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Abstract
Laser Power Converters (LPCs) are components of the laser wireless power transmission (LWPT) system receiving laser power. This paper proposes a comprehensive test method that employs continuous, pulse-pause, and short-time techniques to evaluate the performance of six-junction GaAs LPCs operating with an optical input at 808 nm. Additionally, we investigate the performance of LPCs with different areas and achieve a conversion efficiency over 60%. Furthermore, we apply LPCs with varying areas to wireless information transmission and successfully achieve a response time of 1.7 µs.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Laser wireless power transmission technology has gained significant prominence across various fields, owing to its remarkable attributes such as high power density and long-distance transmission capabilities [1–3]. The laser wireless power transmission system mainly consists of a laser at the transmitting end and LPCs at the receiving end. The efficiency of LPCs plays a crucial role in determining the overall optoelectronic performance of the system [4,5].
In recent years, vertically-connected multi-junction LPCs have garnered considerable attention due to their immense potential for high performance [6–9]. In 2022, our research team reported the development of six-junction LPCs, featuring an area of 1 cm2 by continuous laser irradiation testing, achieving an impressive efficiency of 50% and an output power of 15 W [10]. Furthermore, in the same year, Simon et al. reported the results of their study on five-junction LPCs, exhibiting an area of 0.03 cm2, an efficiency of 74.7% attained through cryogenic testing conducted at 150 K [11].
This paper investigates the device performance of LPCs with varying areas under continuous, pulse-pause, and short-time test conditions, resulting in an efficiency over 60% with an area of 0.25 cm2. Additionally, we apply these LPCs to wireless information transmission, achieving a response time of 1.7 µs.
2. Materials and methods
The photovoltaic vertical six-junction structure was designed to operate at 808 nm, and the Beer-Lambert law is used to calculate the absorber thickness of each subcell with an absorption coefficient of 14,000 cm-2 [8], where each subcell absorbs approximately 1/6 of the incident laser. Specifically, the thickness of the GaAs absorber subcell used here is 147 nm, 180 nm, 210 nm, 327 nm, 559 nm, and 2555 nm.
Each subcell has a Si-doped GaAs emitter and a C-doped GaAs base sandwiched between the InGaP window and back surface field layers. The p++-AlGaAs/n++-AlGaAs tunnel junctions connect six subcells vertically and transparently to the input laser beam [10].
The epitaxial layers are grown using commercial production Aixtron Metal Organic Chemical Vapor Deposition (MOCVD) reactors. Trimethylindium (TMIn) and trimethylgallium (TMGa) were utilized as the sources for group III elements, while AsH3 and PH3 were used as the sources for group V elements. The GaAs LPC wafers were grown at a temperature of 690°C and a pressure of 50 mbar, with C (CBr4 source), Si (Si2H6 source) and (or) Te (DeTe source) were used as p-type and n-type dopants, respectively [12].
The LPC wafers were processed into chips with a side length of 1 and 0.5 cm, as shown in Fig. 1, with TiO2/SiO2 dual-layer as anti-reflection coatings (ARCs), the measured surface reflectivity of the reference LPCs around 808 nm was less than 1%. LPCs were placed on copper-plated ceramic heat sinks with silver paste between them to improve thermal conductance.
The I-V characteristics of the LPCs were obtained using a Keithley 2601B source-meter. Continuous, pulse-pause, and short-time test methods were employed to evaluate the performance of the LPCs. Short-time tests were completed within 1 s, and pulse-pause tests with a duration of 5 ms and pauses ranging from 0 to 60000 ms were performed. This approach allowed for efficient data acquisition while minimizing potential temperature-related distortions [11,13,14]. The temperature of the LPCs was precisely controlled using a heating plate with a high level of accuracy, achieving temperature regulation within ± 0.5°C. This meticulous control ensured consistent and reliable temperature conditions throughout the experimental process.
3. Result and discussion
The pulse-pause test involves controlling the duration of laser irradiation on the LPC and the pause time between irradiation cycles. Its purpose is to effectively dissipate heat within the LPC, achieving a lower thermal equilibrium and enabling the LPC to approach its theoretical performance limit under ideal conditions. However, the heat generated varies depending on the incident laser power. Therefore, it is crucial to experimentally determine an appropriate pulse-pause duty cycle that maximizes heat dissipation at a specific incident laser power.
To assess the effectiveness of the pulse-pause test method in dissipating heat at different incident laser powers, we experimented with varying pulse-pause duty cycles. Suppose the pulse-pause test can dissipate the heat at higher incident laser power. In that case, the pulse-pause duty cycle test is also able to dissipate the heat brought about by the lower incident laser power to the LPC, so the I-V curve of the LPC was tested at five different pulse-pause duty cycles at a high incident laser power of 8 W. The LPC parameters were plotted and extracted in Fig. 2 and Table 1, respectively. The onset of the saturation is not resolved very well due to the fluctuations in the incident laser power at pulse-pause test mode, as shown in Fig. 2 (c) and (d).
Fig. 2. Under pulse-pause test conditions. (a) IV curves for different pause times. (b) Pause time versus open-circuit voltage. (c) Pause time versus short-circuit current. (d) Pause time versus conversion efficiency.
Table 1. Pulse-pause test of 808 nm LPCs parameters under different pause conditions
Figure 2(a) illustrates the I-V curves obtained for different pulse-pause duty cycles. Each duty cycle represents the duration of laser irradiation and the corresponding pause time. For instance, “5-0” signifies that the laser irradiates for 5 ms, one IV point is tested, and there is no pause before testing the next data point. “5-1000” indicates that the laser irradiates for 5 ms, one point is tested, and then there is a pause of 1000 ms to dissipate heat before the subsequent test, resulting in a duty cycle of 0.498%. Similarly, “5-10000”, “5-30000”, and “5-60000” correspond to pulse-pause duty cycles of 0.050%, 0.017%, and 0.008%, respectively.
Analysis of the I-V curves reveals that, under the same test conditions, the pulse-pause test demonstrates greater improvement than the continuous test (5-0), particularly in the open-circuit voltage. Table 1 provides a comprehensive overview: At an incident laser power of 8 W, the pulse-pause test exhibits an efficiency nearly 2% higher than the continuous test, with an open-circuit voltage that is 0.104 V higher and a short-circuit current that is 23 mA higher.
The I-V curves for tests conducted at pulse-pause duty cycles of 0.498%, 0.050%, 0.017%, and 0.008% at laser power nearly overlap, suggesting that the pulse-pause test at a duty cycle of 0.498% sufficiently dissipates the heat generated by 8 W laser irradiation on the LPC. Figures 2(b), 2(c), and 2(d) demonstrate that when the pulse-pause duty cycle is 0.498%, the Voc, Isc, and η exhibit significant improvements compared to the continuous test. However, when the pause time is further increased to allow for additional heat dissipation, theoretically leading to increased Voc, Isc, and η values, the actual measurements show no substantial changes. This indicates that, under the current test conditions, the heat generated by 8 W laser irradiation on the LPC for 5 ms dissipates within a pause of 1000 ms. Hence, a duty cycle of 0.498% is sufficient for conducting the pulse-pause test at 8 W.
The performance of the LPC was evaluated using three different test methods: continuous test, short-time test, and pulse-pause test (duty cycle of 0.498%) at various incident laser powers. Analysis of the I-V curves in Fig. 3(a) reveals noticeable differences in device performance when measured using different test methods at the same incident laser power.
Fig. 3. (a) Comparison of IV curves for different powers under three test methods. (b) Relationship between incident power and photoelectric conversion efficiency under three test methods. (c) Relationship between incident power and short-circuit current under three test methods. (d) Relationship between incident power and open-circuit voltage under three test methods.
Compared to continuous testing, short-time testing increases open-circuit voltage and short-circuit current. Additionally, the pulse-pause test produces greater open-circuit voltage and short-circuit current enhancement than the other two test methods. The results are shown in Fig. 3, and the corresponding parameters are extracted and summarized in Table 2. Figure 3(b) shows the efficiency improvement achieved through the pulse-pause test, particularly at low incident laser power. For instance, the efficiency increases from 45.94% to 55.82% at an incident power of 1.28 W. Meanwhile, Fig. 3(d) demonstrates the significant boost in open-circuit voltage achieved through the pulse-pause test compared to the continuous test. Under continuous test conditions, the ability to dissipate heat is reduced, leading to a decrease in Voc and FF, which in turn reduces efficiency.
Table 2. Comparison of LPC parameters at different incident laser powers for the three test methods
Two types of LPCs with dimensions of 1 cm × 1 cm and 0.5 cm × 0.5 cm were fabricated to investigate the influence of LPC area on device performance. Pulse-pause tests (duty cycle of 0.498%) were conducted at various incident laser powers, and the corresponding I-V curves are presented in Fig. 4, while the extracted parameters are summarized in Table 3. Figure 4(a) reveals that the LPC with a 0.5 cm × 0.5 cm area performs better for the same incident laser power, exhibiting greater enhancements in both Isc and Voc. This suggests that reducing the LPC area can yield improved performance characteristics. Figure 4(b) demonstrates that the efficiency of the LPC varies with the incident laser power, following a similar trend for both area sizes. The LPC with a 0.25 cm2 area achieves an impressive efficiency over 60%. Under this area condition, the efficiency of this paper are at a high level when compared with the reported results [15]. Figure 4(c) illustrates that Isc increases rapidly with incident laser power for the LPC with a 0.25 cm2 area, indicating its superior current generation capabilities. Additionally, Fig. 4(d) compares the variation in Voc between the two LPCs at different powers. The smaller device area is observed to correspond to a lower reverse saturation current (I0). According to Eq. (1), this leads to a higher open-circuit voltage for the LPC with a 0.25 cm2 area.
For a LPC with junction number N, the current can be expressed as:
Fig. 4. (a) Comparison of IV curves for different powers under two LPC area conditions. (b) Incident power versus photoelectric conversion efficiency under two LPC area conditions. (c) Incident power versus short-circuit current under two LPC area conditions. (d) Incident power versus open-circuit voltage under two LPC area conditions.
Table 3. Comparison of parameters with different powers for different LPC areas
The values of series resistance (Rs) and shunt resistance (Rsh) can be determined through a fitting process. Subsequently, the ideal factor (n), can be obtained by substituting the values of Is and Rsh into Eq. (3).
The I-V curves of both the 0.25 cm2 and 1 cm2 area LPCs were subjected to pulse-pause measured. These curves were then fitted to determine the values of Is, Rs, Rsh, and n, as depicted in Fig. 5. The reverse saturation current for the 0.25 cm2 LPC is an order of magnitude lower than that of the 1 cm2 device, indicating that decreasing the device area results in a decrease in dark current. Furthermore, the series resistance of the 0.25 cm2 device is smaller, and the parallel resistance is greater compared to the 1 cm2 device, resulting in reduced leakage current. It is worth noting that the value of the ideal factor is close to 2 for both the 1 cm2 and the 0.25 cm2 devices, and the ideal factor for the 1 cm2 device is 1.88, which is smaller than the 1.96 for the 0.25 cm2 device.
Fig. 5. (a) 0.5 cm*0.5 cm LPC parameters. (b) 1 cm*1 cm LPC parameters, the inserted picture shows the shape of the beam.
The response time of an LPC plays a critical role in signal detection. A slower response time introduces delays in these processes, limiting the data transmission rate. Moreover, regarding signal distortion and crosstalk, the response time is closely linked to the rise and fall time of the signal. In high-speed communication systems, where the signal rise and fall times are extremely short, a prolonged response time of the LPC can result in inaccurate tracking of input signal variations. Consequently, this discrepancy can lead to signal distortion and crosstalk.
In this study, we investigate the response time of LPCs with varying areas and power densities. Response time is defined as the time required from 10% to 90% of the rising edge of the response voltage curve of the LPC’s output. Figure 6 presents the modulated signal waveforms of LPCs receiving optical power densities of 80 mW/cm2, with LPC areas of 0.10 cm2, 0.25 cm2, and 1.00 cm2, corresponding to response times of 1.7 µs, 1.8 µs, and 2.0 µs, respectively. The noise level is less than 5 × 10−3. Additionally, when the LPCs receive an optical power density of 110 mW/cm2, the response times of the modulated signal waveforms were 1.8 µs, 2.0 µs, and 2.1 µs, respectively (not shown in the figure). These results are summarized in Table 4. We observe that smaller LPC areas facilitate faster collection processes by comparing LPCs receiving the same optical power density but with different areas. Moreover, the smaller area reduces capacitance, enabling quicker charge-charging and discharging processes, thus accelerating the response time. It is important to note that when the laser excites electrons and holes, these carriers require time to move from the space charge region to the electrode region. Furthermore, within the same LPC area in Table 4, lower laser power densities lead to reduced carrier generation, thereby shortening the time required for charge transfer and resulting in a shorter response time.
Table 4. Comparison of response times for different areas and laser power conditions
4. Conclusion
This study provides test methods for investigating the performance of the six-junction GaAs LPCs with an optical input at 808 nm that uses continuous, pulse-pause, and short-time techniques. The performance of LPC in various areas was also examined, yielding a conversion efficiency over 60%. Moreover, we employ LPCs with varying areas for wireless information transmission, resulting in a response time of 1.7 µs obtained.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. C. Algora, I. García, M. Delgado, R. Peña, C. Vázquez, M. Hinojosa, and I. Rey-Stolle, “Beaming power: Photovoltaic laser power converters for power-by-light,” Joule 6(2), 340–368 (2022). [CrossRef]
2. K. Jin and W. Zhou, “Wireless laser power transmission: A review of recent progress,” IEEE Trans. Power Electron. 34(4), 3842–3859 (2019). [CrossRef]
3. P. Sprangle, B. Hafizi, A. Ting, and R. Fischer, “High-power lasers for directed-energy applications,” Appl. Opt. 54(31), F201–F209 (2015). [CrossRef]
4. S. Fafard, D. Masson, J.-G. Werthen, J. Liu, T.-C. Wu, C. Hundsberger, M. Schwarzfischer, G. Steinle, C. Gaertner, and C. Piemonte, “Power and spectral range characteristics for optical power converters,” Energies 14(15), 4395 (2021). [CrossRef]
5. S. Fafard and D. P. Masson, “Perspective on photovoltaic optical power converters,” J. Appl. Phys. 130(16), 160901 (2021). [CrossRef]
6. M. C. York and S. Fafard, “High efficiency phototransducers based on a novel vertical epitaxial heterostructure architecture (VEHSA) with thin p/n junctions,” J. Phys. D: Appl. Phys. 50(17), 173003 (2017). [CrossRef]
7. S. Fafard, F. Proulx, M. C. York, L. Richard, P.-O. Provost, R. Arès, V. Aimez, and D. P. Masson, “High-photovoltage GaAs vertical epitaxial monolithic heterostructures with 20 thin p/n junctions and a conversion efficiency of 60%,” Appl. Phys. Lett. 109(13), 131107 (2016). [CrossRef]
8. D. Masson, F. Proulx, and S. Fafard, “Pushing the limits of concentrated photovoltaic solar cell tunnel junctions in novel high-efficiency GaAs phototransducers based on a vertical epitaxial heterostructure architecture,” Progress in Photovoltaics 23(12), 1687–1696 (2015). [CrossRef]
9. C. Guan, L. Li, H.-M. Ji, S. Luo, P. Xu, Q. Gao, H. Lv, and W. Liu, “Fabrication and characterization of a high-power assembly with a 20-junction monolithically stacked laser power converter,” IEEE J. Photovoltaics 8(5), 1355–1362 (2018). [CrossRef]
10. Y. Gou, H. Wang, J. Wang, R. Niu, X. Chen, B. Wang, Y. Xiao, Z. Zhang, W. Liu, H. Yang, and G. Deng, “High-performance laser power converts for direct-energy applications,” Opt. Express 30(17), 31509–31517 (2022). [CrossRef]
11. S. Fafard and D. P. Masson, “74.7% Efficient GaAs-Based Laser Power Converters at 808 nm at 150 K,” Photonics 9(8), 579 (2022). [CrossRef]
12. Y. Gou, H. Wang, J. Wang, H. Yang, and G. Deng, “High performance p++-AlGaAs/n++-InGaP tunnel junctions for ultra-high concentration photovoltaics,” Opt. Express 30(13), 23763–23770 (2022). [CrossRef]
13. H. Helmers, E. Lopez, O. Höhn, D. Lackner, J. Schön, M. Schauerte, M. Schachtner, F. Dimroth, and A. W. Bett, “68.9% Efficient GaAs-Based Photonic Power Conversion Enabled by Photon Recycling and Optical Resonance,” Physica Rapid Research Ltrs 15(7), 2100113 (2021). [CrossRef]
14. S. Reichmuth, H. Helmers, C. Garza, D. Vahle, M. De Boer, L. Stevens, M. Mundus, A. Bett, and G. Siefer, “Transient IV measurement set-up for photovoltaic laser power converters under monochromatic irradiance,” in 32nd European Photovoltaic Solar Energy Conference, (2016), 5–10.
15. S. Fafard and D. Masson, “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 10(8), 940 (2023). [CrossRef]
