Covalent Linking Greatly Enhances Photoinduced Electron Transfer in Fullerene-Quantum Dot Nanocomposites: Time-Domain Ab Initio Study Vitaly V. Chaban, †,‡ Victor V. Prezhdo, § and Oleg V. Prezhdo* ,† † Department of Chemistry, University of Rochester, Rochester, New York 14627, United States ‡ MEMPHYS - Center for Biomembrane Physics, Odense M. 5230, Denmark § Institute of Chemistry, Jan Kochanowski University, 25-406 Kielce, Poland ABSTRACT: Nonadiabatic molecular dynamics combined with time-domain density functional theory are used to study electron transfer (ET) from a CdSe quantum dot (QD) to the C 60 fullerene, occurring in several types of hybrid organic/inorganic nanocomposites. By unveiling the time dependence of the ET process, we show that covalent bonding between the QD and C 60 is particularly important to ensure ultrafast transmission of the excited electron from the QD photon-harvester to the C 60 electron acceptor. Despite the close proximity of the donor and acceptor species provided by direct van der Waals contact, it leads to a notably weaker QD-C 60 interaction than a lengthy molecular bridge. We show that the ET rate in a nonbonded mixture of QDs and C 60 can be enhanced by doping. The photoinduced ET is promoted primarily by mid- and low- frequency vibrations. The study establishes the basic design principles for enhancing photoinduced charge separation in nanoscale light harvesting materials. SECTION: Energy Conversion and Storage; Energy and Charge Transport A great variety of solar cell designs are proposed and investigated worldwide. 1-6 Photosynthetic membranes, conjugated polymers, sensitizer chromophores, inorganic semi- conductors, and other materials serve as light-harvesting antennas in liquid-junction, 7 hybrid organic, 8 thin film, 9 and other types of photovoltaic and photocatalytic devices. Utilization of nanoscale architectures in light energy conversion into electrical and chemical energy has emerged as an alternative to single-crystalline devices. Low material cost, easy methods of fabrication, high light absorption cross sections, and ability to tune continuously the optical response make inorganic semiconductor nanocrystals attractive candi- dates for photon harvesting. 7,10-12 To achieve photoinduced charge separation, nanocrystals can be coupled to wide band gap inorganic semiconductors, such as titanium and zinc oxides, 13-15 replacing molecular chromophore in dye-sensitized semiconductor solar cells. 2,4,16 Nanoscale carbon, including graphene, 17 carbon nanotubes, 18 and fullerenes, 19 represents another class of materials that are actively explored for solar energy applications. Hybrid organic/inorganic composites, combining nanoscale carbon with semiconductor quantum dots (QDs), are synthetized and tested for light-energy- harvesting purposes. Recently, a number of groups have produced and characterized experimentally hybrid inorganic/organic QD/ fullerene nanocomposites for solar energy applications. 19-25 A variety of architectures are being explored, including both mechanical mixtures of fullerenes and QDs 20-23,25 and covalently linked composites. 19,24 Bang and Kamat 19 reported utilization of fullerenes in QD solar cells by placing a blend of CdSe QDs and C 60 on an optically transparent electrode using electrophoretic deposition. By functionalizing C 60 with a thiol compound, they linked it covalently to the QD and investigated the photoinduced electron transfer (ET) from the QD to C 60 using time-resolved transient absorption spectroscopy. The covalent linking provided a significant improvement in the charge separation and photoconversion efficiency compared with the previous work utilizing a mechanical blend. 20 The current letter reports state-of-the-art time-domain ab initio simulations of the photoinduced ET in a series of fullerene-CdSe QD nanocomposites, including both covalently linked and mechanically mixed compounds. Directly mimicking the recent experimental work of Kamat and coworkers 19 in real time and at the atomistic level, the study provides detailed insight into the charge-separation processes in the novel nanoscale composite material. Good agreement with the experimental data is achieved. The simulations show that both binding energy and electronic donor-acceptor coupling between the two species are much weaker in the mechanical mixture, despite the significantly shorter distance between the QD and C 60 relative to the covalently linked couple. The noncovalent interaction and ET rate between the QD and C 60 is enhanced by doping the latter with Li. The study characterizes the vibrational modes that promote the photo- Received: November 17, 2012 Accepted: December 10, 2012 Published: December 10, 2012 Letter pubs.acs.org/JPCL © 2012 American Chemical Society 1 dx.doi.org/10.1021/jz301878y | J. Phys. Chem. Lett. 2013, 4, 1-6 Downloaded via GUIZHOU UNIV on November 9, 2019 at 08:22:42 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.