A novel class of photovoltaic cascade structures is introduced which features multijunction upper subcells. These superstructure high efficiency photovoltaics (SHEP's) exhibit enhanced upper subcell spectral response because of the additional junctions which serve to reduce bulk recombination losses by decreasing the mean collection distance for photogenerated minority carriers. Two possible electrical configurations were studied and compared: a three-terminal scheme that allows both subcells to be operated at their individual maximum power points and a two-terminal configuration with an intercell ohmic contact for series interconnection. The three-terminal devices were found to be superior both in terms of beginning-of-life expectancy and radiation tolerance. Realistic simulations of three-terminal AlGaAs/GaAs SHEP's show that one sun AMO efficiencies in excess of 26 percent are possible

Leburton, J. P.

So, L. C.

Wagner, M.

English

NASA Technical Reports Server

N87-26421
SUPERSTRUCTURE HIGH EFFICIENCY PHOTOVOLTAICS*
M.Wagner, L.C. So, and J.P. Leburton
University of Illinois
Urbana, Illinois
A novel class of photovoltaic cascade structures is introduced which features multijunction upper
subcells. These Superstructure High Efficiency Photovoltaics (SHEP's) exhibit enhanced upper subcell spec-
tral response because of the additional junctions which serve to reduce bulk recombination losses by
decreasing the mean collection distance for photogenerated minority carriers. Two possible electrical
configurations were studied and compared: a three-terminal scheme that allows both subcells to be
operated at their individual maximum power points and a two-terminal configuration with an intercell
ohmic contact for series interconnection. The three-terminal devices were found to be superior both in
terms of Beginning-of-Life efficiency and radiation tolerance. Realistic simulations of three-terminal
A1GaAs/GaAs SHEP's show that one sun AMO etficiencies in excess of 26% are possible.
INTRODUCTION
Although considerable effort has been devoted to the development of monolithic III-V cascade cells
with two energy gaps, experimentally attainable conversion efficiencies have, to date, been substantially
lower than the best single-gap cells. The highest reported efficiency for a two-terminal AIGaAs/GaAs cas-
cade cell under one-sun AMO conditions is 15.1% [ref. 1]. Lattice-mismatched A1GaAs/InGaAs cells have
achieved efficiencies of only 13.6% (under one sun AM1.5 conditions) [ref. 2].
There are two principal obstacles which must be surmounted before practical monolithic cascade cells
can become realizable: upper subcell quality must be improved and a low-resistance intercell ohmic con-
tact (IOC) technology must be developed [ref. 1]. In this paper we present a novel class of cascade cells
designed to overcome both problems. These superstructure high efficiency photovoltaics (SHEP's) feature
an upper subcell with multiple junctions for improved performance. Previous work by the authors has
shown that multiple junctions in a single-gap cell can substantially improve spectral response, particularly
for A1GaAs cells of the composition needed for the upper subcell of a cascade device.
A two-terminal configuration is possible for the SHEP's, but they lend themselves most naturally to a
three-terminal configuration. This might seem a serious drawback since three-terminal photovoltaics have
been dismissed as impractical by many authors [refs. 1,2A]: however, this apparent difficulty lead us to an
approach which circumvents the need for an IOC with its attendant electrical and optical losses. Comple-
mentary pairs (npn and pnp) of three-terminal cells in which the upper-subcell short-circuit current of
one is matched to the lower-subcell short-circuit current of the other can be used to obtain a two-terminal
output. Figure 1 shows the electrical configuration and one possible physical configuration of this scheme.
We found only one reference to this idea in the literature [ref. 5], so it appears to have been largely over-
looked.
* Work supported by the National Aeronautics and Space Administration
65
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DEVICE STRUCTURE AND MODELLING
Cross-sectional views of two possible SHEP structures are shown in figure 2. The five- and seven-
layer structures allow both the upper-subcell emitter and lower-subcell base to be p-doped, which is desir-
able for optimum spectral response (electron diffusion lengths are usually longer than hole diffusion
lengths in III-W compounds). Additional junctions in the upper subcell of the seven-layer structure
prevent transport of collected carriers in the direction transverse to the layers, as occurs in conventional
solar cells. Instead, carriers must be transported in the layer planes to heavily doped contact regions
extending vertically through the superstructure. These regions are known as selective electrodes since they
connect to layers of like doping, while forming reverse-biased junctions (during normal operation) with
layers of opposite doping type. By allowing collected carriers to be transported in the layer planes, selec-
tive electrodes make it possible to incorporate any number of epitaxial layers in the upper subcell. Fabri-
cation of selective electrodes in structures with doping concentrations similar to what one would expect for
photovoltaics ( > 1018cm-3) was recently reported [ref. 6].
The performance potential of the cells was evaluated by means of a detailed computer model, which
allows a wide variety of multilayer, multi-bandgap structures to be investigated. Effects of the selective
electrode regions on the minority-carrier distribution are assumed to be negligible, permitting a one-
dimensional analysis. Doping and composition profiles are assumed to be uniform within each layer and
abrupt at junctions. These assumptions allow closed-form expressions to be used for the contributions of
each layer to short-circuit current, injection current, and space-charge recombination, so that layer
thicknesses can be rapidly optimized. The optimization criterion used was to maximize beginning-of-life
(BOL) efficiency with the subcells operating at their respective maximum-power points. This criterion does
not yield a precise estimate of cell performance in a complementary configuration, which will require the
pnp and npn cells to be simultaneously optimized because of the current-matching requirement already
discussed. Itowever. it will provide a basis for comparing the five- and seven-layer structures as well as a
rough prediction of the efficiency in the complementary configuration (within 1% AMO).
The current-voltage (I-V) characteristics of the three-terminal SHEP's are represented by an Ebers-
Moll model. The coupled-diode equations are:
hv I
x,. x,, 2_E e 2 (1)
J l(v 1.v 2)=J_c 1--Jo ne l_jo 12e Je'_ -v
m Vb m 1
x . . kv2 -- _.Vl ,r.,J 2(v l?' 2)-_Jsc --do e --Jo e --z.2 21 22 in Jgrm
)k_' 2
2
e
Vbn _--V 2
(2)
where h = q/kT and the subscript one refers to the upper subcell. The terms Jsc I and Jsc 2 represent the
upper and lower subcell short-circuit current densities, respectively. Coefficients Jon" J°_2" J°2_ and Jo22
determine the dependence of the injected component of the dark current on the subcell terminal voltages.
Coupling of the I-V equations arises from interaction of the injected minority-carrier populations across
the heterojunction separating the upper and lower subcells. The space-charge recombination component of
the dark current is represented by the remaining terms. Each summation is over the homojunctions con-
tained in the corresponding subcell. Note that it is necessary to have a term for each junction because, in
general, the barrier potentials (Vb, ,'s ) will not be the same for all junctions in a particular subcell. If they
were identical, the voltage dependent terms would factor out and one could simply sum over the Je,,,'s to
obtain a composite coefficient as in the case of the short-circuit and injected current terms. To compute the
Je,,, "s we use Choo's theory of space-charge recombination for abrupt, asymmetrical junctions [ref 7].
66
Expressions for the contributions of the window, emitter (upper subcell), and base (lower subcell) to
the short-circuit and injected currents are well known and have been published elsewhere [8]. Contribu-
tions of layers bounded above and below by homojunctions are easily derived from the minority-carrier
continuity and current-density equations with the boundary condition of zero excess carrier density at the
depletion-region edges. The contributions of layers adjacent to the isotype heterojunction which separates
the upper and lower subcells are considerably more complicated to calculate because one must account for
interaction between the minority-carrier populations in the layers above and below the heterojunction. The
barrier seen by minority carriers at the heterojunction is the junction built-in potential. This holds under
all bias conditions since doping concentrations in the layers forming the heterojunction are sufficiently high
that junction bias will be effectively zero for typical current densities. The boundary conditions on the
minority- carrier populations are therefore
- exp[q(EF, -(AE_ + EF2))/kT] (1)
Dp,VP,,, = Dt2VP,, 2 (2)
The fermi levels EF, and EF2 are measured from the respectivevalence band edges. Subscript one denotes
the higher gap layer of the upper subcell. These boundary conditions are nearly identical to those
described for low-high junctions [ref. 9]. The only difference is the /kE v term which must be included in
the expression for the junction barrier potential to account for the valence band edge discontinuity at the
heterojunction. We assume that AE v is 40% of the energy gap difference in accordance with recent experi-
mental work [ref. 10].
For all designs studied, the top layer is specified to be a 300 _k Alo.gGA 0,1As window with a surface
recombination velocity of 106cm Is. The antireflection coating is Si 3N4 and grid obscuration is assumed to
be 4%, attainable with existing grid array technology [ref. 11]. The absorption coefficient is modelled as in
reference 12.
RESULTS AND DISCUSSION
We found that efficiencies of optimized SHEP structures vary gradually with upper subcell composi-
tion (figure 3). Broad peaks occur at AlAs mole fraction x = 0.36 for both the five- and seven-layer struc-
tures with maximum BOL one-sun AMO effi_iencies predicted to be slightly in excess oa" 26%. Dependence
of efficiency on composition is expected to become more critical when the current-matching constraints of
the complementary design are imposed. The spectral response of a five-layer SHEP with optimum upper
subcell composition (x = 0.36) is shown in figure 4. Curves for the individual layer contributions include
both depietion and quasi-neutrai region response.
The performance, as diffusion lengths are degraded, of two SHEP structures and a five-layer, series-
connected A1GaAs-GaAs cascade cell is shown in figure 5. With space-charge recombination suppressed
(fig. 5a) the seven-layer SHEP has considerably higher BOL efficiency than the five-layer structure. This is
because the injection component of the dark current depends strongly on layer thicknesses, so that the
effect of thinner layers in the upper subcell more than offsets the effect of having two additional junctions
in that subcell. Unfortunately, the space-charge recombination component of the dark current, which tends
to dominate the injection component at the maximum- power point in AlGaAs cells, is quite independent of
layer thicknesses. The seven-layer SHEP is, therefore, at a slight disadvantage in BOL efficiency because of
the two additional junctions if the effects of space-charge recombination are included (fig. 5b), but its
overall performance is still seen to be superior to that of the five-layer SHEP.
Although simulation of a complementary pair has not yet been performed, the efficiency is expected
to be very close to that of an A1GaAs-GaAs cascade cell with an ideal IOC. The curves in figure 5 were
calculated using this assumption of an electrically and optically lossless 1OC. The BOL efficiency of this
67
cell is slightly lessthan27%without space-charger combination,which issomewhatlowerthan thevalue
of 27.6%found in an earlier study of AIGaAs-GaAscascadecells(this study neglectedeffectsof space-
chargerecombination)[13]. The optimum upper-subcell composition predicted by our model (x = 0.38)
was also found to be somewhat lower than that of the previous study (x = 0.41). These differences are
probably accounted for by the assumption in our model that diffusion lengths fall exponentially with
increasing AlAs mole fraction, which tends to skew the optimum upper-subcell composition to smaller
values.
It is interesting to compare the degradation curves of the two five-layer structures. The slopes are
seen to be almost identical. This shows that the number of layers is more important than electrical
configuration (two-terminal vs. three-terminal) in determining radiation tolerance. It should be noted that
the IOC of the series-connected cell is assumed to remain ideal even as layer diffusion lengths are degraded.
We therefore anticipate that complementary pairs of multilayer SHEP's will decisively outperform cascade
cells with monolithic IOC's, because of the additional layers and the lack of an IOC which could undergo
degradation.
CONCLUSIONS
We have presented a class of structures designed to surmount the principal difficulties facing the
development of practical lattice-matched monolithic cascade cells. Upper subcell quality is improved by
multiple junctions, and the problem of an IOC is completely circumvented by using complementary pairs
of three-terminal cells. Although simulation of complementary cells has not yet been performed, results
from series-connected structures with an ideal IOC indicate that BOL etficiencies in excess of 25% under
one-sun AMO conditions should be attainable. With higher concentration factors we anticipate conversion
efficiencies approaching 30%. The seven-layer SHEP appears most advantageous, at present, for space appli-
cations because of its superior tolerance to radiation degradation. If the effects of space-charge recombina-
tion were reduced, which would entail reduction of deep level impurities and vacancies, structures with
even greater numbers of thinner layers could substantially increase performance.
References
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6.
Hutchby, J. A.: Markunas, R. J.; Timmons, M. L.; Chiang, P. K.; and Bedair, S. M.: A Review of
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York, 1985), p. 20.
Lewis, C. R.; Ford, C.W.; Virshup, G. F.; Arau. B. A.; Green, R. T.; and Werthen, J. G.: A Two-
Junction Monolithic Cascade Solar Cell in a Lattice-Matched System. Proc. 18th IEEE Photovoltaic
Spec.Conf. (IEEE, New York, 1985), p. 20.
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Choo, S. C.: Carrier Generation-Recombination in the Space-Charge Region of an Asymmetrical p-n
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133 (1986).
Werthen, J. G.; Virshup, G. F. ; Ford. C. W.; Lewis, C. R.: and Hamaker, A. C.: 21% (One-Sun air
mass zero) 4 crn 2 GaAs Space Solar Cell. Appl. Phys. Lett. 48, 74 (1986).
Hutchby, J. A.; and Fudurich, R. L.: Theoretical Analysis of Al x Ga z-, As-GaAs Graded Bandgap
Solar Cell. J. Appl. Phys. 47, p. 3140 (1976).
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(a)
! r
i !
I Vl arV 2
I iv, T-
,_'I
! ,
....... J L ........ .I
npn pnp
Cell Cell
(b)
I °i ..........
r --J I I t
LP- 2772
Figure 1. Complementary interconnection scheme for three-terminal photovoltaics showing a) electrical
configuration and b) one possible physical configuration.
GoAs,
L
LP--2649
Figure 2. Cross-sectional views of five- and seven-layer SItEP structures, lsotype heterojunctions are
represented by dashed lines.
69
4O
3C
t I I I
With Space-Charge Recombination
---- Without Space-Charge Recombination
1 Total
2 Top Cell
5 Bottom Cell
l
/_,j
¢_ I I I I
VO 0 1 02 0:5 04 0.5
AlAs Mole Fraction in Top Cell _,-2,2,
Figure 3. Performance of seven-layer SttEP structures under one-sun AM() illumination at 300 K.
1.O
0.4
0.2
0
2OO
i 1 w I w I i
400 6OO
Wavelength
800 1000
(nm) Ls-2,,5
Figure 4. Spectral response of a five-layer SIIEP with upper subcell composition x = 0.36. Individual
layer contributions are shown as well as overall spectral response. Layers one, two, and three
are the window, emitter, and base, respectively, of the upper subcell; layers four and five are
the emitter and base of the lower subcell.
70
29
"-_..., (a)
28 _°_,.o..""°"'"°"''°"'-..o,"x=0 36
_.,, _ ""°--.o..
_ _',,.,,_ "_,_x =0.36 "_"
: "%'t3_,%
%°%'0,_, %,
o----_Five-LayerSHEP .o.,.,
o-.-.-oFive-Layer Series ,o,.,,
o.....oSeven-L0yer SHEP
21 i I i I , i , I ,
1.0 1.4 1.8 2.2 2.6 5.0
Diffusion Length Degradation Factor
LS-277"7
>,27
(.,)
(,-.
.__ 26
LLJ
0
_E 25
<Z
(-
= 24
or)
I
t-
O 23
22
' I i I _ I i I l
tbJ
Five-Layer SHEP
__.. o...-...o Five- Layer Series -
•. o----.oSeven- Layer SHEP
•,, _._..
(- .,, _.Q_..
._- _ .,__.
o24 - N',,
;_ ""_', "-._.,x:0.36
_23- "_. _ "_.
'_. _"% "o,.
<,_ "% x:0.56-o,, ".o,.,
o_22 : " "_! -- IQb
,_ x O.58"x,q(--
f ,,.,,...,.,41
19 J I i I I I i I i "
1.0 1.4 1.8 2.2 2.6 3.0
Diffusion Length Degradation Factor
l.S- 2776
Figure 5. Performance as diffusion lengths are degraded of five- and seven-layer SHEP designs and a
five-layer series-connected cascade cell with ideal lOC a) with space-charge recombination
suppressed and b) including space-charge recombination.
71


Superstructure high efficiency photovoltaics

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