Open Access
Add to BibSonomy
Abstract
High-performance laser power converters are crucial for laser wireless power transmission systems. Through the optimization of the resistive thermal annealing temperature applied to the laser power converter, the conversion efficiency reaches 55.0%. For 830 nm laser irradiation, the conversion efficiency further elevates to 59.3%. The potential for improvement remains substantial, with an anticipated increase to 63.8% achievable through the optimization of current matching at this specific wavelength. Moreover, the reliability of the laser power converter is demonstrated by its ability to 1,000 hours of operation at an elevated temperature of 180°C.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Laser wireless power transmission is an emerging technology that uses laser as a carrier to realize long-distance transmission of electrical energy [1]. This technology achieves cable-free and electromagnetic isolation and has broad application prospects [2]. Laser wireless power transmission technology can rapidly rebuild the power supply for disaster relief sites and local battlefield equipment [3]. It can also provide continuous wireless power supply for equipment such as unmanned aerial vehicles and space vehicles [4]. The considerable promise of this technology has garnered substantial attention from scholars globally [5]. In recent years, as laser wireless power transmission technology has progressed, the primary impediment to optimizing the efficiency of these systems lies in the performance of laser power converters at the receiving end.
Laser power converters are systematically categorized based on substrates: GaAs-based [6,7], Ge-based [8], InP-based [9], and organic [10]. Further classification is undertaken with respect to their structural composition: single junction, vertical multi-junction, horizontal multi-junction [2], and according to their output power: regular-power format, medium-power format, high-power format [11]. Additionally, they are distinguished by their wavelength range, spanning from 630 to 1550 nm [11]. Among these, the investigation into 8xx nm GaAs-based vertical cascade multi-junction laser power converters noteworthy. This configuration demonstrates a remarkable output power of 30 W [5], with the laser power converters, sized at 0.03 cm2, achieving an impressive conversion efficiency of 74.7% under the conditions of 150 K [12]. Notably, the conductive properties of the electrode play a pivotal role in determining the current extraction capability, thereby directly impacting the output performance of the laser power converters. Thermal annealing of the electrode material is critical in influencing the electrode resistance, and in particular, the temperature and time of thermal annealing are key factors affecting is these devices.
This study explores the impact of thermal annealing on the electrode resistance of laser power converters, aiming to decrease resistivity through the optimization of annealing temperatures. Consequently, the conversion efficiency of these laser power converters demonstrates noteworthy advancements, reaching 57.1% at 808 nm and 59.3% at 830 nm. Moreover, the reliability of the laser power converters is investigated by operating continuously for over 1,000 hours at an elevated temperature of 180°C.
2. Experimental
The GaAs vertical series six-junction structure is depicted in detail in the referenced work [3]. The determination of absorber thickness for each sub-cell is conducted employing Beer-Lambert's law. Notably, the absorption coefficient is 14,000 cm-2, with each sub-cell efficiently absorbing approximately 1/6 of the incident laser. Each individual sub-cell features a Si-doped GaAs emitter and a C-doped GaAs base, sandwiched between an InGaP window and a back surface field layer. A p++-AlGaAs/n++-AlGaAs tunnel junction facilitates the interconnection between the six sub-cells, establishing a vertical and transparency to the incident laser beam [13].
The epitaxial layers were grown using aixtron metal organic chemical vapor deposition (MOCVD) reactors [14]. Trimethylindium (TMIn) and trimethylgallium (TMGa) served as sources for group III elements, while AsH3 and PH3 were employed as sources for group V elements. The laser power converter wafers were grown at a temperature of 690°C and a pressure of 50 mbar. In the doping process, C (CBr4 source), Si (Si2H6 source), and/or Te (DeTe source) were utilized as p-type and n-type dopants, respectively.
The fabricated laser power converter device underwent conventional photovoltaic fabrication processes, including photolithography and wet etching. The front contact metal, consisting of Ti/Pt/Au, was deposited through thermal evaporation, while the back received the AuGe/Ni/Au contact metal deposition. A TiO2/SiO2 double layer is used as the anti-reflective coating (ARC), and the measured surface reflectance of the reference laser power converter is less than 1% at around 808 nm.
Laser power converters were placed on copper-plated ceramic heat sinks with silver paste between them to improve thermal conductance. A heating plate precisely controlled the laser power converter temperature with an accuracy of ±0.5°C. I-V measurements were performed using an 808 nm and 830 nm laser and a Keithley 2601B source meter.
3. Result and discussion
The ohmic resistivity of laser power converters metal electrodes and semiconductor materials directly determines the performance of laser power converter devices. Therefore, it is extremely important to study the impact of annealing conditions on laser power converters performance. The prevailing methodology for such investigations involves the transmission line model, wherein, under constant current conditions, the respective resistances are derived by measuring the voltage between contact points. Subsequently, various resistances, including parasitic resistance, are deducted from the overall resistance through judicious approximations and assumptions, yielding contact resistivity.
The transmission line model method comprises several categories, including the rectangular transmission line model, circular dot transmission line model, and circular transmission line model. Among these, the circular dot transmission line model, also known as the circular transmission line method (CTLM), holds a distinct advantage. This advantage lies in the simplicity of sample preparation, as it does not require the insulation of the sample. Moreover, the CTLM model exhibits minimal deviation between theoretical and actual values of contact resistivity. Hence, this study employs the dot transmission line model, specifically the CTLM, to characterize the contact interface between N-type doped GaAs and Ge/Au/Ni/Au.
In Fig. 1(a), the depicted sample structure comprises a layering of Ge (40 nm)/Au (80 nm)/Ni (35 nm)/Au (50 nm) evaporated onto the N-type doped GaAs substrate. To prevent the device from potential surface damage resulting from multiple tests during the testing process, a passivation layer consisting of Ti (50 nm)/Pt (100 nm)/Au (100 nm) is deposited on the surface. Figure 1(b) presents a visual image of the device, while Fig. 1(c) provides a group image within the depicted device. Each group comprises six distinct rings, with the fixed diameter (r0) of the inner circle maintained at 149.7 µm. The inter-circle distance, denoted as dTLM, varies from 8.9 µm to 39.5 µm between the inner and outer circles.
Fig. 1. (a) The device structure on N-type doped GaAs substrate for measuring the ohmic contact resistivity. (b) The image of the device. (c) One group of circular rings under a microscope. Each group comprises six distinct rings, with the fixed diameter (r0) of the inner circle maintained at 149.7 µm. The inter-circle distance, denoted as dTLM, varies from 8.9 µm to 39.5 µm between the inner and outer circles.
Utilizing the four-point probe testing method facilitates the measurement of voltage-current characteristics, enabling the calculation of resistance (R). In accordance with the circular transmission line method (CTLM) [15], the relevant equation can be expressed as follows.
In the given equation, R represents the total resistance, which can be obtained through the measurement of current and voltage across the sample using the four-probe method. RS corresponds to the sheet resistance, and r0 represents the diameter of the inner circle. LT denotes the transmission length, which refers to the length of the effective contact area extending from the gap. The calculation formula for LT is provided as Eq. (2):
where ρc represents the contact resistivity. When d<<r0, Eq. (1) can be simplified to Eq. (3):Based on Eq. (3), a linear regression can be applied, with the independent variable on the horizontal axis being ln(1 + d/r0), and the dependent variable on the vertical axis being R. The slope of this fitted line corresponds to RS/(2π), and the intercept is given by RSLT/(πr0). This regression analysis facilitates the calculation of the contact resistivity.
Three groups of samples were selected, and the annealing temperatures were conducted at 380°C, 400°C, and 420°C, respectively, and the annealing time was 45 minutes. Following the completion of the annealing process, the contact resistivity was assessed and computed utilizing the CTLM model fitting. The outcomes of these evaluations are graphically presented in Fig. 2, with detailed resistivity data provided in Table 1.
Fig. 2. The fitting results of the contact resistivity for the samples under different annealing conditions: (a) 420°C−45 min, (b) 380°C−45 min (c) 400°C−45 min. (d)Contact resistivity
Table 1. The contact resistivity and sheet resistance of the samples under different annealing conditions
Figure 2 demonstrates that the test data of the three sample groups exhibit good fitting results, showing the expected linear relationship. Combining with Table 1, it can be observed that the annealing condition of 400°C for 45 minutes yields the minimum contact resistivity of the annealed samples, reaching 6.5 × 10−7 Ω·cm2. This value indicates that the deposited metal material exhibits ideal ohmic contact with the N-type doped GaAs. Therefore, the subsequent device fabrication will adopt the annealing condition of 400°C for 45 minutes.
After the same batch of six-junction GaAs laser power converter wafers (after metallization) were annealed at 380°C and 400°C, they were tested under the same conditions and the distribution of device performance on the wafer was compared, as shown in Fig. 3, 44 devices were selected for testing at the same position on each wafer. It was found that the highest conversion efficiency was 51.0% with the annealing temperature at 380°C, while at 400°C it increased to 58.8%. The average value improved from 46.1% at 380°C to 55.0% at 400°C, and performance uniformity on the 400°C wafer also improved. This improvement may comes from the improvement of the fill factor (FF).
Fig. 3. Map and statistics of the FF and PCE for two six-juncttion GaAs laser power converter wafers. FF distribution results with different annealing temperatures: (a) 380°C, (b) 400°C, (c) Box and whisker charts of FF at different annealing temperatures. PCE distribution results with different annealing temperatures: (d) 380°C, (e) 400°C, (f) Box and whisker charts of PCE at different annealing temperatures. The box defines the 25th and 75th percentiles and the median value of the set, the hollow square is the mean value, and the whiskers show the range. The optical input power was 7.5 W.
The performance of the laser power converters annealed at 400°C was investigated. As shown in Fig. 4(a), when exposed to 808 nm laser irradiation, the conversion efficiency was 57.1%, while under 830 nm laser irradiation conditions, the efficiency increased to 59.3%. A discernible difference in the short-circuit current (Isc) of the laser power converters is evident from the I-V curve, particularly noticeable under the 830 nm laser condition. However, it is noteworthy that the FF is significantly lower under 830 nm compared to 808 nm. This discrepancy in FF is primarily attributed to the fact that the laser power converters are specifically designed based on the absorption wavelength of the GaAs material at 808 nm. When exposed to an 830 nm laser, a current mismatch occurs, leading to a reduction in fill factor.
Fig. 4. (a) Performance comparison of six-junction GaAs laser power converters at 808 nm and 830 nm.The optical input power was 7.36 W and the cell size was 1 cm2. (b) EQE measurement.
Fig. 5. Stability measurement. The output power is continuously measured under 21.3 W at 180°C (pink line), and 2.1 W and 10.0 W at 150°C (green line). The cell size was 1 cm2.
In order to verify that the above short-circuit current changes are related to the spectral response of the laser power converters, the external quantum efficiency (EQE) of the laser power converters was conducted, as shown in Fig. 4(b). The EQE of the laser power converter at 830 nm is approximately 92.1%, while at 808 nm, it is around 89.4%. This shows that the spectral response of the laser power converters at 830 nm is higher. This discrepancy indicates a higher spectral response of the laser power converter at 830 nm, providing an explicable rationale for the observed higher short-circuit current under 830 nm laser irradiation compared to 808 nm.
Considering the higher EQE at 830 nm, if the design is tailored to optimize the absorption coefficient at this wavelength, the anticipated conversion efficiency is projected to reach an impressive 63.8%. This underscores the significance of aligning the laser power converter's spectral response with the incident wavelength for optimal performance, as supported by the EQE analysis.
This study also assessed the reliability of the laser power converters subsequent to annealing at 400°C for 45 minutes. The laser power converters underwent exposure to both 2.1 W and 10.0 W laser irradiation at an ambient temperature of 150°C, as shown in Fig. 5, demonstrating continuous operation for 1200 hours. In a more demanding scenario where the incident laser power was elevated to 21.3 W, coupled with an ambient temperature of 180°C, the laser power converter's output power exhibited a negligible decrease of less than 2.5% even after sustained operation for over 1000 hours. To our knowledge, this result is the first report of laser power converters reliability at these temperatures.The above results show that the laser power converters can maintain good output stability under long-term, high-temperature, and high-power laser irradiation.
4. Conclusion
This paper investigates the impact of thermal annealing of electrode resistance on the performance of six-junction GaAs laser power converters. Notably, the efficiency exhibits a substantial enhancement, progressing from 46.1% at an annealing temperature of 380°C to 55.0% at 400°C. Furthermore, under 830 nm laser irradiation, the efficiency of the laser power converter subjected to annealing at 400°C reaches 59.3%. With current matching at this wavelength, the anticipated efficiency is projected to reach an impressive 63.8%. Moreover, the reliability of the laser power converter was demonstrated by its ability to operate at high temperatures of 180°C for 1,000 hours and 150°C for 1,200 hours.
Funding
National Natural Science Foundation of China (62301347); China Postdoctoral Science Foundation (2022M722243); Fundamental Research Funds for the Central Universities (2023SCU12010).
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.
Reference
1. K. Jin and W. Zhou, “Wireless laser power transmission: A review of recent progress,” IEEE Trans. Power Electron. 34(4), 3842–3859 (2019). [CrossRef]
2. C. Algora, I. García, M. Delgado, et al., “Beaming power: Photovoltaic laser power converters for power-by-light,” Joule 6(2), 340–368 (2022). [CrossRef]
3. Y. Gou, H. Wang, J. Wang, et al., “High-performance laser power converts for direct-energy applications,” Opt. Express 30(17), 31509–31517 (2022). [CrossRef]
4. Y. Kim, H.-B. Shin, W.-H. Lee, et al., “1080 nm InGaAs laser power converters grown by MOCVD using InAlGaAs metamorphic buffer layers,” Sol. Energy Mater. Sol. Cells 200, 109984 (2019). [CrossRef]
5. 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]
6. Y. Gou, H. Wang, J. Wang, et al., “High-performance laser power converts for wireless information transmission applications,” Opt. Express 31(21), 34937–34945 (2023). [CrossRef]
7. Y. Gou, H. Wang, J. Wang, et al., “1064 nm InGaAs metamorphic laser power converts with over 44% efficiency,” Opt. Express 30(23), 42178–42185 (2022). [CrossRef]
8. V. P. Khvostikov, S. V. Sorokina, K. О. А, М. V. Nakhimovich, et al., “Ge-Based Photovoltaic Laser-Power Converters,” IEEE J. Photovoltaics 13(2), 254–259 (2023). [CrossRef]
9. S. Fafard and D. P. Masson, “High-Efficiency and High-Power Multijunction InGaAs/InP Photovoltaic Laser Power Converters for 1470 nm,” Photonics 9(7), 438 (2022). [CrossRef]
10. Y. Wang, Z. Zheng, J. Wang, et al., “Organic laser power converter for efficient wireless micro power transfer,” Nat. Commun. 14(1), 1 (2023). [CrossRef]
11. S. Fafard and D. P. Masson, “Perspective on photovoltaic optical power converters,” J. Appl. Phys. 130(16), 160901 (2021). [CrossRef]
12. 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]
13. Y. Gou, H. Wang, J. Wang, et al., “High performance p++-AlGaAs/n++-InGaP tunnel junctions for ultra-high concentration photovoltaics,” Opt. Express 30(13), 23763–23770 (2022). [CrossRef]
14. Y. Gou, J. Wang, Y. Cheng, et al., “A Modeling and Experimental Study on the Growth of VCSEL Materials Using an 8× 6 Inch Planetary MOCVD Reactor,” Coatings 10(8), 797 (2020). [CrossRef]
15. 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,” Prog. Photovoltaics 23(12), 1687–1696 (2015). [CrossRef]
