A simple open tube diffusion technique for the fabrication of n+p junction solar cells is described. Large area (greater than 0.25 square cm) solar cells have been made by this process with a photovoltaic conversion efficiency of 15.2 percent under simulated AMO illumination. An ideality factor is 1.04 and a saturation current density of 9.6 times 10 to the minus 16th power A/square cm have been observed for these cells. These are the lowest (best) values reported to date for diffused structures in bulk InP

Borrego, J. M.

Bothra, S.

Ghandhi, S. K.

Parat, K. K.

English

NASA Technical Reports Server

N87-26444
SOLAR CELLS IN BULK InP USING AN OPEN TUBE DIFFUSION PROCESS*
K.K. Parat, S. Bothra, J.M. Borrego and S.K. Ghandhl
Rensselaer Polytechnic Institute
Troy, New York
This paper describes a simple open tube diffusion technique for the fabrication
of N+P junction solar cells. Large area (>0.25 cm 2) solar cells have been made by
this process with a photovoltaic conversion efficiency of 15.2% under simulated AMO
illumination. An ideality factor is 1.04 and a saturation current density of 9.6 x
10-16 A/cm 2 have been observed for these cells. These are the lowest (best) values
reported to date for diffussed structures in bulk InP.
INTRODUCTION
At the present time there is large interest in the use of indium phosphide on
solar cells for space applications. Besides being a direct band gap material with an
energy gap of 1.34eV which is close to the one required for maximum efficiency under
AMI illumination, it offers the possibility of low surface recombination velocity if
properly passivated (ref. 1) and it has shown less degradation to gamma ray radiation
than Si or GaAs cells of comparable junction depths (ref. 2).
InP solar cells previous reported had been fabricated by either epitaxial growth
or by thermal diffusion in a closed box system (ref. 3, 4). In this paper we des-
cribe the fabrication and photovoltaic characteristics of n+p InP solar cells using a
simple open tube diffusion process. Advantages of this technique are its case of
implementation and potential for low cost.
DEVICE FABRICATION
The p-type InP substrates used for the fabrication were pre-polished LEC grown**
2" diameter wafers (i00) misorientated 2 ° towards (110) and doped with Zn to a
concentration of 5-6 x 1016 cm -3. After degreasing the slices were etched in order to
remove any residual poslishing damage. Before loading in the evaporator, the wafers
were etched in 10% H3PO 4 to remove any surface damage. A gallium sulphide (99.99%
purity) film was evaporated onto the frontside which was used as the source for sul-
phur doping. To prevent degradation of the InP surface during diffusion the InP
substrate was coated on top and back with a 0.5 um thick film of SiO 2. The silicon
dioxide film was deposited on the wafer in a CVD reactor by the pyrolysis of silane
and oxygen in an argon ambient. During the SiO 2 deposition the samples were held at
325°C. The sulphurdiffusions were carried out for 20 to 60 minutes at 700°C in an
open tube furnace with nitrogen gas flowing in order to prevent back diffusion of
air. After diffusion the SiO 2 and Ga2S 3 films were removed and a back ohmic contact
* Work performed under NASA-Lewis Grant No. NAG-3-604.
**Crystacom Inc., Mountain View, CA 94043.
295
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made by evaporating a film of Au - 5% Zn and alloying it at 420°C in forming gas.
The non-optimized top metallic grid was defined photolithographically and electro-
plated Au was used as the contact metal for the top n+ layer. The active area of
the cell was delineated by a second photolithography step and it was mesa etched
using a solution of iodic acid. The total area of the cell was 0.313 cm 2 which
after substracting the area of the top metallic grid gives 0.265 as the active area
of the cell. Since the refractive index of InP is approximately 3.5, the optimum
values of the refractive index of a single layer AR coating is 1.9 which makes SiO
a suitable choice for this purpose. An SiO layer of approximately 750 A was
evaporated over the whole cell to serve as an AR coating. No attempts were made to
passivate the top n+ surface of the cell. A schematic diagram of the cell structure
is shown in Fig. i.
ELECTRICAL AND PHOTOVOLTAIC CHARACTERISTICS
Electrical characterization of the finished cells involved measurement of their
photovoltaic performance of large area cells, their quantum efficiency as a function
of wavelength and in I vs V. Small area diodes were used for diagnostic purposes
specially for determining the doping concentrations and doping profile in the vici-
nity of the junction. The photovoltaic performance of the large area solar cells was
measured under simulated AM1 conditions in our laboratory and under simulated AM0 at
NASA* and it is summarized in Table I using the active area of the cell. The short
circuit is larger under AMO conditions than under AMI illumination but the fill fac-
tor and efficiency remain the same. Figure 2 shows the in I vs V characteristics of
a large area cell. For low forward biases the forward characteristics are determined
by recombination in the depletion layer. At larger currents the in I vs V charac-
teristics are linear and show an ideality factor of n = 1.04 and saturation current
density of Jo = 9.6 x 10 -16 A/c m2. The very low value of the saturation current
density is responsible for the high open circuit voltage observed. The value of the
ideality factor close to 1 indicates that the forward current characteristics are
dominated by recombination in the neutral regions and not in the depletion layer of
the junction which indicates a clean junction. The high value of the fill factor is
a consequence of the ideality factor being close to 1 and that the top n+ layer is
heavily doped so the series resistance is low even for the shallow junction use_
Figure 3 shows the external quantum efficiency as a function of wavelength for
typical AR and a non-AR coated cell. For the non-AR coated cell the quantum effi-
ciency remains essentially constant for 500 nm to 850 nm where the efficiency drops
very sharply and it is essentially zero at 920 nm which corresponds to the energy
gap of InP.
The external quantum efficiency of the AR coated cell shows that the efficiency
improves between 550 nm and 900 nm. This increase in efficiency is responsible for
an increase of 37% in the short circuit current between AR coated and non-AR coated
cells.
* The AMO measurements were performed by Dr. I. Weinberg of NASA Lewis Research Cen-
ter, Cleveland, Ohio.
296
COMPUTER SIMULATION
In our measurements the internal quantum efficiency _ int was obtained from the
measuredexternalquantum efficiency _ ext through the equation:
int = n ext/(l-R) (i)
where R is the reflection coefficient. This internal quantum efficiency was compared
to a computer simulation in order to determine important cell parameters. The in-
ternal quantum efficiency at short wavelengths is determined mainly by the junction
depth and the top surface recombination velocity. The hole diffusion length in the
top n+ layer does not appear to affect the quantum efficiency as long as the diffu-
sion length is longer than the junction depth. At long wavelengths the internal
quantum efficiency depends upon the depletion layer width and the electron diffusion
length in the p-region.
Figure 4 shows the comparison between a computer simulation and the measured
internal quantum efficiency of one of the non AR coated cells. The same figure
shows that the values of the parameters for a good match were junction depth of 0.06
_m top surface recombination velocity of 2 x 106 cm/sec, depletion layer width of
0..26 _m and electron diffusion length of 2.5 _m. Of the above parameters the
depletion layer width was determined by measuring the depletion layer capacitance
of small area diodes.
CONCLUSION
We have demonstrated an open tube diffusion technique for fabricating high
efficiency n+p InP solar cells. Detailed evaluation of the electrical and photo-
voltaic characteristics of the cells shows that the fabrication techniques used give
a clean junction with an ideality factor close to 1 and a very low value of the
saturation current density. Comparison of the measured and simulated spectral
response as a function of wavelength allowed the determination of important device
and material parameters.
REFERENCES
i. H.C. Casey, Jr; and E. Buehler: Evidence for Low Surface Recombination on n-type
InP. App. Phys. Lett. Vol. 30, No. 5, March 1977, pp 247-249.
2. A. Yamamoto; M. Yamaguchi; and C. Uemura: High Conversion Efficiency and High
Radiation Resistance InP Homojunction Solar Cells. App. Phys. Lett. Vol. 44,
No. 6, March 1984, pp. 611-613.
3. C.C. Shen; and K.Y. Choi: High Conversion Efficiency p-n+ InP Homojunction Solar
Cells. Electron Devices Lett. Vol, EDL-6, No. 2, February 1985, pp. 78-80.
4. A. Yamamoto; M. Yamaguchi; and C. Vemura: Frabrication of High Efficiency n+-p
Junction InP Solar Cells by Using Group VI6 Element Diffusion into p-type InP,
IEEE Trans. on Elec. Dev. Vol. ED-32, No. 12, December 1985, pp. 2780-2786.
297
1(NASA)
TABLE I
Photovoltaic Performance of
Jsc
(mA/cm 2)
24.7
31.01
Bulk n+p
Voc
V
0.800
0.815
InP Cells.
FF
0.815
0.826
Eff.
(7o)
16.3
15.2
DEVICE STRUCTURE
AR COATING ( 750 A )
ELECFROPLATED Au.
DIFFUSED LAYER ( 400 - 600 A )
OHMIC CONTACT ( Au-Zn )_......_.__
BULK P- lnP ( 2E16 -- 9EI6 )
MESA ETCH DEFINED
Fig. 1 - Schematic
* DEVICE AREA 0.313 CM
Diagram of Solar Cell Structure
298
1E-1
IE-2
IE-3
_ IE-4
E--
Z
m
_" 1E-5
1E-6
IE-7
1E-8
KK 88B
1.04
I t
................................. n--!;63 ............................
,/'/ Io = 3 E-16 .
I I I / I /' I I I I I
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.(
VOLTAGE (V)
2AREA = 0.313 cm
Jo = 9.58 E-16 A /cm 2
Fig. 2 - inl vs V characteristics of large area solar cell
299
>.
r.)
Z
u
KK88B
AR_OATED
NON ARCOATED
I t I ' I i I i I i I
400 500 600 700 800 900
WAVELENGTH (nm)
Fig. 3 - Quantum efficiency as a function of wavelength
for n+p InP cells.
1ooo
0.9
0.8
0.7
• 0.6
r,
0.5
Z
,,_ 0.4
0.3
Z 0.2
0.1
0.0 !
300 1000
CELL NO. KK-117A ___m_m_+_"_ °
N
o.. EXPEmMEN'rAL /
400 500 600 700 g00 900
WAVELENGTH ( nM )
Fig. 4 - Comparison between measured and simulated
internal quantum efficency for a non AR
coated InP cell.
300


Solar cells in bulk InP using an open tube diffusion process

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