Study On The Preparation And Properties Of P3HT/PDMS Stretchable Organic Semiconductor Blending Films

Conjugated polymers are being widely used in flexible electronics due to their many advantages such as low cost,intrinsic softness and large-area processability.In recent years,with the rapid progress on internet of things,wearable and portable devices and soft robots,organic functional materials are required not only to have good optoelectronic performance but also to have the mechanical flexibility and deformability to meet the response to dynamic environment during service.However,there is inherent competition between the deformability and photoelectric properties of conjugated polymers in terms of underlying microstructures.Ordering and crystallization of polymer conjugated segments are the structural basis of charge transport and high-performance optoelectronic functions,but simultaneously bring about brittleness and rigidity to the films,that is detrimental to flexibility of the polymer.Hence,balance of mechanical and electronic properties in conjugated polymers is of great significance for the application of organic flexible electronic devices.Blending conjugated polymer with organic elastomer is possibly the most straightforward and facile way to balance these two properties.However,the inherent immiscibility and phase separation of these two kinds of polymers in condensed state lead to a challenge to the solution processing and the structural control of the blending films.The excellent electrical properties call for a crystalline network of the semiconducting polymer chains with high connectivity,while the flexibility of the blend film requires simultaneously an elastomer chain network constructed to load effectively external stress and strain.Therefore,the ideal phase separation structure of the conjugated polymer/elastomer blend film is a highly interconnected and interpenetrating dual networks.In this sense,controlling the crystallization of conjugated polymer semiconductor and the phase separation behavior of the blend solution is the key to the preparation of the blending films with balanced electrical and mechanical properties.In this thesis,p-type semiconductor poly(3-hexylthiophene)(P3HT)and elastic insulating polymer poly(dimethylsiloxane)(PDMS)were selected as a prototypically investigated system.Two different solvents(chloroform and toluene)were used to control the crystallization and aggregation of P3HT solution,and the surface energy of substrate in the spin coating process was modified to control the phase separation behavior of the blending films.It was shown that the substrate with high surface energy induced typically a bilayer vertical phase separation structure of the blending films in which P3HT layer segregated towards the interface with the substrate.However,on the substrate with low surface energy,the blending films tend to form a PDMS/P3HT/PDMS sandwich structure.The degree of vertical phase separation of the blending films was further affected by the aggregation state of P3HT.In contrast to coils formation in chloroform solution,P3HT chains in toluene solution are apt to crystallize to nanowire configurations.Owing to much slower freedom of motion of the nanowires,an interpenetrating double networks structure form in the P3HT/PDMS blending films on the low surface energy substrate.This interconnected double-networks structure allows not only maintained electrical properties,but also highly improved stretchability.The main contents and results can be summarized as following:(1)Firstly,the effects of the surface properties of the substrate on the phase separation and electrical properties of P3HT/PDMS/chloroform blends were studied.The bottom-gate,bottom-contact OFET devices were used to characterize the electrical properties based on the bare SiO2/Si(surface energy is relatively high)and the octadecyltrichlorosilane(ODTS)-treated SiO2/Si(surface energy is relatively high)substrates.The vertical and lateral phase separation structure of P3HT/PDMS blending films based on bare SiO2/Si and ODTS-treated SiO2/Si substrates were examined by X-ray photoelectron spectroscopy(XPS),scanning electron microscope(SEM)with energy dispersive x-ray spectroscopy(EDS)mapping and optical microscope(OM).On bare SiO2/Si polar substrate(surface energy is relatively high),due to the lower surface energy of PDMS and relatively high polarity of P3HT,the bilayer structure of PDMS at the top and P3HT at the bottom is formed,which ensures the effective charge transport channel between the source and drain of the bottom-contact OFETs.Even when the content of P3HT is as low as 2.4wt%,owing to the phase separation and connectivity of P3HT,the blending films exhibited the average mobility around 4.26×10-3 cm2 V1 s-1,which is similar to that of the pure P3HT film.However,on ODTS-treated SiO2/Si nonpolar substrate(surface energy is relatively low),the PDMS/P3HT/PDMS sandwich structure is formed.Due to the existence of insulating PDMS between the source and drain,the average mobility of blending films dropped rapidly as the content of P3HT decreased.It is shown that the surface energy of the substrate has a significant effect on the phase separation structure and electrical properties of the blending films.(2)P3HT one-dimensional nanowires were prepared in marginal solvent toluene and blended with PDMS to prepare the stretchable semiconductor film.The effect of the change of P3HT aggregation morphology on the phase separation structure of the blending films was investigated.P3HT nanowires/PDMS blends were spin-coated onto bare SiO2/Si(surface energy is relatively high)substrate and low surface energy PDMS stretchable substrate.OFETs based on P3HT/PDMS blending films were tested to evaluate the electronic properties of the films.The phase separation structures,surface morphologies and the deformation under stretching were characterized by XPS,atomic force microscopy(AFM),and OM,respectively.Similarly,on the SiO2/Si substrate with relatively high surface energy,P3HT nanowires tend to aggregate at the substrate interface to form a P3HT/PDMS stratified structure.However,on the PDMS substrate,because the freedom of motion of the nanowires is much slower than that of the coiled molecular chain,the PDMS/P3HT/PDMS stratified structure driven by the low substrate surface energy is not as obvious as that in chloroform solution,the double networks structure of P3HT nanowires and PDMS molecular chains interpenetrating is formed.The double networks structure affords the blending films not only similar field-effect mobility in a wide range of P3HT content(?5 wt%),but also a much enhanced stretching performance.At 100%strain,compared with pure P3HT film,the blending films has no obvious cracks and its mobility decreases by only an order of magnitude,which is much better than the stretchability of pure film.In the repeated stretching cycles,the mobility of the blended film was always higher than that of the pure film.This double networks structure driven by the surface energy of substrate may provide an efficient way to prepare flexible orgonic semiconductor films with excellent mechanical and electrical properties.(3)In order to further improve the performance of the P3HT/PDMS blending films,P3HT nanoparticles were added to the blending films.Similar to the rubber filler network,the addition of P3HT nanoparticles not only can maintain the electrical properties of the blending films,but also may strengthen the mechanical properties of the blending films.In the preliminary work,P3HT nanoparticles were successfully prepared by a two-step precipitation method.The method of spin-coating nanoparticle suspension first and then nanowires solution was used to study the changes in electrical properties before and after composite.It was found that the combination of nanoparticles and nanowires had a certain increase in mobility compared to pure nanowires.This may result from the more continuous P3HT conductive pathways between source and drain with the introduction of nanoparticles.