| Organic semiconductor materials,including organic polymer materials and organic small molecule materials,have been widely concerned in the field of semiconductor optoelectronics because of their advantages of easy adjustment of optical band gap,low cost and large area processing on flexible substrate.Compared with inorganic semiconductors,organic semiconductors have strong electron-lattice interaction,which makes the carriers in organic semiconductor no longer be electrons or holes,but form solitons,polarons,bipolarons.In addition,"self-trapping" excited states,such as excitons and biexcitons,can also be formed when organic semiconductors are photoexcited or electrically excited.These unique features make it has a wealth of electrical,optical,magnetic and other functional characteristics.Organic functional devices,such as organic light-emitting devices and organic photovoltaic devices,have shown broad application prospects and huge market value in energy conversion.Exciton migration and charge transfer play an important role in the functional process of organic devices.Among them,the ultrafast directional migration of excitons modulated by strain engineering provides a new strategy for the functional optimization of organic systems.However,the dynamical modulation at the nanoscale with real-time and-space characterizations remains a challenge.In addition,the phenomena of ground state charge transfer under negative interfacial energy gap and excited state charge transfer under near-zero band offset have been found experimentally,but the underlying physical mechanism is still controversial.Therefore,how to modulate exciton migration and understand the mechanism of interfacial charge transfer in organic systems will be the focus of our attention.Based on this,the extended Su-Schrieffer-Heeger(SSH)model combined with the non-adiabatic quantum dynamics method was used in this work to carry out theoretical research on the strain modulation of ultrafast exciton migration and interfacial charge transfer mechanism in organic systems.The specific research contents and results are as follows:1.Migration dynamics of excitons/biexcitons induced by a funnel-like nonuniform compression strain over organic polymersManipulation of exciton migration is of great significance for photoelectric control of organic systems.The advantage of organic semiconductor lies in its strong electron-lattice interaction,which results in strong binding energy of excitons.Therefore,compared with inorganic semiconductor,organic semiconductor is easier to achieve exciton modulation at room temperature.Recently,strain engineering has provided a new strategy to control ultrafast exciton migration in organic systems.The atomically thin materials and organic materials have strong flexibility,plasticity and are not easy to fracture,and can sustain a large enough strain to achieve the expected modulating effects.However,the dynamical modulation at the nanoscale with real-time and-space characterizations remains a challenge.Therefore,we establish an organic polymer model with a funnel-like nonuniform compression strain created,where a quantum model combined with the nonadiabatic dynamical method was employed.The results show that both an exciton and a biexciton,pre-generated near and/or within the strain range,will migrate toward the strain center with the speeds up to 7-10 nm/ps.We attribute the present exciton/biexciton migration mechanism to the strain-induced gradient of exciton/biexciton energy generation(or driving force).Furthermore,taking exciton migration as an example,we clarified that the strain-induced exciton migration can be further optimized by tuning the strain parameters(e.g.,the strain gradient and range)as well as the polymer parameters(e.g.,the initial interchain distance and the e-l interaction constant).These findings contribute to the understanding of strain-modulated photoelectric processes in different semiconductor materials,thus shed light on promoting the application of strain engineering in device innovations.2.Mechanism study on ground state charge transfer in organic polymer heterojunctionOrganic molecular doping has great potential to improve the conductivity of organic systems.Experimental studies have reported that the main mechanism is the spontaneous ground state charge transfer between different organic molecules.However,the understanding of the physical mechanism of ground state charge transfer is still unclear.The early understanding believed that the bottom level of conduction band(LUMO)of acceptor(A)molecule was lower than the top level of valence band(HOMO)of donor(D)molecule,which was the premise of ground state charge transfer.However,some experiments have shown that ground state charge transfer can occur when the LUMO level is slightly higher than the HOMO level.Based on this,the polymer D/A heterojunction model is constructed theoretically.By emphasizing the characteristics of strong electron-lattice interaction of the system,the ground state charge transfer phenomenon and physical mechanism under different interface energy gaps are revealed from two aspects of electron energy and lattice energy.It is found that after intermolecular charge transfer in the negative interface energy gap,although the electron energy of the system increases,the lattice energy decreases significantly,resulting in the decrease of the total energy of the system after charge transfer.Furthermore,this paper illustrates the results that intermolecular ground state charge transfer can be optimized by adjusting the elastic constant of the acceptor,e-l interaction constant of the acceptor and the intermolecular distance.These findings reveal the physical mechanism of ground state charge transfer in D/A system from the perspective of lattice energy for the first time,and provide theoretical guidance for modulating the electrical properties of organic systems through organic molecular doping.3.Energy correction of excited state charge transfer energy gap in organic polymer/small molecule D/A heterojunctionPolymer donor/small molecule acceptor heterojunction system as photovoltaic layer has been widely used in efficient organic solar cells,and the excited state charge transfer at its interface is the core of photovoltaic process.It is generally believed that the band offset between donor/acceptor(D/A)provides the driving force for the transfer of electrons or holes at the interface in the excited state.In recent years,experiments have reported that effective hole charge transfer can occur in an efficient polymer donor/non-fullerene small molecule acceptor photovoltaic system,even if the hole transfer energy order is very small or even negative.It can be seen that whether the interface band offset is the driving force source of excited state charge transfer in polymer donor/small molecule acceptor system is controversial.Based on this,two kinds of D/A heterojunction models,polymer donor/fullerene acceptor and polymer donor/non-fullerene acceptor,are theoretically constructed in this work.The phenomenon and physical mechanism of excited state charge transfer at different band offsets are revealed from two aspects of electronic energy and lattice energy.It is found that the band offset of interfacial charge transfer defined only from the perspective of electronic energy is not accurate enough.It is necessary to take into account the changes of lattice energy before and after charge transfer,and modify the band offset of excited state interfacial charge transfer from the perspective of total energy.In addition,the factors affecting the modified band offset of excited hole charge transfer in the polymer donor/non-fullerene acceptor system are discussed,including electron push-pull potential of acceptor molecules,intermolecular distance,elastic constant and electron-lattice interaction of donor molecules.These results provide a theoretical basis for the further optimization of polymer donor/small molecule acceptor heterojunction system in photovoltaic applications. |
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