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UBC Theses and Dissertations

Chemical vapor deposited single layer graphene as transparent electrodes for flexible photovoltaic devices Jiang, Zenan


Graphene has attracted intensive attention for various electronic applications in the past decades given its unique properties. The synthesis of graphene by chemical vapor deposition (CVD) on copper foil provides the opportunity to deliver large-area, high quality, and continuous graphene films. The metal foil can be removed with a wet etching process. The transferred CVD graphene films can be integrated into existing semiconductor device manufacturing platforms, or into low-cost roll-to-roll manufacturing of flexible electronics. Since graphene is a two-dimensional material, the optical, mechanical, and electrical properties can easily be altered with surface modification. Copper etchants used in graphene transfer process can lead to films with different levels of doping and mechanical strength. The topology and temperature dependent electrical properties of transferred graphene using three different etchants were investigated. All of the graphene samples demonstrate a doping level above 10¹³ cm-³. The graphene films prepared with cupric sulfate solution presents the most uniform and continuous layer, with the least density of defects. Metallic and organic residues, defects and grain boundaries, as well as intercalated water molecules, attribute to the variation in conductivity and permittivity of the films. By coating the films with charge selective materials, graphene sheets with improved sheet resistance and transparency of about 90% were fabricated. The hole-selective transparent conductors show about 50% reduction in resistivity. All the samples demonstrate high stability with repeated bending of over 800 cycles. Organic photovoltaic (OPV) devices using the hole-selective graphene transparent conductors as electrodes were fabricated on plastic substrates. Less than 5% fluctuation in power conversion efficiency (PCE) was noticed when the devices were bent up to 130 degrees. As an extension of this work, the photovoltaic characteristics of inverted OPV devices fabricated with AlxZn(1-x)O as an electron transport layer with Al fraction of up to 11% were reported. The light-soaking effect can be eliminated by using more than 4% of Al doping. All devices demonstrate PCE over 3.4% with air-stability of over 150 days. The light-soaking mechanism is investigated by employing a numerical simulation on the devices.

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