Study On Crosslinkable Organic Solar Cells Acceptor Materials

Since human activities and industrial development need a lot of energy,it is urgent to find a sustainable and environment-friendly energy source and develop related utilization technologies.Different from other energy types,solar energy has features in terms of inexhaustible,clean,non-pollution,and no territory dependent,and thus has been recognized as an idea candidate for next generation energy.As one of solar energy utilization technologies,organic solar cells（OSCs）have attracted much attention in recent years because of their flexibility,light weight,translucency,low cost,capability for large area module production and so on.In the past decade,OSCs have achieved a big progress in power conversion efficiency（PCE）,in which the record of single junction devices has reached 15.7%,while that of tandem cells exceeding 17%.However,for practical applications,OSCs need to solve the stability problem at the meantime.Among a number of factors that induce OSC device instable,one main account originates from bulk heterojunction（BHJ）active layer formed by physically blending donor and acceptor material.During daily use,the donor and acceptor components tend to segregate into a large phase separation structure,leading to the reduction in exciton diffusion efficiency and finally in device performance.Such phenomenon is particularly severe in the devices using fullerene derivatives as acceptor component.To address this problem,the most valid strategy up to date is to modify photovoltaic materials with crosslinkable functionalities and use them in the BHJ active layer.After favorable BHJ microstructure is achieved,crosslinking reaction can be triggered to fix the obtained structure and thus improve device stability.In fact,this strategy has been applied on both donor and acceptor materials,and achieves remarkable effect.In the past,a number of crosslinkable/polymerizable fullerene derivatives were synthesized by hydrolyzing[6,6]-phenyl-C₆₁-butyric acid methyl ester(PC₆₁BM),a representative fullerene acceptor in the field,and then reacting with certain regent carrying crosslinkable functional units.But none of them,when lonely used as an acceptor material for OSCs,displayed photovoltaic performance comparable to PC₆₁BM.Recently,[6,6]-phenyl-C₆₁-butyl acrylate(PC₆₁BA),an acrylated fullerene derivative bearing similar molecular shape and size as PC₆₁BM,was developed in our group.The so-prepared solar cells showed similar efficiency as the PC₆₁BM-based devices,but displayed much enhanced device stability.Along this line,seven fullerene detivatives having an acrylate functionality,[6,6]-phenyl-C_61/71-ethyl acrylate(PC_61/71EA),[6,6]-phenyl-C_61/71-propyl acrylate(PC_61/71PrA),[6,6]-phenyl-C₇₁-butyl acrylate(PC₇₁BA)and[6,6]-phenyl-C_61/71-pentyl acrylate(PC_61/71PeA),were newly synthesized and studied systematically in this thesis work.The aim of the work is to show light on the effect of C₇₀ in place of C₆₀,and the effect of different side chain lengths of fullerene derivatives.In the second chapter,poly（3-hexylthiophene）（P3HT）,a representative donor material in the field,was used to combine with the synthesized fullerene acrylates for fabrication of solar cells.It was found that the devices based on C₇₀ derivatives were more efficient than those based on the corresponding C₆₀ compounds.And their efficiencies showed a gradually increasing tendency upon the increment of fullerene side chain length.Thus,the device based on PC₇₁PeA,which has six methylene units in the side chain,displayed the largest PCE（4.12%）in the family.It is also higher than PC₇₁BM-based cell（4.06%）.The investigation on device stability found that solar cells based on PC₇₁EA,PC₇₁PrA,PC₇₁BA,and PC₇₁PeA after heating at 150 ^oC for 35 h maintained 38%,79%,75%,and 92%efficiency of their original best values,respectively.In comparison,the PC₇₁BM device only kept 12%efficiency under the same treatment.The result indicates that C₇₀ acrylates do improve device stability.The best case is that PC₇₁PeA device still displayed a PCE of 3.78%after 35hour heating.In the third chapter,[（2,5-bis（2-hexyldecyloxy）phenylene）-alt-（5,6-difluoro-4,7-di（thiophen-2-yl）benzo[c]-[1,2,5]thiadiazole）](PPDT2FBT-V₁₀)bearing 10%vinyl-terminated side chains was used as donor material to combine with C₇₀ acrylates for fabrication of solar cells capaple of whole crosslinking among donor and acceptor components.It was found that the so-prepared devices exhibited higher efficiencies that those based on P3HT.Meanwhile,when the side chain of fullerene acrylates elongated,a similar increasing tendency of PCE was observed in the system.The largest value in the family was 5.60%,which was displayed by PC₇₁PeA device.It is also higher than that of PC₇₁BM device（5.31%）.