ARTICLE IN PRESS Physica E 41 (2008) 15– 17
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Quantum-dot intermediate-band solar cells with inverted band alignment A. Franceschetti a,, S. Lany a, G. Bester b a b
National Renewable Energy Laboratory, Golden, CO 80401, USA ¨ r Festko ¨rperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany Max-Planck-Institut fu
a r t i c l e in fo
Article history: Received 16 April 2008 Accepted 20 May 2008 Available online 26 July 2008
The intermediate-band concept was proposed over a decade ago as a possible route to increase the efﬁciency of single-junction solar cells. Despite a number of experimental attempts to realize this concept, no efﬁciency improvement over conventional single-junction solar cells has so far been demonstrated. This is likely due to the fact that the intermediate band itself acts to enhance electron–hole recombination. In this work we propose a novel intermediate-band solar-cell architecture based on doped semiconductor nanostructures having an inverted type-I band alignment with the surrounding host. The recombination of carriers in the nanostructures is prevented by ultra-fast charge transfer to the host, thereby removing the main obstacle to achieve high conversion efﬁciency. & 2008 Elsevier B.V. All rights reserved.
PACS: 73.21.La 73.63.Kv Keywords: Intermediate-band solar cells Quantum dots
Third-generation photovoltaic devices will require both low manufacturing cost ($125/m2 or less) and high solar-energy conversion efﬁciency (50% or more) in order to become competitive with conventional energy sources . The maximum theoretical efﬁciency of conventional single-junction solar cells is 31% under one-sun illumination (Shockley–Queisser limit ) and 41% under full solar concentration . There are two fundamental factors that limit the efﬁciency of single-junction solar cells: (i) photons of energy hooEg, where Eg is the semiconductor band gap, are not absorbed, and therefore do not generate carriers that could in principle contribute to the photocurrent and (ii) the excess energy ho Eg of absorbed photons is converted to heat by phonon emission, instead of being used to create additional electron–hole pairs. Multijunction solar cells use a sequence of single-junction cells to increase the harvest of photons. While multi-junction solar cells can achieve efﬁciencies as high as 40% , they require complex and costly device architectures and delicate interconnects. An alternative approach that avoids the serial connection of single-junction units is the intermediate-band (IB) solar cell, based on a concept originally proposed by Wolf , and more recently expanded by Luque and Martı´ . In IB solar cells an intermediate electronic band is introduced in the band gap of a semiconductor host. The IB acts as a ‘‘stepping stone’’ that allows absorption of sub-band-gap photons of energy ho4Emin, where Emin is the smaller of EVI and EIC (see Fig. 1). Under the assumption
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(A. Franceschetti). 1386-9477/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2008.05.023
that the IB does not create non-radiative recombination centers for photo-generated electron–hole pairs, the maximum efﬁciency of IB solar cells was calculated by Luque and Martı´ [5,6] to be 46% at one-sun illumination and 63% at full solar concentration. Different approaches to create an IB in a semiconductor host have been proposed in the literature [7–12]. One such approach is to dope III–V semiconductor materials with transition-metal impurities, such as Ti impurities substituting for As (P) in GaAs (GaP)  or Cr impurities substituting for Ga in GaP . A second approach to IB formation is via isovalent impurity doping of II–VI or III–V semiconductors [9,10]. Yu et al.  showed that the incorporation of small quantities of oxygen impurities substituting for Te in Zn1 yMnyTe leads to the formation of an IB in the Zn1 yMnyTe host. Similarly, the incorporation of nitrogen into GaAs1 yPy was shown  to split the conduction band into two sub-bands, leading to the formation of an IB. A third approach consists of embedding a dense array of quantum dots in a semiconductor host . The quantum-dot levels are electronically coupled, leading to the formation of one or more IBs in the band gap of the semiconductor host. Martı´ et al.  have recently demonstrated optical absorption from the IB to the conduction band in an array of self-assembled InAs quantum dots embedded in (Al,Ga)As. Despite these recent advances in the fabrication and characterization of IB semiconductors, no IB solar-cell device has so far unequivocally demonstrated solar-conversion efﬁciency exceeding that of the semiconductor host material (i.e., without the IB). This is most likely due to the fact that the conditions assumed in Ref.  are not fulﬁlled in real devices. Speciﬁcally, the IB is likely to enhance carrier recombination, which constitutes a net loss in
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A. Franceschetti et al. / Physica E 41 (2008) 15–17
Fig. 1. Schematic band-alignment diagram of intermediate-band (IB) solar cells . An IB is present in the band gap between the valence band (VB) and the conduction band (CB) of the semiconductor host.
the detailed balance equations . For example, in the case of bulk semiconductors doped with transition-metal ions [7,8] or with isovalent impurities [9,10], the defect levels that form the IB may act to kill photo-generated carriers via multi-phonon emission [13,14]. In the case of quantum-dot arrays embedded in a semiconductor host [11,12], the quantum-dot levels that form the IB are not electronically separated from the barrier levels, due to the presence of multiple quantum-conﬁned levels as well as wetting-layer levels. As a result, photo-generated electrons and holes can quickly relax, via phonon emission, from the barrier into the quantum dot, where they recombine efﬁciently [15,16]. In this work, we propose a new IB solar-cell architecture that is speciﬁcally designed to remove the unwanted side effect of carrier recombination through the IB. Our IB solar-cell concept is shown schematically in Fig. 2. The main components of the proposed solar-cell architecture are: (i) An array of semiconductor nanostructures (e.g. quantum dots or quantum wires) embedded in a semiconductor host. These nanostructures are designed to have an inverted type-I band alignment with the surrounding host, meaning that the conduction-band minimum (CBM) of the nanostructures is higher in energy than the CBM of the host, and the valence-band maximum (VBM) of the nanostructures is lower in energy than the VBM of the host (see Fig. 2). Note that the band alignment of Fig. 2 is opposite to that of conventional type-I nanostructures used in quantum-dot IB solar cells [11,12]. (ii) The semiconductor nanostructures contain an IB that is energetically located in the nanostructure band gap, and spatially localized in the interior of the nanostructure (see Fig. 2). The purpose of the IB is to increase the electron–hole photo-generation rate by allowing absorption of sub-band-gap photons (as in conventional IB solar cells). The purpose of the inverted type-I band alignment between the nanostructures and the surrounding host is to efﬁciently remove photo-generated carriers from the nanostructures, and inject them in the surrounding host, before they can recombine through the IB. In the proposed structure, carrier transport occurs in the semiconductor host; the nanostructures may act as scattering centers, but their effect on conductivity should be small as long as the density of defects at the nanostructure/host interface is low. The efﬁciency of the solar-cell architecture of Fig. 2 is expected to be higher than that of the host material, because the nanostructures act as a source of additional carriers, generated by sub-band-gap absorption through the IB. For this purpose, it is necessary that a signiﬁcant fraction of the impinging sub-bandgap photons of energy Emin ohooEhost are absorbed in the g nanostructures (absorbance 1). This may require a dense array
Fig. 2. Schematic diagram of the proposed IB solar-cell architecture. Note the inverted type-I band alignment between the nanostructure and the surrounding host semiconductor.
Table 1 Possible nanostructure/host material combinations for inverted-type-I IB solar-cell architectures Host material
Al0.38Ga0.62As Al0.25In0.75P Ga0.63In0.37P Ga0.55In0.45N Al0.62In0.38As Al0.79Ga0.21Sb Al0.87In0.13Sb GaP0.39As0.61
Al0.54Ga0.46As Al0.34In0.66P Ga0.78In0.22P Ga0.65In0.55N Al0.69In0.31As Al0.90In0.10Sb Al0.95In0.05Sb GaP0.53As0.47
In all cases, EghostE1.90 eV and EgnanoE2.10 eV (not including strain). The materials parameters were obtained from Refs. [19,20].
of nanostructures as well as strong optical absorption between the band edges and the IB. In the following, we discuss possible materials and geometry combinations that can be conducive to the band alignment depicted in Fig. 2.
(i) Nanostructure/host combinations. The valence-band and conduction-band offsets between the nanostructures and the semiconductor host (DEV and DEC in Fig. 2) should be sufﬁciently large to prevent signiﬁcant overlap of the electron and hole wave functions of the surrounding matrix with the IB inside the nanostructure, and to prevent thermal excitation of electrons and holes into the nanostructures. At the same time, the band offsets should be sufﬁciently small to allow for efﬁcient absorption of sub-band-gap photons. Valence-band and conduction-band offsets of 50–100 meV should be optimal for device operation at room temperature. Table 1 lists a few possible candidates to realize the proposed band alignment. In Table 1, we have assumed that the optimal band gap of the host is 1.9 eV . In the calculation of the band gaps we have neglected the effects of strain and assumed that the substrate is lattice matched to the barrier material. If a different substrate is chosen, the gap values should be reevaluated using the appropriate deformation potentials and strain. The nanostructure array can be fabricated for example by the Stranski–Krastanow growth method , which requires lattice mismatch, or by the droplet epitaxy method , which allows the formation of
ARTICLE IN PRESS A. Franceschetti et al. / Physica E 41 (2008) 15–17
quantum dots not only in lattice-mismatched but also in lattice-matched systems such as GaAs/AlGaAs. (ii) Nanostructure/IB combinations. The purpose of the IB nanostructures in the solar-cell architecture of Fig. 2 is to increase the production of carrier via absorption of low-energy, subband-gap photons. The IB can be realized for example by doping the nanostructures with transition-metal impurities, as suggested in Refs. [7,8], or by isovalent doping, as demonstrated in Refs. [9,10]. Following Ref. , the ideal position of the IB in the nanostructure band gap is approximately 0.7 eV from one of the band edges, i.e. Emin0.7 eV. To prevent carrier recombination through the IB, two conditions must be met: (a) Carriers generated in the nanostructures must migrate to the host on a time scale shorter than typical recombination times via the IB. This implies that the nanostructures must be small (a few nm) in at least one or two dimensions. (b) The IB states must be spatially localized inside the nanostructures, so that they have little or no overlap with the band-edge states of the semiconductor host. This implies that the nanostructures must be selectively doped to form the IB. Wei and Forrest  recently considered IB solar cells consisting of an array of quantum dots embedded in a semiconductor host (with a conventional type-I band alignment), and addressed the problems caused by carrier trapping into the dots and subsequent carrier recombination. They suggested surrounding the quantum dots with high-band-gap ‘‘fences’’ that would provide a repulsive potential for the carriers generated in the host. One of the drawbacks of this approach is that these fences would extend over the entire device, and thus hinder the transport of photogenerated carriers towards the electrodes. Also, recapture of carriers generated inside the nanostructures (the same carriers supposed to yield an increase in efﬁciency) is exacerbated by the presence of fences. Indeed, recent experimental studies of this IB concept—using InGaAs quantum dots surrounded thin GaAsP fences—have shown no increase in efﬁciency compared to a reference test cell without nanostructures . None of the
problems introduced by the presence of fences in the IB architecture of Ref.  are present in our IB architecture. In summary, we have proposed a new approach to IB solar cells that is speciﬁcally designed to remove the unwanted side effect of carrier recombination through the IB. This is accomplished by embedding the IB in nanostructures having an inverted type-I band alignment with the semiconductor host. Electron–hole recombination in the nanostructures is prevented by the rapid expulsion of carriers into the surrounding host. This work was funded by the US Department of Energy, Ofﬁce of Science, Basic Energy Sciences, under Contract no. DE-AC3699GO10337 to NREL. References  A. Luque, A. Martı´, A.J. Nozik, MRS Bull. 32 (2007) 236.  W. Shockley, H.J. Queisser, J. Appl. Phys. 32 (1961) 510.  R.R. King, D.C. Law, K.M. Edmondson, C.M. Fetzer, G.S. Kinsey, H. Yoon, R.A. Sherif, N.H. Karam, Appl. Phys. Lett. 90 (2007) 183516.  M. Wolf, Proc. IRE 48 (1960) 1246.  A. Luque, A. Martı´, Phys. Rev. Lett. 78 (1997) 5014.  A. Luque, A. Martı´, Prog. Photovolt. Res. Appl. 9 (2001) 73.  P. Wahnon, C. Tablero, Phys. Rev. B 65 (2002) 165115.  C. Tablero, Phys. Rev. B 72 (2005) 035213.  K.M. Yu, W. Walukiewicz, J. Wu, W. Shan, J.W. Beeman, M.A. Scarpulla, O.D. Dubon, P. Becla, Phys. Rev. Lett. 91 (2003) 246403.  K.M. Yu, W. Walukiewicz, J.W. Ager III, D. Bour, R. Farschchi, O.D. Dubon, S.X. Li, I.D. Sharp, E.E. Haller, Appl. Phys. Lett. 88 (2006) 092110.  A. Martı´, N. Lo´pez, E. Antolı´n, E. Ca´novas, C. Stanley, C. Farmer, L. Cuadra, A. Luque, Thin Solid Films 511–512 (2006) 638.  A. Martı´, E. Antolı´n, C.R. Stanley, C.D. Farmer, N. Lo´pez, P. Dı´az, E. Ca´novas, P.G. Linares, A. Luque, Phys. Rev. Lett. 97 (2006) 247701.  D.V. Lang, C.H. Henry, Phys. Rev. Lett. 35 (1975) 1525.  A. Luque, A. Martı´, E. Antolı´n, C. Tablero, Physica B 382 (2006) 320.  G. Wei, S.R. Forrest, Nano Lett. 7 (2007) 218.  V. Aroutiounian, S. Petrosyan, A. Khachatryan, Sol. Energy Mater. Sol. Cells 89 (2005) 165.  D. Leonard, M. Krishnamurthy, C.M. Reaves, S.P. Denbaars, P.M. Petroff, Appl. Phys. Lett. 63 (1992) 3203.  N. Koguchi, S. Takahashi, T. Chikyow, J. Cryst. Growth 111 (1991) 688.  I. Vurgaftman, J.R. Meyer, L.R. Ram-Mohan, J. Appl. Phys. 89 (2001) 5815.  I. Vurgaftman, J.R. Meyer, J. Appl. Phys. 94 (2003) 3675.  A.G. Norman, private communication.