Performance Comparison of Top and Bottom Contact 

Gallium Arsenide (GaAs) Solar Cell

by Naresh C Das

ARL-TR-7054 September 2014

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Army Research Laboratory

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ARL-TR-7054 September 2014

Performance Comparison of Top and Bottom Contact 

Gallium Arsenide (GaAs) Solar Cell

Naresh C Das

Sensors and Electron Devices Directorate, ARL

Approved for public release; distribution unlimited. 
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Performance Comparison of Top and Bottom Contact Gallium Arsenide (GaAs)

Solar Cell

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Naresh C Das

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14. ABSTRACT

We fabricated single-junction solar cell on epitaxially grown n-type gallium arsenide (GaAs) substrate. We used a germanium 

(Ge)/gold (Au)/nickel (Ni)/Au metal contact from the top side on a highly doped n-type epitaxial layer as well as the bottom 

side on a n-type GaAs substrate. We observed 10–15% increase in solar cell power when the top contact is used for the n-type 

GaAs epi layer compared to the bottom side n- type GaAs substrate. Solar cell fill factor, sheet, and shunt resistances are same 

for both the top and bottom contact type devices. We conclude that to achieve higher power, it is advantageous to use an n-type 

contact from a highly doped top n epitaxial layer rather than a bottom n-type GaAs substrate.

15. SUBJECT TERMS

Gallium arsenide, GaAs solar cell, efficiency, top contact, anti reflection coating

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Naresh C Das

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Contents

Lists of Figures iv

List of Tables iv

1. Introduction 1

2. Experimental 1

3. Results and Discussion 3

4. Conclusions 5

5. References 6

List of Symbols, Abbreviations, and Acronyms 7

Distribution List 8
iv

Lists of Figures

Fig. 1 Cross-sectional view of GaAs solar cell..............................................................................2

Fig. 2 Photograph of the processed solar cell ................................................................................3

Fig. 3 Dark current of both the top and bottom n-type contact solar cells ....................................3

Fig. 4 Current vs. voltage of solar cells with AM1.0 solar cell irradiation ...................................4

Fig. 5 Power versus voltage for top and bottom contact solar cells ..............................................5

List of Tables

Table 1 GaAs single-junction solar cell structure..........................................................................2
1

1. Introduction

The performance of state-of-the-art III-V gallium arsenide (GaAs) solar cells is excellent. Due to 

their direct bandgap electronic structure, they are able to absorb 97% of the AM1 radiation in a 

thickness of 2 μm. Another important application of GaAs solar cells is in the concentrated solar 

power area with an irradiation intensity of more than 1000 suns. The Spire GaAs solar cell with 

27.6% at 255 suns has the highest efficiency reported for a single-junction solar cell operating 

under concentrated light.1 Many attempts have been made to improve the GaAs solar cell 

performance by using a single double-layer anti-reflection coating (ARC) as well as surface 

engineering including nanostructure formation.2 Yu et al. achieved a 28% enhancement in solar 

cell power by using indium tin oxide nano columns on GaAs solar cells by oblique-angle 

deposition.3 Most of the reported results use GaAs cells with bottom contact metal deposited by 

blanket deposition on the bottom surface of the GaAs substrate. This method is easy and the 

most cost-effective way of solar cell fabrication. Though very few papers reported GaAs solar 

cell with a top n-type contact,4 to the best of our knowledge, no one has made a comparative 

study of solar cell performance with a bottom n-type contact and a top n-type contact on highly 

doped epitaxial layer structures. 

Here we compare solar cell performance with either a top or bottom n-type contact. We found 

that there is about 10–15% increase in solar energy harvesting power when a top n-type contact 

is used compared to a bottom n-type contact device. 

2. Experimental

The single-junction GaAs solar cell p- on n-type structure is fabricated using the molecular beam 

epitaxy (MBE) technique. Table 1 shows the details of the epitaxial structure used to fabricate 

the solar cell. After initial cleaning, a lightly doped 200-Å n-type GaAs layer is deposited at 

590 °C. Following the first deposition, the temperature is lowered to 580 °C and the rest of the 

layers are deposited as shown in Table 1. Solar cell device fabrication starts with etching of the 

mesa region using inductively coupled plasma (ICP) etching with boron trichloride (BCl3) gas 

until the bottom n-type GaAs layer is reached. We use RF1 and RF2 powers of 150 and 500 W, 

respectively. A 30-s chemical etch (hydrochloric acid [HCl]: water [H2O]-1:1) is used to remove 

any oxide prior to the ICP etch. We carefully performed the etching experiment, so that we 

would not over etch and go beyond the n+ bottom layer. For the p-type contact metal, we created 

a titanium (Ti)/platinum (Pt)/gold (Au) (300/500/2500 Å) three-layer deposition process using an 

electron beam evaporation tool. Following the creation of the double-layer ARC with silicon 

dioxide (SiO2)/silicon nitride (Si3N4) and a window opening, we used a germanium (Ge)/Au/
2

nickel (Ni)/Au (300/500/200/2500 Å) four-layer metal film for the n-contact on both the top and 

bottom layers. We used a 60-s rapid thermal anneal (RTA) process at 380 °C to anneal the 

contacts. 

Table 1 GaAs single-junction solar cell structure

Material 

Composition

Doping level

(cm–3)

Thickness

(Å)

Comment;

Function; Repeats

GaAs P = 2E19 200 Tsub = 580 oC

Al0.8Ga0.2As P = 5E18 300 Tsub = 580 oC

GaAs

Emitter P = 5E18 4,000 Tsub = 580 oC

GaAs Undoped 10,000 Tsub = 580 oC

GaAs

Base N = 2E17 20,000 Tsub = 580 oC

Al0.2Ga0.8As N = 5E18 300 Tsub = 580 oC

GaAs N = 5E18 2,000 Tsub = 595 oC

GaAs substrate N +

Figure 1 shows the cross-sectional view of the solar cell. As seen in the figure, we can have 

access to apply bias to the n-type contact either through the top epitaxial layer or the bottom ntype substrate. The top n-type contact connects to the heavily doped n-layer, whereas the bottom 

contact connects to the lightly doped GaAs substrate material. The top n-type contact has a ring 

structure all around the p-type mesa region. As also seen in Fig. 1, the p-type metal has a grid 

pattern so that the photo-generated carriers can be collected with minimum drift from the origin 

of their creation. 

Fig. 1 Cross-sectional view of GaAs solar cell

A photograph of part of the processed chip is shown in Fig. 2. Here we show the top and bottom 

contact pads that we use on the probe station to test the solar cell’s performance. Since the device 

has ARC, the top mesa surface exhibits a black color. The gap between the edge of the mesa 

region and the n-type metal pad is 20 μm. Similarly, the gap between the p-type metal and the 

edge of the mesa region is 10 μm. 
3

Fig. 2 Photograph of the processed solar cell

3. Results and Discussion

In Fig. 3, we show the dark current versus voltage of the top and bottom contact devices. The 

dark and illuminated current versus voltage (I-V) measurements were taken using an Agilent HP 

4156 C parametric analyzer. The dark current is plotted in the logarithmic scale. We didn’t see 

any appreciable difference for dark currents between the two types of devices. Hence, the 

number of defects responsible for dark current for both types of configurations is the same. Also 

in the subthreshold region of the device’s operation (0.5 to 1.0 V), the slope of the current for 

both devices is the same. Hence, the sheet and shunt resistances of the devices are the same. 

Fig. 3 Dark current of both the top and bottom n-type contact solar cells

The current versus voltage for both the top and bottom contact devices at AM1.5G is shown in 

Fig. 4. An unfiltered Newport solar simulator provided broadband illumination with an 

equivalent AM1.5G illumination intensity of approximately 1.0 sun (100 mW/cm2

). We tested

10-9

10-8

10-7

10-6

10-5

0.0001

-2 -1.5 -1 -0.5 0 0.5 1

bot. cont.

top cont.

Voltage (V)

Top n-contact 

Ge/Au/Ni/Au

Top p-contact

Ti/Pt/Au metal
4

these devices using a probe station. The short circuit current (Isc) for the top contact device is 

26.5 mA/cm2, which is very close to the world record Isc number for a GaAs single-junction 

device. We observed about a 13% increase in the short circuit current for the top contact device 

compared to the bottom contact device. The open circuit voltages (Voc) for both the 

configurations have same value of 0.82 V. As also seen in Fig. 4, the subthreshold slope of the 

I-V curves is the same for both devices, showing that the sheet resistances of these devices are 

the same.

Fig. 4 Current vs. voltage of solar cells with AM1.0 solar cell irradiation

The solar cell power versus voltage is shown in Fig. 5. The data in Fig. 4, in the voltage range of 

0 to Voc, are used to plot the power versus voltage curves in Fig. 5. We observed about an 11% 

increase in peak power for the top contact devices compared to the bottom contact devices. This 

is a significant improvement in solar power harvesting capability with a minimal process change. 

The solar cell external power conversion efficiencies of the bottom and top contact devices are 

14.8% and 16.5%, respectively. However, these numbers are well below state-of-the-art GaAs 

solar cell efficiency values, so we have more room for improvement in the fabrication process 

and material growth of our devices. The result is nonetheless very encouraging, as we received 

these significant device performances in our first attempt. The design of the epitaxial layer 

thickness, doping, and the p-contact layer grid patterns are a few parameters that we will try to 

optimize to improve the overall solar cell efficiency. 

-0.03

-0.02

-0.01

0

0.01

0.02

-2 -1.5 -1 -0.5 0 0.5 1

bot. cont.

top cont.

Voltage (V)
5

Fig. 5 Power versus voltage for top and bottom contact solar cells

We presented here our findings on solar cell performance with top and bottom contact 

configurations for an n-type contact in a solar cell device. From Figs. 4 and 5, we found out that 

the slopes of the I-V curves were the same for both types of device. Hence, we conclude that the 

series resistances of the solar cells for both configurations are the same. We believe the 

difference in the short circuit currents, as seen in Fig. 4, may be due to the higher number of 

carriers available for the top contact devices as compared to the bottom contact devices due to 

the high n-type doping in the epitaxial layer as compared to the doping of an n-type GaAs 

substrate. We believe the measurements of carrier lifetime and density in the epitaxial layer will 

be more rewarding.

4. Conclusions

We reported a 10–15% increase in GaAs solar cell harvesting power when the device used a top 

n-type contact configuration rather than a bottom n-type contact configuration. Our observed 

short circuit current of 26.5 mA/cm2 is very close to the state-of-the-art Isc value for GaAs singlejunction solar cell devices. Since most of the solar cell production runs use a bottom n-type 

contact configuration, our results will be useful in redesigning the solar cell structure to produce

devices featuring a top contact configuration to harvest additional power.

0

0.005

0.01

0.015

0.02

0 0.2 0.4 0.6 0.8 1

bot. con. power

top con. power

Voltage (V)
6

5. References

1. Green MA, et al. Solar cell efficiency tables (version 16). Prog. Photovolt. 2000;8:377–383.

2. Yeh L-K et al. Adv. Energy Mater. 2011;1:506–510.

3. Yu P, Chang CH, Chiu CH, Yang CS, Yu JC, Kuo HC, Hsu SH, Chang YC. Adv. 

Mater.2009;21:1618.

4. Subramanian G. et al. Appl. Phys. Left. 1991;58(22):2541.
7

List of Symbols, Abbreviations, and Acronyms

ARC anti-reflection coating 

Au gold

BCl3 boron trichloride

GaAs gallium arsenide 

Ge germanium

H2O water

HCl hydrochloric acid

ICP inductively coupled plasma 

I-V current versus voltage 

MBE molecular beam epitaxy 

Ni nickel

Pt platinum 

RTA rapid thermal anneal

Si3N4 silicon nitride 

SiO2 silicon dioxide 

Ti titanium 
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