(Invited) Minimizing Thermal Losses at Quantum Dot-Metal Oxide Interfaces for Solar Energy Conversion

Abstract

The sensitization of mesoporous oxides by semiconductor quantum dots (QDs) represents an appealing route for the development of low-cost solutions for solar energy production and storage (e.g. solar cells and fuel architectures). In order to boost photoconversion efficiency at these technologically relevant interfaces is important to minimize energy losses upon photon absorption; Energy losses that are intimately linked to kinetic completion at the relevant donor-acceptor interface [1]. Two main thermal energy losses can be identified following the absorption of a photon at the QD-oxide interface: firstly, any excess energy of the impinging photon vs the QD absorption onset is generally wasted as heat within the QD donor; secondly, the excess energy between QD electron donating state and the oxide accepting state (bottom of the conduction band) constitutes as well a thermal loss (now within the oxide) which reduces the available energy in the system to produce work. Reducing thermal losses associated with the latter mechanism is mandatory for allowing these architectures to reach efficiencies up to ~33% (the so called Shockley-Queisser limit). Preventing charge carrier cooling within the QD absorber after photon absorption, e.g. by enabling hot electron transfer toward the oxide, could – in principle - pave the way for developing QD dot based hot carrier solar cells with efficiencies beyond the Shockley-Queisser limit.Here I will present, from a charge carrier dynamics perspective, our studies aiming bypassing thermal losses at QD-oxide interfaces. First I will show how attempts to reduced QD-oxide donor-acceptor interfacial energetics by exploiting QD dipolar molecular capping can be in vain in strongly coupled QD-oxide sensitized systems [2]. This is linked to Fermi level pinning at the QD-oxide interface. I will show how this effect can be bypassed by inserting an insulating layer between QD donor and oxide acceptor. In the second part of my talk, I will discuss results demonstrating room-temperature hot electron transfer (HET) in strongly coupled QD-sensitized mesoporous oxides [3]. We quantify HET rates and show that the HET efficiency is determined by interfacial kinetic competition that can be optimized by both, maximizing QD-oxide coupling and/or suppressing electron cooling within the QD. Implications of these results for the challenging development of ultra-high efficiency solar converters will be discussed.