Using Novel Fast Removable Solvent Additives Enables Convenient Fabrications Of Highly Efficient Thick-film Polymer Cells

Bulk heterojunction（BHJ）polymer solar cells（PSCs）have attracted much attention due to advantages of flexibility,light weight,and large area that is compatible to roll-to-roll printing,which would impart the green energy technology with low cost potential.These advances were described in Charpter 1.In the meantime,the development of solvent additives towards morphological control of BHJ active layers was analyzed in Charpter 2.Toward full roll-to-roll solution-processing of BHJ PSCs,the vacuum drying process to remove a solvent additive with a high bp.,eg.DIO,is unfavorable and should be avoided.Removing DIO via a solvent washing process is an alternative consideration;however,the extra washing processing would increase cost.Therefore,a fast removable solvent additive during spin-casting active layer would be more perfect for the roll-to-roll printing.In Charpter 3,introducing CBA as a new solvent additive for CB,a common host solvent,was demonstrated.With a lowΔT_bp.of about 80°C for the CB:CBA solvent group,CBA could be fast removed during spin-coating PTB7:PC₇₁BM active layers with film thickness range from 100 to 300 nm,thus additional treatments such as vacuum drying and inert solvent washing were not required.The PTB7-based PSCs showed power conversion efficiencies（PCEs）of 9.11%,8.24%,and 7.11%for the CBA-processed 100,200,and 300 nm active layers,respectively,much higher than 7.53%,5.71%,and 4.93%for corresponding DIO control devices with vacuum drying.Thus the CBA-processed active layers might sufficiently supply thickness windows of 140-60 nm and 200-100 nm for PSCs with target PCEs of more than 8%and 7%,respectively.The results suggest that the PTB7-based active layer is still a candidate for the large area and high speed roll-to-roll production of PSCs.Relative to DIO,much better nanoscale morphologies for 200 and 300 nm thick active layers could be realized by CBA,due to well suppressing the proposed excessive solvent vapor annealing effect for DIO cases.In charpter 4,three fast removable solvent additives CBE,CBA,and CBK withΔT_bp.of63,80,and 98°C relative to CB,respectively,were applied for BHJ morphology control of PTB7-Th:PC₇₁BM active layer.For a D/A ratio of 1:1.5,the CBE-,CBA-,and CBK-processed PTB7-Th:PC₇₁BM active layers in the absence of vacuum drying process could show PCEs of9.66%,9.43%,and 9.12%,respectively.Particularly,the CB/CBE case,with a very lowΔT_bp.of 63°C,further challenges theΔT_bp.limit of the CB/CBA case.For a D/A ratio of 1:2,CBE-processed thicker PTB7-Th:PC₇₁BM active layers of 230 nm could display an even higher PCE of 10.12%than PCE of 9.51%for the 110 nm thick active layer through the traditional spin-coating process.More importantly,from 110 to 300 nm,the CBE-processed PTB7-Th:PC₇₁BM active layers showed PCEs over 9%,from which the thickness tolerance of PTB7-Th:PC₇₁BM active layer could be improved.Our results suggest that CBE would be a promising solvent additive to supply more convenient roll-to-roll processing of high performance active layers.In Charpter 5,it was found that non-aromatic solvent additives such as DES and EOS in combination with non-halogenated trimethylbenzene（TMB）host solvent could well regulate the morphology of FBT-Th₄（1,4）:PC₇₁BM.The Open circuit voltage of the solar cell can reach0.8 V,which is highest for the solar cell based on the active layer materials.Especially for EOS,adding a very small volume fraction of 0.6%can greatly improve the morphology.From 100 to500 nm,the solar cells can maintain a high fill factor more than 70%.The PCE of the solar cell shows more than 10%for the 300 nm thick active layer.Therefore,TMB and EOS are promising solvent additives that can match TMB for environmentally friendly fabrications of PSCs.We have also studied the OFET mobility of bulk heterojunction films with or without an additive.We found that using CBK as the solvent additive could improve the wettability of TMB solution on the OTS modified SiO₂ substrate.