Study of the properties of solar cells based on $a$-Si : H-$p$–$i$–$n$ structures by admittance spectroscopy

Properties of solar cells based on $a$-Si : H-$p$–$i$–$n$ structures are studied by admittance spectroscopy. The responses of the density of states in the $(i)a$-Si:H layers and $a$-SiC:H layers in the $p$-type region of the structure are distinguished in the admittance spectra. The density of states in the middle of the mobility gap for $(i)a$-Si:H is estimated to be 5 $\times$ 10$^{16}$ cm$^{-3}$ eV$^{-1}$. It is shown that this value increases during the course of photoinduced degradation to $\sim$ 10$^{17}$ cm$^{-3}$ eV$^{-1}$. For the wide-gap $a$-SiC:H layers, the observed response of the density of states in the valence-band tails made it possible to estimate the lower bound for the density of states at the Fermi level (10$^{18}$ cm$^{-3}$ eV$^{-1}$) and to find the Fermi level position to be 0.4 eV above the valenceband edge. The suggested procedure can be used to optimize the design of solar cells in order to improve their efficiency.