| Organic solar cells(OSCs)have drawn much research attention due to their advantages in achieving light weight,flexible,semi-transparent and low-cost devices.Ternary OSCs as an efficient way to improve the performance of OSCs have raised wide interest of scientists and a substantial amount of research works have been carried out on this issue.However,there are still some limitations and challenges for further development of ternary OSCs: 1.Current ternary OSCs is still limited because most ternary OSCs use thin active layers of about 100 nm in thickness,which is not sufficient to absorb all photons in their spectral range and may also cause problems for future roll-to-roll mass production that requires thick active layers;2.The reason of continuously tunable open-circuit voltage(VOC)in some ternary OSCs is still in doubt;3.The research of the application of ternary OSCs in non-fullerene OSCs which have more potential to achieve higher performance and the exploration of its regularity and working mechanism is still lacked.Therefore,our research works in this thesis are mainly focus on the above issues.In the first part,we have devoted ourselves to solve the problem of the thickness of active layer for ternary OSCs.First,we doped the polymer based on naphtho[1,2-c:5,6-c]bis[1,2,5]thiadiazole with p-type dopant(F4-TCNQ)to enhance the hole mobility.Though it enhanced the power converted efficiency(PCE)of the thin-film device,it did not achieve efficient thick-film device.Then,we introduced a nematic liquid crystalline small molecule(BTR)into PTB7-Th:PC71BM binary system as a second donor to fabricate ternary OSCs for the first time.The addition of BTR into the PTB7-Th:PC71BM system can decrease the π-π stacking distance,enlarge the coherence length,and enhance the domain purity of the blend film,thus significantly increasing the hole mobility of the ternary active layer.Finally,we achieved a high-performance thick-film(optimized thickness 250 nm)ternary OSC with a best PCE of 11.4%,which is,to the best of our knowledge,one of the highest-efficiency ternary OSCs reported to date.Furthermore,this ternary system is the first reported highperformance OSC with a thick active layer that can be processed at room temperature and that can meet the needs of future roll-to-roll production.Therefore,our results showed that the introduction of a highly crystalline small molecular material into ternary OSCs is an efficient way to achieve high-performance thick-film devices.In the second part,we used two oligomers(S2 and S7)based on 5,6-bis(octyloxy)-4,7-di(thiophen-2-yl)[c][1,2,5]thiadiazole as donors and PC71 BM as acceptor to fabricate ternary OSCs for the first time,tried to explore the regularity and the reason of the tunable VOC.We have found that VOC was continuously increased due to the increasing energy of the charge transfer state(ECT)of the active layers with the increasing content of S2.Further investigation suggested that VOC was also continuously tunable upon change in ECT in a ternary blend system that consisted of S2 and its corresponding polymer(P11):PC71BM.It is interesting to note that higher power conversion efficiencies can be obtained for both S2:S7:PC71BM(20wt%S2)and S2:P11:PC71BM(40wt%S2)ternary systems compared with their binary systems,which can be ascribed to an improved VOC due to the higher ECT and an improved fill factor due to the improved hole mobility resulted from the improved film morphology upon the incorporation of S2.These findings provide a new guideline for the future design and post-treatment of conjugated polymers for achieving higher performance of OSCs.In the third part,we have studied the influence of the regularity of the polymers based on 5-fluoro-2,1,3-benzothiadiazole(FBT)asymmetric unit in the performance of non-fullerene OSCs and explored its application in ternary OSCs.We have found that the regioregular polymer molecule has stronger interaction with small molecular acceptor,which lead to a smaller phase separation size for more efficient exciton dissociation.Futhermore,the regioregular polymer has higher hole mobility for faster charges transportation and extraction and less recombination.Finally,the device based on the regioregular polymer has higher shortcircuit current density(JSC)and PCE than that based on the random polymer.However,when we tried to introduce the third components with different absorption range into the regioregular polymer: PC71 BM blend film to fabricate ternary OSCs,it did not improve the performance of the devices.Therefore,to improve the performance of non-fullerene OSCs with ternary way,we may not only consider the complementary absorption issue,but focus more on the control film morphology after introducing the third component.There are still more research works need to be finished to explore its regularity and working mechanism. |
