Numerical modeling of polycrystalline copper indium selenium(2) based hetrostructure solar cells
CuInSe$\sb2$ is a good candidate for thin-film solar cells because of its low cost, manufacturability and good absorption of light. However, its material properties and physics are not well understood. In this thesis, the numerical modeling of thin-film poly-crystalline CIS based solar cells is presented. The computer simulations of the solar cells are carried out by simultaneously solving the three fundamental equations describing the behavior of semiconductor devices, the Poisson and current continuity equations.
Numerical modeling helps explain why most CuInSe$\sb2$ based solar cells exhibit either high short-circuit current/low open-circuit voltage or low short-circuit current/high open-circuit voltage. If there is a slightly less doped region near the junction, compared to the doping level of bulk CIS, the width of depletion region is effectively the same as the less doped layer. When the depletion region width is wide, there is more space charge recombination, thus low open-circuit voltage, and the collection field extends further resulting in high short-circuit current.
It has been experimentally observed that there are two distinctively different types of illuminated I-V curves: one that develops a kink near the open-circuit voltage and then resumes its exponential increase, and another in which the current increase slows down with increasing applied voltage near the open-circuit voltage and never resumes its exponential increase. The numerical simulation results show that these non-exponential illuminated I-V curves can be obtained when there is a secondary parasitic junction. When the second junction is at the heterojunction interface, either caused by a conduction band offset or by interface states, the current resumes its exponential increase after showing a kink in the I-V curve. However, the current never resumes its exponential increase when the second parasitic junction is due to a Schoktty diode at the back contact.