Electron–hole exchange blockade and memory-less recombination in photoexcited films of colloidal quantum dots

• Published in: Nature physics Vol. 13; no. 6; pp. 604 - 610
• Main Authors: Fidler, Andrew F., Gao, Jianbo, Klimov, Victor I.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.06.2017; Nature Publishing Group; Nature Publishing Group (NPG)
• Subjects: 140/125; 639/624/399/1017; 639/925/357/1017; Activation energy; Atomic; Carriers; Classical and Continuum Physics; Colloids; Complex Systems; Condensed Matter Physics; Dynamics; Electrons; Exchange; Mathematical and Computational Physics; Molecular; Optical and Plasma Physics; Photocurrent; Photoelectric effect; Physics; Physics and Astronomy; Quantum dots; solar (photovoltaic), solar (fuels), solid state lighting, bio-inspired, electrodes - solar, defects, charge transport, materials and chemistry by design, optics, synthesis (novel materials), synthesis (scalable processing); Theoretical
• ISSN: 1745-2473; 1745-2481
• DOI: 10.1038/nphys4073

Understanding charge transport and recombination dynamics in photoexcited colloidal quantum dot (QD) solids is key to their applications in optoelectronic devices. Towards this end, we conduct transient photocurrent studies of films of electronically coupled, device-grade PbSe QD films. We observe that the photocurrent amplitude detected following excitation with a short 100 fs pulse is virtually temperature independent down to 6 K, suggesting a tunnelling mechanism of early-time photoconductance. The later-time signal exhibits clear signatures of thermal activation with characteristic energies that are surprisingly robust and independent of the exact type of QD surface treatment. We attribute this behaviour to the involvement of intrinsic fine-structure states and specifically the electron–hole exchange interaction, which creates an energetic barrier to electron–hole separation between adjacent QDs. At room temperature, which is well above the largest activation energy, relaxation of photoconductivity is dominated by non-geminate recombination involving mobile band-edge carriers of one sign and low-mobility carriers of the opposite sign (pre-existing and photoexcited) residing in intragap states. This process leads to memory-less dynamics when the photocurrent relaxation time is directly linked to the instantaneous carrier density. Understanding the recombination dynamics in quantum dots is crucial for their use in optoelectronic devices. A photocurrent spectroscopy study shows how two distinct relaxation mechanisms are at play over different timescales.