Alkali elements are the “salt in the soup” in the manufacture of CIGS thin-film solar cells. Even the smallest amounts of sodium (0.3 per mill) help to increase efficiency by up to 50%. For a long time, sodium was regarded as the most important alkali element used to optimise efficiency of polycrystalline CIGS solar cells. In 2013, however, it was discovered that the heavier alkali element potassium also leads to a significant increase in efficiency. The last efficiency records of up to 22.6% were achieved by ZSW with the aid of a rubidium fluoride post-treatment. Here it was observed that the heavier alkali element rubidium displaces the lighter sodium from the CIGS layer.
In order to better understand this process, the distribution of the alkali metals was measured with three-dimensional spatial resolution (with 3D ToF-SIMS). The measurement (see fig. below) shows that rubidium is highly concentrated at the grain boundaries, but is also present in the volume of the grains up to a concentration of 16 ppm (parts per million), which corresponds to the usual doping concentrations of semiconductors. Diffusion measurements showed that rubidium diffuses more strongly along the grain boundaries when sodium was already present in the layer. The heavier rubidium mainly displaces the lighter sodium from the grain boundaries and is thus thought to better passivate the grain boundaries (see figure below right). The latest analytical methods, such as mass spectrometric depth profiling (ToF-SIMS), are extremely helpful in better understanding small processes microscopically.