3.2 Contact resistance of a-Si:H(i)/MoOx/ICO/Ag stack
The contact resistance related to the carrier selection transport layer has a significant impact on carrier collection efficiency and solar cell performance. Therefore, investigation of contact resistances of the SCH solar cells, especially the MoOx HTL side (back side here), is imperative. The main methods for contact resistance measurement include the rectangular transmission line method (TLM)[31], the Cox and Strack method (CSM)[32], and the extended CSM (ECSM)[33]. The electrode structure for CSM and ECSM are the same and it is employed here for contact resistance measurement as shown in Fig. 3 (a). The a-Si:H(i), a-Si:H(p+), TCO and Ag electrodes were deposited successively on the rear side of a p-type silicon wafer to form an ohmic contact. Front side of the p-type silicon wafer was deposited with a-Si:H(i), MoOx, TCO, and Ag in sequence with a metal mask. The purpose of using p-type Si wafer is to reduce the junction effect.
The I-V curves corresponding to c-Si(p)/a-Si:H(i)/MoOx/ICO/Ag contacts with different electrode diameters are plotted in Fig. 3(b). The obvious rectification characteristic indicates that ohmic contacts are not formed. The CSM method is not appropriate for the device featuring junction resistance. Therefore, the ECSM method which can eliminate the impact of rectification effect is used to analyze the contact resistance of the c-Si(p)/a-Si:H(i)/MoOx/ICO/Ag stacks.