce efficient organic solar cells. Usually, this is accomplished by blending the donor and acceptor molecules into a bulk heterojunction. In this article, we investigate an alternative approach to cell design: planar mulitilayer hetrojunctions with efficient energy transport to a central reaction center. We use an experimentally verified Monte Carlo model of energy transport to show that an appropriately engineered planar multilayer stack can achieve power conversion efficiencies comparable to those of the best bulk heterojunction devices. The key to this surprising performance is careful control of the optical properties and thicknesses of each layer to promote Förster resonance energy transfer from antenna/transport layers to a central reaction center. We provide detailed design rules for fabricating efficient planar heterojunction organic cells