Effect Of External Field On The Ordered Structure Of Molecular Chains And Mobility In Conjugated Polymers (P3HT,PTB7)

Conjugated polymers have been widely considered for many aspects,such as organic field effect transistor(OFET),organic light-emitting diode(OLED)and organic solar cell(OSC)because of their solution processiblity,good film-forming and large area machinability.As kinds of classic conjugated polymers,the structures and performances of poly(3-hexylthiophene)(P3HT)and poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b’]dithiophene-2,6-diyl][3-fluoro-2-[(2ethylhexyl)carbonyl]thien o[3,4-b]thiophenediyl]](PTB7)have been studied deeply by scientists.P3 HT is semi-crystalline polymer and “hairy-rod” like polymer,it has rigid main chains and flexible side chains.P3 HT has a simple chemical structure and highly ordered structures.PTB7 is a kind of low band gap photovoltaic materials,it can achieve high photoelectric conversion efficiency and thus has good application prospect.There are three important aspects could achieve perfect photovoltatic device: high mobility material,highly ordered structure of polymer and good structure of device.The conformation can influence the electronic structure and photophysical property in polymers and it is important to control the order degree of conjugated polymers,but scientists often neglect it in fabricating photoelectric devices.As a soft matter,conjugated polymer elicits a strong response when it is affected by a weak external field.In this research,we use external field(electric field,solvent,temperature,etc.)to induce chains conformation change into more ordered structures in P3 HT and PTB7 solutions and films,which can increase hole mobility of polymers and improve the performance of photoelectric device fundamentally.The work is divided into the following three parts.In the first work,we use the external electric field,a kind of directional external field,to change the chain conformation of polar P3 HT.We compared the responses of chain conformation on external electric field in P3 HT with different molecular weights and researched the mechanism.It was found that the external electric field could induce P3 HT chains to orient into more ordered structures along its direction,which was benefit for charge transportation.Besides,it was found that molecular weight,as a kind of polymer intrinsic property,could decide the responses of P3 HT chain conformation on electric field.This work contributed to the understanding of the effect of the external electric field on chain conformation with different Mws in terms of polymer physics and provided a basis for fabricating photoelectric devices with high charge carrier mobility and efficiency.In the second work,different external fields were used to control the size of ordered aggregation in P3 HT and PTB7 solutions.We compared the difference of chain conformation change between P3 HT and PTB7 under the same external field and analyzed the above reasons.The dynamic evolution from chain disorder to order of PTB7 condensed state structures under the external fields had been revealed through high resolution transmission electron microscope(HR-TEM).Firstly,solvent field could impact the aggregation structure of P3 HT and PTB7.It was found that poor solvent could increase the interaction between polymer chains rather than the interaction between polymer chain and solvent molecules.It made rigid P3 HT chains increase π-π stacking and form large-size ordered aggregation.However,flexible PTB7 chains entangled with each other and formed amorphous state in poor solvent.Secondly,temperature field could impact the aggregation structure of P3 HT and PTB7.It was found that lowering temperature could decrease the solubility of poor solvent,which could further increase the π-π stacking between P3 HT chains and form larger-size ordered aggregation.Decreasing temperature could also compact the curled and entangled PTB7 chains more tightly,which increased the π-π stacking between PTB7 segments to form local ordered aggregation.In addition,there was hysteresis of condensed state structure of P3 HT and PTB7 in poor solvent and hence the aggregation could be kept at room temperature.Thirdly,concentration field could impact the aggregation structure of P3 HT and PTB7.The main chains of P3 HT and PTB7 became more stretched in lower concentration solutions,which have longer conjugated main chains and a larger size of aggregation.In the third work,we added some P3 HT into PTB7 solution and spin coated the mixed solution into films.It was found that the conjugated main chain of PTB7 became longer and more ordered with the addition of P3 HT.PTB7 achieved the largest size of ordered aggregation when P3 HT was wt.30%.We applied an external electric field on P3HT/PTB7 mixed film during annealing.It was found that P3 HT molecular chains could align along the direction of electric field.In the mean time,P3 HT could drive PTB7 chain conformation change under the external electric field,while the external field could not impact PTB7 conformation in its neat film.The integral orderness of mixed film could be enhanced because of the synergistic effect between P3 HT and PTB7 under the external electric field.The orderness of mixed film was benefit for charge transportation,which was important to fabricate ternary solar cells.Based on the condensed state physics of polymers,external field induced polymer chains to orient into more ordered structures,which was benefit for improving mobility and photoelectric conversation efficiency.It was found that the intrinsic physical property could influence the performance of conjugated polymer observably.Through controlling external field to increase the chain orderness is of importance to establish physical basis for the molecule design and synthesis of materials to enhance device efficiency based on condensed matter physics of conjugated polymer.