Regulation Of Mechanical Properties Of Conjugated Materials In Organic Solar Cells

Organic solar cells(OSCs)desire the merits of light weight,fabrication via solution process and flexibility,which can be used as the power supply facility in flexible electronics.Mechanical properties play the important role in flexible OSCs,which will determine their stability during bending,stretchable and recovered environment.Therefore,photoactive layers that usually contain conjugated materials as electron donor and acceptor in OSCs should have excellent mechanical properties for flexible application.The mechanical properties for conjugated materials include modulus,crack of onset(COS),toughness and so on.However,conjugated materials usually show poor mechanical properties due to their rigid conjugated backbones.In order to adjust the mechanical properties of photoactive layers,in this thesis,two methods were introduced,via introducing insulating polymers as the third component and using long alkyl side units to construct new conjugated polymers.The studies include,(1)the photoactive layers based on PM6 as donor polymer and BTP-BO-4Cl as acceptor were selected,in which two insulating polymers,PS and SBS,were introduced as the third component.The effect of PS and SBS on the morphology,mechanical properties and photovoltaic performance was studied in detail.It was found that,when enhancing the content of PS and SBS as additive,the efficiencies were gradually drop.This was mainly due to the reduced electron mobilities as confirmed by SCLC measurement,resulting in severe charge recombination.The PM6:BTP-BO-4Cl thin films without or with PS or SBS then performed tensile test via float-on-water(FOW)method,in which PS and SBS were found to have the detrimental effect on the mechanical properties of the photoactive layers.This detrimental effect was due to the immiscibility between PS/SBS and PM6:BTP-BO-4Cl blend,as confirmed by their Flory-Huggins parameter analysis.(2)another strategy by using long alkyl side units on the conjugated polymers was then initiated to improve the mechanical properties.In this part,a typical donor polymer PM6 was selected,and the side units were tuned from short 2-ethylhexyl units to long 2-octyldodecyl units.The monomers were then used to perform Still polymerization,and three conjugated polymers,PLC-L,PLC-M and PCL-H with the number-average molecular weight of 38.8 k Da,81.1 k Da and 178.3 k Da,were obtained via controlling the polymerization condition.These polymers as donor and BTP-BO-4Cl as acceptor were applied into OSCs,in which PM6 based cells provided a high efficiency of 16.56%.For comparison,PLC-L exhibited 6.61% in solar cells,while the efficiency was enhanced to 13.24% in PLC-H based solar cells.The distinct photovoltaic performance in these polymers was due to the microphase separation,in which PM6 based blends have good phase separation with small domain,but PLCs show large domain size in blends.This would be detrimental to the exciton diffusion into the donor/acceptor interface,resulting in low efficiencies in solar cells.It is worthy to note that,PLCs with long side units showed high opencircuit voltages in solar cells,which is due to the longer distance between donor and acceptor with reduced non-radiative recombination losses.The mechanical properties based on these materials by using tensile test via FOW method were then performed,in which PM6 showed a COS of 10.4%.To be surprised,PLCL,M and H polymers exhibited high COS of 40%,160% and 180%,which is much higher than PM6.Therefore,it can be concluded that,introducing longer alkyl side units into conjugated polymers is an efficient method to improve their mechanical properties,which have the potential application in flexible OSCs.