Study On The Photovoltaic Performance And Mechanism Of High Efficiency Ternary Polymer Solar Cells

Polymer solar cells（PSCs）have attracted considerable attention because of their unique advantages such as flexibility,low cost and light weight etc.In recent years,ternary strategy has been proven to be an effective way to improve the performance of PSCs,where ternary device with complementary absorptions are usually used in a single active layer,resulting in high short-circuit current density(J_sc)and PCE in devices However,there are still some limitations and challenges for further development of ternary PSCs:lower PCE and poor stability.Herein,this paper has carried on in-depth research on the above issues,and is committed to the improvement of the performance and stability of the ternary devices.Therefore,the following research works have been done:In the second chapter,we mainly focused on solving the problems of processing difficulties and poor stability of thick film polymer materials with strong aggregation ability in traditional fullerenes system.First,the performance of the device was improved by introducing a liquid crystal small molecule BTR in the PffBT4T-2OD:PC71BM system.Due to the strong aggregation of polymer PffBT4T-2OD,the device must be processed under 110^oC,and the corresponding room-temperature processing devices showed a poor performance.However,owing to the decrease of polymer aggregation intensity in this ternary system,the incorporation of BTR into the PffBT4T-2OD:PC71BM system can decrease the lamellar distance and improve the phase separation of the resulting blend films,and resulting in a much improved mobility of the ternary active layer.Furthermore,this ternary strategy shows an improved stability and a well general applicability for other systems that need hot-processing.When replacing the FBT-based polymer PffBT4T-2OD with NT-based polymer PNTT,the optimized ternary device obtained an improved performance with a PCE of 11.44%,which is,as far as we know,one of the highest-efficiency thick film PSCs reported to date.And the corresponding large area devices obtained a higher PCE of 7.07%.Our results provide an effective and useful approach to fabricate high performance thick-active-layer PSCs that processed at RT,and then it may meet the needs of future roll-to-roll production.In the third chapter,we attracted much attention on lower the photon utilization rate and poor stability problems of nonfullerene acceptor.A series of high-efficiency and air stable inverted ternary PSCs based on a host binary system of PBDTTT-EF-T:ITIC were fabricated by employing a wide-bandgap donor PCDTBT as the third component.The photon harvesting,exciton dissociation,charge transport and collection of the ternary PSCs are simultaneously optimized by regulating the content of two donors.At the same time,the PCE value of the ternary devices in the thickness range of 70-300 nm is still more than 90%of its initial PCE,so the large-area devices also can obtain a better device performance.Furthermore,both of the inverted and conventional ternary PSCs own better stability compared with the related binary PSCs in air conditions.This ternary strategy can be further confirmed by the use of a similar acceptor IEICO-4F to replace the ITIC acceptor in the conventional ternary PSCs.The champion PCE value of 12.15%are achieved for ternary PSCs,which is one of the highest value for ternary nonfullerene PSCs.In the fourth chapter,we integrated the advantages of the previous two chapters,the high performance ternary devices based on the blend acceptor of fullerenes PC71BM and non fullerenes ITIC was fabricated through adjusting the blending proportion of acceptors and the stability of ternary device was investigated at same time.The conventional PCDTBT:PC71BM-based devices obtained a PCE of 7.00%.By optimizing the content of ITIC,the ternary device showed the best PCE of 8.27%increased by 18%.The PCDTBT,ITIC and PC71BM have the ideal cascade energy level structure,which is beneficial to the separation and transport of carriers.The introduction of ITIC can improve the absorption of the binary device,which can improve the photon utilization ratio of the blend film.At the same time,the ternary device showed better stability of the active layer,and the PCE of the device only decreased by about 10%after 180 min thermal annealing at 130 ^oC.Due to the ternary device with high exciton generation rate,exciton dissociation,the charge transfer and extraction efficiency,it was contributed to improve the J_SCC of devices and then improve the device performance.These results show that the use of ITIC nonfullerene acceptors as the third component in fullerene system is an effective method to improve the device performance,which can effectively improve V_OC,J_SC and the thermal stability of the PSCs.In the fifth chapter,we adopted appropriate small molecules DRCN5T as the third component and then it was added to the polymer PBDTTT-EF-T:PC71BM blending system.In the process of device preparation,we also found that selecting the appropriate thermal annealing temperature and the solvent annealing time played an important role in device performance.The device based on PBDTTT-EF-T:PC71BM achieved a PCE of 9.74%,While for the ternary device with DRCN5T,the PCE was nearly unchanged or even worse than binary device.However,after the thermal annealing（TA）and chloroform（CF）solvent anneal（SVA）,the performance of the ternary device was significantly improved.The ternary device containing 10%DRCN5T obtained a PCE from 9.73%to 10.51%,and a FF of 75.80%,such a high FF in PSCs is not common indicating a strongly suppressed recombination behavior and a high carrier mobility.It was found that the TA+SVA treatment can promote the crystallization of DRCN5T,which made the arrangement of small molecule become more orderly,and then promoted the crystallization of PBDTTT-EF-T become more orderly,finally,a strong absorption and high mobility was available for ternary devices.All in all,the overall performance of the ternary device improved significantly with the synergistic effect of TA and SVA,and the FF of the device can remain at a higher level（over 73%）.These results show that choosing the reasonable optimization method of morphology on the third component molecules in the active layer is an effective way to further improve the performance of the ternary device.