  Numerical simulation of a solar cell can provide various information that can
be useful to maximize its power conversion efficiency (PCE). In that respect we
carried out a set of numerical simulation using AFORS-HET simulation program.
Separately, in order to get a better understanding, the optical absorption in
individual layers devices were analyzed. Current-voltage characteristic curve
of a reference cell (Cell-A) was used as the starting device. The PCE of the
reference device was $8.85\%$ with short circuit current density $J_{sc}$ of
15.43 mA/cm$^{2}$ and fill factor (FF) of $68.3\%$. However, it was noticed
that the reference cell had high parasitic optical absorption at the window
layer and the device structure was also not optimized. After suitable
optimization the PCE of this device (Cell-B2) improves to $11.59\%$ ($J_{sc}$
and FF of 13.0 mA/cm$^{2}$ and $87\%$ respectively). The results show that the
effective optical absorption in the active layer can be improved significantly
by optimizing the device structure. The short wavelength response can be
improved by reducing the parasitic optical absorption by the doped window
layer, while its long wavelength response improves by raising effective
absorption length of the active layer. Furthermore, its optimum thickness, for
the highest possible PCE, is found to be dependent upon the material
properties, more importantly on its defect density.
