Polymer design to promote low work function surfaces in organic electronics

https://doi.org/10.1016/j.progpolymsci.2020.101222Get rights and content

Abstract

Organic electronics (OEs) are advantageous for their mechanical flexibility, light weight, and easy solution processability over large areas, all ideal characteristics for next generation portable, cost-effective flexible electronics. The interlayer materials present between the organic active layer and electrodes are critical elements for promoting efficient device operation. Recently, solution-processable polymers have been investigated extensively as efficient electrode interlayer materials for applications in OE devices. We begin this article by describing energy level alignment mechanisms associated with contact of the polymer interlayer and metal electrode. We then overview the latest progress where polymer interlayer materials are used in efficient OE device fabrication. We discuss critical properties of these polymer interlayers, including (1) polymer interactions with the metal surface; (2) the effect of energy levels and electronic structures of these polymers on interfacial modification; and (3) the effect of charge transport and conductivity of the polymer interlayer on device operation. Our motivation is to describe our current understanding, recent progress, and outstanding issues of polymer interlayers in OEs, and propose future potential directions and opportunities.

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).

References (79)

  • Y.H. Zhou et al.

    Inverted and transparent polymer solar cells prepared with vacuum-free processing

    Sol Energy Mater Sol Cells

    (2009)
  • C. Wang et al.

    Semiconducting pi-conjugated systems in field-effect transistors: a material odyssey of organic electronics

    Chem Rev

    (2012)
  • Y. Huang et al.

    Bulk heterojunction solar cells: morphology and performance relationships

    Chem Rev

    (2014)
  • L. Lu et al.

    Recent advances in bulk heterojunction polymer solar cells

    Chem Rev

    (2015)
  • L. Dou et al.

    Low-bandgap near-IR conjugated polymers/molecules for organic electronics

    Chem Rev

    (2015)
  • D. Di Carlo Rasi et al.

    Advances in solution-processed multijunction organic solar cells

    Adv Mater

    (2019)
  • P. Cheng et al.

    Next-generation organic photovoltaics based on non-fullerene acceptors

    Nat Photon

    (2018)
  • L. Ye et al.

    Molecular design toward highly efficient photovoltaic polymers based on two-dimensional conjugated benzodithiophene

    Acc Chem Res

    (2014)
  • J. Wang et al.

    Triarylamine: versatile platform for organic, dye-sensitized, and perovskite solar cells

    Chem Rev

    (2016)
  • J. Hou et al.

    Organic solar cells based on non-fullerene acceptors

    Nat Mater

    (2018)
  • C. Yan et al.

    Non-fullerene acceptors for organic solar cells

    Nat Rev Mater

    (2018)
  • C. Wang et al.

    Organic semiconductor crystals

    Chem Soc Rev

    (2018)
  • J.H. Dou et al.

    A cofacially stacked electron-deficient small molecule with a high electron mobility of over 10 cm2 V−1 s−1 in air

    Adv Mater

    (2015)
  • A.F. Paterson et al.

    Recent progress in high-mobility organic transistors: a reality check

    Adv Mater

    (2018)
  • J.H. Jou et al.

    Approaches for fabricating high efficiency organic light emitting diodes

    J Mater Chem C

    (2015)
  • Y. Liu et al.

    All-organic thermally activated delayed fluorescence materials for organic light-emitting diodes

    Nat Rev Mater

    (2018)
  • Z. Yang et al.

    Recent advances in organic thermally activated delayed fluorescence materials

    Chem Soc Rev

    (2017)
  • Y. Cui et al.

    Over 16% efficiency organic photovoltaic cells enabled by a chlorinated acceptor with increased open-circuit voltages

    Nat Commun

    (2019)
  • L. Meng et al.

    Organic and solution-processed tandem solar cells with 17.3% efficiency

    Science

    (2018)
  • J.E. Anthony

    Organic electronics: addressing challenges

    Nat Mater

    (2014)
  • X. Peng et al.

    Low work function surface modifiers for solution-processed electronics: a review

    Adv Mater Interfaces

    (2018)
  • C.C. Chueh et al.

    Recent progress and perspective in solution-processed Interfacial materials for efficient and stable polymer and organometal perovskite solar cells

    Energy Environ Sci

    (2015)
  • H. Ma et al.

    Interface engineering for organic electronics

    Adv Funct Mater

    (2010)
  • Z. Hu et al.

    Water/alcohol soluble conjugated polymers for the interface engineering of highly efficient polymer light-emitting diodes and polymer solar cells

    Chem Commun

    (2015)
  • Z.H. Hu et al.

    Energy-level alignment at the organic/electrode interface in organic optoelectronic devices

    Adv Funct Mater

    (2016)
  • Z. He et al.

    Recent advances in polymer solar cells: realization of high device performance by incorporating water/alcohol-soluble conjugated polymers as electrode buffer layer

    Adv Mater

    (2014)
  • C. Duan et al.

    Recent advances in water/alcohol-soluble pi-conjugated materials: new materials and growing applications in solar cells

    Chem Soc Rev

    (2013)
  • Z. Yin et al.

    Interfacial materials for organic solar cells: recent advances and perspectives

    Adv Sci

    (2016)
  • F. Huang et al.

    Water/alcohol soluble conjugated polymers as highly efficient electron transporting/injection layer in optoelectronic devices

    Chem Soc Rev

    (2010)
  • M. Graetzel et al.

    Materials interface engineering for solution-processed photovoltaics

    Nature

    (2012)
  • D. Li et al.

    Printable transparent conductive films for flexible electronics

    Adv Mater

    (2018)
  • Z.A. Page et al.

    Fulleropyrrolidine interlayers: tailoring electrodes to raise organic solar cell efficiency

    Science

    (2014)
  • Y. Liu et al.

    Dual functional zwitterionic fullerene interlayer for efficient inverted polymer solar cells

    Adv Energy Mater

    (2015)
  • S. Braun et al.

    Energy-level alignment at organic/metal and organic/organic interfaces

    Adv Mater

    (2009)
  • Q. Bao et al.

    Regular energetics at conjugated electrolyte/electrode modifier for organic electronics and their implications on design rules

    Adv Mater Interfaces

    (2015)
  • C.Z. Li et al.

    Doping of fullerenes via anion-induced electron transfer and its implication for surfactant facilitated high performance polymer solar cells

    Adv Mater

    (2013)
  • K. Zhang et al.

    Highly efficient inverted polymer solar cells based on a cross-linkable water-/alcohol-soluble conjugated polymer interlayer

    ACS Appl Mater Interfaces

    (2014)
  • S. Dong et al.

    Cross-linkable and dual functional hybrid polymeric electron transporting layer for high-performance inverted polymer solar cells

    Adv Mater

    (2017)
  • B.H. Lee et al.

    Multi-charged conjugated polyelectrolytes as a versatile work function modifier for organic electronic devices

    Adv Funct Mater

    (2014)
  • Cited by (0)

    View full text