Efficient Multiple Exciton Generation Observed in Colloidal PbSe Quantum Dots with Temporally and Spectrally Resolved Intraband Excitation Minbiao Ji, †,‡ Sungnam Park, † Stephen T. Connor, § Taleb Mokari, | Yi Cui, ⊥ and Kelly J. Gaffney* ,† PULSE Institute SLAC National Accelerator Laboratory, Stanford UniVersity, Stanford, California 94305, Department of Physics, Department of Chemistry, and Department of Materials Science and Engineering, Stanford UniVersity, Stanford, California 94305, and Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720 Received January 12, 2009 ABSTRACT We have spectrally resolved the intraband transient absorption of photogenerated excitons to quantify the exciton population dynamics in colloidal PbSe quantum dots (QDs). These measurements demonstrate that the spectral distribution, as well as the amplitude, of the transient spectrum depends on the number of excitons excited in a QD. To accurately quantify the average number of excitons per QD, the transient spectrum must be spectrally integrated. With spectral integration, we observe efficient multiple exciton generation in colloidal PbSe QDs. The generation of cost-effective and environmentally benign sources of energy represents a critical challenge for sustaining and advancing human well fair throughout the world while simultaneously reducing greenhouse gas emissions. Despite the tremendous potential of solar energy, the high cost of solar energy relative to conventional sources of electrical power limits solar energy to a minor component of the present global energy supply. This creates an enormous incentive to develop novel materials and approaches to light harvesting and power conversion. The maximum single band gap solar cell conversion efficiency is calculated to be 31%, termed the Shockley- Queisser limit. 1 This calculation assumes that only one band edge electronic excited state can be generated per absorbed photon, with all photon energy in excess of the band gap energy being dissipated as heat. In bulk inorganic semicon- ductors this limitation has been shown to be an accurate assumption, 2 but the increased interaction between excitons in nanostructured inorganic semiconductors makes multiple exciton generation (MEG) potentially more efficient. 3 Should efficient MEG and carrier extraction be achievable in a solar cell, the theoretical photovoltaic device efficiency could be increased significantly to 43%. 4,5 The report of highly efficient MEG in PbSe QDs by Schaller and Klimov 6,7 has stimulated significant interest in using MEG to improve photovoltaic efficiency. Ellingson et al. 8 also observed efficient MEG in PbSe, and a variety of groups reported efficient MEG in a variety of semiconducting QDs. 9-12 These initial claims, however, have been followed by a series of measurements that observe much weaker or nonexistent MEG. 13-15 Measurements in support of efficient MEG have been based primarily on time-resolved transient absorption (TA) measurements. 6,8,16 A schematic of the experiment and data analysis method utilized to extract exciton multiplicity appears in Figure 1. In Figure 1A, the reduction in ground- state absorption, represented by the crossed-out upward green arrow, and the presence of stimulated emission, represented by the downward green arrow, result in an increased transmission of the probe pulse at the interband transition energy. When more than one exciton resides in a QD, the multiple exciton state decays quickly due to Auger recom- bination on the many tens to hundreds of picoseconds time scale, 5,17-20 until only one exciton remains in the QD. In PbSe, this single exciton state decays radiatively on the hundreds of nanoseconds time scale, 21 generating a nearly time-independent offset on the hundreds of picoseconds time † PULSE Institute, SLAC National Accelerator Laboratory, Stanford University. ‡ Department of Physics, Stanford University. § Department of Chemistry, Stanford University. | Molecular Foundry, Lawrence Berkeley National Laboratory. ⊥ Department of Materials Science and Engineering, Stanford University. NANO LETTERS 2009 Vol. 9, No. 3 1217-1222 10.1021/nl900103f CCC: $40.75 © 2009 American Chemical Society Published on Web 02/18/2009