Revealing Trap States in Lead Sulphide Colloidal Quantum Dots by Photoinduced Absorption Spectroscopy. (5th December 2017)

• Record Type: Journal Article
• Authors: Kahmann, Simon; Sytnyk, Mykhailo; Schrenker, Nadine; Matt, Gebhard J.; Spiecker, Erdmann; Heiss, Wolfgang; Brabec, Christoph J.; Loi, Maria A.
• Is Part Of: Advanced Electronic Materials. Volume 4:Number 1(2018)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2017-12-05
• Subjects: colloidal quantum dots -- mid infrared spectroscopy -- molecular vibrations -- pump‐probe spectroscopy -- trap states; Materials -- Electric properties -- Periodicals; Materials science -- Periodicals; Magnetic materials -- Periodicals; Electronic apparatus and appliances -- Periodicals; 537
• DOI: 10.1002/aelm.201700348
• Languages: English
• ISSNs: 2199-160X

Abstract

Abstract: Due to their large surface to volume ratio, colloidal quantum dots (CQDs) are often considered to exhibit a significant amount of surface defects. Such defects are one possible source for the formation of in‐gap states (IGS), which can enhance the recombination of excited carriers, i.e., work as electrical traps. These traps are investigated for lead sulphide CQDs of different size, covered with different ligands using a mid‐infrared photoinduced absorption (PIA) technique. The obtained PIA spectra reveal two distinct absorption bands, whose position depends on the particle size, i.e., the electronic confinement in the CQDs. Smaller particles exhibit deeper traps. The chemical nature of the capping ligand does not affect the resulting position other than due to its change in confinement, but better passivating species lead to smaller signals. Furthermore, ligand specific narrow lines observed are superimposed on the broad electronic background of the PIA spectra, which is attributed to Fano resonances caused by the interplay of the narrow molecular vibrations and the continuum of trap states. Mid‐infrared photoinduced absorption represents a valuable tool to unravel distributions of IGS in CQDs and allows for an assessment of the quality of ligand exchanged films. These findings have implications for understanding the performances of CQD‐based (opto‐) electronic devices, such as solar cells, transistors, or quantum dot light emitting diodes, which are limited by frequent carrier trapping events. Abstract : Trap states in PbS colloidal quantum dots are investigated through mid‐infrared pump‐probe spectroscopy. Two distinct photoinduced absorption bands are observed for a variety of ligands and quantum dot sizes. Native oleic acid exhibits the lowest concentration of surface traps, whereas those ligands, relevant for device manufacturing, display a significantly increased response.