Controlling (hot) electron transfer in quantum dot heterojunction films

Gianluca Grimaldi^a, Arjan Houtepen^a, R.W. Crisp^a, S. Ten Brinck^b, F. Zapata^b^,^c, M. van Ouwendorpa^a, M. van den Brom^a, N. Renaud^a, N. Kirkwood^a, W. Evers^a^,^d, S. Kinge^c, I. Infante^b, L.D.A. Siebbeles^a

^aDelft University of Technology, The Netherlands, Julianalaan, 136, Delft, Netherlands

^bVrije University (VU) Amsterdam, De Boelelaan 1081, Amsterdam, Netherlands

^cNetherlands eScience Center, 1098 XG Ámsterdam, Países Bajos, Amsterdam, Netherlands

^dDelft University of Technology, The Netherlands, Julianalaan, 136, Delft, Netherlands

^eToyota Motor Europe, Materials Research & Development, Hoge Wei 33, B-1930 Zaventem, Belgium

Materials for Sustainable Development Conference (MATSUS)
Proceedings of nanoGe Fall Meeting 2018 (NFM18)

S5 Charge Carrier Dynamics at the Nanoscale

Torremolinos, Spain, 2018 October 22nd - 26th

Organizers: David Egger, Arjan Houtepen and Freddy Rabouw

Oral, Gianluca Grimaldi, presentation 344
DOI: https://doi.org/10.29363/nanoge.nfm.2018.344
Publication date: 6th July 2018

Control over the energy alignment of Quantum Dots (QD) heterostructures can be used to unlock new functionalities for QD based optoelectronic devices: from improved carrier separation in type-II heterostructure, to control over hot-carrier transfer in type I heterostructures and lowered Carrier Multiplication threshold in quasi-type-II heterostructures.

We investigated the carrier dynamics in QD heterojunction films composed of PbSe and CdSe QDs. We demonstrate that such films tend to form a type I band alignment in which fast and efficient hot electron transfer from PbSe QDs to CdSe QDs is observed by transient absorption (TA) measurements. The efficiency of the hot electron transfer process increases with excitation energy as a result of the more favorable competition between hot-electron transfer and electron cooling. The experimental picture is supported by time-domain density functional theory calculations, showing that electron density is transferred from lead selenide to cadmium selenide quantum dots on the sub-picosecond timescale. Hot-electron solar cells have been proposed as a route towards higher efficiency solar cells, and our observation reveals the possibility to achieve and control hot-electron transfer via energy-structure engineering in QD heterojunctions.

We next attempted to switch the energy alignment between the PbSe QDs and CdSe QDs, using the size-dependence of their energy structure as well as using tailored ligands to shift the energy levels through their surface dipoles. Spectroelectrochemical measurements reveal that we can shift the type I alignment to a type II alignment and TA measurements demonstrate a much-improved efficiency of “cold” electron transfer.

We thus proved that a combination of size-variation and control over surface-passivation allows to span the range between type-I and type-II alignment. One particularly interesting configuration is that of quasi-type-II alignment, where the conduction electron levels are resonant, as this could potentially be used for optimal Carrier Multiplication.

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