Research On Flexible Transparent Electrodes And Their Applications In Organic Solar Cells

Organic solar cells（OSCs）have broad application prospects in the future due to their merits of light weight,high mechanical flexibility,low cost and so on.Recently,with the development of high-efficiency nonfullerene（NF）active layer,the power conversion efficiency（PCE）of small-area single-junction OSCs has exceeded 19%on glass substrates.However,the development of flexible OSCs is still slow,the PCE is about 16%and 10%for small-area（<0.1 cm²）and large-area（>1 cm²）,respectively.One of the main reasons is the lack of flexible transparent electrodes with low square resistance,low surface roughness,and high optical transmittance.To address these issues,this thesis focuses on the research of design and optimization of high-quality flexible transparent electrodes for high-performance flexible OSCs.The main contents are shown as follows:（1）Fabrication of all-solution processed OSCs by using water transfer active layer and asynchronous etching method.Poly（3,4-ethylenedioxythiophene）-polystyrenesulfonic acid（PEDOT:PSS）as a transparent electrode is widely used in fullerene OSCs.However,in the study,we found that the additive 1,8-diiodooctane（DIO）in the NF active layer（PM6:IT-4F）could induce the chemical reduction of PEDOT:PSS electrode under acidic atmosphere.The reduction results in a decrease in work function of PEDOT:PSS,which leads to poor hole transport and collection,and poor performance.The incompatibility of PEDOT:PSS as a top electrode with high-efficiency NF active layer limits its application in NF OSCs.To address this issue,an ultrathin（15 nm）active layer without DIO was inserted between the initial active layer and PEDOT:PSS electrode,the chemical interaction between DIO and PEDOT:PSS electrode was successfully avoided.Further,we developed an asynchronous method（laser etching and phosphoric acid removal of PSS）to pattern the PEDOT:PSS bottom electrode,an all-solution processed OSC shows PCE of8.1%based on the PM6:IT-4F active layer.（2）Development of an ultrathin Ag electrode and fabrication of high-efficiency and flexible OSCs.Nonfullerene acceptors（NFA）,the state-of-the-art active layers absorb photons up to near-infrared region（950 nm）.However,PEDOT:PSS electrode has a strong absorption in this region,which limits the PCE of NF OSCs.To solve the above problems,we adopted ultrathin Ag as the flexible transparent electrode（FTE）.Organic metal peroxide（OMP）was proposed as a growth surface of Ag film.The percolation threshold thickness of Ag on OMP is reduced to 2 nm.Then,optimizing the thickness of antireflection layers（PEI-Zn）based on the transfer matrix method,which achieved outstanding optical performance（average transmittance of 80%）and mechanical flexibility.At the same time,it also exhibits low surface roughness（0.19 nm）.Finally,based on the optimized ultrathin Ag electrode,an ultrathin flexible OSC with an efficiency up to 14.4%was fabricated.In addition,the micrometer fold was created by a pre-stretched VHB elastomer.PCE of the cells increased from 14.4%to 16.3%.（3）Fabrication of large-area flexible OSC modules with high efficiency.The fabrication of large-area flexible devices has become possible,the roughness and sheet resistance of the bottom electrodes limit the further development of large-area devices.Based on the ultrathin Ag electrode developed in the previous chapter,through the design and optimization of the module structure,we obtained a single-junction sub-cell with a width of 0.5 cm,and the width of P1,P2,and P3 are 200μm,350μm,and 200μm,respectively.The geometric fill factor（GFF）of the module reaches 86%.Then,we fabricated large-area flexible OSC module with the structure of PET/FTE/PM6:BTP-e C9:PC₇₁BM/MoO₃/Ag.Based on the ultrathin Ag transparent electrode,it shows a PCE of 12.1%for the large-area（23.3 cm²）flexible module.