Effects of oxygen during close-spaced sublimation of CdTe solar cells

The presence of oxygen during close-spaced sublimation (CSS) deposition of CdTe has been regarded as necessary for high-efficiency CdS/CdTe solar cells. To date, research has focused on the effect of oxygen on the acceptor density in the CdTe absorber. We find that although oxygen may influence the doping density, a perhaps more significant effect is its influence on nucleation and growth of the film. Oxygen partial pressures of 0 to 24 torr and total pressures of 5 to 45 torr (balance He) have been explored for CSS depositions. We find that increasing the O-2 partial pressure increases the density of CdTe nucleation sites, thereby suppressing pinhole formation and likely reducing defects at the junction. However, increasing O-2 also tends to decrease grain size and faceting in the films. In addition to influencing the film properties, O-2 adversely affects the source material producing non-uniformly oxidized surfaces that reduce deposition rates and impair device uniformity and run-to-run reproducibility. We have also determined that the graphite susceptors typically used with CSS deposition convert a portion of the oxygen ambient to CO and CO2 during deposition. The amount converted is highly dependent on the type and age of the graphite susceptor used. The COx byproducts are not efficiently incorporated in the CdTe film and do not influence the deposition as oxygen does. We describe the effect of the changing oxygen partial pressure during the course of deposition. Finally, we report high-efficiency solar cells (12.8% AM1.5) made without oxygen.