The Study of Eco-Friendly Solvent Processed Active Layers for Highly Efficient Organic Solar Cells

Author:Liu Chang

Supervisor:chen jun wu


Degree Year:2019





Organic solar cells(OSCs)have drawn significant attention as a new generation of thin film photovoltaic cells in recent years,due to the advantages of simple preparation process,low cost,light weight and big potential to fabricate flexible devices.In-depth developing of both organic photovoltaic materials and devices have boosted the power conversion efficiency(PCE)to exceed 16%.In addition,significant progress in fast removable solvent additives,thick-film processing and long-term stability provides the possibility for the industrialization of OSCs.However,most efficient OSCs are fabricated with halogenated solvents with high toxicity,such as chlorobenzene(CB),dichlorobenzene(DCB),chloroform(CF)and1,2,4-trichlorobenzene(TCB),which would result in health hazarding and environment pollution.Therefore,environment-friendly solvents are essential for large-scale OSC production in the future.In chapter 2,we summarized the progress of using environment-friendly solvents to prepare bulk heterojunction(BHJ)active layers.In this dissertation,several new types of eco-friendly solvents were applied to fabricate both fullerene and non-fullerene based devices,which could show great potential in the preparation of highly efficient OSCs.In chapter 2,terpinolene(TPO),a natural and safe substance for food and cosmetic additives,was selected as a solvent to replace halogen solvents such as CB for solution processing of OSCs.We calculated Hansen solubility parameters(HSPs)of TPO,and found that Distance in Hansen Space(Ra)and Relative Energy Difference(RED)of TPO to PC71BM acceptor were suitable to be a good solvent.With well-known PTB7 as the polymer donor and minor 1,8-diiodooctane(DIO)as the solvent additive,high quality BHJ film could be fabricated by using TPO as the solvent.In OSC devices,the TPO-processed PTB7:PC71BM active layers could show PCEs of 6.42%.What’s more,TPO-processed PTB7-Th-based OSC also showed PCEs of 7.01%.The results indicate the great promise of TPO in the OSC fabrication.In chapter 3,we utilized binary solvent strategy to improve the OSC performance.The combination of 1,4-dioxane(DIOX)and TPO gives a new type of non-chlorinated and non-aromatic binary solvent,whose optimized ratio of 58:42 is obtained via the calculation of Hansen solubility parameters and further confirmed by actual solubility tests.As a result,the binary mixture processed PTB7:PC71BM and PTB7-Th:PC71BM active layers in the conventional OSCs showed PCEs of 7.6%and 8.3%,respectively,while the PCEs could be elevated to 8.30%and 9.39%in inverted OSCs,respectively.All the photovoltaic performances are comparable or even slightly higher than the corresponding CB cases.Our results demonstrate the big potential of the mixed non-chlorinated and non-aromatic solvents towards environment-friendly fabrication of highly efficient OSCs.In chapter 4,we explored 2-isopropyl anisole(2-IPrA)as a new type of non-halogen but aromatic eco-friendly solvent to process FBT-Th4(1,4)and its derivative PFBT4TDT90-EH10based active layers in thick-film fullerene OSCs.The active layers were optimized by the selection of both main solvents and solvent additives.In inverted OSCs,the 300 nm thick FBT-Th4(1,4)and PFBT4TDT90-EH10 based active layers showed PCEs of 9.03%and8.09%,respectively.The PCEs are comparable with that achieved with the optimized DCB:CB mixture.In addition,2-IPrA also afforded a PCE of 7.55%for the J51:ITIC:IEIC based ternary non-fullerene OSCs.In chapter 5,we chose indan with as a new type of non-halogen but aromatic eco-friendly solvent to process PTQ10:ITIC-4Cl based non-fullerene active layer,in the absence of any solvent additive.In inverted OSCs,an optimized PCE of 13.93%could be achieved,which was the highest efficiency for OSCs being processed with an eco-friendly solvent.The efficiency is also much higher than 13.08%,13.43%,and 10.91%achieved for the chloroform(CF),xylene(o-XY),and carbon disulfide(CS2)processed active layers,respectively.In addition,more than 11%PCEs could be realized for the Indan processed other non-fullerene systems including PTB7-Th:IEICO-4F and PM7:ITIC-4F.The results indicate that indan is one of the most promising eco-friendly solvent in processing of highly efficient and stable OSCs.