Study On The Properties Of Excited States Modulated By Nonuniform Electric Field In Organic Solar Cells

Organic semiconductors,including small molecules and conjugated polymers,are widely used in sensors,lasers,optical switches and solar cells due to their light weight,flexibility,and simple preparation.Unlike inorganic semiconductors,organic semiconductors have strong electron-lattice interactions,which can lead to lattice distortion caused by doped electrons and holes,thus forming self-trapped elementary excitations,such as solitons,polarons and bipolarons.In addition,neutral excited states,such as excitons and biexcitons,formed by light or electric excitation,are also bound by the local lattice potential field in addition to the Coulomb interaction between electrons and holes.These unique features make it has a wealth of optical,electrical,magnetic and other functional characteristics.At present,solar cells using organic semiconductors as photovoltaic layer have shown a broad prospect in energy utilization.However,the low photovoltaic efficiency and poor stability of the device hindered its marketization process.An important direction to solve these problems is to have a deeper understanding of the basic photovoltaic process.In particular,how to modulate the behavior of excited states in organic solar cells.Modulating the photoexcitation process of organic molecules is essential to optimize the photovoltaic process of organic solar cells.Although experimental studies have shown that temperature effect has a certain influence on light absorption,the detailed related dynamics processes(including excitation dynamics and internal conversion dynamics)are still unclear.In particular,charge separation is the core process of organic photovoltaics.In recent years,with the development of femtosecond ultrafast technology and transient detection technology,some of the latest charge separation phenomena are difficult to explain by traditional exciton theory.For example,transient photo-absorption spectroscopy experiments show that in some highly efficient organic photovoltaic systems,up to 70%of the charge can be separated within～100 fs.According to the exciton theory,it is impossible for excitons to be transported to the donor/acceptor interface in femtosecond scale.At present,the cause of ultrafast charge separation is still unclear.In addition,the charge separation process is generally considered to be dominated by the cold charge transfer state or hot charge transfer state formed at the donor/acceptor interface.However,for a given organic photovoltaic system,which mechanism plays a leading role,and what are the modulation means and mechanisms for the charge separation process dominated by these two mechanisms?Clarifying the above issues will provide theoretical basis and modulation basis for further improvement the photovoltaic efficiency of organic solar cells.In organic solar cells,the electric field plays a key role in modulating the excited states,and the form of electric field is often nonuniform in space.On the one hand,the disordered molecular orientation,various defects and the accumulation of charge at the D/A interface in the organic photovoltaic layer may induce the nonuniform electric field;on the other hand,by adding electrical materials(such as polar additives,ferroelectric polymers,etc.)to the photovoltaic layer,the distribution and intensity of the nonuniform electric field can also be artificially generated or modulated.In addition to the nonuniform electric field in space,the electric field component of the light field in the process of photoexcitation is nonuniform on the time scale.We can introduce the femtosecond pulsed electric field in the form of Gaussian to simulate the external light field and study its modulation on the photoexcitation process.In this dissertation,we use the tight binding SSH model combined with the non-adiabatic quantum dynamics method to study the properties of excited states modulated by nonuniform electric field in organic solar cells.The outline and the main conclusions of the studies are as follow.1.Photoexcitation dynamics induced by femtosecond pulsed electric field in organic solar cellsBy introducing a femtosecond pulsed electric field to simulate the light field,and taking into account the temperature effect,the photoexcitation dynamics in the polymer molecule and its heterogeneous interface are studied respectively.Our results unravel that the temperature effect has little effect on the light absorption efficiency of various excited modes in polymer molecules,but it has a significant regulatory effect on the hot excited state internal conversion relaxation after stimulated absorption.In order to explain the source of sub-bandgap light absorption signals at the organic heterointerface,the photoexcitation dynamics in the organic donor/acceptor system is further simulated.The results show that there are two different modes of electron transition in the system,and their transition energies are both lower than the energy gap of the monomer,which is called sub-bandgap excitation or transition.By simulating the evolution of the electron occupation number of band-edge level with time and the charge transfer dynamics at the donor/acceptor interface,it is clearly confirmed that the low-energy transition of the sub-bandgap originates from cold charge transfer absorption,while the high-energy transition comes from hot charge transfer absorption.In addition,we clarified the influence of interface structures and photoexcitation conditions on the charge transfer absorption efficiency and absorption position.These findings provide theoretical guidance for the future experiment regulation of the photoexeitation and internal conversion processes in organic solar cells.2.Transport and dissociation dynamics of excited states driven by spatially nonuniform electric field in organic solar cellsBy introducing a spatially nonuniform electric field,we systematically studied its influence on the dynamics of excited states.The results show that both the exciton and the biexciton in polymers can achieve directional and ultrafast transport under the drive of a nonuniform electric field.In particular,the direction of transport of biexcitons is found to be opposite to that of excitons,and the underlying mechanism is attributed to their reversely polarized behaviors under an external electric field.Furthermore,according to this conclusion,we design the ultrafast regulation of organic luminescence by nonuniform electric field,and predict the regulation results,so as to be confirmed by experiments.In addition,the nonuniform electric field also has a significant regulatory effect on the cold charge transfer state at the donor/acceptor interface,and it is found that its dynamic evolution is closely related to the electric field strength distribution.For the cold charge transfer state initially generated in the strong field region,it will be directly dissociated into free charges;while in weak field regions,before the desired charge separation,it will undergo an ultrafast transport process along the donor/acceptor.Finally,we reveal the transport mechanism of the cold charge transfer state and discuss the influence of acceptor molecular aggregation on the transport of the cold charge transfer state.The above research has certain reference value for explaining some of the latest charge separation phenomena.3.Re-excitation dynamics of excited states induced by femtosecond pulsed electric field in organic solar cellsThe contribution of hot excitons or hot charge transfer states to charge separation has been experimentally confirmed by using pump-push-photocurrent/probe(PPP)spectroscopy.Although some theoretical methods have been developed to explain the results of PPP spectroscopy,the influence of some intrinsic factors of the system on the re-excitation dynamics of the excited state is still unclear.By constructing relevant physical models and considering the light field,we separately studied the re-excitation dynamics of the exciton in polymer molecules and the cold charge transfer state at donor/acceptor interfaces.It is found that both the exciton and the cold charge transfer state can be re-excited to high energy states(including hot excitons and hot charge transfer states)under the action of a femtosecond pulsed electric field.The transition modes and probabilities are primarily determined by the pulse energy.Furthermore,by calculating the intensity of the critical dissociation field under different pulse intensities,the contribution of the charge delocalization effect in the hot excited state to the charge separation is proved.This work not only reasonably explains the experimental results,but also provides an effective means to modulate the charge separation.