R.R. King, H.L. Cotal, D.R. Lillington, J.H. Errner, N.H. Karam. Spectrolab Inc., 12500 .... data is courtesy of. David King, Sandia National L.aboratories. 966 ...
Ge CONCENTRATOR
CELLS FOR Ill-V MULTIJUNCTION
DEVICES
D.J. Friedman, J.M. Olson, S. Ward, T. Moriarty, K. Emery, Sarah Kurtz, A. Duda National Renewable Energy Laboratory (NREL), 1617 C,ole Blvd., Golden, CO 80401 USA R.R. King, H.L. Cotal, D.R. Lillington, J.H. Errner, N.H. Karam Spectrolab Inc., 12500 Gladstone Ave, Sylmar, CA 91342 USA
ABSTRACT
contributioli to the overall three-junction device efficiency would be expected to increase with concentration to as much as -15% at 1000 suns for an ideal device. On the other hancl, a poor Ge junction can adversely affect the performance of the entire three-junction cell.
We identify a failure mode due to a photoactive back contact for Ge concentrator solar cells. This problem manifests itself as a leveling off and subsequent decrease of open-circuit voltage (V,,) as the concentration increases above -20 suns. Correction of this problem yields a much improved Ge cell for which V,, increases in an almost ideal n=l manner from 0.2 volts at one sun to 0.4 volts at 1400 suns. This cell’s fill factor remains at or above its one-sun value up to 500 suns, confirming that this cell is fully suitable for high-concentration use. We show that solving the back-contact problem can significantly improve the high-concentration performance of GalnP/GaAs/Ge three-junction solar cells.
The Ge junction is thus an important part of GalnP/GaAs/Ge cells under concentration, but the concentrator performance of the Ge junction has been little studied to dlate. Here, we identify a failure mode for the Ge junction, a photoactive back contact, that manifests itself only under concentration. We then demonstrate that correction of this problem produces a Ge cell capable of near-ideal performance up to concentrations above 1000 suns. Finally, we show the consequences of these results for GalnP/GaAs/Ge three-junction concentrator cefls. DE\IICE FABRICATION
AND MEASUREMENT
INTRODUCTION Ge single junctions were fabricated at NREL by growing n-type GalnP epilayers epitaxially on p-type Ge substrates by metal-organic vapor-phase epitaxy (MOVPE). The resulting junctions are found to be diffused n-GalnP/n-Ge/p-(Ge homojunctions rather than n-GalnP/pGe heteroliunctions. Planar backside contacts and front grid contacts weire applied by electroplating Au. For initial versions of the device, the back surface of the wafer was etched -0.3pm deep before the Au back contact was applied, giving a contact that appeared (under room light illumination) to be adequately ohmic. More will be said below about this contact. The resulting device structure is shown in Fig. l(b). One-sun IV characteristics were measured on a continuous-illumination solar simulator. Concentrator IVs were measured on a pulsed (“flash”) solar simulator. These measurements were selectively compared with measurements under continuous concentrated illumination. For the concentrator measurements, a linear relation between the short-circuit current (J,,.) and concentration is assumed [4], so that the concentration ratio was estimated from the ratio of J,, under concientration to the one-sun J,,.
Recently, a GalnP/GaAs/Ge three-junction solar cell, designed and grown at Spectrolab and processed at NREL, has set a record efficiency of 32.3% (AMl.5 direct, 440 suns) [l-3]. The device structure is shown schematically in Fig. 1 (a). A key part of this structure is the Ge third junction, which can contribute close to 10% of the total device efficiency at one sun. Since the Ge bandgap is much lower than that of GalnP or GaAs. its relative
1
grids
r
GalnP cell
0.25~pm contact 0.25~pm window 0.06~pm emitter
H
tunnel junction GaAs cell
170~pm base
(a) F$nP/GaAs/Ge
RESULTS AND DISCUSSION
Au back contact (b) Ge cell
Fig. 2(a) shows the open-circuit voltage (V,,) vs. concentration (i.e., vs. J,,) for a Ge cell processed with the initially-used back contact processing described above. The data for this configuration of the device are labeled
Fig. 1. Schematic cross sections of (a) the GalnP/GaAs/Ge three-junction solar cell, and (b) an expanded view of the Ge third junction in the configuration discussed in this paper. The drawings are not to scale.
“bad back contact” in the figure. (The “good back contact” data will be discuissed below.)
O-7803-5772-8/00/$1 0.00 0 2000 IEEE
965
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At concentrations below 20 suns, V,, increases with J,, in a manner close to that expected for the ideal-diode relation V,, = r-&T/e In[(J,,/J,)+l], with n=l. However, at higher concentrations, there is a radical departure from the ideal behavior: V, levels off and subsequently decreases as the concentration is further increased. The decrease in ‘V, suggests the presence of another junction opposed to i.he intentionally-grown junction, which turns on only at high light intensities. It is helpful to note that similar behavior has been observed for Si concentrator cells, and was traced to a parasitic junction at the back contact [5]. To investigate whether this mechanism could explain the behavior of Fig. 2, we took a piece of wafer with a Ge junction grown nominally identical to the device in Fig. 2, etched off the intentionally grown junction from the front side and plated a planar contact on it, and put grid contacts on the other side (the “back” side). The illuminated -one-sun IV curves were measured for these backside devices; one such IV curve is shown in Fig. 3. The contact is in fact a photoactive junction, which would act as a buckilng junction relative to an intentionallygrown junction on the front side of the wafer. However, it is so leaky and its V,, is so small at one sun that it is not easily distinguished1 from an ohmic contact. After this backside-junction IV curve was measured, the gold grids were etched off, and the wafer underneath was etched
-0.01
0.00 0.02 0.01 volts Fig. 3. IV curves for device formed by plating grids to backside of Ge wafer on which a Ge junlction with a GalnP window layer was grown on the front side, and then etched off. The sign of the voltage is opposite to that of the intentionally-grown junction. several pm deep. New grids were then applied, and their IV curve measured. This curve is also shown in Fig. 3; it is ohmic, showing that etching the Ge wafer backside removed the parasitic junction. This suggests very strongly that the origin of the parasiitic junction is diffusion of materials in the reactor ambient, presumably the Ge ntype dopants phosphorus or arsenic, into the back of the Ge wafer. The reason that the parasitic backside junction affects the device performance only under concentrated illumination is that the light must pass through the thickness of the wafer (170 pm, in the case of the device of Fig. 2) to within a diffusion length of the back side to be collected by the back junction. That this is possible at all is due to the fact that the absorption coefficient of Ge between its 0.67-eV indirect gap and its 0.8-eV direct gap is very low, so that photons in this energy range have a chance of making it through thle wafer. Figure 4 shows the modeled internal quantum efficiency of an ideal backside Ge device, illustrating that very small photocurrents are to be expected for a backside junction at one sun.
v-I-=
.--
1.o
- 0.8 Co 2 - 0.6 g 2 Fig. 2. (a) V,, and (b) fill factor vs. concentration for a typical Ge device (no. MB640-2399#11). The concentration ratio labeling the upper axis was derived by dividing the J,, from the lower axis by the one-sun J,,. The device was originally processed and measured with the “bad” back contact, then reprocessed and remeasured with an altered, “good” back contact, as described in the text. The dashed line in (a) indicates how V,, would increase with concentration for an ideal cell with ideality factor n=l. The dashed line in (b) shows a modeled fill factoir calculated from a simple lumpedresistance model of series resistance from the emitter and grids; resistance parameters for these were measured from test structures on the same wafer as the device. The data points marked with asterisks were measured under continuous illumination, while the other data were measured1 under flash illumination.
- 0.4 3 !s - 0.2 g I ~1 I 0.0 1.1 1.2 1 .o 0.8 0.9 photon energy (eV) Fig. 4. Modeled internal quantum efficiency (QE) of an ideal Ge backside device, and transmittance of a typical Fresnel lens. The QE calculation assumes a 50-pm diffusion length in the Ge wafer, typical for these cells (note that changing the diffusion length changes the amplitude of the QE curve but does not significantly alter its shape). The transmittance data is courtesy of David King, Sandia National L.aboratories. o.ool
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Coincidentally, many commonly-used acrylic Fresnel concentrating lenses have a strong transmission minimum in this spectral region [6], as shown in Fig. 4. This coincidence raises a measurement issue: lowering the incident light intensity in the 0.67-0.8 eV range will lower the back-junction photocurrent and thus suppress the V,droop effect seen in Fig. 2. Therefore, the extent to which this effect is observed will depend on whether or not acrylic Fresnel lenses are used to concentrate the light. As an aside, note that the lens transmission valleys shown in Fig. 4 would be problematic for future-generation GalnP/GaAs/l -eV-cell/Ge four-junction cells, for which the current requirements for the bottom two cells will be very demanding due to the requirement of current-matching with the top cells [7]. This suggests that such four-junction devices might prove more compatible with reflective-optics concentration than with Fresnel-lens concentration. The cure for the V,, rollover illustrated in Fig. 2 is simply to remove the surface of the backside of the Ge wafer, either by etching or mechanical polishing, before applying the back contact. For the device of Fig. 2, the back contact and several pm of the underlying Ge wafer were scraped off, and new backside metallization was applied. The concentrator performance of the resulting backside-contact-corrected “good back contact” device is shown in Fig. 2 (filled circles). The dependence of V,, on J,, is now almost perfectly described by an ideal n=l characteristic, all the way up to concentrations of 1400 suns, confirming that the V,, behavior for the data labeled “bad back contact” in Fig. 2 was indeed due to a photoactive back junction. Note that the device now achieves a V,, of 0.4 V at 1400 suns. Fig. 2 also shows the fill factor vs J,, for this same device, along with a calculated fill factor assuming a simple lumped-resistance model of series resistance from the emitter and grids; resistance parameters for these were measured from test structures on the same wafer as the device. With the good back contacts, the agreement between the measured and modeled fill factor is excellent, indicating that there are no significant losses internal to the device - i.e., essentially all the loss in fill factor is due to losses in the emitter and grids, losses that would not come into play for a Ge device serving as a third junction in a GalnP/GaAs/Ge stack. The results
above 10
indicate
that, with a proper
suns
contact, thle Ge cell is suitable for high-concentration operation as the third junction of the GalnP/GaAs/Ge structure. Figure 5 shows V,, vs. concentration for a Spectrolab-grown GalnP/GaAs/Ge three-junction ceil very similar to the device of Ref. [I]. The back contact of this device was initially fabricated similarly to the “bad-backcontact” device of Fig. 2, without aidequate etching of the Ge wafer back surface. Consequently it shows the same characteristic behavior of a dip in V,, with increasing concentration, as shown in Fig. 5 with open circles. For this device, it proved possible to remove the back (contact, etch about 1.5 plm from the Ge wafer back surface, and then plate on a new back contact. The dependence of V,, on J,, (i.e., on concentration) for the device with the back contact fixed is shown in Fig. 5 with filled circles. Also shown with a solid line is the VocJ,, dependency corresponcling to a model three-junction device with ideality factor n=l.l for each junction. Clearly, fixing the back contact results in near-ideal concentrator behavior. CONCLUSIONS With the elimination of an unintended diffused photoactive back junction, we have demonstrated a Ge third junc:tion whose near-ideal behavior under concentration makes this device fully suitable as a highperformance third junction for colncentrations well over 1000 suns. Eliminating the photoactive back junction significantly improved the high-concentration performance of the recently-demonstrated record 32%-efficient threejunction GalnP/GaAs/Ge concentrator solar cell [l]. REFERENCES
PI H.L. Cotal, D.R. Lillington, J.H. Ermer, R.R. King, S.R.
Kurtz, D.J. Friedman, J.M. Olson, S. Ward, A. Duda, K.A. Emery, and T. Moriarty, “Triple-junction solar cell efficiencies above 32%: the promise and challenges of their application in high concentration-ratio PV systems,” this conference proceedings. PI N.H. Karam, R.R. King, M. Haddad, J.H. Ermer, H. Yoon, H.L. Cotal, R. Sudharsanan, J.W. Eldredge, K. Edmondson, D.E. Joslin, D.D. Krut, M. Takahashi, W.T. Nishikawa, M. Gillanders, J. Granata, P. Hebert, and D.R. Lillington, “Recent B.T. Cavicchi, Developments in High-Efficiency Gao slnoaP/GaAs/Ge Dual- and Triple-Junction Solar Cells: Steps to NextGene&ion PV Cells,” to be published in Solar Energy Mater. Solar Cells (2000). [31 R.R. King, N.H. Karam, J.H. Ermer, M. Haddad, P. Colter, T. Isshiki, H. Yoon, H.L. Cotal, D.E. Joslin, D.D. Krut, R. Sudharsanan, K. Edmondson, B.T. Cavicchi, and D.R. Lillington, “Next-generation highefficiency Ill-V multijunction solar cells,” this conference proceedings. of the Isc vs. irradiance 141 J.M. Gee, “Characterization relationship for silicon and Ill-V concentrator ceils,” 19th /EEE PVSC, 1987, pp. 1390-l 395. RR. King, Spectrolab (unpublished). ;:i Lens transmission data. provided by D. King, Sandia Nationlal Laboratories. 171 S.R. I0” 2.7 2.6 100
1000 J,, (mA/cm’)
three-junction Fig. 5. V,, vs. J,, for a GalnP/GaAs/Ge device (device no. 9X-5672-9A#6) before and after the back contact is fixed.
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