Influence of Alkali Doping on the Grain Boundaries in CIGS Studied by Electrical and Capacitive Methods

Although the impact of alkali doping on the properties of CIGS solar cells has been a subject of ongoing debate for many years now, most research still focuses simply on the comparison between alkali-rich and alkali-free devices, without looking into the impact of the different alkali concentrations. We are convinced that in order to better understand the pathways of change and the specific factors impacting it, one has to study them over a broad range of different samples with controlled properties.<br/><br/>In this work, we present a systematic study of CIGS solar cells and corresponding thin films doped with a controlled concentration of sodium and potassium spanning three orders of magnitude. In case of thin films conductivity measurements were made, and on solar cells Deep Level Capacitance Profiling (DLCP) was used to estimate the doping level. Characterization of both cells and thin films together allows for further insight into the specific pathways of alkali impact – i.e. through distinguishing between the effects taking place on the interface and in the absorber – be it in its bulk or at the grain boundaries. Moreover, as the alkalis are known to segregate primarily on grain boundaries, a series of samples prepared with different evaporation methods, resulting in different grain sizes, were studied. Among all of these sample series, the dependence of conductivity, barrier height and free hole concentration on the alkali concentration (Na or K) follow a similar trendline, suggesting a common cause. One of effects that could lead to such result is the alkali-induced passivation of defects at the grain boundaries. As such, the values of energy barriers from conductivity measurements and the hole concentrations obtained from DLCP were then confronted with the results of 1D SCAPS simulations. The influence of donor defects at the grain boundaries on the barrier height and the average concentration of holes were studied. The simulation results were consistent not only qualitatively but also with good quantitative accuracy with the conductivity barrier heights and the free hole concentration values measured by DLCP.<br/><br/>All of those results point to the beneficial alkali effect being primarily linked to the passivation of donor defects and subsequent reduction of the energy barriers at the grain boundaries.