A monolithic two-terminal InP/InGaAsP tandem solar cell was successfully fabricated. This tandem solar cell consists of a p/n InP homojunction top subcell and a 0.95 eV p/n InGaAsP homojunction bottom subcell. A patterned 0.95 eV n(+)/p(+) InGaAsP tunnel diode was employed as an intercell ohmic connection. The solar cell structure was prepared by two-step liquid phase epitaxial growth. Under one sun, AM1.5 global illumination, the best tandem cell delivered a conversion efficiency of 14.8 pct

Chang, P. T.

Emery, K. A.

Shen, C. C.

English

NASA Technical Reports Server

N91-30216
Two-terminal Monolithic InP-based Tandem Solar Cells with Tunneling
Intercell Ohmic Connections
c. c. Shen and P. T. Chang
Center for Solid State Electronics Research
Arizona State Unversity, Tempe, AZ 85287
K. A. Emery
Solar Energy Research Institute, Golden, CO. 80401
SUMMARY
We have successfully fabricated a monolithic two-terminal InP/InGaAsP tandem solar cell. This tandem
solar cell consists of a p/n InP homojunction top subcell and a 0.95 eV p/n InGaAsP homojunction bottom
subcell. A patterned 0.95 eV n+/p + InGaAsP tunnel diode was employed as intercell ohmic connection. The
solar cell structure was prepared by two-step liquid phase epitaxial growth. Under one sun, AM1.5 global
illumination, our best tandem cell delivered a conversion efficiency of 14.8%.
INTRODUCTION
Although multijunction solar cells have exhibited higher conversion efficiency than single junction solar
cells in recent years, its full potential is far from being realized. Most of the R & D work on III-V tandem solar
cells is based on GaAs and its related ternary compound semiconductors. The highest conversion efficiency
reported for a monolithic two-terminal tandem solar cell under one-sun, AM1.5 global illumination is about
27% [1][2]. InP-based multijunction tandem solar cells have not received much attention until recently. InP and
its related ternary and quaternary compound semiconductors such as InGaAs and InGaAsP offer desirable
combinations of energy bandgap values and show great promises for multijunction tandem solar cell
applications; especially for tandem solar cells which consist of three subcells.
One of the key components for a two-terminal monolithic mulitijunction tandem solar cell is a low-loss,
highly conductive intercell ohmic contacts (IOCs) for the series connection of individual subcells. Two types
of IOCs are considered to be feasible for monolithic tandem solar cells. They are tunnel junctions and metal
interconnects (MICs). Both tunnel junctions and metal interconnects have been used as IOCs for GaAs-
based multijunction tandem cells [1] [2] [3]. For InP-based mutijunction solar cells, only MICs have been used
recently as IOCs for the InP/InGaAs tandem solar cells developed by M. Wanlass and coworkers at SERI [4].
In this work, we have developed a 0.95 eV n+/p + InGaAsP tunnel diode and successfully employed it as IOCs
for a monolithic two-terminal InP/InGaAsP tandem solar cell. Details of the device structure, device
fabrication procedures and device performance are discussed in following sections.
DEVICE STRUCTURE, EPITAXIAL GROWTH AND DEVICE FABRICATION
A schematic cross section of a two-terminal monolithic InP/InGaAsP tandem solar cell is shown in Fig.
1. The entire tandem cell structure was grown on a (100) oriented n-type InP substrate by using two-step
liquid-phase epitaxial (LPE) growth. The two subcells are connected with a patterned 0.95 eV n+/p + InGaAsP
tunnel junction. The patterned n+/p + InGaAsP tunnel junction covers about 12.5% of the total surface area.
The patterning of the tunnel junction was accomplished by using photolithography and selective wet chemical
etching. A p-lnP layer was grown on top of the InGaAsP lower cell and used as an etch-stop layer to facilitate
the patterning processes. A patterned p+-InGaAs layer was employed as the contacting layer for the front
grid contacts. The front contacts cover about 12.5% of the surface area. The complete solar cells are of mesa
type, with a total surface area of 4 mm 2. Single layer o! Sb20 3 was used as antireflection coating.
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https://ntrs.nasa.gov/search.jsp?R=19910020902 2020-03-19T17:29:31+00:00Z
In theearly stage of this work, n+/p + In0.53Ga0.47As tunnel junctions were considered for IOCs for
the series connection of the InP subcell and the InGaAsP subcell. In0.53Ga0.47As has a direct energy
bandgap of 0.75 eV and is lattice matched to InP, which makes it an attractive material for tunnel junctions.
In our previous work [5], we have investigated the electrical properties of p+/n + In0.53Ga0.47As tunnel
junctions prepared by LPE. Since then, we have made better p+/n + In0.53Ga0.47As tunnel junctions with
higher conductivities and larger peak current densities. We have also developed 0.95 eV p+/n + InGaAsP
tunnel junctions by LPE. Their electrical properties are listed in Table 1, along with several different type of
tunnel junctions which have been used as IOCs for GaAs-based tandem solar cells. Although In0.53Ga0.47As
tunnel junctions offer the highest peak current densities as well as the largest conductivities, we found that it
was difficult to incorporate them into our tandem solar cell structures by LPE, since melt-back problem during
LPE growth forbids the direct growth of InP on In0.53Ga0.47As. As a result, we switched to InGaAsP
tunnel junctions and were successful in using them as IOCs.
The thickness and doping concentration of individual epitaxial layer for the first and second LPE
growth are listed in Table 2. The LPE growth procedures for the p/n InP upper cell and the p/n InGaAsP
lower cell were similar to what we reported previously [6] [7]. Typical growth temperatures for the first and
second growth are 6300C-6250C and 5800C-5500C, respectively. In order to simplify the InP growth
processes for the InP/InGaAsP tandem structure, the p-lnP region and the p-lnGaAsP region were not
grown epitaxially as we did for the single-junction InP and InGaAsP solar cells but were created by in-diffusion
of p-type impurities (Zn) during the growth period. Consequently, both subcells are essentially of diffused
junction type rather than grown junction type. The depth of both p/n diffused junctions is about one micron,
no attemps were made to optimize the junction depth for maximum conversion efficiencies. The processing
sequence for the fabrication of the InP/InGaAsP tandem solar cell is illustrated in Fig. 2.
Several InP/InGaAsP tandem solar cells were evaluated at SERI under 1 sun, global AM1.5
illuminations. Shown in Figure 3(a) and 3(b) are the light I-V characteristics for the two most efficient
InP/InGaAsP tandem solar cells we have tested so far. The open-circuit voltage (Voc), short-circuit current
(Jsc) and Fill factor exhibited by the best device were 1.363 V, 13.85 mA/cm 2 and 0.786, respectively. The
total-area power conversion efficiency was 14.8%. The second best tandem solar cell which was processed
from the same wafer exhibited a conversion efficiency of 14.6%. The Voc, Jsc, and Fill factor exhibited by
this second best device were 1.383 V, 12.94 mNcm 2 and 0.816, respectively. The relative quantum efficiency
of the InP upper cell and the 0.95 eV InGaAsP lower cell is shown in Fig. 4.
DISCUSSION
Because of the current matching requirements imposed on a series-connected two-terminal tandem
solar cell, its output current is limited by the subcell which generates the least current. For the two-terminal
InP/InGaAsP tandem solar cell reported here, the output current is limited by the InGaAsP lower subcell. The
short-circuit current density of 13.85 mA/cm 2 exhibted by the 14.8% efficient InP/InGaAsP tandem solar cell
is comparable in value to what has been reported on the most efficient monolithic Ga0.51n0.5P/GaAs tandem
solar cell and the most efficient AI0.37Ga0.63As/GaAs tandem solar cell [1] [2]. Our results thus confirm that
the 0.95 eV InGaAsP solar cell performs quite well underneath an InP solar cell when they are connected in
series. With the incorporation of another 1.93 eV wide bandgap solar cell such as AIGalnP solar cell as the
top subcell, with the InP subcell in the middle and the InGaAsP subcell at the bottom, the total conversion
efficiency can be expected to increase to 30% or more, assuming the output voltage can be enhanced without
sacrificing the output current.
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REFERENCES:
1. J.M. Olson,S. R. Kurtz,A. E.KibblerandP. Faine,"A27.3%efficientGa0.51n0.5P/GaAstandemsolar
cell,"Appl.Phys.Lett.56, 623, (1990).
2. B.- C. Chung,G. F. Virshup,S. Hikido,and N. R. Kaminar,"27.6%efficiency(1 sun, air mass1.5)
monolithicAI0.37Ga0.63As/GaAstwo-junctioncascade solar cell with prismatic cover glass," Appl. Phys.
Lett. 55, 1741, (1989).
3. C. Amano, H. Sugiura, M. YamGuchi, and K. Hane, "Fabrication and numerical analysis of AIGaAs/GaAs
tandem solar cells with tunnel interconnections," IEEE Trans. on Electron Device, ED-36, 1026, (1989).
4. M.W. Wanless, J.S. Ward, T.A. Gessert. K.A. Emery, G.S. Horner and T.J. Coutts, "Development of
High-Performance GalnAsP Solar Cells for Tandem Solar Cell Applications," Proceeding of 21st IEEE
Photovoltaic Specialist Conference, (IEEE, New York 1990). p. 172.
5. C.C. Shen, P.T. Chang, and K.Y. Choi, "An In0.53Ga0.47As tunnel diode for monolithic multijunction
solar cell applications," Proceeding of 20th IEEE Photovoltaic Specialist Conference, (IEEE, New York
1988) p 771.
6. K.Y. Choi and C.C. Shen "p/n InP homojunction solar cells with modified contacting scheme by liquid
phase epitaxy," J. Appl. Phys. 63, 1198, (1988).
7. C.C. Shen, P.T. Chang and K.Y. Choi,"Photovoltaic properties of 0.95 eV p/n InGaAsP homojunction
solar cells prepared by liquid-phase epitaxy," IEEE Trans. on Electron Device, ED-37, 464, (1990).
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lnP cell (
lnGaAsP cell {
AR coating
front contact
p'-InGaAs contact layer
p-InP
n-InP
n*-InP buffer
--ff /p"-InGaAsP tunnel junction
p-InP etch stop layer
p-InGaAsP
n-InGaAsP
ff -InP
back contact
Fig. 1. Schematic cross section of a two-terminal, monolithic InP/InGaAsP tandem cell.
Table 1. Electrical properties of various types of tunnel Junctions
Material System Peak current density Resistivity Ref.
Jp (A/cm 2) R (10 -3 ,_-cm 2)
p+-GaAs/n+-GaAs
p+-GaAs/n+-GaAs
p+-GaAs/n+-GaAs
p+-GaAs/n+-GaAs
p+-Ge/n+-Ge
p+/n +-ln0.53Ga0.47A s
p+/n+-lnGaAsP
34 3.0 a
15 4.7 b
23 1.7 c
45 2.0 d
3 10 e
793 0.4 This Work
28 2.0 This Work
a. D.L. Miller, S. W. Zehr, and J. S. Harris, Jr., J. Appl. Phys., Vol. 53, pp. 744-748, 1982.
b S.K. Ghandhi, R T. Huang, and J. M. Borrego, AppL Phys. Lett., Vol. 48,415-416, 1986.
c. R.E. Hayes et al., Solar Cells, Vol. 15, pp. 231-238, 1985.
d. P. Basmaji et al., J. Appl. Phys., Vol. 62, pp. 2103-2106, 1987.
e. P. K. Chiang, M. L. Timmons, G. G. Fountain, and J. A. Hutchby, Proc. 18th IEEE Photovoltaic Specialist
Conf., pp. 562-567, 1985.
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Table 2. Thicknesses and doping concentrations of epitaxial layers in the InP/InGaAsP
tandem cell.
Layer Thickness Doping Concentration
(_tm) (cm-3)
First Crystal Gro_h
undoped InGaAsP 3.0 3.0 x 1017
p-InP 1.0 2.5 x 1018
p+-InGaAsP 0.5 2.0 x 1019
n+-InGaAsP 0.5 2.0 x 1019
Second Crystal Growth
n+-InP 2.5 7.0 x 1018
undoped InP 2.5 1.0 x 10 ]7
p+-InGaAs 1.3 6.0 x 1018
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n'/p '-InGaAsP
p-lnP
undoped-lnGaAsP
n ÷-InP substrate
(a) First LPE Growth (b) Patterning of IOC Layers
(c) Second LPE Growth
13"-InGaAs
p-lnP
undoped-lnP
n*-InP buffer
p-lnP
undoped-lnGaAsP
n'-InP substrate
I
(d) Metallization
p* -InGaAs
p-lnP
undoped-lnP
n'-InP buffer
p-lnP
undoped-lnGaAsP
n ,-InP substrate
F-
I
/
(e) Cell Isolation (f) Contact-Layer Removal and
AR Coating Deposition
Fig. 2. The Processing sequence for the fabrication of the InP/InGaAsP tandem solar cell.
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0.4
<
E
v 0.3
t_
'_ 0.2
0.1
0.6
0.5 I
0.0
-0.1
-X
Voc = 1.363 volts
Jsc = 13.85 mA/cm:
- Fill factor = 78.65 %
Efficiency = 14.8 %
I
VOLTAGE (volts)
(a)
0.6
0.5
0.4
v
Z (].2
L_
0.0
X
Voc = 1.383 volts
Jsc = 12.94 mA/cm:
Fill factor -- 81.59 %
Efficiency = 14.6 %
-0.1
-0.2. J ! I I _ t , I 1 I 1 I 1 I r
I
VOLTAGE (volts)
(b)
Fig. 3. Light I-V characteristics under 1 sun, AM1.5 global illuminations for two InP/InGaAsP
tandem solar cells. The total surface area is 0.04 cm 2.
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100_
80
_ 60
40
0.3 0.5 0.7 0.9 1.1 1.3 1.5
Wavelength (lam)
Fig. 4 Spectral response of the InP upper subcell and the 0.95 eV InGaAsP lower
subcell.
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Two-terminal monolithic InP-based tandem solar cells with tunneling intercell ohmic connections

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