The theory is explained in a self-contained fashion in:

• Agapito, L. A., Cheng, H. P. Journal of Physical Chemistry C 2007, Issue 37 September

For additional background refer to:

• Krstic, P. S.; Zhang, X. G.; Butler, W. H. Physical Review B 2002, 66.
• Sanvito, S.; Lambert, C. J.; Jefferson, J. H.; Bratkovsky, A. M. Physical Review B 1999, 59, 11936.