26th European Photovoltaic Solar Energy Conference and Exhibition CHARACTERIZATION OF MULTILAYER LUMINESCENT SOLAR CONCENTRATORS

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1 6th Europan Photovoltaic Solar Enrgy Confrnc and Exhibition CHARACTERIZATION OF MUTIAYER UMINESCENT SOAR CONCENTRATORS A. Bozzola, S. Flors Daorta, M. Galli, M. Patrini,.C. Andrani, A. Alssi, R. Fusco, A. Proto, P. Scudo Dipartimnto di Fisica A. Volta and UdR CNISM, Univrsità dgli Studi di Pavia, Italy ENI S.p.a. Rsarch Cntr for Non-Convntional Enrgis Istituto ENI Dongani, Novara, Italy ABSTRACT: W prsnt an xprimntal study of multilayr luminscnt solar concntrators (SCs) basd on spctroscopic charactrization and xtrnal quantum fficincy masurmnt, which is assumd as figur of mrit for this work. W focus on two typs of SCs: polymric and liquid-basd. Our aim is to study th photovoltaic convrsion of solar nrgy by mans of solar spctrum splitting by th SC, to valuat th ffcts of dys concntrations on absorption of sunlight and on slf-absorption of mittd fluorscnc, and finally to valuat th ffcts of SCs sizs on thir prformanc. Singl-layr and bi-layr SCs ar considrd, basd on on or mor typs of fluorscnt dys. Kywords: Concntrators, photoluminscnc, spctral rspons. INTRODUCTION uminscnt solar concntrators (SCs) ar hybrid photovoltaic dvics in which sunlight in absorbd by organic or inorganic fluorophors (dys) disprsd in a solid or liquid matrix (Fig. ). Emittd fluorscnc is guidd within th matrix bulk by total intrnal rflctions until it rachs its dgs, whr photovoltaic (PV) clls ar applid [-5]. In principl, SCs hav many advantags ovr convntional PV clls. Sinc th dg ara of PV clls is much smallr than th SC s surfac ara, th amount of rquird smiconductor matrial is far rducd, lading to a rduction of th total dvic costs. Furthrmor, sinc absorption of sunlight and fluorscnc mission ar distinct procss (th lattr bing govrnd only by total intrnal rflction insid th SC), ths dvics can act as concntrators for both dirct and diffus sunlight without rquiring any auxiliary tracking systms. This fatur allows to rduc th dvic costs with rspct to common concntrator systms which mak us of mirrors or lnss to track th sun in its apparnt motion so to harvst only dirct sunlight. Finally, SCs can b mad smi-transparnt by a propr choic of dys absorbing only in th ultraviolt (UV) and high-nrgy visibl (VIS) spctral rangs, and this fatur is dsirabl for applications in windows, grnhouss and othr buildingintgratd photovoltaic structurs. In this work w propos an xprimntal charactrization of SCs by mans of xtrnal quantum fficincy () masurmnts. From th, short-circuit currnt dnsity J sc can b drivd according to th formula + J sc = ( λ) bs ( λ) dλ, whr b is th incidnt AM.5 solar photon flux, s rportd in Fig. in arbitrary units (for quantitativ data s, for xampl, [6]). In our work spcial attntion is paid to th following charactrizing faturs of SCs: (i) ffcts of dy absorption in diffrnt spctral rangs of th solar spctrum; (ii) ffcts dy concntrations on absorption of incidnt sunlight and slf-absorption of mittd fluorscnc; (iii) ffcts of SCs siz on th prformanc of final dvics. () Figur : Schmatic rprsntations of a singl-layr SC coupld with silicon PV clls at th dgs, and of a bi-layr SC. SC configurations undr xam: polymric (c), and liquid-basd SC (d). DEVICES UNDER INVESTIGATION SCs undr invstigation ar rprsntd in Fig.. W study both singl-layr and bi-layr SCs, which ar shown, rspctivly, in Figs. and. Singl-layr SCs can b dopd with on or mor typs of fluorscnt dys, and ar particularly suitd for th production of smi-transparnt dvics. In our work w considr thr typs of organic fluorscnt dys, brifly dnotd as A, B and C [7]. Absorption is dscribd in trms of th molar xtinction cofficint ε which has dimnsions of [cm - M - ] (whr M is th molar concntration of th dy in th matrix matrial), whil mittd fluorscnc is dscribd in trms of th normalizd mission spctrum b, AYER AYER (c) h n d h d a SC Incidnt sunlight n n n n (d) a PV Clls n d 59

2 6th Europan Photovoltaic Solar Enrgy Confrnc and Exhibition ε (M - cm - ) ε (M - cm - ) ε (M - cm - ) Wavlngth (nm) , AM.5 Photon Flux (arbitrary units),,5 DYE A (c) Wavlngth (nm),5,,8,6,4,,,8,6,4, b (nm - ) b (nm - ) b (nm - ) Figur : Molar xtinction cofficints ε (black solid lins) and normalizd mission spctra b (rd dashd lins) for dy A, dy B and dy C (c). which satisfis th condition: + b ( ) λ dλ = Optical spctra of molar xtinction cofficints ε and normalizd mission b ar shown in Figs., and (c) for dys A, B and C rspctivly. For singl-layr SCs w considrd th cass of a combination of dys A and B, dy B alon or dy C alon. Bi-layr SCs ar shown in Fig.. In this cas th top layr is dopd with a combination of dys A and B or dy B alon, whil th bottom layr is dopd with dy C alon. In trms of layr s structur, two configurations hav bn tstd, namly polymric SCs and liquid-basd SCs, which ar shown in Figs. (c) and (d) rspctivly. Polymric SCs ar mad of squar slabs of poly mthyl - mthacrylat (PMMA) in which fluorscnt dys ar disprsd. Th rfractiv indx n d of PMMA is qual to.5, which givs a rflctanc R~.4 at th intrfac btwn PMMA and air. Th thicknss h of th PMMA slabs is.65 cm, and it is qual to th width of th PV clls applid at th dgs. Th sid lngth of th slabs is dnotd with a, and it is qual to a multipl of th lngth of th applid PV clls (. cm). For th cas of polymric SCs, all four latral dgs ar covrd with PV clls, but, in ordr to rduc th numbr of PV clls, som latral dgs can b covrd with mirrors as wll. iquid-basd SCs ar shown in Fig. (d) and ar obtaind by filling a squard quartz couvtt of rfractiv indx n ~.45 with a solution of,-dichlorobnzn (th solvnt, whos rfractiv indx n d is qual to.55) and () fluorscnt dy. Th thicknss h of th couvtt is th sam of polymric slabs, whil th intrnal width d is qual to. cm. This implis that th concntrations of fluorscnt dys hav to b 6.5 tims highr in th couvtt with rspct to PMMA slabs in ordr to rach th sam dgr of absorption. For both polymric and liquid-basd SCs, latral PV clls ar kpt in optical contact with th dgs of th SCs by mans of a transparnt indx-matching gl, which prvnts strong rflction losss at th intrfac. 3 EXTERNA QUANTUM EFFICIENCY FOR THE DEVICES 3. Thortical Modl In ordr to driv an xprssion for th of SC dvics, it is convnint to dcompos thir global working mchanism into thr stps and to analyz thm sparatly, assuming that only on kind of fluorscnt dy is disprsd in th SC. Th global working mchanism can b dcomposd into: Absorption of sunlight by fluorscnt dys; Emission and propagation of fluorscnc through th SC; Photovoltaic convrsion of fluorscnc at th dgs of th SC. Absorption of sunlight is th first working stp and th main physical quantitis involvd ar rflctanc R of th intrfac btwn SC and air, molar xtinction cofficint ε and molar concntration C of th disprsd dy. Th absorption probability for incidnt photons at wavlngth λ can thus b xprssd as P R ln() Cε ( λ ) h abs ( λ) = [ ( λ)][ ] Emission of fluorscnc is dtrmind by th quantum yild QY of th dy, by its mission pattrn (which has sphrical symmtry for th dys undr xamination [7]) and by th normalizd mission spctrum b. Propagation of fluorscnc through th SC, instad, is dscribd by th fraction f of fluorscnc that can rach th dgs by total intrnal rflction and by th collction probability P c. For th cas of polymric SCs with all four dgs covrd by PV clls, th fraction f is qual to.74, and it is dtrmind only by th rfractiv indcs of th matrix matrial and air. Th collction probability P c is dtrmind by slf-absorption of th mittd fluorscnc, by absorption of th matrix matrial, and by th losss driving from scattring insid th SC and at th surfac (whr a fraction of fluorscnc can scap). In an idal SC, P c is takn qual to unity, but in a ral dvic it is always lss than unity. Furthrmor, th gratr th SC, th smallr th collction probability will b, sinc losss bcom mor rlvant for longr propagation distancs (in this contxt w rfr to it as siz ffcts). From th abov considrations it follows that mittd photons at wavlngth λ can rach th dgs of th SC with a probability P g givn by P ( λ ') = QY f P ( λ ') b ( λ ') dλ ' g c Photovoltaic convrsion of th fluorscnc that rachs th dgs of th SC is dtrmind by th xtrnal (3) (4) 6

3 6th Europan Photovoltaic Solar Enrgy Confrnc and Exhibition quantum fficincy PV of th applid PV clls, nglcting rflction losss at th intrfacs btwn SC and PV clls. From th abov considrations, it follows that th global xtrnal quantum fficincy for a (m) = Pabs(m) singl-layr SC dvic (Fig ) with a singl fluorscnt dy can b xprssd as # Pg(m')PV(m') = 6- 6 # 6 m'>m m'>m (5) Whn two or mor typs of fluorscnt dys ar disprsd in th sam slab (for xampl, in our cas, dy A and dy B), som changs hav to b applid in Eq. (5) to tak into account cross-absorption, namly th fact that a rlvant fraction of fluorscnc photons mittd by dy A can b r-absorbd by dy B bfor raching th latral PV clls. Cross absorption as wll as slfabsorption ar rathr difficult to valuat analytically. A possibl solution is to valuat thm numrically, using, for xampl, a Mont Carlo approach [8]. For a bi-layr SC dvic (Fig., whr both layrs ar assumd to b dopd with a singl dy) diffrnt xprssions can b drivd for th of th two layrs. For th top layr, Eq. (5) is valid. For th bottom layr, Eq. (5) has to b multiplid by a factor ln() Cεh [ R] [ ] which taks into account that photons rflctd or absorbd in th top layr cannot contribut to th of th bottom layr. HAOGEN IGHT SOURCE OPTICA CHOPPER MONOCHROMATOR REFERENCE c-si CE Figur 3: Schm of th xprimntal stup usd for xtrnal quantum fficincy masurmnt. BS AMPIFIER OCK-IN AMPIFIER AMPIFIER to PC DEVICE UNDER INVESTIGATION 3. Exprimntal stup for masurmnt Th xprimntal stup usd for th optical charactrization of SC dvics is shown in Fig. 3. Th xtrnal quantum fficincy SC of th studid dvics is obtaind comparing th short-circuit currnt dvlopd by th dvics with that of a calibratd, rfrnc c-si PV cll (whos xtrnal quantum fficincy is dnotd as rf ) undr illumination with monochromatic light. A halogn lamp is usd as light sourc, and a monochromator is usd to produc th monochromatic bam at a givn wavlngth λ. Bfor ntring th monochromator, light is focusd on th ntranc slits by mans of a pair of fusd silica lnss ( in Fig. 3) and modulatd with an optical choppr, which is phas-matchd with a lock-in amplifir. W us modulatd light and lock-in amplifir in ordr to minimiz ffcts producd by xtrnal light, which causs spurious DC currnt contributions whn dvics ar in short-circuit conditions. Monochromatic light scaping th monochromator is snt onto a bam splittr (BS in Fig. 3), whos rflctanc R BS and transmittanc T BS ar known. Rflctd light is thn focusd on th rfrnc cll, whil transmittd light is dirctd onto th dvic undr invstigation, providing its uniform illumination and prvnting dirct illumination of th latral PV clls. Short-circuit currnts (which contain both DC and AC contributions) dvlopd by th rfrnc cll and by th dvic ar convrtd to voltag signals V rf (λ) and V SC (λ) and pr-amplifid with gains qual to G rf and G SC rspctivly. Thn signals ar snt to th lock-in amplifir, which slcts only th AC componnts that contain th physical signals of intrst. With a simultanous masurmnt of th voltags V rf (λ) and V SC (λ), on can obtain th xtrnal quantum fficincy SC (λ) of th dvic undr invstigation as SC Th spctral rang availabl for masurmnt is 35- nm and it is dtrmind by th rang of calibration valus of th rfrnc c-si cll. 4 RESUTS RBSGrf ISC ( λ) ( λ) = rf ( λ) T G I ( λ) BS SC rf 4. Extrnal quantum fficincy for singl-layr SCs W study singl-layr SCs dopd with on typ of fluorscnt dy (B or C) or with two typs of dys (A and B). Rsults of masurmnts for liquid-basd SCs dopd with dy B and C ar shown in Figs. 4 and 4 rspctivly, for diffrnt concntrations C of th dys. Both dvic rsponss hav common faturs. For low concntrations, th is mostly dtrmind by th molar xtinction cofficint ε, with a pak at th absorption maximum (at λ=45 nm for dy B, and at λ=5 nm for dy C). Incrasing th concntration, th absorption saturats and th rachs a platau. Actually, as it is vidnt from th data, th is not prfctly constant, and this is du to slf-absorption of th mittd fluorscnc. To undrstand this last point, on has to tak into account that fluorscnc is mittd at lowr nrgy than th xcitation nrgy, or, in othr words, th fluorscnc wavlngth λ is largr than xcitation wavlngth λ. Whn on xcit th SC with a short wavlngth λ, fluorscnc photons ar mittd ovr th whol mission spctrum b, and th ovrlap btwn b and ε dtrmins slf-absorption. On th othr hand, for longr xcitation wavlngths λ, fluorscd photons will b mittd at longr wavlngths λ, and a smallr fraction of thm will b mittd in th (6) 6

4 6th Europan Photovoltaic Solar Enrgy Confrnc and Exhibition ,5,4,3,,,,5,4,3,, C [ppm/wight] C [ppm/wight] , Wavlngth (nm) Figur 4: Extrnal quantum fficincy for dy B and for dy C in,-dichlorobnzn in liquid-basd SCs for diffrnt concntrations of th dys. ovrlap rgion btwn ε and b, thus rducing slfabsorption and giving an highr. Optimal concntrations C for dys ar thos which produc th maximum short-circuit currnt dnsitis J sc, and thy dpnd on th siz of th SC, giving highr C for smallr dvics, and lowr C for biggr ons, sinc slfabsorption losss ar mor rlvant in th lattr cas. Whn concntrations ar incrasd abov th optimal on, th spctrum bcoms slightly broadr, but th cntral platau starts dcrasing du to incrasd slfabsorption losss which ar not compnsatd by furthr gains, sinc absorption is alrady saturatd. Rsults on liquid-basd singl-layr SCs ar summarizd in Fig. 5. s at optimal concntrations ar rportd in Fig. 5 for SCs dopd with dy A and dy B (blu lin), dy B alon (black lin) and dy C alon (rd lin). As vidnt, whn dy A is addd to dy B in a liquid SC, th is incrasd in th UV spctral rang. Howvr, sinc th solar photon flux is rathr poor at high nrgy, only a vry small incras in short-circuit currnt dnsity is producd with rspct to dy B alon. Th for SCs with dy C alon is lowr than that of dy B, and this is du to th fact that fluorscnc quantum yild QY for dy C in,-dichlorobnzn is just.7, compard with.95 of dy A and B in th sam solvnt [7]. Starting from th data of Figs. 4 and 4, short-circuit currnt dnsitis J sc hav bn calculatd according to Eq. () and ar rportd in Fig. 5 for diffrnt concntrations of dy B and dy C. As it is vidnt, in spit of its lowr QY, dy C dvlops highr J sc than dy B, and this is du to th fact that th solar photon flux is richr in th absorbing rang of dy C than in that of dy B. J sc (ma/cm ),5,4,3,,, Wavlngth (nm) 4 3 DYE A C [ppm/wight] Figur 5: Extrnal quantum fficincy for liquidbasd, singl-layr SC with dy A and dy B (blu lin), dy B (black lin) and dy C only (rd lin) at optimal concntrations. Short-circuit currnt dnsity J sc dvlopd by singl-layr SC as a function of dy s concntration.,,8,6,4, Edg c-si PV Clls Modl SC Ara = 4.84 cm SC Ara = 9.36 cm SC Ara = cm, NO Back-Rflctor, Wavlngth (nm) Figur 6: Extrnal quantum fficincy for polymric SCs with dy A and dy B at optimal concntrations for diffrnt SC sizs. Rsults of masurmnts for polymric SCs of diffrnt sizs dopd with dy A and dy B ar shown in Fig. 6, togthr with th masurd PV of th applid latral c-si PV clls (top lin). As vidnt from th data, th dcrass for biggr dvics, sinc propagation losss bcom mor rlvant. Furthrmor, for th cas of polymric SCs, dy A dos not produc any particular ffct in th, diffrntly from th cas of liquid-basd SCs, as it is vidnt comparing Fig. 5 and Fig. 6. For comparison, also th calculatd with Eq. (5) for SCs with dy B only and without any loss is rportd 6

5 6th Europan Photovoltaic Solar Enrgy Confrnc and Exhibition with a solid gry lin: th modl can fit quit wll xprimntal data at high nrgy, vn if it dos not tak into account dy A. A possibl xplanation of this bhavior is that th polymric matrix matrial (PMMA) is not compltly transparnt in th UV spctral rang, diffrntly from th cas of fusd silica and,- dichlorobnzn, which ar transparnt for wavlngths abov 3 nm. This causs incidnt light to b absorbd by PMMA rathr than by dy A, giving lowr for th dvics in th UV spctral rang 4. Extrnal quantum fficincy for bi-layrd SCs W study liquid-basd bi-layrd SCs in which th top layr (layr ) is dopd with dy A and dy B at optimal concntrations, whil th bottom layr (layr ) is dopd with dy C. Rsults of masurmnts ar shown in Fig. 7: th of layr is plottd in black, and it is th sam of Fig. 5 (blu lin), whil th of layr is masurd for diffrnt concntrations of dy C. As vidnt from th data, th pak of th is lowr than that of th top layr, du to th lowr QY of dy C. Furthrmor, th whol spctrum is narrowr in th cas of dy C in layr, sinc most of th incidnt light at wavlngths λ blow 55 nm is absorbd within th top layr and dos not contribut to th of th bottom layr. From masurmnts, th short-circuit currnt dnsity J sc is calculatd according to Eq. () for diffrnt concntrations of dy B and dy C, and rsults ar shown in Fig. 7. Th J sc dvlopd by layr is th sam of Fig. 5 and it is rprsntd with a black lin. Th J sc for th bottom layr is rprsntd by a rd lin, and is found to b lss than on half of th J sc of layr du to a partial ovrlap of th absorption spctra of dys B and C. 5 CONCUSIONS Photovoltaic dvics mad of singl-layr and bi-layr SCs coupld to c-si solar clls hav bn invstigatd and charactrizd by mans of xtrnal quantum fficincy () masurmnts. First, liquid-basd SCs hav bn considrd, in ordr to study th ffcts of slfabsorption and to dtrmin th optimal concntrations for fluorscnt dys. Onc th optimal concntrations hav bn found, polymric SCs hav bn invstigatd with th sam procdur, paying attntion to th ffcts of th SCs siz on its. For singl-layr SC dvics, masurmnts ar in good agrmnt with our thortical modl. SCs dopd with dy C show highr short-circuit currnt dnsitis with rspct to thos dopd with dy B, vn if dy C has a lowr fluorscnc quantum yild with rspct to othr dys. This is bcaus absorption of dy C is shiftd towards th rd, whr th solar photon flux is richr. This rsult suggsts that nw and improvd fluorscnt dys hav to b synthsizd in ordr to absorb in th low nrgy visibl and infrard spctral rangs. Nw organic dys as wll as inorganic compounds, lik smiconductor quantum dots, ar currntly bing invstigatd for this purpos. Finally, it is vidnt that slf-absorption, scattring losss and bulk absorption within th matrix rprsnt svr limits for dvlopmnt of larg-ara SCs. For this rason much attntion will b dvotd to th synthsis of nw fluorscnt dys with a gratr shift btwn absorption and mission, as wll as in th production of high-quality, high transparncy polymric matrials. J sc (ma/cm ),5,4,3,,, Wavlngth (nm) C (ppm/wight) Figur 7: Extrnal quantum fficincy for -layrs SC with dy A and dy B at optimal concntrations in th first layr (black lin), and dy C in th scond layr (rd lin) for diffrnt concntrations of dy C. Shortcircuit currnt dnsity J sc dvlopd by -layrs SC as a function of dys concntration. 6 REFERENCES AYER : DYE A + B Optimal concntration AYER (): C [ppm/wight] AYER : AYER : [] J.S. Batchldr, A.H. Zwail, and T. Col, Appl. Opt. 8, 39 (979); ibid., 3733 (98). [] W.G.J.H.M. van Sark t al., Opt. Exprss 6, 773 (8). [3].H. Slooff t al., Phys. Status Solidi (RR), 57 (8). [4] J.C. Goldschmidt t al., Solar Enrgy Matrials & Solar Clls 93, 768 (9). [5] P.F. Scudo,. Abbondanza, R. Fusco, and. Caccianotti, Solar Enrgy Mat. Solar Clls 94, 4 (). [6] AM.5 solar spctrum irradianc data: [7] A. Alssi, R. Fusco, A. Proto, G. Schimprna, P. Scudo, Composizioni fotoluminscnti pr convrtitori di spttro a migliorata fficinza, Italian patnt application numbr MI9A796. [8] S. Flors Daorta, M. iscidini,.c. Andrani, P. Scudo and R. Fusco in Procding of th 6 th Europan Photovoltaic Confrnc and Exhibition, Sssion Rfrnc CV.3., Abstract No. 8, Hamburg (). 63