文献翻译内量子效率为100体相异质结太阳能电池.docx
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文献翻译内量子效率为100体相异质结太阳能电池
Bulkheterojunctionsolarcellswithinternal
quantumefficiencyapproaching100%
内量子效率接近100%的体相异质结太阳能电池
Wereportthefabricationandmeasurementofsolarcellswith6%powerconversionefficiencyusingthealternatingco-polymer,poly[N-900-hepta-decanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole)(PCDTBT)inbulkheterojunctioncompositeswiththefullerenederivative[6,6]-phenylC70-butyricacidmethylester(PC70BM).ThePCDTBT/PC70BMsolarcellsexhibitthebestperformanceofanybulkheterojunctionsystemstudiedtodate,withJSC510.6mAcm22,VOC50.88V,FF50.66andhe56.1%underairmass1.5global(AM1.5G)irradiationof100mWcm22.Theinternalquantumefficiencyiscloseto100%,implyingthatessentiallyeveryabsorbedphotonresultsinaseparatedpairofchargecarriersandthatallphotogeneratedcarriersarecollectedattheelectrodes.
我们报道了一种能量转换效率为6%的太阳能电池的制作和测量,这种太阳能电池体相异质结材料为PCDTBT和PC70BM。
这种PCDTBT/PC70BM材料体相异质结电池是目前所有体相异质结太阳能电池中性能最好的,在AM1.5G、100mW/cm-2光照下,短路电流10.6mAcm-2,开路电压50.88V,填充因子50.66、效率56.1%。
Polymerbulkheterojunction(BHJ)solarcellsbasedoncompositesofanelectron-donatingconjugatedpolymerandanelectron-acceptingfullereneofferpromisefortherealizationofalow-cost,printable,portableandflexiblerenewableenergysource1–4.AlthoughBHJsolarcellperformancehassteadilyimproved,withpowerconversionefficiencies(PCE;he)approaching5%,furtherimprovementsinefficiencyarerequiredforlargescalecommercialization5–7.
聚合物体相异质结太阳能电池是基于电荷给提共轭聚合物和电荷受体富勒烯混合制作的。
这种太阳能电池具有成本低、可印刷、便携和柔性等优势。
尽管BHJ太阳能电池性能缓慢提高:
能量转换效率接近5%,但若大规模的商业应用需要进一步提高其性能。
Ratherthanusingasinglejunctionarchitecture,thefundamentalBHJconceptinvolvestheself-assemblyofnanoscaleheterojunctionsbyspontaneousphaseseparationofthedonor(polymer)andtheacceptor(fullerene).Asaresultofthisspontaneousphaseseparation,charge-separatingheterojunctionsareformedthroughoutthebulkofthematerial.
BHJ不是单层结构,它是通过给体(聚合物)和受体(富勒烯)自发相分离构成的纳米级别的异质结。
所以自发相分离、电荷异质结分离是有机异质结材料一直讨论的。
Overthepastdecade,researchhasfocusedonregio-regularpoly(3-hexylthiophene)(P3HT)asthestandardelectron-donatingmaterialinpolymerBHJsolarcells,withimportantprogresshavingbeenmadeinunderstandingthedevicescienceandtheassociatedimprovementsindeviceefficiency.Relativelyhigh-performancepolymerBHJsolarcellsmadefromamixtureofP3HTand[6,6]-phenylC61butyricacidmethylester(PCBM)havebeenreported,withmaximumPCEsofhe?
4–5%(refs5–7).AlthoughapproachestoimprovingtheefficiencyofP3HT/PCBMcellsarestillbeingreported,therelativelylargebandgapofP3HT(_1.9eV)limitsthefractionofthesolarspectrumthatcanbeharvested,andtherelativelysmallenergydifferencebetweenthetopofthep-band(highestoccupiedmolecularorbital,HOMO)ofP3HTandthelowestunoccupiedmolecularorbital(LUMO)ofthefullereneacceptorresultsinalowopen-circuitvoltage,VOC=0.6V.Thesefundamentalenergiesdefinedbytheelectronicstructureofthesemiconductingpolymer(theenergygapandtheHOMOenergy)mustbedecreasedinorderachievepolymerBHJsolarcellswithPCEsof6%andhigher.
在过去的十年里,P3HT作为BHJ重要的电荷给体材料,人们对其进行了大量的研究,并在理解器件工作原理方面取得重大进展,同时器件工作效率也相应的得到提高。
P3HT和PCBM混合制成的BHJ太阳能电池的最高效率是4-5%。
尽管有报道了提高P3HT/PCBM太阳能电池性能的方法,但是P3HT相对宽的带隙(1.9ev)是限制光吸收的主要因素。
P3HT的π键和富勒烯受体LUMO之间较小的能量差导致了较低的开路电压:
Voc=0.6v。
为了使BHJ太阳能电池达到6%或者更高效率,这些通过半导体聚合物电子结构定义的基本能级必须减小。
Recently,severalclassesoflow-bandgappolymershavebeendevelopedtobetterharvestthesolarspectrumwithdeeperHOMOenergiesthatcanpotentiallyincreaseVOC(refs8–12).Thesepolymersaredesignedtomakeuseofinternalchargetransferfromanelectron-richunittoanelectron-deficientmoietywithinthefundamentalrepeatunit.Amongthem,alternatingco-polymersbasedonpoly(2,7-carbazole)derivatives11,12,withasuiteofelectron-deficientmoietiestochoosefrom,areparticularlyinteresting(seeFig.1).Thedifferentelectron-deficientmoietiescanbeusedtotunetheelectronicenergygapofthesemiconductingpolymer,whilethedeepHOMOofthecarbazoleleadstohighervaluesforVOC(ref.11).Theimpliedflexibilityinthesynthesiscanleadtobothasmallerbandgapthatenablestheharvestingofalargerfractionofthesolarradiationspectrum,andadeeperHOMOenergythatincreasestheopencircuitvoltageofthephotovoltaicdevice.IntheirinitialreportofthesynthesisanddeviceperformanceofPCDTBT,LeclercandcolleaguesdemonstratedaPCEof3.6%
fromaBHJcellwithVOCvalueapproaching0.9V.
最近,有人研究出一系列的低带隙聚合物,这些聚合物具有更低的HOMO能级,能够吸收更多的光,从而提高开路电压。
设计这些聚合物时可以把基本重复单元中的电荷由富电子集团转移到缺电子部分。
···聚合物材料合成具有灵活性,所以可以使材料具有窄带系能够吸收更多的光,还可以使HOMO能级更深以提高器件的开路电压。
先前Leclerc和他的同事报道了PCDTBT的合成以及器件性能,利用PCDTBT制备的BHJ太阳能电池的效率达到3.6%,开路电压接近0.9v。
Wereportheresolarcellswith6%PCEfromBHJcompositescomprisingPCDTBT/[6,6]-phenylC71butyricacidmethylester(PC70BM)withshort-circuitcurrentJSC?
10.6mAcm22,opencircuitvoltageVOC?
0.88VandfillfactorFF=0.66underairmass1.5global(AM1.5G)irradiationof100mWcm22.Theinternalquantumefficiency(IQE)iscloseto100%,implyingthatessentiallyeveryabsorbedphotonresultsinaseparatedpairofchargecarriersandthatallphotogeneratedcarriersarecollectedattheelectrodes.
我们报道了利用PCDTBT和PC70BM制备的BHJ太阳能电池,在AM1.5G、功率为100mWcm-2的光照条件下测量:
短路电流为10.6mAcm-2,,开路电压0.88v,填充因子FF为0.66,内量子效率接近100%,表明电池吸收的每一个光子都会产生一对载流子,电极收集了所有光生载流子。
Titaniumoxideopticalspacerandholeblockinglayer
二氧化钛光空间与空穴阻挡层
Historically,arelativelylowPCEhasbeendemonstratedinpolymersolarcellsmadefrompolymersthatmakeuseoftheinternalchargetransferconcept,includingPCDTBT11,12.ThislowPCEhasbeen
limitedbytherelativelylowphotocurrentobtainedfromthesedevices.InBHJcells,thephotocurrentgenerationisgovernedbytwomainfactors13,14:
(i)thefractionalnumberofabsorbedphotonsintheactivelayer(relativetothetotalfluxofphotonsfromthesolarspectrum)and(ii)theIQEdefinedbythefractionofcollectedcarriersperabsorbedphoton.Inprinciple,onecansimplyincreasethethicknessoftheactivelayertoabsorbmorelight.However,becauseoftherelativelylowcarriermobilityofthedisorderedmaterials(castfromsolutionwithsubsequentphaseseparation),increasingthethicknessincreasestheinternalresistanceofthedevice.Consequently,thefillfactortypicallyplummetsasthethicknessisincreased.Basedonthissimpleanalysis,weconsiderthefollowingapproachtowardsobtaininghigherphotocurrent:
maximizingthephotonabsorptionforafixedactivelayerthicknesswhilesimultaneouslyimprovingtheIQE.
以前,为解释聚合物太阳能电池相对低的能量转换效率利用内部电荷转移的概念,包括利用PCDTBT制作的BHJ太阳能电池。
这些器件相对较低的光电流导致了较低的能量转换效率。
在BHJ太阳能电池中,光电流的产生主要受到两个因素影响:
(i)活性层吸收的光子数(相当于吸收的光通量)和(ii)由每吸收一个光子产生的载流子数第一而来的内量子效率。
原则上,可以简单的增加活性层的厚度以吸收更多的光。
但是,由于无序材料(····)相对较低的载流子迁移率,增加厚度的同时会增加器件内部的电阻。
因此,填充因子会随着厚度增加而大幅减小。
基于这些分析,我们认为接下来的方法会提高光生电流:
确定合适的活性层厚度以达到最大的光子吸收效率,同时提高内量子效率。
Toincreasethephotocurrentwhilekeepingthethicknessfixed,weusedanopticalspacerbetweenthephoto-activelayerandthetopelectrode;becauseoftheopticalspacer,themaximumlightintensityisredistributedtobewithintheactivechargeseparatingBHJlayer.Theutilityoftheopticalspacerhasbeenreproducedinrecentpublications15.Inparallel,bychoosingoptimalconditionsforprocessing,wehavedemonstratedananoscaleBHJmorphologythatresultsinnearly100%IQE.ThisdualfocusedapproachappliedtoPCDTBT/PC70BMresultsinPCE,ne_6%;thehighestvaluereportedtodateforpolymerBHJsolarcells.
在确定适合的活性层厚度时为了增加光生电流,我们在光活性层和顶电极之间添加一个光空间。
由于光空间的存在,光能够在BHJ太阳能电池的活性层中实现再分配。
以往的文献中有用过光空间的。
我们选择优化的制作调解,验证了纳米级别BHJ形貌的太阳能电池可以达到100%的内量子效率。
这两种方法使得PCDTBT/PC70BMT太阳能电池效率超过6%,这是目前报道的BHJ太阳能电池最高的效率。
Figure1showsthestructureoftheBHJdevicetogetherwiththemolecularstructuresandanenergyleveldiagramofthecomponentmaterials.Fromthefundamentalphysicsoftheopen-circuitvoltageassociatedwiththedonor–acceptorheterojunctionandtheempiricalrelationshipdemonstratedinrefs16–18,therelativelydeepHOMOenergyofPCDTBT,_5.5eV,shouldresultinahigheropen-circuitvoltage.Moreover,becausethesphericalsymmetryofthefullerenehasbeenliftedinPC70BM(comparedtoPCBM),thePCDTBT/PC70BMBHJmaterialhashigherabsorptionand,consequently,
enhancedphotocurrent.
图1是BHJ太阳能电池的结构、分子能级结构和组成器件的材料的能级图。
从开路电压和给体-受体异质结的物理关系与在文献中证明的经验关系可以看出,PCDTBT相对低的HOMO能级使得开路电压提高。
然而,由于PC70BM中没有球对称的结构(相对于PCBM),PCDTBT/PC70BM材料的BHJ太阳能电池光吸收效率更高,从而提高了光生电流。
Thesolutionprocessibletitaniumsub-oxide(TiOx)layerwasintroducedasanopticalspacer21andasaholeblocker23(seeSupplementaryInformation)betweentheBHJlayerandthetopmetalelectrode.TheTiOxlayerredistributesthelightintensitywithintheBHJbychangingtheopticalinterferencebetweentheincidentlightandthelightreflectedfromthemetalelectrode24,25.Astheactivelayerthicknessdecreases,theintensityofreflectedlightincreases,andtheopticalinterferenceeffectbecomesmorepronounced.Hence,weexpectthattheefficacyoftheTiOxlayerwillbehigherforthinneractivelayers.HoleblockingbytheTiOxisalsomoreimportantforthinner-filmdevices.InthePCDTBT:
PC70BMsolarcellsreportedhere,thethicknessoftheactivelayeris_80nm,andtheTiOxlayerthicknessis_10nm.Withthisconfiguration,weareabletomakegooduseoftheopticalspacerbyavoidingdestructiveinterferencewithinthechargeseparatinglayerbetweentheincidentlightandthelightreflectedfromthealuminium–TiOxinterface.Inaddition,thebottomoftheconductionbandofTiOxmatchestheLUMOofPC70BM.Finally,therelativelyhighelectronmobilityofPC70BMandthehole-blockingfea