S olution-processed solar cells offer the promise of low cost, large-area processing, and, prospectively, high

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1 OPEN SUBJECT AREAS: NANOPARTICLES APPLIED OPTICS ELECTRICAL AND ELECTRONIC ENGINEERING SOLAR CELLS Folded-Light-Pth Colloidl Quntum Dot Solr Cells Ghd I. Koleilt*, Illn J. Krmer*, Chris T. O. Wong, Susnn M. Thon, André J. Lelle, Sjoerd Hooglnd & Edwrd H. Srgent Deprtment of Eletril nd Computer Engineering, University of Toronto, 1 King s College Rod, Toronto, Ontrio M5S 3G4, Cnd. Reeived 28 Mrh 213 Aepted 2 June 213 Pulished 9 July 213 Colloidl quntum dot photovoltis omine low-ost solution proessing with quntum size-effet tuning to mth sorption to the solr spetrum. Rpid dvnes hve led to ertified solr power onversion effiienies of over 7%. Nevertheless, these devies remin held k y ompromise in the hoie of quntum dot film thikness, lning on the one hnd the need to mximize photon sorption, mndting thiker film, nd, on the other, the need for effiient rrier extrtion, onsidertion tht limits film thikness. Here we report n rhiteture tht reks this ompromise y folding the pth of light propgting in the olloidl quntum dot solid. Using this method, we hieve sustntil inrese in short-iruit urrent, ultimtely leding to improved power onversion effiieny. Correspondene nd requests for mterils should e ddressed to E.H.S. (ted.srgent@ utoronto.) * These uthors ontriuted eqully to this work. S olution-proessed solr ells offer the promise of low ost, lrge-re proessing, nd, prospetively, high solr power onversion effiienies 1 4. Colloidl quntum dots (CQDs) hve ttrted interest s photovolti light hrvesters euse, in ddition to eing proessed from the solution phse, their ndgp n e tuned vi syntheti ontrol over nnoprtile size to optimize their spetrl properties in the sorption of sunlight 5 8. Unfortuntely, stnding in the wy of higher solr power effiieny is the ft tht CQD mterils tody exhiit limited trnsport lengths 9 11 ompred to trditionl, high-temperture-proessed ulk semiondutors. There exists therefore n urgent need to enhne sorption in given thikness of CQD solid. Optil enhnements of sorption in ultrthin film semiondutor photovoltis offer venues to overoming limited eletroni trnsport in these mterils. Progress hs reently een mde in this diretion y engineering oth the geometri nd eletroni lndspes 16 in CQD devies, s well y improving the eletril k ontts t the semiondutor/metl interfe 17,18. Plsmoni sorption enhnements were reently shown 19, nd multijuntion rhitetures offer rodnd pproh to inresing solr ell performne 2,21. Results In the present work, we sought to inrese the intertion of light with the soring CQD medium y pssing light through the CQD lyer multiple times. Multi-pss optil mngement shemes hve een shown to e effetive in rystlline silion nd dye-sensitized solr ells 25. We would fold the light pth in order to propgte light through the tive lyer more thn the typil two norml-inidene psses. Figure 1 shows flt depleted heterojuntion 3 devie tking dvntge of only two psses of light while folded-light-pth () devie (Figure 1) n tke dvntge of 6 psses, effetively soring ll photons hving energy greter thn the CQD ndgp. Figure 1 illustrtes the geometri onstrints to hieving periodilly repeting struture. Equtions relting ngles h 1, h 2 nd lengths x 1, x 2, x 3, A, A tilt, L, t nd t tilt n e found in SI 1. We sought to ssess quntittively the extent to whih the strtegy ould enhne sorption, hene urrent. We egn y exploring the spetrl ehviour in the se of 45u tilt ngle. We show in Figure 2 the perentge ontriution of eh sequentil pss in oth the flt nd the rhitetures. Wheres only first nd seond psses n e onsidered in the flt se (grey nd lk dshed lines, respetively), we onsidered up to six full psses in the se, wherein the length of eh pss is sled up y ftor of 1=sinh 2. The ontriution of the dditionl psses through the CQD lyer trnsltes into dded photon urrent through spetrl multiplition with the AM 1.5 solr photon fluene spetrum 26 (Figure 2, lue region versus ornge region). We then proeeded to explore the impt of different hoies of tilt ngle. Figure 2 ompres the two pss nd six pss totl hievle urrent s funtion of tilting ngle. The optiml lulted ngle is pproximtely 5u, yielding net potentil short-iruit urrent density (J SC ) enhnement of 3 ma?m 22. SCIENTIFIC REPORTS 3 : 2166 DOI: 1.138/srep2166 1

2 Figure 1 Configurtion nd geometri onsidertions of CQD solr ells. () Conventionl doule-pss CQD solr ell. () Periodi rrngement of CQD solr ell whih tkes dvntge of multiple psses. () Geometri onsidertions for periodiity nd multipss sorption. More detils n e found in setion SI 1. We frited devies where the CQD mterils were synthesized nd pplied using the highest-photovolti-performne previouslypulished hemistry nd mterils proessing 8. For oth the onventionl flt devie nd the devie, the eletron-epting TiO 2 lyer ws deposited y RC mgnetron sputtering 27. Msks were developed to ommodte the evportion of the vrious metlli mirrors nd eletroni ontts (SI 2) for the devie. Briefly, edge mirrors of titnium, gold nd silver were deposited, followed y titni on the fluorine-doped tin oxide (FTO) plne. Following n queous titnium hloride tretment nd therml nneling, we deposited k mirror onsisting of gold nd silver. The CQD lyer ws then deposited in lyer-y-lyer fshion using solid stte merptopropioni id (MPA) tretment. Finlly n elongted top ontt/mirror onsisting of molydenum trioxide, gold nd silver ws deposited. Controls employed onventionl ontts s desried previously 8. For more informtion, see methods. The sorption hrteristis of the flt nd devies were then explored. First, to look for qulittive evidene of multipss, we provide in Figure 3 (inset) photogrph of smple illuminted through its perture t 45u tilt, leding to internl refletions nd visul indition of t lest six totl psses through the film. This piture shows psses 1, 3 nd 5 seen through the top of the devie. Note tht the tive re of the devie is defined y the light perture, not the ontt re 28. The sorption spetr of Figure 3 inlude oth the totl devie sorption inluding the glss, FTO nd TiO 2 lyers for the norml nd 45u ngled se; nd lso the sorption ontriution of the CQD film lone. The mesured sorne inludes effets from oth the desired quntum dot film sorption, nd lso the undesired sustrte nd eletrode sorption. The externl quntum effiieny (EQE, Figure 3) of the devie, on the other hnd, fetures only the enefits of the inrese in quntum dot film sorption. In the infrred spetrl rnge, where doule-pss sorption is the lowest, the mesured se improvement is the gretest, nd the enhned sorption nd EQE follow one nother most losely. In the visile spetrl rnge, the enefits of the re rued in ft in the first pss (i.e. it is not multipss/folding, ut only tilting, tht enefits EQE): sorption ours loser to the front hrge-seprting interfe, leding not to higher sorption, ut to higher internl quntum effiieny in the tilted se. Convolving these EQE urves with the AM 1.5 solr spetrum nd integrting genertes expeted J SC vlues of 25 ma?m 22 to 21 ma?m 22 in the nd flt devies, respetively. To evlute the impt on solr power onversion effiieny, we now onsider s well the implitions on voltge of the rhiteture. Colleting urrent from the multipss devie neessittes n inrese in eletrode re per sun-fing re of solr ell. Thus, in the eqution for open-iruit voltge (V OC ) s funtion of shortiruit urrent (I SC ), drk sturtion urrent (I ) nd Temperture (T) 29 V OC ~ kt q ln I SC z1 ð1þ I were k is the Boltzmnn onstnt nd q is the elementry hrge, I is inresed y the re of the eletrode. In our est ells, our typil ontt strip re ws over 7 times lrger thn the perture re due to geometri onsidertions, yielding n expeted drop in V OC from our typil flt ontrol of.61 V to.56 V (i.e. loss of kt=q ln7 s predited in eqution 1). The 15% expeted improvement in J SC exeeds this 8% loss in V OC. Experimentlly, the devies exhiited n enhned urrent ompred to plnr ontrols of 24.6 ma?m 22 versus 2.2 ma?m 22 (Figure 4,), n inrese of 4.4 ma?m 22. The derese in voltge (green ompred to red mrkers in Figure 4) lso followed the predited derese (Figure 4e) from.61 V to.56 V. Overll, the power onversion effiieny, g, reported in Figure 4 nd d, improved to AM1.5 solr g of 7.8 (the fill ftor differenes re negligile s they fll within the mesurement error). For referene, Figure 4f nd g illustrte the expeted gin in urrent nd g s funtion of eletrode pixel strip length when onsidering purely optil light pture. Clerly notle is the 2 pss regime under,3.25 mm, the 4 pss regime from,3.25 mm to,6 mm nd the 6 pss regime from,6 mmto,8.75 mm. Susequent psses were not onsidered s they ontriute very little to dditionl J SC. Disussion It is evident tht ny up-front sorption y the sustrte nd trnsprent ontts would ompound unfvourly in multipss ontext. To quntify this, we turned to the sme model employed in Figure 2 while ounting for the glss, FTO nd TiO 2 lyers of our typil depleted heterojuntion devies. The eletron-epting SCIENTIFIC REPORTS 3 : 2166 DOI: 1.138/srep2166 2

3 Perent Asored (%) (μa m 2 nm 1 ) (ma m 2 ) AM pss 6 pss pss 3 6 pss pss 2 pss Tilt Angle, θ (deg) 6 1 # of psses Figure 2 Theoretil ontriutions of multiple psses on J SC. () Asorption ontriution of the first two psses (grey nd lk dshed lines) of flt DH CQD devie nd of the first six psses (olored lines) of DH CQD devie. () Spetrl urrent ontriution of the first two psses nd susequent four psses of DH CQD devie. The two-pss flt ontriution is shown s dshed lk urve. Grey re represents the AM 1.5 spetrum. () Tilting ngle impt on totl potentil output urrent onsidering two psses (ornge urve) nd six psses (lue urve). TiO 2 is,5 nm thik nd is formed from olloidl pste. Figure 5, d nd g-left highlight the ontriution of eh of these lyers, highlighting the impt of eh in 45u tilted ontext. The thik TiO 2 lyer lone sors the photon equivlent of 6 ma?m 22 of short-iruit urrent (red r, Figure 5g), leving the CQD lyer with est-se 26 ma?m 22 J SC (yellow r, Figure 5g). By employing thinner, sputtered TiO 2 lyer (Figure 5, e nd g-middle), the devie improves its est-se J SC to 3 ma?m 22. This is still fr from the theoretil idel 1.3 ev ndgp mximum of 36 ma?m 223. To further illustrte the importne of ll lyers in the stk, we lso modeled the est-se senrios for ll lyers in the stk, inluding thinner glss, highly trnsprent indium tin oxide (ITO), sputtered TiO 2 s well s perfet refletor t the top ontt over ll wvelengths of interest (Figure 5, f nd g-right). In this se, only Fresnel refletions t the front surfe of the ell nd the sorption oeffiients of the CQD lyer itself limit sorption. Through this nlysis, it is possile to hieve over 34 ma?m 22 of J SC. This third ondition, wherein the tive lyer hs ess to nerly ll inident photons, suggests further venue to optimizing solr ell employing folded light pth. The inoupling nd trpping of Figure 3 Asorption nd externl quntum effiieny of devies. () Devie (solid lines) nd CQD film (dshed lines) sorption urves for flt (red) nd (green) smples. () Externl quntum effiieny of flt nd devies. ( inset) Photogrph of tilted smple illuminted through the devie perture t 45u with thin strip of white light demonstrting the pturing of multiple psses of light in this se the third nd fifth psses n e lerly oserved. For photovolti mesurements, the light perture defined the tive re dimensions 28. () Asorption nd EQE enhnement of devie s ompred to flt devie. Internl quntum effiieny urves n e found in setion SI 3. light into the tive lyer is ruil to pprohing omplete sorption. It would e ttrtive to tilor the lyers ldding the CQD mteril suh tht, following the first pss, light remined trpped within the CQD lyer, wherein it would e wveguided nd sored. In the sene of more strongly soring mterils, folding the light pth within the tive lyer of solr ell remins promising mens to overome the sorption versus extrtion ompromise. Methods Synthesis of PS quntum dots nd metl hlide tretment. PS quntum dots were synthesized ording to previously pulished method 31. A solution-phse metl hlide tretment (CdCl 2 ) ws then rried out following previously pulished method 8. Speifilly, the metl hlide preursor (1 ml of CdCl 2 )nd tetrdeylphosphoni id (TDPA) were dissolved in oleylmine with Cd:TDPA molr rtio. This mixture ws introdued into the CQD retion flsk fter the sulfur soure injetion during the slow ooling proess. A 651 P:Cd molr rtio ws dopted during the synthesis. At 3 35uC, the nnorystls were isolted y the ddition of 6 ml of etone then sujeted to entrifugtion. The nnorystls were then purified y dispersion in toluene nd re-preipittion with mixture of etone/ SCIENTIFIC REPORTS 3 : 2166 DOI: 1.138/srep2166 3

4 24 2 V OC (mv) e Current Density, J (ma m 2 ) J SC (ma m 2 ) d f g 4 8 η (%) Voltge, V (V) Time, t (s) Pixel Length, L (mm) Figure 4 Mesured nd theoretil photovolti devie performne. () J-V hrteristis of flt nd tilted smples illustrting,4 ma?m 22 enhnement in J SC nd,.5 V redution in V OC. (,,d) Stti V OC, J SC nd g mesurements, respetively, onfirming lk of rpid degrdtion nd trnsient rtefts in flt nd devie performne. (e,f,g) Theoretil implitions of inresed pixel strip length on V OC, J SC nd g. methnol (151 volume rtio), then re-dissolved in nhydrous toluene. The solution ws further wshed with methnol two more times efore finlly dispersing it in otne t onentrtion of 5 mg/ml. Sustrte preprtion. Clened FTO-oted glss sustrtes (Pilkington, TEC 15) with 45u ut edge were employed in this study. We first deposited the edge mirror y e-em evportion of 5 nm of Ti t 1 Å/s followed y e-em evportion of 5 nm of Au t 1.5 Å/s followed y 1 nm of thermlly evported Ag t 2 Å/s. 5 nm TiO 2 (Kurt J. Lesker) ws deposited to over the sustrte nd its mirrored edge y RC mgnetron sputtering t room temperture under Ar t 5 mtorr t rte of.2 Å/s. Both deposition systems were Angstrom Engineering Åmod deposition systems loted in n Innovtive Tehnology nitrogen filled gloveox. The sustrtes were then treted with 12 mm TiCl 4 solution t 7uC for 3 min followed y rinse with deionized wter nd nneling step on hot plte t 52uC for 45 min in ir mient. The deposition of the ottom mirror ws lstly done y e-em evportion of 5 nm of Au t 1.5 Å/s followed y 1 nm of thermlly evported Ag t 2 Å/s. Perent Asored (%) (μa m 2 nm 1 ) (ma m 2 ) d AM 1.5 CQDs TiO 2 FTO Glss g e f TiO 2 Pste Sputtered TiO 2 Optimisti TCO Figure 5 Asorption ontriutions of onstituent solr ell lyers. (,,) Perentge sorption ontriution nd (d,e,f) spetrl J SC ontriution of glss (nvy), FTO (lue), TiO 2 (red) nd CQDs (yellow) for devies employing TiO 2 pste (,d), sputtered TiO 2 (,e) nd n optimisti sustrte/ eletrode omintion (,f). (g) Totl sored photon equivlent J SC for ll three ses. SCIENTIFIC REPORTS 3 : 2166 DOI: 1.138/srep2166 4

5 Devie frition. PS CQD films were deposited using lyer-y-lyer spinoting proess under n mient tmosphere. For eh lyer, the CQD solution (5 mg/ml in otne) ws deposited on the sustrte nd spin-st t 25 RPM for 1 s. Solid-stte lignd exhnge ws performed y flooding the surfe with 1% v:v MPA in methnol for 3 s efore spin-oting dry t 25 RPM for 1 s. Two rinses with methnol were used to remove unound lignds. Eh devie onsisted of 1 lyers. The top ontts were finlly deposited nd onsisted of 1 nm thermlly evported MoO 3 deposited t rte of.2 Å/s, followed y e-em deposition of 5 nm of Au deposited t 1.5 Å/s, nd finlly 12 nm of thermlly evported Ag deposited t 2. Å/s. AM 1.5 photovolti performne hrteriztion. Current voltge dt were mesured using Keithley 24 soure meter. The solr spetrum t AM1.5 ws simulted to within lss A speifitions (less thn 25% spetrl mismth) with xenon lmp nd filters (SieneTeh; mesured intensity of 1 mw?m 22 ). The soure intensity ws mesured with Melles-Griot rodnd power meter through horizontl retngulr.2 m 2 perture. We used n perture slightly smller thn the top eletrode to void overestimting the photourrent: the entire photon fluene pssing through the perture ws ounted s inident on the devie for ll nlyses of J SC nd EQE 28. The spetrl mismth of the system ws hrterized using lirted referene solr ell (Newport). The totl AM 1.5 spetrl mismth - tking into ount the simultor spetrum nd the spetrl responsivities of the test ell, referene ell, nd rodnd power meter - ws re-mesured periodilly nd found to e,11%. This multiplitive ftor, M 5.89, ws pplied to the urrent density vlues of the J V urve to most losely resemle true AM 1.5 performne 32. The unertinty of the urrent voltge mesurements ws estimted to e 67%. EQE mesurements. Externl quntum effiieny mesurements were otined y pplying hopped (22 khz) monohromti illumintion (4 W xenon lmp through monohromtor with order-sorting filters) ollimted nd ofoused with 1 Sun intensity white-light soure on the devie of interest. The power ws mesured with lirted Newport 818-UV nd Newport 818-IR power meters. The response from the hopped signl ws mesured using Stnford Reserh Systems lok-in mplifier t short-iruit onditions. The unertinty in the EQE mesurements ws estimted to e 68%. Asorption mesurements. Asorption spetrosopy ws rried out using Cry 5 UV-vis-IR Sn photospetrometer with n tthed integrting sphere. The devies were ngled t 45u to mimi sorption in tilted ontext. 1. O Regn, B. & Grtzel, M. A low-ost, high-effiieny solr ell sed on dyesensitized olloidl TiO 2 films. Nture 353, (1991). 2. Shheen, S. E. et l. 2.5% effiient orgni plsti solr ells. Applied Physis Letters 78, (21). 3. Pttntyus-Arhm, A. G. et l. Depleted-Heterojuntion Colloidl Quntum Dot Solr Cells. ACS Nno 4, (21). 4. Todorov, T. K., Reuter, K. B. & Mitzi, D. B. High-Effiieny Solr Cell with Erth- Aundnt Liquid-Proessed Asorer. Advned Mterils 22, E156 E159 (21). 5. Luther, J. M. et l. Shottky Solr Cells Bsed on Colloidl Nnorystl Films. Nno Lett. 8, (28). 6. Luther, J. M. et l. Stility Assessment on 3% Bilyer PS/ZnO Quntum Dot Heterojuntion Solr Cell. Advned Mterils 22, (21). 7. M, W. et l. Photovolti Performne of Ultrsmll PSe Quntum Dots. ACS Nno 5, (211). 8. Ip, A. H. et l. Hyrid pssivted olloidl quntum dot solids. Nture Nnotehnology 7, (212). 9. Krmer, I. J. & Srgent, E. H. Colloidl Quntum Dot Photovoltis: A Pth Forwrd. ACS Nno 5, (211). 1. Zhitomirsky, D. et l. Colloidl Quntum Dot Photovoltis: The Effet of Polydispersity. Nno Letters 12, (212). 11. Guyot-Sionnest, P. Eletril Trnsport in Colloidl Quntum Dot Films. J. Phys. Chem. Lett. 3, (212). 12. Leshkies, K. S., Jos, A. G., Norris, D. J. & Aydil, E. S. 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Nno Lett. 13, (213). 2. Choi, J. J. et l. Solution-Proessed Nnorystl Quntum Dot Tndem Solr Cells. Advned Mterils 23, (211). 21. Wng, X. et l. Tndem olloidl quntum dot solr ells employing grded reomintion lyer. Nture Photonis 5, (211). 22. Sinton, R. A., Kwrk, Y., Gn, J. Y. & Swnson, R. M perent silion onentrtor solr ells. IEEE Eletron Devie Letters 7, (1986). 23. Cmpell, P. & Green, M. A. Light trpping properties of pyrmidlly textured surfes. Journl of Applied Physis 62, (1987). 24. Heine, C. & Morf, R. H. Sumirometer grtings for solr energy pplitions. Appl. Opt. 34, (1995). 25. Rühle, S., Greenwld, S., Koren, E. & Zn, A. Optil Wveguide Enhned Photovoltis. Opt. Express 16, (28). 26. Stndrd Tles for Referene Solr Spetrl Irrdines: Diret Norml nd Hemispheril on 37u Tilted Surfe. (28). doi:1.152/g173-3r Koleilt, G. I. et l. A Donor-Supply Eletrode (DSE) for Colloidl Quntum Dot Photovoltis. Nno Lett. 11, (211). 28. Snith, H. J. The perils of solr ell effiieny mesurements. Nture Photonis 6, (212). 29. Nelson, J. The Physis of Solr Cells. (Imperil College Press, 23). 3. Henry, C. H. Limiting effiienies of idel single nd multiple energy gp terrestril solr ells. Journl of Applied Physis 51, (198). 31. Hines, M. A. & Sholes, G. D. Colloidl PS Nnorystls with Size-Tunle Ner- Infrred Emission: Oservtion of Post-Synthesis Self-Nrrowing of the Prtile Size Distriution. Advned Mterils 15, (23). 32. Kroon, J. M., Wienk, M. M., Verhees, W. J. H. & Hummelen, J. C. Aurte effiieny determintion nd stility studies of onjugted polymer/fullerene solr ells. Thin Solid Films 43 44, (22). Aknowledgements This pulition is sed on work in prt supported y Awrd No. KUS , mde y King Adullh University of Siene nd Tehnology (KAUST). We thnk Angstrom Engineering nd Innovtive Tehnologies for useful disussions regrding mteril deposition methods nd ontrol of gloveox environment, respetively. G.I.K. knowledges NSERC support in the form of Alexnder Grhm Bell Cnd Grdute Sholrship. The uthors knowledge the Interntionl Coopertion of the Kore Institute of Energy Tehnology Evlution nd Plnning (KETEP) grnt funded y the Kore government Ministry of Knowledge Eonomy (212T1174). The uthors would lso like to knowledge the tehnil ssistne nd sientifi guidne of E. Plmino, L. Levin, R. Wolowie nd D. Kopilovi. Author ontriutions G.I.K. nd S.H. oneived of the projet nd undertook initil experiments. G.I.K., I.J.K. nd C.T.O.W. designed, frited, hrterized nd optimized devies. A.J.L. nd C.T.O.W. optimized the eletrodes. I.J.K. developed the optil model. S.M.T. ided in the design of experiments projet oordintion. I.J.K. nd E.H.S. wrote the mnusript. All uthors edited nd reviewed the mnusript. Additionl informtion Supplementry informtion ompnies this pper t sientifireports Competing finnil interests: The uthors delre no ompeting finnil interests. How to ite this rtile: Koleilt, G.I. et l. Folded-Light-Pth Colloidl Quntum Dot Solr Cells. Si. Rep. 3, 2166; DOI:1.138/srep2166 (213). This work is liensed under Cretive Commons Attriution- NonCommeril-ShreAlike 3. Unported liense. To view opy of this liense, visit SCIENTIFIC REPORTS 3 : 2166 DOI: 1.138/srep2166 5