Acknowledgements
This work was supported in part by grants from the
National Science Foundation (Nos. NSF-NIRT-ECS-
0609064 and NSF-EPCOR EPS-0447479) and by grants
from the Kentucky Science and Engineering Foundation
(Nos. KSEF–148-502-02-27 and KSEF-148-502-03-68).
The authors would like to thank PV measurements,
Inc for quantum efficiency measurement