title: Origin of Unusual Excitonic Absorption and Emission from Colloidal Ag2S Nanocrystals: Ultrafast Photophysics and Solar Cell.
abstract: Colloidal Ag2S nanocrystals (NCs) typically do not exhibit sharp excitonic absorption and emission. We first elucidate the reason behind this problem by preparing Ag2S NCs from nearly monodisperse CdS NCs employing cation exchange reaction. It was found that the defect-related midgap transitions overlap with excitonic transition, blurring the absorption spectrum. On the basis of this observation, we prepared nearly defect-free Ag2S NCs using molecular precursors. These defect-free Ag2S NCs exhibit sharp excitonic absorption, emission (quantum yield 20%) in near-infrared (853 nm) region, and improved performance of Ag2S quantum-dot-sensitized solar cells (QDSSCs). Samples with lower defects exhibit photoconversion efficiencies >1% and open circuit voltage of ∼0.3 V, which are better compared with prior reports of Ag2S QDSSCs. Femtosecond transient absorption shows pump-probe two-photon absorption above 630 nm and slow-decaying excited state absorption below 600 nm. Concomitantly, open-aperture z-scan shows strong two-photon absorption at 532 nm (coefficient 55 ± 3 cm/GW).
authors.affiliation: Department of Chemistry, Indian Institute of Science Education and Research (IISER) , Pune 411008, India.
authors.forename: Wasim J
authors.initials: WJ
authors.lastname: Mir
authors.affiliation: Department of Chemistry, Indian Institute of Science Education and Research (IISER) , Pune 411008, India.
authors.forename: Abhishek
authors.initials: A
authors.lastname: Swarnkar
authors.affiliation: Department of Physics, Indian Institute of Science Education and Research , Bhopal 462023, India.
authors.forename: Rituraj
authors.initials: R
authors.lastname: Sharma
authors.affiliation: Department of Chemistry, Indian Institute of Science Education and Research (IISER) , Pune 411008, India.
authors.forename: Aditya
authors.initials: A
authors.lastname: Katti
authors.affiliation: Department of Physics, Indian Institute of Science Education and Research , Bhopal 462023, India.
authors.forename: K V
authors.initials: KV
authors.lastname: Adarsh
authors.affiliation: Department of Chemistry, Indian Institute of Science Education and Research (IISER) , Pune 411008, India.
authors.forename: Angshuman
authors.initials: A
authors.lastname: Nag
keywords: Ag2S nanocrystals; cation exchange; excitonic absorption and emission; femtosecond transient absorption; quantum-dot-sensitized solar cell