| Organic solar cells have been widely concerned by researchers because of their advantages such as cleanliness,low cost,simple process,easy processing and wide source of raw materials.It is very important to optimize the structure of organic polymer materials to improve the photoelectric conversion efficiency of organic solar cells.Donor-acceptor alternating conjugated polymers,in which conjugated blocks of different electron affinity are arranged alternately along the main chain of the polymers,are excellent organic electronic material.The interaction between electron rich units and electron deficient units in conjugated polymers can regulate the photovoltaic and electronic properties of polymers.The research on the internal photophysical mechanism of conjugated polymer materials can clarify the interaction between electron rich and electron deficient units,which is of guiding significance for the development of efficient organic photovoltaic materials and the improvement of the performance of organic solar cells.Here,combining femtosecond time-resolved transient absorption spectroscopy and steady-state spectroscopy,we track ultrafast intramolecular exciton dissociation and excited state relaxation dynamics in organic solar cell donor materials HSD and HXS copolymers.HSD conjugated polymers are composed of carbazole as electron donor unit,5,6-bis(n-octyloxy)-2,1,3-benzothiadiazole as electron deficient unit and thiophene unit as ? bridge.HXS conjugated polymers were prepared with carbazole(CBZ),5,11-dihydroindolo[3,2-b]carbazole(DCBZ),phenanthrocarbazole(PCBZ)as electron donating units and5,6-bis(n-octyloxy)-2,1,3-benzothiadiazole derivatives as electron deficient units.The main work can be divided into the following two parts.In the first part,we investigated the ultrafast intramolecular exciton relaxation dynamics of HSD series alternating conjugated polymers using femtosecond time-resolved transient absorption spectroscopy.Intramolecular exciton dissociation is critical for high efficient mobile charge carrier generations in organic solar cells.However,the regulation of ? bridges on exciton dissociation dynamics in Donor-?-Acceptor alternating conjugated polymers remain still unclear.Based on the measurement and analysis of the steady-state and transient absorption spectra of HSD alternating conjugated polymers connected by different ? bridges,it is found that the addition of thiophene unit as ? bridges will lead to the red shift of steady-state absorption spectrum.At the same time,the transient spectrum of the sample has an extra spectral signal,which is the bridge-specific charge transfer state(CT').This CT' state results in a new intramolecular exciton dissociation pathway of HSD-B and HSD-C.Depending on the proportion of CT'state and nongeminate recombination are important step for the understanding of high power conversion efficiencies in HSD-B than in HSD-C.We propose that this bridge-specific exciton dissociation pathway plays an important role in ultrafast intramolecular exciton dissociation of organic photovoltaic material D-?-A alternating conjugated polymers.In the second part,we studied the ultrafast dynamics of HXS series alternating conjugated polymers.It is found that different donor materials will affect the excited state relaxation mechanism of the copolymer.The donor material of PCBZ has no obvious dependence on the alkyl side chain,and different alkyl substituent have a weak effect on the properties of PCBZ copolymer.However,The donor material of DCBZ shows obvious alkyl side chain dependence.The substitutions of branched and straight chains in the alkyl side chain affect the degree of electron conjugation along the aromatic main chain of DCBZ copolymer,which leads to different excited state relaxation paths of the copolymer.Our findings provide some ideas for the synthesis of efficient donor materials in organic photovoltaic devices,and provide an important photophysical basis for optimizing the performance of organic solar cells. |
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