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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 redfilter for 100 hours the Ns·d is directly proportional to the filmthickness. This indicates that the light-induced defects are created inthe bulk of a-Si:H films. The spin density Ns before light illuminationand the light-induced spin density 6Ns decrease with increasing depositiontemperature (5, 6). The a-Si:H films deposited at low temperaturehave inhomogeneous structural properties and large amount of hydrogenatoms evolved at relatively low temperature of ~350oC. Therefore,this result suggests that the density of source for light-induced de~ectstends to increase with losing structural homogeneity and seems tobe related to the fractional hydrogen evolved at low temperature in aSi:H (6).Figure 2 shows the change of spin density Ns by prolonged lightillumination of AMI.5 (100mW/cm2 ). The spin density Ns increases withincreasing time up to 100 hours and saturates at about 300 hours. Thesaturated value of Ns, Nsoo,decreasesperature. We suppose that the:saturation of Ns results fromwith increasing deposition tem10  the limited density of thesource for the light-induceddefects and the density of thesource for the light-induceddefects decreases with increasingdeposition temperature. Wehave made a numerical analysison the saturation behavior ofthe dangling bond density byprolonged illumination accordingto the kinetic model proposedby Eser (7) in which thetotally degraded state