Photoluminescence properties of sodium incorporated in CuInSe2 thin films

CuInSe2 films with Cu/In ratios of m = 0.83-0.99 have been deposited on glass substrates by the co-evaporation method using a bi-layer process. The effects of sodium in these films have been investigated by comparing the electrical and optical properties of the films with and without the Na2Se incorporation. Two donor-acceptor pair emissions, [D, A](alpha) at 0.89-0.94 eV and [D, A](beta) at 0.84-0.87 eV, were typically observed in the photoluminescence spectra. The relative intensity of [D, A](alpha) to [D, A](beta) was found to depend strongly on both the Cu/In ratio and Na incorporation. For the films with m = 0.94, [D, A](alpha) was predominantly observed regardless of Na incorporation. For m less than or equal to 0.9, [D,A](beta) became dominant for the films without Na, while [D, A](alpha) remained as the dominant emission for the films with Na. A correlation between the prominence of [D, A](alpha) and resistivities below 2 x 10(3) Omega . cm was found, suggesting a reduced compensation by suppressed Se vacancy donors due to the presence of Na.