Rachel M. Howden

Polymer‐Free Near‐Infrared Photovoltaics with Single Chirality (6,5) Semiconducting Carbon Nanotube Active Layers

We demonstrate a polymer-free carbon-based photovoltaic device that relies on exciton dissociation at the SWNT/C(60) interface, as shown in the figure. Through the construction of a carbon-based photovoltaic completely free of polymeric active or transport layers, we show both the feasibility of this novel device as well as inform the mechanisms for inefficiencies in SWNTs and carbon based solar cells.

Organic Solar Cells with Graphene Electrodes and Vapor Printed Poly(3,4-ethylenedioxythiophene) as the Hole Transporting Layers

For the successful integration of graphene as a transparent conducting electrode in organic solar cells, proper energy level alignment at the interface between the graphene and the adjacent organic layer is critical. The role of a hole transporting layer (HTL) thus becomes more significant due to the generally lower work function of graphene compared to ITO. A commonly used HTL material with ITO anodes is poly(3,4-ethylenedioxythiophene) (PEDOT) with poly(styrenesulfonate) (PSS) as the solid-state dopant. However, graphenes hydrophobic surface renders uniform coverage of PEDOT:PSS (aqueous solution) by spin-casting very challenging. Here, we introduce a novel, yet simple, vapor printing method for creating patterned HTL PEDOT layers directly onto the graphene surface. Vapor printing represents the implementation of shadow masking in combination with oxidative chemical vapor deposition (oCVD). The oCVD method was developed for the formation of blanket (i.e., unpatterened) layers of pure PEDOT (i.e., no PSS) with systematically variable work function. In the unmasked regions, vapor printing produces complete, uniform, smooth layers of pure PEDOT over graphene. Graphene electrodes were synthesized under low-pressure chemical vapor deposition (LPCVD) using a copper catalyst. The use of another electron donor material, tetraphenyldibenzoperiflanthene, instead of copper phthalocyanine in the organic solar cells also improves the power conversion efficiency. With the vapor printed HTL, the devices using graphene electrodes yield comparable performances to the ITO reference devices (η(p,LPCVD) = 3.01%, and η(p,ITO) = 3.20%).

oCVD poly(3,4-ethylenedioxythiophene) conductivity and lifetime enhancement via acid rinse dopant exchange

Reduced sheet resistance and longer film stability of oCVD (oxidative chemical vapour deposition) PEDOT films were achieved by including a post-process acid rinse step in the production of the thin films. PEDOT films were rinsed in multiple concentrations of hydrobromic acid, sulfuric acid, and hydrochloric acid to test the effect of acid rinsing on sheet resistance, doping concentration, chemical composition, optical transmittance, and film morphology. XPS, FTIR, Raman spectroscopy, and XRD measurements were taken to determine the morphology and composition of the rinsed films. On average, rinsing films in HCl, HBr, and H2SO4 produced conductivity increases of 37%, 135%, and 117%. The dc to optical conductivity ratio, σdc/σop, was increased to 6, 12, and 10, for HCl, HBr, and H2SO4 rinsed films respectively as compared to σdc/σop = 4 for MeOH rinsed films. This study found evidence of dopant exchange within the films facilitated by the acid rinsing step, as well as complete removal of residual iron chloride oxidant. The acid rinse step also resulted in improved film conductivity stability at elevated temperatures. The XRD measurements in particular show signs of semi crystallinity in the PEDOT film after acid rinsing in comparison to an amorphous structure observed before this step. In this study, acid rinsing applied as a post-process step alters thin PEDOT films in ways that enhance their ability to function as electrode materials in photovoltaic devices.