Polymers via chemical vapor deposition and their application to organic photovoltaics

Abstract

There is emerging interest in the ability to fabricate organic photovoltaics (OPVs) on flexible, lightweight substrates, which could lower the cost of installation and enable new form factors for deployment. However, substrate and material choices are often limited by compatibility with common processing agents such as heat and solvents. Here, we explore oxidative chemical vapor deposition (oCVD) as an all-dry, vacuum-based method for processing conjugated polymer device layers for organic photovoltaics. The entire process occurs via the vapor phase and under conditions of low temperature and low energy input, enabling conformal coverage and film deposition on delicate substrates (e.g., papers, plastics, and textiles). Moreover, oCVD offers the well-cited benefits of vacuum processing, including parallel and sequential deposition, well-defined thickness control and uniformity, and inline integration with other standard vacuum processes (e.g., vacuum thermal evaporation). Conductive poly(3,4-ethylenedioxythiophene) (PEDOT) layers deposited by oCVD are explored for a variety of roles within vacuum-processed OPVs, including as a transparent anode, as an anode buffer layer on ITO transparent electrodes, and as a cathode buffer layer in electrically inverted devices. By using in situ shadow masking, the oCVD PEDOT electrodes are vapor-patterned over large areas to monolithically fabricate OPV circuits directly on a variety of common paper substrates. The resulting paper photovoltaic arrays power common electronic displays in ambient indoor lighting and can be tortuously flexed and folded without loss of function. Further, optically inverting the device structure (by positioning the oCVD PEDOT electrode on top of the device and illuminating from above) improves performance with non-transparent substrates; power conversion efficiencies just under 3% are demonstrated, including up to 2% on common paper substrates. We also show applicability of an oCVD semiconductor, unsubstituted polythiophene, as the photoactive electron donor in all-vacuum-processed polymer heterojunction OPVs.