Using computer modeling analysis in single junction a-SiGe:H p–i–n solar cells

In this article we discuss basic aspects of single junction a-SiGe:H $\mathrm{p–i–n}$ solar cells by coupling computer simulations with experimental characteristics. We are able to fit the dark illuminated current–voltage characteristics and the spectral response curves of a-SiGe:H $\mathrm{p–i–n}$ structures in the initial state, modeling the density of dangling bonds in each device layer by using either uniform density profiles or the defect pool model. Although we can fit these experimental curves with any of these two electrical models, band gap profiling in the a-SiGe:H intrinsic layer leads to improvement of the solar cell performance only when the defect pool model is implemented in our simulations. A U-shaped band gap profile is tailored in our samples by a staircase band gap profile composed of (i) several front band gap graded steps, (ii) one lowest band gap region, and (iii) several back band gap graded steps. Only by using the defect pool model are we able to predict an optimum thickness for the front band gap graded steps and for a buffer layer located at the $\mathrm{p/i}$ interface. Furthermore, using the defect pool model, the simulation predicts that higher efficiencies in single junction a-SiGe:H $\mathrm{p–i–n}$ solar cells can be achieved by depositing nonuniform graded steps, i.e., thicker band gap graded layers besides the lowest band gap a-SiGe:H layer and thinner band gap graded layers besides the $\mathrm{p/i}$ and $\mathrm{i/n}$ interfaces.