Relationship Between Condensed Structure And Intrinsic Stretchability Of Conjugated Polymer Film

Conjugated polymers are ideal materials for the preparation of organic field effect transistors(OFETs),organic light-emitting diodes(OLEDs),organic solar cells(OPVs),and stretchable electronic devices due to their low-cost,solution-rocessable,and large-scale processable properties.Stretchable devices require conjugated polymer films to maintain excellent electrical properties under deformations.However,some current D-A conjugated polymers with high carrier mobility are less flexible.It is well known that charge transportation relies on the intra-chain π-electron delocalization and inter-chain electron hopping within π-π stacking,and the mechanical stretchability depends on the energy dissipation of the polymer chain conformation and chain stacking under external stress.Therefore,film morphology is a critical factor that affects mechanical and electrical properties.The key problem lies in the requirement for conjugated polymers to simultaneously have high carrier mobility,high ductility,and reversibility,which presents a challenge for conjugated polymer morphology design and modulation.First,we prepared a series of films with different crystalline morphologies to address this issue through solution modulation and thermal annealing.Then,the evolution of the microstructure upon film deformation was investigated to propose the structure-mechanism-property model.Finally,devices with optimized charge mobilities and stretchability were obtained by tuning the film crystallinity.The main findings are as follows.1.The crystallinity of high molecular weight poly[2,5-bis(4-decyltetradecyl)pyrrolo[3,4-c]pyrrole-1,4-(2H,5H)-dione-(E)-1,2-di(2,2’-bithiophen-5-yl)ethene(DPP-TVT)films was regulated by the solvent-side chain interaction parameters.Firstly,a range of solvents with different compatibility with the DPP-TVT side chains was selected based on Hansen solubility parameters.As Ra(solvent-side chain)decreased,the backbone conformation underwent a coil-extend-collapse transition;meanwhile,the solutions show similar short-range π-π aggregation with decreasing long-range ordering formed by side chain assembly.Spin-coating could preserve these features,resulting in a range of films with similar short-range aggregation and decreasing relative degree of crystallinity from 3.1 to 1.2.The microstructure evolution and the failure mechanism of DPP-TVT films under tensile deformation were investigated by HR-TEM.The results show that the crystalline regions and tie-chains have significant effects on the mechanical behavior of the conjugated polymer films.(1)Under a single stretch,the films of low crystallinity underwent uniform orientation deformation,films with moderate crystallinity formed fibrous networks,and films of high crystallinity formed discontinuous fibrous networks with cracks.(2)Plastic deformation during stretching leads to wrinkles during recovery.Under compression forces,the tie chains drove the overall reorientation of the fibrous network and enabled the semi-crystalline films to accommodate the residual strain generated during stretching.In contrast,lacking a stress transfer pathway in uniformly deformed low crystallinity films prevented the molecular chains from reorienting,causing the formation of folds;in highly crystalline films,the discontinuous fibrous network structure induced local stress concentration,leading to the formation of large-size folds.(3)The fibrous network structure of semi-crystalline films could maintain structural stability and resist fatigue damage during cyclic stretching.3.The DPP-TVT films of medium crystallinity had both high charge mobility and cyclic deformation stability.We have systematically investigated the mechanical properties,tensile device performance,and durability of films with different crystallinity morphology by performing tensile mechanical properties tests and preparing tensile transfer OFET devices.The results showed that:(1)As the film crystallinity decreased,the fracture strain increased from 57%to 89%,while the charge mobility of the unstrained film decreased from 0.45 cm2V-1s-1 to 0.28 cm2V-1s1.(2)After stretching to 100%,the charge mobility of film with low crystallinity increased from 0.28 cm2V-1s-1 to 0.47 cm2V-1s-1 due to chain alignment;the charge mobility of the high crystallinity films decreased by 50%due to cracking.;the charge mobility of medium crystallinity films kept steady.(3)After 100 times 50%strain,the charge mobility of the low crystallinity films decreased by an order of magnitude.While the medium crystallinity films showed no declination in charge mobility(>0.45 cm2V-1s-1)after both single and cyclic stretching.