Influence Of Fabrication Methods On The Performance Of Organic Solar Cells

Solar cell,which generates the electricity directly from sunlight,is expected to play a major role in solving the global energy crisis in an environmental friendly and sustainable way.According to the different active layer materials,solar cell can be divided into two types,which are inorganic solar cell and organic solar cell(OSC).Among them,OSC has attracted a great deal of attantions owing to several advantages,including wide range of applicable materials,low-cost,and compatible with flexible substrates.Recently,with the continuous development of novel efficient materials,new design of device architecture,and deeper investigation into the physical process of the photovoltaic mechanism,the state of art OSCs have achieved a power conversion efficiency(PCE)over 13%.However,high-cost fabrication processing and low device stability still asks for further optimization of film surface treatment.In this case,this thesis introduced the spray coating technology with the novel in situ annealing treatment,to substitute the traditional spin coating method.The mechanism of in situ annealing has been well investigated,paving the way for the industrial fabrication of OSCs.Simultaneously,the mechanisms of several film surface treatments were also studied,including solvent vapor annealing(SVA),solvent proceeding additive,and crystallizing agent treatment,laying the theoretical foundation for the record-breaking high efficient OSCs.1.By introducing the in situ annealing treatment in the spray coating processing,the mechanism of in situ annealing on the performance of OSCs based on the poly-3(hexylthiophene)(P3HT): [6,6]-phenyl-C61-butyric acid methyl ester(PC60BM)active layer has been systematically investigated.The results showed that the solution droplet could cover the whole substrate when spray rate was set to be 0.3 ml/min,leading to a homogeneous smooth film surface.In addition,the increased in situ annealing temperature promoted the P3 HT crystallization perpendicular to the substrate direction,which accelerated the hole transfer and decreased the charge recombination by balancing the charges mobility.The PCE of in situ annealed OSC was 2.7%,which was comparable with the post annealed devices.Consequently,the in situ annealing enhanced the potential of spray coating process for large-scale OSC production than post-annealing,for not only consumed less materials but also saved a large amount of processing time.2.Efficient spray coated inverted OSCs based on poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate)(PEDOT:PSS)hole transport layer(HTL)were investigated by incorporating in situ annealing treatment.The results revealed that the relatively smooth and dense spray cast film could be obtained by using a spray height of 18.5 cm together with a spray rate of 0.3 ml/min.By introducing ternary solvent system of islpropanol(IPA): deionized water(DIW):(PEDOT:PSS),the large scale aggregation issue has been addressed.Performance enhancement could be attributed to the improvement of the vertical phase separation of PEDOT:PSS and the reduction of residual humidity inside the HTL.Compared to the conventional annealed OSC,the in situ annealed OSC exhibited a 25.5% and 47.7% enhancement in FF and PCE,respectively.In addition,the lifetime of in situ annealed OSC was also improved by a factor of 2.3.The conductive patterns based on silver nanoparticles were prepared by in situ sintering method,and its mechanism had been systematically studied.It was found that the addition of 45% ethanol to optimize silver nanoparticles precursor could effectively reduce the surface tension of the mixed solution droplets while maintaining the balance of the Marangoni flow during the evaporation process.Through the introduction of in situ sintering process,we successfully fabricated the conductive circuit pattern through spray coating method.In situ sintering method can not only accelerate the volatilization of polymer stabilizer,but also limit the damage of the high temperature on polymer chains.When the temperature was controlled at 140 °C,the in situ sintering took only 20 s to obtain a circuit with a Rsq of 6 ?/sq,which was 1/6 of the conventional sintering process.Finally,we fabricated the OSCs based on in-situ sintered electrode,giving rise to a PCE of 2.75%.4.Effects of SVA on the performance of the OSC based on the poly [[4,8-bis[(2-ethylhexyl)oxy] benzo [1,2-b:4,5-b'] dithio-phene-2,6-diyl] [3-fluoro-2-[(2 ethylhexyl)carbonyl] thieno [3,4-b]-thiophenediyl]](PTB7):[6,6]-phenyl-C71 butyric acid methyl ester(PC70BM)active layer have been investigated by applying different polar solvents,including methanol,ethanol,dimethyl sulfoxide,acetone and isopropanol.The result showed that both the solubility parameters(?)and viscosity of the solvent were playing an important role in controlling the morphology of PTB7:PC70BM blend.Especially,the OSC treated by methanol SVA exhibited a remarkable enhancement of PCE from 6.55% to 8.13%.The performance improvement was mainly due to the formation of the nanoscale crystallization of PTB7:PC70BM blend and the moderated aggregation of PC70 BM,resulting in efficient charge separation,balanced charge transport and suppressed charge recombination.5.Novel solvent additive diphenyl ether(DPE)was introduced to modify the PTB7:PC70BM active layer.By characterizing the surface morphology,optical information and electronic parameters,the mechanism of DPE was systematically analyzed.When PTB7:PC71BM was added with 4% DPE,the interfacial contact resistance decreased dramatically,which meant the better order of PTB7 chains were realized.In addition,DPE could introduce much condense polymer-fullerene conjugated network,providing the efficient charge transfer and resist the large aggregation of PC70 BM.Compared with the OSC without solvent additive,DPE treated OSC showed a 300% enhancement on the PCE(PCE=5.92%).Meanwhile,the DPE significantly shortaged the charge extraction time by promoting the PTB7 crystallization,which shows a great potential in the thick active layer system.The optimized film thickness has been increased from 100 nm to 140 nm.More importantly,PCE can maintain 90% even further increase the film thickness to 180 nm,paving the way for the future industrial fabrication of OSC.6.A crystallizing-agent,2,4-bis-[(N,N-diisobutylamino)-2,6-dihydroxyphenyl]-4-(4-diphenyliminio)squaraine(ASSQ),was used in the poly [(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo [1,2-b:4,5-b?] dithiophene)-co-(1,3-di(5-thiophene-2-yl)-5,7-bis(2-ethylhexyl)benzo [1,2-c:4,5-c?] dithiophene-4,8-dione)](PBDB-T): 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno [2,3-d:2',3'-d']-s-indaceno [1,2-b:5,6-b'] dithiophene(ITIC)based non-fullerene OSC for the first time.Through detailed morphology characterization,it observed that addition of 4wt% ASSQ assisted ITIC organizational order and promoted PDBD-T:ITIC aggregation in the preferential face-on orientation.In addition,the ASSQ and PBDB-T showed efficient exciton dissociation in the ternary blend over F?rster resonance energy transfer(FRET).By using surface potential and solubility measurements,it was exhibited that a ASSQ-ITIC co-crystalline can facilitate a significant improvement in the device PCE,from 8.98% to 10.86%.In summary,the influence of different preparation methods and treatment processes on the OSC performance has been studied,including spray coating,in situ annealing,in situ sintering,solvent vaporation annealing,solvent additive and crystallizing-agent,which laid a solid foundation for the realization of the high efficiency OSC.