Browsing by Author "Sharma, S."

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Open Access
Fermi level pinning ınduced by doping in air stable n type organic semiconductor
(American Chemical Society, 2020) Sharma, S.; Ghosh, S.; Ahmed, T.; Ray, S.; Islam, S.; Salzner, Ulrike; Ghosh, A.; Seki, S.; Patil, S.
Doping of organic semiconductors enhances the performance of optoelectronic devices. Although p-type doping is well studied and successfully deployed in optoelectronic devices, air stable ntype doping was still elusive. We succeeded with n-type doping of organic semiconductors using molecular dopant N-DMBI under ambient conditions. Strikingly, n-type doping accounts for a gigantic increase of the photoconductivity of doped thin films. Electrical and optical properties of the n-doped molecular semiconductor were investigated by temperature dependent conductivity, electron paramagnetic resonance (EPR), and flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurements. A significant reduction and saturation in activation energy with increasing doping level clearly suggests the formation of an impurity band and enhancement in carrier density. Computational studies reveal the formation of a charge transfer complex mediated by hydrogen abstraction as the rate-determining step for the doping mechanism. The colossal enhancement of photoconductivity induced by n-doping is a significant step toward optoelectronic devices made of molecular semiconductors.
Open Access
Synthesis and characterization of quinoidal diketopyrrolopyrrole derivatives with exceptionally high electron affinities
(American Chemical Society, 2017) Ray, S.; Sharma, S.; Salzner, U.; Patil, S.
Open-shell singlet biradicaloids are short-lived intermediates, but they exhibit fascinating properties for spin-based devices. Therefore, understanding the nature of their electronic structure and stability is critical for harnessing them in optoelectronic or spintronic devices. Toward this goal, we have synthesized a series of diketopyrrolopyrrole-based quinoidal molecules to investigate the contribution and relative importance of the biradical form on the ground-state electronic structure and distribution of spin density. Possibility of crossover from a closed-shell to an open-shell structure with increase in the C=C/C-C conjugation length was investigated. The ground-state properties were systematically investigated by nuclear magnetic resonance (NMR) spectroscopy, single-crystal X-ray diffraction, and electrochemical studies. Furthermore, n-doping has been carried out in solution at ambient conditions to understand the nature of doped species and demonstrate air stability. Doped species were probed by UV-visible and electron spin resonance (ESR) spectroscopy to unambiguously establish the generation of anionic species in solution. Experimental results are complemented by theoretical calculations to provide insight into the trend toward biradicaloid spin states with increasing conjugation length.