May be carbon conductive paste.

There is some more information:

"Screen Printed Thin Film CdS/CdTe Solar Cell"

Nobuo Nakayama, Hitoshi Matsumoto, Akihiko Nakano, Seiji Ikegami, Hiroshi Uda1 and Toshio Yamashita

Abstract: Thin film CdS/CdTe solar cells with an efficiency of 6.3 % have been prepared on a borosilicate glass substrate of 4s×4 cm2 by successively repeating screen printing and heating (sintering) of each paste of CdS, CdTe and C. The CdS paste consists of CdS, CdCl2, GaCl2 and propylene glycol (PG). The CdTe paste contains CdCl2 and PG, and the C paste contains PG and a small amount of acceptor impurity. During the heating of C paste, an n•CdS/p•CdTe heterojunction is formed. In the most efficient cell, the peak of electron voltaic effect exists within 1 µm of the CdTe side from the CdS/CdTe metallurgical boundary. The C electrode cell is more stable than the previous Cu2Te electrode cell for an accelerated life test.

URL: http://jjap.jsap.jp/link?JJAP/19/703/

DOI: 10.1143/JJAP.19.703

But nevertheless it is not clear what is "C paste"?

May be that is carbon paste only. They might have used it as a conducting electrode. Go through with the attached paper.

@Nagaraja: Thank you for the paper. It is very useful. Really C-paste means Carbon paste that forms carbon electrode on the back side of the cell.