Jk. Rath et al., MICROCRYSTALLINE N-LAYER AND P-LAYER AT THE TUNNEL JUNCTION OF A-SI-HA-SI-H TANDEM CELLS/, Journal of non-crystalline solids, 230, 1998, pp. 1282-1286
Completely microcrystalline layers have been incorporated at the tunne
l junction of a tandem cell in the configuration SnO2:F/p-a-SiC:H/i-a-
Si:H/n-mu c-Si:H/p-mu c-Si:H/i-a-Si:H/n-aSi:H/Ag and an efficiency of
9.89% has been achieved. We present here our experimental findings and
our modelling predictions using the computer programme AMPS. A wide b
and gap a-Si:H buffer layer between p-mu c-Si:H and i-layer a-Si:H was
necessary for obtaining good performance of the bottom cell. The spec
tral response in the blue region is reduced with the increase of buffe
r layer thickness and thickness as small as 1.0 nm was necessary to ac
hieve the best efficiency. Computer prediction shows that recombinatio
n in the p-mu c-Si:H could be very high when no buffer is used because
of the combination of low band gap and high (> 10(17) cm(-3)) defect
density. Buffer layer prevents electron back diffusion into the mu c-S
i:Pi p-layer. Computed light current voltage characteristics did not c
hange significantly when tunnelling across the thin buffer layer was i
ncorporated in the model. To achieve larger efficiencies in the tandem
configuration, insertion of oxide layers between a-Si:H layer and n-m
u c-Si:H layer of the top cell and between n-mu c-Si:H layer and p-mu
c-Si:H layer was necessary, Our simulations show that, (a) the recombi
nation rate is increased at the junction due to lower mobility gap of
the microcrystalline layers, (b) the density of trapped carriers is re
duced with microcrystalline junction, not only in the p- and n-layers
but also in the i-layers, (c) the recombination rate at the microcryst
alline junction is increased with an oxide layer at the p-mu c-Si:H an
d n-mu c-Si:H interface showing an improvement of both FF as well as V
-oc. (C) 1998 Elsevier Science B.V. All rights reserved.