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Multiple Exciton Generation in Semiconductor Nanocrystals: Toward Efficient Solar Energy Conversion

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Multiple Exciton Generation in Semiconductor Nanocrystals: Toward Efficient Solar Energy Conversion Early View publication on www.interscience.wiley.com (issue and page numbers not yet assigned; citable using Digital Object Identifier – DOI) Laser & Photon. Rev., 1–23 (2008) / DOI 10.1002/lpor.200810013 1 Abstract Within the range of photon energies illuminating MEG the Earth’s surface, absorption of a photon by a conventional photovoltaic semiconductor device results in the production of a single electron-hole pair; energy of a photon in excess of the semiconductor’s bandgap is efficiently converted to heat through electron and hole interactions with the crystal lattice. Recently, colloidal semiconductor nanocrystals and nanocrystal films have been shown to exhibit efficient multiple electron-hole pair generation from a single photon with energy greater than E E twice the effective band gap. This multiple carrier pair process, g hν referred to as multiple exciton generation (MEG), represents one route to reducing the thermal loss in semiconductor solar cells and may lead to the development of low cost, high efficiency solar energy devices. We review the current experimental and theoretical understanding of MEG, and provide views to the near- term future for both fundamental research and the development of working devices which exploit MEG. Absorption of a single photon with energy in excess of two times the band gap, Eg, produces multiple excitons at the band edge. TEM picture of typical PbSe nanocrystals. © 2008 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Multiple exciton generation in semiconductor nanocrystals: Toward efficient solar energy conversion Matthew C. Beard* and Randy J. Ellingson* National Renewable Energy Laboratory, Golden, CO 80401, USA Received: 27 March 2008, Revised: 13 May 2008, Accepted: 26 May 2008 Published online: 16 July 2008 PACS: 78.47.jk, 78.67.Bf, 82.53.Mj 1. Introduction Queisser limit) [3], and ∼ 47% of the incident power would turn into heat within the cell. The majority of losses result Conventional single-junction photovoltaic (PV) devices when a photon is absorbed with energy greater than the such as those based on crystalline silicon routinely attain bandgap of Si and the resultant carrier undergoes fast cool- a relatively high efficiency of ∼ 22%, albeit with signifi- ing via phonon scattering and emission (Fig. 1a). Therefore, cant manufacturing costs associated with starting material surpassing the Shockley-Queisser limit requires an expan- purification, high-temperature processing steps, and strict sion of the PV device complexity to efficiently convert a parameter control. At present, the record laboratory effi- wider range of the solar spectrum. The primary example ciency for a crystalline Si (c-Si) solar cell stands at 24.7% involves the use of multiple absorber materials with vary- under standard AM1.5G illumination [1]. The same type ing bandgaps (largest bandgap as the top absorber); such a of solar cell reaches slightly higher conversion efficiency device more efficiently converts a wide range of photon en- when used in a concentrator design (27.6% record effi- ergies and effectively reduces conversion efficiency losses ciency) [2]. A thermodynamically ideal Si solar cell operat- to thermal energy. Specifically, the use of triple-junction ing under conventional assumptions of one electron-hole device designs has enabled an integrated solar cell based pair per absorbed photon would convert about 33% of the on GaInP, GaInAs, and Ge to attain an efficiency of 40.7% incident AM1.5G sunlight to electrical power (Shockley- for concentrated sunlight [4]. However, the III-V multi- * Corresponding authors: e-mail: matt [email protected], randy [email protected] © 2008 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2 M. C. Beard and R. J. Ellingson: Multiple exciton generation in semiconductor nanocrystals Hot charge carriers (a) For Si (Eg = 1.1 eV) at T = 300 K, AM1.5G e- electron loses energy to ηmax = 32.9% y phonons Losses g r transmission = 18.7% e - n e heat = 46.8% E radiative em. = 1.6% hν p-type usable photo- n-type voltage (qV) Figure 1 (online color at: www.lpr-journal.org) hole loses (a) Schematic of a traditional p − n junction h+ energy to 1 e--h+ pair/photon solar cell. Light that is absorbed with photon phonons energies greater than the bandgap produces car- riers (electron and holes) with excess kinetic energy. These ‘hot’ carriers lose energy by scat- 0.7 T = 3000k (b) (c) e 0.45 tering and emitting phonons with the lattice. 0.6 0.40 Heat loss makes up ∼ 47% of the energy con- version losses. (b) If the carriers could be ex- 0.35 Te = 900k y 0.5 c tracted with no heat loss the thermodynamically n e 0.30 T = 600k i Mmax allowed conversion efficiency could be doubled e c i 0.4 f f 0.25 to ∼ 67% [8], Te is the hot carrier temperature. E ficiency T = 300k e (c) Enhancement expected from an increased 5 Ef 0.3 . 0.20 1 photocurrent due to carrier multiplication [9], M 0.15 A N is the Shockley- Queisser limit, M=2 is for 0.2 0.10 M=2 extracting 2 carriers per absorbed photon at 0.1 Ehv > 2Eg, and Mmax is extracting M car- 0.05 N riers at Ehv > MEg. In (b) the photo-voltage 0.0 is increased while in (c) the photo-current is en- 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0 0.5 1.0 1.5 2.0 Bandgap (ev) Bandgap (ev) hanced. junction concentrator cells carry a very high manufacturing within the solar spectrum. However, substantial gains would cost due to the high number of layers (each requiring a sep- result from a PV device for which each absorbed photon arate growth step) and the vacuum requirements for high of sufficient energy efficiently generates multiple electron- quality epitaxial crystal growth. hole pairs which also contribute efficiently to the collected An ideal technology would circumvent heat losses in a photocurrent (Fig. 1c). relatively simple device design, such as a single junction The unique optical properties of quantum-confined solar cell. Such a device would utilize the excess energy semiconductor nanocrystals (NCs), including atomic-like of hot-carriers (Fig. 1b) either by extracting them prior electronic structure and size-dependent bandgap energies, to thermalization (hot-carrier solar cells) [5] yielding a make them interesting materials for potential application in higher photo-voltage, or by generating multiple electron- optoelectronic devices such as solar cells. Large intra-band hole pairs per photon via a carrier multiplication process energy gaps equivalent to several optical phonon quanta to enhance the photocurrent [6, 7]. If carriers can be ex- open up at small NC sizes, suggesting the possibility that tracted without heat losses, the thermodynamically allowed carrier cooling rates may slow due to the decreased effi- efficiency would approximately double to ∼ 67% under ciency of simultaneous multi-phonon scattering events (an standard AM1.5G illumination (Fig. 1b) [8]. Under con- effect referred to as the phonon bottleneck) [5]. Slowed centrated sunlight, this value can even be higher. A carrier cooling would enable interfacial charge transfer and/or im- multiplication process where an electron with sufficiently pact ionization processes (carrier multiplication rates) to high kinetic energy can generate one or more additional compete with the rate of carrier relaxation, potentially im- electron-hole pairs through a scattering process known as proving the performance of photoconversion devices. In impact ionization would increase the theoretically attain- addition, spatial confinement results in strong carrier-carrier able efficiency by ∼ 30% from 33 to 43% (Fig. 1c) [9]. Im- interactions within NCs (an electron-hole pair confined to pact ionization does not occur in conventional PV devices a NC is typically referred to as an exciton). Observations with a rate sufficiently high to enhance device efficiency of photoluminescence (PL) blinking observed in single-NC © 2008 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.lpr-journal.org Laser & Photon. Rev. (2008) 3 measurements [10] were ascribed to the process of Auger Recently, MEG in semiconductor NCs has received ionization [10, 11] in which exciton-exciton annihilation considerable attention and has brought a renewed interest (Auger recombination) ejects an electron out of the NC in understanding and measuring the dynamics of photo- core, resulting in a charged nanocrystal for which emission generated carriers in semiconductor NCs. We review the is quenched. The strong Coulomb interactions correlate current status of MEG and related carrier dynamics in the with the prominent role played by Auger recombination context of solar energy conversion. The outline of our pre- within NCs occupied by more than one exciton. The PL sentation is as follows: In Sect. 1.1 we provide a brief intro- blinking effect, together with the prospect of a phonon bot- duction to semiconductor NC photophysics as background tleneck to slow carrier cooling, led Nozik to propose that for the remaining article; for more detailed reviews we the inverse of Auger recombination, impact ionization, may refer readers to the many books and review articles on occur with higher efficiency in semiconductor NCs [5, 12]. this topic [17–19]. Sect. 1.2 introduces readers to femtosec- ond transient absorption measurements in semiconductor Subsequent to Nozik’s prediction of enhanced impact NCs. Sect. 2.1 provides background information on charge ionization in quantum-confined semiconductor NCs, exper- carrier dynamics in bulk semiconductor crystals. Sect. 2.2 imental measurements on colloidal solutions of PbSe, PbS, addresses carrier cooling in semiconductor NCs. Sect. 2.3 and PbTe NCs revealed evidence of the single-photon gen- introduces photon absorption statistics and multiexciton eration of multiple excitons following pulsed excitation at recombination characteristics of NCs. Sects. 2.4 and 2.5 high photon energy and low intensity, as evidenced by the cover MEG in NC samples: measurement, analysis, and signature of Auger recombination within the exciton pop- other aspects.
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