Chlorine passivation of grain boundaries in cadmium telluride solar cells

Abstract

Cadmium telluride is the most commercially important second generation thin-film photovoltaic, with a record solar cell conversion efficiency of 22.1%. However as-deposited cells are 5% efficient and require a cell activation treatment with ${\mathrm{CdCl}}_{2}$ at about 400 ${}^{\ensuremath{\circ}}\mathrm{C}$ to reach commercially viable efficiencies. Such a treatment is a routine process during CdTe module manufacturing. However, the precise mechanisms at work for this remarkable efficiency enhancement are not well understood. In this paper, atomistic modeling techniques are used to improve the fundamental understanding of the structural and electronic properties of CdTe by modeling the effects of chlorine and other elements with their interaction with extended defects and grain boundaries (GBs). Studies at high spatial resolution with nanoscale secondary ion mass spectrometry, transmission electron microscopy (TEM), and energy dispersive x-ray analysis show that chlorine atoms are concentrated at grain boundaries in CdTe after the ${\mathrm{CdCl}}_{2}$ treatment. Density functional theory calculations show that both ${\mathrm{Cl}}_{\mathrm{Te}}$ and ${\mathrm{Cl}}_{i}$ are stabilized at the grain boundaries compared to bulk CdTe. Similar defect formation energies of these defects suggest both will be present at the grain boundaries. As expected, four single-particle levels are present in the $\mathrm{\ensuremath{\Sigma}}3$ (112) GB band gap, which explains the low efficiencies prior to treatment. ${\mathrm{Cl}}_{\mathrm{Te}}$ substitutions passivate one of these levels and partially passivate another two. Remarkably, further addition of ${\mathrm{Cl}}_{i}$ fully passivates the remaining single-particle levels. This passivation of single-particle levels is most likely to be the primary cause of the efficiency enhancement on chlorine treatment. Further to this, alternative halogens were then trialed as activation treatments. All halogens show similar electronic effects and their defect formation energies follow ionic radii trends.