Influence of perfluorinated ionomer in PEDOT:PSS on the rectification and degradation of organic photovoltaic cells

Abstract

Poly (3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is widely used to build optoelectronic devices. However, as a hygroscopic waterbased acidic material, it brings major concerns for stability and degradation, resulting in an intense effort to replace it in organic photovoltaic (OPV) devices. In this work, we focus on the perfluorinated ionomer (PFI) polymeric additive to PEDOT:PSS. We demonstrate that it can reduce the relative amplitude of OPV device burn-in, and find two distinct regimes of influence. At low concentrations there is a subtle effect on wetting and work function, for instance, with a detrimental impact on the device characteristics, and above a threshold it changes the electronic and device properties. The abrupt threshold in the conducting polymer occurs for PFI concentrations greater than or equal to the PSS concentration and was revealed by monitoring variations in transmission, topography, work-function, wettability and OPV device characteristics. Below this PFI concentration threshold, the power conversion efficiency (PCE) of OPVs based on poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) are impaired largely by low fill-factors due to poor charge extraction. Above the PFI concentration threshold, we recover the PCE before it is improved beyond the pristine PEDOT:PSS layer based OPV devices. Supplementary to the performance enhancement, PFI improves OPV device stability and lifetime. Our degradation study leads to the conclusion that PFI prevents water from diffusing to and from the hygrosopic PEDOT:PSS layer, which slows down the deterioration of the PEDOT:PSS layer and the aluminum electrode. These findings reveal mechanisms and opportunities that should be taken into consideration when developing components to inhibit OPV degradation. 35-38 whilst oxidation, delamination and interfacial effects may occur at the electrodes. 39-45 Metal ion diffusion from the electrodes and changes in morphology have also been reported in both the active and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layers. C. T. Howells et al. 2/14organic optoelectronics between indium tin oxide (ITO) anodes and active layers. [54][55][56][57] With its metal properties, 58-62 PEDOT:PSS and similar conducting layers are part of the contact and are sometimes called hole injection layers in light emitting devices. By analogy, in this solar cell work, PEDOT:PSS is then referred to as a hole extraction layer (HEL) rather than a hole transport layer or interlayer. The large ionisation potential promotes an Ohmic contact and improves the built-in electric field, whilst high conductivity and transparency ensure minimal resistive and optical losses, respectively. 53,63,64 The HEL also helps to prevent metal ion diffusion from the ITO into the active layer and the cathode from short-circuiting the anode. 50 The PSS is a water-soluble polyelectrolyte that serves as a charge balancing dopant during the polymerisation of EDOT monomers. It oxidises and stabi...

Highly Efficient Nonfullerene Organic Photovoltaic Devices with 10% Power Conversion Efficiency Enabled by a Fine‐Tuned and Solution‐Processed Hole‐Transporting Layer

“…[ 14,17,30 ] Currently, the best polymer donor reported for collocating NFAs requires an extremely deep highest occupied molecular orbital ( HOMO ) level that is more negative than −5.4 eV to maximize the built‐in voltage of devices. [ 9–11,16,17,31 ] The low HOMO level of the active layer therefore forms a massive energy barrier between the HOMO of the electron donor and the work function (WF) of PEDOT:PSS (WF ≈ −5.0 eV is mostly reported in the literature), [ 32–34 ] which results in the poor electrical performance of inverted devices. Nevertheless, only a few studies have reported an inverted OPV device using an NF‐based BHJ layer with an alternative solution‐processed HTL for industrial module production.…”

Enhanced Thermoelectric Performance and Lifetime in Acid-Doped PEDOT:PSS Films Via Work Function Modification

“…19,[23][24][25][26][27] Although p-type organic semiconductors are generally regarded as air-stable, there are a few degradation pathways such as water absorption and diffusion of molecules with small sizes or low boiling point, which leads to the creation of trap states or morphological changes in the microstructure of the OSC. [28][29][30] Formic acid (FA) is a common dopant for PEDOT:PSS, which makes a clear example of such a mechanism. Its low boiling point leads to evaporation of the FA and thus losses in electrical conductivity.…”

Recent Advances of Synthesis, Properties, Film Fabrication Methods, Modifications of Poly(3,4‐ethylenedioxythiophene), and Applications in Solution‐Processed Photovoltaics

“…The W F of PEDOT:PSS mixed with PFI increased from 4.71 to 5.41 eV, which leads to the subtle increase in V OC from 0.59 to 0.62 V for P3HT:PC 61 BM based OSCs. [255] For PerSCs, PFI-PEDOT:PSS displayed improved V OC of 0.05-0.15 V. [256,257] However, it should be noted that the increased work function does not necessarily improve the V OC . Interaction between HTL and active layer is important for charge collection.…”