High efficiency solar cells working under ultra-high concentrations (>;1000X) have been shown to be a promising solution to decrease the cost of PV electricity, increase the efficiency and circumvent the material availability restrictions for massive PV penetration. A detailed analysis of the limitations of our current triple junction solar cell (36.2% at 700X), in the quest to maximize efficiency at 1000X, shows that the main improvements to tackle are: a) implementation of a high band gap tunnel junction; b) increase the band gap of the top cell; c) fine current matching tune; d) enhancement of the front contact process. This constitutes our roadmap to reach an efficiency over 41

Algora del Valle, Carlos

Barrigón Montañés, Enrique

García Vara, Iván

Rey-Stolle Prado, Ignacio

Archivo Digital UPM

Roadmap towards efficiencies over 40% at ultra-high concentrations (> 1000 suns)

Servicio de Coordinación de Bibliotecas de la Universidad Politécnica de Madrid

ROADMAP TOWARDS EFFICIENCIES OVER 40% AT ULTRA-HIGH 
CONCENTRATIONS (> 1000 SUNS) 
 
E. Barrigon, I. Rey-Stolle, I.García, and C. Algora  
Instituto de Energía Solar – Universidad Politécnica de Madrid 
ETSI de Telecomunicación; Avda. Complutense 30; 28040 Madrid (Spain) 
Phone: +34 91 3367232; Fax: +34 91 5446341; Email: algora@ies-def.upm.es 
 
 
ABSTRACT 
 
High efficiency solar cells working under ultra-high 
concentrations (>1000X) have been shown to be a 
promising solution to decrease the cost of PV electricity, 
increase the efficiency and circumvent the material 
availability restrictions for massive PV penetration. A 
detailed analysis of the limitations of our current triple 
junction solar cell (36.2% at 700X), in the quest to 
maximize efficiency at 1000X, shows that the main 
improvements to tackle are: a) implementation of a high 
band gap tunnel junction; b) increase the band gap of the 
top cell; c) fine current matching tune; d) enhancement of 
the front contact process. This constitutes our roadmap to 
reach an efficiency over 41%. 
 
INTRODUCTION 
 
Record efficiencies of triple junction solar cells have been 
historically achieved within the range going from 300 to 
500 suns: 40.8% at 326 X by NREL [1]], 41.1% at 454 X 
by Fraunhofer Institute [2], 41.6% at 364 X by Spectrolab 
[3], 42.3% at 406 X by Spire [4] and 43.5% at around 400 
suns by Solar Junction [5]. In most cases, the efficiency 
dramatically drops at concentrations of 1000 suns or 
higher. Alternatively, the operation at ultra-high 
concentrations (> 1000 suns) is expected to produce the 
following benefits: 
a) Cost reduction as consequence of a decrease of the 
solar cell impact on the final cost of the concentrator 
system [6], 
b) Increase of the raw material availability whose most 
severe limitation seems to be that of germanium 
substrates which is currently the preferred one for 
MJSCs [7]. Other materials used in the MJSC 
manufacturing such as TMGa for the MOVPE growth 
has increased recently its price as a consequence of 
the rapid penetration of LED backlighting modules in 
LCD TVs which absorbs most of the production of 
MOVPE consumables to date. Therefore, an 
increased concentration would relax this cost stress. 
c) Efficiency increase of the MJSCs with concentration 
as a result of the increase of its open circuit voltage 
[8] if its series resistance is kept low. 
However, there are some concerns about going to higher 
concentrations than 500 X such as; 1) high concentrations 
usually lead to chromatic aberration in the optics; 2) such 
high concentrations raise doubts about cooling, series 
resistance and stability of MJSC; and 3) although the cost 
impact of the MJSCs is reduced, an increase on cost of 
other elements such optics and tracking appears because 
of the need of a higher pointing accuracy. However, all 
these issues seem to be already properly faced [9]. 
 
Accordingly, the strategy of IES-UPM has been the design 
and manufacturing of III-V solar cells for operation at ultra-
high concentrations. In fact, we have developed and 
manufactured a GaInP/GaAs dual-junction solar cell which 
exhibits an efficiency of 32.6% for a concentration range 
going from 499 to 1026 suns. This efficiency is the world 
record efficiency for a dual-junction solar cell [10]. 
Besides, the efficiency is still as high as 31% at 3000 
suns. We have extended this strategy to lattice-matched 
GaInP/Ga(In)As/Ge triple junction solar cells (TJSC). At 
the current state of development, our cells show an 
efficiency of 36.2% at 700 suns [11]. 
 
Furthermore, the active area of our cells is about 1 mm2, 
which is close to the optimum value, in accordance to the 
compromise situation imposed by the series resistance, 
perimeter recombination, packaging cost and heat 
dissipation for an operation of the device at 1000 suns. 
The resulting geometrical dimensions of the device make 
it appropriate to use the “LED-like approach" [6] for the 
post-growth processing of the semiconductor structure. 
 
For the sake of obtaining high efficiencies at ultra-high 
concentrations, we analyze in this paper the factors 
limiting the efficiency of our current triple junction solar 
cells. If these limiting factors (namely a low band gap top 
cell, an absorbing tunnel junction and a significant current 
mismatch) are overcome, the theoretical modeling carried 
out, based on realistic technological improvements, shows 
that an efficiency over 41% at 1000 suns is reachable. The 
first improvements have been already experimentally 
achieved and are presented here. 
 
RESULTS 
 
Figure 1 summarizes the external quantum efficiency 
(EQE) of the lattice matched (LM) GaInP/Ga(In)As/Ge 
triple junction solar cells manufactured in-house (EQE of 
Ge subcell not plotted for the sake of clarity).  
 
The overall result is that the device is middle cell limited 
with a short circuit current of JSC = 13.3 mA/cm
2 (AM1.5D 
ASTM G173 normalized to 1000 W/m2), being the current 
in the top cell JSC,TC = 14.2 mA/cm
2. This situation reveals 
a relatively large current mismatch of around 6.5% that 
978-1-4244-9965-6/11/$26.00 ©2011 IEEE 000775
can be easily corrected adjusting the thickness of the top 
and middle subcells. 
 
 
 
Figure 1.  External quantum efficiency (EQE) of the 
triple-junction solar cells (TJSC) developed. The red 
line represents the modeling of such EQE. The EQE of 
the Ge subcell in the TJSC is not shown for clarity. 
 
Figure 2 presents the concentrator results of the triple-
junction solar cells developed, showing that the efficiency 
is over 36% for concentrations between 550 and 850 
suns, peaking at 700X with a 36.2% [11]. 
 
 
 
Figure 2.  Concentration response of the triple 
junction solar cell whose EQE is shown in figure 1. 
 
DEVICE IMPROVEMENT ANALYSIS 
 
A detailed analysis of the solar cells developed has been 
carried out in order to establish a clear diagnosis of their 
limitations and, consequently, a roadmap to attain 
efficiencies over 40% at 1000 suns. Figure 3 visually 
illustrates such roadmap, by means of iso-efficiency 
contours of 1000-sun JSC and VOC that yield efficiencies 
from 36% to 40% at 1000 suns. This figure assumes that 
the FF evolution in Fig 2 can be maintained (namely 
FF=86.0% at 1000 suns) as improvements in JSC and VOC 
are materialized (i.e. junction quality and series resistance 
remain unaffected). Additionally, it is also assumed the 
linearity of JSC vs concentration (hence the following 
discussions will be based on the one-sun JSC or JSC,1X). In 
this figure, the current design is marked as a filled black 
circle. Other markers represent some improved designs 
that steadily approach the η = 40% target.  
 
 
 
Figure 3.  Iso-efficiency contours of JSC and VOC at 
1000 suns to attain 36% to 40% efficiency at 1000 suns 
(FF is assumed to be the experimental value in fig.2, 
FF= 86.0%). The corresponding JSC/VOC pairs for 
current result (Fig 2) and several improvements on 
this design are also indicated. 
 
The modeling of the EQE of the device (fig. 1) revealed a 
defective design in the middle cell. On the one hand, its 
thickness (2500 nm) was clearly insufficient and some 
interferometric reflections in upper layers were causing 
unexpected reflection losses. These facts were corrected 
in Design #1 and subsequently current matching at  
JSC,1X = 14.1 mA/cm
2 can be attained (corresponding EQE 
depicted in Figure 4) together with a slight increase in VOC 
(∼20 mV).  
 
Design #2 implements a high band gap cathode 
(n++GaInP) in the top tunnel junction and a slight 
correction in the thickness of the top cell. With these 
changes the Isc can be further increased to JSC,1X = 14.4 
mA/cm2, which in turn causes a slight increase in VOC.  
 
Design #3 represents the use of surfactant-mediated 
growth to raise the band gap of the top cell to ∼1.81 eV 
and subsequent rearrangement of thicknesses for current 
978-1-4244-9965-6/11/$26.00 ©2011 IEEE 000776
matching. This approach would increase the VOC in some 
60 mV and JSC would be raised to JSC,1X = 14.55 mA/cm
2.  
 
Finally, Design #4 includes a final tuning of the 
window/emitter interface of the top cell, and a new 
increase in the top cell band gap to 1.83 eV. These 
changes would elevate the short circuit current to 
JSC,1X = 14.60 mA/cm
2, while a further boost in VOC can be 
achieved to reach VOC = 3.206 V and thus get a final 
efficiency of 40.3% at 1000X. In this final stage our design 
would show slightly lower figures than those reported for 
the record lattice matched triple junction cell [3] at 1000X 
(JSC,1X=14.67 mA/cm
2, VOC = 3.270 V, EgTC=1.84 eV), 
though with a significantly higher fill factor (86% vs. 83%). 
The evolution of the EQE throughout all these designs can 
be followed in figure 4. 
 
300 400 500 600 700 800 900
0
20
40
60
80
100
Wavelength [nm]
E
xt
er
n
al
 Q
u
an
tu
m
 E
ff
ic
ie
n
cy
 [
%
]
 
 
Current result
Design #1
Design #2
Design #3
Design #4
 
 
Figure 4.  Evolution of the EQE of the top and middle 
cell from Design 1# to Design#4. 
 
Last, there is further room for improvement regarding the 
FF. In the world record dual-junction solar cell, the FF 
achieved at 1000X was 88% [12], 2% higher than the 
experimental value of our solar cells, considered for this 
device analysis. We believe that this difference is due to a 
defective processing of the front contact. When such high 
FF are reproduced in the triple junction, its efficiency will 
be 41.2% at 1000X. 
 
EXPERIMENTAL PROGRESS 
 
The experimental implementation of the aforementioned 
roadmap is not straight forward. However significant 
progress has been made so far. For instance, 
improvements in the middle cell have been implemented 
resulting in a significant increase in its JSC.(~1 mA/cm
2). 
On the other hand, Figure 5 shows the J-V curve of the 
high band gap tunnel junction implemented (with an 
AlGaAs-based anode and a GaInP-based cathode), as 
grown and after a 30-min long anneal at 675ºC in order to 
simulate the thermal load suffered by the tunnel structure 
inside the MOVPE reactor when a complete triple junction 
structure is grown. The “as grown” device presents a very 
high peak current (3 kA/cm2) and a low series resistance 
(2.8·10-4 Ω·cm2) while JSC is reduced to about 730 A/cm
2 
and the series resistance is raised to 1.5·10-3 Ω·cm2 after 
the annealing process. However, the values of these 
figures of merit are still good enough to use these tunnel 
junctions in the final triple junction solar cell structure. For 
instance, in the inset of Figure 5, it can be seen that the 
voltage drop at a current injection of 14 A/cm2 (the 
expected JSC of a TJSC working at 1000X) is merely 20 
mV.  
 
 
 
Figure 5.  J-V curve of the high band gap tunnel 
junction implemented, for the cases “as-grown” and 
annealed 30 min at 675 ºC.  
 
SUMMARY AND CONCLUSIONS 
 
High efficiency solar cells working under ultra-high 
concentrations (>1000X) seem to be a promising solution 
to decrease the cost of PV electricity, increase the 
efficiency and circumvent the material availability 
restrictions for massive PV penetration. A detailed 
analysis of the limitations of our current triple junction solar 
cell (36,2% at 700X) in the quest to maximize its efficiency 
at 1000X, has led to the description of a roadmap to reach 
an efficiency over 41%. In order to follow this roadmap, a 
proper high band gap tunnel junction for ultra-high 
concentrations has already been successfully developed. 
The other required improvements are currently under 
development. 
 
REFERENCES 
                                                 
[1] J.F. Geisz, D. J. Friedman, J. S. Ward, A. Duda, W. J. 
Olavarria, T. E. Moriarty, J. T. Kiehl, M. J. Romero, A. G. 
Norman, and K. M. Jones, “40.8 efficient inverted triple-
junction solar cell with two independently metamorphic 
junctions”, Appl. Phys. Lett. 93, 2008, pp. 123505. 
 
[2] W. Guter, J. Schöne, S. P. Philipps, M. Steiner, G. 
Siefer, A. Wekkeli, E. Welser, E. Oliva, A.W. Bett and 
Frank Dimroth, “Current-matched triple-junction solar cell 
reaching 41.1% conversion efficiency under concentrated 
sunlight”, Appl. Phys. Lett. 94, 2009, pp 223505. 
978-1-4244-9965-6/11/$26.00 ©2011 IEEE 000777
                                                                               
 
[3] R.R. King, A. Boca, W. Hong, X.-Q. Liu, D. Bhusari, D. 
Larrabee, K. M. Edmondson, D. C. Law, C. M. Fetzer, S. 
Mesropian, and N. H. Karam, "Band-gap-engineered 
architectures for high-efficiency multijunction concentrator 
solar cells," 24th European PV Solar Energy Conference 
and Exhibition, 2009, pp. 55-61 
 
[4] M. A. Green, K. Emery, Y. Hishikawa and W. Warta, 
Solar cell efficiency tables (version 37), Prog. Photovolt: 
Res. Appl. 19, 2011, pp. 84–92 
 
[5] http://www.compoundsemiconductor.net/csc/news-
details.php?cat=news&id=19733359 
 
[6] C. Algora, "Very high concentration challenges of III-V 
multijunction solar cells”, in “Concentrator Photovoltaics”, 
A. Luque & V. Andreev (eds.), Springer (Germany) 2007. 
 
[7] A. Feltrin and A. Freundlich, "Material Challenges for 
Terawatt Level Deployment of Photovoltaics," 2006 IEEE 
and 4th World Conference on Photovoltaic Energy 
Conversion, 2006, pp. 2469-2472. 
 
[8] G. L. Araújo in “Physical limitations to photovoltaic 
energy conversion”, Ed. A. Luque and G.L. Araujo, Adam 
Hilger (1990). 
 
[9] Carlos Algora and Ignacio Rey-Stolle, The interest and 
potential of ultra-high concentration, in New concepts for a 
next generation of photovoltaics,Springer-Verlag (in press) 
 
[10] M. A. Green, K. Emery, Y. Hishikawa and W. Warta,, 
Solar cell efficiency tables (version 36), Prog. Photovolt: 
Res. Appl. 18, 2010, pp. 346–352. 
 
[11] I. Rey-Stolle, C. Algora, E. Barrigon, I. Garcia, P. 
Espinet and E. Garcia Tabarés, “Status of ultra-high 
concentrator multijunction solar cell development at IES-
UPM”, 25th European PVSC, Sept 2010, pp. 152-155 
 
[12] I. García, Rey-Stolle, B. Galiana and C. Algora, “A 
32.6% efficient lattice-matched dual-junction solar cell 
working at 1000 suns”, Appl. Phys. Lett. 94, 053509 
(2009) 
978-1-4244-9965-6/11/$26.00 ©2011 IEEE 000778


http://oa.upm.es/11717/1/INVE_MEM_2011_93445.pdf

Roadmap towards efficiencies over 40% at ultra-high concentrations (> 1000 suns)

Abstract

Similar works

Full text

Available Versions

Archivo Digital UPM

Servicio de Coordinación de Bibliotecas de la Universidad Politécnica de Madrid