Demonstration of a $\text{G}\text{aS}\text{b}/\mathrm{Ga}\text{A}\text{s}$ Quantum Dot Intermediate Band Solar Cell Operating at Maximum Power Point

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Demonstration of a $G aS b / Ga A s$ Quantum Dot Intermediate Band Solar Cell Operating at Maximum Power Point

I. Ramiro, J. Villa, J. Hwang, A. J. Martin, J. Millunchick, J. Phillips, and A. Martí

Phys. Rev. Lett. 125, 247703 – Published 9 December 2020

Abstract

Intermediate band solar cells (IBSCs) promise high efficiencies while maintaining a low device structural complexity. A high efficiency can be obtained by harvesting below-band-gap photons, thus increasing the current, while at the same time preserving a high voltage. Here, we provide experimental proof that below-band-gap photons can be used to produce nonzero electrical work in an IBSC without compromising the voltage. For this, we manufacture a $GaSb / GaAs$ quantum-dot IBSC. We use light biasing and make our cell operate at the maximum power point at 9 K. We measure the photocurrent response to absorption of photons with an energy of less than 1.15 eV while the cell is operating at 1.15 V. We also show that this result implies the existence of three quasi-Fermi levels linked to the three electronic bands in our device, as demanded by the IBSC theory to preserve the output voltage of the cell.

Received 31 May 2020
Revised 23 September 2020
Accepted 3 November 2020

DOI:https://doi.org/10.1103/PhysRevLett.125.247703

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Optoelectronics Photovoltaic effect

Photovoltaic absorbers Quantum dots

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

I. Ramiro ^1,*, J. Villa ¹, J. Hwang ², A. J. Martin³, J. Millunchick³, J. Phillips ², and A. Martí ¹

¹Instituto de Energía Solar, Universidad Politécnica de Madrid, 28040 Madrid, Spain
²Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan 48109, USA
³Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA

^*i.ramiro@ies.upm.es, he/him/his

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Vol. 125, Iss. 24 — 11 December 2020

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Figure 1
(a) Simplified band diagram of the IB material in an IBSC under operation. The photocurrent originates from below-band-gap photon absorption (via transitions 1 and 2) in addition to above-band-gap absorption (transition 3). The corresponding three band-to-band recombination processes, $1^{'}$ , $2^{'}$ , and $3^{'}$ , are also indicated. Three QFLs, $μ_{C}$ , $μ_{V}$ , and $μ_{I}$ , are associated to the CB, the VB, and the IB, respectively. The photogenerated current $J$ is delivered at a voltage $V = (μ_{C} - μ_{V}) / e$ . (b) Simplified band diagram of an IB material implemented with $GaSb / GaAs$ QDs, exhibiting a type II (staggered) band lineup with confined hole states serving as the IB. For simplicity, only three confined states have been indicated.
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Figure 2
(a) Layer structure of the $GaSb / GaAs$ QD-IBSC studied. Background colors illustrate the doping character of each layer: red for $n$ type, blue for $p$ type. (b) Calculated band diagram, at room temperature, of the sample studied. The Fermi level $E_{F}$ is set to the zero-energy value. The shadowed area highlights the QD region. The CB and VB have been calculated, using the PC1D software [19], for a structure similar to (a) but without QDs. The top edge of the IB (red line) lies in a quasi flat band region.
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Figure 3
Current-voltage curve (red circles) at $T = 9 K$ of the $GaSb / GaAs$ QD-IBSC studied, under excitation with a continuous laser diode with a photon energy of 1.93 eV. The electrical power $P$ produced by the solar cell is plotted with blue triangles. $V_{o p} = 1.15 V$ is the operating voltage at which the photocurrent measurement of Fig. 4 is taken.
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Figure 4
(a) Spectral photocurrent of the $GaSb / GaAs$ QD-IBSC studied. Red solid line: measurement with light bias (1.93 eV laser) using the setup described in (b). Black dashed line: conventional photocurrent measurement at short circuit ( $R_{load} = 0 Ω$ ) and without light bias. The energy regions corresponding to response to transitions 2 and 3 are also indicated. The inset shows the EL spectrum of the sample for an injection current of $0.23 A {cm}^{- 2}$ . All measurements are taken at 9 K. (b) Diagram of the spectral photocurrent setup used for measuring (a) as described in the main text.
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Figure 5
Processes that could explain the photocurrent measured in Fig. 4 under monochromatic illumination with photons of an energy $E_{C I} < E < E_{C V}$ . (a) Tunneling. (b) Thermal excitation. (c) and (d) Auger generation. (e) Photon absorption. (f) Reduced $CB \to IB \to VB$ recombination.
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Figure 6
Electronic bands and their associated QFLs at ${e V}_{o p} > E_{C I}$ in the $GaSb / GaAs$ QD-IBSC studied, for the case of (a) $μ_{I} = μ_{V}$ and (b) $μ_{I} \neq μ_{V}$ . Vertical arrows indicate below-band-gap photon absorption via transition 2. In (a) red crosses illustrate prevented absorption owing to the lack of electrons in the IB states with an energy greater than $μ_{I}$ .
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Demonstration of a GaSb/GaAs Quantum Dot Intermediate Band Solar Cell Operating at Maximum Power Point

I. Ramiro, J. Villa, J. Hwang, A. J. Martin, J. Millunchick, J. Phillips, and A. Martí

Phys. Rev. Lett. 125, 247703 – Published 9 December 2020

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Demonstration of a $G aS b / Ga A s$ Quantum Dot Intermediate Band Solar Cell Operating at Maximum Power Point