Quantum confined nanocrystals and nanostructures for high efficiency solar photoconversion Matthew C. Beard NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable
2 Technological Goal: Produce solar power at a cost equivalent to coal ($0.02/kWh) MC - $2.70/Wp SP - $3.80/Wp SIII 21-46 c/kwh SI 37-81 c/kwh Average US rate ~ 9 c/kwh Hillhouse and Beard, Solar Cells from colloidal nanocrystals: Fundamentals, materials, devices, and economics, COCIS, 14,245,2009
Hot Carrier Utilization: Scientific Challenge 3 Hot charge carriers hv e - electron loses energy to phonons e - For Si (E g = 1.1 ev) at T = 300 K, AM1.5G h max = 32.9% Losses transmission = 18.7% heat = 46.8% radiative em. = 1.6% p-type n-type usable photovoltage (qv) h + hole loses energy to phonons 1 e - -h + pair/photon
Two ways to utilize hot carriers Hot Carrier Photoconversion (Higher PhotoVoltage) Impact Ionization in QDs (Higher Photocurrent) Lower ideal Bangap Ross & Nozik, J. Appl. Phys., 53, 3813 (1982) Nozik, Physica, E14 (2002), 115 4
Introduction to Quantum Dots When R a B, discrete, atomic-like electronic states and tunable band gap bulk Density of States n=1 n=2 n=3 n=4 QD Energy E 1 R 2 60 Å 5
6 Why study Quantum Confined Semiconductors for solar energy conversion? 1. Tunable and controllable properties: bandgap, engineered carrier relaxation, transport, catalytic activity, and doping 2. Nanoparticles as building blocks to function solids: added degrees of freedom in approaches to solar conversion, storage, and manipulation 3. New physics: MEG, phonon bottleneck, plasmonics, nanoscale charge-transfer
PbSe QDs are 2x more efficient than bulk PbSe 7 Comparing Multiple Exciton Generation in Quantum Dots To Impact Ionization in Bulk Semconductors: Implications for Enhancement of Solar Energy Conversion, Beard, M.C., Midgett, A.G., Hanna,M.C., Luther, J.M., Hughes, B.K., Nozik, A.J, NL, 10, 3019, 2010 Multiple Exciton Generation in Semiconductor Quantum Dots, Beard, M.C., JPCL, 2, 1282, 2011 PbSe NRs Nano Lett, 11, 3476, 2011 PbSe QDs (McGuire) Nano Lett, 10, 2049, 2010 PbSe bulk (M. Bonn) Nat. Phys. 5, 811, 2009
Strategies for Incorporating QDs into Solar Cells 8 QDs need to be active medium Architecture should not destroy Quantum size effects Excitons need to be separated prior to Auger recombination Excitons must be separated into free-charge carriers and transported to respective electrodes A.J. Nozik, Physica E, 14, 115, (2002) A.J. Nozik, Nano Lett 10, 2735, (2010) Nozik suggested three general strategies for incorporating QDs into Solar Cells
Incorporate QDs and QD-layers in tranditional solar cell configuration such as p-i-n, p-n, or Schottky-Junctions 9 The QDs must be electronically coupled to each other to promote electron and hole transport Ideally miniband formation, but not necessary y Major focus of NREL efforts
Device Configuration 10
NREL Certified QD Solar Cells Voc = 588 mv Jsc = 8.93 ma/cm 2 FF = 56% PCE = 2.94% J. Luther, J. Gao, et al., Adv. Mat. 22,3704(2010). 1 st all QD certified device. High V oc confirms QD11 confinement effect. 11
Stability test J. Luther, J. Gao, et al., Adv. Mat. 22,3704(2010). Test under ambient air for 1000 hours 12
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14 Peak EQE values greater than 100% O. Semonin, et al., Science, 334, 1530 (2011).
15 Peak EQE values greater than 100% O. Semonin, et al., Science, 334, 1530 (2011).
16 Peak EQE values greater than 100% O. Semonin, et al., Science, 334, 1530 (2011).
17 Peak EQE values greater than 100% O. Semonin, et al., Science, 334, 1530 (2011).
18 Peak EQE values greater than 100% O. Semonin, et al., Science, 334, 1530 (2011).
19 Peak EQE values greater than 100% O. Semonin, et al., Science, 334, 1530 (2011).
20 Compare to Best Solar Cells 7 E g (160 nm Si photodetectors: Canfield L.R., et. al., Metrologia, 35, 329, (1998)
Quantum Efficiency (%) Sun (a.u.) Spectral Photocurrent (ma cm -2 ev -1 ) Benefit to Photocurrent 140 120 100 EQE EQE/A MEG Region (~1 ma/cm 2 ) Solar Intensity Photons s -1 m -2 ev -1 3.0 2.5 2.0 ~4% boost to photocurrent Compared to < 1% in an optimized Si Solar cell 80 60 40 20 0 2.0 Extra Current (ma cm -2 ev -1 ) 2.5 3.0 Photon Energy 3.5 1.5 1.0 0.5 0.0 To achieve maximum benefit need to further increase MEG efficiency 21
22 Conclusions and Summary MEG is a potential way to increase solar energy conversion in QD solar cells QD solar cells can be constructed with high photocurrent and high IQE s MEG is at least 2X as efficient as II in bulk materials MEG has been observed in photocurrent measurements
23 Quantum Dot Team Members Justin Johnson (NREL) Tavi Semonin (CU/Physics) Aaron Midget (CU/Chemistry) Barbara Hughes (CU/Chemistry) Jianbo Gao (NREL) Hugh Hillhouse (U of Washington) Matt Bergren (CSM/Physics) Hsiang Yu Chen (NREL) Joey Luther (NREL) Center for Advanced Solar Photophysics Victor Klimov (Los Alamos) Art Nozik (CU/NREL) Sasha Efros (NRL) Matt Law (UC - Irvine) Danielle Smith (NREL) Jayson Stewart (LANL) Funding: DOE Office of Basic Energy Sciences Solar Photochemistry Program (Isolated Colloidal Quantum Dots) and Center for Advanced Photophysics (Quantum Dot Solar Cells) -- EFRC
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