On The Morphology Control Of Organic Bulk Heterojunction Solar Cells

Organic solar cells?OSCs?have garnered considerable research interest because of their prominent merits,such as low cost,mechanical flexibility and solution processing in the past decades.Thepower conversion efficiency?PCE?of OSCs had exceeded 12% with tremendous efforts.Although the morphology of active layer significantly impacts the device performance has been proven in numerous reports,there didn't exist a widely applicable rule on the morphology control.The morphology remains complex and a limiting factor for further development of the OSCs.In this thesis,we studied the morphology control of active layer based on the working mechanism of bulk heterojunction OSCs.The morphology manipulation of active layer mainly focus on the following aspects:the domain length scale,the donor-acceptor interface,the interpenetrating network and the phase purity.We first examined the morphology of complex ternary solar cell system,then the molecular orientation control in p-DTS?PTTH₂?₂/N2200 blends was adjusted by the heterogeneous nucleation.The main results are as follows:1.The current understanding of the active layer morphology in ternary OSCs is superficial owing to the much more variables and complexities compared to that of binary OSCs.The PTB7-Th:PF12TBT:PC₇₁BM ternary system with complementary polymer absorption spectra and efficient energy transfer from PF12 TBT to PTB7-Th was anticipated an outstanding performance.However,only limited improvement in PCE was achieved when the ternary devices were processed from CB/DIO,which is because large PC₇₁ BM domains formed with the addition of amorphous PF12 TBT and the PF12 TBT molecules embedded in large PC₇₁ BM domains served as trap sites.Constraining the formation of large PC₇₁ BM domains and avoiding the PF12 TBT molecules being fully embedded in PC₇₁ BM domains are needed for a better performance.Polymer preaggregation before liquid-liquid phase separation is beneficial to construct the bicontinuous interpenetrating networks with proper phase-separated domains.Therefore,para-xylene was introduced into CB/DIO to weak polymer-solvent interaction for enhanced PTB7-Th solution-phase aggregation.The enhanced PTB7-Th aggregation in CB/PX/DIO solution restricted the extent of liquid-liquid phase separation and a well-developed polymer network formed with improved PTB7-Th crystallinity which prevented large PC₇₁ BM domains from forming via fluid-phase Ostwald ripening in the liquid-solid phase separation stage.The appearance of PF12 TBT emission and raised PTB7-Th emission in ternary film processed from CB/PX/DIO suggested less PF12 TBT molecules were embedded in PC₇₁ BM domains and the energy transfer from PF12 TBT to PTB7-Th was more efficient.Thus,PCE increased from 8.09% for binary blends processed from CB/DIO to 9.28% for ternary blends processed from CB/PX/DIO.Our work reveals the significance of ternary blend morphology for device performance,and highlights the potential for further boosting the performance of ternary OSCs by morphology control.2.The molecular orientation of the donor molecules and acceptor molecules significantly influence the carrier mobility and the dissociation efficiency of excitons.However,the understanding of molecular orientation is rather phenomenological and no principle has been proposed that can guide the molecular orientation control.We proposed that the molecular orientation would be influenced by the manner of nucleation.Due to the large difference in melting temperature between p-DTS?PTTH₂?₂andN2200,the step crystallization is feasible and the manner of p-DTS?PTTH₂?₂nucleation of can be controlled by the temperature.We demonstrated that the heterogeneous nucleation of p-DTS?PTTH₂?₂ induced by the N2200 crystal lead to the face-on orientation of p-DTS?PTTH₂?₂,while homogeneous nucleation of p-DTS?PTTH₂?₂ would adopt edge-on orientation.The nucleation barrier of homogeneous nucleation when crystalline from solution was adjusted by controlling the solution aggregation of p-DTS?PTTH₂?₂ and N2200.Thus,the sequence of nucleation can be altered.When nucleation of p-DTS?PTTH₂?₂ is prior to that of N2200,the p-DTS?PTTH₂?₂ crystal induced the heterogeneous nucleation of N2200,leading to the edge-on orientation of N2200.The composition of the blends also impact the orientation due to liquid-liquid phase separation in solution.We drew the phase diagram of p-DTS?PTTH₂?₂/N2200 system according to the influence of solvent and composition on nucleation sequence.Our work provided a new manner for adjusting the orientation in donor/acceptor blends,which improved the current understanding of molecular orientation.