33. References. 35. Annex: 'Research Methodologies' report ... versatile! tool! able! to! incorporate! easily! new! characterization! techniques! in! future! reviews,!
MASTER'IN'NANOTECHNOLOGY'AND'MATERIALS'SCIENCE'
MASTER'THESIS' ' ' ' ' '
Design and Implementation of a Photocurrent Mapping Tool for Organic Solar Cells '
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Author:'Javier'Fernández'Tejero'(NIU:'2142019)' Director:'Dr.'Mariano'CampoyKQuiles'(ICMAB)' Tutor:'Dr.'Eva'Pellicer'(UAB)' Institution:'Institut'Ciència'Materials'de'Barcelona'(ICMAB)' Date:'September'2014'
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' ' ' ' ' ' ' ' ' ' ' ' ' A"mis"padres"y"a"mi"hermana" por"su"paciencia"y"cariño," ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' '
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' Acknowledgments' '
' First' of' all' I' would' like' to' thank' this' great' opportunity' to' Dr.' Mariano' CampoyKQuiles.' Thanks' to' him' I' learned' more' than' I' expected' in' only' one' semester,' and' with' his' patience' and' sense' of' humor' I' discovered' a' new' way' to' work'in'a'great'ambient.'Thanks'Mariano!' ' Fortunately,' from' my' BSc' thesis' in' physics' at' MATGAS' 2000' AIE,' I' knew' other' great'researcher'from'the'group,'Dr.'Alejandro'Goñi.'He'always'can'help'you'in'any' problem'that'you'have,'providing'solutions'with'a'smile.'One'more'time,'thank'you' Ale!'Your'contributions'have'been'essential'in'my'BSc'and'MSc'thesis.' ' During' the' week' meetings' at' Institut' Ciència' de' Materials' de' Barcelona' (ICMABK CSIC)' I' have' learned' lots' of' things,' not' only' in' the' field' of' organic' solar' cells,' becoming'more'and'more'interesting'my'time'in'the'research'group.'Thanks'to'this' project' I' had' the' pleasure' of' meeting' great' professionals' from' around' the' world' who'are'also'great'persons.'Ignasi,'Miquel,'Isabel,'Pablo,'Bernhard,'Ahmed,'Laura,' Damià,' Xabi,' Katiane,' Joanna,' Daiki,' Hideki…' thank' you' very' much' for' your' help' and'advices,'and'for'make'me'feel'always'part'of'the'research'group.'Huge'thanks' to'all'the'great'ICMAB'family.' ' I'would'like'to'thank'the'constant'support'received'from'the'Universitat'Autònoma' de' Barcelona' (UAB),' as' much' to' the' master' coordinator' Dr.' Xavier' Cartoixà' as' to' my'tutor'Dra.'Eva'Pellicer.' ' Finally,' mention' that' without' the' support' and' love' of' my' parents' and' my' sweet' sister'I'never'could'have'come'this'far.'You'are'the'reason'why'I'fight'every'day.' ' ' ' ' ' ' ' ' ' '
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' Contents' ' ' '
Summary' ' ' ' ' ' ' ' 1' Introduction'' ' ' ' ' 1.1' Organic'solar'cells' ' ' ' " " Bulk"heterojunction"concept"" " " Material"requirements" " " " Organic"solar"cell"performance" ' 1.2' LightKbeam'induced'current'(LBIC)' ' ' Gradient"samples" " " ' 1.3' Objectives'of'this'project' ' ' ' ' 2' Experimental'details' ' ' ' 2.1' LBIC'design' ' ' ' ' 2.1.1' Optical'system' ' ' ' 2.1.2' Description'of'the'selected'components' ' ' Optical"fiber" " " " " " Fiberport" " " " " " Cage"Cube"system" " " " " Beamsplitters"" " " " " Photodiode" " " " " " Camera" " " " " " LED"light"source"(camera)" " " " Objective"lens"turret" " " " " Objective" " " " " " Sample"holder" " " " " Motorized"stage" " " " 2.2' LBIC'assembly'and'adjustment' ' ' 2.3' Software'design' ' ' ' ' ' 3' Results'and'discussion' ' ' ' 3.1' LBIC'setup' ' ' ' '
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3.2' LBIC'resolution' ' ' ' ' 3.3' Future'checks'and'improvements' ' ' ' 4' Conclusions' ' ' ' ' ' References' ' ' ' ' ' ' Annex:'‘Research'Methodologies’'report' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' '
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! Summary! ! ! ! ! ! ! ! ! ! ! A! new! light+beam! induced! current! (LBIC)! automatized! mapping! system! is! developed!allowing!the!acquisition!of!photoresponse!and!external!quantum!efficiency! maps!to!improve!the!organic!solar!cells!performance.!This!system!is!designed!to!be!a! versatile! tool! able! to! incorporate! easily! new! characterization! techniques! in! future! reviews,! i.e.! reflectivity,! electroluminescence! or! photoluminescence.! The! study! of! samples! with! thickness,! morphology! or! annealing! gradients! using! this! LBIC! characterization! device! will! yield! new! results! in! the! promising! research! of! efficient! plastic!solar!cells.! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !
1!
! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !
2! !
! ! ! ! ! ! Chapter!1.!Introduction! ! ! The! field! of! organic! solar! cells! has! been! advancing! quickly! and! has! increased! drastically! during! the! past! fifteen! years! in! terms! of! reported! solar! cell! performance!and!publications!(Fig.%1.1).!The!field!has!advanced!in!many!areas!that! demonstrate!increased!power!efficiencies!to!around!10%!for!laboratory!devices[1].! !
!
Figure%1.1.*%Increase%in%web%of%science*indexed%papers%on%polymer%solar%cells,%2000%to%2013.%[2]%
! There! have! been! a! number! of! reviews! discussing! the! development! of! organic! photovoltaics!(OPVs)!and!presenting!the!key!breakthroughs!in!the!short!history!of! the! OPV! field[3F11].! Although! polymer! solar! cells! have! been! developing! rapidly,! there!are!still!three!issues!that!are!not!fully!resolved:!the!improvement!of!device! photoconversion! efficiency! (PCE)! above! 15%,! the! extension! of! device! lifetimes! beyond!10!years,!and!the!largeFscale!production!(Fig.%1.2).!! 3!!
Master!Thesis!
! There! are! a! number! of! loss! mechanisms! that! result! in! low! PCEs,! and! currently,! most!research!groups!in!the!field!are!struggling!to!overcome!these!mechanisms!to! improve!efficiency.!More!and!more!groups!are!addressing!the!stability!issue,!and! they! have! reported! various! approaches! for! improving! device! lifetimes.! Yet! the! improvements!are!minor,!and!more!studies!are!needed.!Furthermore,!experiments! show! that! when! a! good! polymer! is! developed! and! when! the! small! model! devices! based! on! such! a! polymer! deliver! high! efficiencies,! it! is! still! a! large! practical! challenge!to!transfer!the!device!structure!and!performance!to!a!viable!largeFscale! production!process,!i.e.!rollFtoFroll!(R2R)!process[12F14].! !
!
Figure%1.2.*%Key%factors%on%OPVs%research[15].%
! !
1.1!Organic!solar!cells! ! ! The! key! for! higher! internal! PCE! in! organic! solar! cells! is! to! combine! an! electron!donating!and!an!electron!accepting!material!together!to!provide!a!strong! local!driving!force!for!charge!separation.!For!an!initially!generated!singlet!exciton! to!be!efficiently!separated!into!an!electron!and!hole,!the!exciton!needs!to!be!close! to!a!donor/acceptor!interface.!Typical!singlet!exciton!diffusion!distances!are!in!the! range!of!10!nm,!and!excitons!within!that!distance!from!a!donor/acceptor!interface! are! likely! to! be! separated! with! maximum! efficiency.! This! presents! a! problem,! as! the! absorption! coefficient! of! polymers,! even! at! optimal! wavelengths,! is! not! high! enough!to!allow!for!complete!absorption!of!the!sunlight!in!a!10!nm!layer.!! ! Bulk!heterojunction!concept! ! Efficiency!loss!in!bilayer!devices!will!therefore!arise!from!either!recombination!of! excitons!far!from!the!donor/acceptor!interface!or!limited!transmission!of!incident! light! through! the! device.! To! solve! this! problem,! the! bulk! heterojunction! (BHJ)! concept! was! introduced.! Here,! the! electron! donating! and! accepting! materials! are! mixed! together,! forming! a! three! dimensional! network! of! interfaces! in! the! bulk! of! the! blend! (Fig.% 1.3).! With! phase! separated! region! sizes! similar! to! the! exciton! diffusion!distance!(10F20!nm),!photon!absorption!events!throughout!the!blend!are!
4! !
Design!and!Implementation!of!a!Photocurrent!Mapping!Tool!for!Organic!Solar!Cells!
! likely!to!result!in!separated!electrons!and!holes.!These!can!then!travel!via!separate! pathways!to!their!respective!contacts,!minimizing!recombination.! !
Figure%1.3.*%BHJ%solar%cell%performance[16].%
!
! The! electron! donating! or! accepting! character! of! a! molecule! in! relation! to! each! other!is!determined!by!the!electron!affinities!of!the!molecules.!The!molecule!with! the! higher! electron! affinity! is! called! an! electron! acceptor,! and! the! molecule! with! lower!electron!affinity!is!called!an!electron!donor.!Electron!affinity!is!defined!as!the! energy! difference! between! the! vacuum! energy! and! the! lowest! unoccupied! molecular!orbital!(LUMO)!of!a!neutral!molecule.! ! In! BHJ! organic! solar! cells,! the! donor! material!is!mainly!responsible!for!photon! absorption.! When! the! generated! exciton! diffuses! to! an! interface! with! an! acceptor! material,! the! electron! will! transfer! from! the! LUMO! of! the! donor! to! the! lowerF energy! LUMO! of! the! acceptor! (Fig.% 1.4).! Holes,!which!can!be!viewed!as!an!absence! of! an! electron! in! the! highest! occupied! molecular! orbital! (HOMO),! effectively! having! a! positive! charge,! tend! to! move! upward! in! energy! and! prefer! to! stay! on! the! donor.! Sandwiching! the! active! layer! between! two! electrodes! with! different! work! functions! provides! a! driving! force! Figure%1.4.*%Exciton%dissociation%in%bulk% heterojunction%organic%solar%cells[17].% that!will!cause!the!electrons!and!holes!to! travel!towards!respective!contacts.! ! ! !
5!
Master!Thesis!
! Material!requirements! ! The!main!coefficients!used!to!describe!solar!cell!device!efficiencies!are!the!power! conversion! efficiency! (PCE),! which! measures! how! much! of! the! power! of! the! incident!light!is!converted!to!electrical!power,!the!fill!factor!(FF),!which!measures! how!much!of!the!theoretical!maximum!power!of!the!cell!is!actually!obtained!and! the! external! quantum! efficiency! (EQE),! which! measures! the! fraction! of! incoming! photons!of!a!given!wavelength!converted!to!extracted!electrons.! ! The!EQE!of!a!solar!cell[1]!is!given!by! ! !"! = (1 − !)!"#!! ! ! ! !!!!(Eq.%1.1)! ! where!!!is! reflection! loss! and!!"#!is! the! internal! quantum! efficiency,! or! in! other! words,!the!efficiency!for!an!absorbed!photon!giving!rise!to!a!charge!pair!collected! at!the!electrodes.!There!are!basically!four!processes!involved!in!determining!IQE,! namely! the! photon! absorption! efficiency! (!! ),! the! charge! collection! efficiency! (!!! ),!the!charge!transfer!efficiency!(!!" )!and!the!exciton!diffusion!efficiency!(!!" ).!! ! The!relation!between!these!efficiencies[1]!is!given!by! ! !"# = !! !!! !!" !!" !!! !!!!!!!! ! !!!(Eq.%1.2)! ! There!are!several!criteria!a!semiconducting!organic!polymer!needs!to!meet!to!be!a! good!component!in!the!active!layer!of!an!organic!solar!cell.!The!polymer!working! as!a!donor!needs!to!absorb!as!much!of!the!sunlight!as!possible,!meaning!that!there! should!be!maximum!overlap!between!the!absorption!spectrum!of!the!polymer!and! the!spectrum!of!the!sunlight.!Since!the!intensity!of!the!sun!extends!in!the!infrared! wavelengths,!it!is!important!that!polymers!for!solar!cell!applications!are!tailored!to! have!an!extended!absorption!in!the!infrared.!The!absorption!is!determined!by!the! optical! gap! of! the! polymer,! which! in! this! context! means! the! difference! in! energy! between! the! HOMO! and! LUMO! levels.! Recombination! is! also! a! limiting! factor! during! charge! collection,! when! separated! charges! travel! to! the! electrodes! to! be! extracted.! Low! recombination,! high! mobility! and! suitable! electrodes! are! the! principal!factors!to!control!the!charge!collection!efficiency.!To!enhance!the!charge! transfer! efficiency,! the! regions! in! the! BHJ! should! optimally! be! smaller! than! the! exciton!diffusion!distance!in!the!given!materials.! ! Organic!solar!cell!performance! ! A!solar!cell!in!the!dark!acts!as!a!simple!diode.!The!equivalent!electronic!circuit!that! approximates!the!organic!solar!cell!performance!is!shown!in!Fig.%1.5.!! ! 6! !
Design!and!Implementation!of!a!Photocurrent!Mapping!Tool!for!Organic!Solar!Cells!
!
!
Figure%1.5.*%Operation%principle%of%OPV%devices%represented%by%an%electrical%circuit[18].%
! It!comprises!a!diode!with!ideality!factor!n!and!saturation!current!!! !(current!in!the! dark! at! reverse! bias);! a! source! to! provide! current! that! corresponds! to! photocurrent!!! !generated! during! illumination;!!! !series! resistance! which! takes! into! account! all! the! resistance! at! interfaces! in! the! layers,! the! conductivity! of! the! semiconductors,! and! the! electrodes;! and! shunt! resistance!!!! ,! which! takes! into! account!the!leakage!of!the!current!through!shunts!as!a!result!of!defects!in!the!films.! For!good!performance!of!the!device,!!! !needs!to!be!minimum!and!!!! !maximum.! ! Fig.% 1.6! shows! the! currentFvoltage! characteristics! (IFV! curves)! for! a! typical! solar! cell.!The!figure!presents!the!key!parameters!that!define!the!performance!of!the!cell,! which! are! the! openFcircuit! voltage!!!" ,! shortFcircuit! current!!!" ,! fill! factor!!!,! the! maximum!power!!! !represented!by!the!product!of!the!voltage!and!the!current!at! maximum!power!point!!! !and!!! .!! !
!
Figure%1.6.*%Typical%I*V%curve%for%a%solar%cell%specifying%the%principal%parameters.%
! The! following! expression! defines! the! current! for! the! circuit! and! link! almost! all! parameters!exposed!above[18]:! ! ! = !! !"#
!(!!!!! ) !!! !
−1 +
!!!!! !!!
− !! !!
!
!!!!!(Eq.%1.3)! !
where!!!is!the!elementary!charge!and!!! !!the!thermal!energy.! !
7!
Master!Thesis!
! The! power! produced! by! a! solar! cell! is! defined! as! the! product! of! the! current! and! voltage,!and!the!maximum!power!will!correspond!to!the!maximum!product!!! · !! .! Eq.%1.4!is!the!basic!equation!that!defines!the!PCE!of!the!solar!cell,!which!shows!the! ratio!between!maximum!electric!power!produced!by!the!cell!and!the!power!of!the! incident!light!(!!" )!on!the!cell!with!a!given!active!area!!:! ! !"# ≡ ! = !!
!!" !!"
!!!
!
!!!!
!!!!!!!(Eq.%1.4)!
!! = !!!! !! !
!
!
!!!!!!!(Eq.%1.5)!
!!" !
! where!!!!is!defined!as:! ! ! !
!" !"
! !!!shows! how! much! power! is! in! practice!produced! by! the! cell! with! given!!!" !and! !!" ,!compared!to!the!maximum!theoretically!value.!Parasitic!resistances!!! !and!!!! ! significantly! affect!!!.! Typically,! a! good! operating! organic! solar! cell! delivers!!!! values!in!the!range!of!60F70%[1].! ! As! we! can! see! in! this! section,! the! enhancement! of! the! organic! solar! cell! performance! has! many! parameters! that! must! be! controlled.! Nowadays! the! scientific!community!have!lots!of!characterization!techniques!that!can!be!applied! in! OPVs! research.! Linking! some! of! these! techniques! in! one! versatile! characterization!setup!is!one!of!the!main!objectives!of!this!master!thesis!project.! ! !
1.2!Light?beam!induced!current!(LBIC)! ! ! LBIC! is! the! acronym! of! ‘LightFBeam! Induced! Current’! and! is! a! wellF established! nonFdestructive! technique! for! the! characterization! of!!!!!! semiconductors[19F22].!Basically!it!involves!moving!a!focused!light!source!over!the! surface!of!a!semiconductor!sample!while!measuring!the!current!output!generated! locally!(photoinduced!current).!! ! With!this!technique!we!can!obtain!photoresponse!maps!or!IFV!curves!of!each!point! of!the!sample!and,!as!we!saw!in!the!previous!section,!extract!key!parameters!such! !!" ,!!!" ,!!! ,!!! ,!FF!and!study!how!efficient!is!the!device!performance!as!a!function!of! position.! This!kind!of!measurements!represents!one!of!the!most!important!characterization! techniques! for! solar! cells! research! due! to! the! great! amount! of! information! extracted.! One! can! study! statistical! performance! of! a! cell/module,! inhomogeneities,! defects! and! pinholes,! damages! (Fig.% 1.7),! degradation! mechanisms,!etc.! ! 8! !
Design!and!Implementation!of!a!Photocurrent!Mapping!Tool!for!Organic!Solar!Cells!
!
!
Figure%1.7.*%Example%of%damage/defects%introduced%by%bending%and%crumbling%an%organic% solar%cell%module%and%the%resulting%LBIC%photorensponse%map.[23]%
! However! photoresponse! maps! or! IFV! curves! are! not! the! unique! information! that! we! can! extract! from! a! LBIC! setup.! Coupling! appropriated! measurement! tools! a! LBIC!can!be!used!to!study!other!important!parameters!for!solar!cells!research!as! EQE,! IQE,! electroluminescence! (EL),! reflectivity,! photoluminescence! (PL),! etc.! Automating! and! coordinating! the! different! characterization! techniques,! this! powerful! LBIC! can! extract! a! huge! quantity! of! information! studying! the! relationships!between!them!and!providing!new!advances!in!OPVs!research.! ! Gradient!samples! ! Normally! the! search! for! the! optimum! features! of! a! material! or! device! means! the! study! of! many! samples,! spending! time! and! money! to! create! a! huge! quantity! of! sample!variations.!The!idea!of!the!research!group!to!solve!this!problem!is!generate! samples! with! a! gradient! of! one! of! these! features,! i.e.! thickness,! composition/morphology! (Fig.% 1.8% and! 1.10),! annealing,! etc[24F28].! Studying! photoresponse! or! EQE! maps! of! these! gradient! samples! with! the! LBIC! device! designed!in!this!project!we!can!extract!lots!of!information!in!a!short!time,!deducing! the! appropriate! features! to! improve! the! performance! and/or! stability! of! organic! solar!cells.! During! the! development! of! this! project,! other! members! of! the! research! group! are! working! to! optimize! the! different!techniques!to!create! Figure%1.8.*%Schematics%of%a%photodiode%based%on%a% in! a! controlled! and! gradient%of%two%complementary%wedge%layers%of%CuPc%and% reproducible! way! these! C60[25].% gradient!samples!(Fig.%1.9).!
!
9!
Master!Thesis!
!
Figure%1.9.*%Two%modes%for%the%fabrication%of%gradients:%(a)%letting%the%atmosphere%saturate% first%and%then%introducing%the%sample,%and%(b)%having%the%sample%inside%of%the%recipient%from% the%beginning%and%gradually%removing%the%sample%after%a%given%waiting%time%inside[26].%
!
! !
!
Figure%1.10.*%(a)%Picture%of%a%device%consisting%of%a%P3HT:PCBM%layer%exhibiting%a%gradient%in% morphology%sandwiched%between%electric%contacts%and%(b)%diagram%of%the%morphology[26].%
! !
1.3!Objectives!of!this!project! ! ! The! main! objective! of! this! project! is! to! provide! a! new! and! versatile! characterization! tool! to! the! ICMAB’s! Nanostructured% Optoelectronic% Materials% research! group.! During! this! master! thesis! all! the! structure! of! an! LBIC! has! been! designed,!ordered!and!assembled.!The!design!has!been!thought!to!be!the!first!step! in! the! construction! of! an! expandable! characterization! device! that! (presumably)! will!provide!many!results!for!the!group!in!the!field!of!organic!solar!cells!studying! gradient!samples.!
10! !
Design!and!Implementation!of!a!Photocurrent!Mapping!Tool!for!Organic!Solar!Cells!
! In! the! final! part! of! this! work,! some! aspects! related! to! the! update! of! the! software! used!to!control!the!measurement!are!treated,!as!well!as!an!exposition!of!the!future! checks! and! improvements! that! can! be! made! to! include! new! measurement! techniques!to!the!LBIC!setup.! !
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11!
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12! !
! ! ! ! ! ! Chapter!2.!Experimental!details! ! In!this!chapter!all!the!aspects!related!to!the!design!and!assembly!of!a!light7 beam! induced! current! (LBIC)! automatized! mapping! setup! are! presented.! The! structure!of!this!section!is!divided!in!three!parts:!design!(2.1),!assembly!(2.2)!and! software!update!design!(2.3).! !
2.1!LBIC!design! ! ! During! the! first! months! of! this! project! all! the! efforts! were! dedicated! to! define! the! theoretical! optical! system! needed.! Thanks! to! the! contributions! of! the! research!group!members,!a!first!version!of!the!LBIC!optical!system!was!obtained! (Fig.% 2.1).! This! optical! system! besides! including! the! main! instruments! needed! to! obtain! photoresponse! and! EQE! maps! (blue! lines! in! Fig.% 2.1),! contain! a! camera! system!with!a!variable!illumination!device!(red!lines!in!Fig.%2.1).!The!objective!of! this!camera!is!to!obtain!high7resolution!images!of!the!measured!area!and!help!the! user!to!focus!inside!the!sample!using!the!software!in!the!computer.!! The!theoretical!optical!system!is!explained!below!while!the!technical!specification! of!the!different!parts!are!detailed!in!Section%2.1.2.! ! 2.1.1!Optical!system! ! As!we!can!see!in!the!optical!diagram!(Fig.%2.1)!the!light!generated!from!a!lamp!Oriel! Instruments™!7340,!with!a!selectable!Xenon!or!Halogen!light!source,!arrives!to!the! system!using!an!optical!fiber!connected!to!a!fiberport!with!a!convergent!lens!with! 30! mm! of! focal! distance.! We! can! select! two! different! light! pathways! (Line! 1! and! Line! 2! in! Fig.% 2.1)! depending! if! we! need! light! with! a! specific! wavelength! or! not.! Thanks!to!the!convergent!lens,!the!optical!fiber!outgoing!light!has!a!parallel!beam!
13!!
Master!Thesis!
! that!cross!two!beamsplitters!and!arrives!at!the!end!of!the!system!to!the!objective! used!to!focus!in!the!sample.!! ! The! first! beamsplitter! (Beamsplitter! 1! in! Fig.% 2.1),! with! a! ratio! 50:50! (R:T)! of! reflected!and!transmitted!light,!is!used!to!reflect!part!of!the!light!to!a!photodiode!to! measure! the! initial! intensity! of! light! in! order! to! normalize! the! photoinduced! current! generated! in! the! sample! (i.e.! EQE! measurement).! The! light! beam! pass! through!a!second!beamsplitter!(Beamsplitter!2!in!Fig.%2.1),!with!a!ratio!30:70!(R:T)! and!finally!arrives!to!the!objective!focusing!the!light!in!the!sample.! !
!
!
Figure%2.1.0%Optical%diagram%including%the%direction%of%the%light%for%the%LBIC%measurements% (blue%lines)%and%the%camera%system%(red%lines);%and%general%connection%showing%two%different% light%source%lines%(Line%2%with%monochoromator),%and%the%final%connection%to%the%Keithley% sourcemeter%and%computer.%
! Beamsplitter!2!is!used!basically!for!the!camera!system.!An!array!of!LEDs!connected! to! a! LED! dimmer! is! used! to! control! the! light! intensity! that! arrives! to! the! Beamsplitter! 2.! The! 30%! of! this! light! is! reflected! through! the! objective! and! then! reflected! over! the! sample! and! transmitted! through! the! Beamsplitter! 2! one! more! time.!This!LED!light!beam!is!reflected!by!the!Beamsplitter!1!and!finally!arrives!to!a! convergent! lens,! with! 30! mm! of! focal! distance,! that! generates! the! image! of! the! sample!into!the!camera!sensor.!! Taking! into! account! the! reflected:transmitted! characteristics! of! the! beamsplitters! we!can!calculate!that!the!35%!of!the!light!coming!from!the!fiberport!arrives!to!the!
14! !
Design!and!Implementation!of!a!Photocurrent!Mapping!Tool!for!Organic!Solar!Cells!
! sample.! In! the! camera! system! we! can! modulate! the! intensity! using! the! dimmer! controller!in!order!to!avoid!a!possible!light!damage!on!the!organic!samples.!Once! the! sample! is! aligned,! the! LED! must! be! switched! off! during! the! measurement.! Finally,! the! photocurrent! induced! on! the! sample! is! measured! by! a! Keithley! sourcemeter!and!the!information!transmitted!to!the!computer.! ! In!general!terms!this!is!the!theoretical!optical!system!that!we!need!to!reproduce!on! the!laboratory.!The!next!Section%2.1.2!details!the!specifications!of!each!component! used!to!assembly!our!final!LBIC!setup.! ! ! 2.1.2!Description!of!the!selected!components! ! Optical!fiber! ! In! order! to! drive! the! light! from! the! lamp! to! the! fiberport! with! a! broad! spectral! range!and!minimizing!the!intensity!loss,!we! decided! to! order! an! optical! fiber! customized! for! our! purposes.! The! optical! fiber! created! is! ideal! for! spectroscopy! applications,!with!a!spectral!range!between! 180!and!1150!nm.! ! The!connectors!selected!for!both!sides!were! the! SMA! type! (Fig.% 2.2).! SMA! connectors,! which! are! most! commonly! used! with! multimode!fibers,!have!a!ferrule!design!that! is!ideally!suited!for!large!core!fibers.! ! Figure% 2.2.0% UV% non0solarizing% step0index% Specifications!provided!by!the!company! Thorlabs™!are!presented!in!Table%2.1.! ! ! Core!diameter! Cladding!diameter! Coating!diameter! Coating!material! Operating!wavelength! Operating!temperature! Minimum!bend!radius! Numerical!aperture! Attenuation!
multimode% fiber% with% SMA% connectors% (Thorlabs™% reference% UM2203000 CUSTOM).!
300!±!6!µm! 330!±!7!µm! 370!±!7!µm! Polyimide! 180!–!1150!nm! 765!°C!to!300!°C! 100!x!Clad!diameter!(Momentary)! 300!x!Clad!diameter!(Long!term)! 0.22!±!0.02! 0.32!dB/m!@!250!nm!(Typical)!
Table%2.1.0%Thorlabs™%specifications%for%optical%fiber%UM220300.%
!
15!
Master!Thesis!
! Fiberport! ! As! well! as! the! optical! fiber,! the! fiberport! plays! an! important! role.! We! decided! to! order!a!fiberport!with!differential!adjusters!(Fig.%2.3)!able!to!positioning!with!great! precision!the!light!beam!through!our!LBIC!setup.!Specifically!has!a!coarse!and!fine! adjustment!of!0.4!mm/rev!and!25!µm/rev,!respectively.!! !
!
Figure%2.3.0%XY%translator%with%differential%drives%(Thorlabs™%reference%ST1XYD/M).%
! Cage!cube!system! ! One!of!the!most!important!aspects!discussed!was!the!design!of!a!system!capable!to! performing! the! measurements! manipulating! the! device! as! minimal! as! possible! in! order! to! preserve! the! stability! and! alignment! of! the! system.! After! evaluating! different!possibilities,!we!finally!decided!to!order!a!cage!cube!system!solution!(Fig.% 2.4).!! These! 30! mm! cage! cubes! are! ideal! for! building! optical! setups,! i.e.! beamsplitters,! preserving! the! alignment! of! the! system.! Connecting! two! cubes,! one!for!each!beamsplitter,!we!are!able!to!create!a! versatile! system! capable! to! contain! many! tools,! i.e.!cameras,!detectors!and!light!sources.!! Furthermore,! using! this! versatile! cage! cube! system,! we! can! expand! or! modify! our! optical! system! to! allow! different! measurements,! transforming! the! setup! into! a! more! powerful! Figure% 2.4.0% 30% mm% Cage% System% Cube% 40Way% (Thorlabs™% reference% device.! C4W).% !! Beamsplitters! ! Other!important!aspect!of!the!design!is!the!election!of!the!beamsplitters.!Due!to!the! big! range! of! wavelengths! used! for! the! measurements,! the! beamsplitters! selected! must! have! a! good! stability! of! the! reflected7transmitted! ratio! for! different! wavelengths!(Fig.%2.5).!! 16! !
Design!and!Implementation!of!a!Photocurrent!Mapping!Tool!for!Organic!Solar!Cells!
! ! After!some!discussions!with!the!research!group,!the!selected!beamsplitters!are!the! Thorlabs™! EBS1! (50:50)! and! EBP1! (30:70).! Table% 2.2! presents! the! company! specifications.! ! Splitter!ratio! 50:50!(EBS1)! 30:70!(EBP1)! Angle!of!incidence!(AOI)! 45°! Transmittance:!70!±!10!%! Splitter!ratio!tolerance!for! Transmittance:!50!±!10!%! Reflectance:!50!±!10!%! Reflectance:!30!±!10!%! 450!–!650!nm!(AOI!=!45°)! Diameter! (1”)!∅!25.4!mm!+0.0/70.25!mm! Thickness!(Nominal)! 1!mm! Clear!Aperture! >!90%!Diameter! Temperature!range! 750!to!+!80!°C! Material! Soda!Lime!Glass! Table%2.2.0%Thorlabs™%specifications%for%beamsplitters%EBS1%(50:50)%and%EBP1%(30:70).%
% As! we! can! observe! in! Fig.% 2.5,! these! beamsplitters! present! a! stable! response! (around! 10%! of! tolerance)! for! wavelengths! between! 400! and! 800! nm.! However,! the!response!out!of!this!range!must!be!taken!it!into!account!in!order!to!correct!the! splitter!ratio!of!each!beamsplitter!in!our!measurements.! Other! important! reason! to! select! these! beamsplitters! is! the! relatively! thin! thickness,! which! avoids! possible! replicas! of! the! image! transmitted! to! the! camera! (ghosting),! and! having! two! beamsplitters! of! same! thickness! at! 45°! correct! the! beam!axis!displacement.! !
Figure%2.5.0%Splitter%ratio%at%different%wavelengths%for%beamsplitter%50:50%(EBS1;%Fig.%2.5a)% and%30:70%(EBP1;%Fig.%2.5b).%The%highlighted%regions%represent%the%specified%wavelength% range%(450%–%650%nm)%of%these%beamsplitters.% % % %
!
!
17!
Master!Thesis!
! Photodiode! ! For! the! measurement! of! the! incoming! light! we! need! a! photodiode! sensor! with! a! fast! response! time! and! high! resolution! able! to! detect! light! with! a! big! range! of! wavelengths.! Table%2.3%shows! the! company! specifications! for! the! selected! silicon! photodiode!sensor.! ! Detector!type! Silicon!photodiode!(UV!extended)! Wavelength!range! 200!nm!–!1100!nm! Optical!power!range! 50!nW!–!50!mW! Max!average!power!density! 20!W/cm2! Linearity! ±0.5%! Resolution! 1!nW! Measurement!uncertainty!(for!beam! ±3%!(451!–!1000!nm)! diameter!>!1!mm)! ±5%!(over!rest!of!range)! Response!time!
! Camera! ! The!camera!selected!for!the!LBIC!setup!is!a!high7resolution!CMOS!(Complementary% Metal% Oxide% Semiconductor)! camera! with! 1280! x! 1024! pixel! resolution! and! a! maximum!full!frame!rate!of!25!fps.!This!is!a!color!camera!with!an!IR!filter!in!front! of!the!sensor.!! ! By!connecting!this!camera!to!our!cage!cube!system!we!will!be!able!to!obtain!high7 resolution!images!of!the!sample!and!thus!align!it!with!precision.!Table%2.4!shows! the!company!specifications!for!this!color!CMOS!camera.! ! Sensor!type! CMOS! Pixel!resolution! 1280!x!1024! Optical!sensor!class! 1/3”,!Color! Sensor!model!(Mikron)! MT9M131! Sensor!size! 4.61!mm!x!3.69!mm! Pixel!size! 3.6!µm!x!3.6!µm! Full!frame!rate!(max)! 25!fps! Table%2.4.0%Thorlabs™%specifications%for%color%CMOS%camera%DCC1645C.%
! ! ! ! 18! !
Design!and!Implementation!of!a!Photocurrent!Mapping!Tool!for!Organic!Solar!Cells!
! LED!light!source!(camera)! ! Thorlabs™! LED! array! source! selected! (LIUCWHA)! for! camera! visualization! has! a! broadband!white!output!with!an!intensity!of!3.0!W/cm2.!The!array!is!composed!of! 20! LEDs! mounted! close! to! each! other! and! protected! by! special! LED! driver! integrated!circuits.! ! To!control!the!light!intensity!and!avoid!a!possible!damage!of!the!organic!samples! we! decided! to! order! and! assemble! a! LED! dimmer! from! the! company! JKL! components™.! ! Objective!lens!turret! ! One! of! the! challenges! of! this! project! is! to! create! a! versatile! LBIC! device.! For! this! reason!we!decided!to!include!an!objective!lens!turret!to!mount!different!objectives! and! lenses! to! have! a! wide! variety! of! magnifications! without! adjusting! the! optical! setup.!The!selected!Thorlabs™!objective!lens!turret!(OT1)!with!four!RMS7threaded! ports!can!be!mounted!directly!in!the!cage!cube!system.! ! Objective! ! The! proper! selection! of! the! objective! is! of! vital! importance.! Due! to! the! encapsulation!of!some!organic!samples!studied,!we!decided!to!order!an!objective! with! ultra7long! working! distance! from! the! company! Mitutoyo™! (Fig.% 2.6)! able! to! measure!through!glass.!! ! This! apochromat! objective! has! a! magnification! of! 20x! infinity! corrected! and! is! designed!for!bright!field!observation!at!long!working!distance.! !
Figure%2.6.0%Mitutoyo™%20x%objective%for%bright%field%observation%with%ultra0long%working% distance%(M%Plan%Apo%SL20x).%
!
!
!
19!
Master!Thesis!
! Magnification! Numerical!Aperture!(N.A)! Working!distance!(mm)! Focal!length!(mm)!(λ=550!nm)! Resolution!limit!(µm)!(λ=550!nm)! ±!Depth!of!field!(µm)! Mass!(g)!
20x! 0.28! 30.5! 10! 1.2†! 3.5! 240!
Table%2.5.0%Mitutoyo™%specifications%for%objective%M%Plan%Apo%SL20x.%
! To! have! a! different! field! of! view,! several! convergent! lens! mounted! in! tube! lens! were!also!assembled!in!the!objective!lens!turret.! ! Sample!holder! ! The! samples! created! at! ICMAB! for! this! study! normally! are! integrated! on! a! chip! with!six!different!contacts!(Fig.%2.7a)!and!two!ground!contacts.!The!research!group! has! a! home7made! sample! holder! contact! selector! consisting! in! six! pins,! one! for! each!contact,!and!a!manual!selector!for!each!one!(Fig.%2.7b).!! ! Using!this!sample!holder!we!can!include!manually!each!part!of!the!sample!in!the! circuit,! testing! all! the! parts! separately.! The! signal! is! then! sent! to! a! Keithley! sourcemeter.! ! !
Figure%2.7.0%(a)%Example%of%sample%with%six%different%contacts%(106)%and%two%grounds%(I0II)% and%(b)%home0made%sample%holder%with%a%contact%selector.%
!
! ! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! †!The!calculus!and!some!other!aspects!related!to!the!resolution!limit!of!the!objective!are!treated!in! Chapter%3.2%LBIC%resolution.! 20! !
Design!and!Implementation!of!a!Photocurrent!Mapping!Tool!for!Organic!Solar!Cells!
! Motorized!stage! ! The!study!of!samples!with!gradients!is!one!of!the!main!applications!of!the!research! group!for!this!LBIC!device.!Thickness!or!composition!gradients!across!one!sample! can! provide! more! results,! and! faster,! than! one! type! of! sample! for! each! measurement.! In! order! to! study! these! gradients! we! need! to! achieve! 2D! maps! of! measurements!to!evaluate!the!response!of!each!part.! To!solve!this!requirement!we!decided!to!order!a!XYZ!motorized!translation!stage.! It! consists! in! three! individual! translation! stages! (Fig.%2.8a)! assembled! to! obtain! a! 3D! movement! for! the! sample! holder! (Fig.% 2.8b).! Each! motor! can! be! computer! controlled! allowing! totally! automatized! measurement! maps! maintaining! the! optical!focus!all!the!time!using!the!Z7motor.! !
!
Figure%2.8.0%(a)!Thorlabs™%MTS%Series%Motorized%Translation%Stage%(MTS25/M0Z8)%and%(b)% XYZ%configuration.% %
Each!motorized!stage!provide!smooth!and!low7friction!movement,!been!50!nm!the! minimum! achievable! movement.! The! travel! range! for! each! motorized! stage! is! 25! mm,!been!able!to!reach!every!part!of!the!sample!(25!x!25!mm).!Furthermore,!other! important!specifications!are!given!in!Table%2.6.! ! Travel!range! 25!mm!(0.98”)! Min!achievable!incremental!movement! 0.05!µm! Min!repeatable!incremental!movement! 0.8!µm! Home!location!accuracy! ±!4.0!µm! Velocity!(max)! 2.4!mm/s! Velocity!stability! ±!0.25!mm/s! Acceleration!(max)! 4.5!mm/s2! Table%2.6.0%Thorlabs®%specifications%for%MTS%Series%Motorized%Translation%Stage%%%% MTS25/M0Z8.%
!
21!
Master!Thesis!
!
2.2!LBIC!assembly!and!adjustment! ! After! a! few! meetings! we! finally! realized! the! order! list! with! all! the! components!described!above.!! Once! we! have! the! LBIC! assembled,! with! the! aid! of! the! members! of! the! research! group,!we!could!adjust!each!LBIC!part!to!bring!the!light!beam!through!the!system! to!the!sample!in!the!best!conditions.! ! The!first!step!was!the!adjustment!of!the!outgoing!fiberport!light!to!have!an!intense! and! centered! light! beam.! The! following! step! was! the! correct! positioning! of! the! beamsplitters,!starting!with!the!beamsplitter!1!(50:50)!(Fig.%2.1)!testing!the!correct! position!with!the!maximum!response!of!the!photodiode.!Finally,!using!the!camera,! we!could!fix!the!second!beamsplitter!(30:70)!to!have!a!good!position!of!the!sample! image!in!the!camera!chip.! ! !
2.3!Software!update!design! ! The! research! group! had! a! home7made! software,! realized! with! the! programming!environment!LabVIEW,!to!control!the!LBIC!and!EQE!measurements! in!one!dimension!(Fig.%2.9).!! However,! this! new! LBIC! dispositive! requires! a! software! modification! to! control! and!automatize!the!measurements!in!two!dimensions.! !
Figure%2.9.0%Screen%capture%of%the%software%first%version.%
22! !
!
Design!and!Implementation!of!a!Photocurrent!Mapping!Tool!for!Organic!Solar!Cells!
! One!of!the!main!challenges!of!this!software!update!is!maintaining!the!optical!focus! and!correct!for!a!possible!tilt!of!the!sample!during!the!entire!measurement.!! ! To! solve! this! problem! the! software! requires! the! user! at! the! beginning! to! locate! three! different! points! in! the! sample,! the! top! two! corners! and! the! lower! central! point,! with! the! correct! focus! (Z! value)! in! each! point! (P1,! P2! and! P3! in! Fig.%2.10).! These!three!points!in!focus!are!used!to!calculate!the!mathematical!plane!where!all! the!points!are!in!focus,!or!in!other!words,!the!XYZ!value!of!each!point!of!the!plane.! This! first! step! applies! to! the! whole! sample,! it! means! the! chip! containing! the! six! different!sample!contacts!(Fig.%2.10)!and!is!not!necessary!to!change!these!values!if! the!measurements!are!made!without!manipulating!physically!the!sample!holder.! !
!
Figure%2.10.0%Relative%position%of%the%three%points%(P1,%P2%and%P3)%to%calculate%the%correct% focus%plane%and%tilt%of%the%sample,%and%initial%point%of%measurement%(P4).%
! After! this! initial! step,! the! software! requires! the! user! the! type! of! measurement! (LBIC!or!EQE!in!this!case)!and!the!initial!XY!coordinates!of!the!first!measurement! point!(P4!in!Fig.%2.10).!Finally!the!last!step!required!is!the!length!in!X!and!Y!of!the! sample!to!measure,!as!well!as!the!measurement!step!in!X!and!Y.! ! With! these! previous! indications! the! software! knows! exactly! the! position! of! each! part!of!the!sample!and!can!correct!the!focus!along!the!measurements.!This!update! of!the!software!improves!the!measurement!accuracy!in!one!dimension,!included!in! the! previous! software,! and! allows! the! acquisition! of! LBIC! and! EQE! measurement! maps.! ! It!is!important!to!mention!that!this!is!only!the!design!of!the!software!update,!the! situation! of! the! software! at! the! end! of! this! master! thesis! (September! 2014)! is! incomplete!due!to!problems!of!amount!of!time!to!achieve!the!final!version!of!the! software.!Nowadays!members!of!the!research!group!are!developing!this!design!to! implement!these!features!to!the!previous!software!version!(Fig.%2.9).!
!
23!
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24! !
! ! ! ! ! ! Chapter!3.!Results!and!Discussion! ! In!this!chapter!we!expose!the!final!state!of!the!LBIC!device!assembled!and! adjusted! and! the! changes! suffered! respect! the! initial! design! (3.1),! some! aspects! related! with! the! resolution! (3.2)! and! finally! some! of! the! future! checks! and! improvements!for!this!setup!are!exposed!(3.3)!in!order!to!proof!the!versatile!and! powerful!tool!that!can!be!the!LBIC!setup!designed!in!this!project.! !
3.1!LBIC!setup! ! ! This!section!contains!some!pictures!of!the!final!LBIC!dispositive!assembled! and!adjusted,!as!well!as!indications!of!each!component!detailed!in!section!2.1.2.!! ! Regarding! the! light! source! used! for! the! LBIC! setup,! the! research! group! have! a! double! lamp! Oriel! Instruments™! 7340! with! a! selectable! Xenon! or! Halogen! light! source.! The! light! outgoing! from! this! lamp! can! be! directly! guided! through! the! optical! fiber! to! the! fiberport! to! acquire! IPV! curves! of! the! sample! using! the! LBIC! setup.! Furthermore,! the! light! provided! by! the! lamp! can! be! directed! to! a! monochromator! Horiba! Jobin! Ybon™! MicroHR! enabling! the! study! of! external! quantum!efficiency!(EQE)!for!different!wavelengths.! ! The!sample!pin!selector!(Fig.%2.7(b))!is!connected!to!a!Keithley!series!2400!source! measure! unit! instrument,! providing! precision! voltage! and! current! sourcing! photoinduced! on! the! sample.! At! the! same! time,! the! measure! unit! Keithley! is! connected!to!the!computer!containing!the!control!software!to!manage!the!results.! ! All!the!system!is!connected!to!a!computer!to!control!the!translation!stage,!camera! and!measurement!tools.!
25!!
Master!Thesis!
! !
Figure%3.1.0%Description%and%comparative%between%initial%and%final%LBIC%setup:%(a)%initial% optical%diagram%designed%during%the%first%months%of%the%project;%(b)%final%optical%diagram% including%as%a%improvement%a%diaphragm%(fiberport)%and%a%convergent%lens%(LED);%(c)%final% LBIC%setup%top%view%showing%the%principal%components.%
26! !
!
Design!and!Implementation!of!a!Photocurrent!Mapping!Tool!for!Organic!Solar!Cells!
! As!a!comparative,!Fig.%3.1(a)!shows!the!initial!LBIC!optical!design!that!we!present! at!the!beginning!of!this!project!(Section%2.1.1)!and!Fig.%3.1(b)!the!final!LBIC!optical! diagram!improved.!! We!decided!to!assemble!a!diaphragm!after!the!fiberport!to!control!the!size!of!the! light!beam!and!a!convergent!lens!after!the!LED!light!source!to!correct!the!outgoing! divergent! light! minimizing! internal! reflections! in! the! system.! Furthermore,! we! ordered! a! new! piece! to! fix! the! relative! position! of! the! cage! cubes,! improving! the! stability!of!the!system!and!preserving!the!alignment.!Finally,!Fig.%3.1(c)!shows!a!top! view!of!the!final!LBIC!setup!indicating!each!component.!Using!this!comparative!we! can! observe! the! design! evolution! along! the! project! and! perfectly! identify! all! the! components!understanding!the!light!path!through!the!system.! ! As!discussed!previously,!the!final!LBIC!setup!has!mounted!an!objective!lens!turret! containing! different! lens! and! objectives,! including! our! main! ultraPlong! working! distance!objective!(Fig.%3.2).!
Figure%3.2.0%LBIC%lateral%view%showing%the%3D%motorized%stage%and%the%main%objective% assembled%in%the%turret%lens.%
!
!
!
27!
Master!Thesis!
!
!
Figure%3.3.0%Controllers%top%view%and%description.% %
Finally,! Fig.% 3.3! shows! all! the! manual! controllers! of! our! setup,! the! sample! pin! selector! to! choose! the! part! of! the! sample! that! we! want! to! measure,! the! LED! dimmer! to! control! the! LED! light! intensity! (preserving! possible! damages! on! the! organic!sample)!and!the!X,!Y!and!Z!motorized!stage!controllers,!being!possible!the! sample!position!control!manually!or!through!the!computer.!
! 3.2!LBIC!resolution! ! ! A!key!factor!to!understand!how!powerful!is!our!device!is!the!light!spot!size! that!we!can!generate!over!the!sample.!A!smaller!light!spot!means!more!resolution! in! our! measurement! maps.! In! general! terms,! the! two! features! that! limit! our! resolution!are!the!light!spot!size!and!the!minimum!step!of!the!motorized!stage!(in! our!case!50!nm).! ! In!a!theoretical!way,!we!can!calculate!the!resolution!limit!(RL)!of!our!objective,!for! a! light! wavelength! of! 550! nm,! knowing! by! the! company! specifications! that! the! numerical!aperture!(NA)!is!0.28!(Table%2.5):! ! !"
!" = !" =
(!!"!!")(!.!") (!.!")
= 1.2!!"!! !
!
(Eq.%3.1)!
! where! λ! is! the! light! wavelength! and! C! is! the! constant! of! proportionality! with! a! value!of!0.61.!It!is!important!to!note!that!this!is!the!maximum!resolution!that!our! objective!can!achieve!with!a!light!wavelength!of!550!nm.!
28! !
Design!and!Implementation!of!a!Photocurrent!Mapping!Tool!for!Organic!Solar!Cells!
! In!order!to!test!the!real!spot!size!of!our!setup!in!working!conditions,!we!captured! some!images!with!the!LBIC!camera!system!of!the!light!spot!over!organic!solar!cells! samples! with! different! grid! patterns! (Fig.%3.4).! Knowing! the! grid! width! (w! in! Fig.% 3.4(a)! and! (b))! we! can! extrapolate! the! size! of! our! light! spot! in! 20! µm! approximately.! !
!
Figure%3.4.0%Size%comparative%between%LBIC%light%spot%and%two%different%patterned%grids.%(a)% and%(b):%diagrams%of%structure%of%the%patterned%grids%(honeycomb%and%linear%shape,% respectively)[29];%(c)%and%(d):%optical%images%of%organic%solar%cells%with%honeycomb[29]%and% linear%grids[30],%respectively;%(e)%and%(f):%optical%images%obtained%with%our%LBIC%setup%showing% a%light%spot%size%of%20%µm%approximately.%
! ! ! !
29!
Master!Thesis!
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3.3!Future!checks!and!improvements! ! ! During! the! following! months! after! this! master! thesis! project! the! research! group! should! to! check! some! aspects! of! the! system! to! ensure! the! perfect! working! conditions.!Some!of!these!aspects!are:! ! ·!Photodiode! calibration.! To!know!exactly!the!changes!induced!in!the!light!that! pass! through! the! setup! is! important! to! acquire! a! wavelength! vs.! intensity! graph,! comparing! the! photodiode! response! when! reminds! in! his! operating! position! and! when!is!positioned!in!the!sample!position.!This!calibration!step!provides!essential! information!of!the!changes!in!the!light!by!the!setup!components,!i.e.!beamsplitters.! ! ·! Resolution.! The! size! of! the! spot! light! should! be! measured! with! detail! and! not! only!in!a!comparative!way.!During!the!first!measurements!the!role!played!by!the! minimum! step! of! the! motorized! stage! and! the! spot! size! should! be! checked! to! improve!the!resolution!of!the!system.! ! ·!Stability.!Since!the!LBIC!setup!is!assembled!over!an!optical!table!shared!by!other! experimental!setups,!it!is!important!to!check!the!stability!of!the!system!in!working! conditions!to!minimize!possible!noise!in!the!measurements.! ! Moreover,! this! project! is! the! first! step! in! the! construction! of! a! powerful! multifunction! measurement! tool.! Then,! we! present! some! of! the! measurement! maps!that!can!be!made!with!this!LBIC!device!only!including!a!few!new!components! to!our!assembly.! ! ·!Reflectivity.!As!we!can!see!in!Fig.%3.5,!including!a!third!beamsplitter!inside!a!new! cage!cube!and!another!photodiode,!we!can!measure!the!reflectivity!of!our!samples.! Reflectivity!is!the!square!of!the!magnitude!of!the!reflection!coefficient,!which!can! be!expressed!as!a!complex!number!as!determined!by!the!Fresnel!equations.! ! This! kind! of! measurement! can! extract! information! about! the! behaviour! of! each! part! of! the! sample! under! illumination,! showing! which! parts! absorb! more! efficiently!light[31P32].! ! ·! Electroluminescence! (EL).!Is!a!phenomenon!in!which!a!material!emits!light!in! response! to! an! electric! current.! Applying! an! electric! current! to! the! sample! and! using! the! same! assembly! described! in! Fig.%3.5,! we! can! detect! with! the! additional! photodiode!the!electroluminescence!at!each!point[33P35].! ! ·!Photoluminescence!(PL).!Is!a!phenomenon!in!which!a!material!emits!light!upon! absorption! of! light! with! higher! energy! due! to! radiative! recombination! between! electrons!and!holes.!Observation!of!photoluminescence!at!a!certain!energy!can!be! 30! !
Design!and!Implementation!of!a!Photocurrent!Mapping!Tool!for!Organic!Solar!Cells!
! viewed!as!indication!that!excitation!populated!an!excited!state!associated!with!this! transition! energy! obtaining! information! of! the! material! studied[36P37].! Due! to! the! low!signal!obtained!with!this!technique,!for!this!kind!of!measurement!we!need!to! include!a!lockPin!in!the!assembly!to!reduce!the!ratio!between!signal!and!noise.! !
Figure%3.5.0%Optical%diagram%including%the%possibility%of%reflectivity,%photoluminescence%and% electroluminescence%measurements.%
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! ! ! ! ! ! Chapter!4.!Conclusions! ! ! This! master! thesis! report! exposes! the! complex! process! of! developing! a! custom! light7beam! induced! current! characterization!device! from! zero.! Discussing! with! the! research! group! the! most! actual! points! of! the! research! in! the! field! of! organic!solar!cells,!exposed!in!the!introduction!chapter,!we!were!able!to!design!a! highly! useful! device! capable! to! characterize! different! types! of! samples.! The! importance!of!this!new!LBIC!setup!lies!mainly!in!his!versatility,!able!to!incorporate! easily!new!characterization!modules!and!allowing!the!study!of!gradient!samples!to! optimize!in!a!few!steps!the!critical!aspects!of!organic!solar!cells!performance.! ! Studying! carefully! the! needs! that! must! be! covered! by! this! LBIC,! and! after! three! orders!to!Thorlabs™!and!some!other!problems!related!to!the!optical!stability,!we! finally! assemble! all! the! components! and! measurement! dispositives.! Chapter( 2! exposes! all! these! components! explaining! the! reason! of! choosing! each! of! them.! In! the!particular!case!of!the!optical!fiber!(Table(2.1),!the!offer!that!we!can!find!in!the! Thorlabs™!catalogue!not!contain!an!optical!fiber!able!to!cover!our!needs,!and!we! decided! to! order! a! customized! one! designed! expressly! for! our! new! characterization!setup.! ! This! new! device! allows! the! acquisition! of! measurement! maps! using! the! 3D! motorized! stage! assembled.! To! exploit! this! possibility! is! necessary! to! implement! this! new! feature! to! the! previous! software.! This! report! also! exposes! the! design! of! the!LBIC!software!update!in!order!to!automatize!the!measurements!correcting!the! optical! focus! and! tilt! of! our! samples.! Unfortunately,! due! to! time! constraints,! this! report! ends! in! the! design! of! the! software! update,! however! other! members! are! nowadays!finishing!the!programming!software,!and!probably!before!the!end!of!the!
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Master!Thesis!
! year! the! research! group! will! be! ready! to! extract! the! first! results! with! this! new! setup.! ! Thanks!to!this!opportunity!afforded!by!Dr.!Mariano!Campoy7Quiles,!I!have!learned! not! only! the! technical! aspects! of! the! organic! solar! cells! research! but! the! exciting! and!complex!process!to!create!a!totally!new!characterization!device!that!I!hope!to! give!many!results!for!the!Nanostructured!Optoelectronic!Materials!research!group! of! ICMAB.! This! different! master! thesis! project! brings! me! new! abilities! and! a! different!perspective!of!the!whole!research!process.!
34! !
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A.M.! Ballantyne,! T.A.M.! Ferenczi,! M.! Campoy?Quiles,! T.M.! Clarke,! A.! Maurano,! K.H.! Wong,! W.! Zhang,! N.! Stingelin?Stutzmann,! J.S.! Kim,! D.D.C.! Bradley,!J.R.!Durrant,!I.!McCulloch,!M.!Heeney,!J.!Nelson.!Understanding!the! influence!of!morphology!on!poly(3?hexylselenothiophene):PCBM!solar!cells.! Macromolecules(2010;!43:!1169?1174.! J.! Cabanillas?Gonzalez,! O.! Peña?Rodríguez,! I.S.! López,! M.! Schmidt,! M.I.! Alonso,! A.R.! Goñi,! M.! Campoy?Quiles.! Organic! position! sensitive! photodetectors! based! on! lateral! donor?acceptor! concentration! gradients.! Applied(Physics(Letters(2011;!99:!103305.! M.! Campoy?Quiles,! V.! Randon,! M.M.! Mróz,! M.! Jarzaguet,! M.! Garriga,! J.! Cabanillas?González.! Continuous! lateral! gradients! in! film! morphology! for! position! sensitive! detection! and! organic! solar! cell! optimization.! Organic( Photonics(and(Photovoltaics(2013;!11?23.! T.! Kirchartz,! T.! Agostinelli,! M.! Campoy?Quiles,! W.! Gong,! J.! Nelson.! Understanding! the! thickness?dependent! performance! of! organic! bulk! heterojunction! solar! cells:! The! influence! of! mobility,! lifetime! and! space! charge.!The(Journal(of(Physical(Chemistry(Letters(2012;!3:!3470?3475.! J.!Cabanillas?Gonzalez,!M.!Schmidt,!O.!Peña?Rodríguez,!M.I.!Alonso,!A.R.!Goñi,! M.! Campoy?Quiles.! Effect! of! structure! and! interlayer! diffusion! in! organic! position! sensitive! photodetectors! based! on! complementary! wedge! donor/acceptor! layers.! Journal( of( Nanoscience( and( Nanotechnology( 2013;! 13:!1?6.! I.!Burgués?Ceballos.!Towards!industrial!viability!of!organic!solar!cells:!ITO? free,!green!solvents!and!technological!aspects!for!upscalability.!PhD!thesis.! Universitat(Autònoma(de(Barcelona,!2014.! I.!Burgués?Ceballos,!N.!Kehagias,!C.M.!Sotomayor?Torres,!M.!Campoy?Quiles,! P.D.!Lacharmoise.!Embedded!inkjet!printed!silver!grids!for!ITO?free!organic! solar! cells! with! high! fill! factor.! Solar( Energy( Materials( &( Solar( Cells( 2014;! 127:!50?57.! S.W.! Liu,! C.C.! Lee,! J.C.! Huang,! W.C.! Su,! C.F.! Lin,! C.T.! Chen,! J.H.! Lee.! Low? reflectance! organic! light?emitting! diode! embedded! with! organic! solar! cell.! Symposium(Digest(of(Technical(Papers(2012;!42:!1773?1775.! G.! Dennler,! K.! Forberich,! M.C.! Scharber,! C.J.! Brabec,! I.! Tomis,! K.! Hingerl,! T.! Fromherz.!Angle!dependence!of!external!and!internal!quantum!efficiencies! in! bulk?heterojunction! organic! solar! cells.! Journal( of( Applied( Physics( 2007;! 102:!054516.! U.! Rau.! Reciprocity! relation! between! photovoltaic! quantum! efficiency! and! electroluminescent! emission! of! solar! cells.! Physical( Review( B( 2007;! 76:! 085303.!
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K.! Tvingstedt,! K.! Vandewal,! A.! Gadisa,! F.! Zhang,! J.! Manca,! O.! Inganäs.! Electroluminescence! from! charge! transfer! states! in! polymer! solar! cells.! Journal(of(the(American(Chemical(Society(2009;!131:!11819?11824.! S.W.! Liu,! C.F.! Lin,! C.C.! Lee,! W.C.! Su,! C.T.! Chen,! J.H.! Lee.! High! open?circuit! voltage! planar! heterojunction! organic! photovoltaics! exhibiting! red! electroluminescence.! Journal( of( the( Electrochemical( Society( 2012;! 159:! H191?H194.! N.A.! Nismy,! K.D.G.I.! Jayawardena,! A.A.D.T.! Adikaari,! S.R.P.! Silva.! Photoluminescence!quenching!in!carbon!nanotube?polymer/fullerene!films:! Carbon!nanotubes!as!exciton!dissociation!centres!in!organic!photovoltaics.! Advanced(Materials(2011;!23:!3796?3800.! Y.!Park,!K.S.!Choi,!H.!Yang,!S.Y.!Kim.!Dependence!of!organic!photovoltaic!cell! efficiency! on! the! photoluminescence! wavelength! of! core/shell! quantum! dots.!Thin(Solid(Films(2013;!537:!217?220.!
!!!!!!!!! Master!in!Nanotechnology!and!Materials!Science! ! ! ! ! ! !
Master Thesis Annex: Design and Implementation of a Photocurrent Mapping Tool for Organic Solar Cells !
Research!Methodologies! 201352014!
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Author:!Javier!Fernández!Tejero!(NIU:!2142019)! Director:!Dr.!Mariano!Campoy5Quiles!(ICMAB)! !
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Contents!
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1! !
Presentation!of!the!research!topic!
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Key!factors:!efficiency,!stability!and!process! Photovoltaic!process!in!organic!solar!cells! Objectives! ! ! ! ! !
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! ! ! 2! State?of?the?art! ! ! ! ! ! ! ! 2.1! Morphological!and!chemical!characterization!methods! ! ! Atomic!force!microscopy!(AFM)! ! ! ! ! ! Scanning!electron!microscopy!(SEM)! ! ! ! ! X5Ray!photoelectron!microscopy!(XPS)! !!!!! ! ! ! MicroRaman!spectroscopy!!!!!!!!! ! ! ! ! ! 2.2! Electrical!characterization!methods! ! ! ! ! I5V!curves! ! ! ! ! ! ! ! ! Internal!and!external!quantum!efficiency!(IQE/EQE)! !!!!!!!!!!! ! Light!beam5induced!current!(LBIC)! ! ! ! ! Conductive!and!photoconductive!atomic!force!microscopy! ! ! 3! Project!planning!and!methodology! ! ! ! ! ! Task!1.5!Design! !!!!!!!!! ! ! ! ! ! ! Task!2.5!Assembly! ! ! ! ! ! ! ! Task!3.5!Software!development! !!!!!! ! ! ! ! Task!4.5!Calibration! ! ! ! ! ! ! ! Task!5.5!Reference!measurements! ! ! ! ! ! ! 4! Timing!of!tasks! !!!!! ! ! ! ! ! ! ! References! ! ! ! ! ! ! ! ! ! ! ! ! !
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Design'and'Implementation'of'a'Photocurrent'Mapping'Tool'for'Organic'Solar'Cells'
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! 1!–!PRESENTATION!OF!THE!RESEARCH!TOPIC! ' ' After' the' discovery' of' polymer' materials' with' semiconducting' properties' at' the' 70’s' of' the' XXth' century,' such' materials' have' been' intensively' studied' for' various' electronic' applications,' especially' in' the' past' decade.' The' field' of' organic'''' photovoltaics[A1JA2]' (OPVs)' (Fig.% 1)' has' been' growing' quickly' and' showing' promise' as' a' lowJcost'PV'technology.' ' '
Figure%1.*%Plastic%solar%cell.%
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' ' Reasons'for'the'increased'interest'in'OPVs'can'be'summarized'as'follows:'polymers'allow' for' fast,' simple,' lowJcost,' and' largeJvolume' processing.' In' other' words,' polymers' can' be' solution'processed,'which'refers'to'the'printing'or'coating'of'solar'cells'using'rollJtoJroll' (R2R)'machinery,'similar'to'the'way'in'which'newspapers'are'made[A3JA4].'' ' ' Key!factors:!efficiency,!stability!and!process! ' The'power'conversion'efficiency'(PCE)'is'of'course'highly'important'in'order'to'compete' with'the'traditional'silicon'technology,'however'at'least'two'other'important'factors'must' be'considered'for'the'success'of'OPVs'(Fig.%2).'These'aspects'are'the'stability,'the'cost'and' the' processing[A5JA6].' In' general' terms' these' requirements'must'be'applied'for'any'device'under' study,'but'in'the'case'of'OPVs'are'critical.'' Organic' materials' are' by' nature' more' susceptible' to' chemical' degradation' than' inorganic' materials' and' the' understanding' of' the' degradation' processes' are' a' key' factor' to' develop' OPVs' with' good'work'lifetimes.'Additionally,'the'optimization' of' the' manufacturing' process,' becoming' environmentJfriendly'and'economic,'is'essential'for' the' largeJscale' production' and' commercialization' of'OPVs.' Figure%2.*%Key%factors%on%OPVs%research.[A5]%
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Master%Thesis%Annex:'Research'Methodologies'
' Photovoltaic!process!in!organic!solar!cells! ! A'typical'PV'process'involves'the'creation'of'free'carriers'from'incident'photons'(Fig.%3).' The' external' quantum' efficiency' (EQE)' as' a' function' of' wavelength' (!)' is' the' ratio' between' the' collected' photogenerated' charges' and' the' number' of' incident' photons,' ultimately'being'the'product'of'four'efficiencies'(!):'absorption'(A),'exciton'diffusion'(ED),' charge'separation'(CS)'and'charge'collection'(CC).' ' !"! ! = !! ! · !!" ! · !!" ! · !!! (!)'' ' ' (Eq.%1)' ' '
Figure%3.*%Physics%and%energy%diagram%of%polymer*fullerene*based%OPVs.[A7]'
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' Objectives! ' The' aim' of' this' master' thesis' is' the' design,' assembly' and' calibration' of' a' computerJ controlled' 3D' Light' BeamJInduced' Current' (LBIC)' dispositive' able' to' map' the' principal' parameters' described' previously,' in' order' to' study' different' characteristics,' such' as' nanomorphology,'homogeneity,'stability,'graded'and'patterned'structures,'etc.'' ' The'final'objective'of'this'project'is'the'measurement'of'samples'with'different'gradients' (composition,'thickness,'etc.).'Using'the'new'LBIC'dispositive'we'will'be'able'to'extract'a' huge'quantity'of'conclusions'of'the'device'performance'and'stability'to'find'the'optimum' characteristics.''
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Design'and'Implementation'of'a'Photocurrent'Mapping'Tool'for'Organic'Solar'Cells'
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2!–!STATEGOFGTHEGART! ! ! ' The' overall' conversion' efficiency' of' a' photovoltaic' device' depends' on' the' efficiency'of'every'point'of'the'photoactive'layer,'including'the'edges'or'areas'close'to'the' contacts.' By' understanding' the' correlation' between' the' morphological/chemical' properties' and' the' electrical' response' we' can' optimize' the' final' performance' of' OPVs' devices.'As'we'mentioned'in'the'previous'section,'stability'is'also'an'important'factor.'The' existence'of'local'defects'may'serve'as'a'seed'of'a'future'degradation'adversely'affecting' the'solar'cell'global'properties.'' ' Experimental' techniques' suitable' to' map' the' spatial' distribution' of' morphological,' chemical' and' electrical' parameters' provide' valuable' information,' and' thus' help' to' improve'the'technology'for'the'production'of'efficient'and'reproducible'organic'solar'cells.' Nowadays' there' are' many' techniques' to' imaging' the' principal' parameters' with' great' spatial' resolution.' Some' of' these' techniques' are' summarized' in' this' chapter,' ordered' by' types'of'measurement'(electrical'or'morphological/chemical)'and'describing'his'principal' characteristics.'Furthermore,'some'examples'of'application'in'OPVs'research'are'shown.' ' ' '
2.1!–!Morphological!and!chemical!characterization!methods!
! While' EQE' or' IJV' curve' measurements' provide' useful' information' on' the' photovoltaic' properties' other' methods' provide' information' (directly' or' indirectly)' on' either' morphology' or' chemistry,' or' a' combination' of' both[A9JA15].' Some' of' the' principal' morphological'and'chemical'characterization'methods'are'summarized'in'Table%1.' ' ! Technique! Bulk!analysis! Surface! 2D!imaging! NonG analysis! destructive! ✓' ✓' AFM! ' ' ✓' ✓' SEM/TEM! ' ' ✓ ✓ XPS! ' ' ✓' ✓' XRD! ' ' ✓' ✓' ✓' Profilometry! ' ✓' ✓' ✓' Raman! ' ✓' ✓' ✓' Photoluminescence! ' ✓' ✓' ✓' Ellipsometry! ' Table%1.*%Morphological%and%chemical%characterization%methods%triggering%important%advances%in% actual%OPVs%research.%
! Atomic!force!microscopy!(AFM)! ! AFM' is' an' imaging' technique' that' typically' maps' the' surface' with' respect' to' surface' topography.' AFM' can' cover' only' a' limited' analysis' area,' but' has' an' excellent' lateral' and' height' resolution.' The' typical' application' for' AFM' with' regard' to' OPVs' has' been' to' monitor'the'nanostructure'of'bulk'heterojunction'blend'films'in'order'to'optimize'the'OPV' efficiency.' ' ' ' '
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Master%Thesis%Annex:'Research'Methodologies'
' Scanning!electron!microscopy!(SEM)' ' SEM' is' also' an' imaging' technique' and' has' become' a' standard' tool' for' visualizing' morphology' in' OPVs' (Fig.%4).' Just' as' AFM' the' typical' application' for' SEM' with' regard' to' OPVs' has' been' to' monitor' nanostructures' in' order' to' improve' the' OPV' efficiency.' Both' techniques,' AFM' and' SEM,' can' also' be' used' to' extract' information' about' the' chemical' composition'of'the'samples,'for'example,'using'AFM'in'different'modes.' ' '
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Figure%4.*%Top%views%of%films%cast%from%toluene%with%various%weight%ratios%of%MDMO*PPV%to%PCBM% using%SEM.[A14]%
! ! XGRay!photoelectron!microscopy!(XPS)! ! XPS' provides' chemical' information' via' measured' core' electronJbinding' energies.' Is' an' elemental'analysis'and'provides'valuable'quantitative'information'on'the'chemical'state,' which' is' especially' useful' for' example' with' respect' to' carbon' when' studying' OPV' degradation.' ! MicroRaman!spectroscopy! ! MicroRaman' spectroscopy,' is' a' spectroscopic' technique' used' to' observe' vibrational,' rotational,'and'other'lowJfrequency'modes'in'a'system,'and'relies'on'inelastic'scattering'of' monochromatic' light.' Raman' spectroscopy' offers' several' advantages' for' microscopic' analysis.'The'spectra'can'be'collected'from'a'very'small'volume'(
