Materials for Transparent Electrodes: From Metal Oxides to Organic Alternatives

Abstract

Nowadays, opto-electronic devices, such as displays, are omnipresent in our daily life. A crucial component of these devices is a transparent electrode, which allows the in-and out-coupling of light. With the goal of optimizing the electrode characteristics and improving device efficiencies, many approaches for the fabrication of thin, transparent conducting films have been studied. This review gives an overview of the different material classes which have been 2 used as transparent electrodes, ranging from metal oxides, such as Indium Tin Oxide, metal and carbonaceous nanostructures, to conducting polymers and composites. For every material class a brief description of the fundamental principles, processing routes and the latest achievements is given. Furthermore, the different electrodes are compared regarding their opto-electronic performance, flexibility and surface roughness. Ultimately, advantages and drawbacks of the respective electrodes are discussed. This critical comparison of fundamentally different transparent conducting materials allows, on one hand, to make a sensible choice of electrode for specific applications, and, on the other hand, to point out scientific challenges that must still be addressed.

Solution‐Processed Semitransparent Organic Photovoltaics: From Molecular Design to Device Performance

“…Although conventional opaque solar cells exhibit high efficiencies, they require dedicated space and additional installation efforts in the form of metal frame constructions on the roof or façade; a general disadvantage within the BIPV context ( Figure ). The research topic of semitransparent organic solar cells (OSCs) addresses the aforementioned issues by offering solutions, such as the implementation of flexible semitransparent OSCs into glass windows and roofs of new or existing buildings, greenhouses, vehicles, or mobile electronic devices, thereby transforming them into energy harvesting multifunctional units . Additionally, OSCs can be tuned to absorb specific frequencies such as UV, vis, or near‐IR and they can be easily integrated with other devices, such as to drive electrochromic devices that modulate the level of interior light …”

The damaging effects of the acidity in PEDOT:PSS on semiconductor device performance and solutions based on non-acidic alternatives

“…As solution-processing is one of the main advantages of using PEDOT:PSS, this article presents analogues or alternatives that can be solution-processed. If one is looking for a review on how to enhance the conductivity of PEDOT:PSS then this has been nicely summarised by Xu et al, 11 whilst a comprehensive overview of transparent electrode materials in general can be found in a recent article by Cloutet et al 12 Many of the approaches taken to enhance the conductivity of PEDOT:PSS involve treatment with acids such as formic acid, 13 sulfuric acid, 14 phosphoric acid 15 or the addition of high boiling point solvents, such as DMSO 16 or NMP. 17 The strong acidity of high conductivity grades of PEDOT:PSS could be problematic for stability or limit the choice of compatible active materials, yet as these developments are relatively recent the literature has not reported these obvious concerns.…”

Controlled formation of Schottky diodes on n-doped ZnO layers by deposition of p-conductive polymer layers with oxidative chemical vapor deposition

“…To overcome this challenge, hole conductive organic materials can be used to form diode-like heterostructures with n-doped ZnO layers. Advantages of conducting polymers comprise their chemical and mechanical stability in combination with a high degree of flexibility and processability at ambient temperatures [3]. One very promising polymer is poly(3,4-ethylenedioxythiophene) (PEDOT), which is holeconductive, has reported conductivities>6000 S cm −1 [4], is largely transparent in the visible spectral range and highly flexible [5].…”

“…One very promising polymer is poly(3,4-ethylenedioxythiophene) (PEDOT), which is holeconductive, has reported conductivities>6000 S cm −1 [4], is largely transparent in the visible spectral range and highly flexible [5]. Among many other applications, PEDOT is already used in transparent electrodes [3], in energy conversion and storage devices [6] or for thermoelectrics [7].…”

“…By adding an anionic polyelectrolyte to stabilize the otherwise insoluble PEDOT in aqueous environments, the water-soluble PEDOT:PSS (PSS: polystyrene sulfonate) can be obtained. The additive mostly used is the insulating PSS which leads to an energy barrier for charge transport in the film [3]. In addition, due to the high acidity of PEDOT:PSS, strong etching effects are expected on many surfaces like ZnO in acidic environments.…”