Structural Control And Charge Transport Of Graphene-Semiconducting Polymer Composite Films

Semiconducting polymers possess great potentials for the applications in flexible and wearable electronics due to their mechanical flexibility,light-weight and low-cast fabrication of large-area devices.Among them,recently the donor-acceptor(D-A)copolymers have been applied extensively for organic solar cells and organic field effect transistors(OFETs)owing to their excellent electrical and optical properties.However,the practical applications were severely restricted by their low carrier mobility and poor environmental stability compared to inorganic counterparts.The two-dimensional(2D)materials such as graphene exhibit intrinsic high electrical conductivity as well as excellent chemical and thermal stability.Therefore integrating polymer semiconductors with 2D materials into the polymer-2D hybrid structures is promising to overcome aforementioned difficulties.Among them,incorporating the 2D sheets as filler in the polymer matrix via solution mixing,presents a facile and scalable approach for the integrating.Furthermore,effective control of molecular orientaion and packing of the films is crucial to enhance device performance of oragnic semiconductors.So far various deposition strategies have been developed to achieve chain alignment of the conjugated polymers.However successful examples are still scarce for film preparation of the aligned polymer-2D composites presently.Graphene and chain backbones of the D-A copolymers possess large-size ?-conjugated plane thus are expected to exhibit strong interaction with magnetic field,which enables magnetic manipulation of molecular orientation during film growth.In this work,two high-mobility D-A copolymers DPP-2T or P(NDI20D-T2)are utilized to blend with graphene sheets,to explore the method of high magnetic field induction to control the molecular chain orientation and structural order of the blended film during the film growth process,thereby controlling and improving the carrier transport capacity of the film.Some of important results have been achieved as follows:1.An improved liquid-phase exfoliation process is taken to obtain the graphene sheets dispersed uniformly in dichlorobenzene.The exfoliated graphene sheets exhibit lateral size of 200 nm-1.0?m and the few-layer structure with low density of defects.Highly homogenous solutions of the semiconducting polymer/graphene blend have been prepared by mixing graphene dispersion with the P(NDI20D-T2)(or DPP-2T)solution.The solutions were employed to prepare the polymer-based OFETs via spin-coating process.It is found that a 3-4 fold enhancement of carrier mobility has been achieved by the incorporation of graphene nano-sheets into the matrix of both P-type DPP-2T or N-type P(NDI20D-T2).The variation of mobility with channel length was also investigated.Our results reveal the role of the graphene sheets as the electrical conducting bridges,by which good conducting pathways are formed at the boundaries of crystalline polymer domains via graphene sheets to facilitate carrier transport between polymer domains.2.The aligned films of the P(NDI20D-T2)/graphene composites have been achieved by solution drop-cast under high magnetic field(DC-HMF)(8T).Film structures were examined via polarized UV-vis absorption spectra and grazing-incidence X-ray diffraction(GIXRD).It reveals that the incorporation of small amount of graphene sheets improves the degree of chain alignment of the composite films compared to pristine P(NDI20D-T2).The OFETs based on the aligned composite films exhibit a remarkable enhancement of electron mobility and mobility anisotropy(up to 8.3),relative to the devices of pure P(NDI20D-T2).On the other hand,charge transport properties of the composite films are also improved via the drop-cast growth under a rotating magnetic field,which originates from the enhanced face-on packing of P(NDI20D-T2)by magnetically controlling the orientation of conjugated planes of the backbones.A 10-fold mobility enhancement is achieved compare to the P(NDI20D-T2)devices.In order to elucidate the improved chain alignment and charge transport,we suggest that the ensembles comprising of the graphene sheets and chain aggregates adhered,possess much larger anisotropy of magnetic energy than single chain aggregates to drive magnetic alignment of the polymer chains.Furthermore,close ?-? stacking between the conjugated plane of graphene and chain backbones leads to the formation of fast conduction pathways at the interface between the polymer domains and the graphene to facilitate.Our work demonstrate that the strategy for graphene-assisted magnetic alignment via solution process affords a facile and effective route for film structural control of semiconducting polymers,to improve electrical properties of organic devices.3.In order to eliminate the inhomogeneity on morphology and thickness occurred on the DC-HMF polymer films,solvent-vapor annealing of the as-spun films under HMF(SVA-HMF)is utilized,to achieve successfully the large-area highly aligned P(NDI20D-T2/graphene composite films with high uniformity on film morphology and thickness.As evidenced from polarized light microscopy,AFM and polarized UV-vis spectra,chain backbones of P(NDI20D-T2)are highly aligned along the direction of magnetic field during SVA,and the degree of chain alignment is further improved compared to the composite film via DC-HMF.The OFETs based on the SVA-HMF composite films,exhibit an enhanced electron mobility compared to the devices of the DC-HMF and spin-cast films,as well as extremely large anisotropy of carrier mobility(ca.81).We propose that a number of small-size aggregates of P(NDI20D-T2)chains are re-generated in the swollen wet films caused by solvent permeation during SVA-HMF.These aggregates could adhere to the graphene planes to form the ensembles which possess much larger magnetic energy gain than single chain aggregates to drive the rotation of chain aggregates,thus the facilitated chain alignment.The SVA-HMF approach is well compatible with common processing of organic semiconductor devices,therefore will have a promising potential on low-cost high-performance organic electronics applications.