Right On Time: Ultrafast Charge Separation Before Hybrid Exciton Formation

TL;DR

This paper reveals that ultrafast charge separation occurs at organic/ZnO interfaces but is followed by electron recapture and trapping, explaining low efficiencies and guiding future hybrid solar cell design.

Contribution

It identifies and quantifies the ultrafast charge separation and subsequent electron recapture processes at organic/ZnO interfaces using advanced spectroscopy and calculations.

Findings

Charge separation occurs within 350 fs.

Electrons are recaptured on a 100 ps timescale.

Electrons are trapped in a long-lived hybrid exciton state.

Abstract

Organic/inorganic hybrid systems offer great potential for novel solar cell design combining the tunability of organic chromophore absorption properties with high charge carrier mobilities of inorganic semiconductors. However, often such material combinations do not show the expected performance: while ZnO, for example, basically exhibits all necessary properties for a successful application in light-harvesting, it was clearly outpaced by TiO$_2$ in terms of charge separation efficiency. The origin of this deficiency has long been debated. This study employs femtosecond time-resolved photoelectron spectroscopy and many-body ab initio calculations to identify and quantify all elementary steps leading to the suppression of charge separation at an exemplary organic/ZnO interface. We demonstrate that charge separation indeed occurs efficiently on ultrafast (350 fs) timescales, but that electrons are recaptured at the interface on a 100 ps timescale and subsequently trapped in a strongly bound (0.7 eV) hybrid exciton state with a lifetime exceeding 5 $\mu$s. Thus, initially successful charge separation is followed by delayed electron capture at the interface, leading to apparently low charge separation efficiencies. This finding provides a sufficiently large timeframe for counter-measures in device design to successfully implement specifically ZnO and, moreover, invites material scientists to revisit charge separation in various kinds of previously discarded hybrid systems.