Conversion Efficiency of 45.0% in InGaP/InGaAs/Ge Triple-Junction Solar Cells for Laser Power Beaming

Abstract

Optical wireless power transmission systems are attracting attention as a new power transmission technology because they can supply power wirelessly over long distances. In this study, we investigated InGaP/InGaAs/Ge triple-junction solar cells simultaneously irradiated with three laser beams with wavelengths of 635 nm, 850 nm, and 1550 nm to improve photoelectric conversion efficiency. As a result, a photoelectric conversion efficiency of 45.0% was obtained under three laser irradiations with a total incident laser power of 1.77 W/cm². The results showed the possibility of a high-efficiency optical wireless power transmission system by simultaneously irradiating laser beams with different wavelengths onto multi-junction solar cells, which could be installed in automobiles as a new system that complements solar power generation for daylighting.

1. Introduction

Wireless technology for power supply is expected to be a key technology in the ongoing shift to wireless information and communication. Although electromagnetic induction wireless power transmission has already been commercialized for some electronic devices, it has problems, such as a transmission distance of less than a few centimeters and leakage of electromagnetic waves. On the other hand, optical wireless power transfer is attracting attention as a new wireless power transmission technology because of its ability to transfer power over long distances.

Optical wireless power transmission (OWPT) systems supply electrical power by irradiating laser or LED light onto photovoltaic cells and converting the light into electric power. This system has the advantages of long-distance and high-power transmission and can be researched in a various wavelength range by combining various light sources and photovoltaic cells [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17]. Table 1 shows the conversion efficiency of photovoltaic cells in various wavelength ranges [5,6,7,8,9,10,11,12,13,14,15,16,17].

On the other hand, there are also reports of research aimed at increasing efficiency by absorbing the light of multiple wavelengths using new structures such as intermediate-band structures [18] and nanostructures [19,20,21].

We focused on multi-junction photovoltaic cells, which will likely be installed in vehicles in the future, to improve the efficiency of optical wireless power transmission systems. If this can be realized, the battery will be charged under sunlight during the daytime and under laser light at night or in poor weather conditions.

In the case of multi-junction photovoltaic cells, high photoelectric conversion efficiency is expected, as the voltage values obtained in each sub-cell are summed up. Fafard et al. irradiated a 12-junction GaAs device with an 811 nm laser beam of about 40 W, having a photoelectric conversion efficiency of 61% [15]. They also reported a conversion efficiency of 51.1% with a 10-junction InGaAs device under a 1466 nm laser beam of 69 W/cm² [15]. Wang et al. irradiated an eight-junction InGaAs cell with a 1520 nm laser beam of 53.9 W/cm² and reported a photoelectric conversion efficiency of 36.9% [16]. Helmers and the University of Ottawa reported a 45.6% efficiency using a 10-junction InGaAs cell under a 1522 nm laser light [17].

As shown above, high photoelectric conversion efficiency has been obtained in optical wireless power transmission using multi-junction photovoltaic cells, and high efficiency of optical wireless power transmission systems can be expected.

We focused on triple-junction solar cells, which are expected to generate ultra-high efficiency under solar illumination. These triple-junction solar cells have been reported to have high photoelectric conversion efficiencies of more than 30% under solar irradiation, and around 40% under concentrated solar irradiation [22,23,24,25]. This high efficiency is expected to lead to various applications in space and mobility. Plug-in hybrid vehicles equipped with InGaP/GaAs/InGaAs triple-junction solar cells have been tested on public roads since 2019, and research towards their practical use is underway [26]. On the other hand, solar cells have difficulty generating electricity in rain, cloudy weather, and at night, which poses a challenge to the stable electricity supply. Therefore, there is a need for power supply systems that can generate a certain amount of power even under adverse conditions. In this study, we verified an optical wireless power supply system in which three laser beams with wavelengths of 635 nm, 850 nm, and 1550 nm are simultaneously irradiated onto InGaP/InGaAs/Ge triple-junction solar cells.

2. Materials and Methods

In this study, InGaP/InGaAs/Ge 3-junction solar cells were simultaneously irradiated with laser beams of three different wavelengths. Figure 1 shows the experimental configuration of the optical wireless power transmission. A photodiode power sensor measured the incident laser power P_in. Laser beams of wavelengths 635 nm and 850 nm were measured using sensors with corresponding wavelengths of 400–1100 nm (S130C, THOULABS; uncertainly = ±3%), and laser beams of 1550 nm using sensors with corresponding wavelengths of 700–1800 nm (S132C, THOLABS, Newton, NJ, USA; uncertainly = ±5%). The output power P_out of the solar cells was measured with a source measure unit (B2901A, KEYSIGHT, Santa Rosa, CA, USA).

The InGaP/InGaAs/Ge triple-junction solar cells we used were commercially available products (3C36C, Azur Space, Heilbronn, Germany). The prepared solar cell had a photosensitive area of 6.8 × 7.0 mm². It exhibited a photovoltaic conversion efficiency of 33.1%, having a short-circuit current density J_sc of 14.8 mA/cm², an open-circuit voltage V_oc of 2.57 V, and a fill factor FF of 87.0% under simulated sunlight irradiation of 100 mW/cm², as shown in Figure 2. The 3-junction solar cell was equipped with a copper heat sink. The temperature was maintained at 23 °C using a water-cooled recirculating chiller. The laser sources were a 635 nm fiber laser (MDL-D-635, CNI, CHINA; power stability = ± 3%), an 850 nm semiconductor laser (WSLD-850-001-1, WAVE spectrum, CHINA), and a 1550 nm semiconductor laser (L1550G1, THOLABS, Newton, NJ, USA). The photoelectric conversion efficiency η_pv was calculated as P_out/P_in.

Therefore, the error in the voltage obtained with this experimental system was due to the resolution of the source measure unit and can be less than ±0.02%. However, the values of a generated current and a conversion efficiency include errors of up to ±8%, mainly caused by power meter errors (±5%) and laser output fluctuations (±3%).

The 1550 nm laser beam was vertically irradiated to the solar cells, while the 635 nm and 850 nm laser beams were irradiated at an incident angle of about 3°. In this optical wireless power transmission system using 3-junction solar cells, it can be considered that as the incident angle increases, the total conversion efficiency decreases. Therefore, we investigated the dependences of the incident angles of the laser beams on the conversion efficiencies of 3-junction solar cells [27]. In this measurement, the incident angle of the 1550 nm laser was fixed at 0°, one of the incident angles of the 635 nm and 850 nm lasers was fixed at 3°, and the other was varied from 3° degrees to 15°. These results are demonstrated in Section 3.1.

The three laser beams were shaped into a square through an aperture mask (6.0 × 6.0 mm²) and simultaneously irradiated onto the solar cells.

Triple-junction solar cells achieve maximum photoelectric conversion efficiency when current values generated by the three cells are equal. Therefore, each laser output was adjusted so that current values in each cell were equal. The number of electrons N_i generated in each cell is shown in Equation (1) as the external quantum efficiency EQE(λ), the laser incident power P_λ, and the photon energy E_λ of the irradiated laser.

N_i = (P_λ/E_λ) × EQE(λ)(1)

From this formula, the power ratio of each laser was calculated so that the number of electrons generated in each sub-cell was equal. The EQE(λ) at each laser wavelength was EQE₆₃₅ = 78.2%, EQE₈₅₀ = 84.7%, and EQE₁₅₅₀ = 64.4%, respectively. These values were obtained from the wavelength dependence of EQE, which are demonstrated in Section 4.2.

Using Equation (1), the power ratio of each laser is shown by Equation (2).

P₆₃₅:P₈₅₀:P₁₅₅₀ = 2.0:1.4:1.0(2)

Each laser power was adjusted by fixing the power of the 1550 nm laser due to its limited output. The 850 nm laser’s power was set lower than the power ratio in Equation (2), while the 635 nm laser’s power was set conversely higher. First, the 635 nm and 1550 nm lasers’ powers were fixed, and the 850 nm laser’s power was gradually increased. Second, the 850 nm laser’s power was fixed when the photoelectric conversion efficiency was at its highest. Next, the 850 nm and 1550 nm lasers’ powers were fixed, and the 635 nm laser’s power was gradually reduced. The 635 nm laser’s power was then fixed when the photoelectric conversion efficiency was at its highest.

3. Results

3.1. Laser Incidence Angle Dependence

Figure 3 show the laser incident dependencies of η_pv and I_sc, V_oc, and FF in a triple-junction solar cell under three laser beam irradiations with a total laser power of 196 mW [27]. The conversion efficiency η_pv values at both 635 nm and 850 nm decreased with an increased incident angle. The decrease in η_pv at 635 nm was due to decreases in I_sc and FF. However, the decrease η_pv at 850 nm was mainly due to decreases in FF. It is thought that this difference in I_sc between 635 nm and 850 nm was caused by the incident angle dependence of reflectance at the incident laser wavelength.

In any case, the decrease in η_pv with the incident angle from 3° to 7° was suppressed to a low absolute value. In this OWPT system with a triple-junction solar cell and three lasers, the incident laser angle might be allowed up to 7° without a significant decrease in the η_pv.

3.2. Photoelectric Conversion Efficiency

Figure 4 shows a graph of the photoelectric conversion efficiency η_pv versus total incident light power. Although the value of η_pv at 100 mW/cm² under simulated sunlight irradiation was 33.1%, the value of η_pv under simulated sunlight with an intensity of 523.6 mW (corresponding to 11.0 sun) was 37.3%. These efficiencies were measured using a concentrated optical system using a xenon lamp, AM1.5G filter, ND filter, condensing lens, and homogenizer. On the other hand, under three laser irradiations, a maximum photoelectric conversion efficiency of 45.0% was obtained at P_in = 636.2 mW (1.77 W/cm²), composed of 635 nm laser power of 336.0 mW, 850 nm laser power of 180.0 mW, and 1550 nm laser power of 120.2 mW. Figure 5 shows the I-V curve with 45.0% efficiency, which exhibited values of 119.1 mA/I_sc, 2.861 V/V_oc, and 84.0%/FF.

Comparing the η_pv values under simulated sunlight irradiation and laser beam irradiation, it was found that the η_pv under laser beam irradiation was more than 7% higher than it was under simulated sunlight. This conversion efficiency of 45.0% is comparable to values obtained for other multi-junction cells using the same materials, as shown in Table 1, and is a unique value obtained by simultaneously irradiating multiple wavelengths.

4. Discussion

4.1. Photoelectric Conversion Efficiency

Figure 4 shows that η_pv increased as the laser incident power increased. The photoelectric conversion efficiency η_pv can be shown by Equation (3).

η_pv = P_out/P_in = I_sc × V_oc × FF(3)

In this equation, I_sc is the short circuit current, V_oc is the open circuit voltage, and FF is the fill factor. Figure 6 shows graphs of I_sc, V_oc, and FF versus laser incident power.

Figure 6a shows that the value of I_sc increased proportionately to the laser incident power. This indicates that the EQE of the three sub-cells remained unchanged up to a laser incident power of 0.6 W and that current matching with the three lasers can be reproduced up to an incident power of 0.6 W. Figure 6b shows that V_oc increased in proportion to the logarithm of the laser incident power. This occurred because V_oc has the property of increasing in proportion to the logarithm of I_sc. Figure 6c shows that the value of FF was almost constant in the incident power. The reason for this constant FF is considered to be the thick film thickness of the surface finger electrode. For solar cells for solar illumination, the FF tends to decrease as irradiation intensity increases due to Joule heat losses caused by increased generated current. However, the prepared solar cell is a concentrator cell with a 6.5 μm thick surface electrode designed to suppress FF degradation under high-intensity irradiation.

Optical wireless power transmission using InGaP/InGaAs/Ge triple-junction solar cells is expected further to improve η_pv at a higher incident power, above 0.6 W, because I_sc, V_oc, and FF showed no decreasing trend up to an incident power of 0.6 W (1.77 W/cm²).

4.2. Current-Matching

The incident powers of the 635 nm and 850 nm lasers in current matching were obtained at about 2.8 and 1.5 times the incident power of the 1550 nm laser, respectively. However, these values were larger than the calculated power ratio given by Equation (2). This was because some of the 635 nm and 850 nm laser lights passed through to the next bottom sub-cell without being absorbed in InGaP and InGaAs sub-cells. Figure 7 shows the external quantum efficiency (EQE) of a cell with the same structure as the solar cell we used, which was measured using a spectral sensitivity-measuring device (CEP-25 ML, BUNKOUKEIKI, Tokyo, Japan). Solid lines show the EQE of each sub-cell, and dotted lines exhibit the center wavelengths of the three lasers used in this study. Focusing on the EQE of the InGaP cell, the EQE increased with an increasing wavelength of up to 600 nm.

On the other hand, from 600 nm onwards, the EQE decreased with increasing wavelength. This was also seen in other sub-cells, with InGaAs cells decreasing from around 830 nm. The EQE at 635 nm was 8.8% in the InGaAs cell and 19.9% at 850 nm in the Ge cell. This indicates that light was transmitted when light with energies close to the band gap was incident. For example, as the EQE of InGaP was low, at 635 nm, and the EQE of InGaAs was 8.8% at 635 nm, we estimate that about 5–10% of light might have been transmitted to the next sub-cell. When laser light is transmitted, the transmitted light is absorbed in the bottom sub-cell. Therefore, the 635 nm and 850 nm laser beams would have required a higher incident power than the calculated value. This result indicates that when simultaneously irradiating multi-junction solar cells with lasers of different wavelengths, the light transmission component in each sub-cell should be considered to match the currents generated by each sub-cell.

4.3. High-Temperature Operation

When installing this system in a car, using a water cooler is difficult. At high temperatures, the following two effects are expected to appear as the band-gap energy of each cell becomes narrower. First, as shown in Figure 7, EQE characteristics will shift to longer wavelengths, and the EQE value at each incident wavelength in each sub-cell will change (almost increase). Second, the open circuit voltage V_oc will decrease. These two effects are expected to change photoelectric conversion efficiency, and the results are significant, so we would like to take measurements at high temperatures next time in the near future.

4.4. Uniform and Non-Uniform Irradiation

Table 2 shows the intensity distributions of the 635 nm, 850 nm, and 1550 nm laser beams at about 60 mW. The 635 nm and 850 nm laser beam intensity distributions were measured with a CCD Camera Beam Profiler (BC106N-VIS/M, THOURLABS, Newton, NJ, USA), and the 1550 nm intensity distribution was measured with a Slit Beam Profiler (BP209-IR/M, THOURLABS, Newton, NJ, USA). The intensity distributions of the 635 nm and 1550 nm laser beams were non-uniform, while that of the 850 nm laser beam was close to uniform. The beam size of the incident light was about 36 mm² for all three laser beams. We discuss the phenomenon of reduced conversion efficiency in the case of multi-junction solar cells when the incident laser intensity has a non-uniform distribution in two dimensions using Figure 8.

When the peak positions of the 635 nm and 850 nm laser beams were misaligned, the positions of the generated carrier distributions in the InGaP sub-cell and the InGaAs sub-cell were also misaligned in two dimensions. In this case, carrier loss was caused by an imbalance between the number of electrons and holes at the tunnel junction interface and heat loss due to the lateral movement of electrons and holes near the tunnel junction. As a result, non-uniform light decreased the efficiency due to carrier and heat loss.

If three laser beams can be aligned and irradiated uniformly, further improvement in conversion efficiency can be expected. A method using a fly-eye lens has been proposed and demonstrated as a solution to this problem [28,29,30,31,32]. We also adopted this method and conducted experiments using the optical system shown in Figure 9 [28]. Three beams through two fly-eye lenses of 40 mm × 39 mm in size, a condenser lens, and the same sized metal mask as shown in Figure 1 were irradiated to an InGaP/InGaAs/Ge triple-junction solar cell, having a relatively uniform distribution. These fly-eye lenses had anti-reflection coatings for 400–700 nm. Therefore, the optical loss was as low as 5% for the 635 nm laser beam passed through a single fly-eye lens, but high for the 850 nm and 1550 nm laser beams.

Table 3 shows the intensity distribution with and without fly-eye lenses. These distributions of three laser intensities were measured at 60 mW for the 635 nm and 850 nm lasers and at 30 mW for the 1550 nm laser. As shown in Table 2, fly-eye lenses improved beam shapes at 635 nm and 850 nm [28]. However, the beam shape at 1550 nm still needed to be improved. This was due to the difference in focal lengths depending on the wavelength. Therefore, the fly-eye lens design should be developed to improve the uniformity of the three laser beams.

Although these optical loss problems were unresolved, measurements of conversion efficiencies with and without fly-eye lenses were possible because these problems did not interfere with evaluating the value of P_in/P_out. Measured efficiencies are plotted as a function of the total incident laser power in Figure 10 [28]. Conversion efficiency was found to be improved by the use of fly-eye lenses. However, the effect of the uniform three laser beam irradiations due to the fly-eye lenses was obtained as about 1.4–1.7% in efficiency. As the incident intensity of the laser increased, the effect of introducing the fly-eye lenses became progressively less effective. We speculate that this may have been due to the decrease in the uniformity of the laser distribution as the incident intensity increased. In addition, optical loss increased when fly-eye lenses were introduced into this optical system. The optical loss was estimated to be 18.8% for the 635 nm laser beam, 29.0% for the 850 nm beam, and 57.5% for the 1550 nm beam. These optical losses were measured by the ratio of the output intensity of the laser to the transmitted intensity of the laser light after passing through the optical system, which included a condenser lens and two fly-eye lenses. In particular, the anti-reflection coating on the fly-eye lenses was designed for visible light in the 400–700 nm range, resulting in significant reflection losses at 850 nm and 1550 nm. Therefore, an advanced optical system to make uniform light is necessary.

5. Conclusions

Optical wireless power transmission using In-InGaP/InGaAs/Ge triple-junction solar cells simultaneously irradiated with laser beams of 635 nm, 850 nm, and 1550 nm wavelengths was investigated. As a result, a photoelectric conversion efficiency of 45.0% was obtained at an incident power of P_in = 636 mW (1.77 W/cm²). It was found that a portion of the laser incident light on the triple-junction solar cell is considered to have transmitted through the next sub-cell. Each laser incident power must be adjusted to account for the transmittance of the laser light to achieve current matching. Based on this point, further efficiency improvement is expected by further high-intensity injection and optimization of the structure. These results indicate the possibility of optical wireless power transmission by irradiating multi-junction solar cells with lasers of different wavelengths, which is expected to be applied to automobiles as a new lighting system to complement solar power generation.

Author Contributions

Conceptualization, all authors; methodology, all authors; validation, S.U.; formal analysis, M.K.; investigation, M.K., S.S., N.M. and T.S.; data curation, M.K.; writing—original draft preparation, M.K.; writing—review and editing, S.U.; visualization, M.K.; project administration, S.U.; funding acquisition, S.U. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data that support the findings of this study are available from the corresponding author upon reasonable request. Data are not publicly available because the data size is large, and actual measurement data are diverse and complex.

Acknowledgments

The authors wish to acknowledge M. Yamaguchi from the Toyota Technological Institute and K. Araki from Miyazaki University for their assistance and encouragement.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

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Figures and Tables

Figure 1. Experimental configuration.

Figure 2. I-V curve of InGaP/InGaAs/Ge 3-junction solar cell at AM1.5G.

Figure 3. Laser incidence angle dependences of (a) ηpv and Isc, (b) Voc, and (c) FF [27].

Figure 4. Photoelectric conversion efficiencies under three laser irradiations and under simulated sunlight irradiation.

Figure 5. I-V curve of InGaP/InGaAs/Ge triple-junction solar cell under 636.2 mW laser irradiation.

Figure 6. Dependence of characteristic parameters of (a) Isc, (b) Voc, and (c) FF on incident laser power.

Figure 7. EQE of InGaP/InGaAs/Ge three-junction solar cell.

Figure 8. Images of generated carrier flows under non-uniform laser irradiations.

Figure 9. Experimental configuration with two fly-eye lenses.

Figure 10. Photoelectric conversion efficiencies under three laser irradiations with and without fly-eye lenses.

Table 1

Conversion efficiency of photovoltaic cells in various wavelength ranges.

Ref.	Wavelength (nm)	Cell	Incident Power	Distance (m)	ηpv (%)
[5]	1020	Si	1 W/cm2	-	45.1
[6]	858	GaAs	11.4 W/cm2	-	68.9
[7]	1310	InGaAsP	5.9 W/cm2	-	52.8
[8]	1550	InGaAs	-	-	36
[9]	1550	InGaAs/reflector	-	-	53.7
[10]	980	m*-InGaAs	42.9 W/cm2	-	55.2
[11]	1064	m*-InGaAs	14.2 W/cm2	-	54.7
[12]	532	InGaP	0.27 W/cm2	80	40
[13]	638	InGaP	1.5 W/cm2	0.3	46.7
[14]	635	InGaP	53.5 W/cm2	10	37.2
[15]	811	12J-GaAs	~40 W	-	61
[15]	1466	10J-InGaAs	69 W/cm2	-	51.1
[16]	1520	8J-InGaAs	53.9 W/cm2	-	36.9
[17]	1522	10J-InGaAs	-	-	45.6

(m*; metamorphic).

Table 2

Laser intensity distribution images.

	635 nm	850 nm	1550 nm
2D	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]
3D	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]

Table 3

Laser intensity distribution images with and without fly-eye lenses.

	635 nm (60 mW)	850 nm (60 mW)	1550 nm (60 mW)
w/o fly-eye lenses	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]
with fly-eye lenses	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]	[Image omitted. Please see PDF.]

References

1. Zhao, M.; Miyamoto, T. 1 W High Performance LED-Array Based Optical Wireless Power Transmission System for IoT Terminals. Photonics; 2022; 9, 576. [DOI: https://dx.doi.org/10.3390/photonics9080576]

2. Qaraqe, M.; Usman, M.; Serbes, A.; Ansari, I.S.; Alouini, M.S. Power Hotspots in Space: Powering CubeSats via Inter Satellite Optical Wireless Power Transfer. IEEE Internet Things Mag.; 2022; 5, pp. 180-185. [DOI: https://dx.doi.org/10.1109/IOTM.001.001.2200071]

3. Baraskar, A.; Yoshimura, Y.; Nagasaki, S.; Hanada, T. Space solar power satellite for the Moon and Mars mission. J. Space Saf. Eng.; 2022; 9, pp. 96-105. [DOI: https://dx.doi.org/10.1016/j.jsse.2021.10.008]

4. Landis, G.A. Laser Power Beaming for Lunar Polar Exploration. Proceedings of the 2020 AIAA Propulsion & Energy Forum and Exposition; Online, 24–26 August 2020; Paper AIAA-2020–3538 [DOI: https://dx.doi.org/10.2514/6.2020-3538]

5. Green, M.A.; Zhao, J.; Wang, A.; Wenham, S.R. 45% efficient silicon photovoltaic cell under monochromatic light. IEEE Electron Device Lett.; 1992; 13, pp. 317-318. [DOI: https://dx.doi.org/10.1109/55.145070]

6. Helmers, H.; Lopez, E.; Höhn, O.; Lackner, D.; Schön, J.; Schauerte, M.; Schachtner, M.; Dimroth, F.; Bett, A.W. 68.9% efficient GaAs-based photonic power conversion enabled by photon recycling and optical resonance. Phys. Status Solidi RRL Rapid Res. Lett.; 2021; 15, 2100113. [DOI: https://dx.doi.org/10.1002/pssr.202100113]

7. Beattie, M.N.; Helmers, H.; Forcade, G.P.; Valdivia, C.E.; Höhn, O.; Hinzer, K. InP-and GaAs-based photonic power converters under O-band laser illumination: Performance analysis and comparison. IEEE J. Photovolt.; 2022; 13, pp. 113-121. [DOI: https://dx.doi.org/10.1109/JPHOTOV.2022.3218938]

8. Fafard, S.; Masson, D.; Werthen, J.G.; Liu, J.; Wu, T.C.; Hundsberger, C.; Schwarzfischer, M.; Steinle, G.; Gaertner, C.; Piemonte, C. et al. Power and Spectral Range Characteristics for Optical Power Converters. Energies; 2021; 14, 4395. [DOI: https://dx.doi.org/10.3390/en14154395]

9. Helmers, H.; Schauerte, M.; Höhn, O.; Beattie, M.N.; Wilson, D.P.; Hinzer, K.; Lackner, D. C-band Photonic Power Converters based on InGaAs Absorber in Substrate and Thin-film & Single- and Multi Junction Configurations. Proceedings of the 5th Optical Wireless and Fiber Power Transmission Conference, OWPT2-02; Yokohama, Japan, 18–21 April 2023.

10. Helmers, H.; Höhn, O.; Lackner, D.; Schygulla, P.; Klitzke, M.; Schön, J.; Pellegrino, C.; Oliva, E.; Schachtner, M.; Beutel, P. et al. Advancing solar energy conversion efficiency to 47.6% and exploring the spectral versatility of III-V pho tonic power converters. Phys. Simul. Photonic Eng. Photovolt. Devices XIII SPIE; 2024; 12881, pp. 6-15. [DOI: https://dx.doi.org/10.1117/12.3000352]

11. Pellegrino, C.; Helmers, H.; Ohlmann, J.; Höhn, O.; Lackner, D. High-Efficiency 1064 nm Metamorphic Photonic Power Converters for Spacecraft Wireless Power Transfer. Proceedings of the 13th European Space Power Conference; Elche, Spain, 2–6 October 2023; [DOI: https://dx.doi.org/10.1109/ESPC59009.2023.10298134]

12. Steinsiek, F.; Weber, K.H.; Foth, W.P.; Foth, H.J.; Schafer, C. Wireless power transmission experiment using an airship as relay system and a moveable rover as ground target for later planetary exploration missions. Proceedings of the 8th ESA Workshop on Advanced Space Technologies for Robotics and Automation; Noordwijk, The Netherlands, 2–4 November 2004; pp. 1-10.

13. Kurooka, K.; Honda, T.; Komazawa, Y.; Warigaya, R.; Uchida, S. A 46.7% efficient GaInP photonic power converter under high-power 638 nm laser uniform irradiation of 1.5 W cm⁻². Appl. Phys. Express; 2022; 15, 062003. [DOI: https://dx.doi.org/10.35848/1882-0786/ac67bb]

14. Wong, Y.L.; Shibui, S.; Koga, M.; Hayashi, S.; Uchida, S. Optical Wireless Power Transmission Using a GaInP Power Converter Cell under High-Power 635 nm Laser Irradiation of 53.5 W/cm². Energies; 2022; 15, 3690. [DOI: https://dx.doi.org/10.3390/en15103690]

15. Fafard, S.; Masson, D. Vertical Multi-Junction Laser Power Converters with 61% Efficiency at 30 W Output Power and with Tolerance to Beam Non-Uniformity, Partial Illumination, and Beam Displacement. Photonics; 2023; 10, 940. [DOI: https://dx.doi.org/10.3390/photonics10080940]

16. Wang, A.C.; Sun, Y.R.; Yu, S.Z.; Yin, J.J.; Zhang, W.; Wang, J.S.; Fu, Q.X.; Han, Y.H.; Qin, J.; Dong, J.R. Characteristics of 1520 nm InGaAs multijunction laser power converters. Appl. Phys. Lett.; 2022; 119, 243902. [DOI: https://dx.doi.org/10.1063/5.0073806]

17. Helmers, H.; Hohn, O.; Tibbits, T.; Schauerte, M.; Noman Amin, H.M.; Lackner, D. Unlocking 1550 nm Laser Power Conversion by InGaAs Single- and Multi-Junction PV Cells. Proceedings of the IEEE 49th Photovoltaics Specialists Conference; Philadelphia, PA, USA, 5–10 June 2022.

18. Prete, P.; Lovergine, N. Dilute nitride III-V nanowires for high-efficiency intermediate-band photovoltaic cells: Materials requirements, self-assembly methods and properties. Prog. Cryst. Growth Charact. Mater.; 2020; 66, 100510. [DOI: https://dx.doi.org/10.1016/j.pcrysgrow.2020.100510]

19. Prete, P.; Wolf, D.; Marzo, F.; Lovergine, N. Nanoscale spectroscopic imaging of GaAs-AlGaAs quantum well tube nanowires: Correlating luminescence with nanowire size and inner multishell structure. Nanophotonics; 2019; 8, pp. 1567-1577. [DOI: https://dx.doi.org/10.1515/nanoph-2019-0156]

20. Yang, I.; Zhang, X.; Zheng, C.; Gao, Q.; Li, Z.; Li, L.; Lockrey, M.N.; Nguyen, H.; Caroff, P.; Etheridge, J. et al. Radial Growth Evolution of InGaAs/InP Multi-Quantum-Well Nanowires Grown by Selective-Area Metal Organic Vapor-Phase Epitaxy. ACS Nano; 2018; 12, pp. 10374-10382. [DOI: https://dx.doi.org/10.1021/acsnano.8b05771]

21. LaPierre, R.R. Theoretical conversion efficiency of a two-junction III-V nanowire on Si solar cell. J. Appl. Phys.; 2011; 110, 014310. [DOI: https://dx.doi.org/10.1063/1.3603029]

22. Green, M.A.; Dunlop, E.D.; Yoshita, M.; Kopidakis, N.; Bothe, K.; Siefer, G.; Hao, X. Solar cell efficiency tables (version 62). Prog. Photovolt. Res. Appl.; 2023; 31, pp. 651-663. [DOI: https://dx.doi.org/10.1002/pip.3726]

23. Schygulla, P.; Müller, R.; Höhn, O.; Schachtner, M.; Chojniak, D.; Cordaro, A.; Dimroth, F. Wafer-Bonded Two-Terminal III-V//SI Triple-Junction Solar Cell with Power Conversion Efficiency of 36.1% at AM1. 5G. Prog. Photovolt. Res. Appl.; 2024; [DOI: https://dx.doi.org/10.1002/pip.3769]

24. Dimroth, F.; Guter, W.; Schöne, J.; Welser, E.; Steiner, M.; Oliva, E.; Bett, A.W. Metamorphic GaInP/GaInAs/Ge triple-junction solar cells with ≫ 41% efficiency. Proceedings of the 2009 34th IEEE Photovoltaic Specialists Conference (PVSC); Philadelphia, PA, USA, 7–12 June 2009; pp. 001038-001042. [DOI: https://dx.doi.org/10.1109/PVSC.2009.5411199]

25. Yamaguchi, M.; Takamoto, T.; Araki, K.; Kojima, N. Recent results for concentrator photovoltaics in Japan. Jpn. J. Appl. Phys.; 2016; 55, 04EA05. [DOI: https://dx.doi.org/10.7567/JJAP.55.04EA05]

26. Yamaguchi, M.; Masuda, T.; Araki, K.; Ota, Y.; Nishioka, K. Impact and recent approaches of high-efficiency solar cell modules for PV-powered vehicles. Jpn. J. Appl. Phys.; 2022; 61, SC0802. [DOI: https://dx.doi.org/10.35848/1347-4065/ac461b]

27. Koga, M.; Shibui, S.; Omori, S.; Takahashi, R.; Matsuoka, N.; Uchida, S. Incident laser angle dependence on the conversion efficiency of 3-juncton solar cells for optical wireless power transmission. Proceedings of the International Photovoltaic Science and Engineering Conference, TuP-22-20; Nagoya, Japan, 13–17 November 2022.

28. Koga, M.; Matsuoka, N.; Shibui, S.; Uchida, S. Investigation of high efficiency laser wireless power transmission using InGaP/InGaAs/Ge 3-junction solar cells. Proceedings of the 4th Optical Wireless and Fiber Power Transmission Conference, OWPT8-03; Online, 18–20 April 2022.

29. Asaba, K.; Miyamoto, T. Relaxation of Beam Irradiation Accuracy of Cooperative Optical Wireless Power Transmission in Terms of Fly Eye Module with Beam Confinement Mechanism. Photonics; 2022; 9, 995. [DOI: https://dx.doi.org/10.3390/photonics9120995]

30. Katsuta, Y.; Miyamoto, T. Design, simulation and characterization of fly-eye lens system for optical wireless power transmission. Jpn. J. Appl. Phys.; 2019; 58, SJJE02. [DOI: https://dx.doi.org/10.7567/1347-4065/ab238b]

31. Tai, Y.; Miyamoto, T. Experimental Characterization of High Tolerance to Beam Irradiation Conditions of Light Beam Power Receiving Module for Optical Wireless Power Transmission Equipped with a Fly-Eye Lens System. Energies; 2022; 15, 7388. [DOI: https://dx.doi.org/10.3390/en15197388]

32. Asaba, K.; Moriyama, K.; Miyamoto, T. Preliminary Characterization of Robust Detection Method of Solar Cell Array for Optical Wireless Power Transmission with Differential Absorption Image Sensing. Photonics; 2022; 9, 861. [DOI: https://dx.doi.org/10.3390/photonics9110861]