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Rapid, Enhanced IV 

Characterization of MultiJunction PV Devices Under 

One Sun at NREL

Preprint

Tom Moriarty, Ryan France, and Myles Steiner

National Renewable Energy Laboratory

Presented at the 42nd IEEE Photovoltaic Specialists Conference

New Orleans, Louisiana

June 14–19, 2015

Conference Paper

NREL/CP-5J00-64416

September 2015
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Rapid, enhanced IV characterization of multi-junction PV devices under 

one sun at NREL 

Tom Moriarty, Ryan France, Myles Steiner

National Renewable Energy Laboratory, Golden, CO 80401 USA

Abstract — Multi-junction technology is rapidly advancing, 

which puts increasing demands on IV characterization resources. 

We report on a tool and procedure for fast turn-around of IV 

data under the reference conditions, but also under controlled 

variations from the reference conditions. This enhanced data set 

can improve further iterations of device optimization.

I. INTRODUCTION

IV curves of PV devices under reference conditions are an 

essential metric for device performance and an essential tool 

for optimizing the design of succeeding generations of the 

particular technology. Conceptually, and in practice, this has 

been a straight forward process for single-junction devices

through the use of the spectral mismatch correction factor 

method [1]. That is, use a reference cell with known QE, 

measure the QE of the test device, measure the spectrum of 

the simulator, and use these known quantities to calculate a 

spectral mismatch correction factor, M. This factor is used to

adjust the total simulator irradiance, without changing its 

spectral shape, such that the test device will respond under the 

simulator spectrum as it would have responded under the 

reference spectrum.

This process is considerably more difficult and time 

consuming for multi-junction devices. A spectral mismatch 

correction factor must be calculated for each junction, and in 

general M will be different for each junction. Consequently, 

the spectral shape of the simulator must be adjusted as well at 

its total irradiance. It is usually very impractical to make 

multiple measurements under non-standard conditions because 

building the needed spectra tends to be very time consuming.

NREL’s One-Sun Multi-Source Simulator (OSMSS) is a 

tool that greatly speeds this process. The OSMSS and the 

procedures used to build effective reference spectra have been 

previously described [2], and it has now been successfully 

used for up to six junction devices.

Here, we first present a process to vary the spectrum to one 

junction in a controlled fashion while keeping the effective 

irradiance to the remaining n-1 junctions at the reference level. 

IV data collected under these circumstances is proving useful 

in optimizing new iterations of 4J devices [3]. We also 

present IV data for a multi-junction device at several 

temperatures and irradiances. In this case the simulator 

spectrum was recreated for each temperature to provide the 

effective reference irradiance for the temperature varying QEs.

II. BUILDING MODIFIED SPECTRA

Let sim( and ref( be normalized simulator and 

reference spectra, and let Qi( be the normalized QE for 

junction i of multi-junction device. If a test device is to 

behave under the simulator spectrum as it would under the 

reference spectrum then this is a necessary condition:

( ) ( )

( ) ( ) = ( ) ( )( ) ( ) (1)

Define

= ( ) ( )

( ) ( ) ×

( ) ( )

( ) ( ) (2)

Then the simulator spectrum shape will adequately 

reproduce the effect of the reference spectrum when Rij = 1 for 

all i and j. A method for automated adjustment of the OSMSS

spectrum to meet this condition is described in [2], but any 

method on any simulator that successfully sets Rij = 1 for all i

and j will work.

Information on the behavior of individual junctions can be 

obtained by measuring IVs of the test device under various 

alternative conditions, such as temperature or spectrum. 

Alternative spectra can be treated as simple variations of the 

effective irradiance to each junction, i, by some factor, i, 

Then in general, the test device will behave under the 

simulator spectrum as it would under the desired modified 

conditions when...

( , ) ( )

( , ) ( ) = ( ) ( , )

( ) ( , ) (3)

In which case...

= ( , ) ( )

( , ) ( ) ×

( ) ( , )

( ) ( , ) (4)

A useful approach to spectrum variation is to vary the 

irradiance to only one junction, say junction k. Then from (4),

the desired spectrum will be achieved when = , 

= , and = 1 for , .

The OSMSS automated spectrum building algorithm has 

been modified to build spectra with specified 1.
2

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III. VARIED SPECTRUM DATA EXAMPLE

The NREL fabricated 4-junction inverted metamorphic (4JIMM) device, MN187, has been measured with the irradiance 

to each junction varied from 0.8 suns to 1.2 suns (AM1.5 

Direct) while keeping the irradiance to the remaining three 

junctions at 1 sun. Fig. 1 shows the QEs of this 4-junction 

device overlaid with the OSMSS spectra that were built to 

satisfy = , = , and = 1 for , 1,

where varies from 0.8 to 1.2 . That is, the effective 

irradiance to the top junction was varied from 0.8 suns to 1.2 

suns, while the effective irradiances to the other junctions 

were kept at one sun. This process was repeated for the other 

three junctions as well.

Figure 1. QEs for 4J-IMM and spectra built to vary effective 

irradiance to the top junction while keeping effective irradiance to 

junctions 2 through 4 at the effective reference irradiance.

Note that although the spectra in Fig 1 all provide the same 

effective irradiance to junctions 2 through 4, they do not have 

exactly the same shape in the response range of those 

junctions. However, the criteria that = , = ,

and = 1 are satisfied. There are an infinite number of 

ways to satisfy these criteria – it is only necessary to find one 

in the subset attainable by the simulator.

Figs 2, 3, 4, 5 & 6 show the changes in Isc, Fill Factor, 

Vmax, Imax and Pmax, respectively resulting from the varied 

spectra.

Figure 2. Isc vs. the percent of the effective reference irradiance to 

the selected junction. The effective reference irradiance to all other 

junctions is 100%

Figure 3. Isc vs. the percent of the effective reference irradiance to 

the selected junction. The effective reference irradiance to all other 

junctions is 100%

Figure 4. Vmax vs. the percent of the effective reference irradiance 

to the selected junction. The effective reference irradiance to all 

other junctions is 100%.

Figure 5. Imax vs. the percent of the effective reference irradiance to 

the selected junction. The effective reference irradiance to all other 

junctions is 100%.
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Figure 6. Pmax vs. the percent of the effective reference irradiance 

to the selected junction. The effective reference irradiance to all 

other junctions is 100%.

III. VARIED TEMPERATURE DATA EXAMPLE

Measuring the IV response under the reference spectrum at

varying temperatures is usually not a challenging undertaking 

for a single junction device with good thermal contact to a 

temperature controlled stage. It is considerably more 

complicated to properly make these measurements for a 

multjunction device because the temperature dependent QEs 

force a rebuild of the simulator’s spectral shape for every 

temperature. NREL’s traditional hardware and spectral 

mismatch correction method could require hours of set-up 

time to build a new spectrum at for each temperature for a 

three junction device. It was not practical for a device with 

more than three junctions and was never attempted.

This is now a straight forward and rapid process using the 

OSMSS. Fig 7 shows the QEs at three temperatures for a 

three junction GaInP/GaAs/GaInAs device (MH814)

fabricated at NREL.

Figure 7. QEs for GaInP/GaAs/GaInAs (ID: MH814) at three 

temperatures

Fig 8 shows the resulting unscaled spectra that the OSMSS 

built to measure the IV of the device at the three temperatures 

under the effective AM1.5 global spectrum at 1000 W/m2

. 

Note the great variation of the spectra corresponding to the 

temperature changes. Nevertheless, the criteria that = 1 

for all i and j are satisfied. 

Figure 8. 

IV. CONCLUSION

NREL’s OSMSS’s ability to rapidly build spectra opens up 

new IV characterization paths that were previously 

impractical. The data in Figs. 2 through 6 were collected in 

about three hours. Each data point in those figures represents 

an IV curve under simply defined, tightly controlled, unique 

irradiance conditions. Collecting the same set of data on this 

four junction device using NREL’s traditional hardware and 

the spectral mismatch technique would have taken months – if 

it were possible at all.

Other possible research avenues include rapidly building 

representative spectra for simulator based yearly energy yield 

measurements. This should be possible by selecting the 

proper non-unity values for i and j in (4) to reproduce the 

effect of each spectrum.

REFERENCES

[1] C.R. Osterwald, “Translation of device performance 

measurements to reference conditions,” Solar Cells, 18, pp 269-

279, 1986 

[2] T. Moriarty, J. Jablonski, K. Emery, “Algorithm for Building a 

Spectrum for NREL’s One-Sun Multi-Source Simulator,” 

Proceedings of the 38th IEEE Photovoltaic Specialists 

Conference (PVSC), June 2012, pp1291-1295 

[3] R. M. France, J. F. Geisz, I. García1, M. A. Steiner, W. E. 

McMahon, D. J. Friedman, T. E. Moriarty, C. Osterwald, J. S. 

Ward, A. Duda, M. Young, and W. J. Olavarria, “Design 

flexibility of 4-junction ulta-high efficiency inverted 

metamorphic solar cells”, submitted to 42nd IEEE PVSC, 2015.