Towards highly efficient thin-film solar cells with a graded-bandgap CZ TSSe layer. Part II: Piecewise-homogeneous bandgap grading

In Part I, we optoelectronically optimized a thin-film solar cell with a graded-bandgap CZTSSe photon-absorbing layer and a periodically corrugated backreflector, using the hybridizable discontinuous Galerkin (HDG) scheme to solve the drift-diffusion equations. The efficiency increase due to periodic corrugation was minimal, but significant improvement was achieved with a nonlinearly graded bandgap. Due to occasional failures of the HDG scheme from negative carrier densities, we developed a new computational scheme using the finite-difference method, which also reduced the overall computational cost of optimization. Later, a normalization error was discovered in the electrical submodel in Part I, but it did not alter the overall conclusions. We have now re-optimized the solar cells with (i) a homogeneous bandgap, (ii) a linearly graded bandgap, or (iii) a nonlinearly graded bandgap, and (iv) a piecewise-homogeneous bandgap which is easier to implement than a continuously graded bandgap. An efficiency of 13.53% is predicted with a three-layered piecewise-homogeneous CZTSSe layer. Furthermore, concentrating sunlight by a factor of one hundred can boost the efficiency to 16.70% with the piecewise-homogeneous bandgap.