Question

Make a table that summarizes results over the last four years (2013-2017 only) in understanding degradation in amorphous silicon solar cells. The table should have two columns: (1) Brief description of the main results in the paper, and (2) Reference for the paper

Answer 1

Light-induced degradation of hydrogenated amorphous silicon (a-Si:H) solar cells has been modeled using computer simulations. In the computer model, the creation of light-induced defects as a function of position in the solar cell was calculated using the recombination profile. In this way, a new defect profile in the solar cell was obtained and the performance was calculated again. The results of computer simulations were compared to experimental results obtained on a-Si:H solar cell with different intrinsic layer thickness. These experimental solar cells were degraded under both open- and short-circuit conditions, because the recombination profile in the solar cells could then be altered significantly. A reasonable match was obtained between the experimental and simulation results if only the mid-gap defect density was increased. To our knowledge, it is the first time that light-induced degradation of the performance and the quantum efficiency of a thickness series of a-Si:H solar cells has been modeled at once using computer simulations.

Since Fuji Electric put a-Si:H solar cells on the consumer market

in 1980, intensive efforts have been paid to use the solar cells for

power generation. The conversion efficiencies of 11.5% and 7.5% (8.5%

in active-area efficiency) have been attained for lcm2 and 30x40cm2

cells (1) and fabrication processes for 40x120cm2 cells have been developed

(2). As for the stability of a-Si:H solar cells, the lightinduced

metastable performance degradation is a remaining problem though

several techniques for suppressing the degradation have been found (3).

The mechanism of this degradation is usually assumed to be related to

the light-induced changes in photoconductivity (4) resulting from the

light-induced creation of dangling bonds.

2. LIGHT-INDUCED DEFECTS IN a-Si:H FILMS

We have made a series of studies on the light-induced change of

spin density in discharge-produced a-Si:H films by ESR (5, 6). In this

section we will further proceed the discussion on the light-induced

defects. Figure 1 shows the product of spin density Ns and film thickness

d, Ns·d (spins/cm2 ), of a-Si:H films deposited at 200°C as a function

of film thickness. The preparation conditions for specimens are"

essentially identical to those described in previous papers (5, 6).

After the annealing of 200°C for 4 hours the Ns.d of a-Si:H films is

independent of the film thickness. This suggests that the defects in aSi:H

after the annealing are mostly surface or interface defects.

On the other hand, after the illumination of AMl.5 light through red
filter for 100 hours the Ns·d is directly proportional to the film
thickness. This indicates that the light-induced defects are created in
the bulk of a-Si:H films. The spin density Ns before light illumination
and the light-induced spin density 6Ns decrease with increasing deposition
temperature (5, 6). The a-Si:H films deposited at low temperature
have inhomogeneous structural properties and large amount of hydrogen
atoms evolved at relatively low temperature of ~350oC. Therefore,
this result suggests that the density of source for light-induced de~ects
tends to increase with losing structural homogeneity and seems to
be related to the fractional hydrogen evolved at low temperature in aSi:
H (6).
Figure 2 shows the change of spin density Ns by prolonged light
illumination of AMI.5 (100mW/cm2 ). The spin density Ns increases with
increasing time up to 100 hours and saturates at about 300 hours. The
saturated value of Ns, Nsoo,decreases
perature. We suppose that the
:saturation of Ns results from
with increasing deposition tem10  the limited density of the
source for the light-induced
defects and the density of the
source for the light-induced
defects decreases with increasing
deposition temperature. We
have made a numerical analysis
on the saturation behavior of
the dangling bond density by
prolonged illumination according
to the kinetic model proposed
by Eser (7) in which the
totally degraded state