Enhanced Molecular Structure And Film Morphology For High-Performance Optical/electrical Devices Of Semiconducting Polymers

Great progress has been made on organic semiconductors(OSCs)for its application in flexible electronics owing to the light-weight,mechanical flexibility as well as solution-processable and low-cost manufacture of large-area devices.By synthesizing new materials,optimizing device structures and the deposition methods,the mobility of organic field-effect transistors(OFETs)and the power conversion efficiency(PCE)of the organic solar cells are always being updated.Although many achievements have been made in the field of OFETs and OSCs,the performance of the D-A copolymers is far from satisfactory compared to inorganic semiconductors.It is verified that the performance of organic semiconductors devices is closely related to the molecular structure and film morphology.Based on the above ideas,firstly,we describe a straightforward method to achieve large-area highly aligned film of a diketopyrrolopyrrole-bithiophene polymer(PDPP2TBT)by solvent vapor annealing(SVA)of the as-spun films under high magnetic field,meanwhile,the optoelectric properties of aligned films and the charge transport mechanism were intensively analyzed.Secondly,the blend films were prepared by blending a narrow bandgap polymer PDPP2TBT into inert polystyrene(PS)matrix,the film structure and the optoelectric performance of which were tested and analyzed,respectively.In addition,nonfullerene polymer solar cells are fabricated by using PBDB-T:BT-CIC as the active layers,which was then optimized by adding the second donor PTB7-Th.Finally,we finely controlled the structure of the all-polymer bulk heterojunction by solvent vapor annealing(SVA)of the as-spun films under high magnetic field.The main research results are as follows:1.Controling the film structure of copolymer PDPP2TBT by solvent vapor annealing(SVA)of the as-spun films under high magnetic field.In order to solve the problem of achieving uniform films with homogeneous thickness and morphology using the drop-casting method in the previous literature,SVA of the as-spun films of semiconducting polymer under HMF was firstly performed.The structural characterizations disclose that the chain backbones of PDPP2TBT are highly aligned to the applied magnetic field during SVA with the films exhibiting high crystallinity.A mechanism is proposed to explain magnetic alignment,and it is inferred that the film swells in the solvent vapor to form a large number of small-sized polymer chain aggregates,which leads to the molecular chain alignment and film orientation in the magnetic field.The optoelectrical properties of the aligned film are intensively studied by preparing bottom-gate/bottom-contact and top-gate/bottom-contact devices.It is shown that the hole mobility of the aligned f1lms can up to 1.56 cml/Vs extracted from the saturated regime for the parallel device,a nearly six-fold enhancement compared with the isotropic device.The anisotropy of hole mobility??/?? is ca.3,where ??,?? are defined as the mobility of parallel and perpendicular devices.In addition,the top-gate/bottom-contact devices exhibit an obvious bipolar transport performance,indicating the fewer electron defects at the PDDP2TBT/PMMA interface.To further study the mechanism of the carrier transport,we investigated the temperature-dependent FET mobility.It reveals that a remarkable lowering of thermally activated energy for carrier hopping in the aligned film.The results indicate the formation of rapid intra-chain conduction pathway parallel to the chain alignment direction,which originates from the alignment-induced backbone extension and enhanced order of inter-chain packing.Thanks to the compatibility with the semiconductor device manufacturing processes,the SVA-HMF is expected to be a simple and effective way to optimize the optoelectric performance of organic semiconductors.2.Preparation of blends film field-effect transistors by blending a narrow bandgap copolymer PDPP2TBT with a low-cost insulator polymer polystyrene(PS).It is found that the absorption of the PDPP2TBT/PS films becomes weaker gradually with the decrease of PDPP2TBT,and the PDPP2TBT/PS blends exhibits a high transmittance of nearly 100%across the whole visible range when the semiconductor polymer content is as low as 0.5 wt.%.The high transmittance in the visible light range provides a promising way for its application in transparent electronic devices.We investigated the relationship between charge mobility,contact resistance,and on/off ratios versus with the semiconductor polymer content.It is shown that the parameters of blend films device with a content of 0.5 wt.%-1.0 wt.%PDPP2TBT are significantly better than pristine PDPP2TBT devices.Mobility as high as 0.63 cm2V-1s-1 extracted from the saturation region,On/off ratios of 106 and the turn-on voltage(Von)close to zero of the optimized PDPP2TBT/PS devices is achieved,respectively.Film morphology was investigated by the AFM measurements to analyze the correlation between electrical performance and topographic structure.The high performance of the PDPP2TBT/PS blends is attributed to the the formation of the nano-network fiber structure by the addition of insulator polymer polystyrene(PS),and the nano-network fiber structure is more conducive to carrier transport than the randomly arranged PDPP2TBT films.The above work is of great significance for developing low-cost,high-performance transparent electronic devices.3.Ternary polymer solar cells are fabricated by adding the second donor PTB7-Th into the nonfullerene PBDB-T:BT-CIC active layer.It is found that the power conversion efficiency(PCE)of the ternary system is much better than that of binary solar cells owing to the enhancement of short-circuit current density and fill factor.The best PCE can be up to 11.85%via incorporating 20 wt.%PTB7-Th in donors.According to the characterization of light absorption,film structure and morphology and the measurement of device electrical properties,the enhancement of PCE can be concluded as follows:1)With incorporating PTB7-Th into PBDB-T:BT-CIC,the ternary active layer can broaden the absorption range of the solar spectrum;2)By adding PTB7-Th into the binary system,the morphology and phase separation of the system was optimized,which facilitates the carrier separation and transport.4.The molecules orientation and crystallinity in polymer semiconductor is crucial to the carrier separation and transport.All-polymer cells are constructed by selecting a P-type polymer PBDB-T as the donor and N-type polymer P(NDI20D-T2)as the acceptor.We first optimized the morphology of the bulk hetrojunction by solvents,donor/acceptor ratio,and additives,and the optimal PCE can be up to 4.93%.Then,the film structure was regulated by solvent vapor annealing(SVA)of the as-spun films under high magnetic field.The structural and morphology characterization indicate that the preparation of aligned film with high crystallinity,which is beneficial to the carrier transport and can be more likely contribute for the performance of the OSCs devices.The method for controlling the molecular structure and orientation of the bulk hetrojunction provides a route for enhancing the performance of the OSCs.