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Letter Cite This: ACS Photonics XXXX, XXX, XXX−XXX
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Plasmonically Enhanced Spectral Upconversion for Improved Performance of GaAs Solar Cells under Nonconcentrated Solar Illumination
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Huandong Chen,† Sung-Min Lee,†,# Angelo Montenegro,‡ Dongseok Kang,† Boju Gai,† Haneol Lim,† Chayan Dutta,‡ Wanting He,† Minjoo Larry Lee,∥ Alexander Benderskii,‡ and Jongseung Yoon*,†,§ Departments of †Chemical Engineering and Materials Science, ‡Chemistry, and §Electrical Engineering, University of Southern California, Los Angeles, California 90089, United States ∥ Departmet of Electrical and Computer Engineering, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States # School of Materials Science and Engineering, Kookmin University, Seoul 02707, Republic of Korea S Supporting Information *
ABSTRACT: Spectral upconversion has the potential to compensate for sub-bandgap transparency of single-junction solar cells. Here a composite module of GaAs solar cells is presented that can improve their one-Sun photovoltaic performance by capturing long-wavelength photons below the bandgap via plasmonically enhanced spectral upconversion. Ultrathin, microscale GaAs solar cells released from the growth wafer and etched with a bottom contact layer are printed on a polymeric waveguide containing NaYF4:Er3+, Yb3+ upconversion nanocrystals (UCNC), coated on a plasmonic reflector composed of hole-post hybrid silver nanostructure. The photovoltaic efficiency of GaAs microcells on a UCNC-incorporated plasmonic substrate is increased by ∼6.4% (relative) and ∼11.8% (relative), respectively, compared to those on a nanostructured silver reflector without UCNC and on a plain silver reflector with UCNC, owing to the combined effects of local electric-field amplification to enhance the absorption of UCNC, augmented upconverted emission via coupling into radiative modes, as well as waveguided photon concentration. KEYWORDS: III−V solar cells, GaAs solar cell, spectral upconversion, surface plasmon resonance, nanomembrane
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intrinsically low intensity of natural sunlight at long wavelengths and correspondingly low efficiency of the nonlinear optical process.13,14 In this regard, we have recently demonstrated an approach that can address these difficulties by utilizing a plasmonically engineered reflector composed of hole-post hybrid silver nanostructure to realize strongly improved performance of ultrathin silicon solar cells by spectral upconversion.15,16 The nanostructured plasmonic reflector enabled the enhancement of both the absorption and the emission processes of upconversion luminophores by surface plasmon resonance, resulting in strongly augmented absorption of overlying ultrathin silicon solar cells for longwavelength photons near 980 nm.15−17 Motivated by these advances, here we report a composite solar module designed for overcoming the thermodynamic limit of GaAs solar cells under nonconcentrated solar illumination by exploiting plasmonically enhanced spectral upconversion, in conjunction with printing-enabled modular materials assemblies that
II−V compound semiconductors represent materials of vital importance for realizing ultrahigh efficiency photovoltaic systems owing to a number of distinct advantages such as direct and widely accessible bandgap energy, excellent photophysical properties to generate, transport, and collect charge carriers by interacting with sunlight, and ability to form monolithically grown multijunction tandem systems.1−3 In particular, gallium arsenide (GaAs), with record-high efficiency and high power-to-weight ratio, is a promising materials candidate that outperforms silicon for terrestrial photovoltaic applications.4−6 While the bandgap energy (Eg ∼ 1.42 eV, ∼875 nm) of GaAs is near-optimal against solar spectrum as a single-junction absorber, the maximum efficiency is still thermodynamically limited by the inability to utilize photons with energies lower than the bandgap.7,8 In this regard, spectral upconversion has been proposed as a remedy to alleviate such optical losses associated with sub-bandgap transparency by converting light with energies lower than the bandgap to higher energy photons that can be readily absorbed by the solar cell.9−12 Although conceptually straightforward, the practical application of spectral upconversion in photovoltaic energy conversion has been hindered because of the © XXXX American Chemical Society
Received: September 4, 2018
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DOI: 10.1021/acsphotonics.8b01245 ACS Photonics XXXX, XXX, XXX−XXX
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Figure 1. (a) Schematic illustration of fabrication processes for a plasmonically enhanced upconversion solar module of ultrathin GaAs solar cells. (b) Optical micrographs of GaAs microcells after top-contact metallization (top left), isolation (top right), undercut etching (bottom left), and bottom contact etching after the pick-up (bottom right). (c) Optical micrographs of ultrathin GaAs microcells printed on plain and nanostructured silver substrates.
nm, Si-doped, 2 × 1018 cm−3), n-GaAs emitter (50 nm, Sidoped, 2 × 1018 cm−3), p-GaAs base (150 nm, Be-doped, 3 × 1017 cm−3), p-Al0.3Ga0.7As back-surface field (100 nm, Bedoped, 1 × 1019 cm−3), p-GaAs bottom contact (1300 nm, Bedoped, 4 × 1019 cm−3), and AlAs sacrificial layer (200 nm, undoped), grown on a semi-insulating GaAs wafer by molecular beam epitaxy (MBE).20 The fabrication process began with the deposition of n-type metal contact (Pd/Ge/Au = 5 nm/35 nm/80 nm, Figure S1), followed by the etching of top contact (i.e., GaAs), except the region (∼480 × 35 μm2) covered by the metal (top left, Figure 1b). To take full advantage of spectral upconversion for GaAs solar cells printed on a luminescent medium, a properly configured cell design is critical. Specifically, the heavily doped bottom contact layer in conventional GaAs solar cells needs to be thinned down or eliminated to minimize parasitic optical losses and thus maximize the absorption of upconverted photons through the bottom surface of printed solar cells. As such, a p-type ohmic contact was intentionally formed on a Al0.3Ga0.7As backsurface field layer, while the optically thick (∼1.3 μm) GaAs bottom contact layer was removed by wet chemical etching after the release of solar cells from the growth wafer. Because of the high doping concentration in the BSF layer, an ohmic
synergistically combine plasmonic nanostructure, high-efficiency upconversion luminophore, and specialized epitaxial design of GaAs solar cells. In the following, systematic studies of optical and electrical properties, and photovoltaic performance of ultrathin GaAs solar cells integrated in the plasmonically engineered composite substrate at various materials configurations, together with numerical optical modeling, quantitatively elucidate the origin of improved photovoltaic performance and provide optimal design rules for the reported system. Figure 1a shows a schematic illustration of fabrication processes for a composite solar module of ultrathin (i.e., emitter + base = 200 nm) GaAs solar cells, configured to collect sub-bandgap (i.e., λ > ∼875 nm) photons through plasmonically enhanced spectral upconversion. Although microscale cell configuration was employed here for the proof-of-concept demonstration, the reported materials and fabrication strategies are readily applicable to conventional centimeter or wafer-scale devices.18,19 GaAs solar microcells have been fabricated by adapted procedures from our previous work.6,20,21 The epitaxial stacks of ultrathin GaAs solar cells employed in this study are composed of n-GaAs top contact (200 nm, Si-doped, 5 × 1018 cm−3), n-Al0.4Ga0.6As window (40 B
DOI: 10.1021/acsphotonics.8b01245 ACS Photonics XXXX, XXX, XXX−XXX
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Figure 2. (a) Tilt-view scanning electron microscope (SEM) image and schematic illustration of fabricated hole-post hybrid silver nanostructure, where geometric parameters for optimization including diameter and height of nanoholes (Dhole and hhole) and nanoposts (Dpost and hpost) are also shown. (b) Measured (solid line) and calculated (dotted line) total (i.e., sum of specular and diffuse) reflectance spectra of nanostructured plasmonic substrate in air and after the coating of upconversion nanocrystals (UCNC) and PMMA at near-normal incidence (θ = 8°). (c) Contour plots of electric-field intensity profiles of hole-post hybrid silver nanostructures under normally incident illumination at wavelengths of 690 and 980 nm in the surrounding medium of air (top) and PMMA (bottom), respectively, where phole = 700 nm, Dhole = 490 nm, hhole = 550 nm, Dpost = 340 nm, and hpost = 260 nm. (d) Emission spectra of NaYF4: Yb3+, Er3+ nanocrystals coated on plain and nanostructured silver reflectors, with PMMA and SU-8 as superstrate media, where a 968 nm continuous-wave (CW) laser (∼12 W/cm2) was illuminated at near-normal incidence (∼5°), and the emitted light was collected by a spectrometer. The inset shows the corresponding photographic images of upconversion luminescence.
upconversion nanocrytals in toluene was spin-coated onto the metallic reflector, followed by the successive depositions of poly(methyl methacrylate) (PMMA, ∼3 μm) and thermally curable epoxy (SU-8, ∼5 μm) as a transparent waveguiding medium.16 The fully functional GaAs solar cells were then printed onto the engineered upconversion substrate using a thin (∼1 μm) photocurable adhesive. Figure 1c shows optical micrographs of a GaAs solar cell printed on a plain and nanostructured silver substrate, respectively. Figure 2a shows a tilt-view scanning electron microscope (SEM) image of fabricated hole-post hybrid silver nanostructure, where the measured period (phole), diameter (Dhole), and height (hhole) of nanohole are ∼700, ∼495, and ∼546 nm, respectively, while the diameter (Dpost) and height (hpost) of nanopost are ∼336 and ∼266 nm. These dimensions are close to the calculated optimal designs (Dhole = 490 nm, hhole = 550 nm, Dpost = 340 nm, and hpost = 260 nm), as determined by 3D full-wave numerical modeling based on finite-difference timedomain (FDTD) method, where details of calculation methods are available elsewhere.16 Figure 2b depicts measured and calculated reflectance spectra of the nanostructured silver substrate at near-normal incidence (θ = 8°) in air as well as after the coating of upconversion medium (i.e., UCNC and PMMA). The “dips” of measured (solid line) reflectance spectra resulting from surface plasmon resonance shifted from ∼690 to ∼980 nm after the spin-coating of polymeric upconversion layer owing to the change of refractive index of surrounding medium of silver nanostructure and corresponding shift of resonance wavelengths, which was also
behavior was still obtained (Figure S2). Notably, the recessed p-type contact (∼240 × 40 μm2) was made within the cell boundary to avoid the undesired fracture during the pick-up and printing processes (top right, Figure 1b). After the delineation of active junction area (∼500 × 500 μm2) and isolated microcell layout (∼580 × 580 μm2; top right, Figure 1b), the sacrificial layer (i.e., AlAs) was removed under a polymeric anchor structure by delivering dilute hydrochloric acid under a polymeric anchor structure to yield “printable” arrays of ultrathin GaAs microcells (bottom left, Figure 1b), where the etch holes were made outside the junction area to avoid undesired defects to the active layers.19,22 The fully undercut-etched GaAs solar cells were then released from the growth wafer using an elastomeric stamp made of polydimethylsiloxane (PDMS), followed by the selective removal of the bottom contact layer by wet chemical etching (bottom right, Figure 1b). As upconversion luminophores, hexagonal-phase NaYF4:Er3+, Yb3+ nanocrystals were synthesized by previously reported schemes. 15,23 The upconversion nanocrystals (UCNC) can absorb sub-bandgap photons around 980 nm and convert them into red (∼660 nm) and green light (∼540 nm) that can be readily absorbed by GaAs solar cells.11,15 To obtain meaningful enhancement of cell performance via spectral upconversion under one-Sun illumination, a plasmonically engineered reflector incorporating hole-post hybrid silver nanostructure was fabricated by softimprint lithography and angled electron-beam evaporation of metals on a silicon substrate.15,16 Subsequently, a solution of synthesized C
DOI: 10.1021/acsphotonics.8b01245 ACS Photonics XXXX, XXX, XXX−XXX
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Figure 3. (a) Schematic illustration of the experimental setup for measuring photovoltaic performance of GaAs solar cells integrated in a composite solar module, where the same illumination area (∼2 × 2 mm2) was implemented using an anodized metallic plate with a square-shaped hole and a printed GaAs solar cell was located around the center of illuminated area. (b) Representative current density (J)-voltage (V) curves of ultrathin GaAs solar cells at various substrate configurations including on a plain silver reflector (red line), nanostructured silver reflector (green line), plain silver reflector with UCNC (blue line), and nanostructured silver reflector with UCNC (orange line), and growth wafer (black line). Corresponding plots of average short-circuit current density (Jsc) and open-circuit voltage (Voc) extracted from these curves are shown in (c) and (d). Error bars show maximum and minimum values of the error from the average value.
Table 1. Average Values of Short-Circuit Current Density (Jsc), Fill-Factor (FF), Open-Circuit Voltage (Voc), and Efficiency (η) Extracted from JV Characteristics in Figure 3ba Jsc (mA/cm2) on wafer plain Ag w/o UCNC plain Ag w/UCNC Ag NS w/o UCNC Ag NS w/UCNC
Voc (V)
η (%)
FF
avg
Err+/Err−
avg
Err+/Err−
avg
Err+/Err−
avg
Err+/Err−
−10.8 −15.9 −15.9 −16.8 −17.8
0.2/0.2 0.1/0.1 0.2/0.2 0.1/0.1 0.1/0.2
0.937 0.960 0.961 0.962 0.965
0.002/0.003 0.001/0.002 0.002/0.002 0.001/0.001 0.001/0.001
0.786 0.776 0.777 0.772 0.777
0.004/0.006 0.003/0.004 0.004/0.004 0.002/0.002 0.002/0.003
7.9 11.8 11.9 12.5 13.3
0.2/0.1 0.1/0.1 0.2/0.2 0.1/0.1 0.1/0.1
a
Maximum (Err+) and minimum (Err−) values of the error from the average (avg) are also shown.
(with vs without UCNC). Given that the printed microcell is surrounded by an open illuminated area (i.e., not covered by solar cells) on a module substrate, the effect of waveguided photon flux needs to be also considered.15,26−28 Light incident outside the active cell area can be coupled to guided modes in a polymeric waveguide and make an additional contribution to the absorption of a printed GaAs solar cell. Furthermore, as previously reported,16,29 upconversion nanocrystals used in this study can also provide downshifting luminescence (i.e., fluorescence) by absorbing a short-wavelength portion of solar illumination and emitting longer wavelength light. For upconversion nanocrystals coated in the area not covered by solar cells, the incident solar light can therefore undergo both upconversion and downshifting processes. A fraction of emitted light generated from these processes can be coupled into waveguided photon flux to the printed solar cell. In this context, for the accurate comparison of performance among different configurations of composite substrates, we implemented the same illumination area (∼2 × 2 mm2) using an
quantitatively consistent with the calculated spectra (dotted line) and contour plots of electric-field intensity (Figure 2c) obtained from 3D full-wave optical modeling based on a finitedifference time-domain method (FDTD). Before integrating GaAs solar cells, the effectiveness of a fabricated plasmonic reflector to enhance processes of spectral upconversion was confirmed by measuring the emission spectra of upconversion nanocrystals on plain and nanostructured silver reflectors (Figure 2d). As expected, the intensity of upconversion luminescence on the nanostructured silver reflector was strongly increased due to the effects of local electric-field amplification to enhance the absorption of upconversion luminophores as well as augmented upconverted emission via resonant coupling of surface plasmon polariton into radiative scattering modes.15,24,25 Photovoltaic performance of printed ultrathin GaAs solar cells was characterized under simulated AM1.5G solar illumination (1000 W/m2) at various configurations of reflector (planar vs nanostructured) and luminescent medium D
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Figure 4. (a) Top-view schematic illustration of confined illumination area (Cr/Au = 10 nm/150 nm; ∼620 × 620 μm2) implemented by photolithography, metal deposition, and liftoff processes to limit the contribution of waveguided photon flux to the photocurrent. (b) Average Jsc of printed GaAs microcells in various substrate configurations with and without the confined illumination. Error bars show maximum and minimum values of the error from the average value. (c) Representative current JV curves of ultrathin GaAs solar cells on a growth wafer (red line) and printed cells on a plain silver reflector with (green line) and without (blue line) the removal of the bottom contact layer. (d) Corresponding average Voc extracted from (c) and reverse-bias saturation current (I0) obtained from fitting the dark IV data using a single diode model in log-scale. Error bars show maximum and minimum values of the error from the average value.
performance to those on a transparent printing medium (i.e., without UCNC) because of the lack of surface plasmon resonance and negligible generation of upconverted photons under one-Sun illumination. By contrast, when a nanostructured silver reflector was used, the short-circuit current density of ultrathin GaAs microcells on an upconversion medium (orange line) was noticeably higher by ∼6.0% and ∼11.9% compared to those on a nanostructured silver reflector without UCNC (green line) and on a plain silver reflector with UCNC (blue line), respectively. The absolute enhancement of Jsc by the effect of spectral upconversion (i.e., between Ag NS with UCNC and without UCNC) is ∼0.7 mA/cm2, while there is virtually no enhancement on a plain silver reflector. As previously mentioned, the waveguided photon flux from surrounding area also partly accounts for the observed photocurrent enhancement, where both upconversion (i.e., by long-wavelength portion of solar illumination near 980 nm) and downshifting (i.e., by short-wavelength portion of solar illumination) processes can take place. To quantify such contribution of the “indirect” photon flux waveguided from the surrounding area of solar cell in the reported system, we performed measurements with a confined illumination area (∼620 × 620 μm2) by a lithographically defined metal aperture (Cr/Au = 10 nm/150 nm) such that the waveguiding effect is minimized (Figure 4a). As depicted in Figure 4b, the Jsc with 2 × 2 mm2 illumination area increased by ∼1.8−2.0% and ∼3.4% in plain and nanostructured silver reflectors, respectively, compared to those measured under confined (∼620 × 620 μm2) illumination. As expected, a nanostructured silver reflector provided a higher waveguided photon flux due to the diffuse reflection. We also examined the effect of removing
anodized metallic plate with a square-shaped hole, where a printed GaAs solar cell was located around the center of illuminated area (Figure 3a). Figure 3b shows a representative current density (J)-voltage (V) curves of an ultrathin GaAs solar cell printed on a composite substrate. Corresponding short-circuit current density (Jsc) and open-circuit voltage (Voc) extracted from these data are summarized in Figure 3c,d and Table 1. When GaAs solar cells were released from the growth wafer and printed onto a composite substrate, their photovoltaic performance always improved regardless of substrate configurations, owing to the increased light absorption upon the removal of the growth substrate that only permits a single optical path and thus limits the absorption of optically thin solar cells. The short-circuit current density of printed cells on a plain silver reflector without UCNC (red line) increased by ∼47% compared to that measured on the growth wafer (black line), where a fraction of long-wavelength photons incident directly onto the cell area and not absorbed by the solar cell was reflected back at the rear surface for the additional chance of being absorbed. On the other hand, the solar light illuminated outside the cell area can be also waveguided through a polymeric printing medium to partially contribute to the absorbed photon flux of the printed solar cell. In this regard, a nanostructured silver reflector (green line) provided higher improvements in Jsc than a plain silver reflector (red line), which is attributed to the oblique-angle scattering of incident solar light by the hole-post silver nanostructure and more effective coupling into guided modes by satisfying the condition of total internal reflection. As upconversion nanocrystals were incorporated, cells printed on a plain silver reflector (blue line) exhibited comparable E
DOI: 10.1021/acsphotonics.8b01245 ACS Photonics XXXX, XXX, XXX−XXX
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GaAs bottom contact layer upon the Jsc and Voc of printed solar cells as summarized in Figure 4c. When a released cell is printed on a plain silver reflector without etching the 1.3 μm thick GaAs contact layer (blue line), the increase of Jsc is nearly negligible due to the severe recombination of photogenerated carriers in the heavily doped contact layer.20,21 Notably, despite such little enhancement of Jsc, the open-circuit voltage (Voc) still increased by ∼3 mV (Figure 4d). This enhancement of Voc is attributed to the effect of a low index medium below the printed solar cell and resultant increase of external luminescence efficiency (ηext) for the emitted photons through the front surface of the cell, consistent with the following expression of Voc,30,31 Voc = Vdb + =
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Sung-Min Lee: 0000-0001-9446-9122 Chayan Dutta: 0000-0003-4839-2245 Jongseung Yoon: 0000-0001-5342-1181 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Authors gratefully acknowledge support from National Science Foundation (ECCS-1202522, ECCS-1509897, CBET1707169). The authors thank Donghai Zhu and John Curulli for help in using facilities at Keck Photonics Laboratory and Center for Electron Microscope and Microanalysis (CEMMA) at USC.
kT ln(ηext) q
yz kT ijj Jsc lnjjj + 1zzzz j J0 z q k {
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where Vdb, k, T, q, and J0 are open-circuit voltage for the detailed balance limit, Boltzmann’s constant, absolute temperature, electronic charge, and reverse-bias saturation (or dark) current density, respectively. Such an increase of ηext upon printing means more efficient photon recycling that can lead to greater quasi-Fermi level splitting and thus increase of Voc.4,30,32 The corresponding reverse-bias saturation current (I0) obtained from fitting the dark IV data using a single-diode equation33 (Figure S3 and Table S1) decreased from ∼4.7 × 10−13 A (on wafer) to ∼3.0 × 10−13 A (printed on plain Ag), thereby supporting this analysis. On the other hand, when the bottom contact layer was completely removed by wet chemical etching (green line in Figure 4), the long wavelength light that was not absorbed and reflected either at the rear surface of the solar cell or by the underlying silver reflector can readily reach to the base for reabsorption without parasitic losses, which results in the substantial enhancement of both Jsc and Voc. In summary, the results presented here illustrate a type of composite solar module that can achieve improved performance of single-junction GaAs solar cells by additionally capturing sub-bandgap long-wavelength photons via spectral upconversion. Specialized epitaxial design and printing-enabled integration of GaAs solar cells onto nanostructured silver reflector coated with an upconversion medium enabled the additional boost of photovoltaic performance by combined effects of plasmonically enhanced spectral upconversion, photon recycling, and waveguiding. Although GaAs was used for the above demonstration, we anticipate that the reported fabrication concepts and design strategies can be also applicable to broad classes of solar cell materials and upconversion luminophores to provide a practical route to address the thermodynamic limit of single-junction solar cells.
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REFERENCES
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.8b01245. Experimental methods, transmission line model (TLM) measurements of p- and n-type contacts, fitting curves by a single-diode equation, and extracted diode characteristics (PDF). F
DOI: 10.1021/acsphotonics.8b01245 ACS Photonics XXXX, XXX, XXX−XXX
ACS Photonics
Letter
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DOI: 10.1021/acsphotonics.8b01245 ACS Photonics XXXX, XXX, XXX−XXX
