Py6S v1.0 was described in Wilson (2012). You must cite this paper if you use Py6S as part of any research which you then publish.

Wilson, R. T., 2012, Py6S: A Python interface to the 6S radiative transfer model, Computers and Geosciences, 51, p166-171, PDF

A list of papers citing Py6S is available here.