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Contract No. DE-AC36-99-GO10337 

W.E. McMahon, K.E. Emery, D.J. Friedman, 

L. Ottoson, M.S. Young, J.S. Ward, C.M. Kramer, 

A. Duda, and Sarah Kurtz 

Prepared for the 31st IEEE Photovoltaics Specialists 

Conference and Exhibition 

Lake Buena Vista, Florida 

January 3–7, 2005

February 2005 • NREL/CP-520-37376 

An On-Sun Comparison of 

GaInP2/GaAs Tandem Cells 

with Top Cell Thickness Varied 
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AN ON-SUN COMPARISON OF GaInP2/GaAs TANDEM CELLS 

WITH TOP CELL THICKNESS VARIED 

W.E. McMahon, K.E. Emery, D.J. Friedman, L. Ottoson, M.S. Young, 

J.S. Ward, C.M. Kramer, A. Duda, and Sarah Kurtz 

National Renewable Energy Lab, 1617 Cole Blvd, Golden, CO 80401 

ABSTRACT

This study compares the on-sun performance of a set 

of GaInP2/GaAs tandem cells with different GaInP2 top-cell 

thicknesses. Because high-efficiency III-V cells are best 

suited to concentrating photovoltaic (CPV) applications, 

the cells were mounted on a two-axis tracker with the incident sunlight collimated to exclude all except the direct 

beam. Current-voltage (I-V) curves were taken throughout 

the course of several days, along with measurements of 

the direct solar spectrum. Our two major conclusions are: 

(1) GaInP2/GaAs tandem cells designed for an "air mass 

1.5 global" (AM 1.5G) or a "low aerosol optical depth" (Low

AOD) spectrum perform the best, and (2) cells can be 

characterized indoors and modeled using outdoor spectra 

to predict the correct result. These results are equally

valid for GaInP2/GaAs/Ge triple-junction cells. 

INTRODUCTION

To maximize the performance of GaInP2/GaAs tandem cells and GaInP2/GaAs/Ge triple-junction cells, the 

top GaInP2 cell must be "thinned" slightly to allow some 

above-band-gap photons to pass through to the GaAs 

bottom cell. Because the solar spectrum changes 

throughout each day, the optimal top-cell thickness (ttop) 

constantly changes. Nonetheless, when tandem cells are 

manufactured for use in a concentrator system, a single 

ttop must be specified. This study is intended to aid in ttop

selection for CPV applications. 

The first half of this paper compares the performance 

of real GaInP2/GaAs tandem cells as a function of ttop, 

under direct outdoor sunlight. Measurements were taken 

over the course of several days to study how changing 

spectral content affects performance. In general, we find 

that GaInP2/GaAs tandem cells designed for an AM 1.5G 

or a Low AOD spectrum perform the best. These results 

agree well with a previous theoretical study [1], in which 

we simulated the performance of GaInP2/GaAs tandem 

cells under "standard day conditions" as a function of ttop. 

In the second half of this paper, we obtain the same 

results theoretically by carefully characterizing the same 

set of cells and applying a simple device model. This 

modeling also allows us to theoretically decouple the topand bottom-cell photocurrents, so as to clearly illustrate 

the importance of "current matching" for tandem cell performance. 

Measurements were made on clear days in Golden, 

Colorado (40°N, 105°W, 1830m). To a first approximation, 

they represent the spectral variation of sunlight at a typical 

concentrator site in the southwestern United States. 

EXPERIMENTAL DETAILS

To give some background, the model results in Fig. 1 

show how performance should vary with ttop under four 

standard reference spectra [2]. For each spectrum, there 

is an optimal ttop. 

For this study, we grew a set of tandem cells (named 

'A' - 'E') with five different top-cell thicknesses, with a ttop

range spanning all foreseeable applications. Approximate 

Fig. 1. Calculated power produced by a GaInP/GaAs/Ge 

triple-junction cell as a function of ttop for Air Mass 0, 1.5 

Global, 1.5 Direct, and Low Aerosol Optical Depth standard reference spectra. Relative ttop values are shown, 

normalized to the optimal AM0 ttop. Although the power 

produced by a GaInP/GaAs tandem cell will be less, the 

optimal ttop for any given spectrum will not change. Approximate ttop values for cells A - E are indicated. 

1 
ttop values for these cells are shown along the bottom axis. 

Cell A has the thinnest ttop and is well suited to "blue-rich" 

space applications. Cell E has the thickest ttop and is better for "red-rich" morning and evening light. The other 

three cells have intermediate ttop values which are compromises between midday power production and overall 

daily energy production. As a gauge of experimental error, 

two cells were grown with the median ttop (C1 and C2). 

Cell C should perform best under a Low-AOD spectrum [3] 

proposed for concentrator applications. A "clear sky" direct spectrum which is similar to the Low-AOD spectrum 

has also been proposed [4]. 

Fig. 2. Measured powers for cells A - E on Sept. 23-24, 

2004, at NREL, in Golden, Colorado. Because no morning data were taken the first day, data from the morning of 

the second day are substituted. Cells are labeled in the 

order of performance midday and during the evening. The 

cell area for each cell is 0.253 cm2

. 

The cells were then mounted on a two-axis tracker, 

with the incident sunlight collimated to exclude all except 

the direct beam. The collimators followed the design 

shown in the annex of Ref. [5], with a 5° field of view. No 

protective glass, antireflection coatings, nor bypass diodes 

were used. 

The cells were actively cooled to a nominal temperature of 25° to 30° C, and I-V measurements were made for 

each cell throughout the day. To facilitate cell modeling, 

the direct solar spectrum was measured concurrently using a collimated spectrometer mounted on a two-axis 

tracker. 

EXPERIMENTAL RESULTS

Figure 2 shows the power produced by each cell on a 

particularly clear (blue-rich) day. Cell B (designed for 

~AM1.5G) was best for midday power production, whereas 

cell E performed best during the morning and evening. 

Cell A (designed for ~AM0) is out-performed by other cells 

throughout the day. Although not shown here, the measured midday power for cells B and C during slightly hazy,

partly cloudy days was approximately the same. 

The power produced by each cell over the course of 

the day shown in Fig. 2 was integrated to determine its 

daily energy (Table 1). If cells C1 and C2 are averaged, 

the daily energies for cells B and C are about the same. 

On a slightly hazy, less blue-rich day, cell C is favored. 

MODEL DETAILS

To better understand the experimental results, we applied a simple device model to the same set of cells. The 

semi-empirical model we used needed three inputs: (1) 

the measured outdoor spectra as a function of time of day, 

(2) the measured top- and bottom-cell external quantum 

efficiencies as a function of photon wavelength, and (3) 

the thicknesses of the top and bottom cells, used only to 

estimate their dark currents. 

Some representative direct spectra taken during the 

test day. are shown in Fig. 3. To discuss these spectra, it 

is useful to divide the spectra into "blue" photons above (in 

energy) the GaInP band gap and "red" photons between 

the GaAs and GaInP band gaps. The ratio of blue to red 

light is highest during the midday hours, favoring a tandem 

cell with a thin top cell. In the morning and evening, the 

direct spectra become "red-rich" so a thicker top cell is 

better. In a triple-junction GaInP/GaAs/Ge cell, the Ge 

junction is generally over-supplied with photons below the 

GaAs band gap. For this reason, the conclusions of this 

paper apply equally to GaInP/GaAs/Ge triple-junction 

cells. 

Cell ttop

(relative) 

Energy

(Wh) 

Relative

Energy

(%) 

Design

Spectrum 

(approx.) 

A 0.93 0.4626 89.51 AM 0

B 1.40 0.5140 99.46 AM 1.5 G 

C1 1.73 0.5082 98.34 Low AOD

C2 1.73 0.5168 100.00 Low AOD

D 2.02 0.5092 98.53 --

E 2.85 0.4988 96.52 --

Table 1. Daily energy calculated by integrating the 

measured power for each 0.253-cm2 cell over the test day

shown in Fig. 1. The ttop values are approximate. 

2 
Fig. 4. Measured external quantum efficiencies for the top 

and bottom cells. Cell C is represented by C2. (Cell C1 

data is not shown.) 

A small non-physical sub-band-gap top cell response was 

removed by smoothly setting the top cell QE to zero for 

wavelengths greater than ~700 nm. 

Fig. 3. Measured direct spectra labeled by time of day

during the test day. The photon flux is expressed as mA 

equivalent for 100 percent conversion of photons to 

photocurrent, per cm2

 area and nm of wavelength. The 

GaAs and GaInP band gaps are indicated with vertical 

dashed lines. Morning spectra are plotted with solid lines. 

Afternoon and evening spectra are plotted with dashed 

lines. 

Fig. 5. Model results for the test day shown in Fig. 2. 

Gaps in the curves occur whenever the direct spectra 

was not recorded. 

As an aside, it is interesting to note the differences 

between morning and afternoon spectra. In each plotted 

pair, (7:00 and 17:00, for example), the afternoon spectrum (dashed) is more red-rich. This is not too surprising, 

since the atmospheric conditions in the morning are generally different from those in the afternoon and evening. 

Measured top- and bottom-cell external quantum efficiencies (QEs) are shown in Fig 4. The top-cell QE was 

measured by using a bias light to over-supply the bottom 

cell with photocurrent. The tandem cell is therefore 

strongly top-cell limited, so the response to a second light 

source can be monitored as a function of photon wavelength to obtain the top-cell QE. A similar procedure is 

used to measure the bottom-cell QE. A description of this 

technique can be found in ref. [6]. 

Above the GaInP band gap, both the top- and bottomcell QEs change with ttop. As the ttop increases (from A to 

E), the top cell QE increases and the high-energy tail of 

the bottom cell QE decreases. 

Below the GaInP band gap, there is no optical absorption by the top cell, so the bottom-cell QE does not change 

with ttop. However, multiple reflections of sub-band-gap 

light through the top GaInP cell leads to interference 

fringes in the bottom-cell QE. As ttop increases, the period 

of these fringes decreases. Because the amplitude of 

these fringes is fairly small, they should not complicate our 

results. For anti-reflection coated cells, these fringes and 

their effects will become completely negligible. 

3 
MODEL RESULTS 

The QEs of Fig. 4 were convoluted with the measured 

direct spectra of Fig. 3 to give decoupled top- and bottomcell short circuit currents [Jsc(top) and Jsc(bottom)]. These 

Jsc values were then fed into standard diode equations to 

generate tandem-cell I-V curves. The resulting power at 

the maximum power point for each I-V curve is plotted in 

Fig. 5. These results and the experimental results (Fig. 2) 

are in very good agreement. This sort of modeling can 

therefore serve as a quick and convenient supplement to 

actual outdoor cell measurements under various conditions. 

Modeling can also be used to illustrate how current 

matching changes throughout the day, directly affecting 

the tandem-cell performance. The ratio 

Jsc(top)/[Jsc(top)+Jsc(bottom)] is plotted in Fig. 6 as a function of time of day for each cell. Maximum performance 

occurs when this ratio is 0.5 (dashed line). This can be 

confirmed by noting that midday power performance in 

Figs. 2 and 5 is (from best to worst): B, C, D, E, A. This 

correlates well with the midday distance from the dashed 

line at 0.5 in this figure. 

However, cell C produces more energy than cell B 

over the course of the day (Table 1). This is because the 

curve for cell C crosses 0.5, and is therefore closer to 0.5 

for a longer period of time. 

CONCLUSIONS

We have measured and modeled the on-sun performance of GaInP/GaAs tandem cells under direct illumination 

for concentrator applications. This study gives direct support for the use of the Low AOD spectrum to design cells 

for maximum daily energy and midday power. A similar 

"Clear Sky" standard spectrum, and even the AM 1.5G 

spectrum will also work quite well. The AM 1.5D spectrum 

is a poor choice, unless maximizing morning and/or evening power production is a priority. The similarity between 

the observed and modeled performance validates the use 

of modeling for design of concentrator cells. 

Fig. 6. Modeled decoupled short-circuit currents (plotted 

as a ratio) during the test day. The best power production is for a current-matched cell with a ratio of 0.5. 

ACKNOWLEDGEMENTS 

The authors would like to thank Bill Marion for providing the "standard day" spectra used in this study. This 

work was performed at the National Renewable Energy

Laboratory under DOE contract DE-AC36-99GO10337. 

REFERENCES

[1] W.E. McMahon, Sarah Kurtz, K. Emery, and M.S. 

Young, "Criteria for the Design of GaInP/GaAs/Ge TripleJunction Cells to Optimize their Performance Outdoors," 

Proc. of the 29th IEEE PV Specialists Conference, 2002, 

New Orleans, LA, pp. 931-934. 

[2] ASTM Standard Tables for Reference Solar Spectral 

Irradiance at Air Mass 1.5: Direct Normal and Hemispherical for a 37° Tilted Surface, Standard G159-99, 

American Society for Testing and Materials, West Conshohocken, PA (1999). 

[3] K. Emery, D. Myers and S. Kurtz, "What is the Appropriate Spectrum for Characterizing Concentrator Cells?," 

Proc. of the 29th IEEE PV Specialists Conference, 2002, 

New Orleans, LA, pp. 840-843. 

[4] D.R. Myers, K. Emery and C. Gueymard, "Proposed 

Reference Spectral Irradiance Standards to Improve Concentrating Photovoltaic System Design and Performance 

Evaluation," Proc. of the 29th IEEE PV Specialists Conference, 2002, New Orleans, LA, pp. 923-926. 

[5] Standard ASTM E1125-99, "Standard Test Method for 

Calibration of Primary Non-Concentrator Terrestrial Photovoltaic Reference Cells Using a Tabular Standard," American Society for Testing and Materials, West Conshohocken, PA. 

[6] S.R. Kurtz, K. Emery and J.M. Olson, "Methods for 

Analysis of Two-Junction, Two-Terminal Photovoltaic Devices," Proc. of the First World Conference on Photovoltaic 

Energy Conversion, p1733 (1994). 

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An On-Sun Comparison of GaInP2/GaAs Tandem Cells with Top 

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14. ABSTRACT (Maximum 200 Words)

This study compares the on-sun performance of a set of GaInP2/GaAs tandem cells with different GaInP2 top-cell 

thicknesses. Because high-efficiency III-V cells are best suited to concentrating photovoltaic (CPV) applications, the 

cells were mounted on a two-axis tracker with the incident sunlight collimated to exclude all except the direct beam. 

Current-voltage (I-V) curves were taken throughout the course of several days, along with measurements of the 

direct solar spectrum. Our two major conclusions are: (1) GaInP2/GaAs tandem cells designed for an "air mass 1.5 

global" (AM 1.5G) or a "low aerosol optical depth" (Low AOD) spectrum perform the best, and (2) cells can be 

characterized indoors and modeled using outdoor spectra to predict the correct result. These results are equally 

valid for GaInP2/GaAs/Ge triple-junction cells. 

15. SUBJECT TERMS

PV; on-sun performance; tandem cells; top-cell thickness; high-efficiency III-V cells; concentrating photovoltaic 

(CPV); Current-voltage; low aerosol optical depth (Low AOD); triple-junction; 

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