Showing papers by "Bright Walker published in 2013"
TL;DR: In this article, the substitution of the heteroatoms in the aromatic end-groups of three diketopyrrolopyrrole containing small molecules is investigated to evaluate how such substitutions affect various physical properties, charge transport, and the performance in bulk heterojunction solar cells.
Abstract: Substitution of the heteroatoms in the aromatic end-groups of three diketopyrrolopyrrole containing small molecules is investigated to evaluate how such substitutions affect various physical properties, charge transport, and the performance in bulk heterojunction solar cells. While the optical absorption and frontier orbital energy levels are insensitive to heteroatom substitution, the materials' solubility, thermal properties, film morphology, charge carrier mobility, and photovoltaic performance are altered significantly. Differences in material properties are found to arise from changes in intra- and intermolecular interactions in the solid state caused by heteroatom substitution, as revealed by the single crystal structures of three compounds. This study demonstrates a systematic investigation of structure–property relationships in conjugated small molecules.
179 citations
TL;DR: In this paper, a series of solution-processable, small-molecule, donor materials based on an architecture consisting of two diketopyrrolopyrrole (DPP) cores with different aromatic π-bridges between the DPP units and different end-capping groups was reported.
Abstract: We report a series of solution-processable, small-molecule, donor materials based on an architecture consisting of two diketopyrrolopyrrole (DPP) cores with different aromatic π-bridges between the DPP units and different end-capping groups. In general, this architecture leads to desirable light absorption and electronic levels for donor materials. Out of the compounds investigated, a material with a hydrolyzed dithieno(3,2-b;2′,3′-d)silole (SDT) core and 2-benzofuran (BFu) end capping groups leads to the most favorable properties for solar cells, capable of generating photocurrent up to 800 nm while producing an open-circuit voltage of over 850 mV, indicating a small loss in electrical potential compared to other bulk heterojunction systems. Device properties can be greatly improved through the use of solvent additives such as 2-chloronaphthalene and initial attempts to optimize device fabrication have resulted in power conversion efficiencies upwards of 4%.
119 citations
TL;DR: Three stereoisomers of DPP(TBFu)2 are separated and identified to investigate the effects of stereoisomerism on crystal structures and the optoelectrical properties.
Abstract: Three stereoisomers of DPP(TBFu)2 are separated and identified to investigate the effects of stereoisomerism on crystal structures and the optoelectrical properties. The crystal structures and FET mobility are sensitive to stereoisomers, in which the mesomer possesses the highest carrier mobility and the greatest crystallization tendency to dominate the crystallization in spin-cast films of the as-synthesized stereoisomeric mixture.
82 citations
TL;DR: In this article, the effect of solvent additives on the performance of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) field effect transistors (FETs) was investigated.
Abstract: The effect of solvent additives on the performance of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) field effect transistors (FETs) was investigated. Hole mobilities increased from 0.10 cm2 V−1 s−1 for pristine devices to 0.73 or 0.71 cm2 V−1 s−1, when TIPS-pentacene FETs were processed with diphenyl ether (DPE) or chloronaphthalene (CN), respectively. In order to examine the impact of additives on the surface morphology, molecular ordering and crystallinity of TIPS-pentacene, scanning electron microscopy (SEM), X-ray diffraction (XRD) and optical microscopy measurements were carried out. Appropriate amounts of additives were found to induce the formation of well-ordered crystalline domains in TIPS-pentacene films, resulting in enhanced hole transport as well as consistent device performance. Additionally, reduced contact resistances were observed in devices processed with additives compared to neat TIPS-pentacene FET devices. Our findings indicate that the use of solvent additives constitutes a new and effective methodology for the fabrication of OFETs with improved performance.
41 citations
TL;DR: The ILMs layer minimizes the contact resistance between PbS and PCBM due to the improved compatibility between the two layers, confirmed as a decrease in charge transfer resistance, as measured by electrical impedance spectroscopy.
Abstract: We investigated the effect of ionic liquid molecules (ILMs) in hybrid quantum dot-organic solar cells (HyQD-OSCs). The insertion of an ILM layer between PbS and phenyl-C61-butyric acid methyl ester (PCBM) can shift the band edge of PCBM closer to the vacuum level of PbS due to spontaneous dipole polarization. Because of this new architecture, improvements in device performance were achieved, including increases in open-circuit voltage (VOC, from 0.41 V to 0.49 V), fill factor (FF, from 0.48 to 0.59), and power conversion efficiency (PCE, from 1.62% to 2.21%), compared to reference devices under AM 1.5G illumination at 100 mW cm–2. We observed that treatment of the PbS layer with ILMs causes a significant increase in work function from 3.58 eV to 3.93 eV. Furthermore, the ILMs layer minimizes the contact resistance between PbS and PCBM due to the improved compatibility between the two layers, confirmed as a decrease in charge transfer resistance, as measured by electrical impedance spectroscopy.
37 citations
TL;DR: In this article, two kinds of conjugated copolymers, poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(3,6-dithien-2-yl,2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4dione)] (P1) and poly [N- 9′-hexadecyl-2.5]-phenyl)-2.7]-carbaz
Abstract: Two kinds of novel carbazole and diketopyrrolopyrrole based conjugated copolymers, poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(3,6-dithien-2-yl-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione)] (P1) and poly[N-9′-(3,5-bis(octyloxy)phenyl)-2,7-carbazole-alt-5,5-(3,6-dithien-2-yl-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione)] (P2), were synthesized via a Suzuki cross coupling reaction. The optical and electrical properties of these polymers, in which the pendent groups on the carbazole moiety were modified, were investigated. Aryl substituted P2 shows a narrower band gap (1.61 eV), higher hole mobility (4.4 × 10−3 cm2 V−1 s−1) and more planar backbone structure than alkyl substituted P1. The modification of side groups significantly affected their surface morphologies. The device performance of solar cells based on these polymers and a fullerene acceptor was characterized.
20 citations
TL;DR: X-ray photoelectron microscopy images show the distribution of each phase of the blend films upon the use of additives, which results in a well-percolated network through the active layer.
Abstract: The effect of additives such as 1,8-diiodooctane (DIO) and 1-chloronaphthalene (CN) on the electronic structures, charge transport and phase separation of small-molecule-based bulk heterojunction (BHJ) films was investigated. Charge transport properties of the BHJ layers significantly changed via the introduction of additives, even though the molecular energy levels remained unchanged. X-ray photoelectron microscopy (XPM) images show the distribution of each phase of the blend films upon the use of additives. The CN additive, in particular, results in a well-percolated network through the active layer.
16 citations
TL;DR: In this paper, an organic solar cell was fabricated on hybrid transparent conductive films consisting of multilayer graphene (MLG) and metal grids, and the optical transmittance was found to be 87% at a wavelength of 550 nm, while the sheet resistance was measured to be 28 ± 7.9 Ω/square.
Abstract: Organic solar cells (OSCs) were fabricated on hybrid transparent conductive films consisting of multilayer graphene (MLG) and metal grids. MLG was transferred onto Ag grids to form hybrid transparent conductive films. The optical transmittance was found to be 87% at a wavelength of 550 nm, while the sheet resistance was measured to be 28 ±7.9 Ω/square. The device characteristics of OSCs prepared on the hybrid films include an open circuit voltage of 0.58 V, a short circuit current of 8.05 mA/cm2, a fill factor of 51%, and a power conversion efficiency (PCE) of 2.38%. The PCE shows 11% improvement compared with that of OSCs fabricated using MLG films without Ag grids. This improvement can be attributed to the reduced sheet resistance of the hybrid film. These results indicate that hybrid films comprising MLG deposited on Ag grids constitute a promising transparent electrode for improving performance in OSCs.
8 citations
2 citations