Mukundan Thelakkat | University Of Bayreuth, Germany (original) (raw)

Papers by Mukundan Thelakkat

The Journal of Physical Chemistry Letters, 2014

Conjugated poly(3-hexylthiophene) (P3HT) chains are known to exist at least in two distinct confo... more Conjugated poly(3-hexylthiophene) (P3HT) chains are known to exist at least in two distinct conformations: a coiled phase and a better ordered aggregated phase. Employing steady state absorption and fluorescence spectroscopy, we measure the course of aggregation of P3HT in tetrahydrofuran (THF) solution within a temperature range of 300 K to 170 K. We show that aggregation is a temperature controlled process, driven by a thermodynamic order−disorder transition. The transition temperature increases with the molecular weight of the chains and can be rationalized in the theory of Sanchez. This implies a smearing out of the phase transition in samples with increasing polydispersity and erodes the signature of a first order phase transition. The detection of a hysteresis when undergoing cooling/heating cycles further substantiates this reasoning.

Macromolecules, 2004

... Kun-Rung Lin, Chin-Chuan Chang, Ying-Hsueh Chang Chien, Man-kit Leung, Shun-Wei Liu, Jiun-Haw... more ... Kun-Rung Lin, Chin-Chuan Chang, Ying-Hsueh Chang Chien, Man-kit Leung, Shun-Wei Liu, Jiun-Haw Lee and Kuo-Huang Hsieh. Langmuir 2010 26 (7), 5147-5152. ... Zhongyuan Zhou, Jaclyn L. Brusso and Steven Holdcroft. Chemistry of Materials 2010 22 (7), pp 2287–2296. ...

Synthetic Metals, 1999

Polymeric triphenyldiamines (p-TPD)s and naphthylphenyldiamines (p-NPD)s were synthesized by Ullm... more Polymeric triphenyldiamines (p-TPD)s and naphthylphenyldiamines (p-NPD)s were synthesized by Ullmann reaction between bis(N,N-diaryl)amines and aryldiiodides in presence of phase-transfer catalyst, 18-crown-6. The polymers obtained are soluble in THF, CHCl3 etc and possess Tgs in the range of 206°C to 232°C and form stable and amorphous thin films. The HOMO energy values determined from cyclic voltammetry measurements lie between −5.10 and

Macromolecules, 2004

... Kun-Rung Lin, Chin-Chuan Chang, Ying-Hsueh Chang Chien, Man-kit Leung, Shun-Wei Liu, Jiun-Haw... more ... Kun-Rung Lin, Chin-Chuan Chang, Ying-Hsueh Chang Chien, Man-kit Leung, Shun-Wei Liu, Jiun-Haw Lee and Kuo-Huang Hsieh. Langmuir 2010 26 (7), 5147-5152. ... Zhongyuan Zhou, Jaclyn L. Brusso and Steven Holdcroft. Chemistry of Materials 2010 22 (7), pp 2287–2296. ...

Macromolecular Materials and Engineering, 2002

5.5-inch panel thin OLED display with a quarter-VGA resolution (240 6 320 pixels) and a brightnes... more 5.5-inch panel thin OLED display with a quarter-VGA resolution (240 6 320 pixels) and a brightness of 200 cd/m 2 (Sanyo-Kodak).

Organic Letters, 2010

Boron-dipyrrin dyes, through rational design, yield promising new materials. With strong electron... more Boron-dipyrrin dyes, through rational design, yield promising new materials. With strong electron-donor functionalities and anchoring groups for attachment to nanocrystalline TiO 2 , these dyes proved useful as sensitizers in dye-sensitized solar cells. Their applicability in a solid-state electrolyte regime offers additional opportunities for practical applications.

physica status solidi (RRL) - Rapid Research Letters, 2011

Thin Solid Films, 2010

Hole transport properties of three different side chain poly(triarylamines) have been determined ... more Hole transport properties of three different side chain poly(triarylamines) have been determined by means of the analysis of steady-state current-voltage characteristics using co-planar diode structures. The interpretation is based on space-charge limited models with field-dependent mobility. Mobilities between~10 − 8 and 10 − 6 cm 2 V − 1 s − 1 are obtained. The highest mobility is achieved for poly(tetraphenylbenzidine) devices and the lowest for poly(triphenylamine) devices. Electron-rich methoxy substituents increase the mobility of poly(triphenylamine)s. A comparison of the mobility values with those obtained using organic field-effect transistors is also given.

The Journal of Physical Chemistry C, 2008

We present a photophysical and device-based investigation of a new bipyridyl-NCS ruthenium comple... more We present a photophysical and device-based investigation of a new bipyridyl-NCS ruthenium complex sensitizer with an extended π system, in both sensitized TiO 2 and incorporated into solid-state dye-sensitized solar cells. We compare this new sensitizer to an analog dye without the extended π system. We observe very similar excited-state absorption spectra and charge recombination kinetics for the two systems. However, the π-extended senstizer has a phenomenally enhanced molar extinction coefficient which translates into far greater light harvesting and current collection in solid-state dye-sensitized solar cells. We also infer from transient photovoltage measurements that positioning the pendent extended π system away from the TiO 2 surface has induced a favorable dipole shift, generating enhanced open-circuit voltage. The resulting power conversion efficiency for the solar cell has been increased from 2.4% to 3.2% when comparing the new sensitizer to an analogy with no pendent group.

Synthetic Metals, 1999

Low molecular weight and polymeric imides are synthesized by attaching perylene units as substitu... more Low molecular weight and polymeric imides are synthesized by attaching perylene units as substituents or by incorporating in aromatic main chain copolymers to achieve high photoluminescent efficiency. In order to improve electron injection and transport properties 1,3,4oxadiazole comonomers with high electron affinities are used. The best solid state photoluminescence is obtained in copolymers with 0.5 mol% perylene content. The low molecular weight compounds exhibit high electron affinity with LUMO values in the range of -3.9 eV as determined by cyclic voltammetry. Electroluminescent devices with copolymers in PVK blends show characteristic red emission from perylene which is nearly identical with its photoluminescence and the efficiency of this device, ITO/copolymer+PVK blend(lOOnm)/Al, depends upon the PVK content.

Physical Chemistry Chemical Physics, 2011

Fig.S1 shows the life time measurements for HW456 dye. The potential step is from -0.4 V to +0.45... more Fig.S1 shows the life time measurements for HW456 dye. The potential step is from -0.4 V to +0.45 V vs Fc/Fc + . 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 0.0 0.2 0.4 0.6 0.8 1.0 HW456-TiO 2 film +0.45 V vs Fc/Fc + Time, t (min) Normalized ΔOD λ=770 nm Figure S1 Lifetime measurements for HW456 on nanocrystalline TiO 2 film in non-aqueous solution (0.1 M TBAP in acetonitrile).

Organic Electronics, 2007

The structural and optical properties of semiconductor block copolymers containing triphenylamine... more The structural and optical properties of semiconductor block copolymers containing triphenylamine as hole transport material and perylene bisimide as dye and electron transport material are reported. The polymers were prepared by nitroxide mediated controlled radical polymerisation and characterized with GPC, DSC, and TGA. The electrochemical properties as determined by cyclic voltammetry show the HOMO and LUMO values of the block copolymers to be À5.23 eV and À3.65 eV, respectively. The perylene bisimide units aggregate by p-p stacking which could be analyzed with wideangle X-ray scattering. The absorption and fluorescence properties of the perylene bisimide polymers and monomers in solution and film were investigated. It could be shown that they are strongly influenced by intramolecular coupling between different perylene bisimide units in polymers. The block copolymers exhibit a microphase separation on a nanometer scale with a constant perylene bisimide domain width of 13 nm and lengths of up to several micrometers.

Macromolecules, 2010

Molecular weight and stereoregularity affect the morphology and the crystallinity of conjugated p... more Molecular weight and stereoregularity affect the morphology and the crystallinity of conjugated polymers and are thus of pivotal relevance for the mobility of charge carriers in electro-optical device applications. Currently, poly(3-akylthiophenes) are discussed as one of the most promising classes of materials for photovoltaic applications. Here, we use 13 C solid-state NMR to determine the crystallinity and details on crystalcrystal phase transformations in regioregular head-to-tail poly(3-hexylthiophene) of well-defined molecular weight and demonstrate that the crystallinity was previously severely underestimated. Typical crystallinities are at least around 37% for the lowest molecular weights and as high as about 64% upon increasing MW, corresponding to a corrected maximum value for the reference melting enthalpy of ΔH m ¥ ≈ 37 J/g for use in DSC experiments. Using 1D 13 C spectra and 2D experiments that measure the strength of 13 C-1 H dipolar couplings, we observe that the crystal-crystal phase transition between a 3D-and a 2D-ordered crystalline phase at around 60°C entails a structural disordering process of the alkyl side chains, while not affecting rigidity and conformation of the backbones. This phase transition is suppressed at higher molecular weights due to a kinetic suppression of the formation of the alkyl-ordered low-temperature phase.

Macromolecules, 2010

We report on structural investigations of a series of regioregular poly(3-hexylthiophene) with we... more We report on structural investigations of a series of regioregular poly(3-hexylthiophene) with well-defined molecular weight (5-19 kg/mol) using DSC, small angle and wide-angle X-ray scattering, and AFM. With increasing temperature, we identify three ordered phases, namely 3D crystalline, 2D crystalline with disordered side chains, and a layered phase of smectic symmetry, followed by complete melting. Although all samples crystallize in extended chain conformation, the lower molecular weight material exhibits a lower crystallinity, most likely caused by noncrystallizable end groups. The crystallinity increases strongly with increasing molecular weight, which could be a possible explanation for the known dependence of charge transport properties on molecular weight.

Macromolecules, 2010

We report the quantitative conversion of bromine end groups in regioregular poly(3-hexylthiophene... more We report the quantitative conversion of bromine end groups in regioregular poly(3-hexylthiophene)s (P3HTs) and the characterization of the resulting monocarboxylated P3HTs (P3HT-COOHs) carrying one carboxylic acid group at their chain ends. The monocarboxylation for three different chain lengths is carried out, and the resulting P3HT-COOHs are characterized with size exclusion chromatography, matrix-assisted laser desorption ionization spectroscopy with time-of-flight detection mass spectroscopy, and UV-vis spectroscopy. The thermal properties and crystallinity in bulk and thin films were studied in a comparison between P3HT and P3HT-COOHs. Differential scanning calorimetry and wide-angle X-ray scattering support the increasing crystallinity for the higher molecular weight samples. Preliminary OFET measurements show a good charge carrier mobility in the range of 10 -3 cm 2 /(V s) for P3HT-COOHs with molecular weights of 5000 and 10 800 g/mol.

Macromolecular Materials and Engineering, 2002

5.5-inch panel thin OLED display with a quarter-VGA resolution (240 6 320 pixels) and a brightnes... more 5.5-inch panel thin OLED display with a quarter-VGA resolution (240 6 320 pixels) and a brightness of 200 cd/m 2 (Sanyo-Kodak).

The Journal of Physical Chemistry Letters, 2014

Conjugated poly(3-hexylthiophene) (P3HT) chains are known to exist at least in two distinct confo... more Conjugated poly(3-hexylthiophene) (P3HT) chains are known to exist at least in two distinct conformations: a coiled phase and a better ordered aggregated phase. Employing steady state absorption and fluorescence spectroscopy, we measure the course of aggregation of P3HT in tetrahydrofuran (THF) solution within a temperature range of 300 K to 170 K. We show that aggregation is a temperature controlled process, driven by a thermodynamic order−disorder transition. The transition temperature increases with the molecular weight of the chains and can be rationalized in the theory of Sanchez. This implies a smearing out of the phase transition in samples with increasing polydispersity and erodes the signature of a first order phase transition. The detection of a hysteresis when undergoing cooling/heating cycles further substantiates this reasoning.

Macromolecules, 2004

... Kun-Rung Lin, Chin-Chuan Chang, Ying-Hsueh Chang Chien, Man-kit Leung, Shun-Wei Liu, Jiun-Haw... more ... Kun-Rung Lin, Chin-Chuan Chang, Ying-Hsueh Chang Chien, Man-kit Leung, Shun-Wei Liu, Jiun-Haw Lee and Kuo-Huang Hsieh. Langmuir 2010 26 (7), 5147-5152. ... Zhongyuan Zhou, Jaclyn L. Brusso and Steven Holdcroft. Chemistry of Materials 2010 22 (7), pp 2287–2296. ...

Synthetic Metals, 1999

Polymeric triphenyldiamines (p-TPD)s and naphthylphenyldiamines (p-NPD)s were synthesized by Ullm... more Polymeric triphenyldiamines (p-TPD)s and naphthylphenyldiamines (p-NPD)s were synthesized by Ullmann reaction between bis(N,N-diaryl)amines and aryldiiodides in presence of phase-transfer catalyst, 18-crown-6. The polymers obtained are soluble in THF, CHCl3 etc and possess Tgs in the range of 206°C to 232°C and form stable and amorphous thin films. The HOMO energy values determined from cyclic voltammetry measurements lie between −5.10 and

Macromolecules, 2004

... Kun-Rung Lin, Chin-Chuan Chang, Ying-Hsueh Chang Chien, Man-kit Leung, Shun-Wei Liu, Jiun-Haw... more ... Kun-Rung Lin, Chin-Chuan Chang, Ying-Hsueh Chang Chien, Man-kit Leung, Shun-Wei Liu, Jiun-Haw Lee and Kuo-Huang Hsieh. Langmuir 2010 26 (7), 5147-5152. ... Zhongyuan Zhou, Jaclyn L. Brusso and Steven Holdcroft. Chemistry of Materials 2010 22 (7), pp 2287–2296. ...

Macromolecular Materials and Engineering, 2002

5.5-inch panel thin OLED display with a quarter-VGA resolution (240 6 320 pixels) and a brightnes... more 5.5-inch panel thin OLED display with a quarter-VGA resolution (240 6 320 pixels) and a brightness of 200 cd/m 2 (Sanyo-Kodak).

Organic Letters, 2010

Boron-dipyrrin dyes, through rational design, yield promising new materials. With strong electron... more Boron-dipyrrin dyes, through rational design, yield promising new materials. With strong electron-donor functionalities and anchoring groups for attachment to nanocrystalline TiO 2 , these dyes proved useful as sensitizers in dye-sensitized solar cells. Their applicability in a solid-state electrolyte regime offers additional opportunities for practical applications.

physica status solidi (RRL) - Rapid Research Letters, 2011

Thin Solid Films, 2010

Hole transport properties of three different side chain poly(triarylamines) have been determined ... more Hole transport properties of three different side chain poly(triarylamines) have been determined by means of the analysis of steady-state current-voltage characteristics using co-planar diode structures. The interpretation is based on space-charge limited models with field-dependent mobility. Mobilities between~10 − 8 and 10 − 6 cm 2 V − 1 s − 1 are obtained. The highest mobility is achieved for poly(tetraphenylbenzidine) devices and the lowest for poly(triphenylamine) devices. Electron-rich methoxy substituents increase the mobility of poly(triphenylamine)s. A comparison of the mobility values with those obtained using organic field-effect transistors is also given.

The Journal of Physical Chemistry C, 2008

We present a photophysical and device-based investigation of a new bipyridyl-NCS ruthenium comple... more We present a photophysical and device-based investigation of a new bipyridyl-NCS ruthenium complex sensitizer with an extended π system, in both sensitized TiO 2 and incorporated into solid-state dye-sensitized solar cells. We compare this new sensitizer to an analog dye without the extended π system. We observe very similar excited-state absorption spectra and charge recombination kinetics for the two systems. However, the π-extended senstizer has a phenomenally enhanced molar extinction coefficient which translates into far greater light harvesting and current collection in solid-state dye-sensitized solar cells. We also infer from transient photovoltage measurements that positioning the pendent extended π system away from the TiO 2 surface has induced a favorable dipole shift, generating enhanced open-circuit voltage. The resulting power conversion efficiency for the solar cell has been increased from 2.4% to 3.2% when comparing the new sensitizer to an analogy with no pendent group.

Synthetic Metals, 1999

Low molecular weight and polymeric imides are synthesized by attaching perylene units as substitu... more Low molecular weight and polymeric imides are synthesized by attaching perylene units as substituents or by incorporating in aromatic main chain copolymers to achieve high photoluminescent efficiency. In order to improve electron injection and transport properties 1,3,4oxadiazole comonomers with high electron affinities are used. The best solid state photoluminescence is obtained in copolymers with 0.5 mol% perylene content. The low molecular weight compounds exhibit high electron affinity with LUMO values in the range of -3.9 eV as determined by cyclic voltammetry. Electroluminescent devices with copolymers in PVK blends show characteristic red emission from perylene which is nearly identical with its photoluminescence and the efficiency of this device, ITO/copolymer+PVK blend(lOOnm)/Al, depends upon the PVK content.

Physical Chemistry Chemical Physics, 2011

Fig.S1 shows the life time measurements for HW456 dye. The potential step is from -0.4 V to +0.45... more Fig.S1 shows the life time measurements for HW456 dye. The potential step is from -0.4 V to +0.45 V vs Fc/Fc + . 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 0.0 0.2 0.4 0.6 0.8 1.0 HW456-TiO 2 film +0.45 V vs Fc/Fc + Time, t (min) Normalized ΔOD λ=770 nm Figure S1 Lifetime measurements for HW456 on nanocrystalline TiO 2 film in non-aqueous solution (0.1 M TBAP in acetonitrile).

Organic Electronics, 2007

The structural and optical properties of semiconductor block copolymers containing triphenylamine... more The structural and optical properties of semiconductor block copolymers containing triphenylamine as hole transport material and perylene bisimide as dye and electron transport material are reported. The polymers were prepared by nitroxide mediated controlled radical polymerisation and characterized with GPC, DSC, and TGA. The electrochemical properties as determined by cyclic voltammetry show the HOMO and LUMO values of the block copolymers to be À5.23 eV and À3.65 eV, respectively. The perylene bisimide units aggregate by p-p stacking which could be analyzed with wideangle X-ray scattering. The absorption and fluorescence properties of the perylene bisimide polymers and monomers in solution and film were investigated. It could be shown that they are strongly influenced by intramolecular coupling between different perylene bisimide units in polymers. The block copolymers exhibit a microphase separation on a nanometer scale with a constant perylene bisimide domain width of 13 nm and lengths of up to several micrometers.

Macromolecules, 2010

Molecular weight and stereoregularity affect the morphology and the crystallinity of conjugated p... more Molecular weight and stereoregularity affect the morphology and the crystallinity of conjugated polymers and are thus of pivotal relevance for the mobility of charge carriers in electro-optical device applications. Currently, poly(3-akylthiophenes) are discussed as one of the most promising classes of materials for photovoltaic applications. Here, we use 13 C solid-state NMR to determine the crystallinity and details on crystalcrystal phase transformations in regioregular head-to-tail poly(3-hexylthiophene) of well-defined molecular weight and demonstrate that the crystallinity was previously severely underestimated. Typical crystallinities are at least around 37% for the lowest molecular weights and as high as about 64% upon increasing MW, corresponding to a corrected maximum value for the reference melting enthalpy of ΔH m ¥ ≈ 37 J/g for use in DSC experiments. Using 1D 13 C spectra and 2D experiments that measure the strength of 13 C-1 H dipolar couplings, we observe that the crystal-crystal phase transition between a 3D-and a 2D-ordered crystalline phase at around 60°C entails a structural disordering process of the alkyl side chains, while not affecting rigidity and conformation of the backbones. This phase transition is suppressed at higher molecular weights due to a kinetic suppression of the formation of the alkyl-ordered low-temperature phase.

Macromolecules, 2010

We report on structural investigations of a series of regioregular poly(3-hexylthiophene) with we... more We report on structural investigations of a series of regioregular poly(3-hexylthiophene) with well-defined molecular weight (5-19 kg/mol) using DSC, small angle and wide-angle X-ray scattering, and AFM. With increasing temperature, we identify three ordered phases, namely 3D crystalline, 2D crystalline with disordered side chains, and a layered phase of smectic symmetry, followed by complete melting. Although all samples crystallize in extended chain conformation, the lower molecular weight material exhibits a lower crystallinity, most likely caused by noncrystallizable end groups. The crystallinity increases strongly with increasing molecular weight, which could be a possible explanation for the known dependence of charge transport properties on molecular weight.

Macromolecules, 2010

We report the quantitative conversion of bromine end groups in regioregular poly(3-hexylthiophene... more We report the quantitative conversion of bromine end groups in regioregular poly(3-hexylthiophene)s (P3HTs) and the characterization of the resulting monocarboxylated P3HTs (P3HT-COOHs) carrying one carboxylic acid group at their chain ends. The monocarboxylation for three different chain lengths is carried out, and the resulting P3HT-COOHs are characterized with size exclusion chromatography, matrix-assisted laser desorption ionization spectroscopy with time-of-flight detection mass spectroscopy, and UV-vis spectroscopy. The thermal properties and crystallinity in bulk and thin films were studied in a comparison between P3HT and P3HT-COOHs. Differential scanning calorimetry and wide-angle X-ray scattering support the increasing crystallinity for the higher molecular weight samples. Preliminary OFET measurements show a good charge carrier mobility in the range of 10 -3 cm 2 /(V s) for P3HT-COOHs with molecular weights of 5000 and 10 800 g/mol.

Macromolecular Materials and Engineering, 2002

5.5-inch panel thin OLED display with a quarter-VGA resolution (240 6 320 pixels) and a brightnes... more 5.5-inch panel thin OLED display with a quarter-VGA resolution (240 6 320 pixels) and a brightness of 200 cd/m 2 (Sanyo-Kodak).