Synergistic Roll-to-Roll Transfer and Doping of CVD-Graphene Using Parylene for Ambient-Stable and Ultra-Lightweight Photovoltaics

Large area graphene grown by chemical vapor deposition (CVD) has been the main focus of many researchers due to its vast areas of applications such as sensing or photovoltaics. Addressing the main challenges in transfer techniques such as Roll-to-Roll (R2R) process is a critical step for scaling up and commercialization of graphene. In this work, we employ a R2R transfer technique and improve the electrical properties of transferred graphene on flexible substrates using parylene as an interfacial layer. We deposit a layer of parylene on graphene/copper (Cu) foils grown by CVD and laminate them onto EVA/PET. Then, the samples are delaminated from the Cu using an electrochemical transfer process, resulting in flexible and conductive substrates with sheet resistance of below 300 Ω/sq, which is significantly better (4-fold) than the sample transferred by R2R without parylene (1200 Ω/sq). By scanning over different types of parylene (N, C, and D) here, we find that parylene C and D are better candidates than parylene N, given the higher conductivity measured on the as-transferred graphene samples. Our characterization results indicate that parylene C and D dope graphene due to the presence of chlorine atoms in their structure, resulting in higher carrier density and thus lower sheet resistance.