The Effect Of Active Layer Morphology Control On The Thick Film Organic Solar Cells

With the continuous and rapid development of the performance of organic solar cells(OSCs),the research of thick film OSCs has attacted much attention due to the compatibity with the commercialization process of large-area printing.Although the power conversion efficiency(PCE)of high-performance thin film non-fullerene OSCs has exceeded 18%,the efficiency of thick film OSCs develops slowly because the efficiency of charge recombination of thick film OSCs is much higher than that of thin film OSCs.In order to achieve the transition from laboratory scale to industrial high-throughput manufacturing,it is most important to obtain the ideal morphology for the charge generation and charge transport in the thick film OSCs.Therefore,in view of the bottleneck of restricting the industrialization problems of thick film OSCs,the morphology control based on thick film of OSCs is studied.By selecting different co-solvent system,adding the third component and using up-side-down thermal annealing on active layer,the PCE of thick film OSCs is significantly improved.The specific contents are divided into four parts shown as follows.1.Study on the morphology control by using co-solvent system for high efficiency thick film OSCsThe effects of co-solvent on the morphology,crystallization and light intensity distribution of thick film organic solar cells based on polymer of[(5,6-difluoro-2,1,3-benzothiadiazol4,7-diyl)-alt-(3,3000-di(2-octyldodecyl)-2,20,50,200,500,2000-quaterthiophen-5,5000-diyl)](Pff BT4T-2OD):[6,6]-phenyl-C₇₁-butyric acid methyl ester(PC₇₁BM)are studied.By adjusting different co-solvent systems in thick active layer,it is found that the polymer crystallization,fullerene absorption and the morphology of Pff BT4T-2OD:PC₇₁BM are optimized by using a co-solvent of chlorobenzene(CB):dichlorobenzene(DCB):1,8-diiodooctanein(DIO).The optimized active layer thickness of 250 nm is analyzed by using transfer matrix theory,resulting in enhanced fill factor(FF)and PCE of 66.7%and 9.16%,respectively.This phenomenon is due to the fact that about 90%of the incident light can be absorbed in the active layer with a thickness of 250nm,which contributes to the light intensity distribution and exciton generation rate.2.Morphology control effects of ternary blended ICBA toward low-temperature-processed thick film OSCsAn improvement of the PCE of OSCs from 8.06%to 9.81%by blending indene-C₆₀bis-adduct(ICBA)in Pff BT4T-2OD:PC₇₁BM active layer.We find the incorporation of ICBA could lead to the Pff BT4T-2OD:PC₇₁BM solution with less temperature-dependent preaggregation,thus promoting the crystalline and preferable morphology of the active layer.Additionally,we demonstrate the addition of ICBA can increase the charge carrier lifetime,ensuring efficient charge carrier transport while suppressing bimolecular recombination and thus enhance FF.This paper illustrates a valuable strategy toward low-temperature-processed thick film OSCs with high performances.3.Toward efficient thick film OSCs by introducing iridium complex as an aggregation reshaping auxiliaryThe morphology control of thick film OSCs is an important factor to determine their PCE.Particularly,during the building of phase separation,aggregation morphology plays a prominent role in the control of both horizonal and vertical gradient distribution of donor/acceptor(D/A)in active layer.In this work,we introduced a novel iridium complex of(tfmppy)₂Ir(tpip)into the active layer of Pff BT4T-2OD:PC₇₁BM as an“aggregation reshaping auxiliary”to form a long and narrow aggregation shape in both horizontal and vertical directions.Through characterizing the morphology of active layer in details,it was found that the combination of 5%(tfmppy)₂Ir(tpip)assists Pff BT4T-2OD aggregation shape with a 1:2.5 aspect ratio while maintaining high crystallinity.In addition,the results showed that the(tfmppy)₂Ir(tpip)facilitates efficient exciton dissociation and charge transport because of increased contacting area of D/A interface.As a result,the short circuit current(J_SC)and FF performances were both improved contemporaneously,leading to a 20.3%enhancement in PCE.4.Morphology regulation by upside-down thermal annealing based on thick film OSCsThe morphology property of the active layer which is determined by the post-treatment plays a critical role in optimizing the PCE of OSCs.Here,we present a novel upside-down thermal annealing(DTA)method to regulate the morphology of Pff BT4T-2OD:PC₇₁BM and further increase the PCE.Compared with regular thermal annealing treatment,a dramatic 15%enhancement in PCE to 10.4%was obtained after the DTA process,which is attributed to the more orderly packed?-?stacking of the Pff BT4T-2OD network and the redistribution of the PC₇₁BM molecules.Besides,the DTA method is beneficial for boosting the dissociation of excitons and transport of charge carriers.This ideal morphology regulation by DTA method is an effective way to achieve high-performance OSCs.In summary,this work studied the effect of different co-solvents system,adding the third component and using up-side-down thermal annealing on the morphology control of thickness active layer,the PCE of thick film OSCs were significantly improved.This work provided a simple and effective way to promote the commercial application of high performance OSCs.