Properties of Heterogeneous Materials for Organic Electronics: Crystal Structure and Phase Behavior of Thiophene-based Polymers-Aromatic Diimides Blends

Abstract

Organic electronics has been attracting attention both from academic institutions and industry for more than two decades. That interest is due to the unique combination of inherent flexibility of some organic materials and their electronic properties. Such properties enable potential novel applications including e.g. wearable, deformable photovoltaic cells, diode-based lighting, epidermal human-health monitoring systems or artificial skin for robots to name just few.[1] Reaching these challenging horizons requires, however, solving fundamental problems in the field of materials science. Significant number of issues the field of organic electronics is related to the synthesis and processing of materials into the form that is useful in unit electronic devices. In the case of organic semiconductors, it is often a problem of fabrication of thin films. Therefore, conjugated polymers with their good film-forming properties, flexibility, mechanical stability and unique ability to transfer charge carriers, play an important role here.[2] The charge transport characteristics of conjugated polymers can be tuned by blending them with polyaromatic small molecules.[3,4] The conjugated polymer-small molecule binary blends may reveal improved apparent charge carrier mobility (in comparison to the corresponding pure polymers) or ambipolar charge carrier transport in thin-film based transistors.[4,5] Binary blends show also increased power conversion efficiency in photovoltaic cells.[6] There are three major sources that altered/enhanced electronic properties stem from. First: at the molecular level, it is the right match between the electronic band structure of the blend components. Second reason for the enhanced electronic properties of the blends is formation of the heterojunctions (interfaces between the components with unequal band gaps). And the third thing is the formation of percolation networks enabling effective transport of charge carriers through the volume of the film. Significant part of the binary organic semiconductors is based on conjugated polymers and crystalline small-molecular additives. Hence formation of the heterojunction and percolation network can be considered a problem of crystallization-driven phase separation in the blends. Since conjugated polymer-based films are typically fabricated by solution based techniques (like spin- or spray-coating, printing etc.) their formation can be considered a special case of solution crystallization. The time necessary for evaporation of solvent used for fabrication of the film is typically significantly shorter than the time necessary to form stable crystal structure of the components. Therefore the films are often subjected to additional solvent treatment (solvent-vapor annealing) and/or heat treatment in order to have them kinetically equilibrated. The processing procedures applied during and after fabrication of films cause changes in their crystal structure.[7] The altered crystal structure is the reason for changed electronic properties of the post-processed films. Despite, however, that correlation is rather intuitive, researchers pay little or no attention when choosing the parameters of the post-processing procedure – they are often arbitrary selection-based or random. Capturing generalized patterns of crystallization and crystal growth in the binary films should therefore facilitate engineering of properties of the binary blends used in organic electronics. Finding the patterns of properties requires, however, systematic studies on the binary blends.Based on the motivation above, we have performed a systematic study on the blends of poly(3-hexylthiophene) (P3HT) and variable ratios of either of five different polyaromatic diimides: N,N’-n-hexylated pyromellitic diimide (PIRnC6), N,N’-n-butyl-, N,N’-n-hexyl-, and N,N’-n-octyl naphthalene diimide (NDInC4, NDInC6, NDInC8 respectively) and N,N’-n-hexylated perylenediimide (PDInC6). Such blends were already reported for their unique electronic properties in thin-film field effect transistors and photovoltaic cells.[4,8] In our experimental approach we have analyzed bulk crystal structure of the blends and thin films deposited on silicon/silicon dioxide substrates – as in model electronic devices. The crystal structure of the blends was determined by X-ray diffraction (XRD). The XRD was also used to determine thermally-induced growth of crystal domains in the blends and also solvent-vapor-induced structure changes of the films. The latter was additionally studied by scanning electron microscopy. Combination of XRD and differential scanning calorimetry (DSC) enabled determination of the phase transition landscapes in the blends. Analysis of experimental results collected for over 30 different blends enabled correlating the crystal structure and the phase behavior of the blends with molecular structure of the blend components. These results were further compared with charge carrier mobility in the field-effect thin-film transistors based on the above blends. Generally, it was found that the phase transition points of the blends depend on their composition. The phase diagrams of all the blends featured the characteristic lowest melting (isotropisation) points resembling the eutectic point observed in some conventional binary alloys. Because of that similarity we called that point “pseudoeutectic”. The position of the pseudoeutectic points depended on the length of the alkyl chains of the blend and the size of the aromatic cores of the small molecular compound. Based on the analysis of the NDI series it was found that longer alkyl chains caused a shift of the pseudoeutectic points towards lower P3HT content in the blends. Comparison of the datasets for PIR, NDI and PDI-based compounds indicated that increasing the size of the aromatic core caused pseudoeutectic point to move towards the higher P3HT content. Morphological studies on the blends revealed that the pseudoeutectic behavior were related to physical sizes of the crystal domains formed directly after crystallization as well as to their Scherrer’s coherence lengths. Peak charge carrier mobilities were found following the trends for pseudoeutectic points – the mobilities were always the highest around the blend compositions for which pseudoeutectic points were found. Solvent-vapor annealing of the blend-based films caused changes in structure and morphology: a growth of the crystal domains, and also altered also their orientation. Such changes in structure caused non-trivial alterations in charge carrier mobility in the studied blends. In general, in this study we demonstrate how does molecular structure of the components affect crystal morphology and charge transport in homological series of blends based on P3HT and polyaromatic small molecules. Hence, from this perspective, this study can be considered a basis for designing properties of such blends. Acknowledgement This work was supported by the National Science Centre, Poland through the grant UMO-2016/22/E/ST5/00472. Variable-temperature- and grazing-incidence XRD measurements were partly performed using the 10 keV beam at the BL9 beamline at DELTA synchrotron facility in Dortmund (Germany). The Authors thank Dr. Christian Sternemann for support during the experiments at BL9. References[1] Xi, Z., et al., Organic Electronics for a Better Tomorrow:Innovation, Accessibility, Sustainability. A White Paper from the Chemical Sciences and Society Summit (CS3), 2012, 1, 1.[2] Guo, X. Müllen, K., Prog. Polym. Sci., 2013, 38, 1832.[3] Orgiu, E., et al., Chem. Comm., 2012, 48, 1562.[4] Puniredd, S.R., et al., J. Mater. Chem. C, 2013, 1, 2433.[5] Janasz, L., et al., J. Mater. Chem. C, 2018, 6, 7830.[6] Su, G.M., et al., J. Mater. Chem. A, 2014, 2, 1781.[7] Chlebosz, D., et al., Dyes Pigment., 2017, 140, 491.[8] He, Q., et al. Dyes Pigment. 2016, 128, 226