Application Of Silver Nanowire Semi-embedding Polyimide Composite Electrode In Organic Solar Cells

Organic solar cells(OSCs),owing to their desirable characteristics such as lightweight,flexibility,lowcost and solution processing have become a research hotspot.Ultraflexible OSCs based on ultrathin transparent polymer(with a sunstrate thickness of <10 μm)make the characteristics such as lightweight and flexibility of OSCs more outstanding and are suitable for applications where rigid devices can not be qualified,especially as energy supply devices of flexible electronic systems,such as wearable devices,health monitoring,soft robots and portable power supplies.At present,the performance of ultraflexible organic solar cells still lags behind that of rigid cell devices,and the lack of highperformance flexible substrates and flexible electrodes is the key factor that limits the performance improvement.Polyimide(PI)is an ideal choice for flexible substrate due to its excellent mechanical properties,thermal stability,light transmittance,chemical stability and surface smoothness.In terms of electrode selection,silver nanowires(AgNWs)have high light transmittance,conductivity and flexibility,and the solution processing is simple and compatible with the prepapation of flexible OSCs.Therefore,AgNWs is considered as the best alternative of ITO for preparing high-performance flexible electrodes.However,However,to date,AgNWs have been rarely used as the bottom electrode in ultraflexible OSCs,which may be ascribed to the highly rough surface,weak interfacial adhesion,and poor thermal and anticorrosion(such as oxidation and sulfurization)stability via directly dispersing AgNWs on plastic-supported substrates,resulting in an inferior performance and service life.In this thesis,the strategy that semi-embedding AgNWs into PI substrate was adopted to obtain flexible transparent composite electrodes with low surface roughness,high transmittance,thermal stability and mechanical stability.Then the composite electrode was used to construct the ultraflexible OSCs.The main research results are as follows:(1)High-performance ultrathin flexible transparent composite electrode(AgNWs@CPI)was developed by semi-embedding AgNWs into a colorless transparent polyimide(CPI)substrate for application in highly efficient ultraflexible OSCs.First,four CPI samples with number-average molecular weight ranging from 35.9 kDa to 177.5 kDa were prepared and their optical,mechanical and thermal properties were studied in detail.Then,with the semiembedding strategy,an ultrathin composite electrode(with a thickness of ～9μm)via semi-embedding a silver nanowire electrode in a colorless polyimide(CPI)substrate,which simultaneously possessed outstanding performance such as low square resistance,high optical transmittance,smooth surface,and excellent thermal,mechanical,and solution producing stability,was developed,At the same time,the effect of the molecular weight on the minimum thickness that can withstand the following solution production of ultraflexible OSCs was investigated.Based on the ultrathin AgNWs@CPI electrode,ultraflexible OSCs with PCE of 14.37% and outstanding mechanical robustness was constructed,in which the PCE could still maintain above 93% of its initial value after 4000 bending cycles at a bending radius of 0.5 mm.(2)The physically cross-linked polyimide was obtained by constructing strong hydrogen bonds to improve the mechanical,thermal and solvent resistance of the material and to have the performance of filtering ultraviolet light.Combined with the semi-embedding method to obtain ultra-thin composite electrodes,ultra-flexible organic solar cells with mechanical bending stability and UV irradiation stability were successfully prepared.First,hydroxyl and benzimidazole units were introduced into the polyimide structure design to obtain hydrogen-bonded polyimide(HB-PI),and the mechanical properties and solvent resistance of HB-PI film were significantly improved.Moreover,the cut-off wavelength of the film reaches 376 nm,which can filter out 95% of UV light.Secondly,an ultrathin composite electrode(AgNWs@HB-PI with a thickness of about 7 μm)was prepared using the strategy of semi-embedding AgNWs into a flexible substrate.Finally,ultra-flexible OSCs based on AgNWs@HB-PI composite electrodes were constructed,and their photoelectric conversion efficiency reached 13.53%.The efficiency of the ultra-soft OSCs was basically unchanged after 180 min of continuous light exposure under UV light,while the rigid device decreased by 12%.This result indicates that the AgNWs@HB-PI composite electrode can effectively filter UV light and enhance the UV irradiation stability of the device.