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  • Editor: Christian Seassal
  • Vol. 22, Iss. S2 — Mar. 10, 2014
  • pp: A359–A364
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Type II GaSb quantum ring solar cells under concentrated sunlight

Che-Pin Tsai, Shun-Chieh Hsu, Shih-Yen Lin, Ching-Wen Chang, Li-Wei Tu, Kun-Cheng Chen, Tsong-Sheng Lay, and Chien-chung Lin  »View Author Affiliations


Optics Express, Vol. 22, Issue S2, pp. A359-A364 (2014)
http://dx.doi.org/10.1364/OE.22.00A359


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Abstract

A type II GaSb quantum ring solar cell is fabricated and measured under the concentrated sunlight. The external quantum efficiency confirms the extended absorption from the quantum rings at long wavelength coinciding with the photoluminescence results. The short-circuit current of the quantum ring devices is 5.1% to 9.9% more than the GaAs reference's under various concentrations. While the quantum ring solar cell does not exceed its GaAs counterpart in efficiency under one-sun, the recovery of the open-circuit voltages at higher concentration helps to reverse the situation. A slightly higher efficiency (10.31% vs. 10.29%) is reported for the quantum ring device against the GaAs one.

© 2014 Optical Society of America

1. Introduction

Since last decades, due to the diminishing reserve of fossil fuel and the concerns of global warming, the need to develop alternative energy sources becomes great. Solar energy has been regarded as a promising candidate for this purpose. Among various technologies, the quantum structure based solar cell is one of the most important designs to push the power conversion efficiency (PCE) over the classic Shockley-Queisser limit (SQ limit) [1

1. W. Shockley and H. J. Queisser, “Detailed Balance Limit of Efficiency of p-n Junction Solar Cells,” J. Appl. Phys. 32(3), 510–519 (1961). [CrossRef]

,2

2. A. Luque and A. Martí, “Increasing the Efficiency of Ideal Solar Cells by Photon Induced Transitions at Intermediate Levels,” Phys. Rev. Lett. 78(26), 5014–5017 (1997). [CrossRef]

]. The quantum scale structure, such as quantum dots (QDs) or quantum rings (QRs) [3

3. S. M. Hubbard, C. D. Cress, C. G. Bailey, R. P. Raffaelle, S. G. Bailey, and D. M. Wilt, “Effect of strain compensation on quantum dot enhanced GaAs solar cells,” Appl. Phys. Lett. 92(12), 123512 (2008). [CrossRef]

,4

4. R. B. Laghumavarapu, A. Moscho, A. Khoshakhlagh, M. El-Emawy, L. F. Lester, and D. L. Huffaker, “GaSb/GaAs type II quantum dot solar cells for enhanced infrared spectral response,” Appl. Phys. Lett. 90(17), 173125 (2007). [CrossRef]

], can construct an extra absorptive transition by introducing an intermediate band (IB) within the band gap of the host material. This intermediate band solar cell (IBSC) allows the extra electron-hole pairs generated from the two inter-band transitions [2

2. A. Luque and A. Martí, “Increasing the Efficiency of Ideal Solar Cells by Photon Induced Transitions at Intermediate Levels,” Phys. Rev. Lett. 78(26), 5014–5017 (1997). [CrossRef]

], and thus the PCE of the device can surpass the SQ limit. However, since its first introduction in 1997, many attempts to realize such designs have been failed due to serious open-circuit voltage (VOC) reduction of the devices. This drop of VOC, accompanied with limited increase of short-circuit current (JSC), is the main culprit that makes the PCEs of quantum structure embedded solar cells falling behind their single band gap counterparts [5

5. P. G. Linares, A. Martí, E. Antolín, C. D. Farmer, Í. Ramiro, C. R. Stanley, and A. Luque, “Voltage recovery in intermediate band solar cells,” Sol. Energy Mater. Sol. Cells 98, 240–244 (2012). [CrossRef]

,6

6. C.-C. Lin, M.-H. Tan, C.-P. Tsai, K.-Y. Chuang, and T. S. Lay, “Numerical Study of Quantum-Dot-Embedded Solar Cells,” IEEE J. Sel. Top. Quant. 19, 4000110 (2013).

]. Many possible explanations/solutions have been provided, such as the flatness of the intermediate band [2

2. A. Luque and A. Martí, “Increasing the Efficiency of Ideal Solar Cells by Photon Induced Transitions at Intermediate Levels,” Phys. Rev. Lett. 78(26), 5014–5017 (1997). [CrossRef]

], the requirement of half-filled intermediate band [7

7. A. Luque, A. Marti, and C. Stanley, “Understanding intermediate-band solar cells,” Nat. Photonics 6(3), 146–152 (2012). [CrossRef]

], carrier recombination [5

5. P. G. Linares, A. Martí, E. Antolín, C. D. Farmer, Í. Ramiro, C. R. Stanley, and A. Luque, “Voltage recovery in intermediate band solar cells,” Sol. Energy Mater. Sol. Cells 98, 240–244 (2012). [CrossRef]

], the electronic isolation of IB [8

8. N. López, L. A. Reichertz, K. M. Yu, K. Campman, and W. Walukiewicz, “Engineering the Electronic Band Structure for Multiband Solar Cells,” Phys. Rev. Lett. 106(2), 028701 (2011). [CrossRef] [PubMed]

], etc.. To fulfill these conditions is not a trivial job for the device designer.

2. Quantum ring growth and device fabrication

The solar cell wafers were grown in a Riber C21 molecular beam epitaxy (MBE) system and the (100) n-type GaAs substrates were used. Two wafers with and without GaSb QRs were grown and they are referred to as the reference sample and the GaSb QR sample, respectively. The actual epitaxial structures are summarized in Table 1.

Table 1. Detailed Epitaxial Structures of Reference and GaSb QR Samples

table-icon
View This Table
While the total thickness of the lightly-doped n-type layer is kept the same (1020nm), the QR wafer differs from the reference sample by inserting 3 monolayers (MLs) of GaSb QRs at the center. To have the ring structure instead of dot structure, a post-growth Sb soaking with a specific Sb/As ratio is necessary. The growth procedures and the formation mechanisms of GaSb QRs are discussed elsewhere [21

21. W.-H. Lin, K.-W. Wang, S.-W. Chang, M.-H. Shih, and S.-Y. Lin, “Type-II GaSb/GaAs coupled quantum rings: Room-temperature luminescence enhancement and recombination lifetime elongation for device applications,” Appl. Phys. Lett. 101(3), 031906 (2012). [CrossRef]

]. By this growth technique, a GaSb QR layer with ring density around 1.9 × 1010 cm−2 is obtained for the QR sample discussed in this paper [21

21. W.-H. Lin, K.-W. Wang, S.-W. Chang, M.-H. Shih, and S.-Y. Lin, “Type-II GaSb/GaAs coupled quantum rings: Room-temperature luminescence enhancement and recombination lifetime elongation for device applications,” Appl. Phys. Lett. 101(3), 031906 (2012). [CrossRef]

]. The average height and outer diameter of rings are 1.5 and 46.7 nm, respectively [21

21. W.-H. Lin, K.-W. Wang, S.-W. Chang, M.-H. Shih, and S.-Y. Lin, “Type-II GaSb/GaAs coupled quantum rings: Room-temperature luminescence enhancement and recombination lifetime elongation for device applications,” Appl. Phys. Lett. 101(3), 031906 (2012). [CrossRef]

]. Both the reference and the GaSb QR wafers were then went through regular semiconductor process. The p-type metal is Cr/Au and the n-type bottom contact is AuGe/Au, respectively. The H3PO4 / H2O2 / H2O solution was used for mesa etch step which defines a 1mm by 1mm square-sized area. The final step is the anti-reflection coating made of SiO2 100nm to achieve the maximal reception of photons in the visible range.

After the wafer processing, the wafer was cleaved into individual dies and the chip was taken to the regular IV and EQE test under one sun condition. The IV characteristics were taken under the Newport AM1.5G 1000Watts source with Class 1A standard. The currents and voltages were recorded by the HP4156B semiconductor parameter analyzer via a computerized interface. The EQE was taken by Newport 6258 300W Xe lamp with Cornerstone 260 1/4 m monochromator. For the concentration measurement, Keithley 2400 source meter was used to take the IV of the device under test. The concentration was accomplished by a Newport/Oriel 92193 1600Watt Xe lamp with the Newport/Oriel 81030 High Flux Beam Concentrator. All the measurements were performed under a controlled temperature of 25°C.

3. Results and discussion

The PL intensities of the QR sample at low temperature (10K) and room temperature (300K) are shown in the inset of Fig. 2.
Fig. 2 The EQE spectral response of the reference and GaSb QR samples. The inset is the PL data of the QR under 10K and room temperature.
The wavelengths for peak intensities are at 1072 nm (10K) and 1192 nm (300K), respectively. The full-width-at-half-maximum (FWHM) are 101nm (10K) and 175nm (300K), similar to what we reported before [22

22. W.-H. Lin, M.-Y. Lin, S.-Y. Wu, and S.-Y. Lin, “Room-Temperature Electro-Luminescence of Type-II GaSb/GaAs Quantum Rings,” IEEE Photonic Tech. L. 24(14), 1203–1205 (2012). [CrossRef]

].

Figure 2 shows the measured EQE at room temperature. The QR sample has stronger absorption at λ>900nm range, and thus a higher JSC. Compared to the inset PL spectrum, the corresponding peak wavelength falls within the enhanced region of the EQE. Meanwhile, the EQE of the reference GaAs sample falls steeply after 900nm. The one sun IV characteristics is shown in Fig. 3(a).
Fig. 3 The current-voltage (IV) characteristics of the reference and GaSb QR samples under (a) one-Sun and (b) 60 Sun.
As can be seen from the figure, the one-sun VOC reduction is still severe and the PCE of the QR device suffers from this fact. The short-circuit current, on the other hand, shows considerable enhancement (from 16.84mA/cm2 to 18.05mA/cm2, a 7.18% increase). The PCE of the QR sample is 6.85% while the reference sample is 8.81% under one-sun condition. The lower than average PCE of the reference can be expected from EQE measurement in which the visible wavelength range is not very efficient and the overall peak efficiency is low (about 40%). This is mainly due to non-optimized structure of our design. In fact, we have to compromise our device structure due to epitaxial growth limitation.

The devices were then put under the concentrated sunlight test from 1 sun to 90 sun. An example of concentrated IV curve is shown in Fig. 3(b). The measured PCE, and VOC of the best QR and reference devices under different concentrations are plotted in Fig. 4.
Fig. 4 (a) The power conversion efficiencies of the reference and GaSb QR samples versus different concentration factor. The dashed lines are for eye-guiding only. (b) The VOC under the same condition.
From the PCE curve, the QR sample quickly close the performance gap when the pumping intensity goes higher, and eventually surpasses the reference sample at 60 sun test (10.29% vs. 10.31%). The JSC sees a leading edge of QR sample over the whole range of concentration factor. On the VOC, the QR sample outperforms the reference sample at the medium concentration but fall short on the higher intensities. If the fill factor is not considered, and just compare the JSC × VOC between the QR and the reference samples, a 2% to 7% enhancement can be found under the concentrated sunlight and this ratio is below 1 at one-sun condition.

Currently, our devices suffer from the inferior fill factor due to bad series and shunt resistances. Also the overall device structure is not optimized for single junction solar cell due to growth concerns. A switch to metal-organic chemical vapor deposition (MOCVD) growth can possibly solve part of our problems and more cares are necessary to fabricate a high fill factor device in the future. Meanwhile, the QR structure demonstrated its potential to outpace the single band gap device at high concentration and we hope more improvement could lead to its success under one sun condition in the future.

4. Conclusion

In conclusion, we demonstrate a comparison between a single band gap GaAs cell and a type-II GaSb QR solar cell under one sun and concentrated sunlight. The result verifies the theoretical model developed previously. While the device is not optimized for conversion efficiency, the QR sample outperforms its single band gap counterpart at 60-sun illumination condition and the recovery of VOC is observed. We believe the type-II QR structure should be suitable for implementation of the highly efficient IBSC and more studies are underway to improve the device performance in the near future.

Acknowledgments

The authors would like to thank the financial supports from the National Science Council of Taiwan through the grant number: NSC101-2221-E-009-046-MY3.

References and links

1.

W. Shockley and H. J. Queisser, “Detailed Balance Limit of Efficiency of p-n Junction Solar Cells,” J. Appl. Phys. 32(3), 510–519 (1961). [CrossRef]

2.

A. Luque and A. Martí, “Increasing the Efficiency of Ideal Solar Cells by Photon Induced Transitions at Intermediate Levels,” Phys. Rev. Lett. 78(26), 5014–5017 (1997). [CrossRef]

3.

S. M. Hubbard, C. D. Cress, C. G. Bailey, R. P. Raffaelle, S. G. Bailey, and D. M. Wilt, “Effect of strain compensation on quantum dot enhanced GaAs solar cells,” Appl. Phys. Lett. 92(12), 123512 (2008). [CrossRef]

4.

R. B. Laghumavarapu, A. Moscho, A. Khoshakhlagh, M. El-Emawy, L. F. Lester, and D. L. Huffaker, “GaSb/GaAs type II quantum dot solar cells for enhanced infrared spectral response,” Appl. Phys. Lett. 90(17), 173125 (2007). [CrossRef]

5.

P. G. Linares, A. Martí, E. Antolín, C. D. Farmer, Í. Ramiro, C. R. Stanley, and A. Luque, “Voltage recovery in intermediate band solar cells,” Sol. Energy Mater. Sol. Cells 98, 240–244 (2012). [CrossRef]

6.

C.-C. Lin, M.-H. Tan, C.-P. Tsai, K.-Y. Chuang, and T. S. Lay, “Numerical Study of Quantum-Dot-Embedded Solar Cells,” IEEE J. Sel. Top. Quant. 19, 4000110 (2013).

7.

A. Luque, A. Marti, and C. Stanley, “Understanding intermediate-band solar cells,” Nat. Photonics 6(3), 146–152 (2012). [CrossRef]

8.

N. López, L. A. Reichertz, K. M. Yu, K. Campman, and W. Walukiewicz, “Engineering the Electronic Band Structure for Multiband Solar Cells,” Phys. Rev. Lett. 106(2), 028701 (2011). [CrossRef] [PubMed]

9.

C. G. Bailey, D. V. Forbes, R. P. Raffaelle, and S. M. Hubbard, “Near 1 V open circuit voltage InAs/GaAs quantum dot solar cells,” Appl. Phys. Lett. 98(16), 163105 (2011). [CrossRef]

10.

T. Sugaya, O. Numakami, R. Oshima, S. Furue, H. Komaki, T. Amano, K. Matsubara, Y. Okano, and S. Niki, “Ultra-high stacks of InGaAs/GaAs quantum dots for high efficiency solar cells,” Energy & Environmental Science 5(3), 6233–6237 (2012). [CrossRef]

11.

S. Tomic, A. Marti, E. Antolin, and A. Luque, “On inhibiting Auger intraband relaxation in InAs/GaAs quantum dot intermediate band solar cells,” Appl. Phys. Lett. 99(5), 053504 (2011). [CrossRef]

12.

P. J. Carrington, M. C. Wagener, J. R. Botha, A. M. Sanchez, and A. Krier, “Enhanced infrared photo-response from GaSb/GaAs quantum ring solar cells,” Appl. Phys. Lett. 101(23), 231101 (2012). [CrossRef]

13.

T. Tayagaki, N. Usami, P. Wugen, Y. Hoshi, and Y. Kanemitsu, “Enhanced carrier extraction under strong light irradiation in Ge/Si type-II quantum dot solar cells,” in Photovoltaic Specialists Conference (PVSC),201238th IEEE(IEEE, Austin, TX, 2012), pp. 003200–003203. [CrossRef]

14.

A. Alemu, J. A. H. Coaquira, and A. Freundlich, “Dependence of device performance on carrier escape sequence in multi-quantum-well p-i-n solar cells,” J. Appl. Phys. 99(8), 084506 (2006). [CrossRef]

15.

J. Hwang, A. J. Martin, J. M. Millunchick, and J. D. Phillips, “Thermal emission in type-II GaSb/GaAs quantum dots and prospects for intermediate band solar energy conversion,” J. Appl. Phys. 111(7), 074514 (2012). [CrossRef]

16.

V. Popescu, G. Bester, M. C. Hanna, A. G. Norman, and A. Zunger, “Theoretical and experimental examination of the intermediate-band concept for strain-balanced (In,Ga)As/Ga(As,P) quantum dot solar cells,” Phys. Rev. B 78(20), 205321 (2008). [CrossRef]

17.

A. Martí, E. Antolín, E. Cánovas, N. López, P. G. Linares, A. Luque, C. R. Stanley, and C. D. Farmer, “Elements of the design and analysis of quantum-dot intermediate band solar cells,” Thin Solid Films 516(20), 6716–6722 (2008). [CrossRef]

18.

N. Ahsan, N. Miyashita, M. M. Islam, K. M. Yu, W. Walukiewicz, and Y. Okada, “Two-photon excitation in an intermediate band solar cell structure,” Appl. Phys. Lett. 100(17), 172111 (2012). [CrossRef] [PubMed]

19.

S. M. Hubbard, C. G. Bailey, R. Aguinaldo, S. Polly, D. V. Forbes, and R. P. Raffaelle, “Characterization of quantum dot enhanced solar cells for concentrator photovoltaics,” in Photovoltaic Specialists Conference (PVSC),200934th IEEE(IEEE, Philadelphia, PA, 2009), 000090–000095. [CrossRef]

20.

K. Yoshida, Y. Okada, and N. Sano, “Device simulation of intermediate band solar cells: Effects of doping and concentration,” J. Appl. Phys. 112, 084510 (2012).

21.

W.-H. Lin, K.-W. Wang, S.-W. Chang, M.-H. Shih, and S.-Y. Lin, “Type-II GaSb/GaAs coupled quantum rings: Room-temperature luminescence enhancement and recombination lifetime elongation for device applications,” Appl. Phys. Lett. 101(3), 031906 (2012). [CrossRef]

22.

W.-H. Lin, M.-Y. Lin, S.-Y. Wu, and S.-Y. Lin, “Room-Temperature Electro-Luminescence of Type-II GaSb/GaAs Quantum Rings,” IEEE Photonic Tech. L. 24(14), 1203–1205 (2012). [CrossRef]

23.

J. Nelson, The Physics of Solar Cells (World Scientific, 2003).

24.

T. Brunhes, P. Boucaud, S. Sauvage, F. Aniel, J. M. Lourtioz, C. Hernandez, Y. Campidelli, O. Kermarrec, D. Bensahel, G. Faini, and I. Sagnes, “Electroluminescence of Ge/Si self-assembled quantum dots grown by chemical vapor deposition,” Appl. Phys. Lett. 77(12), 1822–1824 (2000). [CrossRef]

25.

T. Gu, M. A. El-Emawy, K. Yang, A. Stintz, and L. F. Lester, “Resistance to edge recombination in GaAs-based dots-in-a-well solar cells,” Appl. Phys. Lett. 95(26), 261106 (2009). [CrossRef]

26.

M. D. Kelzenberg, D. B. Turner-Evans, B. M. Kayes, M. A. Filler, M. C. Putnam, N. S. Lewis, and H. A. Atwater, “Photovoltaic Measurements in Single-Nanowire Silicon Solar Cells,” Nano Lett. 8(2), 710–714 (2008). [CrossRef] [PubMed]

OCIS Codes
(040.5350) Detectors : Photovoltaic
(350.6050) Other areas of optics : Solar energy
(250.5590) Optoelectronics : Quantum-well, -wire and -dot devices

ToC Category:
Photovoltaics

History
Original Manuscript: November 21, 2013
Revised Manuscript: January 23, 2014
Manuscript Accepted: February 4, 2014
Published: February 14, 2014

Citation
Che-Pin Tsai, Shun-Chieh Hsu, Shih-Yen Lin, Ching-Wen Chang, Li-Wei Tu, Kun-Cheng Chen, Tsong-Sheng Lay, and Chien-chung Lin, "Type II GaSb quantum ring solar cells under concentrated sunlight," Opt. Express 22, A359-A364 (2014)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-S2-A359


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References

  1. W. Shockley and H. J. Queisser, “Detailed Balance Limit of Efficiency of p-n Junction Solar Cells,” J. Appl. Phys.32(3), 510–519 (1961). [CrossRef]
  2. A. Luque and A. Martí, “Increasing the Efficiency of Ideal Solar Cells by Photon Induced Transitions at Intermediate Levels,” Phys. Rev. Lett.78(26), 5014–5017 (1997). [CrossRef]
  3. S. M. Hubbard, C. D. Cress, C. G. Bailey, R. P. Raffaelle, S. G. Bailey, and D. M. Wilt, “Effect of strain compensation on quantum dot enhanced GaAs solar cells,” Appl. Phys. Lett.92(12), 123512 (2008). [CrossRef]
  4. R. B. Laghumavarapu, A. Moscho, A. Khoshakhlagh, M. El-Emawy, L. F. Lester, and D. L. Huffaker, “GaSb/GaAs type II quantum dot solar cells for enhanced infrared spectral response,” Appl. Phys. Lett.90(17), 173125 (2007). [CrossRef]
  5. P. G. Linares, A. Martí, E. Antolín, C. D. Farmer, Í. Ramiro, C. R. Stanley, and A. Luque, “Voltage recovery in intermediate band solar cells,” Sol. Energy Mater. Sol. Cells98, 240–244 (2012). [CrossRef]
  6. C.-C. Lin, M.-H. Tan, C.-P. Tsai, K.-Y. Chuang, and T. S. Lay, “Numerical Study of Quantum-Dot-Embedded Solar Cells,” IEEE J. Sel. Top. Quant.19, 4000110 (2013).
  7. A. Luque, A. Marti, and C. Stanley, “Understanding intermediate-band solar cells,” Nat. Photonics6(3), 146–152 (2012). [CrossRef]
  8. N. López, L. A. Reichertz, K. M. Yu, K. Campman, and W. Walukiewicz, “Engineering the Electronic Band Structure for Multiband Solar Cells,” Phys. Rev. Lett.106(2), 028701 (2011). [CrossRef] [PubMed]
  9. C. G. Bailey, D. V. Forbes, R. P. Raffaelle, and S. M. Hubbard, “Near 1 V open circuit voltage InAs/GaAs quantum dot solar cells,” Appl. Phys. Lett.98(16), 163105 (2011). [CrossRef]
  10. T. Sugaya, O. Numakami, R. Oshima, S. Furue, H. Komaki, T. Amano, K. Matsubara, Y. Okano, and S. Niki, “Ultra-high stacks of InGaAs/GaAs quantum dots for high efficiency solar cells,” Energy & Environmental Science5(3), 6233–6237 (2012). [CrossRef]
  11. S. Tomic, A. Marti, E. Antolin, and A. Luque, “On inhibiting Auger intraband relaxation in InAs/GaAs quantum dot intermediate band solar cells,” Appl. Phys. Lett.99(5), 053504 (2011). [CrossRef]
  12. P. J. Carrington, M. C. Wagener, J. R. Botha, A. M. Sanchez, and A. Krier, “Enhanced infrared photo-response from GaSb/GaAs quantum ring solar cells,” Appl. Phys. Lett.101(23), 231101 (2012). [CrossRef]
  13. T. Tayagaki, N. Usami, P. Wugen, Y. Hoshi, and Y. Kanemitsu, “Enhanced carrier extraction under strong light irradiation in Ge/Si type-II quantum dot solar cells,” in Photovoltaic Specialists Conference (PVSC),201238th IEEE(IEEE, Austin, TX, 2012), pp. 003200–003203. [CrossRef]
  14. A. Alemu, J. A. H. Coaquira, and A. Freundlich, “Dependence of device performance on carrier escape sequence in multi-quantum-well p-i-n solar cells,” J. Appl. Phys.99(8), 084506 (2006). [CrossRef]
  15. J. Hwang, A. J. Martin, J. M. Millunchick, and J. D. Phillips, “Thermal emission in type-II GaSb/GaAs quantum dots and prospects for intermediate band solar energy conversion,” J. Appl. Phys.111(7), 074514 (2012). [CrossRef]
  16. V. Popescu, G. Bester, M. C. Hanna, A. G. Norman, and A. Zunger, “Theoretical and experimental examination of the intermediate-band concept for strain-balanced (In,Ga)As/Ga(As,P) quantum dot solar cells,” Phys. Rev. B78(20), 205321 (2008). [CrossRef]
  17. A. Martí, E. Antolín, E. Cánovas, N. López, P. G. Linares, A. Luque, C. R. Stanley, and C. D. Farmer, “Elements of the design and analysis of quantum-dot intermediate band solar cells,” Thin Solid Films516(20), 6716–6722 (2008). [CrossRef]
  18. N. Ahsan, N. Miyashita, M. M. Islam, K. M. Yu, W. Walukiewicz, and Y. Okada, “Two-photon excitation in an intermediate band solar cell structure,” Appl. Phys. Lett.100(17), 172111 (2012). [CrossRef] [PubMed]
  19. S. M. Hubbard, C. G. Bailey, R. Aguinaldo, S. Polly, D. V. Forbes, and R. P. Raffaelle, “Characterization of quantum dot enhanced solar cells for concentrator photovoltaics,” in Photovoltaic Specialists Conference (PVSC),200934th IEEE(IEEE, Philadelphia, PA, 2009), 000090–000095. [CrossRef]
  20. K. Yoshida, Y. Okada, and N. Sano, “Device simulation of intermediate band solar cells: Effects of doping and concentration,” J. Appl. Phys.112, 084510 (2012).
  21. W.-H. Lin, K.-W. Wang, S.-W. Chang, M.-H. Shih, and S.-Y. Lin, “Type-II GaSb/GaAs coupled quantum rings: Room-temperature luminescence enhancement and recombination lifetime elongation for device applications,” Appl. Phys. Lett.101(3), 031906 (2012). [CrossRef]
  22. W.-H. Lin, M.-Y. Lin, S.-Y. Wu, and S.-Y. Lin, “Room-Temperature Electro-Luminescence of Type-II GaSb/GaAs Quantum Rings,” IEEE Photonic Tech. L.24(14), 1203–1205 (2012). [CrossRef]
  23. J. Nelson, The Physics of Solar Cells (World Scientific, 2003).
  24. T. Brunhes, P. Boucaud, S. Sauvage, F. Aniel, J. M. Lourtioz, C. Hernandez, Y. Campidelli, O. Kermarrec, D. Bensahel, G. Faini, and I. Sagnes, “Electroluminescence of Ge/Si self-assembled quantum dots grown by chemical vapor deposition,” Appl. Phys. Lett.77(12), 1822–1824 (2000). [CrossRef]
  25. T. Gu, M. A. El-Emawy, K. Yang, A. Stintz, and L. F. Lester, “Resistance to edge recombination in GaAs-based dots-in-a-well solar cells,” Appl. Phys. Lett.95(26), 261106 (2009). [CrossRef]
  26. M. D. Kelzenberg, D. B. Turner-Evans, B. M. Kayes, M. A. Filler, M. C. Putnam, N. S. Lewis, and H. A. Atwater, “Photovoltaic Measurements in Single-Nanowire Silicon Solar Cells,” Nano Lett.8(2), 710–714 (2008). [CrossRef] [PubMed]

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