Jan 9, 2016 - Abstract. Intermediate band solar cells (IBSC) have emerged as an alternative design for solar cells that can dramatically increase power ...
Absorption characteristics of intermediate band solar cell S. Tomić, N. M. Harrison, and T. S. Jones Citation: AIP Conference Proceedings 1199, 499 (2010); doi: 10.1063/1.3295525 View online: http://dx.doi.org/10.1063/1.3295525 View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/1199?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Quantum efficiency of intermediate-band solar cells based on non-compensated n-p codoped TiO2 J. Chem. Phys. 137, 104702 (2012); 10.1063/1.4750981 Absorption characteristics of a quantum dot array induced intermediate band: Implications for solar cell design Appl. Phys. Lett. 93, 263105 (2008); 10.1063/1.3058716 Intermediate-band solar cells based on quantum dot supracrystals Appl. Phys. Lett. 91, 163503 (2007); 10.1063/1.2799172 Efficient laser textured nanocrystalline silicon-polymer bilayer solar cells Appl. Phys. Lett. 90, 203514 (2007); 10.1063/1.2739365 Efficiency enhancement of ideal photovoltaic solar cells by photonic excitations in multi-intermediate band structures Appl. Phys. Lett. 83, 770 (2003); 10.1063/1.1592881
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Absorption characteristics of intermediate band solar cell S. Tomića), N.M Harrisona,b), and T.S. Jonesc) a)
STFC Daresbury Laboratory, Cheshire WA4 4AD, UK b) Imperial College, London SW7 2AZ, UK c) University of Warwick, Coventry CV4 7AL, UK
Abstract. Intermediate band solar cells (IBSC) have emerged as an alternative design for solar cells that can dramatically increase power conversion efficiency. Here, it is demonstrated that a k.p multiband theory with periodic boundary conditions can easily be applied to predict electronic and absorption characteristics of the semiconductor QD arrays that produces a mini-band (IB) that is located in the forbidden energy gap of the QD material and is separated from valence and conduction band of the barrier material. Analysis of the electronic and absorption structure suggest that the most promising design for an IB material that will exhibit its own quasi-Fermi level is to employ small QDs (~610 nm QD lateral size) arranged in a periodic array. Using bigger (> 20 nm QD lateral size) QDs leads to extension of the absorption spectra into a longer wavelength region but does not provide a separate IB in the forbidden energy gap. Keywords: Semiconductor quantum dots, solar cells, electronic structure. PACS: 71.15; 71.10.
INTRODUCTION The efficiency of a single junction solar cell can be exceeded by splitting the solar spectrum in such a way that each pn junction only converts a narrow spectral region. Theoretically, tandem or multiple junction solar cells with an infinite number of pn junctions can reach the thermodynamic efficiency limits of solar energy conversion. While tandem solar cells can theoretically exceed 50% efficiency, tandems with large numbers of junctions face increasing complexity and materials issues (for instance, the accumulated strain between different layers with different energy gaps), which lead to diminishing efficiency improvements. Therefore, significant attention has been paid to developing new approaches to single solar cells that exceed the efficiency of a conventional pn junction. Intermediate band solar cells (IBSC) have been proposed as an alternative design for the next generation of highly efficient photo-voltaic devices [1]. Quantum nanostructures, such as quantum dots (QD), arranged in superlattice arrays can produce a mini-band (IB) that is located in the forbidden energy gap of the QD material and is separated from valence and conduction band of the barrier material. Additional absorption, from the valence band to the IB and from the IB to the conduction band, allows two photons with energies below the energy gap of the
nanostructure material to be harvested in generating one electron-hole pair, contributing toward high conversion efficiency.
RESULTS AND DISCUSSION The model QD array presented here consists of truncated pyramidal QD’s shape with the truncation factor fixed at t = 0.5, on the top of a one monolayer wetting layer. The energy of the highest frequency Fourier harmonics in calculations were checked, and are well below energy of possible spurious solutions of the constituent materials that exists due to incompleteness of the bulk bands basis (only antibonding s, and bonding px, py, pz states) are used in the 8-band kp method [2]. The periodicity of the QD array is controlled by the vertical dimensions of the embedding box Lz in the z-direction. In the x and y directions we kept periodicity constant and Lx = Ly and chosen to be large enough to prevent lateral electric coupling. To estimate variation of the first few minibands with the vertical periodicity of the QD array, we change the spacing (dz) between QD array layers in the range from 1 nm to 10 nm (distance between bottom of the WL in i+1th and top of the QD's in the ith layer growth). We first analysed the variation of the first three mini-bands with the spacer distance dz, for three
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small QD (~6-10 nm base) are the best candidate for high efficient IBSC because they exhibit a well defined absorption peak related to the IB that is separated from the rest of the absorption spectra by very low density of states. The bigger QD array (~20 nm base) would behave simply as a bulk material with a red shifted absorption spectrum extending absorption capabilities toward higher wavelengths but not providing energy separation for a third quasi Fermi level that would be related to the IB.
representative QD arrays: (a) small QD of h=3 nm height, b=6 nm base length and Lx=20 nm; (b) medium QD of h=6, b=12 nm, and Lx=20 nm; and (c) big QD of h=10 nm, b=20 nm, and Lx=40 nm. In The lower and upper boundary of a miniband corresponds to Kz = 0 and Kz = π/Lz respectively. The width of the e0 miniband at close spacing of dz = 1 nm is: 177 meV for (a), 86 meV for (b), 38 meV for (c) and almost vanishes by dz = 10 nm in all three cases. The trend that e0, e1, and e2 are steeply rising in energy with increasing embedding box size is attributed to the slow decay of the strain caused by QDs in surrounding layers [3]. Also the energy difference between the e1 and e2 miniband (the two lowest p-like states split due to the piezoelectric field induced C2v symmetry) increases with QD size. This is attributed to increase of piezoelectric field with QD size. In the figure 1 the variation of the optical dipole matrix element between the e0 state and five topmost states in the VB is shown for a realistic [4] vertical periodicity of dz = 4 nm between QD sheets. At that distance the e0 mini-band energy width is: 33 meV for (a), 14 meV for (b), and 6 meV for (c). Each state in the VB is labeled according to its dominant character (heavy or light hole) at the band edge. The first two states in the VB are of clear HH character followed by a LH state. It is possible to identify a clear band anticrossing between the hh1 and hh2 at 0.8KzSL that is similar as in the compressively strained QW structures.
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CONCLUSIONS The analysis presented here suggests that an appropriately designed QD array will support wave function delocalization and the formation of an intermediate band. The delocalisation reduces recombination losses between electrons in the IB and holes in the VB providing for a stable partial population of the IB under operating conditions. As the IB band must be separated from the CB of the host material a QD array consisting of relatively small (~ 610 nm lateral size) dots is the most likely candidate structure for a high efficiency solar cell.
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FIGURE 2. Absorption spectra of the three representative QD arrays: (a) small QD’s, (b) medium QD’s, and (c) big QD’s. Dashed line: in-pane polarization (TE); dotted line: perpendicular polarization (TM); solid line: total absorption.
QD- Medium HH1 (h0) HH2 (h1) LH1 (h2) HH3 (h3) LH2 (h4)
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REFERENCES
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1. A. Luque and A. Marti, Phys. Rev. Lett. 78, 5014 (1997). 2. S. Tomić, A. G. Sunderland, and I. J. Bush, J. Mat. Chem. 16, 1963-1972 (2006). 3. S. Tomić, P. Howe, N. M. Harrison, and T. S. Jones, J. Appl. Phys. 99, 093522 (2006). 4. A. Marti et al., Appl. Phys. Lett. 99, 233520 (2007).
1.0
FIGURE 1. Variation of the different dipole matrix
elements between e0 that for IB and five topmost states in the VB inside IB mini-band. Analysis of the absorption spectra for the three representative arrays suggests that QD array with
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