Charge carrier transport in thin conjugated polymer films: influence of morphology and polymer/substrate interactions
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INVITED ARTICLE
Charge carrier transport in thin conjugated polymer films: influence of morphology and polymer/substrate interactions Ban Xuan Dong 1 & J. K. Wenderott 1 & Peter F. Green 1,2 Received: 11 May 2020 / Revised: 5 August 2020 / Accepted: 10 August 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract The performance of conjugated polymer (CP)-based electronic devices relies on optimal charge carrier mobilities, which are determined by monomeric architecture, degree of polymerization, chain conformation, and the nano- and mesoscale morphologies. With regard to the latter, we discuss two effects that have received limited attention in the literature, yet important for device performance optimization: (1) the role of morphological disorder and of CP/substrate interactions on the in-plane and outof-plane carrier transport in CPs; (2) the impact of morphological disorder on charge transfer at the CP/substrate interface. The emergence of film thickness-dependent carrier mobilities, varying over two orders of magnitude within a length scale of 200 nm, and band-bending phenomena, extending tens of nanometers within the CP, are associated with these effects. These findings suggest areas for further research in order to enable widespread applications of next-generation CP-based devices. Keywords Conjugated polymers . Charge transport . Thickness dependence . Band bending . Polymer/substrate interactions
Introduction In recent years, significant scientific advances have been made toward enabling the practical viability of semiconducting polymer devices, including organic solar cells, organic lightemitting diodes, biomedical devices, and sensors [1–12]. Many of these advances have come as a result of a deeper understanding of the mechanisms of carrier transport in organic semiconducting polymers and exploiting molecular design principles to guide chemical synthesis, including the development of processing strategies to control the alignment of polymer chains [1–8]. Today, polymer systems exhibit carrier mobilities exceeding 10 cm2 V−1 s−1, which is higher than that of amorphous silicon [9, 10]. More recently, heating and cooling thermoelectric applications have become of strong interest because conjugated polymers (CPs) not only exhibit high carrier mobilities but also high electrical conductivities upon
* Peter F. Green [email protected] 1
Department of Materials Science and Engineering, Biointerfaces Institute, University of Michigan, Ann Arbor, MI 48109, USA
2
National Renewable Energy Laboratory, 15013 Denver W Pkwy, Golden, CO 80401, USA
doping, naturally low thermal conductivities, and high Seebeck coefficients [11, 12]. The obvious advantages of semiconducting polymeric devices are the mechanical flexibility, low-temperature solution processability, manufacturing at scale, low cost, light weight, and versatility for adoption to various applications. Within the framework of polymer physics, we consider the charge carrier mobility (μ) of a semiconducting polymer a key parameter of
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