Letter pubs.acs.org/NanoLett
Hot Carrier Extraction from Multilayer Graphene Roberto Urcuyo,† Dinh Loc Duong,† Patrick Sailer,† Marko Burghard,*,† and Klaus Kern†,‡ †
Max Planck Institute for Solid State Research, Heisenbergstrasse 1, D-70569 Stuttgart, Germany Institut de Physique, Ecole Polytechnique de Lausanne, CH-1015 Lausanne, Switzerland
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S Supporting Information *
ABSTRACT: Hot carriers in semiconductor or metal nanostructures are relevant, for instance, to enhance the activity of oxide-supported metal catalysts or to achieve efficient photodetection using ultrathin semiconductor layers. Moreover, rapid collection of photoexcited hot carriers can improve the efficiency of solar cells, with a theoretical maximum of 85%. Because of the long lifetime of secondary excited electrons, graphene is an especially promising two-dimensional material to harness hot carriers for solar-to-electricity conversion. However, the photoresponse of thus far realized graphene photoelectric devices is mainly governed by thermal effects, which yield only a very small photovoltage. Here, we report a Gr−TiOx−Ti heterostructure wherein the photovoltaic effect is predominant. By doping the graphene, the open circuit voltage reaches values up to 0.30 V, 2 orders of magnitude larger than for devices relying upon the thermoelectric effect. The photocurrent turned out to be limited by trap states in the few-nanometer-thick TiOx layer. Our findings represent a first valuable step toward the integration of graphene into third-generation solar cells based upon hot carrier extraction. KEYWORDS: hot carrier, graphene, photovoltaic devices, two-dimensional (2D) materials, F4-TCNQ, Gr−TiOx−Ti heterostructures
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solar cell efficiency is still below that needed for technological applications.15 Their atomic layer thickness renders two-dimensional (2D) materials especially suitable for hot carrier manipulation.16−18 In this context, graphene has attracted particularly strong attention due to its zero band gap, high intrinsic carrier mobility, and broadband photon absorption as well as the phonon bottleneck for electrons with low energies (
