Power maximization in III-V sub-millimeter, radial front contacted cells for thin micro-concentrators

Jose L. Cruz-Campa, Anna Tauke-Pedretti, Jeffrey G. Cederberg, Carlos A. Sanchez, Gerald R. Girard, Charles Alford, Brandon A. Aguirre, Ian Luna, Murat Okandan, Jeffrey S. Nelson, and Gregory N. Nielson

Sandia National Laboratories, PO Box 5800 Albuquerque, NM, 87185, USA

Abstract — Sub millimeter scale solar cells coupled with

medium concentration lenses can reduce the balance of system costs of concentrating photovoltaics by creating thin and highly efficient concentrators with relaxed tracking requirements. This paper shows the design, fabrication, simulation, and testing of micro-sized photovoltaics that have unique perimeter front contacts outside the optical collection area. The design of the device considered the need for low resistance current carrying layers while minimizing optical losses. The paper also shows the successful fabrication of InGaAs cells as well as of GaAs cells transferred onto silicon substrates. The simulations and experimental measurements show that small cells of this type 1) suffered from slightly lower voltage levels caused by proportionally larger dark currents 2) peaked their efficiencies at higher concentration levels compared to larger ones 3) performed better overall. Index Terms —solar cell, InGaAs, GaAs, bonded cells, compact

concentrators.

I. INTRODUCTION

Concentrators are a promising PV technology that can make use of high-efficiency III-V based solar cells in an economical way for terrestrial solar PV applications. Some of the most detrimental effects in concentrators are resistive losses, shading caused by gridlines, and the loss in open-circuit voltage due to thermal effects. Microcells can be designed to have a small distance between gridlines with zero shading losses on the cell aperture. Also due to their small size, microcells can dissipate heat faster than larger cells, and thus achieve higher open-circuit voltages. Furthermore, microcells coupled with small lenses with tight focal lengths allow thinner packages that can enhance performance compared to traditional panel modules. Besides improved thermal management and lower series resistance losses, other benefits include partial shading tolerance (from reduntant interconnections) and novel form factors in optics can be achieved. These benefits have been previously discussed in the literature [12-3]. The concept of using microcells coupled with small lenses, proposed by our group in 2009 [4,5], has since seen encouraging results by other groups with a notable achivement by Panasonic and Solar Junction [6]. Our team has previously demonstrated fabrication of functional and sub-millimeter scale Si solar cells with [ 7 ] and without concentration [8] on flexible [9] or rigid formats [10]. We have also made preliminary studies on resistance

considerations for front contact solar cells [11]. In this paper, we present the design, fabrication, performance simulation, and testing of two types of III-V cells with front contacts, no gridlines, and submillimiter scales.

II. CELL DESIGN AND PROCESSING

We chose to explore fabrication of III-V micro cells because of the demonstrated high efficiency of III-V macro-scale multijunction cells. Two different materials were explored for microcell fabrication; InGaAs and GaAs. The structure for the InGaAs cell consisted of an InP substrate followed by a 1.4 µm p+ InGaAs contact, an 80 nm AlGaInAs back surface field (BSF) layer, and a 3 µm p-type InGaAs absorber. The structure was capped with a 300 nm n-type InGaAs emitter and a 90 nm n+ InP contact.

Fig. 1. Structure diagram for InGaAs cell.

The InGaAs homojunction solar cell was grown and fabricated at Sandia National Laboratories using metal-organic chemical vapor deposition at 620°C and 60 Torr total pressure using methyl metal-organics and hydride sources. After the growth of the epitaxial material for the solar cell, the wafer was processed using standard contact lithography to form hexagonal micro cells. The cell mesa (containing the junction, the window layer, and the BSF) was defined using photolithography and reactive ion etch with inductive coupled plasma (RIE-ICP) in HBr:N2 (16.8:9) mixture. Once the mesa was processed, the two sides of the junction are accessible from the wafer’s front and metal was deposited. For the n-type contact (Te doped) a metal stack of Ti/Au/Ag/Au (250/1000/10000/200Å) was used. For the p-contact (Zn

InP substrate (100)

InP buffer InGaAs P++ contact

AlGaInAs P+ BSF

InGaAs P base InGaAs N+ emitter

InP N++ window/contact

metal

metal

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cell aperture

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doped) a metal stack of Ti/Pt/Au (200/400/5000Å) was used. Plated metal was used to ensure step coverage when forming the larger probe pads. Cell aperture sizes ranged from 50 µm to 1.2 mm. The cell mesa is slightly larger than the cell aperture to enable the incorporation of n contacts and the cell itself is larger than the mesa to accommodate p contacts and testing pads.

Fig. 1 shows the layers and processed cross section of the half of a device. Fig. 2 shows pictures of actual fabricated InGaAs cells on InP substrates

Fig. 2. Pictures of fabricated devices (left) camera picture of a quarter 2” InP wafer with different size devices (right) microscope picture of one 400 µm aperture device.

The structure of the GaAs cell is such that it is releasable

from the GaAs substrate allowing wafer reuse. In this case, the GaAs cell is bonded to a silicon substrate. Fig 3 shows the schematic of the structure after it has been bonded to Si and released from the GaAs substrate. The bonded structure consisted of a 675 µm thick silicon substrate followed by the bonding interface. On the III-V side, the structure is formed by a 1 µm p++ GaAs contact, a 50 nm AlGaAs back surface field (BSF) layer, and a 3.1 µm p-type GaAs absorber. The structure was capped with a 100 nm n-type GaAs emitter, a 40 nm n-type InGaP window, a 20 nm n-type AlGaAs window, and a 760 nm n-type InGaP contact.

Fig. 3. Layers forming the bonded GaAs cell.

In order to obtain the structure shown in Fig. 3, the cell was processed after bonding to the silicon wafer. First the mesa is etched to expose the p contact, followed by metallization of the n-contact with Ge/Au/Ni/Au and the p-metal Ti/Pt/Au. Finally, the InGaP contact layer is etched from the cell aperture area. Fig. 4 shows (left) an optical picture of the

devices on the silicon wafer and a scanning electron microscope (SEM) image of one cell (right).

Fig. 4. Pictures of fabricated GaAs devices. Left: optical picture of a test array with devices ranging in size. Each black dot is a GaAs solar cell. Right: SEM picture of one GaAs device.

III. CELL SIMULATIONS

The simulations of the device were done using a simple customized MATLAB code using the single diode model for a solar cell that creates a simulated current-voltage (IV) curve under one sun illumination. From the IV curve, the code calculates the open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF), and efficiency (Eff) of the cell. In these type of cells (front contact), the major resistance losses occur on the contact and window layers (lateral current carrying layers). Also, the n-type metal contact creates a dark area on the cell that increases the dark current. Since the width of the n-contact does not change for the different cell designs, its represents a larger fraction of the obscured area that will be more detrimental for smaller cells. This will only happen when the width of the contacts is kept the same. However other detrimental effects influencing Voc might take place when the collection area is too close to the edge of the cell.

As part of the simulations, we were able to extract the short-circuit current density (not dependent in device size). The current density increases linearly for both the InGaAs and GaAs cell.

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Voc

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Fig. 5. Simulation of the open circuit voltage vs. concentration.

Silicon substrate

Bond interface GaAs P++ contact

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AlGaAs P+ BSF

GaAs P base GaAs N+ emitter

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Fig. 5 shows the simulated voltage vs. concentration response for InGaAs and GaAs cells of 100 and 1200 µm. The resistance for all cases in Fig. 7 was fixed at 5 ohms. From the graph, it can be seen that GaAs (with a higher bandgap) will produce more voltage than InGaAs cells. Also there is a small difference in Voc between 100 µm cells and 1200 µm cells caused by the increased dark current as explained before. It is important to note that the only losses considered in the simulation were the dark current losses and thus the voltages on the simulated devices are higher than real devices. Figure 6 shows the fill factor vs. concentration for different cell apertures for GaAs and InGaAs cells. The resistance for all cases in Fig. 6 was fixed at 5 ohms. For both materials, the tendency is the same. Larger cells will show a more pronounced decrease in fill factor as the concentration of light increases. This is mainly caused due to the loss of squareness of the IV curve because of series resistance.

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100 μm GaAs 100 μm InGaAs600 μm GaAs 600 μm InGaAs1.2 mm GaAs 1.2 mm InGaAs

Fig. 6. Simulation of the fill factor vs. concentration. The simulations reveal a linear increase of current vs. concentration, a logarithmic increase in voltage vs. concentration, and a fill factor vs. concentration that remained constant for small cells and decayed for bigger cells. Fig. 7 summarizes the effect of voltage, current and fill factor on efficiency and shows that smaller cells can achieve higher efficiencies under concentration than bigger cells given that the resistance of key layers is controlled and small.

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2.5 Ohms InGaAs cell 2.5 Ohms GaAs cell10 Ohms InGaAs cell 10 Ohms GaAs cell20 Ohms InGaAs cell 20 Ohms GaAs cell

Fig. 7. Simulation of the efficiency for a 600 µm cell for different series resistance cases for both InGaAs and GaAs cells.

IV. CELL TESTING AND RESULTS

The cells mentioned in section II were grown, processed, and tested. The testing was done under a 1 sun and a multi-suns simulator. The one sun measurements were done using an OAI class AAA solar simulator (from 300 to 1800 nm) with an intensity of 1 sun and spectrum AM 1.5 calibrated using a silicon reference solar cell. The output of the cell was measured using an Agilent B1500a semiconductor device parameter analyzer. The multi-suns measurements were taken using an Alpha Omega flash tester capable of measuring intensities from 30 to 950 suns through a combination of neutral density filters, 3 different lenses, and a variable attenuator.

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J (m

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m2 )

V (Volts)100 μm InGaAs 150 μm GaAs200 μm InGaAs 250 μm GaAs400 μm InGaAs 500 μm GaAs1.2 mm InGaAs 1000 μm GaAs

Fig. 8. Experimental JV curves of InGaAs and GaAs cells for different sizes under one sun light.

The spectrum match calibration for concentrated measurements was done using two reference cells: one for the

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visible part of the spectrum (350-675 nm) and another for the infrared (625-925 nm) part of the spectrum. The linear regression (intercept at 1 sun) of the current of the cell vs. concentration was used to extract the current at one sun from concentration measurements. The one sun value extracted from concentration was calibrated to match the current at 1 sun as measured with the one sun solar simulator.

Fig. 8 shows the 1 sun J-V measurements of the InGaAs cells and GaAs cells. It can be seen that the open circuit voltage declines as the size gets smaller. Again, this was an expected behavior predicted in the simulations attributed to the proportional increase of the shaded area on the top of the mesa. In future designs, we plan to minimize this area to avoid losses due to dark current.

For the concentration tests, curves for open circuit voltage, short circuit current, fill factor and efficiency were obtained for both InGaAs and GaAs cells. The tendency and behavior of the InGaAs cells measured under concentration is very similar to the one for GaAs. For simplicity, only the results for GaAs are shown.

Figure 9 shows the short circuit current density vs. concentration for cells of differen optical apertures. It can be seen that the smaller cells keep the expected linear behavior at all concentration levels, but the larger cells experience losses in current as the concentration increases. Loses might be caused by heat or other parasitic effects.

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J sc

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Concentration (suns)150 microns 500 microns

750 microns 1000 microns Fig. 9. Experimental current density vs concentration curves of GaAs cells for different cell sizes.

Figure 10 shows the open circuit voltage of the cell vs.

concentration for differen optical apertures. It can be seen that the smaller cells have a slightly smaller Voc for all concentration levels compared to bigger cells. The increase in voltage is logarithmic as expected and most of the increase is observed in the first 100 X of concentration.

1

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Voc

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ts)

#suns150 microns 500 microns

750 microns 1000 microns Fig. 10. Experimental Open circuit voltage vs concentration curves of GaAs cells for different cell sizes.

Figure 11 shows the fill factor of the cell vs. concentration

for differen optical apertures. Fig. 11 portrays a stable fill factor around 80% for the 150 µm aperture cells but as the size increases the fill factor is reduced for higher concentrations.

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150 microns 500 microns750 microns 1000 microns

Fig. 11. Experimental fill factor vs concentration curves of GaAs cells for different cell sizes.

As anticipated from the simulations (Fig. 7), the combined

effect of a linear increase of current, a logarithmic increase in voltage and a decay in fill factor can be seen in Fig. 12 for the GaAs cells efficiency. Despite the noise at low concentration levels, it is interesting to observe the higher efficiency achieved by small cells and the decaying efficiency for larger cells, as predicted by simulations.

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0

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Effic

ienc

y (%

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750 microns 1000 microns Fig. 12. Experimental efficiency vs. concentration curves of GaAs cells for different cell sizes.

V. CONCLUSIONS

We presented the designs of two single junction cells (InGaAs and GaAs) grown on wafers and then processed to obtain small sub-millimeter cells. The sub-millimeter devices were successfully processed on the substrate (InGaAs) or bonded to silicon then processed (GaAs). These devices have unique perimeter front contacts outside the collection area. The simulations and actual device measurements agree on the behavior and shape of curves. The results showed that smaller cells obtain lower open circuit voltages but higher efficiencies overall. The lower voltages seen in smaller cells are due proportionally larger shaded area of the n-type metal contact. This effect won’t necessarily occur in other devices since the width of the contacts can be changed proportionally to the size of the cell in future designs. We also showed that higher resistance cells or larger cells maximize their efficiency at lower concentrations with lower absolute efficiencies. Our group believes that these type of devices can potentially obtain higher efficiencies than conventional concentrator cells due to better heat dissipation and lower series resistance while also providing useful system-level benefits.

VI. ACKNOWLEDGEMENTS

We wish to acknowledge the contributions of all the team members of the MEPV project at Sandia. The work was supported by the Laboratory Directed Research and Development (LDRD) program at Sandia National Laboratories. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE- AC04-94AL85000.

REFERENCES

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[3] M. Paire, L. Lombez, F. Donsanti, M. Jubault, N. Péré-Laperne, S, Collin, A, Perona, A, Dollet, J,-Luc Pelouar, D, Lincot, J. Guillemoles, “Cu(In,Ga)Se2 photovoltaic microcells for high efficiency and low material usage”, in Proc. of SPIE Physics, Simulation, and Photonic Engineering of Photovoltaic Devices vol. 8256, 2012 p. 82560

[4] https://share.sandia.gov/news/resources/news_releases/glitter-sized-solar-photovoltaics-produce-competitive-results/#.UvP0cxDwN8E. (accessed May 21, 2014)

[5] G. N. Nielson, M. Okandan, P. J. Resnick, J. L. Cruz-Campa, P. Clews, M. Wanlass, W. C. Sweatt, E. Steenbergen, V. P. Gupta, “Microscale PV Cells for Concentrated PV Applications”, in 24th European Photovoltaic Solar Energy Conference, 2009 pp. 170-173

[6] H. Arase, A. Matsushita, A. Itou, T. Asano, N. Hayashi, D. Inoue, R. Futakuchi, K. Inoue, T. Nakagawa, M. Yamamoto, E. Fujii, Y. Anda, H. Ishida, T. Ueda, O. Fidaner, M. Wiemer, and D. Ueda “Novel Thin Concentrator Photovoltaic With Microsolar Cells Directly Attached to a Lens Array” IEEE Journal Of Photovoltaic, 2013, in press

[7] G N. Nielson, M. Okandan, J. L. Cruz-Campa, A. L. Lentine, W. C. Sweatt, B. H. Jared, P. J. Resnick, B. Kim, B. J. Anderson, V. P. Gupta, A. Tauke-Pedretti, J. G. Cederberg, T. Gu, M. W. Haney, S. M. Paap, C. A. Sanchez, “216 Cell Demonstration Module with Moderate Concentration, ±4° Acceptance Angle, and 13.3 mm Focal Length” in 39th IEEE Photovoltaic Specialist Conference, 2013, in press

[8] G. N. Nielson, M. Okandan, P. Resnick, J. L. Cruz-Campa, T. Pluym, P. J. Clews, E. Steenbergen, V. P. Gupta “Microscale c-Si (c)PV cells for low-cost power”, in 34th IEEE Photovoltaic Specialist Conference, 2009, pp 1816-1821

[9] J. L. Cruz-Campa, G. N. Nielson, M. Okandan, P. J. Resnick, C. A. Sanchez, V. P. Gupta, J. S. Nelson “Ultrathin and Flexible Single Crystal Silicon Mini-Modules” in 27th European Photovoltaic Solar Energy Conference, 2012, pp 2203-2206

[10] J. L. Cruz-Campa, B. Kim, B. Jared, M. Okandan, G. N. Nielson, M. Ballance, C. Nordquist, W. Sweatt, A. Lentine, “New Challenges in Testing and Failure Analysis for Microsystems-Enabled Photovoltaics Modules” Electronic Device Failure Analysis, Vol. 15, pp. 4-9, 2013

[11] A. Tauke-Pedretti, J. Cederberg, G. Nielson, J. L. Cruz-Campa, C. Sanchez,C. Alford, M. Okandan, E. Skogen, A. Lentine “Resistance Considerations for Stacked Small Multi-junction Photovoltaic Cells” in 39th IEEE Photovoltaic Specialists Conference, 2013, in press

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