Quantifying the Influence of Free Carriers and Crystal Polytypes on Silicon PV with Theoretical Characterization

Theoretical characterization of materials using first-principles tools can provide useful information to explain experimental observations and to discover new materials. However, it is challenging to explain the silicon PV characteristics from a purely theoretical perspective because the indirect band gap of silicon requires the consideration of processes beyond direct electron-photon interactions, including phonon-assisted, charged-impurity assisted, and conductivity-induced optical absorption. In our work, we developed the first theoretical characterization tools to quantify both the direct and the various indirect processes of optical absorption in both pure and doped silicon from first principles. Our model is based on density functional theory, many-body perturbation theory and the maximally localized Wannier function approach, and has been implemented in the open-source Electron-Phonon Wannier (EPW) code.<br/>Our model allows us to quantify the impact of free carrier absorption (FCA) as well as of different crystal polytypes on silicon-based PV. We first show that our calculated indirect absorption coefficients of cubic silicon agree well with experimental measurements and that our calculated FCA coefficients agree with experimental measurements over a broad range of carrier densities and photon wavelengths. Our theoretical characterization explains the different mechanisms dominating FCA at different photon wavelengths. Further, we show that our calculated absorption coefficients for the cubic and the recently synthesized 4H silicon predict a higher absorption coefficient for the 4H polytype in the visible and IR region.<br/>Taking a step forward, and combined with models to calculate absorbance in textured films, we calculate the electron-hole pair induced current density, and we show that we can quantify the influence of free carriers and polytypes on the PV performance. We show that the practical impact of FCA on silicon PV is small due to the thin n-type emitter and the low carrier density in p-type base. However, the same analysis shows that the 4H silicon polytype can potentially allow up to five times thinner film compared to conventional cubic silicon and is a promising polytype for silicon-based PV applications.<br/>This work was supported as part of the Computational Materials Sciences Program funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award No. DE-SC0020129. It used resources of the National Energy Research Scientific Computing (NERSC) Center, a DOE Office of Science User Facility supported under Contract No. DE-AC02–05CH11231.