Browsing by Author "Udayakumar, U.K."

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A tale of two organic small molecular hole transporting materials: Showing same extended shelf-life but very different efficiency of inverted MAPbI3 perovskite solar cells
(Elsevier B.V., 2022) Kakekochi, V.; Kuo, D.-W.; Chen, C.-T.; Wolcan, E.; Chen, C.-T.; Udayakumar, U.K.
This paper mainly discusses the structure-property relationship of two donor-acceptor-donor (D–π–A–π–D) type dopant-free hole transporting materials (HTMs) (TPA-TPy and TPA-Py-PTZ) comprising primarily of 2,4,6-trisubstituted pyridine as the acceptor core and 4,4′-dimethoxytriphenylamine as the peripheral donor groups and their use in p-i-n perovskite solar cells (PVSCs). Compared to inferior TPA-Py-PTZ, TPA-TPy has a superior hole extraction and hole transport at the HTM/perovskite interface. The pinhole-free, smooth and dense, fully covered and well-crystallized MAPbI3 perovskite layer on TPA-TPy reduces the carrier recombination and substantially improves the short circuit current density (JSC), open circuit voltage (VOC), and the fill-factor (FF) of MAPbI3 PVSCs. The PVSC employing TPA-TPy as HTM exhibits a power conversion efficiency (PCE) of 15.33% with a JSC of 23.69 mA cm−2, a VOC of 0.95 V, and a FF of 68.10%. Especially, both TPA-TPy and TPA-Py-PTZ PVSCs exhibit a better moisture stability than that of NiOx PVSCs. It is because of the hydrophobic nature of TPA-TPy and TPA-Py-PTZ, which enables the formation of MAPbI3 perovskite layer having a larger grain-size, a less grain boundary, and a less infiltration of moisture. © 2022 Elsevier B.V.
All-Optical Diode Action through Enhanced Nonlinear Response from Polymeric Photonic Crystal Microcavity
(American Chemical Society, 2022) Puthiya Purayil, N.; Kakekochi, V.; Udayakumar, U.K.; Chandrasekharan, C.
The search for materials for nanophotonic devices needs a way to overcome the limitations of weak nonlinear optical response due to reduced size. The requirement of a high-power excitation source for inducing nonlinear effects in the active medium poses a hurdle in realizing all-optical and integrated optoelectronic systems. Nanosized optical materials can be embedded in photonic cavities to enhance its optical nonlinear responses through confinement effects. This report systematically investigates the linear and nonlinear optical properties of polymeric 1D-photonic crystals (PhC) with a defect layer incorporated with an imidazo-thiadiazole derivative small molecule. The nonlinear optical studies were carried out by means of a z-scan technique under nanosecond pulse using a Q-switched Nd:YAG laser operating at 532 nm. The resonant excitation creates strong confinement of light in the photonic microcavity and facilitates strong light-matter interaction that induces nonlinear response at low input intensities. A giant enhancement in the nonlinear absorption and optical limiting action was observed for PhC comparing to the bare sample. The significant enhancement in the absorptive nonlinearity is utilized for realizing a practical, on-chip, passive, all-optical diode with a PhC/Au hybrid structure. The asymmetrical nonlinear absorption of the hybrid structure exhibits non-reciprocal light transmission in the nonlinear regime. The high transmittance contrast ratio for forward/reverse bias operation and cost-effective fabrication methods makes our system a potential candidate toward the realization of compact photonic integrated devices. © 2021 American Chemical Society.
Butterfly-Shaped Thiophene-Pyridine Hybrids: Green Electroluminescence and Large Third-Order Optical Nonlinearities
(Wiley-VCH Verlag info@wiley-vch.de, 2020) Kakekochi, V.; Gangadharappa, S.C.; Nikhil, P.P.; Chandrasekharan, K.; Darshan, V.; Narayanan Unni, K.N.; Udayakumar, U.K.
A set of four symmetric, butterfly-shaped 4-(4-(decyloxy)phenyl)-2,6-di(thiophen-2-yl)pyridine (TPY) derivatives 2TPA-TPY (TPY center and triphenylamine end groups), 2CBZ-TPY (TPY center and N-ethyl carbazole end groups), 2TPY-TPA (triphenylamine center and TPY at the periphery) and 2TPY-CBZ (N-ethyl carbazole center and TPY at the periphery) was synthesized. The molecules show reverse saturable absorption (RSA) which is consistent with two-photon absorption (2PA) associated with excited-state absorption (ESA) when excited using a 532 nm laser beam. The molecules 2TPA-TPY and 2TPY-TPA possess extremely low limiting thresholds of 1.73 and 2.68 J cm?2, respectively. An organic light-emitting diode (OLED) fabricated from 2TPA-TPY exhibits green emission with a maximum luminance of 207 cd m?2, a current efficiency (?CE) of 1.51cd A?1, a maximum power efficiency (?Pmax) of 0.46 lm W?1 and an external quantum efficiency (?EQE) of 0.48 % at 100 cd m?2. © 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Design and synthesis of new low band gap organic semiconductors for photovoltaic applications
(SPIE spie@spie.org, 2014) Murali, M.G.; Udayakumar, U.K.
Donor-acceptor (D-A) conjugated polymers have attracted a good deal of attention in recent years. In D-A systems, the introduction of electron withdrawing groups reduces Eg by lowering the LUMO levels whereas, the introduction of electron donating groups reduces Eg by raising the HOMO levels. Also, conjugated polymers with desired HOMO and LUMO energy levels could be obtained by the proper selection of donor and acceptor units. Because of this reason, D-A conjugated polymers are emerging as promising materials particularly for polymer light emitting diodes (PLEDs) and polymer solar cells (PSCs). We report the design and synthesis of four new narrow band gap donor-acceptor (D-A) conjugated polymers, PTCNN, PTCNF, PTCNV and PTCNO, containing electron donating 3,4-didodecyloxythiophene and electron accepting cyanovinylene units. The effects of further addition of electron donating and electron withdrawing groups to the repeating unit of a D-A conjugated polymer (PTCNN) on its optical and electrochemical properties are discussed. The studies revealed that the nature of D and A units as well as the extent of alternate D-A structure influences the optical and the electrochemical properties of the polymers. All the polymers are thermally stable up to a temperature of 300 Â°C under nitrogen atmosphere. The electrochemical studies revealed that the polymers possess low-lying HOMO energy levels and low-lying LUMO energy levels. In the UV-Vis absorption study, the polymer films displayed broad absorption in the wavelength region of 400-700 nm. The polymers exhibited low optical band gaps in the range 1.70 - 1.77 eV. Â© 2014 SPIE.
Facile synthesis and exploration of excited state assisted two-photon absorption properties of D-A-D type thiophene-pyridine derivatives
(Royal Society of Chemistry, 2020) Kakekochi, V.; Chatnahalli Gangadharappa, S.; Nikhil, P.P.; Chandrasekharan, K.; Wolcan, E.; Udayakumar, U.K.
A series of trigonal-shaped molecules (TPyT and TPyP) and a polymer (P2TPy) with donor-acceptor-donor (D-A-D) structural arrangement were designed with thiophene and 2,4,6-trisubstituted pyridine as the backbone i.e., a 4-(4-(decyloxy)phenyl)-2,6-di(thiophen-2-yl)pyridine core structure, for the first time and synthesized via a microwave assisted one pot reaction. The extension of conjugation of the system by substituting thiophene acrylonitrile (TPyT) and phenyl acrylonitriles (TPyP and P2TPy) via low cost condensation reactions improved the linear optical, electronic and thermal properties of the synthesized compounds. Furthermore, the nonlinear absorption (NLA) was also enriched appreciably by increasing the conjugation. The three compounds showed an "effective two-photon absorption" phenomenon with the NLA coefficient (?eff) of the order of 10-10 m W-1. The suitability to be utilized in the fabrication of all-optical limiting devices is best achieved by polymer P2TPy as it exhibits substantially higher ?eff (7.02 × 10-10 m W-1) and an extremely low optical limiting threshold (1.42 J cm-2). © 2020 The Royal Society of Chemistry and Owner Societies.