GaAs Vertical-Tunnel-Junction Converter for Ultra-High Laser Power Transfer

Celia Outes et al.: GaAs Vertical-Tunnel-Junction Converter for Ultra-High Laser Power Transfer 1  Abstract— High power laser transmission is being intensively researched as a potential solution to transfer power to remote systems, being the power converter (PC) one of the main limiting factor to improve the system efficiency (η). Current PCs are mostly horizontal structures in which the η heavily decreases at large input power. In this work, we propose a novel GaAs-based vertical-tunnel-junction (VTJ) PC suitable for ultra-high (UH) input power density (Pin). This structure does not suffer from η degradation at high Pin because it is designed to have low current density, high output voltage and reduced series resistance (~ 2 orders of magnitude lower than the state-of-the-art PCs). We have demonstrated increasing η with Pin, reaching values higher than 76% at 3000 W·cm-2. This vertical-based architecture enables a new set of potential applications for wireless PC to power remote systems with η exceeding today´s state-of-the-art PC designs. Index Terms— Laser power transfer, power converter, verticalstructure, tunnel-junction, GaAs, series resistance I. INTRODUCTION IRELESS power transfer (WPT) technology has attracted increasing interest, becoming a billion market within the last years [1]. One of the most promising WPT technologies is high-power laser transmission (HPLT) since it offers electrical isolation, reduced electrical noise and electromagnetic interference and the ability to transfer energy without wires. HPLT uses a monochromatic light source, usually a laser, to transfer power to a remote system via a power converter (PC). There is an intensive research underway to increase the efficiency (η) and power transmission of PCs [2]–[5]. However, current devices have their peak η at input powers (Pin) lower than 100 W·cm-2 [6]. For a paradigm shift, the development of new generation ultraefficient PC suitable for converting power in the order of kilowatts with high η is crucial [7]. This would allow to reduce PC surface, to increase the distance range and the number of applications of this technology [8], [9]. Work supported by the Spanish Government, Xunta de Galicia, Junta de Andalucía and FEDER (Fondo Europeo de Desarrollo Regional) funds (PID2019-106497RB-I00, P18-RT-1595, PID2019104834GB-I00, ED431F 2020/008, GRC 2014/008, RYC-2017-23312, RYC-2017-21910) and by the University of Jaén (ERC_2019_1). C. O., E. F. F. and F. A. are with the University of Jaén, Spain). N. S. and A. J. GL. (Email: antonio.garcia.lour[email protected]) are with the University of Santiago de Compostela, Spain. (Corresponding author: Antonio Garcia-Loureiro) Fig. 1. 2D scheme of the VTJ GaAs power converter structure. WP/NA and WN/ND are the width/doping of the p and n-layers respectively, which will be subject of the optimisation. State-of-the-art PCs are mainly GaAs-based horizontal structures [10], [11], [12] in which the efficiency (η) heavily decreases at large Pin values. The limitation of these materials to achieve high η at extreme Pin under real operating conditions are imposed by the unavoidable series resistance (Rs) losses and the high current density of these designs under ultra-high (UH) input powers. To solve these issues, York et al. [13] proposed to vertically stack several NP junctions connected in series, which reduces the current density and increases the operating voltage, achieving a record η of 66% at a Pin of 64.4 W·cm-2. However, this design, as the conventional PCs, still has the electrical contacts on the top and bottom of the device, which implies a trade-off between the shadowing of the front metal-grid and the RS, limiting the η at high and UH Pin [14]. In this work, we introduce a novel GaAs-based verticaltunnel-junction (VTJ) PC that aims to decrease the RS losses, reducing the current density while increasing the output voltage, placing the contacts perpendicularly to the incident light. The proposed design is based on the UH concentrator solar cell structures recently proposed by the authors [15], [16]. In this case, this early design is investigated for the first time for its potential application as PC suitable for UH laser power. The VTJ PC exhibits, increasing η with Pin, reaching values higher than 76% for a Pin of 3000 W·cm-2. II. DEVICE STRUCTURE AND SIMULATION The PC presented in this work is based on a VTJ structure, introduced for Concentrator PhotoVoltaics in [17], composed of two identical GaAs-based PN junctions connected by a highly doped GaAs tunnel junction (TJ). This III-V material has been selected because is widely used in photovoltaic and power converter applications and presents the best known η results in PCs [10], [18]. GaAs Vertical-Tunnel-Junction Converter for Ultra-High Laser Power Transfer Celia Outes, Eduardo F. Fernández, Natalia Seoane, Florencia Almonacid and Antonio J. García-Loureiro W Celia Outes et al.: GaAs Vertical-Tunnel-Junction Converter for Ultra-High Laser Power Transfer 1 TABLE I OPTIMUM VALUES OBTAINED FOR THE VTJ-BASED POWER CONVERTERS Pin λ W H WP WN NA ND η PC1 10 0.847 20.1 7.5 2.6 7.3 2·1016 1014 69.5 PC2 3000 0.849 10.3 10.5 2.5 2.5 3·1017 1014 76.3 Input power density: Pin (W·cm-2), wavelength: λ (µm), width: W (µm), height: H (µm), width of the P/N layers respectively: WP/WN (µm), acceptor/donor concentration: NA/ND (cm-3) and efficiency: η (%). In the VTJ architecture the illumination is perpendicular to the PN junctions, and the contacts are placed laterally to extract the generated current (see the current flow in Fig. 1). Therefore, the limitation imposed by the trade-off between the shadowing of the front metallic contact and the RS of the conventional structures is avoided. In this way, the RS losses diminish and there is not any type of grid resistance or shadow effect. Using this structure, it is possible to increase the area of the device exposed to the light connecting more VTJs via TJ without increasing the current due to the connection in series. The GaAs PC has two subcells, each one composed by two players (p and p+) and two n-layers (n and n+). The p+ and n+ layers have a width (W) of 0.07 µm and are doped to 5·1019 cm-3. Both layers remain fixed during this study. On the other hand, the TJ is composed by a n+/p+ GaAs junction to avoid any mismatching problems in the structure. The TJ layers are doped to 7·1019 cm-3 and have a 25 nm width. Note that in this vertical architecture the current density is very low and therefore the TJ will not be a limiting factor [19]. The PCs have been modelled using Silvaco TCAD [20], a software widely used by the photovoltaic community for designing and optimising solar cells [21], [22] and power converters [13], [23], because it provides realistic results. The Poisson and continuity equations, that relate the electrostatic potential and the carrier densities, are solved self consistently. Different recombination mechanisms (Auger, radiative and Shockley-Read-Hall (SRH)) have been taken in account in our simulations. The contacts are considered ideal and, we do not account for reflections since the incoming light is parallel to them. This approximation is feasible because of the low resistance of the contacts, typically ranging from 10-4 to 10-5 Ω·cm2, compared with the standard technology [24]. All the simulations are 2D (assuming negligible changes in the third dimension), consider uniform, continuous and monochromatic illumination with a wavelength close to the bandgap and a temperature of 298 K. Further details of the simulation methodology and assumptions above can be also found in [25]. III. RESULTS AND DISCUSSION In the present work, we propose two VTJ-based PCs, optimised for a low Pin value of 10 W·cm-2 (PC1) and an UH Pin value of 3000 W·cm-2 (PC2), with the aim of maximising the η and to evaluate the proposed structure for a wide range of operating conditions. The following parameters were optimised: input wavelength (λ), width (W) and doping of the p (NA) and n (ND) layers, and height (H) of the structure, see optimum values in Table I. Record η values of 69.5% at a Pin of 10 W·cm-2 for PC1, and 76.3% at 3000 W·cm-2 for PC2 were obtained. The larger H value observed for PC2 compared to the PC1 value is due to the trade-off between increasing the Fig. 2. Normalised IV curves at different input power densities (Pin) values for PC2. The short-circuit current (ISC), open-circuit voltage (VOC) and maximum power (PMPP) are also included. Fig. 3. Efficiency as a function of the input power density (Pin) in logarithmic scale for the two VTJ-based power converters optimised at Pin values of 10 W·cm-2 (PC1) and 3000 W·cm-2 (PC2), and for several experimental and simulated state-of-the-art PCs. absorption (via enlarging H) and increasing the current density (via reducing H). On the other hand, the larger the Pin the smaller the W of the n-layer to minimise the impact of recombination effects (see Table III for further details), due to the low mobility of minority carriers (holes) in this layer when compared with the p-layer. Fig. 2 shows the normalised IV curves for the PC2 at different Pin including the short-circuit current (ISC), open-circuit voltage (VOC) and the maximum power (PMPP). Note the linear increase of ISC with Pin. Fig. 3 shows η versus of Pin for the two optimised VTJbased PCs, and for experimental [11], [12], [13], [26], [27], (empty symbols) and simulated [6], [13], [26], [27] (filled symbols) state-of-the-art GaAs PCs available in the literature. Note that the simulated and experimental η values in [13] are virtually the same, validating the use of TCAD to evaluate the performance of new architectures and designs. For PC1 η increases with Pin until 100 W·cm-2 reaching a maximum of 71.7%, and then decreases rapidly with the increase of Pin. Note that this same behaviour has also been reported in previous work, see for instance [6], [12], [26], [27]. PC2 achieves slightly lower η values than PC1 at low Pin values (<100 W·cm-2) since it was optimised for an UH Pin value but, maintains a linear increase in η with the logarithmic Pin for all the studied intervals, ranging from 63.8% at 1 W·cm-2 to 76.3% at 3000 W·cm-2. The two PCs proposed in this work have a significantly better η than the state-of-the-art converters for all studied Pin. The current record η for a PC (66.0%) was Celia Outes et al.: GaAs Vertical-Tunnel-Junction Converter for Ultra-High Laser Power Transfer 1 TABLE II INPUT POWER DENSITIES (PIN), SERIES RESISTANCE (RS) AND EFFICIENCY FOR STATE-OF-THE-ART SIMULATED AND EXPERIMENTAL PCS AND THE VTJBASED PCS Pin (W·cm-2) Rs (Ω·cm2) η (%) Ref. Simulated 10 3.4·10-2 56.8 [26] 64.1 1.6·10-1 66.0 [13] Experimental 31.8 1.1·10-1 48.6 [26] 10 - 104 4.7·10-2 -5.6·10-3 51.9-52.9 [11] 36.1 1.9·10-1 42.2 [27] 64.1 1.1·10-1 66.0 [13] 13.2-21.9 8.3·10-2 -6.9·10-2 52.0-51.6 [6] PC1 3000 4.1·10-5 55.0 This work PC2 3000 7.2·10-5 76.3 This work Fig. 4. Fill Factor and open-circuit voltage as a function of input power density in logarithmic scale for the VTJ-based power converters optimised for an input power density of 10 W·cm-2 (PC1) and 3000 W·cm-2 (PC2). achieved at a relatively low Pin of 64.4 W·cm-2 [13], and for this same Pin value, PC1 and PC2 achieve 5.3% and 4.6% larger η values, respectively, even if they have not been optimised for this Pin. At a medium Pin (400 W·cm-2), the η of [27] is 8.5% lower than those of the VTJ-based PCs proposed. For UH Pin (3000 W·cm-2), the experimental maximum η obtained in [12] is 48.5%, a value significantly lower than the 76.3% achieved by PC2. The high η values of the VTJ-based PCs are due to the RS of the vertical architecture (RS losses are given by J·RS2). To justify this statement, the Rs has been calculated using the slope of the IV curve close to the VOC value (see Table II), following the strategy introduced in [28], for both the VTJ and state-of-the-art PCs. The RS of the VTJbased PCs is at least two or three orders of magnitude lower than those of the state-of-art PCs. These results highlight the importance of structures with low RS values, such as the one proposed in this letter, to obtain high η at UH Pin. Fig. 4 shows the dependence of the open-circuit voltage (VOC) and Fill Factor (FF) vs. Pin for PC1 and PC2. VOC linearly grows with the logarithmic increase of Pin for the two PCs, without any type of degradation. However, for PC1, the FF decreases with Pin, ranging from 88.9% at 1 W·cm-2 to 68.6% at 3000 W·cm-2. This reduction at UH Pin in PC1 is mainly attributed due to low value of the shunt resistance (RSH), 2.5·10-2 Ω·cm-2 at 3000 W·cm-2, as estimated following the methodology discussed in [29]. For PC1, the width of the n-layer is 7.3 µm, and at UH Pin the radiative recombination of carriers in this layer notably increases, deteriorating the device performance. Table III shows the η for the two VTJ-based PCs TABLE III EFFICIENCY VALUES FOR DIFFERENT RECOMBINATION SCENARIOS Efficiency (%) Pin (W·cm-2) AR NR Auger Radiative SRH PC1 3000 55.0 77.2 76.6 55.1 77.0 PC2 3000 76.4 79.7 79.6 76.8 78.6 Input power density: Pin (W·cm-2), AR: all recombinations, NR: no recombinations, Auger: only Auger recombination, Radiative: only radiative recombination, SRH: only Schockley-Read-Hall recombination. at 3000 W·cm-2 for different scenarios: all recombinations activated (AR), no recombination effects (NR) and the cases when only Auger, radiative or SRH are considered. Results show that radiative is the dominant effect, reducing the η for PC1 at UH Pin a 22.1% respect the NR case. This damaging effect of the total recombination is due to the large volume in which it can take place. This phenomenon is specific to the vertical architecture since the direction of the incident light is perpendicular to the location of the contacts. In any case, it can be minimised with a good optimisation of the layer dimensions. For instance, PC2 maintains high FF and η in all the Pin range because the recombinations are very low, reducing the η only by a 3.3%. Note that for PC2 the width of the n-layer is 2.5 µm, allowing two orders of magnitude larger RSH (~3 Ω·cm-2). This highlights the importance of optimising the device structure to improve the performance for a particular targeted Pin. Finally, although the manufacturing of the PC architectures is out of the scope of this letter, we would like to provide details about its feasibility. For this architecture, only GaAs material has been considered. This avoids lattice-mismatching problems and facilitates the fabrication process. In this sense, the structure could be monolithically grown as in standard IIIV multi-junction concentrator solar cells. Also, the metallic contacts could be placed on the laterals using the same techniques as in the solar cell technology. After that, the solar cell can be rotated, so the input laser is perpendicular to the current flow. IV. CONCLUSIONS We have proposed a novel GaAs-based vertical-tunneljunction (VTJ) power converter (PC) that shows increasing efficiency (η) values with the input power (Pin), reaching a η of 76.3% for a 3000 W·cm-2 Pin. This design achieves at least 4.6% larger η than that observed in the current record device, which is 66.0%, but only at a Pin of 64.4 W·cm-2. This architecture benefits from low series resistance values (~10-5 Ω·cm2) and no shadowing effects of the front metal grid. In addition, if the structure is optimised for ultra-high (UH) Pin values, it does not suffer from performance degradation at these extremely high input power values. These results open a new route for the use of GaAs VTJ PCs in future highefficiency and high-power remote applications. Celia Outes et al.: GaAs Vertical-Tunnel-Junction Converter for Ultra-High Laser Power Transfer 2 REFERENCES [1] X. Lu, P. 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