knjiga solarna energija dobra

CdTe THIN-FILM SOLAR CELLS 649
The inset tells us quantitatively that considerable current enhancement is possible
by employing a thinner CdS window, and to a lesser extent with a different glass or
improved SnO 2 process. Potential for improvement in J SC with an antireflection coating
or from better collection of the deeply penetrating photons would be even smaller. It is
certainly possible, however, to reduce the larger losses. The record efficiency cell, for
example, has a J SC of 26 mA/cm 2 . Its losses from glass, SnO 2 , and CdS absorption are
each more than a factor of five less than the cell shown in Figure 14.23, and no single
loss factor contributes more than 1 mA/cm 2 .
The capacitance of a solar cell can yield information about extraneous states within
the band gap, and it can often give a credible profile of the carrier density within the
absorber [176]. Figure 14.25(a) shows the measured capacitance of a CdTe cell as a
function of frequency for three biases: 0, −1, and −3 V. The capacitance magnitude is
relatively small, corresponding to a small carrier density and a large depletion width. The
large depletion width implies that most photons are absorbed in a region with an electric
field and hence photocurrent does not vary significantly with voltage [177]. The fact that
the curves are relatively flat over nearly three decades in frequency strongly suggests that
they are not significantly affected by extraneous states. The upturn at high frequencies is
6
C/A
[nF/cm 2 ]
5
4
V = 0
V = −1
3
2
V = −3
1 10 100 1000
Frequency
[kHz]
(a)
5 × 10 15 4
CdTe thickness = 3.3 µm
12 × 10 8 8
A 2 /C 2
[m 4 /F 2 ]
4
Depletion ends
in back contact
Depletion ends
in CdTe
Frequency = 75 kHz
Hole density
[cm −3 ]
3
2
1
p in CdTe layer,
2 × 10 14 cm −3
p in back contact
0
−3 −2 −1 0
Voltage
[V]
0
0 1 2 3 4
Distance from junction
[µm]
(b)
(c)
Figure 14.25
Capacitance measurements and hole-density determination for a CdTe solar cell