Investigation Of The Photophysical Characteirstics Of Low Bandgap Polymer APFO3

Photovoltaic material is the basis of researching microelectronics andoptoelectronics technology. There are a large number of special photovoltaic propertymaterials which are valued and researched by people at present. Because the organicphotovoltaic materials has more advantages than inorganic photovoltaic materials,such as various molecules, being made large area thin film, easy processing, low-cost,non-toxic and so on, people paid attention to it with a lot of interest and researched itfrom the last century30’s, and got a series of discoveries.Currently, the organic material has been widely used in many fields as animportant sort of material with optical activity such as in the organic light emittingdiode (OLED), organic field effect transistor, an organic solar cell, an organicmemory storage, photodynamic therapy and so on.In the study of solar cells，the most commonly used low-band gap conjugatedpolymer was the copolymer which used fluorene or fluorene derivatives as theacceptor groups of electron-donating group, benzothiadiazole or benzothiadiazolederivatives. The material selected in this paper was the low-band gap polymer APFO3.Through the experimental test to APFO3, we have detailed understanding of thephotophysical properties of the excited state, and we could employ the densityfunctional theory (DFT) to know the basic electronic transition mechanism after thephotoexcitation. Overall, the present study has covered several fundamental processestaking place in the APFO3polymer.The analysis of the UV-Vis absorption spectrum and the emission spectrumshows that the absorption spectra of APFO3in three solutions with different polarityare identical, but the emission spectra are significantly different, the emission bandred shifts as the polarity of solvent gradually enhances, implying the Intrachain chargetransfer (ICT) may occur in this π-conjugated polymer. The UV-Vis absorption spectrum and the emission spectrum of APFO3film are very similar to thesolution, but they red shift. According to the theory of molecular transitions, theIntrachain charge transfer (ICT) will disappear at this time.The ground geometry of APFO3oligomer with two repeat units was optimizedby the Gaussian09program package，The calculation results analyzed that the chargecloud in oligomers clearly redistributes through the Intrachain charge transfer (ICT)process after photoexcitation, combined with simulated absorption spectrum and thevalues of oscillator strength and confirmed that the low energy absorption (P1peak)band may be ascribed to the ICT.The results of transient absorption confirm again that ICT indeed exists andcompetes with the vibrational relaxation at the same time, when APFO3is in amonodisperse system. This ICT process would disappear due to the influence ofinterchain interaction when APFO3is in the condensed phase, where the excitondecay would be dominant in the relaxation process after photoexcitation. Thephotoexcitation dynamics of APFO3film blending with PC61BMare presented,which show that the exciton may be dissociated completely as the percentage ofPC61BMreaches~50%. Meanwhile, the photovoltaic performance based on blendheterojunction shows that the increase of photocurrent is little if the percentage ofPC61BMexceeds~50%. Overall, the present study has covered several fundamentalprocesses taking place in the APFO3polymer.