Effect Of Solvent Annealing Under High Pressure On The Performance Of Polymer Solar Cells

The polymer solar cells (PSCs) have been attractive because of the advantages oflow cost, light weight, simple processing and mechanical flexibility. However, thepower conversion efficiency (PCE) of PSCs is still relatively low compared with thatof the inorganic solar cells. There is still a long way before the PSCs are suitable forpractical application. In order to improve the PCE of PSCs, various approaches, suchas improving the device structure, optimizing the electrode buffer layer, the synthesisof new material, improving the microstructure and morphology of the active layer,were studied. It has been demonstrated that the microstructure and morphology of thebulk heterojunction active layer have critical impact on the performance of PSCs.In this thesis, the active layers of PSCs are prepared by solvent annealing underhigh environmental pressures. The evaporation rate of the residual organic solvent isslowed by increase the environmental pressure of solvent annealing. Therefore thedrying time is prolonged to improve the microstructure, morphology and quality ofthe active layers in this way.Firstly, the active layers were prepared by solvent annealing under highenvironmental pressures, and the PSCs with the device structure of ITO/MoO3/P3HT:PC61BM/LiF/Al were fabricated. The experimental results show that theperformances of the devices prepared by solvent annealing under high environmentalpressures have been significantly improved, and the device prepared under thepressure of2.0MPa has the highest PCE of3.69%, about29%higher than thatprepared under normal environmental pressure. In order to investigate the effects ofsolvent annealing under high environmental pressure on the active layers, the XRD,absorption spectra and AFM tests were taken. We also took the EQE measurement ofthe PSCs, and fabricated the electron-only devices and hole-only devices under high environmental pressures. The results show that the absorption of the active layers andthe number of photogenerated carriers are increased by increasing of environmentalpressure of solvent annealing, and that the degree of crystallinity and phase separationof the active layer, and the charge-carrier mobility, which are beneficial for thephotogenerated exciton dissociation and carrier transportation, are also increased.However, further increasing the environmental pressure of solvent annealing willinduce that the degree of phase separation of the active layer is too high, the electronmobility decreases and the recombination rate of the photogenerated exciton increases,making the PCE of the device decreased.Secondly, in order to improve the PCE of PSCs, the low bandgap copolymer ofPTB7and PC71BM were used as the donor and acceptor, and the PSCs with thedevice structure of ITO/MoO3/PTB7:PC71BM/LiF/Al were fabricated. First, the activelayers were prepared by solvent annealing in a glove box to explore the optimum spincoating speed and time of the active layer. Then the active layers were prepared bysolvent annealing under high environmental pressures, and the optimum thickness ofthe active layer was also explored. The results show that the performances of thedevices have been significantly improved by increasing the environmental pressure ofsolvent annealing. The highest PCE of the device is6.39%, when the pressure is3.0MPa and the spin coating speed of the active layer is1200rpm.The method of solvent annealing under high pressure can prolong the dryingtime of the active layer, increase the degree of crystallinity and phase separation ofthe active layer, be beneficial for the photogenerated exciton dissociation and carriertransportation, improve the performance of the device, and has a good applicationprospect.