NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Manufacturing of HighEfficiency Bi-Facial Tandem 

Concentrator Solar Cells 

February 20, 2009 — August 20, 2010 

Steven Wojtczuk

Spire Semiconductor Corporation, LLC

Hudson, New Hampshire 

Subcontract Report

NREL/SR-5200-51767

June 2011
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

National Renewable Energy Laboratory

1617 Cole Boulevard

Golden, Colorado 80401

303-275-3000 • www.nrel.gov

Contract No. DE-AC36-08GO28308

Manufacturing of HighEfficiency Bi-Facial Tandem 

Concentrator Solar Cells 

February 20, 2009 — August 20, 2010 

Steven Wojtczuk

Spire Semiconductor Corporation, LLC

Hudson, New Hampshire 

NREL Technical Monitor: Harin S. Ullal

Prepared under Subcontract No. NAT-9-88012-04

Subcontract Report

NREL/SR-5200-51767

June 2011
This publication was reproduced from the best available copy

submitted by the subcontractor and received no editorial review at NREL.

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1 

Table of Contents

1 Executive Summary ................................................................................................................................. 2

1.1 Final Cell Performance .......................................................................................................... 2

1.2 6MW/Year Capability ........................................................................................................... 3

2 Background .............................................................................................................................................. 3

2.1 Overview of Competing Technologies ................................................................................. 3

2.2 Overview of Spire’s Bi-Facial Growth Process .................................................................... 4

2.2 Commercialization Issues ..................................................................................................... 5

2.3 Space Applications ................................................................................................................ 5

3 First-Order Modeling: Roadmap to 42.5% Cell...................................................................................... 6

4 Cell Discussion......................................................................................................................................... 7

4.1 Anti-Reflection (AR) Coating ............................................................................................... 7

4.2 Top Contact Grid Design ...................................................................................................... 8

4.3 General Overview of Bi-Facial Epigrowth and Cell Process .............................................. 12

4.4 1.9eV InGaP Top Subcell Development ............................................................................. 14

4.5 GaAs Middle Cell Development ......................................................................................... 16

4.5.1 Bandgap Narrowing to Extend Cutoff .......................................................................... 17

4.5.2 Pseudomorphic Absorption Layer to Extend Cutoff .................................................... 17

4.6 Tunnel Junction Development............................................................................................. 19

4.7 Lattice Mismatched InGaAs Bottom Cell Development .................................................... 23

4.7.1 InGaAs Cell Composition (Cutoff Wavelength) .......................................................... 23

4.7.2 General InXGa1-XAs Absorption Model ........................................................................ 24

4.7.3 InGaAs Cell Thickness vs 1xJsc .................................................................................. 25

4.7.4 Use of a Doping Grade (Drift Field) in InGaAs Base to Enhance Diffusion Length ... 25

4.7.5 Final Bottom Cell Epistructures used in Deliveries D09, D10 and D11 and IV Data . 26

4.8 Tandem Cell Data ................................................................................................................ 27

4.8.1 Tandem Illuminated IV Data ........................................................................................ 27

4.8.2 Tandem QE Data .......................................................................................................... 28

4.8.3 Final D11 Epistructure.................................................................................................. 30

5 Commercialization Efforts..................................................................................................................... 30

5.1 Burn-In Data at 85C 200Hrs ............................................................................................... 30

5.2 Commercial Companies Evaluating Spire Tandems ........................................................... 31
2 

1 Executive Summary

Spire Semiconductor made concentrator photovoltaic (CPV) cells using a new bi-facial growth 

process and met both main program goals: 

a) 42.5% efficiency 500X (AM1.5D, 25C, 100mW/cm2

) 

b) Ready to supply at least 3MW/year of such cells at end of program.

We explored a unique simple fabrication process to make a N/P 3-junction InGaP/GaAs/InGaAs 

tandem cells1

1.1 Final Cell Performance

. First, the InGaAs bottom cell is grown on the back of a GaAs wafer. The wafers 

are then loaded into a cassette, spin-rinsed to remove particles, dipped in dilute NH4OH and 

spin-dried. The wafers are then removed from the cassette loaded the reactor for GaAs middle 

and InGaP top cell growth on the opposite wafer face (bi-facial growth). By making the epitaxial 

growth process a bit more complex, we are able to avoid more complex processing (such as large 

area wafer bonding or epitaxial liftoff) used in the inverted metamorphic (IMM) approach to 

make similar tandem stacks. We believe the yield is improved compared to an IMM process. 

After bi-facial epigrowth, standard III-V cell steps (back metal, photolithography for front grid, 

cap etch, AR coat, dice) are used in the remainder of the process. 

NREL measurement of a large (~1cm2) bi-facial growth cell was 42.2% at 500X (Figure 1) with 

a measurement error such that it is arguable that the 42.5% program goal was met. This cell has 

the highest efficiency ever reported by NREL (42.3% at 505 suns, AM1.5D, 25C). 

Figure 1. NREL-measured illuminated IV of record-efficiency Spire Semiconductor cell.
3 

1.2 6MW/Year Capability

An estimate of the cell throughput (>6MW/year) with present equipment and facilities follows:

• Each 0.97cm2 photoarea cell outputs 500X x 0.1W/cm2 x 42.5% or 20.6W. 

• There are 52 0.97cm2

 cells on a 4in wafer, so up to 1070W is available per wafer. 

• The Veeco (Emcore) E450 program epigrowth reactor grows 13 4in wafers per run. 

• Two complete bi-facial growths can be done in two shifts (26 wafers or ~27.8kW/16 hrs). 

• 3000kW/27.8kW/(2-shift day) or 135 days (if 80% yield) are needed to grow cells. 

• A resource efficient batch size in our present non-optimized wafer fab is 13 wafers.

• We could finish 2 wafer batches/day for 135 days to make 3MW of cells (>6MW/year). 

2 Background 

2.1 Overview of Competing Technologies

High efficiency III-V concentrator photovoltaic (CPV) cells are generally based on the two 

junction In0.49Ga0.51P/GaAs cell pioneered by NREL’s Jerry Olson, Sarah Kurtz, et al.2. This cell 

was commercially adapted for use as one sun space solar cells by Spectrolab and Emcore (which 

acquired ASEC/Tecstar). A third junction was added in the Ge wafer used for III-V space cells 

for mechanical robustness. These companies have adapted this triple-junction InGaP/GaAs/Ge 

cell for CPV use, producing fine concentrator cells with the best cells having a record efficiency 

of 41.6% at 364X, 25C3

One approach to improve the InGaP/GaAs/Ge three junction tandem is to adjust the material 

compositions and therefore bandgaps of the upper and middle cell for better current matching, 

trading off the improvement in overall Jsc with some efficiency decrease due to the minoritycarrier lifetime-lowering dislocation defects from the lattice mismatch to the Ge in the upper and 

middle cells (Figure 2). With this approach, In0.65Ga0.35P/In0.17Ga0.83As/Ge metamorphic cells 

with 1.1% mismatch were made with efficiency up to 41.1% at 454X by Fraunhofer

. However, the Ge has too low a bandgap for optimal use in latticematched InGaP/GaAs tandems. The Ge’s high Jsc (~20mA/cm2

 AM1.5D, 100mW/cm2) is 

current mismatched to the InGaP and GaAs subcells (~15mA/cm2) while the low Ge bandgap 

also gives less Voc and a lower FF than a higher, more optimal bandgap bottom subcell. One 

should be able to do better with a different scheme. 

4

. 

Figure 2. Left: lattice-matched InGaP/GaAs/Ge cell. Right: metamorphic cell with better Jsc but 

lower Voc. Dislocations indicated by squiggly lines.
4 

Another approach to achieve higher efficiency is to substitute lattice-mismatched InGaAs for the 

Ge bottom cell (a “Ge-free” approach). The inverted metamorphic (IMM) process (Figure 3) can 

maintain lattice match for the top and middle cell by growing an inverted cell stack with the 

lattice mismatched InGaAs cell last. Dislocations in the InGaAs thread upward during growth, so 

that the top and middle cells can remain defect-free. Geisz et al.5 used a variation of this IMM 

technique with a slightly mismatched middle cell to achieve 40.8% at 326X from a 0.7% 

mismatch In0.49Ga0.51P/In0.04Ga0.96As/In0.37Ga0.63As cell.

Figure 3. IMM process: Inverted growth; wafer bond to carrier; epitaxial liftoff and flip.

There are approaches (such as the VHESC program championed by Dr. Barnett) pursuing a 

mechanical stack multijunction approach6

2.2 Overview of Spire’s Bi-Facial Growth Process

, which promise efficiencies of 55%. These programs 

use a mechanical stack of 4 or more separate cells, or a lateral approach in which the cells are 

placed side-by-side and the spectrum is split and re-directed onto the proper cell. The argument 

for the proposed cell versus such a competitor would be we believe a reliable high yield single 

42.5% cell should be more cost-effective than a 55% mechanically complex approach using 

multiple (4 or more) cell die.

Spire Semiconductor’s bi-facial (epitaxial) growth (BFG) process1 (Figure 4) makes a similar 

InGaP/GaAs/InGaAs tandem cell without using whole-area wafer bonding and epitaxial liftoff 

steps which are not standard III-V cell process steps and may be low yield. The dislocations of 

the lattice mismatched cell are isolated from affecting the middle and top cells on the opposite 

wafer side by the thick GaAs wafer. Also, no carrier wafer is needed as in the IMM approach.

Figure 4. Bi-facial process: grow InGaAs cell on back, rinse and dry wafer to clean particles, flip 

wafer, and grow rest of cell. InGaAs illuminated through wafer.
5 

A bi-facial epigrowth cell was first described by Varian7 in 1990. They discuss an 

AlGaAs/GaAs/InGaAs tandem and clearly describe the idea of "farside" wafer growth, but did 

not actually make a bi-facial cell. In 2005, King8 refers to the Varian work and has a crosssection of the idea of a bi-facial InGaP/GaAs/InGaAs cell, but no cell was made. Finally, 

Wanlass9

2.3 Commercialization Issues

has considered bi-facial cells for InGaAsP/InP wafer/InGaAs tandems.

Commercialization of the bi-facial epigrowth of InGaP/GaAs/InGaAs tandems faces 3 hurdles. 

First, the cells have to be more efficient than the “standard” InGaP/GaAs/Ge tandem technology 

that set a past record efficiency of 41.6% at 364X. We have exceeded this efficiency (42.3% at 

406X, 42.2% at 505X, 25C, AM1.5D). Since we have been working on the basics of the bi-facial 

epigrowth tandem for only about 18 months and have exceeded or equaled the results of the 

InGaP/GaAs/Ge cell technology that has been under development for almost twenty years, we 

feel the approach has promise. There is still potential for efficiency improvement as the subcell

dopings, thicknesses and compositions are not fully optimized. 

Second, the cells have to be as reliable as the InGaP/GaAs/Ge technology. The cells have passed 

minimal reliability requirements of this program and more extensive reliability testing required 

by module manufacturers is on-going. 

Third, the cells must have high yield and be cost-effective. The process is simple compared with 

the IMM approach (no wafer-scale epitaxial liftoff or wafer bonding or carrier substrates) and is 

similar to the standard lattice-matched InGaP/GaAs/Ge process. Since the yield will be similar 

and it comes down to arguments about the cost of GaAs vs Ge wafers and epigrowth time. 

In moderate quantities, GaAs and Ge wafer costs seem similar (~$80/4in wafer in quantities 

>30,000/yr circa 2010). The epigrowth is longer in the bifacial process than for lattice-matched 

InGaP/GaAs/Ge because of time added for the InGaAs cell as opposed to the junction in Ge 

being formed by dopant diffusion during the growth of the III-V cells. This added growth cost 

must be balanced against whatever efficiency benefit is ultimately achieved. However, the 

growth times would be much closer if lattice-mismatched metamorphic InGaP/GaAs/Ge cells 

became the new production standard for Ge-based cells.

2.4 Space Applications

It is unlikely that InGaP/GaAs/InGaAs tandems made with this bi-facial growth process would 

be useful as one-sun space cells. Near-infrared-transparent GaAs wafers are needed so the 

bottom cell can receive light. A Ge wafer, preferred for space applications versus GaAs due to its 

mechanical strength, would absorb this 890-1300nm infrared light, so GaAs wafers must be 

used. Although the densities of GaAs and Ge are similar, the Ge strength allows use of thinner 

wafers and therefore higher W/kg numbers in one-sun systems. However, these cells could be 

useful in space concentrator systems where, for a given power, the required cell area is smaller 

by the concentration factor, and a thicker GaAs substrate on the cell may not be as significant a 

weight penalty versus a one-sun system. 
6 

3 First-Order Modeling: Roadmap to 42.5% Cell

Table 1 shows the final “roadmap” to the 42.5% goal which was updated throughout the program 

and used to indicate which subcells were on target or needed improvement. Table 1 compares 

model vs measured data from single junction (1J) subcells and 3J tandems. The data is 

normalized to 1cm2. 1J subcells are easier to analyze. The GaAs subcell includes both tunnel 

junctions (TJs) and has an integrated InGaP filter of the same thickness as the top cell (to mimic 

TJ dopant diffusion). The InGaAs subcell is illuminated through the GaAs wafer. 

Table 1. “Roadmap” for a >42% Concentrator

Subcell Perfomance Models Top Middle Bottom Comments

Subcell Wafer IDs 1115-2 1051-1 1140-10

Material InGaP GaAs* InGaAs *GaAs has thin 1%InGaAs to extend cutoff

Bandgap (eV) 1.88 1.424 0.93

λI cut-on (nm) 370 670 890 Cut-on and cutoff wavelengths approximate

λF cut-off (nm) 660 890 1330

Upper limit Jsc: (100%) QE (λI to λF) 16.51 15.54 19.27 conditon: AM1.5D 0.1W/cm2

Measured 1xJsc by Illuminated IV (CW) 14.22 14.20 14.32 wafer avg: CW sim adj by 3 filtered ref cells

"Boxcar" Average QE (flat from λI to λF) 0.86 0.91 0.74 (measured Jsc)/(100% QE Jsc)

Jsc Average Loss Analysis

 Grid shadow (4μ lines, 115μ gaps, wings) 0.03 0.03 0.03 SEM: liftoff Au lines 2um on top, 4um at base

Measured Avg. AR Refl. (λI to λF) 0.067 0.019 0.061 avg of Optronics QE test set refl data

20nm InAlP window loss (50% collection) 0.026 0.43mA loss calculated from InAlP n k data

20nm GaAs TJ absorption 0.028 0.44mA loss calculated from GaAs n k data

GaAs wafer absorption 0.067 1.3mA loss calculated from GaAs n k data

Collection (diffusion length) Loss 0.02 0.01 0.10 Estimate: adjusted to fit measured Jsc

Sum of Losses 0.14 0.09 0.26 sum of losses equals to 1 - Boxcar QE

Model 1xJsc (mA/cm2) 14.2 14.2 14.3

500xJsc (A/cm2

) 7.1

Dark current IO prefactor D (A/cm2/K3

) 0.008 0.003 0.035

I

O: DT3

exp(-Eg/kT) (A/cm2) 3.6E-27 6.9E-20 1.8E-10 compare to radiative limit below

Shockley Radiative Limit - absorbing wafer 3.1E-28 1.2E-20 9.4E-13 q (2πkT/h2c3) (nr2+1) Eg2

 exp(-Eg/kT) (A/cm2)

Model 1xVoc (V) at 25C 1.455 1.025 0.468 nr is refractive index in above equation

Model 500xVoc (V) at 25C 1.615 1.184 0.627 D adjusted to fit to measured 500xVoc

Data from: NREL NREL

Process Lot ID: L810 L873

InGaP/GaAs epirun ID: 0952-3 1141-6

InGaAs epirun ID: 0951-3 1140-1

Cell ID on Wafer: 15 43

Model vs Best Cell Data

Tandems model Dlvy D09 Dlvy D11

1xJsc (mA/cm2) 14.2 13.63 14.07 target (model) average vs "best cell" data

500xJsc (A/cm2) 7.1 6.8 7.0

1xVoc (V) 2.948

500xVoc (V) 3.427 3.495 3.468

Rs and FF Analysis

Calc. Emitter R (Ω-cm2) 0.006 Measured 500 Ω/sq emitter Rsh

Calc. Grid R (Ω-cm2) 0.008 Calculated 0.005 Ω/sq Au grid metal Rsh

 Est. 2 TJs (Ω-cm2) 0.003 2.5mΩ for 20nm upper +0.5mΩ for 30nm lower TJ

Sum of series Rs (Ω-cm2

) 0.017

Ideality "n" (3J) 3 assume n=1 per junction

1xFF 0.880 model: v=qVoc/(nkT); r=V/Jm; rs=Rs/r

500xFF 0.862 0.851 0.866 FF = ([v-ln(v+1)]/(1+v)) (1 - rs)

1xEff 0.368

500xEff 0.419 0.41 0.42
7 

Tandem efficiency is determined by the quantum efficiency, dark currents, and series resistance 

of the subcells. Table 1 indicates the average QE for each subcell, obtained as the ratio of the 

measured 1xJsc from a simulator IV measurement and the 1xJsc that would be obtained over the 

response range if perfect QE using ASTM G173 data. Although using the complete spectral 

response is more accurate, the average QE allows for quick back-of-the envelope calculations 

useful during the program. The Jsc loss analysis indicates the relative importance of losses, 

including InAlP window, GaAs tunnel junction layer, and GaAs substrate absorption for the top, 

middle, and bottom cells respectively. Collection efficiency of the InGaAs bottom cell is much 

higher than the other subcells due to dislocations shortening the diffusion lengths. 

The dark current in Table 1 is empirically adjusted to match the measured wafer Voc averages of 

the component subcells at 500X concentration. Subcell dark current for the top and middle cells 

is within 10X of the Shockley-Queisser radiative limit, as modified by Henry10

4 Cell Discussion 

 for an absorptive 

substrate. This means we cannot do much better with Voc except for the InGaAs bottom cell, 

which is 200X higher than the limit due to the presence of dislocations.

4.1 Anti-Reflection (AR) Coating

In this program, we examined the following AR films (refractive indices at 632nm):

• MgF2: n ~ 1.38 (e-beam and thermal)

• SiO2: n ~ 1.46 (PECVD and e-beam)

• Al2O3: n ~ 1.6 (e-beam)

• SiN(H): n adjustable 1.8 to 2.3 (PECVD ); n > 2.0 has absorption below 500nm 

• Ta2O5: n ~ 2.0 

• ZnS: n ~ 2.35 (e-beam and thermal)

• TiO2: n ~ 2.35 (e-beam, using Ti3O5 source) 

Originally, we planned to use a robust PECVD SiO2/SiNH AR coating, but the nonstochiometric SiNH had substantial absorption below 500nm for higher (Si-rich) indices. We 

then decided to use MgF2/ZnS for cells working into air and evaporated SiO2/ZnS for cells under 

glass. The SiO2 AR film will be part of a semi-infinite medium when used with index-matched 

epoxy and a thick coverglass. We experimented with Ta2O5 and TiO2 to see if we could get a 

higher index (and broader bandwidth) coating, but although higher indices have been reported in 

the literature, we obtained lower indices and erratic results compared to the ZnS. We solve the 

issue of patterning the AR by use of an image reversal photolithography step prior to the 

MgF2/ZnS coating. Figure 5 shows the final program AR coat on a three junction tandem, a two 

junction (diagnostic) tandem with no InGaAs bottom cell (note high reflection off metal on back 

of GaAs wafer in IR), and a model. The model InGaAs optical constant data file available is for 

an InGaAs composition of a slightly higher bandgap than the final cells so the IR cutoff is in 

error.
8 

Figure 5. Measured reflectance (120nm MgF2/60nm ZnS) on 3J & 2J tandem vs model. 

4.2 Top Contact Grid Design

We wanted to use Ag for the contact grid of final program cells because it is about 50% more 

conductive than Au and would be more economical in production. However. we were stymied by 

erratic Jsc data for cells with Ag gridlines. We use a self-aligned contact cap etch process in 

which we first create a liftoff profile (dovetail) in photoresist using an ammonia-doped image 

reversal process, evaporate 5μm of metal, and liftoff the excess metal in acetone. Prior to AR 

coating, we etch the heavily doped N GaAs contact cap layer in a selective citric acid etch that 

removes the GaAs everywhere but under the metal gridlines and stops on the InAlP window. We 

do not use the popular ammonium hydroxide/hydrogen peroxide selective etch because it badly 

attacks Ag. However, although this citric process works well for inert Au gridlines, more 

reactive Ag gridlines seem to leave varying amounts of the cap or some other residue next to the 

gridlines (Figure 6). For commercial reasons, we are still working on developing the Ag grid 

process, but the delivered program cells use tapered Au gridlines similar to Figure 7. 

Figure 8 shows a top view of the final program cell. An optimum four busbar design of the type 

shown has appreciably less grid I2

R loss and the same grid shadow and emitter I2R loss than a 

similar simpler optimum two busbar parallel line design employed on occasion earlier in the 

program. The grid design is optimal when the details of the grid line shape and measured metal 

and emitter sheet resistance are taken into account as in the in the grid loss analysis and 

optimization presented below (Figure 8). 
9 

Figure 6. Comparison of 4μm wide Ag and Au gridlines after cap etch and AR coating. Note the 

extra “shadow” around the Ag gridlines that is absent from the Au gridlines after the same cap 

etch. Credit: Spire.

Figure 7. 5μm high Au gridline with 2μm top, 4μm base, and ~0.7μm “wings” off base. 

Credit: Spire.
10

Figure 8. Top view of the four busbar program cell. 

The fill factor analysis used in the roadmap is based on a standard analysis described most 

clearly by Green11. The calculation of the series resistance needed from the grid metal and 

emitter sheet power loss is shown in Figure 9 after the gridline spacing optimization using data 

for cells at the end of the program. The model for the gridline shadow is based on the SEM 

images taken of the program liftoff process Au gridlines, which are tapered (2μm at top and 4μm 

at bottom) as shown in Figure 7. Some of the benefit of these tapered lines (some light hitting 

tapered sidewall is reflected onto cell and not lost) is reduced by the presence of the unfortunate 

“wings” which are roughly 0.7μm per side residue at foot of gridline which contribute shadow 

but no significant current conduction capability. 
11

Figure 9. MathCAD grid design optimization and series resistance Rs of grid and emitter for cell. 

Total Rs does not include tunnel junction here but it is included in Table 1. 
12

4.3 General Overview of Bi-Facial Epigrowth and Cell Process

N-type 4in GaAs wafers were purchased “epi-ready” (on one side) and double-side polished. 

Wafer bow from the 2.5% lattice mismatched InGaAs bottom cell must be controlled. For small 

bow, we derive from Stoney’s formula: Bow = 3⁄4 d2 ε (MF/MS) (tF/tS

2

) where ε is the strain, tF is 

the film and tS is the substrate (wafer) thickness, d is the wafer diameter, and MF and MS are the 

film and wafer biaxial elastic moduli. Since the InGaAs relaxes as dislocations are formed, the 

strain is much less than 2.5%. The formula reveals useful trends such as bow increases with the 

square of the wafer diameter or decreases with the square of wafer thickness. Wafers 625-650μm 

generally have no noticeable bow. Wafers as thin as 350μm could be and were processed (with 

bow on the order of 0.5mm). 

The bi-facial cell substrate needs to be transparent to near-infrared light to illuminate the InGaAs 

cell. N-type GaAs wafers doped ~1017 cm-3 are used. Free carrier absorption for N GaAs is over 

10X less than for P-GaAs. The free carrier absorption coefficient is ~0.6 cm-1 for N GaAs for 

wavelengths between 1 to 2μm

12 and results in less than 4% QE loss for a 650μm wafer. Figure 

10 shows measured transmission loss through a bare GaAs wafer and the calculated transmission 

using data from Palik13. This is uncoated material so there is ~28% Fresnel loss at these 

wavelengths from each interface leading to the ~50% transmission. The abrupt 890nm turn-on is 

not electron free-carrier loss (which varies slowly as λ2) and is due to phonon-related optical 

transitions near the GaAs bandedge. The (100) wafers are cut 10o to the 111A so that the InGaP 

top cell has a higher bandgap (~1.89eV) than if less tilt was used14. 

0.0

0.1

0.2

0.3

0.4

0.5

0.6

800 820 840 860 880 900 920 940 960 980 1000

Transmission

Wavelength (nm)

Figure 10. Measured (blue) vs. theory (red) transmission loss through bare 350μm GaAs wafer.

An InAlGaAs grading layer is grown first on the non-“epi-ready” side on the GaAs wafer 

followed by the lattice mismatched 0.94eV N/P InGaAs cell. Defects from growth on the nonepi-ready backside are unlikely to affect the 2.5% lattice mismatched InGaAs cell material which 

will have 106

-107/cm2 dislocations. After bottom cell growth, the wafers are unloaded into a 

cassette, run through and rinse/spin dryer to remove particles, and are then flipped and reloaded 

into the in the MOCVD reactor. 
13

A tunnel junction, a 1.42eV GaAs middle cell, a tunnel junction, and the final 1.89eV InGaP cell 

are grown lattice matched on the opposite (epi-ready) GaAs wafer surface (Table 2). If InGaPtype grades are used on the cell backside, or if the middle GaAs and upper InGaP cells are grown 

first, the phosphine degrades the opposite GaAs wafer surface so that additional processing (e.g. 

lapping) or protection of that surface would be needed. 

Table 2. Epistructure of NREL-Verified 41% (eft, D09) vs 42.2% (right, D11) 

500X Concentrator Cells 

41% 500X D09 42.2% 500X D11

Best cell L810-952-3-951-3 Thickness Doping L873-1141-6-1140-1 Thickness Doping

1xJsc; Voc; FF 13.6mA/cm2

; 3.495V; 0.851 nm cm-3 14.1mA/cm2

; 3.468V; 0.866 nm cm-3

contact cap N GaAs 60 >1e19 N GaAs 60 >1e19 Te

spacer N GaAs 200 2.0E+18 N GaAs 200 2.0E+18 Si

window N InAl0.5P0.5 20 8.0E+18 N InAl0.6P0.4 20 8.0E+18 Si

emitter N In0.49Ga0.51P 60 1e18 to 8e18 N In0.49Ga0.51P 60 1e18 to 8e18 Si

base P In0.49Ga0.51P 1200 3.0E+16 P In0.49Ga0.51P 1400 3.0E+16 Zn

BSF P In0.49(Al0.3Ga0.7)0.51P 100 8.0E+17 P In0.49(Al0.3Ga0.7)0.51P 100 8.0E+17 Zn

TJ1 P Al0.4Ga0.4As 100 1.0E+20 P Al0.4Ga0.4As 100 1.0E+20 C

TJ1 N GaAs 25 1.4E+19 N GaAs 20 1.4E+19 Te

window N Al0.4Ga0.4As 30 1.0E+18 N Al0.4Ga0.4As 30 1.0E+18 Si

emitter N GaAs 85 1.0E+18 N GaAs 100 1.0E+18 Si

base P GaAs 3500 2.0E+17 P GaAs 4000 2.0E+17 Zn

absorber p+-In0.01GaAs 500 2e17 to 2e18 Zn

BSF P Al0.3Ga0.7As 150 1.0E+18 P Al0.3Ga0.7As 150 1.0E+18 Zn

TJ2 P Al0.4Ga0.4As 100 1.0E+20 P Al0.4Ga0.4As 100 1.0E+20 Zn

TJ2 N GaAs 30 1.4E+19 N GaAs 30 1.4E+19 Si

buffer N GaAs 200 2.0E+18 N GaAs 200 2.0E+18 Si

wafer N GaAs (4in WAF410) 650μm 1.0E+17 N GaAs (4in WAF410) 650μm 1.0E+17 Si

buffer N GaAs 150 2.0E+18 N GaAs 150 2.0E+18 Si

grade 1 N In0.03(Al0.6Ga0.4)0.97As 250 4.0E+17 N In0.03(Al0.6Ga0.4)0.97As 250 4.0E+17 Si

grade 2 N In0.07(Al0.6Ga0.4)0.93As 250 4.0E+17 N In0.07(Al0.6Ga0.4)0.93As 250 4.0E+17 Si

grade 3 N In0.10(Al0.6Ga0.4)0.90As 250 4.0E+17 N In0.10(Al0.6Ga0.4)0.90As 250 4.0E+17 Si

grade 4 N In0.14(Al0.6Ga0.4)0.86As 250 4.0E+17 N In0.14(Al0.6Ga0.4)0.86As 250 4.0E+17 Si

grade 5 N In0.17(Al0.6Ga0.4)0.83As 250 4.0E+17 N In0.17(Al0.6Ga0.4)0.83As 250 4.0E+17 Si

grade 6 N In0.21(Al0.6Ga0.4)0.79As 250 4.0E+17 N In0.21(Al0.6Ga0.4)0.79As 250 4.0E+17 Si

grade 7 N In0.24(Al0.6Ga0.4)0.76As 250 4.0E+17 N In0.24(Al0.6Ga0.4)0.76As 250 4.0E+17 Si

grade 8 N In0.28(Al0.6Ga0.4)0.72As 250 4.0E+17 N In0.28(Al0.6Ga0.4)0.72As 250 4.0E+17 Si

grade 9 N In0.31(Al0.6Ga0.4)0.69As 250 4.0E+17 N In0.31(Al0.6Ga0.4)0.69As 250 4.0E+17 Si

gr10/window N In0.36(Al0.6Ga0.4)0.64As 450 5.0E+17 N In0.36(Al0.6Ga0.4)0.64As 450 5.0E+17 Si

emitter N In0.35Ga0.65As 300 5.0E+17 N In0.35Ga0.65As 300 5.0E+17 Si

base P In0.35Ga0.65As 1700 6.0E+16 P In0.35Ga0.65As 1700 6e16 to 4e17 Zn

BSF P In0.36(Al0.6Ga0.4)0.64As 100 5.0E+17 P In0.36(Al0.6Ga0.4)0.64As 100 5.0E+17 Zn

contact cap P In0.35Ga0.65As 600 2.0E+19 P In0.35Ga0.65As 400 2.0E+19 Zn
14

The wafers were epitaxially grown in a Veeco (Emcore) E450 low pressure MOCVD reactor 

which can load thirteen 4in wafers per run. For these growths the hydrogen mainflow was 

120slpm and pressure was 50torr. Metal-organics used were TMIn, TMGa, TMAl and hydrides 

were 100% arsine and phosphine. Dopant sources were a DMZn bubbler for P-type and silane 

for N-type cell material. Tunnel junctions (TJs) and N GaAs caps used CBr4 and DeTe. Growth 

temperature and V/III ratios were ~60/660C for InGaP, 30/600C for GaAs, and 60/600C for 

In(Al)GaAs, respectively. Growth rates were ~8A/s for InGaP, 20A/s for GaAs, and 10A/s for 

InGaAs. Conditions for tunnel junctions differ and are proprietary. 

The cell process after epigrowth is relatively simple and the main steps are outlined here. The 

backside 600nm thick InGaAs cap is wet etched to remove a layer of InGaAs “damaged” by 

phosphine from the final InGaP growth. CrNiAu is evaporated over the entire wafer backside 

and acts as a back contact for the cell and optical mirror. The Cr sticking layer and Ni solder 

barrier are made thin since they are poor infrared reflectors. 

New positive resist is applied and openings are made for the gridlines in an amine image reversal 

process used to make dovetails for clean metal liftoff. The evaporated gridlines of the high 

efficiency cells reported here are 4-5μm of TiPtAu as shown earlier in Figure 7. 

A second photolithography step is used to define areas over the busbars where the AR coating 

will be removed by liftoff. The GaAs cap is then stripped in a citric-based etch selective against 

the InAlP window. A double layer 120nm MgF2/ 60nm ZnS antireflection coating is evaporated 

(for cells without coverglass) and the AR film over the busbars is lifted off in acetone. 

The wafer fronts are protected with resist, mounted on tape, and then diced. The dicing is the last 

fabrication step and defines the cell junction area. 

4.4 1.9eV InGaP Top Subcell Development

Table 3 shows the epistructure and illuminated IV data of the final single junction InGaP subcell 

closest to that used in the final tandems. A tunnel junction is included so this N/P InGaP top 

subcell was grown on the same lightly doped N GaAs wafers as used for the tandems. 

Table 3. Epistructure of Single Junction InGaP Developmental Subcell 

Epi ID: m2-1115 Temp V/III Bandgap Thickness Doping Comments

Cell Lot: L864 C ratio eV nm cm-3

cap n+-GaAs 600 30 1.42 60 Te, >1e19 heavy doping for non-alloyed TiPtAu ohmic contact

spacer n-GaAs 600 30 1.42 200 Si, 2e18 spacer improves Voc and Jsc

window n-In0.4Al0.6P 660 60 2.23 (x) 20 Si, 8e18 LMM pseudomorphic, shifts cut-on λ 10nm vs LM

emitter n-In0.49Ga0.51P 660 60 1.88 60 Si, 1 to 8e18 graded emitter improves Jsc, Rsh 500Ω/sq

base p-In0.49Ga0.51P 660 60 1.88 1200 Zn, 3e16 changed to 1.4um in final tandems to add 0.2mA to Jsc

BSF p-In0.49(Al0.3Ga0.7)0.51P 660 60 2.0 (x) 100 Zn, 8e17 maximum Zn doping possible

TJ1 p+-Al0.4Ga0.6As 1.92 100 C, 1e20 lower temp and V/III ratio (proprietary)

TJ2 n+-GaAs 1.42 30 Te, 1.4e19 lower temp and flushes (proprietary)

buffer n-GaAs 600 30 1.42 200 Si, 2e18

wafer 100 GaAs cut 10o

 to 111A 1.42 650μm Si, 1e17 offcut helps give higher disordered LM InGaP bandgap

Wafer 1xJsc 14.01mA/cm2 1xVoc 1.408V 1xFF 0.881

Averages At 492 suns: Voc 1.615V FF 0.808
15

In order to increase the 1xJsc by 0.2mA/cm2 to reach the roadmap (Table 1) level, we made the 

base slightly (0.2μm) thicker in the final tandems. The added thickness was based on 

calculations using published lattice matched In0.49Ga0.51P n and k optical constant data from 

Ferrini15. Separately, Dean Levi16 and Sarah Kurtz17 of NREL also supplied data. All data was 

found to be in close agreement (Figure 11). Figure 12 shows the expected 1xJsc variation.

 

Figure 11. Optical constants for In0.49Ga0.51P from Ferrini (F), Levi (L) and Kurtz (K).

Figure 12. Calculated 1xJsc (AM1.5D, 100mW/cm2) vs. total In0.49Ga0.51P cell thickness. 
16

Figure 12 indicated adding 0.2μm to the base should increase the 1xJsc by 0.2mA/cm2 in the 

final delivered program cells. Of course, this will leave the GaAs middle cell with 0.2mA less, 

but the optimization took this into account so that the expected Jsc for all subcells is in excess of 

14.2mA/cm2 in the final design. 

In order to optimize design of the InAlP window used in the InGaP top cell, good data on the 

absorption in the InAlP window was also needed. J.A. Woollam provided the InAlP 

spectroscopic data (Figure 13). A significant amount of the light absorbed in the InAlP window 

is lost to recombination, but not all. In the roadmap (Table 1), we estimate this loss as 50% of the 

indicated Jsc absorption value in Fig. 13, based on the hand-waving argument that half the 

photogenerated carriers will diffuse upwards towards the cell surface and be lost to surface 

recombination, and that the other half will move towards and fall down the potential hill at the 

emitter and be collected. The window is so thin (20nm) that hole diffusion length in the N InAlP 

may not be a major issue since most likely it is well in excess of 20nm. 

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

300 350 400 450 500 550 600 650 700

n and k for InAlP

Wavelength (nm)

Absorbed

InAlP Am1.5D

window 1xJsc

nm mA/cm2

40 1.44

30 1.17

20 0.86

10 0.48

Figure 13. Spectroscopic ellipsometry data for In0.5Al0.5P on 10o cut GaAs wafers. 

4.5 GaAs Middle Cell Development

Although not a task explicitly called out in the original program SOW because of the advanced 

state of GaAs cell technology, we did examine single junction GaAs middle cells to help us 

interpret the tandem cell performance. Two types of GaAs cells were employed in the program. 

A standard N/P GaAs cell was in use for all of the program, until the final delivery. 

We needed to increase the 1xJsc of the middle cell to make progress with the tandem. We 

wanted to increase the InGaP thickness from 1.2 to 1.4μm to increase the top cell Jsc, but this 

would deprive the GaAs cell of ~0.2mA/cm2

. We needed to compensate for this Jsc loss and add 

more so that the middle cell would also have 14.2mA/cm2 under the new InGaP top cell. 
17

4.5.1 Bandgap Narrowing to Extend Cutoff

We noticed the GaAs cells always had QE in excess of 872nm, the cutoff expected for 1.424eV 

GaAs, whether measured by NREL or Spire. We assumed that this “below-bandgap” response 

was possible through bandgap narrowing in doped GaAs. We increased the doping in the base 

and examined the results (Figure 14). Although there was a 0.12mA increase in Jsc in the below 

bandgap QE, the additional shift of the cutoff wavelength was compensated by the lower 

diffusion length due to the heavier doping at shorter wavelengths and therefore was useless.

Figure 14. Increase of long wavelength QE through bandgap narrowing in GaAs base. 

4.5.2 Pseudomorphic Absorption Layer to Extend Cutoff

Next, we thought about lowering the bandgap by compositional change. InGaP/GaAs/Ge cells

add 1% indium to the GaAs to lattice match with the Ge substrate. We did not wish to make a 

thick lattice-mismatched cell mainly because we worried that the tunnel junction dopants would 

diffuse more in such a structure. Instead, we tried to add a pseudomorphic thin 1%InGaAs layer 

at the back of the GaAs cell to boost the long wavelength QE. 1% InGaAs has a bandgap of 

1.415eV and a cutoff of 876nm. Since the AM1.5D spectral slice from 870 to 880nm can add 

0.7mA/cm2 in just that 10nm, we reasoned that the 4nm 872 to 876nm shift could add 

0.28mA/cm2, on the order of the increase needed. In addition, since the bandgap shift was only 

9meV, less than the thermal kT energy of carriers at 26meV, we reasoned there should be no 

significant barrier for collection at the interface. The 1% InGaAs represents a 717ppm mismatch 

to GaAs. The critical thickness (at which it is energetically favorable to generate dislocations to 

relieve stress instead of continuing elastic accommodation) is calculated using the MathewsBlakeslee18 theory in Figure 15 as 0.5μm for 1% (0.01) indium added to the GaAs. 
18

Figure 15. Critical thickness calculation for InXGa1-XAs on GaAs (1 on X-axis  1%). 

We wanted to grow this 1%InGaAs absorbing layer as thick as possible to maximize its optical 

absorption, but yet keep it below its critical thickness. Table 4 shows a comparison of the 

standard GaAs middle cell with the “pseudomorphic” structure used in the final delivery. The 

final subcell (L843-1051) achieved the 14.2mA/cm2 goal. However, the wafer surface of the 

final tandem epigrowths exhibited a slight cross-hatch (Figure 16). Despite adhering to the 

theory, there was some relaxation (the 1%InGaAs long wavelength absorbing layer was NOT 

pseudomorphic). Surfaces without the 1%InGaAs layer are smooth and featureless. However, 

since the Voc was still acceptable and the tunnel junctions (TJs) worked at the 500X level (the 

TJ dopants did not diffuse disastrously due to the dislocations), this middle cell was selected. 
19

Table 4. Epistructure of Single Junction InGaP Developmental Subcell

m2-1007 m2-1048 m2-1051

Single Junction Filtered GaAs Filtered GaAs Filtered GaAs

Test Cells Middle cell Middle cell Middle cell

All Process L843 3.5um base 4.5um base 4um base

(same AR coat) +0.5um 1%InGaAs

material Thick. Doping material Thick. Doping material Thick. Doping

nm cm-3 nm cm-3 nm cm-3

contact cap p+-GaAs 250 >1e19 p+-GaAs 250 >1e19 p+-GaAs 250 >1e19

top cell filter p-InGaP 1260 1.0E+18 p-InGaP 1260 1.0E+18 p-InGaP 1260 1.0E+18

TJ1 p+-Al0.4GaAs 100 1.0E+20 p+-Al0.4GaAs 100 1.0E+20 p+-Al0.4GaAs 100 1.0E+20

TJ1 n+-GaAs 20 1.4E+19 n+-GaAs 20 1.4E+19 n+-GaAs 20 1.4E+19

window n-Al0.4GaAs 30 1.0E+18 n-Al0.4GaAs 30 1.0E+18 n-Al0.4GaAs 30 1.0E+18

emitter n-GaAs 85 1.0E+18 n-GaAs 100 1.0E+18 n-GaAs 100 1.0E+18

base p-GaAs 3500 2.0E+17 p-GaAs 4500 2.0E+17 p-GaAs 4000 2.0E+17

LW absorber p+-In0.01GaAs 500 2e17 to 2e18

BSF p-Al0.3GaAs 150 1.0E+18 p-Al0.3GaAs 150 1.0E+18 p-Al0.3GaAs 150 1.0E+18

TJ2 p+-Al0.4GaAs 100 1.0E+20 p+-Al0.4GaAs 100 1.0E+20 p+-Al0.4GaAs 100 1.0E+20

TJ2 n+-GaAs 30 1.4E+19 n+-GaAs 30 1.4E+19 n+-GaAs 30 1.4E+19

buffer n-GaAs 200 2.0E+18 n-GaAs 200 2.0E+18 n-GaAs 200 2.0E+18

wafer GaAs 1.0E+17 GaAs 1.0E+17 GaAs 1.0E+17

Wafer Avg (52 cells):

1xJsc (mA/cm2

) 13.81 13.98 14.20

1xVoc (V) 1.000 0.997 0.995

1xFF 0.864 0.863 0.866

Suns 490 na 492

Voc (V) 1.184 na 1.181

FF** 0.687 na 0.686

**thin grid metal

Figure 16. Left: Crosshatch on top and middle cell surface vs Right: Bottom cell. Credit: Spire.

4.6 Tunnel Junction Development

There are two tunnel junctions (TJs) in the tandem (Fig. 17). For 500X operation (i.e. ~7A/cm2) 

, we found carbon doped P AlGaAs and Te-doped N GaAs were usable. One TJ is between the 

top and middle cells. The N GaAs thickness in the upper TJ needs to be thin to limit absorption 

of light that could be used by the GaAs cell. The second TJ is under the GaAs middle cell since

we need to use an N GaAs wafer under the N/P GaAs cell to limit free carrier absorption of 

infrared photons for the bottom cell. 

This lower TJ does not need to be thin since few absorbable photons are left after the GaAs cell, 

and the GaAs wafer would absorb them anyway. IV data of stand-alone TJs using the same 
20

AlGaAs P layer but 20nm GaAs N layer (upper TJ) and 30nm GaAs N layer (lower TJ) are 

shown in Table 5. Both TJs have a layer grown on top of them to simulate the heat and duration 

that would occur in a tandem. The sum of the Rs of the two TJs is used in the Table 1 roadmap. 

Figure 17. TJ position in bi-facial tandem.

Table 5. Data on Upper and Lower TJs in Tandem 

ID GaAs TJ Post TJ Test Rs Jp

nm anneal mΩ-cm2 A/cm2

L844-1058 20 1.3um GaAs 660C diode 1 2.64 30.2

upper TJ1 growth rate to diode 2 2.64 37.5

match InGaP diode 3 2.22 46.5

average 2.50 38.1

L707-730 30 1.5um InGaP 660C diode 1 0.59 172

lower TJ2 diode 2 0.48 168

average 0.53 170

Figure 18 shows typical IVs of the tunnel junctions for which data is summarized in Table 5. In 

tandem cell operation, the TJs are forward biased (operate to the right in positive bias). The peak 

current for a 500X tandem (about 7A/cm2

) is exceeded by both TJs, and the data indicate the 

current TJs should be useful well past 1500X. Figure 19 is a plot of the equivalent Jsc loss for the 

GaAs cell vs GaAs N layer TJ thickness integrated between 660 and 880nm. 
21

Figure 18. Typical TJ IVs from data in Table 6.

Figure 19. Jsc loss vs N GaAs layer thickness in upper TJ. 

Finally, Figure 20 shows a rough calculation of the “small signal” (before Jpeak) TJ resistance 

adapted from Wang19. The plot demonstrates how quickly the resistance can vary with doping 

(through change of the depletion width/tunnel barrier). The calculated values of 1-2mΩ-cm

2

roughly agree with the measured data reported above. 
22

Figure 20 Rough calculation of TJ resistance for P AlGaAs/N GaAs (dominated by GaAs). 
23

4.7 Lattice Mismatched InGaAs Bottom Cell Development

Growth of the InGaAs bottom cell was described in Section 4.3. We discovered that the most 

manufacturable growth sequence was to grow an InAlGaAs grading layer first on the back of the 

wafer followed by an InGaAs bottom cell. In this way, the opposite wafer surface is preserved 

for growth. If an InGaP grading layer is used, the opposite wafer surface is severely degraded. A 

SiN layer could be deposited and then removed to protect this surface, but this entails significant 

added processing (a time-consuming PECVD SiN deposition and subsequent wet etch). If the 

GaAs middle and InGaP top cells are grown first, the opposite wafer side is again degraded by 

the InGaP growth and the back surface must either be protected or lapped.

4.7.1 InGaAs Cell Composition (Cutoff Wavelength)

Figure 21 shows a calculation of the 1xJsc that InGaAs bottom cells of various cutoff 

wavelengths could generate. The 1330nm cutoff (about 35%InGaAs) is adequate for our 

roadmap (Table 1) and is at the edge of a absorption peak in the AM1.5D spectrum so that 

extending the cutoff a bit further will not result in significant additional Jsc.

Figure 21. 1xJsc available from InGaAs cells of the indicated cutoff wavelength. 
24

4.7.2 General InXGa1-XAs Absorption Model

In order to design the InGaAs bottom cell, absorption data was needed to estimate a suitable cell 

thickness. The data used a simple standard absorption model20 and is compared with measured 

In0.53Ga0.47As data (from J.A. Woollam) for model verification in Figure 22. 

Figure 22. InGaAs absorption coefficient model used in program. 
25

4.7.3 4.7.3 InGaAs Cell Thickness vs 1xJsc

The above absorption data was used to estimate that the bottom cell thickness to achieve the 

14.2mA/cm2

 1xJsc goal of Table 1 is about 2μm (Figure 23). Although a thicker cell could 

generate more Jsc in theory, in practice the thickness is limited by the dislocation-limited 

~1.6μm diffusion length in the uniform doped base (Figure 24) of the bottom cell as well as by 

wafer bow in the current cells if grown more than 2.5μm thick. 

Figure 23. Calculated 1xJsc possible from InGaAs bottom cell using absorption coefficient data in 

Figure 22. 

4.7.4 Use of a Doping Grade (Drift Field) in InGaAs Base to Enhance Diffusion 

Length

In order to extend the diffusion length, we graded the base doping as illustrated in Figure 24. 

Grading the base doping is sometimes of questionable value when the diffusion length is limited 

by the base doping. In the bottom cell, dislocations, not the doping, limit the lifetime and 

diffusion length, so a doping grade21 can be used to good effect to generate an electric field that 

aids the diffusion toward the collecting junction (”downstream diffusion length”).Table 6 shows 

the measured cell improvement with the insertion of a doping grade in the bottom cell base. The 

cells had the front and middle cells of a tandem grown on them and then etched off subsequently 

so that any degradation of the bottom cells that occurred in the tandem would occur in these test 

cells.
26

Figure 24. QE model fit (blue solid line) to measured data (open circles) for GaAs-filtered InGaAs 

cell with uniform base doping shows a diffusion length Lbase of ~ 1.6μm gives a good fit. Dotted 

curve is expected improvement from base doping grade. Short wavelength cut-on in measured 

data is from GaAs wafer filter (not modeled).

Table 6. Average Wafer (52 Cells) IV Data Showing InGaAs Cell 

Improvement with Base Doping Grade

Lot PH3 660C Grade Base 1x Jsc 1x Voc Suns Voc

expose anneal steps&nm/step cm-3 mA/cm2 V V

L829-0990-1-0992-1 yes yes 10x250 6.0E+16 -13.6 0.372 484 0.607

L829-0991-1-0992-5 yes yes 10x250 6e16 to 4e17 -14.1 0.394 479 0.621

In% (all): last grade step 31-window 36-emitter 35. All emitters 300nm 5e17 cm-3 and bases 1700nm.

4.7.5 Final Bottom Cell Epistructures used in Deliveries D09, D10 and D11 and IV 

Data

Table 7 shows the epistructures and single junction test data for the bottom cells used in the last 

three tandem deliveries (D09, D10, D11) to NREL. Insertion of a doping grade into the base did 

indeed help, although when a graded emitter was tried in addition to the base grading, the Jsc 

was not further improved (L869-1123-4-1120-7, avg 1xJsc 14.17mA/cm2, 595mV Voc). We 

believe this is because the more heavily doped emitter causes some doping related recombination 

(possibly Auger for this lower bandgap material) as opposed to the lighter base dopings where 

the lifetime is mainly limited by dislocation recombination. Finally, we added a 200nm thicker 

final grading layer which seemed to boost the 1xJsc by 0.1mA/cm2 and perhaps the Voc by a bit. 
27

Table 7. Bottom Cell Epistructure Evolution and Average (52 cells) IV data

0953-2-0951-5 0992-5-0991-1 1142-8-1140-8

Single Junction Filtered InGaAs Filtered InGaAs Filtered InGaAs

Test Cells Bottom cell Bottom cell Bottom cell

all treated with NREL: 41% 500X NREL: 41% 500X Spire: 43% 500X

upper cell growths tandem D09 tandem D10 tandem D11

material Thick. Doping Thick. Doping Thick. Doping

nm cm-3 nm cm-3 nm cm-3

contact cap p In0.35Ga0.65As 600 2.0E+19 600 2.0E+19 400 2.0E+19

BSF p In0.36(Al0.6Ga0.4)0.64As 100 5.0E+17 100 5.0E+17 100 5.0E+17

base p In0.35Ga0.65As 1700 6.0E+16 1700 6e16 to 4e17 1700 6e16 to 4e17

emitter n In0.35Ga0.65As 300 5.0E+17 300 5.0E+17 300 5.0E+17

window n In0.36(Al0.6Ga0.4)0.64As 250 5.0E+17 250 5.0E+17 450 5.0E+17

grade step 9 n In0.31(Al0.6Ga0.4)0.69As 250 4.0E+17 250 4.0E+17 250 4.0E+17

grade step 8 n In0.28(Al0.6Ga0.4)0.72As 250 4.0E+17 250 4.0E+17 250 4.0E+17

grade step 7 n In0.24(Al0.6Ga0.4)0.76As 250 4.0E+17 250 4.0E+17 250 4.0E+17

grade step 6 n In0.21(Al0.6Ga0.4)0.79As 250 4.0E+17 250 4.0E+17 250 4.0E+17

grade step 5 n In0.17(Al0.6Ga0.4)0.83As 250 4.0E+17 250 4.0E+17 250 4.0E+17

grade step 4 n In0.14(Al0.6Ga0.4)0.86As 250 4.0E+17 250 4.0E+17 250 4.0E+17

grade step 3 n In0.10(Al0.6Ga0.4)0.90As 250 4.0E+17 250 4.0E+17 250 4.0E+17

grade step 2 n In0.07(Al0.6Ga0.4)0.93As 250 4.0E+17 250 4.0E+17 250 4.0E+17

grade step 1 n In0.03(Al0.6Ga0.4)0.97As 250 4.0E+17 250 4.0E+17 250 4.0E+17

buffer n GaAs 500 2.0E+18 150 150 2.0E+18

wafer 410 100 n GaAs 10o

 to 111A 650μm 1.0E+17 650μm 1.0E+17 650μm 1.0E+17

Wafer Avg

1xJsc (mA/cm2) 13.6 14.11 14.24

500xVoc (V) 0.573 0.621 0.622

4.8 Tandem Cell Data

In the above sections, we have extensively discussed single junction InGaP top, GaAs middle, 

and InGaAs bottom cells, and tunnel junctions where a great deal of care went into optically 

filtering and heat treating them as they would be in the final tandem. We believe much of the 

quick progress we made in this program occurred due to this experimental approach in which it 

was easy to decipher problems in single junction cells than in multijunction cells. The Table 1 

roadmap indicates the tandem cell performance is well predicted by this method. Therefore, in 

this section, we will primarily discuss the tandem results with less analysis (see the sections on 

the individual subcells for that).

4.8.1 Tandem Illuminated IV Data

Figure 25 shows the illuminated IV data as measured by NREL for tandems in D09 and D10. For 

the 5,5mm cells from D10 (Fig. 25a), the average lot efficiency of 1780 cells was 39.9% at 500X 

as measured by Spire Semiconductor, with the best cell measured as 42%. NREL measured the 

best cell as 41.4%. Measurements agree within NREL’s stated error. Figure 19b shows a larger 

1cm2 cell NREL has measured from D09 as 41.0% at 500X, although the efficiency was slightly 

higher at the lower concentration shown. The one sun Jsc’s (1xJsc) shown were calculated from 

the NREL supplied data. Figure 1 in the Executive Summary is the NREL illuminated IV for the 

record cell from D11. 
28

Figure 25. a) Left: 5.5mm 3-junction cell. Epistructure as in Table 3. B) Right: 1cm 3-junction cell. 

Essentially no large improvement was made from D9 to D10; where the only significant epi 

change was the use of a doping grade in the bottom cell to increase its Jsc. This alone was not 

enough to move the efficiency significantly (despite what tandem QE data shows, the InGaAs 

bottom cell was not limiting the Jsc).

We attribute the 1% point efficiency bump in the D11 cells to the significant epistructure 

changes to all three subcells (shown in Table 2) which were all geared towards increasing the 

1xJsc of each subcell: InGaP top cell – thicker base and strained window; GaAs middle cell – 

thicker base and 1%InGaAs absorber region; InGaAs bottom cell – graded base doping to 

increase diffusion lengths and slightly thicker final step (window) in grading layer (dislocation 

reduction) stack. 

4.8.2 Tandem QE Data

Figure 26 shows a measured tandem external QE from NREL for a typical bi-facial cell. We 

have appended a Spire-measured reflectance curve that was taken for this wafer after the 

MgF2/ZnS coating deposition, as well as the NREL IV data for this tandem (at 281X). The 1xJsc 

obtained for each subcell was obtained by integrating the NREL QE data with the AM1.5D 

spectrum with the power normalized to 100mW/cm2

. This particular tandem appears to be topcell current limited. The small gap between the GaAs middle cell and InGaAs bottom cells is of 

most interest and is due to the GaAs wafer absorption discussed earlier. We compensate for this 

loss by extending the InGaAs cutoff wavelength as needed. Figure 27 shows Spire-measured 

external QE for the single-junction subcells employed for the final tandem with a 

120nmMgF2/60nm ZnS coating deposition, The 1xJsc obtained for each subcell was well 

matched. 
29

Figure 26. NREL-measured external tandem QE showing response of InGaP, GaAs, and InGaAs 

subcells and Spire-measured reflectance of a bi-facial tandem.

Figure 27. Spire-measured external QE of single junction InGaP, GaAs, and InGaAs test subcells 

used in final D11 delivery. 
30

4.8.3 Final D11 Epistructure

The final program epistructure is shown in Table 2 above. The InGaP top cells is thicker (1.4μm 

base) and has a doping-graded emitter (perhaps a 1xJsc gain of +0.15mA/cm2

, L847 1047-2 vs 

1043-2) and strained higher bandgap InAlP window (perhaps +0.1mA/cm2

, L847 1065-1 vs 

1043-2), the GaAs middle cell has the 1% InGaAs absorber added at the back of the cell, and the 

bottom cell utilizes a doping grade in the base and a slightly thicker final step of the grade. 

5 Commercialization Efforts

5.1 Burn-In Data at 85C 200Hrs 

Table 8 shows 85C 200 hour burn-in data on ten randomly selected 3 junction tandems. The test 

was within the 10% spec set by NREL. Most of the degradation is from one bad cell (cell 13, of 

course) that went from 39 to 34%. Structurally, the cells have not changed much in the more 

recent deliveries, so we believe the reliability data is still useful. The final D11 cells with the 

“pseudomorphic” 1%InGaAs were checked at 85C for 48hrs and showed no degradation (Table 

9). A shorter burn-in time was used simply because we were near the end of the program, and 

was done simply because we wanted to at least know that the final cells seemed stable. We have 

detailed specifications from Amonix for tests the cells have to meet in order for them to be put 

on-sun by Amonix. The end of the program was dedicated to reaching the efficiency target. We 

will now have some time to set up the reliability tests needed by module makers as we pursue 

commercial opportunities with these bi-facial growth cells.

Table 8. Tandem 85C 200hr Burn-In Data on Early Tandems

L748- Before 85C 200hr Burn-in After 85C 200hr Burn-in Performance

860-6- cell 1xJsc Jsc Voc FF Eff 1xJsc Jsc Voc FF Eff change

858-3 mA/cm2 A/cm2 V mA/cm2 A/cm2 V

1 13.36 6.637 3.384 0.855 0.387 13.41 6.414 3.374 0.846 0.383 ∆Eff/Eff -3.9%

4 13.13 6.605 3.371 0.867 0.384 13.05 6.411 3.365 0.849 0.373 ∆Voc/Voc -0.3%

5 12.85 6.417 3.390 0.880 0.384 13.05 6.476 3.380 0.850 0.375 ∆1xJsc/1xJsc -1.5%

8 12.80 6.497 3.384 0.882 0.382 12.78 6.517 3.381 0.833 0.360 ∆FF/FF -2.1%

9 12.94 6.403 3.356 0.859 0.373 12.87 6.319 3.345 0.846 0.364

13 12.98 6.505 3.364 0.885 0.386 12.08 6.291 3.345 0.848 0.343

14 12.97 6.406 3.353 0.855 0.372 12.09 6.164 3.338 0.857 0.346

16 13.02 6.536 3.369 0.876 0.384 12.91 6.369 3.357 0.868 0.376

18 12.95 6.404 3.354 0.858 0.373 12.83 6.275 3.340 0.845 0.362

20 13.09 6.544 3.358 0.849 0.373 13.00 6.365 3.351 0.843 0.367

average 13.01 6.495 3.368 0.867 0.380 12.81 6.360 3.358 0.848 0.365

std dev 0.16 0.086 0.014 0.013 0.006 0.42 0.103 0.016 0.009 0.013
31

Table 9. Tandem 85C 48hr Burn-In Data on Final (D11) Tandems

5.5mm Jsc Voc Vm Pm FF conc. Efficiency

cells A/cm2 V V W Suns

average 6.809 3.455 3.058 20.244 0.860 471.0 0.430

0 hrs stdev 0.083 0.009 0.024 0.347 0.008 4.5 0.005

median 6.782 3.455 3.061 20.199 0.862 471.5 0.430

count 35 35 35 35 35 35 35

48hrs average 6.811 3.458 3.067 20.337 0.864 471.5 0.431

85C stdev 0.090 0.008 0.030 0.378 0.009 4.1 0.005

median 6.797 3.458 3.069 20.300 0.863 471.6 0.430

count 35 35 35 35 35 35 35

5.2 Commercial Companies Evaluating Spire Tandems

Companies evaluating Spire cells in modules are: 

• Morgan Solar 

• BrightLeaf (was Aquasoladyne) 

Companies evaluating Spire cells for possible use in modules are: 

• Soliant 

• SolFocus 

• Amonix 

All of the above companies have Spire Semiconductor cell samples, but the latter group requires 

more reliability data than we currently have at this point to start module tests.

6 Spire SAI Program Team

Management: Ed Gagnon, Vic Haven 

Business Development: Brad Siskavich

Epigrowth: Phillip Chiu, Xuebing Zhang, Steve Markham, Tri Ta 

Wafer Process: Chris Harris, Daryl Pulver, Dan Stevens, Nathan Bonniah, Yungeng Gao 

Cell Test: Phillip Chiu, Daniel Derkacs

Device Design and Modeling: Steve Wojtczuk, Daniel Derkacs 

Independent Consultant: Mike Timmons 

Principal Investigator: Steve Wojtczuk
32

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