Na incorporation and diffusion in CuIn_1-xGa_xSe₂

CuIn_1-xGa_xSe₂ based solar cells deposited by vapor phase techniques show significant improvement in performance when Na is present at the growth surface. Evidence is just becoming available concerning how the Na participates in the change in device performance. This work reports on the distribution of Na in CIGS in as grown device-type films, in Na-free films implanted with Na and the effects of annealing of these layers. CIGS thin films were deposited by four-source evaporation and analyzed by secondary ion mass spectroscopy (SIMS) and X-ray photoelectron spectroscopy. Samples were ion implanted and analyzed before and after annealing at 420°C. Na segregates rapidly out of CIGS at normal physical vapor deposition temperatures. No Na diffusion at room temperature was observed in samples after six months. The segregation drives Na both to the surface of polycrystalline layers and into the Mo substrate. The segregation ratio for the CIGS/Mo interface is approx. 10 at 400°C and approx. 100 at 600°C. Small Na-rich regions can also occur inside CIGS layers. In spite of these observations, Na leads to a strong (112) preferred orientation of CIGS and apparently improves the performance of devices.