Study Of Quantum Coherence In Organic Photovoltaic Conversion

By the structure of donor-acceptor heterojunction,the organic semiconductors can realize the transformational process from the optical energy to the electric energy.Compared with inorganic solar cells,organic solar cells exhibit large amount of advantages including flexibility,easy for production,low cost...However,the low energy conversion efficiency limits the promotion of the organic solar cells now.Generally speaking,there are four steps in organic photovoltaic conversion:1.After absorbing the photon,the material transits from the ground state to the excited state,producing an electron hole pair.2.The electron hole pair diffuses to the interface of the donor-acceptor.3.The charge transfer process takes place and creates a charge-transfer state.4.The charge-transfer state is further dissociated,which forms free electrons and holes in the end.In this study,we pay close attention to the physical mechanism on step 1 and step 3.With the discovery of the quantum coherence in the system of the natural photosynthesis,more and more emphasis has been put on how to utilize the quantum coherence to improve the efficiency of organic photoelectric conversion.In step 1,because of thermal relaxation,the absorbed photon which is larger than the energy gap can only produce the energy equal to it.However,considering the singlet fission process with quantum coherence,a high energy singlet exciton can convert to two triplet excitons.This multi-exciton generation process can effectively reduce thermosteresis in the system,which has the potential to break the so called S-Q limit.In step 3,as the Coulomb interaction within the electron hole pair in the organic materials is rather large,it is commonly accepted that at least 0.3 eV driving energy is needed for effectively charge-transfer peocess.Nonetheless,the driving energy less than 0.1 eV in the effective photovoltaic conversion organic solar cells has already been found in recent studies,and the charge-transfer process happens at the first one hundred femtosecond,which indicates the necessity in considering about the non-classical quantum coherence to fully comprehend this physical mechanism.For the purpose of exploring the quantum coherence in the organic photoelectric conversion,we build a two-dimensional electronic spectroscopy in the pump-probe configuration.The major results are summarized below:1.For the first time,we successfully measure the weak magnetic dipole interaction strength(0.008 GHz)within the two triplets generated from singlet fission in tetracene single crystals.In quantum mechanics,degenerate energy levels will mix to each other and open an energy gap when considering the interaction perturbation.We adjust the magnitude and direction of the external magnetic field to make the some sublevels of the triplet pairs in tetracene to near level-crossing state.Through measuring the frequency of the quantum beat corresponding to energy gap opened by the magnetic dipole interaction and comparing with the theoretical simulation,we determine that the interaction strength is around 0.008 GHz,which is far less than the interaction strength between the neighboring molecules in the tetracene crystal.This result verifies the important effect that the electron-photon coupling caused exaction delocalization plays an important role in singlet fission process.2.For the first time,we demonstrate the polaron pairs other than the exactions dominate the hole transfer process in an all-polymer solar cell system.Some organic materials can coherently produce two excited states after photon absorption,namely excitons and polaron pairs.Till now,the effect of polaron pairs in the charge transfer process is still unclear.We obtain the dynamic of the polaron paris in the acceptor(N2200)by measuring the transient absorption spectra of the blend(J51/N2200).Comparing the results between the neat film N2200 and the blend,we discover that the polaron pairs in N2200 mainly controls the hole-transfer process,with the exciton hardly involved.To further confirm the experiment finding,we use the tight binding approximation of SSH theory to simulate the charge separation process of excitons and polaron pairs on the donor-acceptor interface.The result verifies that due to the low binding energy of polaron pair,it becomes more effective in charge separation on the donor-acceptor interface.Technically,it will be of great promise if the polaron pairs and exctions can collaborate to jointly promote the efficiency of charge generation,which may be an alternative strategy to improve the device performance.3.For the first time,we observe the coherent hole transfer process in the non-fullerene organic bulk heterojunctions.By two color two-dimensional electronic spectroscopy(2DES),we study a series of non-fullerene polymer-small molecule cells(including J52/ITIC,J51/ITIC,J61/ITIC,J71/ITIC)hole transfer process.All of the four blends' 2D maps show that the hole-transfer has already taken place at the first 20 fs,with long-lasting coherent phonon oscillation.According to the theoretical simulation,the coherent vibronic coupling is the primary cause of the ultrafast hole transfer.Considering the vibronic coupling between the excited state and the hole-transfer state,the 0.3eV driving energy is no longer necessary,which has a great significance in designing new organic solar cells with higher open-circuit voltage,and reduce the energy loss in the charge separation process.