Current loss due to recombination in Cu-rich CuInSe2 solar cells

Abstract

The absorbers in Cu(In,Ga)Se2 solar cells in general are Cu-poor. However, better transport properties and lower bulk recombination in “Cu-rich” material led us to develop “Cu-rich” CuInSe2 solar cells. We expect higher diffusion lengths and better carrier lifetimes for “Cu-rich” CuInSe2 solar cells, resulting in a higher short circuit current of “Cu-rich” solar cells, compared to Cu-poor ones. However, recent investigations show that the current is lower for absorbers grown under Cu-excess compared to Cu-poor absorbers. Therefore, this work investigates both “Cu-rich” and Cu-poor CuInSe2 absorbers, as well as their resulting cells, in order to understand why the “Cu-rich” CuInSe2 solar cells do not show the expected increase in current. While this contribution gives proof that “Cu-rich” based solar cells in fact do have better carrier collection properties, one limitation of “Cu-rich” devices is a very short space charge width associated with a higher doping level. We suggest tunneling enhanced recombination in the space charge region as the most likely cause of the loss in current. This work shows also that the high doping level of the “Cu-rich” film cannot be decreased by controlling the sodium supply.