4 research outputs found

    Bulk Heterojunction versus Diffused Bilayer: The Role of Device Geometry in Solution p-Doped Polymer-Based Solar Cells

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    We exploit the effect of molecular p-type doping of P3HT in diffused bilayer (DB) polymer solar cells. In this alternative device geometry, the p-doping is accomplished in solution by blending the F<sub>4</sub>-TCNQ with P3HT. The p-doping both increases the film conductivity and reduces the potential barrier at the interface with the electrode. This results in an excellent power conversion efficiency of 4.02%, which is an improvement of ∼48% over the p-doped standard bulk heterojunction (BHJ) device. Combined <i>V</i><sub>OC</sub>–light intensity dependence measurements and Kelvin probe force microscopy reveal that the DB device configuration is particularly advantageous, if compared to the conventional BHJ, because it enables optimization of the donor and acceptor layers independently to minimize the effect of trapping and to fully exploit the improved transport properties

    Dynamic Microscopy Study of Ultrafast Charge Transfer in a Hybrid P3HT/Hyperbranched CdSe Nanoparticle Blend for Photovoltaics

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    We present a spectroscopic investigation on a new hyperbranched cadmium selenide nanocrystals (CdSe NC)/poly­(3-hexylthiophene) (P3HT) blend, a potentially good active component in hybrid photovoltaics. Combined ultrafast transient absorption spectroscopy and morphological investigations by means of an ultrafast confocal microscope reveal a strong influence of the complex local structure on the photogenerated carrier dynamics. In particular, we map the electron-transfer process across the hybrid NC/polymer interface, and we reveal that charge separation occurs through a preferential pathway from the CdSe nanobranches to the P3HT chains. Efficient charge generation at the distributed heterojunction is also confirmed by scanning kelvin probe force microscopy measurements

    Synthesis and Photovoltaic Properties of Regioregular Head-to-Head Substituted Thiophene Hexadecamers

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    We describe the expedient synthesis of regioregular thiophene hexadecamers head-to-head (hh) substituted with hexyl and hexylthio grous. The synthesis was carried out by means of a sequence of ultrasound-assisted selective monobrominations and microwave-assisted Suzuki reactions using 4,4,5,5-tetramethyl-1,3,2-dioxaborolane in THF:water. The hexadecamers, which are very soluble in organic solvents, were investigated in solution and thin film by a variety of techniques (UV, PL, CV, X-ray diffraction, FET charge mobility, SKFM) with the aim of elucidating the effect of the sulfur spacer on morphology and functional properties. We show that the sulfur spacer compensates for the decrease in π–π conjugation caused by the hh regiochemistry and that the λ<sub>max</sub> value and redox potentials of the S-alkyl-substituted hexadecamer are similar to those of head-to-tail substituted poly­(3-hexylthiophene). Measurements in field effect transistor devices showed that the alkylthio-substituted hexadecamer is a p-type semiconductor while the alkyl-substituted counterpart in the same conditions is not electroactive. Scanning Kelvin force microscopy measurements showed that a blend of the alkylthio-substituted hexadecamer with PCBM displays photovoltaic behavior under illumination. In agreement with this, a bulk heterojunction cell fabricated employing the same blend displayed near 1.5% conversion efficiency without addition of additives or device optimization

    Nanoscale Characterization and Unexpected Photovoltaic Behavior of Low Band Gap Sulfur-Overrich-Thiophene/Benzothiadiazole Decamers and Polymers

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    Highly soluble air-stable conjugated decamers and polymers (compounds <b>1</b>–<b>6</b>) with alternating “sulfur-overrich” bis­(3,4′-<i>S</i>-alkyl)-2,2′-bithiophenes as the donor units and 2,1,3-benzothiadiazoles as the acceptor units were designed and expediently synthesized with the aid of microwave and ultrasound enabling technologies. Solid-state cyclovoltammetry showed that <b>1</b>–<b>6</b> had oxidation and reduction potentials in the range 0.6–0.9 V and −1/–1.2 V (vs SCE), respectively, with energy gaps below 2 eV. The electronic properties of spin-coated films of pure <b>1</b>–<b>6</b> were investigated with nanoscale resolution by Kelvin probe force microscopy (KPFM). KPFM measurements showed that charge generation and separation were obtained for all films under illumination. Consequently, <b>1</b>–<b>6</b> were tested on single-material organic solar cells (SMOCs). In agreement with KPFM results, photovoltaic behavior was observed for all compounds with power conversion efficiencies in line with the best results obtained so far for the few donor–acceptor molecules already shown to perform in single-component solar cells. To our knowledge, this is the first time in which thiophene–benzothiadiazole co-oligomers and copolymers are shown to be photoactive materials in SMOCs
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