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Multijunction Photovoltaic 

Technologies for HighPerformance Concentrators 

Preprint 

R. McConnell and M. Symko-Davies 

Presented at the 2006 IEEE 4th World Conference on 

Photovoltaic Energy Conversion (WCPEC-4) 

Waikoloa, Hawaii 

May 7–12, 2006 

Conference Paper 

NREL/CP-520-39791 

May 2006 
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MULTIJUNCTION PHOTOVOLTAIC TECHNOLOGIES FOR 

HIGH-PERFORMANCE CONCENTRATORS* 

Robert McConnell and Martha Symko-Davies 

NREL, Golden, CO 80401-3393, USA 

ABSTRACT 

Multijunction solar cells provide high-performance 

technology pathways leading to potentially low-cost 

electricity generated from concentrated sunlight. The 

National Center for Photovoltaics at the National 

Renewable Energy Laboratory has funded different III-V 

multijunction solar cell technologies and various solar 

concentration approaches. Within this group of projects, IIIV solar cell efficiencies of 41% are close at hand and will 

likely be reported in these conference proceedings. 

Companies with well-developed solar concentrator 

structures foresee installed system costs of $3/watt—half of 

today’s costs—within the next 2 to 5 years as these highefficiency photovoltaic technologies are incorporated into 

their concentrator photovoltaic systems. These technology 

improvements are timely as new large-scale multi-megawatt 

markets, appropriate for high performance PV 

concentrators, open around the world.

INTRODUCTION 

Under contract to the U.S. Department of Energy 

(DOE), the National Center for Photovoltaics (NCPV) at the 

National Renewable Energy Laboratory (NREL) houses the 

High-Performance Photovoltaic Project, which supports inhouse and subcontracted research in high-performance 

future-generation photovoltaics (PV), polycrystalline thinfilm PV, and multijunction concentrator devices. This paper 

focuses on the research and development of multijunction 

III-V solar cells that can be incorporated into structures that 

track and concentrate sunlight using either mirrors or 

lenses. The development of these systems—highperformance concentrator PV systems—target the goal of 

the Solar Program in its Multi-Year Technical Plan, “to 

develop technology and red-uce market barriers to the point 

where the cost of solar energy becomes competitive in 

relevant energy markets—principally in the building and 

power plant markets” [1]. This Multi-Year Plan provides a 

good framework to evaluate the significance of the technical 

progress of multijunction concentrator devices. 

The DOE Solar Energy Technologies Program is 

implementing a program management tool called the Stage 

Gate approach [1]. The approach reflects the development 

history of high-efficiency multijunction solar cells. 

Preliminary investigation begun in the early 1980s on 

multijunction solar cells proceeded to an exploratory 

research phase, and then to further development for the 

satellite power market in the 1990s. Near the turn of the 

century, further development occurred within the HighPerformance PV Project, leading to ongoing validations for 

use in high-performance solar concentrators. Now, these 

technologies and systems are approaching commercial 

launch in their first multi-megawatt power plant projects. 

Each gate in this technology development represented an 

evaluation where decisions were made by companies, 

researchers, and program managers to proceed further with 

increasing sums of money. With the imminent commercial 

launch of products, investors are now exploring the 

feasibility and viability of very large multi-megawatt 

concentrator PV projects requiring funding 10 to 100 times 

larger than the early funding of research for III-V 

multijunction cells. This is a Stage Gate characteristic, that 

the importance of the gate evaluations becomes greater for 

the later stages requiring more funding. 

The original implementation and goals for the HighPerformance PV Project fit nicely within the present 

program management framework of the DOE Solar Energy 

Technologies Program and its recently announced Solar 

America Initiative. As mentioned previously, the HighPerformance PV Project goal is consistent with the overall 

goal of the Solar Program that specifically includes largescale power plant markets. Table 1 is taken from the recent 

DOE Solar Energy Technologies Program Plan and 

summarizes the impact of the high-efficiency solar cells on 

system costs for a concentrator PV utility reference system 

[1]. 

System size MW 10 12.5 16 

Module price $/Wdc 4.13 3 1.56 

Cell 

efficiency 

% 26 

(Si) 

32 (MJ) 40 (MJ) 

Module size kWpdc 40 50 64 

Module 

efficiency 

% 20 25 32 

Installed 

system price 

$/Wdc 5.95 4.3 2.52 

LCOE $/kWhac 0.15–

0.27 

0.10–

0.15 

0.06–

0.11 

Table 1. Benchmark (10-MW) system parameters and 

impact of multijunction (MJ) solar cell efficiency on 

concentrator PV utility reference system, as taken from 

Table 3.1.6-5 in reference [1]. 

___________________________________________________ 

*This work has been authored by an employee or employees of the Midwest Research Institute under Contract No. DE-AC36-99GO10337

with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, 

acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce 

the published form of this work, or allow others to do so, for United States Government purposes.

1
The 10-MW system with its benchmarked parameters 

has a module conversion efficiency of 20% based on 

today’s crystalline-silicon solar cells designed for 

concentrators and having nominal 26% efficiencies. The 

12.5-MW system (25% module efficiency) uses 32% III-V 

multijunction cell available in quantity today, whereas the 

16-MW system costs and 32% module conversion 

efficiency are based on 40% III-V multijunction cells. The 

levelized cost of energy (LCOE) for the Phoenix location 

varies according to financial assumptions, but the impact of 

higher efficiency on electricity cost is clear: solar cell 

efficiencies of 41% and higher are worthwhile targets on the 

pathway to significant market deployment and 

commercialization of high-performance concentrator PV 

systems. To give an idea of this technology’s availability, 

the 39% solar cell demonstrated last year by Spectrolab is 

expected to be easily transferable to production at multihundred kW levels. Achieving these targets highlights an 

advantage of concentrator PV systems when successfully 

launched commercially beyond their benchmark values: 1 

MW at one sun of multijunction solar cells can power 1 GW 

of concentrator PV systems at 1000 suns. Similarly, 1 GW 

of multijunction solar cells can power 1 TW of concentrator 

PV systems, revealing a possible pathway to significant 

TW-levels of electricity production.

HIGH-PERFORMANCE PHOTOVOLTAICS PROJECT 

 The High-Performance PV Project has supported 

several III-V multijunction PV projects that were initially 

funded for three years beginning in 2003. We have 

described research results from these projects at earlier 

conferences, and there are several papers in these 

proceedings that describe the most recent findings as these 

projects near completion [2]. 

In 2003, a rigorous evaluation process identified the 

highest-quality research proposals most likely to achieve 

project goals. Seven companies and universities received 

awards, which are summarized in Table 2 along with two 

additional subcontracts. The additional subcontracts were 

directed to SunPower Corporation for low-concentration, 

high-efficiency Si modules and to Underwriters Laboratories 

(originally awarded to Arizona State University) for 

qualifications standards to support industry’s product 

development for commercial launch. 

A second University of Delaware subcontract was 

selected competitively in a separate PV Exploratory 

Research project solicitation and is described in another 

High-Performance PV contribution to this conference. That 

project explores third-generation PV approaches for 

achieving over 50%-efficient solar cells. NREL in-house 

researchers are also developing III-V multijunction and 

other third-generation PV technologies. Technical 

collaboration and coordination of the subcontract projects 

with the NREL in-house research takes many forms, 

including measurements and characterization of materials 

and devices, cooperative R&D agreements, joint 

publications, site visits, and joint participation in related 

projects such as the recent DARPA project for 50% solar 

devices described in a separate presentation at this 

conference. 

III-V MULTIJUNCTION SOLAR CELL R&D 

 Several III-V multijunction solar cell structures have 

been developed and demonstrated under the HighPerformance PV Project, but some still need further 

exploration to achieve even higher efficiencies. The device 

R&D focus is on the materials, device design, antireflective 

coating, and the spectrum. Several of these novel devices 

will be described below. 

Subcontract Title 

Spectrolab, Inc. Ultra-High-Efficiency Multijunction 

Cell and Receiver Module 

California 

Institute of 

Technology 

Four-Junction Solar Cell with 40% 

Target Efficiency Fabricated by 

Wafer Bonding and Layer Transfer 

University of 

Delaware 

Novel High Efficiency PV Devices 

Based on the III-N Material System 

OHIO STATE Optimized III-V Multijunction 

Concentrator Solar Cells on 

Patterned Si and Ge Substrates 

AMONIX Design and Demonstration of a 

Greater than 33% Efficiency HighConcentration Module Using over 

40% III-V Multijunction Devices 

JX Crystals Toward 40%-Efficient Hybrid 

Multijunction III-V Terrestrial 

Concentrator Cells 

Concentrating 

Technologies, 

LLC 

A Scaleable Reflective Optics High 

Concentration PV System 

SunPower 

Corporation 

Low Concentration PV System 

Prototype Assessment of a 3x 

Mirror Module with SunPower’s 20% 

Efficient A-300 Solar Cell 

Underwriters 

Laboratories 

Development of IEC Design 

Qualification Standard for 

Concentrator PV Modules 

Table 2. Concentrator PV subcontracts funded by the 

DOE/NREL High-Performance PV Project. Most of these 

subcontracts end in 2007. The Solar America Initiative in 

2007 offers an opportunity for concentrator PV technologies 

to move beyond the High Performance PV Project phase. 

The high-efficiency cell development work at 

Spectrolab builds on the successes it had with producing 

record-efficiency 3-junction (3J) GaInP / GaInAs / Ge solar 

cells of both lattice-matched and lattice-mismatched 

(metamorphic) designs. 

The metamorphic approach achieves the desired 

subcell bandgaps with compositions that are latticemismatched to the growth substrate, such as 2%-In to 35%-

In GaInAs and GaInP compositions with the same lattice 

constants, while minimizing the dislocations that typically 

come with lattice-mismatched growth. The bandgap can 

also be engineered in some materials by tailoring the 

ordering of elements from the same column of the Periodic 

Table within the crystal lattice—for instance, Ga and In 

ordering on the Group-III sublattice in GaInP. 

2
 Figure 1 plots the light current-voltage (I-V) curves for 

recent record-efficiency concentrator cells, with 39.0% for a 

lattice-matched design and with 38.8% for a metamorphic 

design, essentially reaching parity with lattice-matched 

cells. The 39.0% result achieved in this program was the 

highest conversion efficiency yet measured and 

independently confirmed in 2005 for any PV device; higher 

efficiencies are expected this year [3]. 

Fig. 1. World-record concentrator solar cell efficiencies 

achieved last year by Spectrolab. 

Also last year, the NREL III-V Multijunction group recently 

developed an ultra-thin, inverted, lattice-mismatched GaInP 

/ GaAs / GaInAs solar cell technology. The cell was 

designed for one-sun applications, but had top-surface grid 

metallizations that allowed testing at low concentration 

ratios. The cell showed peak performance for ~5–10 suns 

concentration, yielding a conversion efficiency of 37.9% at 

10.1 suns. At the time, the measurement constituted a new 

world record that, however, only lasted about a month in 

this fast-moving arena of research. It is expected that an 

improved version of this device will soon be demonstrated 

with efficiencies greater than 40% at concentrations of 

about 100 suns [4]. 

 California Institute of Technology, under its 

subcontract, is developing a 4-junction solar cell with 40% 

target efficiency fabricated by wafer bonding and layer 

transfer. To accomplish this, Cal Tech uses wafer-scale 

synthesis of InP / Si, Ge / Si, and GaAs / Si on transferred 

epitaxial template films by layer-transfer fabrication; 

secondly, they grow high-quality epitaxial compound 

semiconductor 2-junction subcells and doubleheterostructures (InGaP / GaAs / Ge / Si and InGaAsP / 

InGaAs / InP / Si) on transferred epitaxial template layers. 

Recent results include wafer-scale layer transfer of Ge / Si 

and InP / Si template layers at the 50-mm wafer diameter, 

with less than 1 nm roughness and less than 0.1 ohm-cm2

interface resistance. They have fabricated active GaInP and 

GaAs double-heterostructures with about 107

/cm2defects on 

Ge / Si templates 

 The University of Delaware, under its subcontract, 

focuses on demonstrating high-performance tandem solar 

cells based on the InGaN material system. Although the 

high-bandgap InGaN alloys are used in commercial lightemitting diodes, there are many unknowns for the lowbandgap In-rich alloys. The approach is to develop highbandgap In-lean InGaN solar cells and identify growth 

techniques and buffer layers for low-bandgap In-rich InGaN 

grown on Ge to improve the understanding of this portion of 

the alloy range. Early results include demonstrating GaN 

solar cells with external quantum efficiencies of >10% when 

measured through metal Au contact; InGaN solar cells with 

7% In that show Voc = 2 V, dominated by a radiative defect 

channel; and growth of InN demonstrated on Ge, with AlN 

and Al as buffers. 

Ohio State, with MIT as a subtier contractor, has been 

developing and demonstrating the use of novel 3-D 

substrate engineering in Si and Ge to grow III-V 

multijunction devices on Si or Ge. The researchers 

demonstrated unprecedented, near-ideal structural 

perfection for a double-junction cell grown on Si using SiGe 

that successfully avoided all mismatch-related defects, such 

as threading dislocations and anti-phase domains. 

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0 0.511.522.5 3

Voltage (V)

Current Density / Incident Intensity (A/W)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Current Density / Incident Intensity (A/W)

Latt.-matched Metamorphic Latt.-matched Metamorphic

Voc 3.089 2.924 V 2.622 2.392 V 

Jsc/inten. 0.1431 0.1545 A/W 0.1437 0.1599 A/W 

Vmp 2.749 2.568 V 2.301 2.055 V 

FF 0.882 0.860 0.850 0.819

conc. 236 241 suns 1.0 1.0 suns

area 0.269 0.254 cm2

 4.0 4.0 cm2 

Eff. 39.0% 38.8% 32.0% 31.3% 

 Aperture-area efficiency at 25C Total-area efficiency at 25C

 AM1.5D, low-AOD spectrum AM1.5G 1 sun = 0.100 W/cm2 

 Concentrator Cells 1-sun Cells

 As mentioned previously, under another NREL 

subcontract for Future Generation PV technologies, the 

University of Delaware is analyzing advanced solar cell 

efficiency approaches such as multiple-energy-level 

quantum dot solar cells, which include realistic loss 

mechanisms, to identify solar designs that have a credible 

potential for >50% efficiency. 

A quantum-dot group from NREL’s Basic Sciences 

Center demonstrated quantum yields last year of up to 

300%, corresponding to three electron-hole pairs per 

photon at photon energies four times the quantum-dot 

bandgap [4]. This discovery may enable more efficient use 

of high-energy photons in the solar spectrum, once these 

materials are incorporated into a device design. 

CONCENTRATOR SYSTEMS R&D 

Several concentrator systems are being developed and 

have been demonstrated under the High-Performance PV 

Project, but they still need further exploration. Emphasis on 

the receiver package focuses on the receiver and system 

size, mounting and interconnects, active vs. passive 

cooling, and reliability and degradation. These subcontracts 

are described below. 

 Amonix Corporation currently markets large-scale units, 

35 kW in size, and is deploying power plant systems 

through a joint venture in Spain following earlier large 

system installations in U.S. utility grids. Under its 

subcontract, Amonix Corp. has been designing and 

fabricating a lens-cell element that will use >40%-efficient 

multijunction solar cells. Amonix envisions that such a solar 

device will be 33% efficient. 

The design of a Fresnel lens concentrator system must 

account for several optics issues, including nonuniformity, 

chromatic aberration, lens absorption, and matching to 

multijunction solar cell response. Based on their new optical 

design, Amonix engineers recently tested a lens-cell 

element that reached an efficiency of 28.2% at operating 

temperature and on-sun, not in the laboratory. 

JX Crystals, under its subcontract, has focused on 

demonstrating a 40%-efficient hybrid InGaP / GaAs-GaSb 

multijunction cell combination in a 33%-efficient 

Cassegrainian PV concentrator panel. They have designed 

and fabricated all module components, including two cell 

packages, three optical components, and two cooling 

3
components for incorporation into their 25-cm x 25-cm 

dichroic Cassegrain module. 

Concentrating Technologies, LLC, has been 

demonstrating a reflective-optics concentrator system using 

passively cooled Spectrolab triple-junction cells at 500x. 

Results include the continuous operation of Spectrolab 

triple-junction-powered units for more than one year. As a 

system test, the most significant accomplishment came 

from improvements to their power-conditioning unit 

interfacing with the Arizona Public Service grid. The 

architecture of the Concentrating Technologies module is 

versatile and allows next-generation solar cells as a “drop 

in” replacement or upgrade. 

 Sunpower Corporation, in collaboration with JX 

Crystals, has completed its work focusing on lowconcentration flat-plate designs using high-efficiency silicon 

cells. With this design, the impact is anticipated to be 

modules at $1.50/watt, instead of today’s $3/watt. The 

advantages to this approach are the ability to compete in 

the PV rooftop market and no need for a solar tracker. 

 An important challenge for concentrating PV (CPV) 

systems is reliability since systems are just beginning to 

enter the market. CPV needs to be reliable so that 

customers will trust the technology and be willing to buy into 

it. Qualification standards provide a series of accelerated 

and stress tests meant to identify system weaknesses 

before products are deployed, thereby providing a measure 

of assurance to both customers and sellers of long-term 

system reliability. In 2001, IEEE qualification standards 

were completed for CPV, and international standards are on 

the way under the aegis of the International Electrotechnical 

Commission. Under a subcontract originally started with 

Arizona State University, Underwriters Laboratories is 

working to develop, coordinate, and publish an IEC 

standard for the design qualification of CPV modules. The 

first international qualifications standard for CPV systems 

was submitted earlier this year for international ballot. 

CONCLUSIONS 

 Theoretical efficiencies are still much higher than 

laboratory multijunction efficiencies. And researchers are 

designing approaches for solar cells with efficiencies 

greater than 50%, while solar cells with 41% efficiency are 

likely to be available imminently in large quantities. The 

impact of high performance on cost of electricity generated 

by solar concentrators is expected to be significant—

specifically, achieving a system price of $2.00/Wp and less 

for CPV systems with 41%-efficient III-V multijunction solar 

cells. 

 Progress toward achieving long-term DOE goals is 

highlighted by the reported advances of high-efficiency 

cells, as well as by advances in concentrator modules 

demonstrating improved reliability and, finally, the 

appearance of large multi-megawatt CPV projects in the 

marketplace. The High-Performance PV Project has been 

focused to ensure that III-V multijunction concentrators 

reach efficiency levels consistent with cost-competitive 

goals and capture a large portion of a significant energy 

market. Utilities around the world are already beginning to 

see concentrator PV as a future energy alternative, one 

capable of meeting terawatts of future electricity production. 

ACKNOWLEDGEMENTS 

This work is supported under DOE Contract No. DEAC36-99GO10337 with NREL. Many people in both 

organizations have contributed to the development and 

implementation of the High-Performance PV Project. The 

authors thank the NREL High-Performance III-V 

Multijunction Concentrator Group and all of the HighPerformance PV subcontractors. 

REFERENCES 

[1] U.S. DOE, Office of Energy Efficiency and Renewable 

Energy, “Solar Energy Technologies Program, MultiYear Program Plan, 2007–11,” DOE/GO-102006-2290, 

January 2006. 

[2] R. McConnell and M. Symko-Davies, “High-Performance 

PV Future: III-V Multijunction Concentrators,” 20th

European Photovoltaic Solar Energy Conference, 2005. 

[3] R. King, D. Law, K. Edmondson, et al, “Metamorphic and 

Lattice-Matched Solar Cells Under Concentration,” IEEE 

4th World Conference on Photovoltaic Energy 

Conversion, May 2006. 

[4] M. Wanlass, P. Ahrenkiel, D. Albin, et al, “Monolithic 

Ultra-Thin GaInP/GaAs/GaInAs Tandem Solar Cells,” 

IEEE 4th World Conference on Photovoltaic Energy 

Conversion, May 2006. 

4