Amaresh Mishra

A new class of π-conjugated polycyclic hydrocarbons that promises interesting electronic properties is presented. The synthesis and extension of the S,N-heteroacene series consisting of only five-membered heterocyclic rings up to a very long, stable, and still soluble decacene SN10 is realized by multiple Pd-catalyzed aminations of halogenated thiophene precursors as key reactions. These novel heteroacenes were characterized by optical spectroscopy and electrochemistry providing interesting structure-property relationships. Nearly complete bond-length equalization in the inner part of the conjugated backbone and an unusual herringbone packing in the solid state underline the structural features of these novel systems.

ABSTRACT Novel low band gap oligothiophenes incorporating S,N-heteropentacene central units were developed and used as hole-transport material (HTM) in solid-state perovskite-based solar cells. In addition to appropriate electronic energy levels, these materials show high photo absorptivity in the low energy region, thus can contribute to the light harvesting of the solar spectrum. Solution-processed CH3NH3PbI3-based devices using these HTMs achieved power conversion efficiencies of 9.5-10.5% in comparison to 7.6% obtained by reference devices without HTM. Photoinduced absorption spectroscopy gave further insight into the charge transfer behavior between photoexcited perovskite and the HTMs.

ABSTRACT Dicyanovinyl (DCV) end-capped oligomers 1–7 containing fused benzo[2,1-b:3,4-b′]dithiophene (BDT) and naphtho[2,1-b:3,4-b′]dithiophene (NDT) as central cores have been synthesized and characterized. These oligomers show excellent thermal stability due to the insertion of the central fused ring system thus allowing purification by gradient sublimation in high yield. With respect to the reference compound, non-fused tetramer DCV4T, the new oligomers showed hypsochromic shifts in absorption and emission spectra and larger band gaps in thin films. Due to high lying LUMO energy levels, they exhibit sufficient energy offset with respect to C60 to allow for efficient charge transfer in organic solar cells. At the same time, low-lying HOMO energy levels result in high open-circuit voltages (VOC). As a result, planar heterojunction (PHJ) solar cells derived from the novel oligomers and C60 provide very high open circuit voltages (VOC) of up to 1.21 V and power conversion efficiencies (PCEs) of up to 2.7%. Bulk-heterojunction (BHJ) devices comprising oligomers 1–4 and C60 display a slightly lower VOC of 1 V leading to efficiencies as high as 3.6%.

ABSTRACT The extension of a series of dithienopyrrole containing A-D-A oligothiophenes for application in solution-processed bulk heterojunction solar cells is described. Using solvent vapor annealing, power conversion efficiencies up to 6.1% are obtained. Exposure of the photoactive layer to chloroform vapor results in increased absorption and ordering of the donor:acceptor blend, as is evident from UV-vis absorption spectroscopy, X-ray diffraction (XRD) spectroscopy, and atomic force microscopy (AFM). The type and position of the solubilizing alkyl chains influences the dissolution, optical, and packing properties of the oligomers. However, despite subtle differences in molecular structure, all electron donors could be implemented in solar cells demonstrating power conversion efficiencies between 4.4 and 6.1%. Upon further optimization of these in-air, processed devices, it is expected that additional improvements in photovoltaic performance can be achieved.

A new class of acceptor-substituted S,N-heteropentacenes is developed for vacuum-processed organic solar cells, providing encouraging power conversion efficiencies of up to 6.5%. Atomic force microscopy (AFM) investigations give a direct correlation between the blend film morphology and the photovoltaic parameters, such as short-circuit current density (JSC ) and fill factor (FF).

Fused S,N-heterohexacene 4 was synthesized by applying Pd-catalyzed tandem Buchwald-Hartwig coupling and further functionalized to corresponding acceptor-capped derivatives 5 and 6 showing bond length equalization in the π-conjugated backbone and intense optical transitions. Organic thin film transistors (OTFTs) based on a vacuum-deposited film of 6 exhibit p-channel charge-carrier mobilities as high as 0.021 cm(2) V(-1) s(-1) and current on/off ratios of 10(5).

Novel A-D-A-D-A-type oligothiophenes incorporating electron-withdrawing benzo[c][1,2,5]thiadiazole (BTDA) as core and trifluoroacetyl (TFA) as terminal acceptor groups have been developed. Vacuum-processed planar heterojunction organic solar cells incorporating these new oligomers as donor and C(60) as acceptor showed very high open circuit voltages up to 1.17 V, resulting in power conversion efficiencies of 1.56% under AM1.5G conditions.

We have synthesized and characterized a series of dendritic oligothiophenes comprising peripheral mannose functionalities by copper-mediated 1,3-dipolar cycloaddition reaction. The hybrids were accessible either by attachment of acetyl-protected mannosides at the dendritic oligothiophene or by the direct synthesis of the deprotected mannose-oligothiophene conjugates by applying heterogeneous reaction conditions. The specific interaction of the functional dendritic hybrids with lectin protein Concanavalin A was investigated by absorption and fluorescence spectroscopic measurements. An enhancement of turbidity was observed due to the specific interaction of the mannosidic dendrimer with Con A. The specific interaction was further confirmed by fluorescence quenching upon addition of the mannosidic hybrid to Con A.

The novel methyl-substituted dicyanovinyl-capped quinquethiophenes 1-3 led to highly efficient organic solar cells with power conversion efficiencies of 4.8-6.9%. X-ray analysis of single crystals and evaporated neat and blend films gave insights into the packing and morphological behavior of the novel compounds that rationalized their improved photovoltaic performance.

ABSTRACT We report the synthesis of new donor–π–acceptor (D–π–A) dyes and their application in dye-sensitized solar cells (DSCs) with nickel(II) oxide (NiO)-based photocathodes. These D–π–A sensitizers incorporate a triphenylamine donor, a bithiophene π-bridge, and a perylenemonoimide (PMI) acceptor group. Two carboxylate groups attached to the triphenylamine afford strong anchoring to the NiO surface. The dyes in this series were varied firstly by the inclusion of an ethynyl linker between bithiophene and the triphenylamine moieties (1vs. 2), thereby increasing the length of the conjugated bridge. Despite very similar optoelectronic properties, the ethynyl-containing dye 2 showed a 25% improvement in power conversion efficiency in p-DSCs compared to 1, mostly attributed to the increased current density. Contrary to initial expectations, there was no major influence of the distance between the PMI unit of the dye and the NiO surface on the photoinduced dye anion lifetime, as measured by nanosecond transient absorption spectroscopy (TAS). Furthermore, altering the position of the alkyl chains on the bridging bithiophene in 3 and 4 resulted in a modest red shift in the dye absorption on account of increased charge delocalisation between the PMI and the π-bridge, owing to a reduced torsion angle between the PMI and the adjacent thiophene unit. Quantum-chemical DFT calculations were performed in order to evaluate these torsion angles and to study their influence on the electron density distribution in the relevant molecular orbitals. These changes of the molecular structure of the isomeric dyes 3 and 4 did not translate into improved photovoltaic performance, which is primarily attributed to lower charge photogeneration rates probed by transient absorption spectroscopy. While for p-type DSCs impressive overall solar-to-electric conversion efficiency of 0.04–0.10% under full sun illumination (simulated AM1.5G sunlight, 100 mW cm−2) and a broad incident photon to current efficiency (IPCE) response (350–700 nm) is demonstrated for these new dyes, the study clearly shows the need for judicious design rules for p-type sensitizers for application in photocathodic DSCs.

A series of novel structurally well-defined oligothienylene-ethynylene-based dendritic macromolecules up to the 3rd generation (G3) were successfully synthesized by a combination of Pd-catalyzed Sonogashira-type cross-coupling and oxidative homocoupling steps. Oxidative homocoupling of dendrons successfully afforded dendrimers up to the 2nd generation (G2). In contrast, the G3 dendrimer was effectively prepared by a four-fold Sonogashira-type cross-coupling reaction. All compounds showed broad and structureless absorption and emission spectra arising from the presence of different pi-conjugated chromophores. With increasing generation, a bathochromic shift of the pi-pi* absorption band and an increase of the absorption coefficient were observed. The insertion of ethynylene groups into the conjugated dendrimer backbone resulted in a hypsochromic shift compared to all-thiophene dendrimers reported earlier by our group. All dendritic compounds are fluorescent and showed moderate quantum efficiencies due to an effective intramolecular charge-transfer (ICT) process. Cyclic voltammetry measurements also revealed the presence of multiple pi-conjugative pathways that show very broad oxidation waves for higher generations. HOMO-LUMO energy levels of these dendrons and dendrimers were estimated from optical and redox measurements and the calculated band gaps were within the range of 3.3 to 2.4 eV, typical for oligo- and polythiophenes. Electrochemical polymerizations of several desilylated compounds were performed and characterization of the films is reported. Preliminary bulk heterojunction solar cells that utilise these ethynylated dendrimers as the donor and [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM[60]) as the acceptor showed moderate efficiencies ranging from 0.18-0.64%.

A series of terminal dicyanovinylene-substituted quinquechalcogenophenes with increasing number of selenophene units is presented, and their spectroscopic, electrochemical, and thermal properties were investigated. Vacuum-deposited bulk-heterojunction solar cells ...

Amaresh Mishra (right) received his Ph.D. degree in 2000 from the Sambalpur University, India, under the coguidance of Prof. GB Behera and Prof. RK Behera, working on dye−surfactant interactions in microheterogeneous media. He started his postdoctoral research career in ...

Novel A-D-A type oligothiophenes incorporating benzothiadiazole (BTDA) and thiadiazolopyridine (TDAPy) as terminal acceptor groups have been developed for small molecule organic solar cells (SMSC). In vacuum-processed planar heterojunction solar cells the TDAPy-based oligomer showed a power conversion efficiency of 3.15% and a high fill factor of 0.67.

Versatile synthesis of fluorescent, carbohydrate-functionalized oligothiophene hybrids via Sonogashira-type cross coupling reactions is reported. The mannose derivatives reveal specific binding with the model lectin from Canavalia enisformis (Concanavalin A).

Pd-catalyzed Sonogashira cross-coupling reactions were used to synthesize novel π-conjugated oligothienylene-ethynylene dendrons and their corresponding terpyridine-based ligands. Their complexation with Ru(II) led to interesting novel metallodendrimers with rich spectroscopic properties. All new compounds were fully characterized by (1)H and (13)C NMR, as well as MALDI-TOF mass spectra. Density functional theory (DFT) calculations performed on these complexes gave more insight into the molecular orbital distributions. Photophysical and electrochemical studies were carried out in order to elucidate structure-property relationships and the effect of the dendritic structure on the metal complexes. Photophysical studies of the complexes revealed broad absorption spectra covering from 250 to 600 nm and high molar extinction coefficients. The MLCT emission of these complexes were significantly red-shifted (up to 115 nm) compared to the parent [Ru(tpy)2](2+) complex.

Fitzner, R., Reinold, E., Mishra, A., Mena-Osteritz, E., Ziehlke, H., Körner, C., Leo, K., Riede, M., Weil, M., Tsaryova, O., Weiß, A., Uhrich, C., Pfeiffer, M. and Bäuerle, P., Dicyanovinyl–Substituted Oligothiophenes: Structure-Property Relationships and Application in Vacuum ...

ZnO powders are used in industry mainly as reinforcing fillers for elastomers and as overvoltage arresters for the protection of electrical circuits. In addition, the direct band gap (3.37 eV) and high exciton energy (60 meV) make ZnO a candidate material for UV optoelectronic ...