Alignment of poly(3,4-ethylenedioxythiophene) polymer chains in photovoltaic cells by ultraviolet irradiation

Abstract

The conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is a promising candidate for developing flexible electronics. The conductivity of this polymer can be increased via simple ultraviolet (UV) irradiation. This process can lead to enhanced performance and stability of organic photovoltaic (OPV) cells. We used micro-Raman spectroscopy, Kelvin probe force microscopy (KPFM), and quantum chemical calculations to map the energy band diagram and the electronic properties of PEDOT:PSS films irradiated with UV at various times. These procedures can explain how the observed changes in the macroscopic electronic properties of OPV cells arose from local changes in the carrier transport at the organic/organic and metal/organic interfaces. Micro-Raman spectroscopy indicates the effect of UV irradiation on PEDOT chains, which suggests a conformation change of PEDOT chains in the thin film. When UV irradiates the PEDOT:PSS film, the quinoid structure becomes the dominant conformation structure in the treated film. This change originates from the interaction between the dipole moment within the conjugate PEDOT chains and the electric field of the UV. Combining experimental results and theoretical calculations, we propose a gradient energy band for UV-treated PEDOT:PSS film that can reduce the barrier of the organic/organic heterojunction and can induce an interface dipole field at the organic/metal interface that simultaneously results in easy carrier extraction from PEDOT:PSS to metal and the enhancement of carrier transport from the active material to PEDOT:PSS.