Spin-enhanced organic bulk heterojunction photovoltaic solar cells

Transcription

1 Received 7 Mr 212 Accepted 3 Jul 212 Pulished 4 Sep 212 DOI: 1.138/ncomms257 Spin-enhnced orgnic ulk heterojunction photovoltic solr cells Ye Zhng 1, Tek P. Bsel 2, Bhoj R. Gutm 2, Xiomei Yng 1, Der J. Mscro 3, Feng Liu 1 & Z. Vly Vrdeny 2 Recently, much effort hs een devoted to improve the efficiency of orgnic photovoltic solr cells sed on lends of donors nd cceptors molecules in ulk heterojunction rchitecture. One of the mjor losses in orgnic photovoltic devices hs een recomintion of polron pirs t the donor cceptor domin interfces. Here, we present novel method to suppress polron pir recomintion t the donor cceptor domin interfces nd thus improve the orgnic photovoltic solr cell efficiency, y doping the device ctive lyer with spin 1/2 rdicl glvinoxyl. At n optiml doping level of 3 wt%, the efficiency of stndrd poly(3- hexylthiophene)/1-(3-(methoxycronyl)propyl)-1-1-phenyl)(6,6)c 61 solr cell improves y 18%. A spin-flip mechnism is proposed nd supported y mgneto-photocurrent mesurements, s well s y density functionl theory clcultions in which polron pir recomintion rte is suppressed y resonnt exchnge interction etween the spin 1/2 rdicls nd chrged cceptors, which convert the polron pir spin stte from singlet to triplet. 1 Deprtment of Mterils Science nd Engineering, University of Uth, Slt Lke City, Uth 84112, USA. 2 Deprtment of Physics nd Astronomy, University of Uth, Slt Lke City, Uth 84112, USA. 3 Deprtment of Mechnicl Engineering, University of Uth, Slt Lke City, Uth 84112, USA. Correspondence nd requests for mterils should e ddressed to Z.V.V. (emil: vl@physics.uth.edu).

2 nture communictions DOI: 1.138/ncomms257 Solr energy hs een identified s the leding renewle energy source to meet the chllenge of incresing energy demnd. Orgnic photovoltics 1 25 (OPV) represent n emerging sector in the photovoltic industry tht hs seen rpid development in recent yers. Compred to the 3% power conversion efficiency (η) vlue reported in 27 (ref. 26), to dte the est OPV solr cell tht employs non-tndem ulk heterojunction (BHJ) structure hs shown η ~8.3% 27. In typicl BHJ rchitecture (see Fig. 1 for typicl OPV device), solvent-cst lyer of π-conjugted polymer nd fullerenederivtive lend is sndwiched etween the cthode nd node. The most studied polymer/fullerene lend with high η-vlue comprises of regio-regulr poly(3-hexylthiophene) (P3HT) (donor D) nd 1-(3-(methoxycronyl)propyl)-1-1-phenyl)(6,6)C 61 (PCBM) (cceptor A), of which chemicl structures re shown in Fig. 1. This lend is known to contin seprte D nd A nno-sized domins tht fcilitte oth chrge photogenertion nd chrge trnsport, nd collection in the lend 3,4. The P3HT donor polymer sors in the UV-visile prt of the solr spectrum tht compenstes the opticl trnsprency of the fullerene molecules in the sme energy rnge. Upon light sorption in the P3HT polymer chins, excitons re initilly photogenerted in the donor nno-domins 28 ; their susequent dissocition t the D A interfces is fcilitted y the energy level difference etween the lowest unoccupied moleculr oritl (LUMO) of the donor nd cceptor, s well s etween their highest occupied moleculr oritl (HOMO) levels. To rech the D A interfces, the excitons first diffuse towrds the polymer domin oundry within ~1 ps 28,29 where upon rrivl they form chrge trnsfer excitons 3,31. Susequently, the chrge-trnsfer excitons seprte into Coulomiclly ound polron pirs (PP), which re the intermedite species t the D A domin interfces hving reltively long lifetime (nmely, few microseconds) 32. At lter time, the PP species my seprte into free electron nd hole polrons ville for chrge trnsport vi the A nd D domins, nd cn e redily collected t the node nd cthode, respectively. One of the min chllenges tht OPV fces t the present time is its low η-vlue compred with other photovoltic devices 1, with the recomintion of PP t D A interfces eing mjor limiting fctor 29, In ddition to optimizing prmeters such s mteril mss rtio, ctive lyer thickness nd nneling temperture, numerous other pproches hve een tried to enhnce the efficiency y improving the device morphology 2 5, engineering new polymer/fullerene mterils 6 11, mnipulting electrode property 12 15, employing tndem cell rchitecture nd enhncing opticl sorption 19. A numer of these pproches involve introducing nno-prticles dopnts (or dditives) into the ctive lyer nd/or friction process 2,3,12,13, In the present work, we demonstrte new method to improve OPV efficiency y doping the device ctive lyer with spin 1/2 rdicls to reduce PP recomintion t the polymer/fullerene interfces. We chieved significnt increse in η y up to ~34% (in cceptorrich devices) reltive to devices sed on pristine lends. We demonstrte tht the spin 1/2 rdicl dditives fcilitte the intersystem crossing of PP from singlet to triplet spin configurtion, therey enhncing PP seprtion into free chrges in the device; this process is unrveled vi mgneto-photocurrent (MPC) of the doped devices. We demonstrte tht the spin 1/2 rdicls my spin flip the cceptor electron spin vi n exchnge mechnism tht requires Glvinoxyl (C 29 H 41 O 2 ) S CH 2 (CH 2 ) 4 CH 3 P3HT/PCBM n LiF/Al Active lyer PEDOT:PSS ITO Glss ITO Active lyer mm LiF/Al 4 8 c J sc (ma cm 2 ) Doping level (wt%) (%) d MPC (%) MPC (%) Doping level (wt%) B (mt) Figure 1 Spin enhncement of OPV solr cell sed on P3HT/PCBM lend doped with spin 1/2 glvinoxyl rdicls. () The OPV cell structure (for friction see Methods). The P3HT repet unit nd PCBM moleculr structure re lso shown in the insets. () The I-V chrcteristics of OPV solr cells of pristine P3HT/PCBM lend (η = 3.4%, dshed line), the lend doped with 3 wt% glvinoxyl rdicls (η = 4.%, solid line) nd the lend doped with 3 wt% precursor (η = 2.8%, dsh dot line) under AM1.5 sun illumintion condition. The inset shows the glvinoxyl moleculr structure. Smll sphere isected y rrow denotes unpired electron. (c) The chnge in OPV device properties with glvinoxyl-dditive concentrtion, J sc (tringles) nd η (squres), re shown versus glvinoxyl wt% in the P3HT/PCBM lend. η of OPV devices doped with glvinoxyl precursor tht does not possess spin 1/2 rdicl is lso shown for comprison (circles). (d) MPC response of OPV devices doped with glvinoxyl (pristine: thick-solid line; 3 wt%: dsh dot line; 5 wt%: dshed line; 1 wt%: thin-solid line) up to field, B of 19 mt. The inset summrizes the MPC vlue t 15 mt versus glvinoxyl wt%.

3 nture communictions DOI: 1.138/ncomms257 ARTICLE resonnt conditions. We elieve tht this method my work with other D A lends if pproprite rdicls in resonnce re found, in concert with other existing methods to yield even higher OPV device efficiencies. Results Glvinoxyl-doped P3HT/PCBM OPV devices. The spin 1/2 rdicl tht enhnces OPV performnce in this work is glvinoxyl (2,6-dit-utyl-α-(3,5-di-t-utyl-4-oxo-2,5-cyclohexdien-1-ylidene)-ptolyloxy), π-conjugted molecule with C 2 symmetry (Fig. 1 inset). The ulky t-utyl groups on the molecule stilize the rdicl y keeping other molecules prt thus preventing further rdicl rdicl spin interction in the solid stte. The unpired electron is deloclized over the entire molecule nd thus its moleculr structure my e regrded s resonnce hyrid of two configurtions hving loclized unpired spin-polrized electron on different oxygens 36. First, we investigted the effect of glvinoxyl doping in the ctive lyer of stndrd P3HT/PCBM device hving 1.2:1 weight rtio. We note tht our stndrd P3HT/PCBM devices were fricted using well-optimized recipe, nd the otined η-vlue is ~3.4%, which is close to the pulished vlue y Plextronics 37 ; it represents n upgrde from our preliminry results 38. Figure 1 shows tht y doping 3 wt% of glvinoxyl, η increses from 3.4 (short circuit current: J sc = 1.4 ma cm 2, open circuit voltge: V oc =.6 V, fill fctor: FF =.56) to 4.% (J sc = 11.3 ma cm 2, V oc =.6 V, FF =.62) exhiiting n 18% enhncement in the power conversion efficiency. This η increse is significntly lrger thn the stndrd devition in η-vlues of our optiml devices ( ± 3%) (Supplementry Tle S1); thus, doping with glvinoxyl unmiguously enhnces the device η-vlue. The 8.7% increse in J sc tht ccounts for out hlf of the improvement in the device η indictes tht crrier photogenertion increses or crrier recomintion is reduced, or oth. Figure 1c summrizes the P3HT/PCBM (1.2:1) device properties for ll investigted glvinoxyl concentrtions ( wt%). The enhncement in J sc nd η induced y the glvinoxyl rdicls peks t ~3 wt% nd grdully vnishes with further incresed doping. Actully, t high doping level ( > 1 wt%) glvinoxyl suppresses the device performnce. The optiml doping concentrtion (~3 wt%) t which η mximizes divides the effect of glvinoxyl dditives into two regimes: n enhncement regime, where η increses with doping, nd suppression regime, where η decreses with doping. We lso performed MPC mesurements on the glvinoxyl-doped OPV devices to unrvel the underlying mechnism for the increse in J sc with wt%. Figure 1d shows the otined MPC response of OPV devices hving vrious glvinoxyl wt%. It is clerly seen tht glvinoxyl dditives reduce the MPC vlue without chnging the field response. It hs een known tht MPC in OPV lends is due to mgnetic field mnipultion of spin triplet nd singlet sttes within the PP species Therefore, the reduction of MPC with wt% shows tht the spin 1/2 rdicls interfere with the intersystem crossing rtes mong the vrious spin sttes of the PP species, reveling the importnce of the glvinoxyl spin rther thn its ility to serve s donor or cceptor. We therefore conclude tht reduced PP geminte recomintion t the D A interfces is responsile for the enhnced crrier photogenertion upon glvinoxyl doping. The remining enhncement in η with the glvinoxyl dditives is due to n increse in FF, which indictes reduced series resistnce tht results from improved crrier trnsport. We note tht the MPC reduction with glvinoxyl wt% follows the sme trend s tht of the OPV enhncement with wt%. Figure 1d inset shows tht the most MPC reduction occurs t 3 wt%; the MPC response comes ck to tht of pristine device t 1 wt%. This further shows the existing correltion etween the spin 1/2 properties of the glvinoxyl dditives nd the OPV enhncement. In the following, we descrie severl control experiments tht we performed for understnding the OPV enhncement upon glvinoxyl doping. Asornce c Norm. XRD (.u.) Wvelength (nm) θ (deg) Photon energy (ev) To investigte whether the enhncement in J sc with glvinoxyl doping is due to n increse in the device ctive lyer sorption, we compred the sorption spectr of the pristine nd glvinoxyl optimlly doped P3HT/PCBM films (Fig. 2). The two spectr re very similr to ech other; nd, in prticulr, the glvinoxyl sorption pek t 43 nm (Fig. 2), which origintes from HOMO LUMO trnsition, is not discerned in the doped smple sorption. We therefore conclude tht the enhncement in J sc is not cused y chnge in sorption with glvinoxyl doping. In Fig. 2, we compre the externl quntum efficiency (EQE) of the pristine nd optimlly doped devices, which is ctully etter comprison thn the sorption spectrum discussed ove. The enhncement in EQE of the glvinoxyl-doped device does not initite t 43 nm where the glvinoxyl sorption is the strongest. Insted, EQE increses cross the entire spectrum. We conjecture tht glvinoxyl does not ct s donor molecule in this lend system. Morphology chnge nd its impct on exciton diffusion towrds the D A interfces nd collection efficiency lso hve n importnt role in determining J sc. To investigte whether the film morphology chnges due to the ddition of glvinoxyl molecules, we compred X-ry diffrction (XRD) ptterns of the pristine- nd glvinoxyldoped P3HT/PCBM films (Fig. 2c). The XRD (1) P3HT nd of oth films is the sme. Using the XRD pek position, its full width t hlf mximum, nd the Scherrer s reltion, we estimte tht the verge P3HT domin size in oth films is ~19 nm. This domin size is idelly suited to the commonly ccepted 1 nm exciton diffusion length in the P3HT domins 32. The reltive intensity of - nd -1 nds in the photoluminescence (PL) spectrum provides nother wy to determine the crystlliztion degree of the P3HT domins 42. The normlized PL spectr of pristine nd doped (3 wt%) P3HT/PCBM films re shown in Fig. 2d. The identicl PL spectr indicte tht the pcking order of polymer chins in the P3HT domins is not ffected y the ddition of glvinoxyl, nd thus the exciton lifetime in the P3HT domins is unchnged. Similr to the XRD nd PL results, the trnsmission electron microscopy (TEM) imges (Supplementry Fig. S1) show no oservle morphology chnge cused y the glvinoxyl EQE (%) Norm. PL (.u.) Wvelength (nm) Figure 2 Opticl nd structure properties of pristine nd glvinoxyldoped (3 wt%) P3HT/PCBM lend film nd OPV device. () The UV/Vis sorption spectrum of pure glvinoxyl (dsh dot line), pristine (dshed line) nd doped (solid line) P3HT/PCBM lend. () The EQE spectrum of OPV solr cells sed on pristine (dshed line) nd glvinoxyl-doped (solid line) P3HT/BCBM lend. (c) The XRD pttern of pristine (green dsh) nd doped (red solid) P3HT/PCBM films. (d) The PL spectrum of pristine (dshed line) nd doped (solid line) P3HT/PCBM. The phonon replics re ssigned. Norm., normlized. d

4 nture communictions DOI: 1.138/ncomms257 c doping. We therefore conclude tht no chnge in film morphology is responsile for the increse in J sc. To further check the importnce of the glvinoxyl spin 1/2 properties rther thn its doping ility, we mesured OPV device performnce with the ddition of glvinoxyl precursor molecule tht hs one extr hydrogen tom, nd thus does not possess spin 1/2 rdicl (its moleculr structure is shown in Supplementry Fig. S2). We note tht the frontl oritls re similr in the two molecules, nd thus the HOMO LUMO levels do not chnge much in going from glvinoxyl to its precursor, s deduced from their similr sorption spectr (Supplementry Fig. S2). In contrst to glvinoxyl doping, we found tht doping with this precursor monotoniclly reduces the OPV device performnce (Fig. 1c). We therefore conclude tht the vile mechnism for the OPV η increse with glvinoxyl dditives is suppression of PP recomintion t the D A interfces due to the spin 1/2 rdicls. Glvinoxyl-doped P3HT-rich nd PCBM-rich OPV devices. To further understnd the effect of glvinoxyl doping, we studied OPV cells sed on P3HT-rich nd PCBM-rich lends. These re P3HT:PCBM t 2:1, 1:2 nd 1:5. In P3HT-rich OPV (Fig. 3), η decreses monotoniclly with doping concentrtion, indicting tht glvinoxyl does not hve n enhncement effect on lend lcking sufficient PCBM cceptor molecules. In contrst, in PCBM-rich system, similr to the stndrd P3HT/PCBM system, oth enhncement nd suppression regimes cn e clerly identified (Fig. 3). At high glvinoxyl concentrtion (25 wt%), the doped device performnce is still much etter thn the pristine device. In ddition, we notice tht the enhncement effect is more significnt nd the optiml glvinoxyl doping level shifts towrds more hevily Figure 3 I-V chrcteriztion of OPV devices sed on pristine nd glvinoxyl-doped donor/cceptor lends under AM 1.5 illumintion. () Polymer donor-rich P3HT/PCBM 2:1 lend t vrious glvinoxyl doping levels: pristine (η = 2.8%, dshed line), 1.5 wt% (η = 1.9%, tringles), 3 wt% (η = 1.6%, dimonds) nd 1 wt% (η = 1.%, circles). () Fullerene cceptor-rich P3HT/PCBM 1:5 lend t vrious glvinoxyl doping levels: pristine (η =.22%, dshed line), 1 wt% (η =.52%, tringles), 15 wt% (η =.96%, solid) nd 25 wt% (η =.77%, circles). (c) Regio-rndom P3HT/PCBM 1.2:1 lend t different glvinoxyl doping levels: pristine (η = 2%, dshed line) nd 3 wt% (η = 8%, solid). (d) MEH-PPV/ PCBM 1:1 lend t different glvinoxyl doping levels: pristine (η =.26%, dshed line) nd 17 wt% (η =.34%, solid). d doping with incresed PCBM content. For OPV sed on P3HT: PCBM 1:2 lend, η increses y 74% (from 1.8 to 1.88%) t optiml doping of 8 wt%; wheres for devices sed on P3HT:PCBM 1:5 lend, η increses y 336% t optiml doping of 15 wt%. In contrst, devices sed on pristine PCBM lone did not show ny performnce enhncement upon glvinoxyl doping. It is thus cler tht glvinoxyl enhncement of OPV devices is effective only in the presence of the P3HT/PCBM interfces. This is consistent with our specultion tht glvinoxyl ssists the PP dissocition process, which occurs exclusively t the D A domin interfces. Glvinoxyl-doped OPV devices with other donors nd cceptors. We crried out dditionl studies of glvinoxyl doping OPV devices fricted using lends of other donor nd cceptor mterils to explore whether the enhncement effect is limited to the P3HT/ PCBM system. We replced the donor polymer in our stndrd P3HT/PCBM lend with regio-rndom P3HT (RR-P3HT), which hs similr chemicl structure to regio-regulr P3HT except tht the hexyl side-chins re rrnged rndomly. In Fig. 3c, we show tht in glvinoxyl-doped (3 wt%) RR-P3HT/PCBM device, J sc improves from.12 to.41 ma cm 2. Coupled with V oc increse from.61 to.72 V, 3% increse in η is oserved. This is not surprising considering tht the dominnt loss mechnism in the RR-P3HT/PCBM lend is PP recomintion t the D A interfces 43, process tht glvinoxyl doping is expected to directly suppress 44. Similr result ws otined when we replced the donor polymer with poly(2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylenevinylene) (MEH-PPV). In Fig. 3d, we show tht J sc of the doped (17 wt%) device increses from 1.47 to 1.78 ma cm 2. With V oc incresing from.63 to.78 V, 3% increse in η is oserved in devices sed on this lend. In contrst, ttempts to enhnce η were unsuccessful in OPV devices sed on lends with cceptor fullerenes other thn PCBM. When we replced PCBM with ispcbm, isdduct nlogue of PCBM or indene-c 6 isdduct (ICBA) tht is high LUMO level fullerene 9, we found tht η decreses monotoniclly with glvinoxyl doping. It is worth mentioning tht when we replced frction of the PCBM cceptor in the lend with ispcbm, the enhncement effect ws still oservle. We thus conclude tht PCBM in the orgnic lend is crucil for effective glvinoxyl doping enhncement. Other rdils tht we tested ut did not show notle enhncement effect in P3HT/PCBM OPV system re: (2,2,6,6- tetrmethylpiperidin-1-yl)oxyl (TEMPO), α,γ-isdiphenylene-βphenylllyl nd is(1-hydroxy-2,2,4,6,6-pentmethyl-4-piperidinyl) oxlte (Supplementry Fig. S3, Supplementry Tle S2 nd Supplementry Methods). Discussion The experimentl evidence indictes tht the cuse for the enhnced η-vlue in the glvinoxyl-doped OPV devices is the reduced PP recomintion rte t the P3HT/PCBM domin interfces due to the spin 1/2 rdicl dditives. We still need to unrvel the mechnism y which glvinoxyl reduces PP recomintion. As glvinoxyl is spin 1/2 rdicl, we propose spin-flip mechnism tht fcilittes PP seprtion t the P3HT/PCBM interfces y converting photogenerted PP from spin singlet to triplet (Fig. 4) vi spin exchnge interction etween the PP nd glvinoxyl. As triplet PP hs longer lifetime thn singlet PP, the enhnced intersystem crossing results in longer-lived species hving etter chnce to dissocite. Consider tht photogenerted exciton in the P3HT domin hs spin-up electron in the LUMO level nd spin-down hole in the HOMO level. Upon rrivl t the D A interfce, the electron trnsfers to the PCBM LUMO level, forming singlet PP (Fig. 4 upper left), with the spin-down hole in the P3HT HOMO level. The singlet PP species cn either dissocite into free crriers (polrons) in the P3HT

5 nture communictions DOI: 1.138/ncomms257 ARTICLE P3HT P3HT Singlet PP Triplet PP LUMO 1.28 ev HOMO 5.95 ev PCBM Spin-flip PCBM Exchnge Interction LUMO 2.55 ev LUMO SOMO Glvinoxyl LUMO 3.6 ev P3HT PCBM Glvinoxyl HOMO 7.2 ev SOMO 7.15 ev HOMO 8.62 ev 8.14 ev Figure 4 Spin-exchnge mechnism nd DFT clcultion. () The spin exchnge mechnism where the photogenerted PP t the D A domin interfce chnges its spin configurtion from singlet to triplet ugmented y the glvinoxyl spin 1/2 rdicl. () The clculted HOMO, LUMO nd SOMO levels of P3HT, PCBM nd glvinoxyl tht show cler resonnce etween the rdicl nd cceptor LUMO levels. nd PCBM seprte domins, or gemintely recomine. When spin 1/2 rdicl such s glvinoxyl, which hs spin-polrized singly occupied moleculr oritl (SOMO) nd LUMO levels with designted spin orienttions, is introduced next to the singlet PP, then it my form complex with PCBM providing spin-down polrized empty LUMO level in resonnce with the spin-up filled PCBM LUMO level next to the chrged PCBM molecule (Fig. 4 right). This medites n exchnge interction etween the up-spin negtive polron nd the virtul down-spin of glvinoxyl LUMO tht flips the polron upspin to down-spin, therey forming lower-energy triplet PP (Fig. 4 ottom left). The PP triplet species hs longer lifetime ecuse it is spin-foridden, hving reduced geminte recomintion rte. This my fcilitte its dissocition into free polrons. The sme mechnism cn e eqully pplied for photogenerted PP with spin-down electron in the P3HT LUMO level, vi exchnge interction with spin-up defined LUMO level of glvinoxyl rdicl. We conclude tht glvinoxyl dditives ctivte spin-flip process tht converts PP species t the D A interfces from spin singlet to triplet, nd this reduces the overll PP recomintion rte. As our experimentl evidence indictes tht this process is effective only with the presence of PCBM, nd is more effective with incresing PCBM concentrtion, we therefore elieve tht glvinoxyl forms complex with the PCBM t the P3HT/PCBM interfces. Also the OPV enhncement is mximum t certin glvinoxyl optiml concentrtion; this cn e understood if nerest neighour glvinoxyl/ PCBM complex molecules t high wt% re spin-pired to form spin singlet. Therefore, overdose of glvinoxyl molecules my reduce their ility to provide the spin-flip mechnism necessry for reducing the PP recomintion rte. For n effective spin exchnge interction, energy resonnce mong the chrge PCBM nd glvinoxyl moleculr levels is crucil. To investigte this constrint, we performed density functionl theory (DFT) clcultions of the ground-stte frontier moleculr oritl levels of P3HT, PCBM nd glvinoxyl using the CAM-B3YP functionl. The level lignments mong P3HT, glvinoxyl nd PCBM re shown in Fig. 4. P3HT LUMO level ( 1.28 ev) is sustntilly higher thn PCBM LUMO level ( 2.55 ev), nd this fcilittes the initil chrge trnsfer tht results in PP formtion t the D A interfces. Importntly, we found tht PCBM LUMO ( 2.55 ev) is within resonnce of the glvinoxyl LUMO ( 3.6 ev), nd this helps mediting the exchnge interction of the spin-flip process. In ddition, the formtion of glvinoxyl/pcbm complex my lso e importnt for the spin-flip process. We note tht the clculted energy levels re for isolted individul molecules, which my chnge in forming the complex due to intermoleculr interction. Nevertheless, our clcultions show tht the energy resonnce etween glvinoxyl nd PCBM is noticely etter thn those etween TEMPO nd PCBM or etween glvinoxyl nd other cceptors such s ispcbm nd ICBA (Supplementry Fig. S4). This is consistent with our experimentl oservtions tht these lterntive D A lends show no significnt OPV η enhncement. Finlly, we note tht spin 1/2 dditives my lso enhnce the electroluminescence (EL) efficiency in orgnic light emitting diodes (OLED). The rtio of singlet/triplet PP in OLED is usully constrined to e 1:3 due to spin sttistics dictted y quntum mechnics. Thus, the numer of singlet PPs tht re precursors to singlet excitons tht ultimtely emit EL is severely limited. This is n especilly cute prolem for OLEDs with lue emission tht relies on singlet excitons to yield EL; in prticulr supplying the missing lue colour for white OLEDs. By dding spin 1/2 rdicls to the ctive lyer, we my chnge the singlet/triplet sttisticl limit, s the rdicl my provide nother intersystem crossing chnnel tht my increse the singlet PP yield on the expense of triplet PP. This my e vile route of future investigtions, nd our results my show the wy to dvnce this reserch direction ecuse we emphsize the need of energy resonnce in the spin exchnge mechnism etween the spin 1/2 dditives nd the polron levels in the ctive lyer. Methods OPV device friction. The BHJ OPV devices investigted in this study were composed of trnsprent indium tin oxide (ITO) node; spin-cst polyethylenedioxythiophene/polystyrene sulphonte (PEDOT/PSS) hole trnsport lyer; n ctive mteril lyer spin-cst from lend of polymer donor, fullerene cceptor nd spin 1/2 rdicl glvinoxyl (when pplicle); nd cpped with LiF/Al cthode, s shown in Fig. 1. The ITO-coted glss sustrtes (Delt Technology, CB-5IN) were clened y ultrsonic tretment (in cetone, detergent, deionized wter nd methnol sequentilly) nd oxygen plsm tretment. The PEDOT/PSS (Clevios, P VP AI 483) lyer ws spin-cst t 5 r.p.m. for 2 s t mient conditions nd trnsferred to nitrogen-filled gloveox (O 2 < 1 ppm) for nneling t 12 C for 3 min. The orgnic lend tht yielded the est device performnce (η~4%) comprises P3HT (Plextronics, Plexcore OS 21), PCBM (purity > 99.9%) nd glvinoxyl (Aldrich, G37). It ws prepred in the following wy: P3HT (16 mg ml 2 ) nd PCBM were dissolved t 1.2:1 weight rtio in 1,2-dichloroenzene. The lend ws heted t 5 C for 3 min nd stirred over night efore mixing with glvinoxyl (3 wt%, defined s percentge of totl P3HT/PCBM weight) nd stirring for 1 dditionl hour. Other P3HT/PCBM lends were prepred y chnging the D A weight rtio wheres keeping the totl P3HT/PCBM mss unchnged.

6 nture communictions DOI: 1.138/ncomms257 The ctive lyer ws spin-cst from the lend t 4 r.p.m. for 6 min nd nneled t 15 C for 3 min. The device friction ws completed y thermlly evporting 1-nm thick film of LiF followed y 1-nm thick film of Al. Ech device ws mm in dimension, s shown in Fig. 1. Finlly, the completed device ws encpsulted under cover glss using UV-curle opticl dhesive (Norlnd, NOA 61). Devices using other donor/cceptor mterils were fricted using the sme protocol. The PCBM-only device film ws spin-cst from 2-mg ml 1 chloroenzene solution t 4 r.p.m. nd ked t 5 C for 2 h. The MEH- PPV/PCBM (1:1 weight rtio) film ws spin-cst from 3-mg ml toluene solution t 2 r.p.m. nd nneled t 12 C for 3 min. OPV device chrcteriztion. Device I-V chrcteristics were mesured using Keithley 236 Source-Mesure unit. The light intensity of the solr simultor, composed of xenon lmp nd n AM1.5G filter, ws clirted t 1 mw cm 2 using pre-clirted silicon PV cell. Monochromtic lights used to mesure device EQE chrcteristics were modulted t 36 Hz nd filtered y KG5 filter efore sending into the photodetector or OPV device. The photodetector (OPV) device output current ws mesured using phse-sensitive lock-in technique. MPC mesurements. When mesuring MPC, the OPV devices were trnsferred to cryostt tht ws plced in etween the two poles of n electromgnet producing mgnetic field, B up to ~2 mt. The devices were illuminted with tungsten lmp nd mesured t zero is using Keithley 236 pprtus, while sweeping the externl mgnetic field. The MPC is defined s MPC(B) = [PC(B)/PC() 1]. Other chrcteriztion methods. The sorption spectrum ws mesured using UV/Vis spectrophotometer (PerkinElmer Lmd 75) t mient. XRD ws otined using the Cu K β line nd collected y diffrctometer (Philips X Pert PRO) in powder diffrction mode. PL emission ws excited y 1 mw continuous wve lser t 488 nm. The emitted light ws collected using sphericl mirror nd dispersed y n Acton 3 spectrometer efore sending to photodetector. TEM imges were tken under defocused ( 25 µm) condition, which is reported to provide enhnced contrst etween the P3HT nd PCBM regions. Films used for sornce, XRD nd TEM mesurements, were prepred in the sme wy s the ctive lyer of the ctul OPV device. Films used for PL mesurements were drop-cst from 16 mg ml 1 P3HT solution nd nneled t 15 C for 3 min. DFT clcultion. The ground stte moleculr oritl levels were clculted y the Gussin 9 progrm 45 with DFT (CAM-B3LYP/6-311g**). The moleculr geometry ws fully optimized t CAM-B3LYP/6-31g* level 46,47. The DFT clcultion for P3HT were performed with periodic oundry conditions 48 (see lso Supplementry Methods). References 1. Li, G., Zhu, R. & Yng, Y. Polymer solr cells. Nt. Photonics 6, (212). 2. Peet, J. et l. Efficiency enhncement in low-ndgp polymer solr cells y processing with lkne dithiols. Nt. Mter. 6, (27). 3. Yo, Y., Hou, J. H., Xu, Z., Li, G. & Yng, Y. Effect of solvent mixture on the nnoscle phse seprtion in polymer solr cells. Adv. Funct. Mter. 18, (28). 4. Perez, M. D., Borek, C., Forrest, S. R. & Thompson, M. E. Moleculr nd morphologicl influences on the open circuit voltges of orgnic photovoltic devices. J. Am. Chem. Soc. 131, (29). 5. Wng, D. H. et l. Sequentil processing: control of nnomorphology in ulk heterojunction solr cells. Nno Lett. 11, (211). 6. Soci, C. et l. Photoconductivity of low-ndgp conjugted polymer. Adv. Funct. Mter. 17, (27). 7. Chen, C. -P., Chn, S. -H., Cho, T. -C., Ting, C. & Ko, B. -T. Low-ndgp poly(thiophene-phenylene-thiophene) derivtives with roden sorption spectr for use in high-performnce ulk-heterojunction polymer solr cells. J. Am. Chem. Soc. 13, (28). 8. Ross, R. B. et l. Endohedrl fullerenes for orgnic photovoltic devices. Nt. Mter. 8, (29). 9. He, Y., Chen, H. -Y., Hou, J. & Li, Y. Indene-C6 isdduct: new cceptor for high-performnce polymer solr cells. J. Am. Chem. Soc. 132, (21). 1. Price, S. C., Sturt, A. C., Yng, L., Zhou, H. & You, W. Fluorine sustituted conjugted polymer of medium nd gp yields 7% efficiency in polymer fullerene solr cells. J. Am. Chem. Soc. 133, (211). 11. Vndewl, K., Tvingstedt, K., Gdis, A., Ingns, O. & Mnc, J. V. On the origin of the open-circuit voltge of polymer-fullerene solr cells. Nt. Mter. 8, (29). 12. Ko, C. J., Lin, Y. K., Chen, F. C. & Chu, C. W. Modified uffer lyers for polymer photovoltic devices. Appl. Phys. Lett. 9, 6359 (27). 13. Hung, J. -H., Fng, J. -H., Liu, C. -C. & Chu, C. -W. Effective work function modultion of grphene/cron nnotue composite films s trnsprent cthodes for orgnic optoelectronics. ACS Nno 5, (211). 14. Berredjem, Y. et l. On the improvement of the efficiency of orgnic photovoltic cells y the presence of n ultr-thin metl lyer t the interfce orgnic/ito. Eur. Phys. J.Appl. Phys 44, (28). 15. Cox, M. et l. Single-lyer grphene cthodes for orgnic photovoltics. Appl. Phys. Lett. 98, (211). 16. Peumns, P., Bulovic, V. & Forrest, S. R. Efficient photon hrvesting t high opticl intensities in ultrthin orgnic doule-heterostructure photovoltic diodes. Appl. Phys. Lett. 76, (2). 17. Kim, J. Y. et l. Efficient tndem polymer solr cells fricted y ll-solution processing. Science 317, (27). 18. Gilot, J., Wienk, M. M. & Jnssen, R. A. J. Optimizing polymer tndem solr cells. Adv. Mter. 22, E67 E71 (21). 19. Prk, S. H. et l. Bulk heterojunction solr cells with internl quntum efficiency pproching 1%. Nt. Photonics 3, (29). 2. Sun, Y. et l. Solution-processed smll-molecule solr cells with 6.7% efficiency. Nt. Mter. 11, (212). 21. Loez, J. M., Andrew, T. L., Bulović, V. & Swger, T. M. Improving the performnce of P3HT fullerene solr cells with side-chin-functionlized poly(thiophene) dditives: new prdigm for polymer design. ACS Nno 6, (212). 22. Prk, M. -H., Li, J. -H., Kumr, A., Li, G. & Yng, Y. Doping of the metl oxide nnostructure nd its influence in orgnic electronics. Adv. Funct. Mter. 19, (29). 23. Deschler, F. et l. Reduced chrge trnsfer exciton recomintion in orgnic semiconductor heterojunctions y moleculr doping. Phys. Rev. Lett. 17, (211). 24. Veysel Tunc, A. et l. Moleculr doping of low-ndgp-polymer:fullerene solr cells: effects on trnsport nd solr cells. Org. Electron 13, (212). 25. Yun, Y. et l. Efficiency enhncement in orgnic solr cells with ferroelectric polymers. Nt. Mter. 1, (211). 26. Green, M. A., Emery, K., Hishikw, Y. & Wrt, W. Solr cell efficiency tles (version 3). Prog. Photovoltics Res. Appl. 15, (27). 27. Green, M. A., Emery, K., Hishikw, Y., Wrt, W. & Dunlop, E. D. Solr cell efficiency tles (version 38). Prog. Photovoltics Res. Appl. 19, (211). 28. Singh, S. et l. Two-step chrge photogenertion dynmics in polymer/fullerene lends for photovoltic pplictions. Phys. Rev. B 85, 2526 (212). 29. Bkulin, A. A. et l. The role of driving energy nd deloclized sttes for chrge seprtion in orgnic semiconductors. Science 335, (212). 3. Benson-Smith, J. J. et l. Formtion of ground-stte chrge-trnsfer complex in polyfluorene/[6,6]-phenyl-c61 utyric cid methyl ester (PCBM) lend films nd its role in the function of polymer/pcbm solr cells. Adv. Funct. Mter. 17, (27). 31. Drori, T., Holt, J. & Vrdeny, Z. V. Opticl studies of the chrge trnsfer complex in polythiophene/fullerene lends for orgnic photovoltic pplictions. Phys. Rev. B 82, 7527 (21). 32. Behrends, J. et l. Direct detection of photoinduced chrge trnsfer complexes in polymer fullerene lends. Phys. Rev. B 85, (212). 33. Ohkit, H. et l. Chrge crrier formtion in polythiophene/fullerene lend films studied y trnsient sorption spectroscopy. J. Am. Chem. Soc. 13, (28). 34. Holt, J., Singh, S., Drori, T., Zhng, Y. & Vrdeny, Z. V. Opticl proes of piconjugted polymer lends with strong cceptor molecules. Phys. Rev. B 79, (29). 35. Howrd, I. A., Muer, R., Meister, M. & Lqui, F. Effect of morphology on ultrfst free crrier genertion in polythiophene:fullerene orgnic solr cells. J. Am. Chem. Soc. 132, (21). 36. Novk, I. & Kovc, B. Electronic structure of glvinoxyl rdicl. Chem. Phys. Lett. 413, (25). 37. Tipnis, R., Lird, D. & Mthi, M. Polymer-sed mterils for printed electronics: enling high efficiency solr power nd lighting. Mteril Mtters 3, (28). 38. Zhng, Y., Hukic-Mrkosin, G., Mscro, D. & Vrdeny, Z. V. Enhnced performnce of P3HT/PCBM ulk heterojunction photovoltic devices y dding spin 1/2 rdicls. Synth. Met. 16, (21). 39. Zng, H., Xu, Z. & Hu, B. Mgneto-opticl investigtions on the formtion nd dissocition of intermoleculr chrge-trnsfer complexes t donor-cceptor interfces in ulk-heterojunction orgnic solr cells. J. Phys. Chem. B 114, (21). 4. Shky, P. et l. The effect of pplied mgnetic field on photocurrent genertion in poly-3-hexylthiophene:[6,6]-phenyl C61-utyric cid methyl ester photovoltic devices. J. Phys. Condens. Mtter 2, (28). 41. Lei, Y. et l. Mgnetoconductnce of polymer fullerene ulk heterojunction solr cells. Org. Electron 1, (29). 42. Clrk, J., Silv, C., Friend, R. H. & Spno, F. C. Role of intermoleculr coupling in the photophysics of disordered orgnic semiconductors: ggregte emission in regioregulr polythiophene. Phys. Rev. Lett. 98, 2646 (27). 43. Guo, J., Ohkit, H., Benten, H. & Ito, S. Chrge genertion nd recomintion dynmics in poly(3-hexylthiophene)/fullerene lend films with different regioregulrities nd morphologies. J. Am. Chem. Soc. 132, (21). 44. Smnt, A. et l. Quenching of excited doulet sttes of orgnic rdicls y stle rdicls. J. Phys. Chem. 93, (1989). 45. Frisch, M. J. et l. Gussin 9, Revision A.2, Gussin, Inc., Wllingford CT, 29.

7 nture communictions DOI: 1.138/ncomms257 ARTICLE 46. Nyyr, I. H. et l. Locliztion of electronic excittions in conjugted polymers studied y DFT. J. Phys. Chem. Lett. 2, (211). 47. Yni, T., Tew, D. P. & Hndy, N. C. A new hyrid exchnge correltion functionl using the Coulom-ttenuting method (CAM-B3LYP). Chem. Phys. Lett. 393, (24). 48. Kudin, K. N. & Scuseri, G. E. Liner-scling density-functionl theory with Gussin oritls nd periodic oundry conditions: Efficient evlution of energy nd forces vi the fst multipole method. Phys. Rev. B 61, (2). Acknowledgements This work ws supported in prt y the DOE Grnt No. DE-FG2-4ER4619 (TEMPO rdicl dditive nd the initil glvinoxyl studies), NSF-MRSEC progrm Grnt No. DMR (model clcultions nd MPC studies) nd NSF Grnt No. DMR (finl glvinoxyl studies). Author contriutions Y.Z., T.P.B. nd D.J.M. were responsile for fricting nd chrcterizing the OPV devices; B.R.G. ws responsile for the MPC mesurements; X.Y. nd F.L. were responsile for the DFT clcultion; nd Z.V.V. ws responsile for the project plnning, group mnging nd writing the finl mnuscript version. Additionl informtion Supplementry Informtion: ccompnies this pper t nturecommunictions Competing finncil interests: The uthors declre no competing finncil interests. Reprints nd permission: informtion is ville online t reprintsndpermissions/ How to cite this rticle: Zhng, Y. et l. Spin-enhnced orgnic ulk heterojunction photovoltic solr cells. Nt. Comm. 3:143 doi: 1.138/ncomms257 (212).