Polymer design to promote low work function surfaces in organic electronics
Graphical abstract
Introduction
State-of-the-art organic electronic and optoelectronic devices, e.g. OFETs, OLEDs, and OPVs, are complex multilayered structures [[1], [2], [3], [4], [5], [6]]. Synthetic tailoring of the chemical structure of organic semiconductors in device active layers offers a distinct advantage in the rational development of materials for organic electronics (OEs) and efficient device performance [[7], [8], [9], [10]]. Supported by decades of fundamental research, organic semiconductors with remarkable properties have been developed. For examples, ambipolar charge carrier mobilities in excess of 10 cm2/V·s and large on/off ratios, have been reported for OFETs [[11], [12], [13]]. High power efficiencies were demonstrated for white and monochromatic OLEDs [[14], [15], [16]]. Moreover, the PCEs of single-junction OPVs and in tandem device architectures now exceed 16 % [17] and 17 % [18], respectively. Although such advances bring the concepts of organic optoelectronic devices closer to commercial viability, the unique competitive advantage of organic materials is found in the potential cost-effectiveness of solution-based processing of high-throughput, large-area devices. Emerging application areas will exploit the light weight, mechanical flexibility, and semi-transparency of OE devices [19].
The interlayer materials that are placed between the organic active layer and charge-collecting electrodes are critical elements in the operation of OE and optoelectronic devices [20,21]. Interlayers reduce the Schottky barrier height, improving charge injection in OFETs and OLEDs [22,23]. In OPVs, the interlayers increase the Vbi in the device active layer, ensuring efficient extraction of photo-generated charge carriers [24]. Inorganic interlayers that require vacuum deposition or high-temperature annealing as device fabrication steps limit scalability and compatibility with high-throughput, energy-efficient, large-area solvent-processing, especially on polymeric substrates which have desirable flexible properties [25,26]. Hence, replacing transition metal oxides (ZnO, TiO2, and MoO3), metal salts (LiF) and low-work-function metals (Ca, Ba, and Al) to realize “all-organic” device architectures is highly desirable [27]. As such, interface engineering, i.e. the development of interlayer materials and their integration into devices, is now recognized as a critically important research area for realizing OEs as a practical technology [28,29].
Solution-processed organic interlayer materials comprise a growing subject of fundamental academic research and practical development, since they allow synthetic tailoring of properties and are compatible with device fabrication approaches, such as slot-die coating, ink-jet printing, and spray-coating [30]. These interlayer materials, that modify the electrode work function, including polyelectrolytes, polymer zwitterions, and neutral fluorocarbon-, ethylene glycol-, and amine-functionalized materials, have been developed for OLEDs, OFETs and OPVs [20]. Not only can they replace the most efficient inorganic interlayer materials, but they can also lead to superior device efficiency and stability. The use of solution-processed organic interlayers in multilayered device structures is enabled by their solubility in solvents that are orthogonal to those that solubilize the organic active layers [31,32]. Although the electronic utility is derived mainly from modification of the electrode work function, these materials are notably multifunctional, with rheological, morphological, electronic, and optical properties that may all be optimized to advantage of device performance.
In this perspective, we discuss the mechanism of metal work function reduction using polymer interlayers and describe recent studies on the mechanism of surface modification based on several material systems, including CPEs, polymeric amines and CPZs. We then summarize comprehensive efforts to develop polymer modification platforms and present the understanding of multiple functionalities of these materials as cathode modification interlayers in BHJ OPVs, OLEDs and OFETs. Our intent is to bridge the knowledge and terminology gaps among materials designers, device engineers, and condensed matter physicists working in this exciting field.
Section snippets
The mechanism of work function reduction
Tremendous research efforts have been devoted to understanding the mechanisms of energy level alignment at the interface where organic semiconductors contact metal electrodes. Several semi-empirical models were proposed to describe such interfaces, including the charge-transfer model and the interfacial doping model [[33], [34], [35], [36], [37]]. However, none fully explains the experimental observations at such interfaces. So, we simply provide a discussion of the mechanisms associated with
Conjugated polyelectrolytes (CPEs)
CPEs are polymers comprised of a π-conjugated backbone with pendant side chains bearing ionic functional groups (Fig. 4). Numerous polyfluorene-based CPEs were found to increase the VOC of solar cell devices when integrated as interlayers between the active layer and metal cathode [26]. Subsequently, polythiophene-based CPEs were developed as cathode interlayers to improve the efficiency of polymer solar cells containing a typical photoactive layer of PCDTBT: PC71BM [45]. Notably, the CPEs
Non-conjugated polymer interlayers
Polyethylene oxide (PEO, Fig. 9) represents the first studied example of a non-conjugated polymer for ITO modification of inverted OSCs. A thin layer of PEO reduced the work function of ITO by up to 0.5 eV as indicated by UPS, promoting electron extraction from the photoactive layer [70]. Later, aliphatic amine-containing polymer interlayers, PEI and PEIE (Fig. 9), were demonstrated to universally reduce the work function of numerous conductive electrodes including metals, metal oxides,
Summary and outlook
Utilization of organic materials in electronic and optoelectronic devices promises to broaden the range of device applications due to low-cost, high-throughput, large-area fabrication of lightweight and mechanically flexible device architectures. However, to take advantage of the unique properties of organic semiconductors, the scalability and compatibility of device fabrication steps require further improvement. Replacement of inorganic interlayers that require vacuum-based deposition or
CRediT authorship contribution statement
Yao Wu: Writing - original draft. Yao Liu: Supervision, Conceptualization, Writing - original draft. Todd Emrick: Supervision, Writing - review & editing. Thomas P. Russell: Supervision, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Y.L. acknowledges the support from the National Natural Science Foundation of China (NSFC) (21875018). Y.W. and T.P.R. were supported by the Office of Naval Research, Materials Division, under contract N00014-17-1-2241. T.E. acknowledges the support from the National Science Foundation (NSF-CHE-1904660).
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