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).

Impact of reaction variables and PEI/l-cysteine ratio on the optical properties and cytocompatibility of cationic Ag2S quantum dots as NIR bio-imaging probes

“…42 For a 4.5 nm Ag 2 S (3-6.5 nm) with emission at 850 nm, biexponential decay with 1 and 11 ns was reported and the later was attributed to the excitonic recombination. 42 In another study, 1.5 nm Ag 2 S with emission maxima at 813 nm was reported to have an average lifetime of 57 ns, which was attributed to the radiative decay related to surface inhomogeneities. 3 Zhang et al reported increasing lifetimes for Ag 2 S-dodecanethiol with increasing crystal size.…”

Size‐Tunable Synthesis of Colloidal Silver Sulfide Nanocrystals for Solution‐Processed Photovoltaic Applications

“…Monoclinic α‐Ag 2 S is a promising candidate in wide application such as quantum dots sensitized solar cell (QDSSCs), photo catalysis and bioimaging due to its narrow band gap (0.9‐1.1 eV), relatively large optical absorption coefficient and its environment friendly nature. Till date, different methods have been developed for the synthesis of Ag 2 S QDs and thus obtained QDs have been used in biomedical applications such as in vitro,, and in vivo,, imaging.…”

Solution-Processed Environmentally Friendly Ag2S Colloidal Quantum Dot Solar Cells with Broad Spectral Absorption

“…The stability of Ag 2 S quantum dots produced by successive ionic layer adsorption and reaction (SILAR) as well as their ability to inject electrons into an n-type semi-conductor, such as TiO 2 , have been investigated [20]. Above 1% solar to electricity power conversion efficiency (PCE) using a device structure similar to the dye-sensitized solar cells has been achieved [21][22][23][24]. However, there have been no reports on solar cells based on solid CQD layers with Ag 2 S, similar to the solid CQD layers used in the most efficient PbS CQD solar cells discussed above.…”