The impact of deliberate sodium incorporation on CuInSe2-based solar cells

Abstract

The beneficial effect of sodium incorporation in CuInSe2-based solar cells is systematically and quantitatively explored in this thesis. For the first time, a range of sodium concentrations that yield optimal device performance is presented. The primary cause of solar-cell performance improvement is shown to be grain-boundary passivation, but a secondary cause in some cases comes about through an alteration of the growth process. A model is presented based on these observations. The parameters most affected by the sodium concentration are open-circuit voltage, fill factor, and dopant density. CuInSe2 thin films and photovoltaic devices are analyzed to determine how much sodium is needed to improve device performance, and to uncover the effect of sodium at grain boundary surfaces and in the bulk material. A broad range of sodium concentrations in CuInSe2 from 0.001 to 0.15 at% Na is found to result in optimal device performance, which includes high efficiency, high fill factor, high dopant density, low series resistance, and good diode quality. Efficiencies improve by as much as 4% when sodium is added. Beyond this range, both device and material properties degrade. It is assumed that this range supplies sufficient sodium for well-passivated grain boundaries, but not so much as to produce secondary phases, which tend to reduce solar cell performance. Diode junction quality improves with the addition of sodium, as observed by improvements in fill factor and diode quality factor. Analysis of the trends observed in the change in diode parameters with increasing sodium concentration reveals that sodium affects both grain boundary and bulk properties. Secondary ion mass spectroscopy and scanning electron microscope data suggest that the sodium most likely resides at grain boundary surfaces. This analysis leads to a model that explains nearly all the changes that occur in the presence of sodium. A grain-boundary passivation model, including (a) the direct effects of sodium and (b) sodium as a catalyst to oxygen, explains the increase in open-circuit voltage, dopant density, and capacitance. It is likely that when sodium is co-evaporated, it (c) affects the bulk layer by layer by altering the growth process, without remaining in the bulk. Adding sodium during CuInSe2 growth can add flexibility in choosing substrate materials and can reduce constraints on CuInSe2 fabrication control, both important points for manufacturers of CuInSe2-based solar cells.