GaSb based Photovoltaic Laser Power Converter for ... - Springer Link

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Original Russian Text © V.P. Khvostikov, S.V. Sorokina, F.Yu. Soldatenkov, N.Kh. ... A photovoltaic laser power converter based on gal lium antimonide was ...
ISSN 10637826, Semiconductors, 2015, Vol. 49, No. 8, pp. 1079–1082. © Pleiades Publishing, Ltd., 2015. Original Russian Text © V.P. Khvostikov, S.V. Sorokina, F.Yu. Soldatenkov, N.Kh. Timoshina, 2015, published in Fizika i Tekhnika Poluprovodnikov, 2015, Vol. 49, No. 8, pp. 1104–1107.

PHYSICS OF SEMICONDUCTOR DEVICES

GaSbbased Photovoltaic LaserPower Converter for the Wavelength λ ≈ 1550 nm V. P. Khvostikov*, S. V. Sorokina, F. Yu. Soldatenkov, and N. Kh. Timoshina Ioffe Institute, St. Petersburg, 194021 Russia *email: [email protected] Submitted December 3, 2014; accepted for publication December 15, 2014

Abstract—The photovoltaic (PV) laserpower converters for the wavelength λ = 1550 nm have been pro duced by diffusion from the gas phase into an nGaSb substrate. PV cells with areas of S = 4, 12.2, and 100 mm2 have been fabricated and tested under a Xe flash lamp. The monochromatic (λ = 1550 nm) effi ciency η = 34.8% is reached on the best samples with S = 12.2 mm2 at a current density of 8 A/cm2. The sta bility of laser power converter and its degradation rate have been estimated at the operation temperature of 120°C. DOI: 10.1134/S1063782615080114

1. INTRODUCTION The development of semiconductor lasers and techniques for the fabrication of photovoltaic laser power converters (LPCs) has resulted in a substantial increase in their efficiency, which opens up extensive opportunities for their practical use in the technology of wireless power transmission to a remote source, with the number of these applications growing annu ally. Under space conditions, the photovoltaic method is promising for the remote supply of electric power to space vehicles [1–4] and the transmission of laser beam from Earth to space stations or the development of solar power stations. According to [3], the efficiency of a system for remote power transmission may reach a value of 15–39% in the near future. Modern semi conductor lasers paired with photodetectors are also rather promising for the development of novel radiolo cation systems for different applications, including those for aircraft. Under terrestrial conditions, LPCs can be used in fiberoptic communication lines [5] or for recharging household electronic devices. The wide variety of applications require LPCs matched in wavelength λ with various laserradiation sources be developed. Recordbreaking values of effi ciency, η > 56% at wavelengths of λ = 820–850 nm and a laserradiation power density of up to 100 W/cm2 have been obtained for LPCs based on AlGaAs/GaAs heterostructures [1, 2, 4, 6–9]. Of particular interest are PV cells developed on the basis of GaInAs/InP for the conversion of Nd:YAG laser beam with a wave length of 1064 nm [10–12]. LPCs based on the InGaAs/InP system are the most widely used in long distance fiberoptic communication lines. These LPCs cover the laseremission spectrum λ = 1.3– 1.55 μm in which the minimum optical loss and the

minimum dispersion in optical fibers have been achieved, which is extremely important for longdis tance fiberoptic communication lines (FOCLs) [9]. The conversion of laser radiation with a wavelength of λ = 1550 nm can also find practical application because of the possibility of its transmission via optical fibers without a substantial loss. A possible variant of an effective photodetector for this monochromatic radi ation is a PV cell based on gallium antimonide [9, 13]. 2. FABRICATION OF PHOTOVOLTAIC LASERPOWER CONVERTERS BY DIFFUSION FROM THE GAS PHASE A photovoltaic laserpower converter based on gal lium antimonide was fabricated by the diffusion of zinc to an nGaSb substrate. A high spectral response at the wavelength λ = 1550 nm is obtained at small dif fusionemitter thicknesses d < 0.3 μm (Fig. 1). There fore, as previously for solar cells based on gallium anti monide [14, 15], doublestage diffusion was used to fabricate the photocell structure via the formation of a thin ptype emitter on the photosensitive surface, with subsequent burial of the diffusion layer to a depth of 1.0–1.5 μm in the subcontact regions to preclude leaks in the p–n junction or the burningthrough of metalli zation layers. The depth of the p–n junction determines not only the spectral sensitivity, but also the sheet resistance along the layer, which is also a characteristic quantity affecting the amount of ohmic loss. In turn, the ohmic loss determines the position of the efficiency maxi mum (see below) and increases in proportion to the squared illumination flux, with the result that it is par ticularly important in largecurrent operation modes of LPCs.

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Spectral response, A/W 1 1.0 2 3 4 0.8 5

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Fig. 1. Spectral response of GaSbbased photovoltaic LPCs with various emitter thicknesses: (1) 1.1, (2) 0.9, (3) 0.62, (4) 0.32, and (5) 0.14 μm.

3. PARAMETERS OF LASERPOWER CONVERTERS WITH DIFFERENT AREAS AND PURPOSES We fabricated and tested PV cells with different areas S = 4, 12.2, and 100 mm2 (Figs. 2, 3, and 4), intended for the conversion of radiation with various power densities. For the best samples (S = 12.2 mm2) measured under a Xe lamp, an efficiency of η = 34.8% was reached at a current density of 8 A/cm2 (Figs. 3 and 4). As the laser power incident on an LPC increases, its area should be reduced to decrease the ohmic loss. Figure 4 shows how the position of the efficiency max imum shifts for PV cells converting laser radiation with various power densities in the range from ~0.5 to ~120 W/cm2. 4. OPTIMIZATION OF LASERPOWER CONVERTERS WITH DIFFERENT AREAS AND PURPOSES With increasing power density of radiation incident on a PV cell, it becomes highly important not only to optimize the surface shading by the contacts, but also to reduce their contact resistance. Therefore, our LPCs differed not only in terms of the area S and con tactgrid configurations, but also in the structure of the front contacts. The Ti–Pt–Au and Ti–Pt–Ag systems have a low contact resistance to pGaSb [16]. The advantage of silvercontaining composites is in the lower resistivity

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Fig. 2. Spectral response of a typical GaSbbased LPC.

of Ag as compared with gold, which leads to a lower sheet resistance of the contact at the same thickness of the metallization layers. In the present study, the Ti and Ptlayer thicknesses were 10–30 and 30 nm, respectively, with the total contact thickness reaching a value of 1.0–1.5 μm. Each of the contact systems made it possible to fabricate converters with a high fill factor (FF) of the current–voltage characteristic and monochromatic efficiency η (Fig. 5), with the forma tion of a contact grid not giving rise to leakage cur rents. The specific features of how a contact grid is formed for GaSb photovoltaic converters with a thin photoactive region (0.3–0.5 μm) were examined in [16]. In the operation of a LPC under continuous expo sure to highpower laser radiation, its working temper ature substantially grows, and, therefore, the reliability of its metallization is particularly important. For largearea PV cells combined into an array, the prob

Efficiency (λ = 1550 nm), %

Based on the balance between the requirements of raising the spectral response and reducing the sheet resistance of the layer, the optimal thickness of pGaSb on a photosensitive surface was chosen to be 0.5 μm.

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Fig. 3. Fill factor FF, opencircuit voltage Voc, and mono chromatic (λ = 1550 nm) efficiency η of an LPC with S = 12.2 mm2. SEMICONDUCTORS

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Fig. 4. Monochromatic (λ = 1550 nm) efficiency η of LPCs in relation to photocurrent density and radiation power density. S: (1) 4, (2) 12.2, and (3) 100 mm2.

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lem of overheating and heat removal will be particu larly topical. The stability of Ti–Pt–Ag both in con tact fabrication and in the operation of a photovoltaic converter under extreme conditions (temperature T ≈ 200°C) substantially surpasses that of Ti–Pt–Au [16, 17]. In [17], we meant by the degradation rate of a photovoltaic converter the period of time during which the decrease in the generated power P does not exceed 20% relative to the initial value P0 and this parameter was determined for a temperature of ~200°C. In the present study, we continued analysis of the behavior of a photovoltaic converter with the given contact systems at rather high operating temperatures Vol. 49

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Fig. 6. Variation of (a) the fill factor FF and (b) opencir cuit voltage Voc of PV cells with different contact systems under heating to ~120°C. Shortcircuit current Jsc = 1 A/cm2. Contact systems: (1) Ti–Pt–Au and (2) Ti–Pt–Ag. FF0 and Voc0 are the initial values before heating.

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Fig. 5. Monochromatic (λ = 1550 nm) efficiency of LPCs with area S = 12.2 mm2 and (1) Ti–Pt–Au and (2) Ti–Pt–Ag contact systems. Total contact thickness 1.0–1.5 μm.

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(~120°C). As expected, the fill factor (FF) of the cur rent–voltage characteristic and the opencircuit volt age Voc are more stable for PV cells with Ti–Pt–Ag contacts (Fig. 6). The data presented above make it possible to estimate the LPC degradation time under the conditions of its heating to 120°C. This period was 1200 h for the Ti–Pt–Au system and more than 9000 h for Ti–Pt–Ag. The results we obtained make it pos sible to prognosticate the stability of characteristics and the efficiency of the prolonged operation of high power PV converters under laser beam. ACKNOWLEDGMENTS The study was supported by the Ministry of Edu cation and Science of the Russian Federation (agreement no. 14.604.21.0089 of June 27, 2014;

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unique identifier of applied scientific studies, RFMEFI60414X0089). REFERENCES 1. J. T. Howell, M. J. O’Neill, and R. L. Fork, in Proceed ings of the 5th Wireless Power Transmission Conference together with 4th International Conference on Solar Power from Space (Granada, Spain, 2004), p. 187. 2. S. D. Jarvis, J. Mukherjee, M. Perren, and S. J. Sweeney, IET Optoelectron. 8, 64 (2014). doi: 10.1049/iet opt.2013.0066 3. V. A. Bogushevskaya, O. V. Zayats, Ya. N. Maslyakov, I. S. Matsak, A. A. Nikonov, V. V. Savel’ev, and A. A. Sheptunov, Tr. Mosk. Aviath. Inst., El. Zh., No. 51 (2012). www.mai.ru/science/trudy/ 4. A. N. Golikov, A. V. Drondin, S. G. Rebrov, and S. V. Yanchur, Izv. Akad. Nauk, Energet., No. 5, 131 (2012). 5. O. N. Krokhin, http://scilance.com/library/book/15640 6. E. Oliva, F. Dimroth, and A. W. Bett, Progr. Photovolt.: Res. Appl. 16, 289 (2008). 7. J. Schubert, E. Oliva, F. Dimroth, R. Loeckenhoff, and A. W. Bett, IEEE Trans. Electron. Dev. 56, 170 (2009). doi: 10.1109/TED.2008.2010603 8. M. Smith, B. Tillotson, J. Oliver, N. Tarasenko, T. Schoelzel, and H. Brandhorst, in Proceedings of the 38th IEEE Photovoltaic Specialists Conference (Austin, TX, USA, 2012), p. 002825. 9. V. M. Andreev, Sovrem. Elektron., No. 6, 20 (2014).

10. R. K. Jain, IEEE Trans. Electron Dev. 40, 1893 (1993). 11. A. E. Marichev and V. P. Khvostikov, in Proceedings of the 15th AllRussia Young Scientists Conference on Phys ics of Semiconductors and Nanostructures, Semiconduc tor Opto and Nanoelectronics (St.Petersburg, 2013), p. 24. 12. A. E. Marichev and R. V. Levin, in Proceedings of the 15th Russian Young Scientists Conference on Physics and Astronomy “Physics, St.Petersburg” (St.Petersburg, 2013), p. 168. 13. V. M. Andreev, V. P. Khvostikov, V. S. Kalinovsky, V. A. Grilikhes, V. D. Rumyantsev, M. Z. Shvarts, V. Fo kanov, and A. Pavlov, in Proceedings of the WCPEC3 (Osaka, Japan, 2003), paper 3PB533. 14. V. M. Andreev, S. V. Sorokina, N. Kh. Timoshina, V. P. Khvostikov, and M. Z. Shvarts, Semiconductors 43, 668 (2009). 15. V. M. Andreev, V. P. Khvostikov, V. D. Rumyantsev, S. V. Sorokina, and M. Z. Shvarts, in Proceedings of the 28th IEEE Photovoltaic Specialists Conference (Alaska, 2000), p. 1265. 16. F. Yu. Soldatenkov, S. V. Sorokina, N. Kh. Timoshina, V. P. Khvostikov, Yu. M. Zadiranov, M. G. Rastegaeva, and A. A. Usikova, Semiconductors 45, 1219 (2011). 17. V. P. Khvostikov, S. V. Sorokina, N. S. Potapovich, F. Yu. Soldatenkov, and N. Kh. Timoshina, Semicon ductors 48, 1248 (2014).

Translated by M. Tagirdzhanov

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