Photoresponse data of high efficiency GaAs solar cells were analyzed using PC-1D V2.1. The approach required to use PC-1D for photoresponse data analysis, and the physical insights gained from performing the analysis are discussed. In particular, the effect of Al(x)Ga(1-x)As heteroface quality was modeled. Photoresponse or spectral quantum efficiency is an important tool in characterizing material quality and predicting cell performance. The strength of the photoresponse measurement lies in the ability to precisely fit the experimental data with a physical model. PC-1D provides a flexible platform for calculations based on these physical models

Addis, F. W.

Dunham, G.

Huber, D. A.

Olsen, L. C.

English

NASA Technical Reports Server

N91-19198
GaAs Solar Cell Photoresponse Modeling Using PC-1D V2.1
D.A. Huber, L.C. OIsen, G. Dunham and F.W. Addis
Washington State University/Tri-Cities
Richhmd, WA
Introduction
In this study, photoresponse data of high efficiency GaAs solar cells have been
analyzed using PC-1D V2.1. This paper discusses the approach required to use PC-
1D for photoresponse data analysis, and the physical insights gained from performing
the analysis. In particular, the effect of AlxGao_×)As heteroface quality has been
modeled. Photoresponse or spectral quantum efficiency is an important tool in char-
acterizing material quality and predicting cell performance [ref. 1]. The strength of
the photoresponse measurement lies in the ability to precisely fit the experimental
data with a physical model. PC-1D provides a flexible platform for calculations based
on these physical models.
Cell Fabrication and Performance
GaAs cell structures studied in this work were P/N homojunctions with
AlxGa0_x)As heteroface layers for front and back minority carrier reflectors. All
cells were 1.5 cm x 1.5 cm and fabricated from films grown on horizontal Bridg-
man wafers by MOCVD. The cells were fabricated from epi-structures supplied by
SPIRE Corp., as well as grown in WSU/TC's SPI-MO CVD 500XT reactor. The
SPIRE epitaxial layers were grown at atmospheric pressure using trimethylgallium,
trimethylaluminum and 10% arsine in H 2. The WSU/TC cells were grown at low
pressure using pure arsine. N-type doping was accomplished using silane and the
p-type doping with dimethylzinc in H2 in both cases. The basic structure described
in Figure 1 was constant for these cells which facilitated side-by-side comparison of
performance and material quality. Contacts for the devices were made by electro-
plating Au for the front contact and Au/Sn for the back contact. After sintering, the
GaAs cap was removed between the contact fingers and a mesa was etched around
the perimeter. The individual cells selected for this study correspond to varying
growth conditions which affect the cell's performance [ref. 2]. These conditions in-
clude growth temperature of GaAs and AlxGa(l_x)As as well as dopant flows and
change of temperature at the heteroface. Efficiencies for these cells range from 18%
to 22% for one-sun AM1.5 illumination. Cell performance is listed in Table 1.
126
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Modeling Photoresponse Data with PC-1D V2.1
The Iowa State codePC-1D utilizes a finite elementnumerical approachto solve
the semiconductorequations [ref. 3]. Version 2.1 allows significant improvements
in the complexity of the problems that can be solved,compared to earlier versions
[ref. 4]. Severalkey additions have provenespeciallyapplicable to the modeling of
GaAs solar cells. The program permits modeling of minority carrier confinement
due to the AlxGa0_x)As heterofacewith recombination at the interface. PC-1D is
equippedwith an internal model for the absorption of AlxGa(l_x)As as a function of
x and permits the useof an external model or absorption data. Up to three regions
of different material parameterscan be used to definethe device,eachwith its own
doping profilesand electronicandoptical properties. The ability to model the grading
of material propertiesat the interfacebetweeneachregion increasesthe usefulnessof
the program to model realistic structures. Recombinationof the electron hole pairs
can be defined in each region by S-R-H band to band transitions or through user-
defineddeeplevel transitions. The recombinationmodel canalso accountfor Auger
recombination,surfacerecombinationat the interfaceswithin the deviceor saturation
current terms.
The first approach at modeling heteroface GaAs solar cells divided the device
into two materials an AlxGa(l_x)As region in front of the GaAs homojunction region.
The grading between these two materials was set at the minimum value of 10 _, an
abrupt interface. This generated photoresponse which was independent of surface
recombination due to the high drift fields produced. Reasonable values for the grading
thickness always led to the same result since the AE G is large. The response of the
cells, however, showed definite signs of front surface recombination. Incorporating
our previously reported "dead layer" model allowed for an additional interface and
recombination current [ref. 2].
For the cells studied, modeling studies with PC-1D predict the necessity of a thin
defective GaAs region located between the emitter and the front AlxGa(l_x)As layer
where defect states result Jr/current loss due to recombination within the region. This
approach models the AlxGa0_x)As window region, a two-region emitter consisting
of the defective layer and a high quality region, and an n-type base region. This is
accomplished using three material parameter files - a wide bandgap AlxGa(l_x)As
material, a defective low-lifetime GaAs material, and a high quality GaAs material.
Figure 2 shows a bandgap diagram illustrating the modeled regions. The effect of the
back AlxGa(l_x)As reflector was modeled only by a surface recombination velocity in
this study since absorption of photons is inconsequential. This allows more flexiblity
in the modeling of the front interface .
127
Results and Discussion
The model wasusedto producea very closefit to the unique photoresponseof
severalcells. Figures3 through Figure 7 showthe experimentaldata for severalcells
as well as the modeled curves for comparison. The values modeled for individual
cells are given in Table 2. The defective region size increaseswith AlxGa(l_x)As
growth temperature, which correlated with the length of transition time between
GaAs growth and AlxGa0_×)As growth. Defective layer thickness varied from less
than 30 _ to greater than 100 _. Front surface recombination velocities ranged from
1 x 104 cm/s to approximately 2 x 105 cm/s. As shown previously, both a surface
recombination and a finite "dead layer" thickness are needed to fit some devices due
to the absorption in the thin defective region [ref. 4]. The modeled structures corre-
sponded well with the layer thicknesses derived from electrochemical C-V profiling.
Earlier cells could only be modeled using window A1 concentrations lower than the
A10.85Ga0.15As specified. AlxGa(l_x)As concentrations were confirmed using surface
photovoltage measurements and reflectance modeling. This was remedied in later
growth runs which produced higher response. The internal model for photon absorp-
tion in AlxGa(l_x)As included in PC-1D gives good magnitudes for the absorption
coefficients but produces a rather jagged appearing curve compared with the smooth
experimental curves. Our previous model based on Mazier's approach [ref. 5] and
reported elsewhere gave somewhat lower but smoother coefficients [ref. 6].
Both defect layer thickness and surface recombination velocity were functions of
growth conditions. Table 3 lists growth conditions for some of the cells studied. These
studies lead to the conclusion that interrupted growth for a change of AlxGa0_x)As
composition or temperature leads to the introduction of defect states which allow
recombination in the vicinity of the heteroface. The cells grown at WSU/TC incor-
porated the results of the studies on earlier cells which produced devices with high
internal photoresponse.
Acknowledgement
The authors wish to thank the Air Force Office of Scientific Research for support
to investigate solar cells based on III-V compounds under Grant AFOSR-89-0182.
128
References
1. ] L. C. Olsen, et al., Space Photovoltaic Research and Technology Conference,
NASA (1986).
2. ] L. C. Olsen, et al., Proceedings of the Nineteenth IEEE Photovoltaic Special-
ists Conf. (1987).
3. ] P. A. Basore, et al., Proceedings of the Eighteenth IEEE Photovoltaic Spe-
cialists Conf. (1988).
4. ] P. A. Basore, et al., Proceedings of the Twentieth IEEE Photovoltaic Spe-
cialists Conf. (1988).
5. ] C. Maziar, Subcontract Report - SERI/STR-211-2512, DE 85000501 (1984).
6. ] L. C. Olsen, et al., Space Photovoltaic Research and Technology Conference,
NASA (1988).
129
TABLE 1
AM1.5 EFFICIENCIES
Cell Voc Jsc Fill Factor Efficiency
(V) mA]cm 2 ( % )
1 .954 22.6 .803 17.3
2 1.02 23.2 .82 19.3
3 1.02 24.91 .851 21.5
4 N/_ 22.1" N_ NA
5 N_ 25.4 * I_, NA
* Active Area Before Antireflection Layer
130
TABLE 2
MODELED PARAMETERS
Cell S(F) L(E) L(B) S(B) X(DL) X(WH) AI (conc)
cm/s microns microns cm/s A A %
1 2 xlo 5 3.0 2.0 1 xlo 4 1 oo 500 65
2 2 xl04 5.0 0.2 1 xlO 7 50 500 70
3 1 xlo 4 5.0 4.0 1 xlO 4 30 500 75
4 1 xl04 >6.0 3.0 1 xl04 0 500 80
5 1 xlo 4 >6.0 4.0 1 xlO 4 0 500 80
TABLE 3
GROWTH PARAMETERS
Cell NA(E) T(E) T(BR) ND(B) T(BASE) T(AIGaAs) X(AIGaAs)
cm -3 °C °C cm -3 °C °C %
1 1 xl018 700 800 1 xl017 650 800 65
2 1 xl018 700 750 1 xl018 750 700 70
3 1 xl018 700 750 3 xl017 700 750 75
4 LP 1 xl018 720 720 1 xl018 720 720 80
5 LP 1 xl018 720 720 3 xl017 720 720 80
131
grid fingers AR coaling
P- lY Pe AI Ga 1-xAS
V
window
N-type AI xGa1.,_xs..._.._._
back retlector
P-GaAs 0.5 p.m
N-GaAs 3.0 p.m
N+ -GaAs Buffer 1.0 _m
Back contact
N+ -GaAs Substrate 300 p.m
P+ -GaAs cap
lOOOA
500 A
_" 1000 A
Figure 1 The basic structure for the high efficiency GaAs solar cells
modeled.
5 6 7
= •
w •
m •
•
; -
; - 0
• w •
Figure 2 Energy bandgap diagram showing regions modeled using
PC-1D.
132
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Figure 4
INTERNAL PHOTORESPONSE
• I ' I ' I I " I
_ m • •
-o • w -
• CELLI DATA
MODELED
11.
I , I , I , I i I
450 550 650 750 850 950
WAVELENGTH (nm)
Photoresponse data and PC-1D model for CELL1
INTERNAL PHOTORESPONSE
// ""
CELL2DATA
MODELED
I , I , I , I
450 550 650 750
WAVELENGTH (nm)
850 9
Photoresponse data and PC-1D model for CELL2
133
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Figure 5
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Figure 6
INTERNAL PHOTORESPONSE
, • • °
" " CELL3 DATA I
• MODELED •
450 550 650 750 850 950
WAVELENGTH (nm)
Photoresponse data and PC-1D model for CELL3
INTERNAL PHOTORESPONSE
• CELL4 DATA
MODELED
i , I i I i I , I - - _.
450 550 650 750 850 950
WAVELENGTH (nm)
Photoresponse data and PC-1D model for CELL4
134
LU
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O
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INTERNAL PHOTORESPONSE
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tO
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WAVELENGTH (nm)
Figure 7 Photoresponse data and PC-1D model for CELL5
135


GaAs solar cell photoresponse modeling using PC-1D V2.1

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