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Keivanidis P.E.,Imperial College London | Ho P.K.H.,National University of Singapore | Friend R.H.,Optoelectronics Group | Greenham N.C.,Optoelectronics Group
Advanced Functional Materials | Year: 2010

Organic photodiodes are presented that utilize solution-processed perylene diimide bulk heterojunctions as the device photoactive layer. The polymer (9,9â-dioctylfluorene-co-benzothiadiazole; F8BT) is used as the electron donor and the N,Nâ-bis(1-ethylpropyl)-3,4,9,10-perylene tetracarboxylic diimide (PDI) derivative is used as the electron acceptor. The thickness-dependent study of the main device parameters, namely of the external quantum efficiency (EQE), the short-circuit current (ISC), the open-circuit voltage (VOC), the fill factor (FF), and the dark current (ID) is presented. In as-spun F8BT:PDI devices the short-circuit EQE reaches the maximum of 17% and the VOC value is as high as 0.8 V. Device ID is in the nA mm-2 regime and it correlates with the topography of the F8BT:PDI layer. For a range of annealing temperatures ID is monitored as the morphology of the photoactive layer changes. The changes in the morphology of the photoactive layer are monitored via atomic force microscopy. The thermally induced coalescence of the PDI domains assists the dark conductivity of the device. ID values as low as 80 pA mm-2 are achieved with a corresponding EQE of 9%, when an electron-blocking layer (EB) is used in bilayer EB/F8BT:PDI devices. Electron injection from the hole-collecting electrode to the F8BT:PDI medium is hindered by the use of the EB layer. The temperature dependence of the ID value of the as-spun F8BT:PDI device is studied in the range of 296-216 K. In combination with the thickness and the composition dependence of ID, the determined activation energy Ea suggests a two-step mechanism of ID generation; a temperature-independent step of electric-field- assisted carrier injection from the device contacts to the active-layer medium and a thermally activated step of carrier transport across the device electrodes, via the PDI domains of the photoactive layer. Moreover, device ID is found to be sensitive to environmental factors. Minimizing the dark. The correlation between EQE and dark current in bulk heterojunction F8BT:PDI photodiodes is studied for a range of photoactive-layer thicknesses and reverse-bias voltages. For a given value of reverse bias operation, the improvement of the EQE reduces and the dark current increases as the photoactive layer becomes thinner. Dark current originates from the injection of electrons in the electron-accepting molecules (PDI) and from the subsequent carrier transport through the PDI network. Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Boucle J.,Optoelectronics Group | Boucle J.,University of Limoges | Snaith H.J.,Optoelectronics Group | Snaith H.J.,University of Oxford | Greenham N.C.,Optoelectronics Group
Journal of Physical Chemistry C | Year: 2010

This work is devoted to the development of hybrid bulk heterojunction solar cells based on porous zinc oxide (ZnO) electrodes and poly(3-hexylthiophene) (P3HT), using simple synthesis procedures and deposition techniques. Starting from ZnO nanocrystals with well-controlled properties, porous ZnO electrodes of suitable porosity are deposited by spin-coating, varying the main experimental parameters such as composition of the initial ZnO formulation and choice of the organic ligand. Significant charge transfer yields are observed in the corresponding solar cells, and the influence of processing conditions on device performance is investigated using conventional techniques as well as transient photovoltage/photocurrent decay measurements. The temperature used to sinter the ZnO electrode is found to be specifically crucial to ensure efficient charge transport in the device while avoiding a loss in interfacial area through nanocrystal coalescence. Using 8 × 13 nm ZnO nanorods, the best device exhibits a power conversion efficiency of 0.35% under 100 mW·cm -2 AM1.5G simulated solar emission. This strategy, using processing in air with simple deposition techniques, competes with related approaches based on nanostructured ZnO processed using more complex procedures. Moreover, device performance and photophysics are found to be greatly influenced by the morphology of the starting ZnO nanocrystals, illustrating that fine control of the inorganic component can effectively tune the performance of hybrid bulk heterojunction solar cells. © 2010 American Chemical Society.

Lombeck F.,Albert Ludwigs University of Freiburg | Lombeck F.,Optoelectronics Group | Matsidik R.,Albert Ludwigs University of Freiburg | Komber H.,Leibniz Institute of Polymer Research | Sommer M.,Albert Ludwigs University of Freiburg
Macromolecular Rapid Communications | Year: 2015

Direct arylation (DA) of 2-chlorothiophene and 2-chloro-3-hexylthiophene with 4,7-dibromo-2,1,3-benzothiadiazole is used to synthesize 4,7-bis(5-chloro-2-thienyl)-2,1,3-benzothiadiazole (TBTCl2) and 4,7-bis(5-chloro-4-hexyl-2-thienyl)-2,1,3-benzothiadiazole (DH-TBTCl2) in one step. Suitable conditions of the Suzuki polycondensations (SPC) of TBTCl2 and DH-TBTCl2 with the carbazole comonomer CbzPBE2 are established, furnishing PCDTBT and P(Cbz-alt-TBT) with high molecular weight and yield. Compared with control samples made from the corresponding dibromides, high-temperature NMR and UV-vis spectroscopy indicate similar properties for PCDTBT but an increased content of Cbz-Cbz homocouplings for P(Cbz-alt-TBT). (Chemical Equation Presented) © 2014 Wiley-VCH Verlag GmbH & Co. KGaA.

Matsidik R.,Albert Ludwigs University of Freiburg | Martin J.,Albert Ludwigs University of Freiburg | Schmidt S.,Albert Ludwigs University of Freiburg | Obermayer J.,Albert Ludwigs University of Freiburg | And 6 more authors.
Journal of Organic Chemistry | Year: 2015

Pd-catalyzed direct arylation (DA) reaction conditions have been established for unsubstituted furan (Fu) and thiophene (Th) with three popular acceptor building blocks to be used in materials for organic electronics, namely 4,7-dibromo-2,1,3-benzothiadiazole (BTBr2), N,N-dialkylated 2,6-dibromonaphthalene-1,4,5,8-bis(dicarboximide) (NDIBr2), and 1,4-dibromotetrafluorobenzene (F4Br2). Reactions with BTBr2, F4Br2, and NDIBr2 require different solvents to obtain high yields. The use of dimethylacetamide (DMAc) is essential for the successful coupling of BTBr2 and F4Br2, but detrimental for NDIBr2, as the electron-deficient NDI core is prone to nucleophilic core substitution in DMAc as solvent but not in toluene. NDIFu2 is much more planar compared to NDITh2, resulting in an enhanced charge-transfer character, which makes it an interesting building block for conjugated systems designed for organic electronics. This study highlights direct arylation as a simple and inexpensive method to construct a series of important donor-acceptor-donor building blocks to be further used for the preparation of a variety of conjugated materials. © 2014 American Chemical Society.

Lombeck F.,Albert Ludwigs University of Freiburg | Lombeck F.,Optoelectronics Group | Komber H.,Leibniz Institute of Polymer Research | Gorelsky S.I.,University of Ottawa | Sommer M.,Albert Ludwigs University of Freiburg
ACS Macro Letters | Year: 2014

Homocouplings are identified as major side reactions in direct arylation polycondensation (DAP) of 4,7-bis(4-hexyl-2-thienyl)-2,1,3-benzothiadiazole (TBT) and 2,7-dibromo-9-(1-octylnonyl)-9H-carbazole (CbzBr2). Using size exclusion chromatography (SEC) and NMR spectroscopy, we demonstrate that both TBT and Cbz homocouplings occur at a considerable extent. TBT homocoupling preferentially occurs under phosphine-free conditions but can be suppressed in the presence of a phosphine ligand. Cbz homocoupling is temperature-dependent and more prevalent at higher temperatures. By contrast, evidence for chain branching as a result of unselective C-H arylation is not found for this monomer combination. These results emphasize that particular attention has to be paid to homocouplings in direct arylation polycondensations as a major source of main-chain defects, especially under phosphine-free conditions. © 2014 American Chemical Society.

Komber H.,Leibniz Institute of Polymer Research | Mullers S.,Albert Ludwigs University of Freiburg | Lombeck F.,Albert Ludwigs University of Freiburg | Lombeck F.,Optoelectronics Group | And 3 more authors.
Polymer Chemistry | Year: 2014

A combined experimental and theoretical study on the synthesis and solution isomerization behavior of main chain copolymers with multiple spiropyran incorporation is presented. A series of alternating copolymers P(SP-alt-C x) of spiropyran (SP) and flexible linkers (Cx, x = 6,8,10) is synthesized by Suzuki polycondensation (SPC). Careful 1H NMR polymer end group analysis is carried out to reveal termination reactions that limit molecular weight. Although methylene indoline and salicyl aldehyde end groups are found arising from SP cleavage during polymerization, acceptable Mn,SEC up to 34 kg mol-1 is achieved. P(SP-alt-C x) can be transformed into the corresponding protonated form of the red alternating merocyanine (MC) polymer, P(MCH+-alt-Cx), in quantitative yield by direct acidification or pulsed ultrasound. The origin of the latter method lies in the sonochemical degradation of chloroform, which provides a continuous source of hydrochloric acid. Deprotonation of P(MCH +-alt-Cx) occurs upon the addition of a base resulting in the blue form P(MC-alt-Cx), as characterized by UV-vis spectroscopy and confirmed by density functional theory (DFT) calculations of model compounds. DFT further reveals P(MCH+-alt-Cx) to be the thermodynamically most stable form. It is proposed that the para-linkage to the phenyl-based comonomer Cx decreases acidity of protonated merocyanine and hence increases stability of P(MCH+-alt-Cx), which poses a marked difference compared to the commonly employed 6-nitro-substituted analogs. © 2014 The Royal Society of Chemistry.

Banger K.,Optoelectronics Group | Warwick C.,Optoelectronics Group | Lang J.,Optoelectronics Group | Broch K.,Optoelectronics Group | And 5 more authors.
Chemical Science | Year: 2016

The ability to deposit high-quality inorganic semiconductors and dielectrics from solution at low process temperatures (∼200 °C) has become a very important research focus. During the course of our investigation, we identify the presence of an induced dipole present in solid state solution processed inorganic oxide insulator layers processed at reduced temperature (200-350 °C) from either molecular precursors, or well-dispersed metal oxide nanoparticles. Chemical composition analysis coupled with electrical measurements shows that the dielectric instability occurs due to proton migration via the Grotthuss mechanism inducing a long lived dipole disorder. Thus we established conditions for suppressing this effect to afford "ideal" high-k dielectric layer. Using this methodology, solution processed all inorganic thin film transistors (TFTs) with charge carrier mobilities exceeding 6 cm2 V-1 s-1 operating at low voltage (5 V) have been achieved. In addition, we show the broad utility of the perovskite high-k dielectric when processed with state of the art polymer and single crystal organic semiconductors yielding mobilities of approx. 7 cm2 V-1 s-1 at only 4 V. These transparent devices demonstrate excellent electrical device stability and a threshold voltage shift of only 0.41 V over 14 h, which is comparable, or better than sputtered oxide films. © 2016 The Royal Society of Chemistry.

PubMed | Imperial College London, FlexEnable Ltd, Optoelectronics Group, Lensfield Road and King Abdullah University of Science and Technology
Type: | Journal: Nature materials | Year: 2016

Due to their low-temperature processing properties and inherent mechanical flexibility, conjugated polymer field-effect transistors (FETs) are promising candidates for enabling flexible electronic circuits and displays. Much progress has been made on materials performance; however, there remain significant concerns about operational and environmental stability, particularly in the context of applications that require a very high level of threshold voltage stability, such as active-matrix addressing of organic light-emitting diode displays. Here, we investigate the physical mechanisms behind operational and environmental degradation of high-mobility, p-type polymer FETs and demonstrate an effective route to improve device stability. We show that water incorporated in nanometre-sized voids within the polymer microstructure is the key factor in charge trapping and device degradation. By inserting molecular additives that displace water from these voids, it is possible to increase the stability as well as uniformity to a high level sufficient for demanding industrial applications.

Huang Y.-S.,Optoelectronics Group | Yang X.,Optoelectronics Group | Schwartz E.,Radboud University Nijmegen | Lu L.P.,Optoelectronics Group | And 11 more authors.
Journal of Physical Chemistry B | Year: 2011

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. 1H 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. © 2011 American Chemical Society.

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