Optimization of Graphene Films for Photovoltaic Application

  • Aamna R. Al Shehhi

Student thesis: Master's Thesis

Abstract

Graphene is a typical material with high mobility of approximately 106 cm2/V.s for a charge carrier concentration of 1012/cm2. The transparency of a monolayer of graphene is about 97-98% and because of the low sheet resistance and the high optical transparency, graphene films can be used in numerous applications such as transparent conductive electrodes. Moreover, Chemical Vapor Deposition (CVD) methods, along with the starting catalyst or metal foils allow fine tuning graphene films properties[1]. In this study, we are optimizing the combination of graphene films and contact metals for Photovoltaic (PV) cell top electrode functionality. One of the most critical extrinsic aspects for PV cell efficiency is the transparency of the top surface layer to transmit maximum light to the junction. While graphene has been used for these applications on organic PV cells (OPV), our target is inorganic PV cells. Indium Tin Oxide (ITO) is typically used in conventional PVs but because of issues such as scarcity of indium and difficulties of patterning as well as processing requirements, there have been ongoing attempts to replace this material. For example, carbon nanotubes and nanowires have been used, but because of the roughness of these films, we are considering graphene in our study[2]. The optimization of the graphene films` growth is achieved using Aixtron CVD reactor by means of various substrates with different surface finishes such as Japanese Small Grain Cu foils, Japanese Large Grain Cu foils, United State Cu foils and 4 6 Cu and Ni wafers. This is followed by the transfer of these films onto glass and/or SiO2 on Si substrates for optical assessment and conductivity measurements respectively. The latter is done using Kelvin and Hall mobility microstructures using a matrix of metal contacts, which will be part of the optimization process. Graphene properties are impacted during the transfer process; in which various solutions (Ammonium per Sulfate and Ferric Chloride) are used to assess the overall impact on the desired properties. Optical measurements of graphene transmission are performed using a solar cell spectral response measurements system in different wavelengths range and fused silica substrate was used as a baseline line for the measurements. For electrical characteristics we are developing a Kelvin structure to measure the conductivity of these films on fused silica substrate. As a last consequence, we conclude that Graphene grown on the Japanese Small Grain Cu substrate has the highest optical transmission with the lowest electrical resistivity out of all the samples grown on different substrates.
Date of AwardJul 2014
Original languageAmerican English
SupervisorIrfan Saadat (Supervisor)

Keywords

  • Chemical Vapor Deposition (CVD); Graphene Films; Indium Tin Oxide; Graphene Industrial Applications.

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