The Reasearch On Improving Performance Of Polymer Solar Cells Via Optimizing Phase Separation Degree Of Active Layer

As a new type of optoelectronic device, polymer solar cells (PSCs) have attracted more and more attention all over the world. In the past decades, the power conversion efficiency (PCE) of PSCs has been almost increased at an annual rate of 1% due to the continually endeavor in exploiting new materials, designing device structure and optimizing device preparation process. Recently, the PCE PSCs has been increased to 11.7%, which was reported by Prof. Yan He in Hong Kong University of Science and Technology. In this thesis, the research was carried out according to the device physics of PSCs, aiming at improving the PCE of PSCs and clarifying the related mechanism.In order to improve exciton dissociation and carrier transport in active layers, the phase separation degree was optimized with solvent additive 1,8-diiodooctane (DIO), mixed solvents and different post-treatment. In addition, appropriate buffer layers were employed to enhance carrier collection at electrodes. For additive-free PSCs, the performance improvement due to optimization of phase separation degree in active layers was realized by treating the active layers with mixed solvents consisting of work solvent and inert solvent. For characterization of vertical phase separation in active layers, the variation of phase separation degree was reversely inferred according to the following characterization:i) the absorption spectra, photoluminescence (PL) spectra, atomic force microscopy images and water contact angles of different aticve layers; ii) the current density-voltage (J-V) curves under the forward/reverse bias of hole-only and electron-only devices. The detailed works were mainly summarized as the five aspects:i) The research on improving performance of PSCs with solvent additive and on related mechanism. The PCE of PSCs was increased from 3.77% to 4.37% by preparing the active layers with poly(diketopyrrolopyrrole-terthiophene):[6,6]-phenyl-C71-butyric acid methyl ester (PDPP3T:PC71BM,1:2, wt/wt) solution with 5 vol% DIO additive. The performance improvement of PSCs was mainly attributed to enhanced exciton dissociation, carrier transport and collection, which was analysed from the J-V curves and external quatum effieciency (EQE) spectra. It means that DIO additive is beneficial to improving phase separation degree of the active layers. In order to verify that the dispersion of PC71BM in the active layers can be adjusted by DIO additive, the variation known that PMMA shows negligible absorption in the visible range due to its large band, the spectra variation of PMMA:PC71BM blend films should be attributed to the change of PC71BM dispersion. For the the PMMA:PC71BM blend film with DIO additive, the absoption intensity was decreased in the visible range, the PL intensity was increased and the emission peaks was blue shift. The similar phenomenon was also observed by decreasing the PC71BM doping ratio in PMMA:PC71BM blend films, showing that PC71BM molecules were more dispersed in the blend film due to the addition of DIO. Several conclusions were obtained according to the phenomenon:the dispersion of PC71BM in the active layers can be adjusted by DIO, part PC71BM may diffuse towards the top surface of active layer along with slow volatilization of the residual DIO, forming vertical phase separation in the active layer and improving the performance of PSCs.ⅱ) The research on improving performance of PSCs by simultaneously employing interfacial dipole layer and DIO additive. For the widely used anode buffer layer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), the PSS- riched near active layer is beneficial to hole extraction. However, it is difficult to adjust molecular arrangement in PEDOT:PSS. In order to induce more PSS" riched close to active layer, LiF dipole layer was sandwiched between indium tin oxid (ITO) and PEDOT.PSS. A series of PSCs were prepared based on PEDOT.PSS, PEDOT:PSS/LiF, LiF/PEDOT:PSS (without annealing) and LiF/PEDOT:PSS (annealing treatment) anode buffer layers, and the related PCEs were 4.77%,5.03%,5.59% and 5.03%, respectively. The hole collection was enhanced for the PSCs based on PEDOT:PSS/LiF buffer layer, however, the performance of PSCs was finitely improved due to random orientation of LiF dipoles. For the LiF ultrathin layer sandwiched between ITO and PEDOT:PSS, the orientation of LiF dipoles was optimized by Coulomb field formed by O2- on surface of ITO and H+ in PEDOT.PSS, and further optimized by annealing treatment. More PSS- was riched close to the active layer due to the guidance of LiF dipoles with optimized orientation, leading to enhanced hole extraction in the related PSCs. Among hole-only device based on different anode buffer layers, the device with LiF/PEDOT:PSS buffer layer showed higher current density under the same applied bias, meaning that this kind of composite buffer layer was beneficial to hole extraction. For the application of the composite electrode in PSCs, it is beneficial to hole collection.ⅲ) The reaserch on balancing carrier transport and improving PCE of PSCs by optimizing phase separation degree in active layers with hot solution. The active layers of PSCs usually show higher hole transport ability than electron. Hence, it is an effective way to improve the performance of PSCs by balancing the carrier transport in the active layers. It is known that electron transport channels and ability can be improved by optimizing the distribution of acceptor material in active layers. In order to form more uniform donor/acceptor distribution and improved bi-continuous interpenetrating networks in PDPP3T:PC71BM (1:2, wt/wt) active layers, the film forming process was shorted by preparing active layers with hot (70℃) solution. Compared with the PCE of PSCs with active layer prepared with room temperature solution, the PCEs were increased from 5.54% to 5.85% and 6.22% for the PSCs with active layers prepared from room temperature solution with DIO additive and 70℃ solution, respectively. The performance improvement of PSCs should be mainly attributed to the more balanced carrier transport. In order to further demonstrate the conclusion, the related hole-only and electron-only devices were prepared to measure carrier transport ability in the active layers prepared with different solutions. The results showed that the active layer prepared with hot solution had higher electron transport ability and more blanced carrier transport, leading to the improved performance of the related PSCs.iv) Improving vertical phase separation of active layers and performance of related PSCs by accelerating volatilization of solvent additive DIO in active layers. The PBDT-TS1:ICBA (1:1.5, wt/wt) or PBDT-TS1:PC61BM active layers were prepared with room temperature solution,70℃ solution, room temperature solution doped with DIO additive (3 vol%) and 70℃ solution doped with DIO additive (3 vol%), respectively. All the active layers were put in vacuum conditions (～1 Pa) quickly to complete drying process. For devices with indene-C60 bisadduct (ICBA) as acceptor, the best PCE of 4.32% was obtained for the device with the active layer prepared from 70℃ solution doped with DIO additive. For devices with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as acceptor, the best PCE of 5.97% was obtained for the device with the active layer prepared from room tempreture solution doped with DIO additive. It means that the migration of acceptors along with DIO additive was affected by the molecular structure of acceptors. The migration of ICBA among the polymer networks may be limited by its symmetrical molecular structure, limiting electron transport in the active layers. The electron-only devices were prepared based on PBDT-TS1:ICBA (1:1.5, wt/wt) or PBDT-TS1:PC61BM (1:1.5, wt/wt) active layer, and the carrier transport ability showed that the electron transport channels were low-effective for the active layers based on ICBA.v) The study on treating the solvent additive-free active layers with mixed solvents consist of work solvent and inert solvent. The phase separation degree in active layers and the performance of PSCs were optimized by adjusting doping ratio of work solvent in mixed solvents and soaking time of mixed solvents. The phase separation degree in PBDT-TS1:PC71BM active layer was adjusted by treating the active layers with MeOH doped with 10 vol% CH2Cl2 or CB, leading to that the PCE of PSCs were improved from 6.69% to 7.21% or 6.69%, respectively. The performance of PSCs was mainly due to the optimization of vertical phase separation in the active layers, which was demonstrated from the change of PL spectra of films, water contact angle of active layers with and without mixed solvent treatment, the change of carrier transport in electron-only device. In view of the solvent additive-free active layers, the treatment on active layer with mixed solvents is effect on optimizing the phase separation in the active layer and improving the stability of PSCs.