Open-Circuit Voltage in AlGaAs Solar Cells With ... - IEEE Xplore

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IEEE JOURNAL OF PHOTOVOLTAICS, VOL. 7, NO. 1, JANUARY 2017

Open-Circuit Voltage in AlGaAs Solar Cells With Embedded GaNAs Quantum Wells of Varying Confinement Depth Martin Elborg, Takeshi Noda, and Yoshiki Sakuma

Abstract—We investigate the photovoltaic properties of AlGaAs solar cells with embedded GaNAs quantum wells (QWs) with N concentrations in the range of 0–3.1%, for which the QW confinement energy can be tuned by adjusting the N concentration. We systematically study the dependence of open-circuit voltage VOC in relation to the lowest band-to-band transition energy. In samples with low N concentrations (shallow QW confinement), VOC degrades and is limited by the lowest transition energy in the solar cell, i.e., the QW transition. With increasing N concentration, N > 0.5% (deep QW confinement), VOC does not degrade further and is no longer limited by the QW transition energy. The highest N sample exhibits a remarkably small offset between the lowest transition energy and the achieved VOC of 0.23 V, which is beyond the detailed balance limit of standard solar cells. VOC dependence is explained by analyzing the current–voltage (I–V) characteristics under different illumination conditions, from which information about the balance of escape and recombination rates of carriers from the QWs is extracted. In the deeply confined QWs, tunneling and thermal carrier escape is completely suppressed, allowing the recovery of VO C . Index Terms—Current–voltage (I–V) characteristics, dilute nitride, intermediate band solar cell (IBSC), open-circuit voltage, quantum well (QW).

I. INTRODUCTION HOTOVOLTAIC technologies have made great advances over the past decades, and laboratory-record solar cells are now approaching their theoretical conversion efficiency limits. A fundamental tradeoff exists between open-circuit voltage VOC and short-circuit current ISC of a solar cell device, which both depend on the bandgap of the material. To overcome these limitations, novel solar cell concepts have been proposed, such as the intermediate band solar cell (IBSC) [1]. The key component of this concept is an intermediate band or intermediate energy states located within the forbidden bandgap of the host solar cell. In such a device, an additional photocurrent can be

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Manuscript received July 19, 2016; revised September 13, 2016; accepted October 3, 2016. Date of publication October 28, 2016; date of current version December 20, 2016. This work was supported in part by the Japan Society for the Promotion of Science KAKENHI Grant 26790007. M. Elborg is with the International Center for Young Researchers, National Institute for Materials Science, Tsukuba 305-0047, Japan (e-mail: [email protected]). T. Noda is with the Photovoltaic Materials Unit, National Institute for Materials Science, Tsukuba 305-0047, Japan (e-mail: [email protected]). Y. Sakuma is with the Epitaxial Nanostructures Research Group, National Institute for Materials Science, Tsukuba 305-0044, Japan (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JPHOTOV.2016.2617040

generated by a two-step excitation of carriers from the valance band (VB) to the conduction band (CB) via intermediate energy states (IB) by the absorption of subbandgap energy photons, while VOC is maintained at the high level of the host solar cell material. To realize the theoretically anticipated function, solar cells with embedded nanostructures have been widely used, in which confined electronic states formed by the nanostructures are utilized as intermediate energy levels. However, in such solar cells, degradation of VOC is commonly observed [2–5]. The challenge for maintaining a high VOC is ascribed to the fact that the lowest transition energy will generally limit the achievable VOC in solar cells [6]. In previous works, the intermediate energy levels of a quantum structure were not sufficiently isolated from CB and VB, which causes VOC to be limited by the transition energy of the quantum structure. Recently, high VOC values with an offset to the lowest optical transition energy as low as 0.3 V have been demonstrated using quantum dots [7] and highly mismatched alloys [8], which exceeds state-of-the-art bulk semiconductor solar cells, where the offset is ∼0.4 V. To understand the relationship between achievable VOC and energetic position of the intermediate energy states, a systematic study of confinement depth is indispensable. Such a study is, however, difficult due to the lack of suitable material systems, from which quantum structures with varying confinement depth can be grown epitaxially. GaNx As1−x is a unique exception for compound semiconductors in that the incorporation of N in GaAs exhibits an extremely large bowing factor [9]. Therefore, the bandgap of GaNAs can be adjusted over a wide range by incorporating low amounts of N and, therefore, allowing the epitaxial growth on GaAs or AlGaAs with only low lattice mismatch. Embedded as quantum wells (QWs) in wider bandgap AlGaAs, this material system is an ideal model system of IBSCs, where the energetic position of the QW energy states acting as intermediate energy states can be precisely tuned. In previous studies, we have already demonstrated a two-step photocurrent generation in this material system [10]. In this work, we systematically investigate VOC of GaNAs/ AlGaAs QW embedded solar cells with varying confinement depth. VOC is analyzed in relation to the optical transition energies present in the structure, which are determined from photoluminescence (PL) experiments. Current–voltage (I–V) characteristics are measured under different illumination conditions to explain the behavior of carriers in bulk AlGaAs and QWs. This treatment is necessary to elucidate the dependence of VOC on confinement depth of embedded quantum structures.

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ELBORG et al.: OPEN-CIRCUIT VOLTAGE IN ALGAAS SOLAR CELLS WITH EMBEDDED GANAS QWS OF VARYING CONFINEMENT DEPTH

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TABLE I LIST OF SAMPLES WITH THEIR RESPECTIVE QW GROWTH MODE, N CONCENTRATION, AND LOWEST PL PEAK EMISSION ENERGY Sample name AlGaAs reference GaAs QWs 0% N GaNAs QWs 0.5% N GaNAs QWs 1.3% N GaNAs QWs 1.9% N GaNAs QWs 3.1% N

Growth interruption

N conc.

PL peak emission energy

– none none 10 s, (3×) 20 s, (3×) 40 s, (3×)

– 0% 0.5% 1.3% 1.9% 3.1%

1.81 eV 1.56 eV 1.43 eV 1.30 eV 1.21 eV 1.12 eV

II. EXPERIMENT Using a Riber molecular beam epitaxy system equipped with a radio frequency plasma source, we fabricated GaNx As1−x QWs with N concentrations ranging from x = 0−3% embedded in a higher bandgap Aly Ga1−y As (y = 0.27) material. The grown samples consist of a p-i-n solar cell structure, in which ten QWs are embedded in the middle of the i-AlGaAs layer. A summary of the fabricated samples with their respective N concentration is given in Table I. The samples were grown on an n-type GaAs (0 0 1) substrate at a substrate temperature of 600 °C. After an initial deposition of 300-nm Si-doped n-type GaAs (doping density 1 × 1018 cm−3 ), a 300-nm-thick n-AlGaAs (5 × 1017 cm−3 ) layer was grown, followed by a 600-nm-thick i-layer. On top, a Be-doped 200nm-thick p-AlGaAs layer (5 × 1017 cm−3 ), 20-nm p+ -AlGaAs layer (2 × 1018 cm−3 ), and a thin capping layer of p+ -GaAs (2 × 1019 cm−3 ) for surface protection were deposited. This sample without embedded quantum structures served as the AlGaAs reference sample. Five samples with embedded GaNx As1−x QWs of varying N concentration were grown. In each sample, ten layers of 4-nm-thick GaNx As1−x QWs separated by 16-nm-thick AlGaAs barriers were deposited at a substrate temperature of 520 °C. For the lowest N concentration of 0%, GaAs QWs were grown without N. A sample with GaNAs with 0.5% N was grown by supplying active N species from a radio-frequency plasma source at a power of 300 W and N2 gas flow rate of 0.06 sccm during QW growth. In this sample, the 4-nm GaNx As1−x QWs were grown continuously at a growth rate of one monolayer per second. Samples with higher N concentrations of 1.3%, 1.9%, and 3.1% were grown by introducing growth interruptions [11] of 10, 20, and 40 s, respectively, after every 1 nm of QW growth. Postgrowth annealing was performed on all samples at 800 °C for 4 min by rapid thermal annealing, which improves the optical quality of the quantum structures. The grown samples were masked by photolithography to deposit a Pt–Ti–Au front contact by sputtering. A second lithographic mask was used to etch around the individual devices. The device area is 0.66 mm2 with a total illumination area of 0.34 mm2 . I–V characteristics were measured using a Keithley 2400 Source Meter as a voltage source and a Keithley 6514 Electrometer as a current meter. Measurements were performed under dark conditions, standard 1-sun solar AM1.5 illumination supplied from a solar simulator, and monochromized light supplied from a halogen lamp and monochromator system. I–V

Fig. 1. HR-XRD pattern of a coupled scan of GaNAs QW embedded solar cells grown using growth interruptions of 0, 10, 20, and 40 s to adjust N concentration.

curves were measured from reverse to forward bias at a speed of 2 s per 0.1-V step. We confirmed that measurement direction did not influence the I–V characteristics. III. RESULTS AND DISCUSSION To determine the N composition of the grown samples, we use high-resolution X-ray diffraction (HR-XRD) measurements in a 2θ-ω coupled scan around the (0 0 4) diffraction peak and fit the measured diffraction pattern to the simulated pattern. The measured diffraction patterns are shown in Fig. 1. With increasing N growth interruption time, the diffraction peak of GaNAs starts to emerge from the GaAs (0 0 4) diffraction peak, first as a shoulder toward higher 2θ values and as a separate peak in the highest N concentration. This is consistent with the shrinking of the GaNx As1−x lattice constant. The determined N concentrations of the GaNAs QWs cover the range of 0–3.1%. The results show that growth interruptions are effective in increasing the amount of incorporated N without changing other growth parameters. The XRD measurements also confirm that QW thickness, AlGaAs barrier thickness, and AlGaAs composition are constant in the sample series with only slight variation between samples. The QW thickness of 4 nm is below the critical thickness for all employed N concentrations [12]. We perform PL measurements to determine QW-confined energy levels and respective carrier confinement depth. Excited by a 532-nm laser, PL emission is observed from AlGaAs with a bandgap energy of 1.81 eV, with only slight variation between samples. Additional emissions from the GaNAs QWs are observed in the QW embedded samples, which red-shift with increasing N concentration from 1.56 eV peak position in the GaAs QW sample down to 1.12 eV in the 3.1% N GaNAs QW sample. The QW PL peak emission energies are summarized in Table I. A more detailed analysis of the optical transitions is given in [13]. To determine the electron and hole confinement energies in GaNAs/AlGaAs QWs, we first analyze the GaAs/AlGaAs

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Fig. 2. Band diagram of (a) AlGaAs reference p-i-n solar cell and (b) sample series with ten layers of GaNAs QWs with 0% to 3.1% N. The optical transition energies of the QWs are given by the PL peak emission energies.

sample. In the case of the well-known GaAs/AlGaAs QW, 0.62 of the bandgap offset between GaAs and AlGaAs forms the CB offset [14] and 0.38 the VB offset. Finite-element method calculation of the electronic states leads to a confined energy state for electrons 141 meV below AlGaAs CB, as well as for heavy holes 121 meV above AlGaAs VB [13]. When N is incorporated into GaAs, the CB edge of GaNAs drastically shifts to lower energies, while its VB edge changes negligibly [9]. Therefore, N incorporation lowers the confined state of electrons, while that of holes remains unchanged. We determine the electron confinement energy by subtracting the QW transition energy determined from PL and hole confinement energy of 121 meV from AlGaAs’s bandgap energy. The confinement depth for electrons in samples with N concentrations of 0%, 0.5%, 1.3%, 1.9%, and 3.1% are 141, 253, 384, 465, and 542 meV, respectively. The band alignment of the sample series is illustrated in Fig. 2, ranging from shallow to deeply confined electron energy states. It should be noted that the band alignment of the 3.1% N GaNAs QW sample is close to the proposed ideal IBSC transition energies [1, 15] with a VB–CB transition of 1.81 eV and QW transition of 1.12 eV for GaNAs QW with 3.1% N, which forms deeply confined electron states 0.54 eV below the CB edge. Only a small energy fraction of ∼120 meV forms a shallow hole confinement. To analyze the photovoltaic properties of the GaNAs QW embedded solar cells, we measure their I–V characteristics under different illumination conditions: monochromized excitation of QWs, monochromized excitation of AlGaAs bulk, and solar AM1.5 illumination. Fig. 3(a) shows the I–V characteristics of the samples under QW excitation of 800 nm (power 4.3 mW/cm2 ). Under this illumination condition, the AlGaAs reference solar cell does not exhibit any photocurrent Iph generation, since the photon energy is below its bandgap energy. In the GaAs QW embedded solar cell, a constant Iph is generated over the measured voltage range. This Iph is generated from photon absorption by the QWs, from which carriers, both electrons and holes, can escape due to their shallow confinement and are collected at the contacts. In contrast, the GaNAs QW solar cells exhibit no Iph generation at short-circuit condition, indicating that carriers recombine within the deeper QWs be-

fore they can escape. Note that Iph is only generated from QW carriers when both the photogenerated electron and hole escape. In the case electrons recombine due to their deeper confinement, this, in turn, results in the same number of holes being lost in the process and, therefore, zero Iph . In general, Iph generation from QW photon absorption is limited by the stronger confined carriers, which in our case are electrons. Preventing thermal and tunneling escape of carriers from the IB energy states is a necessary condition for IBSC operation, since these escape processes are not thermodynamically compatible with the concept [16]. Toward reverse bias, an increase in photocurrent is observed. In the 0.5% N GaNAs QW solar cell, it reaches a saturated level at high reverse bias of –5 V. In higher N samples, photocurrent onset appears at higher negative voltages, from which it increases with decreasing bias similar to the 0.5% N sample but does not reach a saturated level in the measured voltage range. We confirmed that even under such a high reverse bias voltage, leakage current of the devices is negligible with a dark reverse current of -5 V, a reduction in Iph is observed. The reason for this behavior is the relaxation of photogenerated carriers from CB and VB

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into the QWs, from where Iph generation is again governed by the balance of escape and recombination rates inside the QWs. Under short-circuit condition, the GaNAs QW embedded solar cells with 0.5–3.1% N exhibit a reduced Iph generation of IS C ≈ 0.5 μA; however, Iph does not reduce to zero as in the QW excitation case. To understand this Iph generation at short-circuit condition, which is generated even in the 3.1% N GaNAs QW solar cell (where QW excitation shows no carrier escape from the deep QWs), we look at the spatial distribution of photogenerated carriers in the p-i-n solar cell structure. 600 nm monochromized light with an absorption coefficient of 2 × 104 cm−1 in AlGaAs (x = 0.27) [18], [19] will be absorbed according to the Lambert–Beer law. Dividing the solar cell into three regions, we calculate that 56% of electron–hole pairs are generated in the front 440 nm, 16% in the 200-nm QW region, and 18% in the back 500 nm of the AlGaAs solar cell. Ten percent of light is transmitted through the active solar cell structure and absorbed in the GaAs substrate with negligible contribution to photocurrent generation, as confirmed by photocurrent spectrum measurement [13]. We consider two extreme cases: 1) All photogenerated electrons and holes can pass or reescape from the QWs and reach the contacts, and 2) all carriers that pass the QWs relax and recombine. In the first case, 90% of all absorbed photons will contribute to Iph generation, while 10% of photons are lost to transmission. In the second case, Iph can only be generated by carriers, which do not have to pass the QW region, i.e., electrons generated in the bottom part of the solar cell reaching the back contact and holes generated in the front part reaching the front contact. Iph is, therefore, limited by the number of electrons generated in the bottom part (18%). Electrons from front and QW regions will recombine due to negligible transport through the QW region resulting in the loss of the same number of holes. Compared with our experimental results, we find that the ratio of reduced ISC to saturated Iph (0.5 μA/3 μA) matches the ratio of our calculation (18%/90%) and clarifies the origin of the reduced photocurrent from carriers that do not have to pass the QW region. Fig. 3(c) shows the I–V characteristics under 1-sun solar AM1.5 illumination, where carriers are generated above bandgap energy in the AlGaAs bulk solar cell as well as below AlGaAs bandgap in the embedded QWs. The AlGaAs reference solar cell shows a regularly shaped I–V curve with VOC = 1.1 V and ISC = 16.9 μA. When we embed GaAs QWs, ISC increases to 18.2 μA, while VOC decreases to 0.88 V. This is a commonly observed trend in solar cells with shallow embedded quantum structures. The increase in ISC is caused by additional subbandgap energy photon absorption by the QWs and subsequent thermal/tunneling carrier escape (compare photocurrent spectrum in [13]). This demonstrates the potential for higher current generation in IBSCs; however, carrier escape needs to be solely facilitated by second photon absorption in the future. When the embedded QWs are made deeper by incorporating N, VOC of the 0.5% N GaNAs QW sample degrades to 0.74 V, and its I–V characteristics change drastically, exhibiting a step-like dependence, which can now be understood from

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Fig. 5. Dark I–V characteristics. An example for the fitting of the I–V curves is shown for the AlGaAs reference cell with n = 1.91 and I0 = 3.07 × 10−15 A. Fig. 4. Open-circuit voltage plotted as a function of lowest band-to-band transition energy in the device given by the PL peak emission energy.

the previous analysis of both QW and bulk-generated carriers, which occurs simultaneously under full spectrum illumination. At short-circuit condition, the samples exhibit a reduced Iph generation of ISC ≈ 2 μA, which is almost identical for all GaNAs QW embedded solar cells with 0.5–3.1% N, originating from photogenerated carriers that do not have to pass the QW region. Interestingly, VOC in the GaNAs QW samples with 0.5–3.1% does not degrade further, but even slightly recovers with increasing N concentration to 0.84 V in the 3.1% N GaNAs QW embedded solar cell. The I–V characteristics under solar illumination are qualitatively similar to the 600 nm illumination case, with key differences under solar illumination being 1) saturated photocurrent of GaAs QW and 0.5% N GaNAs QW sample exceeding that of the AlGaAs reference due to the additional subbandgap energy photon absorption and subsequent thermal/tunneling escape from the QWs and 2) quantitatively different dependencies of Iph (V ) due to higher carrier concentration, as well as different spatial carrier distribution under solar illumination, which influences escape and recombination rates of carriers in the QWs. The detailed VOC dependence of the samples is plotted in Fig. 4 as a function of the lowest transition energy, showing more clearly the initial decrease and VOC recovery with increasing N concentration. A useful measure to discuss VOC is the bandgap energy–VOC offset introduced by King et al. [6]. In the case of QW embedded solar cells, the lowest transition energy is, however, not the bandgap energy but the QW transition. Modifying this concept to apply to IBSC [8] and solar cells with embedded quantum structures [7], we plot VOC as a function of PL peak emission energy of the lowest band-to-band transition in the device. For our AlGaAs reference solar cell with a bandgap of 1.81 eV and VOC of 1.1 V, the calculated lowest transition energy to open-circuit voltage offset W = E/q – VOC is 0.7 V. This offset is a reasonable value considering that the solar cell is not optimized using back-surface-field layers or other measures to improve VOC . In the GaAs QW embedded solar cell, the lowest transition energy is the QW transition energy of 1.56 eV. W stays almost unchanged at 0.69 V compared with

the AlGaAs reference. When increasing the QW confinement depth by incorporating 0.5% N, W remains constant at 0.69 V. The constant offset W indicates that VOC in these cells is determined by the lowest transition energy as well as the junction characteristics (mainly p and n doping concentrations) of the host AlGaAs solar cell, which is the same in all devices. For higher N concentrations and, therefore, deeper confinements, this trend of constant W is broken and W decreases down to 0.23 V (3.1% N). This value is a remarkably small offset between the lowest transition energy and the achieved VOC , which is beyond the detailed balance limit of standard bulk singlejunction solar cells with Eg = 1.1 eV (W = 0.26 V for detailed balance limit and W = 0.31 V for radiative recombination only limit) [6]. While this QW embedded solar cell can absorb light of photon energies down to the QW transition, its VOC is not limited by this QW transition energy. Note that VOC at room temperature cannot recover completely to the level of the host solar cell, since some degradation due to thermal injection of carriers into the confined energy states [20] will be a remaining degradation mechanism. To confirm that the VOC dependence is not just an experimental artifact under illuminated condition, we analyze the basic diode properties from the dark I–V characteristics, as shown in Fig. 5, using the standard diode equation qV I = I0 exp −1 (2) nkT where I is the current, q is the elemental charge, k is the Boltzmann constant, and T is the temperature. We fit the experimental dark I–V characteristics to determine the dark saturation current I0 and ideality factor n of each sample. Dark I– V curves are fitted for I ≤ 1 μA, for which series resistance in the sample is negligible. A small measurement offset from zero is additionally taken into account. The extracted diode parameters yield an excellent fit (R > 0.9994) and are summarized in Fig. 6, which shows the average and standard deviation measured for several devices of each sample. Using these diode parameters, we can calculate the expected VOC of the device by adding the generated photocurrent Iph = ISC to

ELBORG et al.: OPEN-CIRCUIT VOLTAGE IN ALGAAS SOLAR CELLS WITH EMBEDDED GANAS QWS OF VARYING CONFINEMENT DEPTH

Fig. 6. Reverse saturation current I0 and ideality factor n of the samples fitted from the dark I–V characteristics.

(2) and solving for V = VOC at I = 0. As a result, the experimentally measured VOC are reproduced in great accuracy and are plotted together in Fig. 4. From this analysis, we show that the VOC dependence is an intrinsic diode property of the QW embedded solar cells. Although the physical interpretation of I0 is difficult because of the oversimplification in the diode model, the extracted parameters allow a good characterization of the overall device properties, as well as for comparison to standard diode devices. Our analysis further shows that the experimentally observed VOC corresponds to Iph generation of the reduced ISC , and VOC can further be increased if trapped QW carriers can be efficiently extracted by second photon absorption in the future. IV. CONCLUSION We have systematically investigated the VOC dependence on QW confinement depth using GaNAs/AlGaAs QW embedded solar cells with N concentrations of 0–3.1%. VOC degrades when shallow QWs are not sufficiently isolated from the CB and VB. VOC of such a device is limited by the transition energy of the quantum structure. In deeply confined QWs, where thermal and tunneling escape of carriers is prevented, VOC can exceed the VOC limit of traditional solar cells with the same transition energy of the quantum structure. The transition between shallow and deeply confined QWs occurs in our samples at electron confinement ∼260 meV. A remarkably small offset between the lowest transition energy and VOC of W = 0.23 V is realized in the highest N sample. The results hold great promise to achieve higher solar cell conversion efficiencies, providing that an effective photon-induced extraction can be achieved in the future. GaNAs/AlGaAs has huge potential for practical application and research due to its tunability of transition energies over the spectral range of interest for IBSCs. REFERENCES [1] A. Luque and A. Mart´ı, “Increasing the efficiency of ideal solar cells by photon induced transitions at intermediate levels,” Phys. Rev. Lett., vol. 78, pp. 5014–5017, 1997. [2] G. Jolley, L. Fu, H. F. Lu, H. H. Tan, and C. Jagadish, “The role of inter sub band optical transitions on the electrical properties of InGaAs/GaAs quantum dot solar cells,” Prog. Photovoltaics, Res. Appl., vol. 21, pp. 736–746, 2013. [3] 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., vol. 98, 2011, Art. no. 163105.

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[4] T. Tayagaki, Y. Hoshi, and N. Usami, “Investigation of the open-circuit voltage in solar cells doped with quantum dots,” Sci. Rep., vol. 3, 2013, Art. no. 2703. [5] M. Elborg et al., “Voltage dependence of two-step photocurrent generation in quantum dot intermediate band solar cells,” Sol. Energy Mater. Sol. Cells, vol. 134, 2015, Art. no. 108. [6] R. R. King et al., “Band gap-voltage offset and energy production in next-generation multijunction solar cells,” Prog. Photovoltaics, Res. Appl., vol. 19, pp. 797–812, 2011. [7] K. Tanabe, D. Guimard, D. Bordel, and Y. Arakawa, “High-efficiency InAs/GaAs quantum dot solar cells by metalorganic chemical vapor deposition,” Appl. Phys. Lett., vol. 100, 2012, Art. no. 193905. [8] N. Ahsan, N. Miyashita, K. M. Yu, W. Walukiewicz, and Y. Okada, “Electron barrier engineering in a thin-film intermediate-band solar cell,” IEEE J. Photovoltaics, vol. 5, no. 3, pp. 878–884, May 2015. [9] S. H. Wei and A. Zunger, “Giant and composition-dependent optical bowing coefficient in GaAsN alloys,” Phys. Rev. Lett., vol. 76, 1996, Art. no. 664. [10] M. Elborg et al., “Two-Color photoexcitation in a GaNAs/AlGaAs quantum well solar cell,” Jpn. J. Appl. Phys., vol. 51, 2012, Art. no. 06FF15. [11] T. Mano et al., “Fabrication of GaNAs/AlGaAs heterostructures with large band offset using periodic growth interruption,” Appl. Phys. Exp., vol. 4, 2011, Art. no. 125001. [12] K. Uesugi, N. Morooka, and I. Suemune, “Strain e!ect on the N composition dependence of GaNAs bandgap energy grown on (0 0 1) GaAs by metalorganic molecular beam epitaxy,” J. Crystal Growth, vols. 201/202, pp. 355–358, 1999. [13] M. Elborg, T. Noda, T. Mano, and Y. Sakuma, “Optical transitions in GaNAs quantum wells with variable nitrogen content embedded in AlGaAs,” AIP Adv., vol. 6, 2016, Art. no. 065208. [14] M. O. Watanabe, J. Yoshida, M. Mashita, T. Nakanisi, and A. Hojo, “Band discontinuity for GaAs/AlGaAs heterojunction determined by C-V profiling technique,” J. Appl. Phys., vol. 57, 1985, Art. no. 5340. [15] T. Sogabe, T. Kaizu, Y. Okada, and S. Tomi, “Theoretical analysis of GaAs/AlGaAs quantum dots in quantum wire array for intermediate band solar cell,” J. Renewable Sustain. Energy, vol. 6, 2014, Art. no. 011206. [16] E. Antol´ın et al., “Reducing carrier escape in the InAs/GaAs quantum dot intermediate band solar cell,” J. Appl. Phys., vol. 108, 2010, Art. no. 064513. [17] M. Jo et al., “Impacts of ambipolar carrier escape on current-voltage characteristics in a type-I quantum-well solar cell” Appl. Phys. Lett., vol. 103, 2013, Art. no. 061118. [18] B. Monemar, K. K. Shih, and G. D. Pettit, “Some optical properties of the Alx Ga1 −x As alloys system,” J. Appl. Phys., vol. 47, 1976, Art. no. 2604. [19] D. E. Aspnes, S. M. Kelso, R. A. Logan, and R. Bhat, “Optical properties of Alx Ga1 −x As,” Appl. Phys. Lett., vol. 60, 1986, Art. no. 754. [20] G. Jolley, H. F. Lu, L. Fu, H. H. Tan, and C. Jagadish, “Electron-hole recombination properties of In0.5Ga0.5As/GaAs quantum dot solar cells and the influence on the open circuit voltage,” Appl. Phys. Lett., vol. 97, 2010, Art. no. 123505.

Martin Elborg received the Diploma degree in Electronic and Sensor Materials from Technische Universit¨at Bergakademie Freiberg, Freiberg, Germany, in 2010 and the Ph.D. degree in Materials Science from the University of Tsukuba, Tsukuba, Japan, in 2013. From 2013 to 2014, he joined the Photovoltaic Materials Group, National Institute for Materials Science (NIMS), Tsukuba. In 2015, he became a ICYS Researcher with the International Center for Young Scientists (ICYS), NIMS, working on the epitaxial growth of III–V semiconductor quantum structures and their electro-optical characterization for application in solar cells and quantum information technology.

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Takeshi Noda received the Ph.D. degree in Electronic Engineering from the University of Tokyo, Tokyo, Japan, in 1998. He served as a Research Associate with the University of Tokyo until 2003. He joined the National Institute for Materials Science, Tsukuba, Japan, where he is currently a Group Leader of the Photovoltaic Materials Group and is involved in molecular beam epitaxy and physics of quantum wells and quantum dots, including their applications to solar cells.

Yoshiki Sakuma received the B.E. and M.E. degrees in Electronic Engineering from Tohoku University, Sendai, Japan, in 1985 and 1987, respectively, and the Ph.D. degree in Material Physics from Osaka University, Osaka, Japan, in 1997. From 1987 to 2002, he was a researcher with Fujitsu Laboratories Ltd. In 2002, he joined the National Institute for Materials Science, Tsukuba, Japan, where he is currently a Group Leader of the Epitaxial Nanostructures Research Group. His research interests include growth and physics of III–V semiconductor nanostructures such as quantum dots and their applications to, e.g., single-photon emitters. Dr. Sakuma is a member of the Japan Society of Applied Physics.