In this work, we discuss difficulties we have faced while characterizing non-LM-Ge based CPV modules at the Instituto de Energía Solar, including: 1. For MJ cells with a 1eV bottom, bottom sub-cells may be current matched to the other two cells at reference spectrum. 2.In the case of metamorphic sells, with lower bandgaps in the middle cell, we find that certain spectral artifacts that previously had little influence on the middle cell (and therefore on ratio of JTop to JMid) are now more important

Antón Hernández, Ignacio

Askins, Stephen

Nuñez Júdez, Rubén

Sala Pano, Gabriel

Servicio de Coordinación de Bibliotecas de la Universidad Politécnica de Madrid

Characterizing CPV in a Lattice Mismatched World 
S. Askins, R. Nuñez, I. Anton, and G. Sala 
Instituto de Energía Solar - Universidad Politécnica de Madrid, Ciudad Universitaria, 28040 Madrid, Spain 
 
1. Background 
During the last decade, the explosive growth of multi-junction solar cell efficiency has led to resurgence in concentrator 
photovoltaics (CPV). During this time, the state of the art for characterization of CPV modules has been improving, 
with new equipment, such as solar simulators [1], being introduced, and many articles about characterization methods 
indoors and out, [2]–[4]. This body of knowledge is now being partly codified in a new IEC standard, 62670-3. 
However, during this time almost all extent CPV modules have used lattice-matched triple junction GaInP/GaInAs/Ge 
(LM-Ge) solar cells. This architecture has certain aspects which simplify its spectral characterization: 
 Most importantly, the third junction produces far more current than the other two, so that we can assume that it is 
never the limiting subcell. Thus the light spectrum can be completely characterized by a single spectral matching 
ratio, 
Top
MidSMR , and in general the amount of energy beyond 900nm can be ignored. This holds true for both indoor as 
well as outdoor measurements.  
 The bandgap of the second junction is high enough such that all effects of perceptible water vapor on the solar 
spectrum fall almost entirely into the third junction such that 
Mid
BotSMR  is mostly related to the atmospheric variables 
that Air Mass (AM) and Aerosol Optical Depth (AOD), whereas Perceptible Water Vapor (PWV) is mostly seen to 
affect Mid
BotSMR [5] 
Now, higher-efficiency alternatives to LM-Ge cells are moving from the laboratory to industry. In 2012, Semprius 
signed an agreement with Solar Junction and exclusively use their GaInNAs-based (1eV bottom) LM cells. In 2014, 
Azur Space, switched their commercial line-up of terrestrial multi-junction cells to 42% efficient upright metamorphic, 
with a Germanium bottom, but top and middle subcells with lower bandgaps than that imposed by the Germanium 
lattice constant. Also in that year, the world record was set for the first time by a four-junction wafer-bonded device. It 
is clear that the world is moving on from the LM-Ge cell. 
2. Experimental  
In this work, we discuss difficulties we have faced while characterizing non-LM-Ge based CPV modules at the Instituto 
de Energía Solar, including: 
1. For MJ cells with a 1eV bottom, bottom sub-cells may be current matched to the other two cells at reference 
spectrum. Even in cases where JBot > (JTop = JMid) if the bottom cell fill factor is low, such as in the case of a 
GaInNAs cell, variations the amount of energy in the bottom cell will impact the overall cell IV curve. We find this 
to be especially important in our indoor measurements. Our Xenon source, unfiltered exhibits 
Mid
BotSMR of 
approximately 0.83. In this work we measure a GaInNas-based CPV module using a custom water filter to ensure a 
triple-match ( 1
Top Mid
Mid BotSMR SMR  ) and show that measurements made without this filter may cause estimations of 
module power. (Figures 1 and 2) 
2. In the case of metamorphic sells, with lower bandgaps in the middle cell, we find that certain spectral artifacts that 
previously had little influence on the middle cell (and therefore on ratio of JTop to JMid) are now more important. The 
absorption peak caused by PWV, or in the case of indoor measurements, Xenon’s spectral lines, have a far-greater 
effect on the middle cell than previously. This will cause measurement errors if LM-Ge isotypes are used for 
spectral characterization. (Figures 3 and 4) 
References 
[1] C. Domínguez, I. Antón, and G. Sala, “Solar simulator for concentrator photovoltaic systems,” Opt. Express, vol. 
16, no. 19, pp. 14894–14901, Sep. 2008. 
[2] G. Peharz, G. Siefer, K. Araki, and A. W. Bett, “Spectrometric outdoor characterization of CPV modules using 
isotype monitor cells,” in 33rd IEEE Photovoltaic Specialists Conference, 2008. PVSC ’08, 2008, pp. 1–5. 
[3] M. Muller, B. Marion, S. Kurtz, and J. Rodriguez, “An Investigation into Spectral Parameters as they Impact CPV 
Module Performance,” in AIP Conference Proceedings, 2010, vol. 1277, pp. 307–311. 
[4] C. Domínguez, I. Antón, G. Sala, and S. Askins, “Current-matching estimation for multijunction cells within a 
CPV module by means of component cells,” Prog. Photovolt. Res. Appl., vol. 21, no. 7, pp. 1478–1488, 2013. 
[5] R. Núñez, I. Antón, S. Askins, and G. Sala, “Atmospheric parameters, spectral indexes and their relation to CPV 
spectral performance,” in AIP Conference Proceedings, 2014, vol. 1616, pp. 290–293. 
 
1 1.05 1.1 1.15 1.2 1.25
0.835
0.84
0.845
0.85
0.855
0.86
SMR
F
ill
 F
a
c
to
r
 
 
FF vs. SMRTop
Mid
, Experimental
FF vs. SMRBot
Mid
, Experimental
FF vs. SMRTop
Mid
, Model
FF vs. SMRBot
Mid
, Model
 
Figure 1: Fill factor for a module using GaInNAs bottom cells. The 
SMR on the x-axis show excess of energy as compared to the 
middle. (
Bot
MidSMR is shown rather than 
Mid
BotSMR ). SMR are measured 
with GaInNAs-based isotype cells. We observe that an excess in 
either the top or bottom bands as compared to the middle causes 
approximately the same increase in fill factor, that is the two SMR 
are equally important for a correct measurement of Fill Factor.  
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 20 40 60 80 100
C
u
rr
en
t
Voltage (V)
0.4
0.5
0.6
0.7
85 90 95 100
 
Figure 2: IV curve of the GaInNAs module with normal spectral 
filtering applied (allows 20% excess energy in the bottom cell), dashed 
lines, as compared to that obtained with the custom water filter (triple 
spectral match), solid lines. The inset shows the knee of the curve, 
where the overestimation of FF in the first case, due to over-saturation 
of the bottom cell, is apparent. 
0.99 1 1.01 1.02 1.03 1.04
3.42
3.44
3.46
3.48
3.5
3.52
3.54
SMRMid
Bot
I*
N
 (
A
/1
0
0
0
W
/ m
2
 
Figure 3: The short circuit current normalized to irradiance, filtered for 1 .01
Top
MidSMR   , is shown over a number of days w.r.t. 
Mid
BotSMR . The 
sensor used to measure the SMR values in this case were LM-Ge isotype cells. The inverse relationship scene is due to loss of current in the 
middle cell as PWV increases. In other words, LM-Ge isotype cells cannot correctly characterize the spectrum for modules using these cells.  
400 500 600 700 800 900 1000 1100 1200
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
Wavelength (nm)
 
 TopUMM
Mid
UMM
Bot
UMM
Top
LM
Mid
LM
Bot
LM
AM1.5D
Xenon
PWV Absorption 
Peak
Xenon 
spectral
lines
 
Figure 4: The quantum efficiency of the UMM cells for the module investigated in Figure 3 are compared to those of LM-Ge cells. The spectral 
zone that was previously in the bottom region but is now in the middle region is shaded pale green. It can be seen that certain important spectral 
phenomena are now affecting the middle cell: The first large water absorption peak is now completely in the middle spectral zone, as well as all 
of the Xenon spectral lines. 
 
Custom water filter 
 1Top MidMid BotSMR SMR   
Normal filter 
1
1
1.2
Top
Mid
Mid
Bot
SMR
SMR
 
 
  
 
 


Characterizing CPV in a Lattice Mismarched World

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Characterizing CPV in a Lattice Mismarched World

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