Conductive Polymer Work Function Changes due to Residual Water: Impact of Temperature‐Dependent Dielectric Constant

Abstract

Tremendous progress has been made in improving the performance and functionality of organic electronic devices. To a significant extent, this was possible by tuning the optical properties, electronic energy levels, and, foremost, charge transport properties of conductive polymers, as these are mostly employed as electrical contact layers. It should be noted that most polymers used in this field are semiconductors, and become sufficiently conductive only upon doping. Among the wide range of available polymers, polythiophenes have attracted attention for their moderate conductivity, high transparency, and air stability, which has enabled their implementation in various electronic and optoelectronic devices. [5-7] Molecular doping of polythiophenes, in which a small amount of dopant is added to the host polymer, has proven to be a successful approach to increase the density of mobile charge carriers and thus conductivity above 1 S cm −1. [8-14] A key challenge to the successful application of molecularly doped polymers lies in their relatively poor thermal stability, which ultimately leads to dopant diffusion and desorption at elevated temperatures. [15-17] A conductive polymer from the thiophene family that presently dominates in device applications is p-doped poly(ethylenedioxythiophene) (PEDOT). The breakthrough of Solution-processed conducting polymer thin films are key components in organic and flexible electronic and optoelectronic devices. An archetypal conducting polymer is poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS), which can feature a high work function and thus helps achieving Ohmic contacts for holes with many semiconductors. However, it is known that residual water in PEDOT:PSS films lowers their work function and is detrimental for device lifetime. Our photoelectron spectroscopy experiments reveal that the work function of PEDOT:PSS films containing residual water shows the same trend as function of temperature as does the dielectric constant (ε) of water, in the range between 25 °C and-100 °C. Consistently, it is found from impedance spectroscopy measurements that ε of residual water containing PEDOT:PSS films increases with decreasing temperature. After removal of residual water from PEDOT:PSS films by annealing in ultrahigh vacuum, the work function of thin films is much higher than before (reaching 6.1 eV) and, notably, independent of temperature. In contrast, no indication is found that the presence of residual water has any impact on the electrical conductivity. For a nominally water-free molecularly doped conjugated donor/acceptor copolymer films, a correlation between sample work function and temperature similar to those seen for PEDOT:PSS is found.

Study of light-induced degradation of polymer: fullerene solar cells

“…Several encapsulation studies have been reported which provide a shelf-life of the devices extended up to several thousand hours. 60–72 Such is the case for P3HT:PC 71 BM devices with MoO 3 as HTL. Table S5 (ESI†) shows the photovoltaic parameters for encapsulated P3HT:PC 71 BM/MoO 3 devices.…”

“…Several encapsulation studies have been reported which provide a shelflife of the devices extended up to several thousand hours. [60][61][62][63][64][65][66][67][68][69][70][71][72] Fig. 6…”

Chemically revised conducting polymers with inflammation resistance for intimate bioelectronic electrocoupling

“…The interfacial dipoles between metals and p-type CPs favor decreasing the potential barrier between them [ 117 , 122 ]. Thus, p-type CPs can help achieve ohmic contacts with high work function metals [ 119 , 123 ]. It has also been observed that the total interface dipole can be tuned by adjusting the magnitude of the molecular dipole moments.…”

A Facile and Effective Ozone Exposure Method for Wettability and Energy-Level Tuning of Hole-Transporting Layers in Lead-Free Tin Perovskite Solar Cells

“…PEDOT:PSS work function control is another critical factor to be considered, especially for use as a HTL . Introduction of MoO 3 , and other additives − are some of the well-known methods of controlling the work function and thereby enhancing the charge selectivity. Among these methods, a facile ultraviolet (UV)–ozone treatment − of PEDOT:PSS has proven effective in improving the performance of devices.…”