Field collapse due to band-tail charge in amorphous silicon solar cells Page: 4 of 6
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Ui.
Ui.0. 0-
0.75-
0.70-
0.65-
0.60-
0.55-2 3 4 8 67' 2 3 5"' 2 3 45 67
1 210
Jsc (mA/cm )
Cell 0 Cell O Cell #
Nd 5x1015 0 0
Ea (meV) 27 27 0
Ed (meV) 44 44 0Figure 1. The simulation results of fill factor
as a function of illumination intensity for
three experiments. The designed cell has a
pin structure with i layer thickness of 0.5
mm. The table shows the i layer parameters
for each experiment.
The first experiment is to run a standard cell with
i-layer parameters close to the real cell. For example,
the conduction-band tail width (Ea) is 27 meV, and
the valence-band tail width (Ed) is 44 meV. We
choose a midgap defect density (Nd) of 5 x106 cm3
,which is close to the defect density in an as-grown
cell. As expected, we observe a decrease of FFwith
increasing light intensity, as in the actual cell. The
symbol 0 represents this experiment.
To test whether or not this decrease in FF is
caused by the space charge trapped in the midgap
defects, we remove the defects. We do the second
experiment on this new cell without the defect states.
We use symbol O to denote this run. We find that it
has little effect on FF. It also shows a decrease of IF
with the increasing of light intensity. The data of the
second experiment are almost identical to the first
one. This implies that the space charge in midgap
defect states is so small that it causes insignificant
changes in the electric field in the dark. In fact, we
find that the midgap defect density must be as high
as 1 x 1017 to reduce the fill factor below that in the
standard cell for just AM1 illumination (J.=12
mA/cm2) from the simulation.
We proposed that the decrease in FF is mainly
caused by the space charge in the band-tail states.
We hypothesized that if the cell has no band-tails,
there is no light dependence of the fill factor. The third
designed cell is the one without the midgap defects
and band-tails. The third experiment is denoted by the
# symbol. The results support our ideas. It really
shows less dependence on light intensity than the
standard cell. This indicates that most
photogenerated space charge is trapped in the band-
tails. On the other hand, there is a slight butC-ed3 G
n 2(p, + p) V2(1)
If the perturbation to the system is large enough that
it determines the shape of the electric field, C,, is
given by= A0-75 G0.25
Ch = I'h G0,
Cj X v0.5(2)
where e is the electron charge, G is the
photogeneration rate, V is the applied voltage plus
the built-in voltage, d is the sample thickness, p, is
the electron drift mobility, p, is the hole drift mobility,
and c is a constant. The beauty of having an
analytical solution is that the relationship between
the cause and effect is clearly presented. The theory
predicts that at low light intensity, C,,, increases
linearly with the generation rate and is reciprocal to
the square of the applied voltage. At high light
intensity, C,, increases with a quarter power of the
generation rate and is reciprocal to the square root of
the applied voltage. Photocapacitance also depends
on the drift mobility, but that is not the subject of this
study. Unfortunately there is not an analytic
expression to connect the two regimes.significant illumination dependence of FF at high light
intensity. We attribute this effect to the space charge
of free carriers.
EXPERIMENTAL
Experimentally, we cannot easily measure the
electric field in the i layer, but we can measure the
effect of space charge using the well-established
capacitance technique. We can measure the
photocapacitance (C) [4-6], which is a sensitive
probe of the i-layer field distortion. It measures the
response of space charge to the applied voltage. We
measure capacitance in the dark and under
illumination. We define the photocapacitance by
subtracting the capacitance in the dark (C, ) from its
value in the light. In the following section, we address
the photocapacitance theory first and then show the
experimental results.
It is well known that any space charge has an
associated capacitance [7]. There are two
capacitances in the p-i-n configuration: one for
electrons and one for holes. For simplicity, one can
think of the total photocapacitance as the two
capacitances added in series. Crandall modeled the
photocapacitance previously [8,9]. The Regional
Approximation was applied to solve the transport
equations and analytical solutions could be obtained
only for two extreme cases. If perturbation to the
system is small, the photogenerated space charge is
so small that the electric field is little changed from its
dark value. In this case, C,, is given by
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Wang, Qi; Crandall, R. S. & Schiff, E. A. Field collapse due to band-tail charge in amorphous silicon solar cells, article, May 1, 1996; Golden, Colorado. (https://digital.library.unt.edu/ark:/67531/metadc668581/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.