Measurements

Chapter 7: Transient Photocurrent Measurements
Figure 7.8: Symbols are the normalized photocharge measurements taken on (a) sample
C(V = 0.5 V) and (b) sample D (V = 1 V) at the indicated temperatures. The solid lines
are the corresponding calculations using Eq. 7.4 with the parameters indicated.
due to the absorption depth of the laser, which is 160 nm, about 5% of the charge
is attributed to electron motion 1 and has been subtracted from the transient photocharge.
The portions of these transients at early times close to the electronic
rise-time (t < 5 × 10 −8 s) have been excised, as have the late-time portions where
most holes have been collected. The solid lines in Fig. 7.8 are a fit to the experimental
measurements using the parameters listed in table 7.2. Additionally, the
multiple trapping parameter of a typical a-Si:H sample taken from Dinca et al.
[142] are shown. From the fitting a valence band-tail width of ∆E V = 31 meV for
sample C (∆E V = 32 meV for sample D) was derived, which is much narrower
than the widths of ∆E V = 40−50 meV reported for amorphous silicon [140, 142],
but is still substantially larger than the values as low as 22 meV that are reported
for the conduction band-tail in amorphous silicon [131]. Remarkably, the values
for the band mobility of µ 0 = 1cm 2 /Vs for sample C and µ 0 = 2.3 cm 2 /Vs for
D, are essentially the same as has been reported in amorphous silicon (for both
electrons and holes) [131, 140, 142]. In this context it is interesting to note, that
the attempt to escape frequency ν = 9 × 10 8 s −1 (ν = 5 × 10 9 s −1 ) is approximately
two to three orders of magnitude smaller than values that have been reported for
holes in a-Si:H [140, 142].
7.5 Discussion
Transient photocurrent measurements have been performed on microcrystalline
material prepared under plasma-deposition conditions similar to those where best
solar cell performance can be achieved. As pointed out above, for conventional
1 For a detailed discussion of the topic see section 3.1.4
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