Marked improvement in the stability of small molecule organic photovoltaics by interfacial modification using self-assembled monolayers to prevent indium diffusion into the active layer

This work describes a simple process to improve the stability of small molecule organic photovoltaics (OPVs) that use poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the hole transport layer (HTL), without reducing the power conversion efficiency (PCE). We investigated the effect of self-assembled monolayers (SAMs) on the degradation of electrical properties caused by the diffusion of indium through the matrix commonly used in OPV devices. X-ray photoelectron spectroscopy depth profiles suggested that the acidic PEDOT:PSS etched the indium from the indium tin oxide (ITO) electrode, which resulted in indium diffusion into and chemical interaction with the active layer. Such behavior was responsible for the deterioration of the device. By inserting a self-assembled monolayer (SAM) with carboxylic acid functional groups at the interface between the ITO and PEDOT:PSS layers, such diffusion could be significantly suppressed. The PCE of the pristine OPV diminished to zero at 49 days post-fabrication under ambient conditions. However, the PCE of the SAM-modified device was reduced by only 30% over the same period, and only to 48% after 98 days. Moreover, the SAMs improved the interfacial electrical properties between the ITO and PEDOT:PSS layers, which maintained the PCE of the device. Our findings suggest a simple approach to improve the stability of OPVs that utilize PEDOT:PSS as the HTL.