We report ultra-efficient multiple exciton generation (MEG) for single photon absorption in colloidal PbSe and PbS quantum dots (QDs). We employ transient absorption spectroscopy and present measurement data acquired for both intraband as well as interband probe energies. Quantum yields of 300% indicate the creation, on average, of three excitons per absorbed photon for PbSe QDs at photon energies that are four times the QD energy gap. Results indicate that the threshold photon energy for MEG in QDs is twice the lowest exciton absorption energy. We find that the biexciton effect, which shifts the transition energy for absorption of a second photon, influences the early time transient absorption data and may contribute to a modulation observed when probing near the lowest interband transition. We present experimental and theoretical values of the size-dependent interband transition energies for PbSe QDs. We present experimental and theoretical values of the size-dependent interband transition energies for PbSe QDs, and we also introduce a new model for MEG based on the coherent superposition of multiple excitonic states.

Multiple exciton generation (MEG) is a process that can occur in semiconductor nanocrystals, or quantum dots (QDs), whereby absorption of a photon bearing at least twice the bandgap energy produces two or more electron-hole pairs. Here, we report on photocurrent enhancement arising from MEG in lead selenide (PbSe) QD-based solar cells, as manifested by an external quantum efficiency (the spectrally resolved ratio of collected charge carriers to incident photons) that peaked at 114 ± 1% in the best device measured. The associated internal quantum efficiency (corrected for reflection and absorption losses) was 130%. We compare our results with transient absorption measurements of MEG in isolated PbSe QDs and find reasonable agreement. Our findings demonstrate that MEG charge carriers can be collected in suitably designed QD solar cells, providing ample incentive to better understand MEG within isolated and coupled QDs as a research path to enhancing the efficiency of solar light harvesting technologies.

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We describe the structural, optical, and electrical properties of high-quality films of PbSe nanocrystals fabricated by a layer-by-layer (LbL) dip-coating method that utilizes 1,2-ethanedithiol (EDT) as an insolubilizing agent. Comparative characterization of nanocrystal films made by spin-coating and by the LbL process shows that EDT quantitatively displaces oleic acid on the PbSe surface, causing a large volume loss that electronically couples the nanocrystals while severely degrading their positional and crystallographic order of the films. Field-effect transistors based on EDT-treated films are moderately conductive and ambipolar in the dark, becoming p-type and 30-60 times more conductive under 300 mW cm(-2) broadband illumination. The nanocrystal films oxidize rapidly in air to yield, after short air exposures, highly conductive p-type solids. The LbL process described here is a general strategy for producing uniform, conductive nanocrystal films for applications in optoelectronics and solar energy conversion.

We describe here a simple, all-inorganic metal/NC/metal sandwich photovoltaic (PV) cell that produces an exceptionally large short-circuit photocurrent (>21 mA cm -2 ) by way of a Schottky junction at the negative electrode. The PV cell consists of a PbSe NC film, deposited via layer-by-layer (LbL) dip coating that yields an EQE of 55-65% in the visible and up to 25% in the infrared region of the solar spectrum, with a spectrally corrected AM1.5G power conversion efficiency of 2.1%. This NC device produces one of the largest short-circuit currents of any nanostructured solar cell, without the need for sintering, superlattice order or separate phases for electron and hole transport. Figure 1 shows the structure, current-voltage performance, EQE spectrum, and proposed band diagram of our device. Device fabrication consists of depositing a 60-300 nm-thick film of monodisperse, spheroidal PbSe NCs onto patterned indium tin oxide (ITO) coated glass using a layer-by-layer dip coating method, followed by evaporation of a top metal contact. In this LbL method, 1 a layer of NCs is deposited onto the ITO surface by dip coating from a hexane solution and then washed in 0.01 M 1,2-ethanedithiol (EDT) in acetonitrile to remove the electrically insulating oleate ligands that originally solubilize the NCs (see Supporting Information). Large-area, crack-free and mildly conductive (σ ) 5 × 10 -5 S cm -1 ) NC films result. The NCs pack randomly in the films, are partially coated in adsorbed ethanedithiolate, and show p-type conductivity under illumination. 1 X-ray diffraction and optical absorption spectroscopy established that the NCs neither ripen nor sinter in response to EDT exposure. We have found that using methylamine instead of EDT yields similar device performance (Supporting Information, Figure 1). 2 We have also fabricated working devices from PbS and CdSe NCs (Supporting Information, Figures 2 and 3), which indicates that the approach adopted here is not restricted to EDT-treated PbSe NCs and that it should be possible to improve cell efficiency by engineering the surface of the NCs to attain longer carrier diffusion lengths and higher photovoltages through surface state passivation and prevention of Fermi level pinning.When tested in nitrogen ambient under simulated 1-sun test conditions (100 ( 5 mW cm -2 ELH white light illumination), EDT-treated PbSe devices exhibit large shortcircuit photocurrent densities (J SC ) and modest open-circuit voltages (V OC ) and fill factors (FF), with one of the most efficient devices yielding J SC ) 24.5 mA cm -2 , V OC ) 239 mV, FF ) 0.41 and a mismatch-corrected 3 AM1.5G efficiency of 2.1% (Figure 1a; see Supporting Information regarding spectral mismatch). The mismatch-corrected J SC values of these devices are reproducibly larger than those of other nanostructured solar cells, including the best organic 4 and dye-sensitized devices, 5 which is remarkable considering the unsintered, glassy microstructure of our NC films and the fact that the NCs retain quantum confinement...

Introduction 6873 2. Synthesis of Colloidal Quantum Dots 6874 2.1. Solution Synthesis 6874 2.2. III-V Quantum Dots Grown via Vapor Phase Deposition 6877 3. Relaxation Dynamics of Photogenerated Carriers In QDs 6877 3.1. Experimental Determination of Relaxation/ Cooling Dynamics and a Phonon Bottleneck in Quantum Dots 6878 4. Multiple Exciton Generation (MEG) in Quantum Dots 6878 4.1. MEG in Si QDs 6880 5. Quantum Dot Arrays 6881 5.1. MEG in PbSe QD Arrays 6882 6. Applications: Quantum Dot Solar Cells 6884 6.1. Quantum Dot Solar Cell Configurations 6885 6.1.1. Photoelectrodes Composed of Quantum Dot Arrays 6885 6.1.2. Quantum Dot-Sensitized Nanocrystalline TiO 2 Solar Cells 6885 6.1.3. Quantum Dots Dispersed in Organic Semiconductor Polymer Matrices 6885 6.2. Schottky Junction and p-n Junction Solar Cells Based on Films of QD Arrays 6886 7. Conclusion 6887 8. Acknowledgments 6887 9. Note Added after ASAP Publication 6887 10. References 6887

Multiple exciton generation (MEG) is a process whereby multiple electron-hole pairs, or excitons, are produced upon absorption of a single photon in semiconductor nanocrystals (NCs) and represents a promising route to increased solar conversion efficiencies in single-junction photovoltaic cells. We report for the first time MEG yields in colloidal Si NCs using ultrafast transient absorption spectroscopy. We find the threshold photon energy for MEG in 9.5 nm diameter Si NCs (effective band gap identical with Eg = 1.20 eV) to be 2.4 +/- 0.1Eg and find an exciton-production quantum yield of 2.6 +/- 0.2 excitons per absorbed photon at 3.4Eg. While MEG has been previously reported in direct-gap semiconductor NCs of PbSe, PbS, PbTe, CdSe, and InAs, this represents the first report of MEG within indirect-gap semiconductor NCs. Furthermore, MEG is found in relatively large Si NCs (diameter equal to about twice the Bohr radius) such that the confinement energy is not large enough to produce a large blue-shift of the band gap (only 80 meV), but the Coulomb interaction is sufficiently enhanced to produce efficient MEG. Our findings are of particular importance because Si dominates the photovoltaic solar cell industry, presents no problems regarding abundance and accessibility within the Earth's crust, and poses no significant environmental problems regarding toxicity.

All-perovskite–based polycrystalline thin-film tandem solar cells have the potential to deliver efficiencies of >30%. However, the performance of all-perovskite–based tandem devices has been limited by the lack of high-efficiency, low–band gap tin-lead (Sn-Pb) mixed-perovskite solar cells (PSCs). We found that the addition of guanidinium thiocyanate (GuaSCN) resulted in marked improvements in the structural and optoelectronic properties of Sn-Pb mixed, low–band gap (~1.25 electron volt) perovskite films. The films have defect densities that are lower by a factor of 10, leading to carrier lifetimes of greater than 1 microsecond and diffusion lengths of 2.5 micrometers. These improved properties enable our demonstration of >20% efficient low–band gap PSCs. When combined with wider–band gap PSCs, we achieve 25% efficient four-terminal and 23.1% efficient two-terminal all-perovskite–based polycrystalline thin-film tandem solar cells.