Correlations of Capacitance-Voltage Hysteresis with Thin-Film CdTe Solar Cell Performance During Accelerated Lifetime Testing Page: 4 of 8
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(a)
E,=
2 94 eV
Ea
0.63 ev
26 28 3.0x10
.1)
1/T (K )0.8-
12
10
a
6-K
(b)
.dell El vs delta X
X = Voc
X = Jsc
--X =FF100
60 80
120
Stress Temperature (C)
Fig. 2. Degradation activation energies (a) and correlation analysis (b) of how
efficiency (Eff) changes versus changes in V,,, Js,, and FF.
The moderate correlation of r% with Js, seen at lower stress
temperatures is due to reduced optical attenuation associated
with S-outdiffusion from the CdS. The most important
variable affecting r% at all temperatures, approaching a near
ideal correlation at 120 "C, was FF. FF represents the
efficiency by which photons absorbed in a solar cell are
collected by a combination of field and diffusion-limited
collection mechanisms. Within the space-charge, photo-
generated electrons and holes are swept towards the n and p-
sides of the junction respectively by the built-in field. In the
quasi-neutral region, carrier diffusion length determines
whether they are collected. Recombination of carriers before
collection is the greatest impediment to improving the overall
performance, primarily Vo, for these cells.
The fundamental current-density, J, and voltage, V,
behavior of a solar cell is represented by:(2)
J=SCR + QNR + J_ ph
Rsh fwhere JsCR, JQNR, and Jph represent recombination currents in
the space-charge and quasi-neutral regions of the cell, as well
as the photo-generated current, while RS and Rsh represent
series and parallel (shunt) resistance losses. JSCR and JQNR are
further represented by the following:recombination currents, JQNR, and JSCR are themselves
dependent upon resistive effects as shown in (3) and (4).
The loss parameters JQNR, JSCR, RS, and Rh can be
determined graphically or by direct modeling with programs
like Pspice. Using the latter approach, the percent contribution
each parameter contributes to the forward current (and thus
loss) in the power quadrant for a laboratory made 14.4%
CdS/CdTe solar cell is shown in Fig. 3.
10loo-0 .6
0
0-4
0.2
0.080
60Jqnr only Rs}
JQnr only J,
- Jser oly Rs}
JsCr lonly J,40F
c20
00 0.2 0.4 0.6 08
Voltage (volts
Fig. 3. Percent contribution to forward current losses modeled for a 14.4%
CdS/CdTe cell.
The results shown in Fig. 3 use a model-fitted value of 3
ohms*cm2 for RS which is a reasonable upper value observed
during stress testing of these cells [9]. As seen in this figure,
recombination occurs mostly in the space-charge except near
Vo where recombination in the quasi-neutral region, i.e.,
between the depletion width and back contact begins to
dominate. Note that resistive contributions for both effectively
go to zero at Vo, where J = 0 in equations (3) and (4).
The effect of addressing recombination within the quasi-
neutral region is shown in Fig. 4.
8gx1D0 -
N 60 i C
40
20
E
0
-20
-0.5 0.0 05 1.0 1.5JQNR J Ceq(V-JR,
QNR 01
JSCR 02 kT_1i
where Jol and Jo2 are further dependent upon minority carrier
transport properties.
In the fourth quadrant, maximum power output is achieved
by maximizing the term, Jph, and minimizing the first three
terms in (2), often referred to collectively as the "forward"
current. Each of the forward current terms contributes to
decreased cell performance. It should be noted that theVoltage (volts)
(3) Fig. 4. The effect of reducing quasi-neutral recombination on the V., of CdTe
solar cells (all J terms given in A/cm2).Shown in this figure is the Pspice model simulation used to
extract the loss mechanisms shown in Fig. 3 along with actual
cell data in a conventional J-V diagram. In this case Jol and Jo2
equal 3e-16 and le-09 A/cm2 respectively. Having obtained a
good fit, the model is then perturbed by simulating conditions
where recombination is removed in either the space-charge (Jo2
~ 0) or quasi-neutral (Joi ~ 0) regions. As can be deduced from
Fig. 4, improving CdTe material quality within the depletion
width (space-charge) does not improve cell performance.
However, an additional 60 mV (0.82 to 0.88) can result if
recombination in the quasi-neutral region is reduced.2
C
sLL
0
E
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Albin, D. & del Cueto, J. Correlations of Capacitance-Voltage Hysteresis with Thin-Film CdTe Solar Cell Performance During Accelerated Lifetime Testing, article, March 1, 2011; Golden, Colorado. (https://digital.library.unt.edu/ark:/67531/metadc839762/m1/4/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.