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Vertical Phase Separation in Poly(3-hexylthiophene): Fullerene Derivative Blends and its Advantage for  Inverted Structure Solar Cells By By Zheng Xu, Li-Min Chen, Guanwen Yang, Yue Zheng Wu, Gang Li, Chain-Shu Hsu,   and Hsu,  and Yang  YangChun-Hao Yang* Huang, Jianhui Hou,

conversion efficiency (PCE) for this system reported so far is about 4–5%. A metho method d whichenables whichenablesthe theinves investigat tigation ionof of the theburiedinterfaceswithout buriedinterfaceswithoutalteri altering ng Morphology optimization of the active the properties of the polymer films is used to study vertical phase separation of  layer is an essential way to improve the spin-coated poly(3-hexylthiophene) (P3HT):fullerene derivative blends. X-ray device efficiency. Besides the lateral phasephotoelectron spectroscopy (XPS) and atomic force microscopy (AFM) analysis separated morphology, the vertical distrireveal rev ealss the theP3H P3HT T enr enrich ichmen mentt at thefree (ai (air) r) sur surfac faces es and abu abundan ndance ce of ful fuller lerene ene bution of the components in the blend film is also critical, and vertical phase separaderivatives at the organic/substrate interfaces. The vertical phase separation tion has been suggested in several polymer is attributed to the surface energy difference of the components and their  blend systems,[9–12] as well as P3HT:PCBM interactions with the substrates. This inhomogeneous distribution of the blends.[13–15] Campoy-Quiles et al. recently  used varia variable-a ble-angle ngle spectr spectroscopic oscopic ellips ellipsoodonor and acceptor components significantly affects photovoltaic device performance and makes the inverted device structure a promising choice. metry (VASE) to model the vertical composition profile of P3HT:PCBM thin films and rep report orted ed a con concen centra tratio tion n gra gradie dient  nt  varying from PCBM-rich near the poly (3,4-ethylenedioxythio1. Introduction phene)/poly (styrene-sulfonate) (PEDOT:PSS) side to P3HT-rich Consequently, the regular device Polymer photovoltaic (PV) cells have the advantage of low-cost  adjacent to the free (air) surface. Consequently, structure (Scheme 1a), in which the polymer blend is sandwiched fabric fab ricati ation on and eas easyy pro proces cessing sing.. The sta statete-ofof-the the-ar -artt dev device ice [1,2] between the PEDOT:PSS-coated indium tin oxide (ITO) anode structure is the polymer bulk heterojunction (BHJ), blending and an d lo low w wo work rk fu func ncti tion on me meta tall ca cath thod ode, e, ha hass a no nonn-id idea eall conjugated conjug ated polym polymers ers intim intimately ately with solub soluble le fuller fullerene ene deriv derivaacomposition compo sition profile. Several appro approaches aches have been propo proposed sed tives. An inter interpenetr penetrating ating network of the donor donor–accep –acceptor tor blend to modify mod ify the compos com positi ition on profile pro file to achiev ach ieve e better bet ter device dev ice sand sa ndwi wich ched ed be betw twee een n th thee an anod odee an and d ca cath thod odee of offe fers rs la larg rgee performance. For example, Campoy-Quiles et al. have shown that  interf int erfaci acial al are areaa for effi efficie cient nt cha charge rge sepa separat ration ion and exc excell ellent  ent  the compositional gradient can be switched by modifying the charge cha rge tra transpo nsport, rt, lea leadin ding g to hig high h effi efficie ciency ncy per perfor forman mance. ce. surface energy of the substrate with a self-assembled monolayer Regioregular Regior egular poly( poly(3-hexyl 3-hexylthiophe thiophene) ne) (RR-P (RR-P3HT) 3HT) and fuller fullerene ene (SAM). Wei Wei et al. also introduced a new fullerene derivative derivative with a derivative deriv ative [6,6][6,6]-phenyl phenyl C   butyric butyric aci acid d met methyl hyl este esterr (PC (PCBM) BM) 61 fluorocarbon chain which spontaneously forms a buffer layer blend represents one of the most promising systems. Several [3–6] [7,8] near the metal cathode to impro improve ve the device perfor performance mance..[16] process proce ss condit conditions ions and post-treatments have been proposed to form a nano-scale phase-separated morphology with crystalline P3HT and PCBM domains, and the highest power [*] Prof. Prof. Y. Yang, Z. Xu, L.-M. Chen, Chen, G. Yang Department of Materials Science and Engineering University of California, Los Angeles Los Angeles, CA 90095 (USA) E-mail: [email protected] [email protected] du C.-H. Huang, Prof. C.-S. Hsu Department of Applied Chemistry National Chiao Tung University Hsinchu, Taiwan (Republic of China) Dr. J. Hou, Dr. Y. Wu, Dr. G. Li Solarmer Energy, Inc. 3445 Fletcher Ave El Monte, CA 91731 (USA)

Scheme 1.   a) Schematic depiction depiction of the regular structure and inverted structure of the PV devices. b) Structures of PCBM and FPCBM.

DOI: 10.1002/adfm.200801286

 Adv. Funct. Mater. Mater. 2009 19,  1227–1234  2009,, 19, 1227–1234

 

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As an alter alterna nati tive ve to the regula regularr device device struct structure ure (ITO (ITO// PEDOT:PSS PEDOT :PSS/P3HT:PC /P3HT:PCBM/Ca/Al BM/Ca/Al), ), the inverted device structure, e.g. e. g.,, [I [ITO TO/C /Css2CO3(non-annealed)/P3HT:PCBM/V2O5/Al], as depicted in Scheme 1a, uses the ITO covered with a functional buff bu ffer er la laye yerr as the ca catho thode, de, ha hass al also so be been en stu studie died d by sev sever eral al [17–19] groups. The inve inverte rted d stru structur cturee has the adva advantag ntagee of imp improv roved ed stability by replacing the low work function metal cathode and PEDOT:PSS,whicharebothdetrimentaltodevicelifetime. [20–22] Itis also expected that the inverted device has the advantage over the regular reg ular confi configura guration tion bec because ause of the ver vertica ticall phas phasee sepa separat ration. ion. Recen Re centl tlyy, we re repor ported ted an inv inver erted ted pol polym ymer er sol solar ar cel celll by lo low  w  tempera temp eratur turee anne annealin aling g of thefunctio thefunctionalCs nalCs2CO3 layer.[23] Thedevice showed 4.2% PCE under standard measurement conditions, [24] which overcame the efficiency gap between regular and inverted cells. In order to validate this assumption, detailed study of the vertical vert ical com composi positio tion n pro profile file in the P3HT P3HT:PC :PCBM BM ble blend nd is stil stilll necessary. X-ray photoelectron spectroscopy (XPS) offers a useful tool for determining deter mining the compos composition ition at the sample surfa surfaces. ces. Unlike spectr spe ctrosc oscopi opicc ell ellipso ipsomet metry ry,, in XPSthe wei weight ght or mol molar ar rat ratio io of the compon com ponent entss can be cal calcul culate ated d dir direct ectly ly fro from m the pea peak k int intens ensiti ities es of  individual elements. However, due to the short mean free path of  the photoelectron photoelectrons, s, the probi probing ng depth of XPS is only 6–8 nm. In some cases, ion sputtering is used to investigate the concentration tio n in bul bulk k mat materi erials als and bur buried ied int interf erface aces. s.[25,26] Th Thee io ion n bombardment may cause various artifacts in the analysis region, e.g., chemical bonds breaking, preferential sputtering, interface mixing mix ing and pha phase se for format mation ion or rou roughe ghenin ning, g, etc etc..[27,28] Thus, without careful calibration the concentration in the sample may  differ from the depth profile. Here, we introduce a unique method which lifts off the blend filmsand enab enables les the theinvest investigatio igation n of the burie buried d inter interfaceswithout  faceswithout  alteri alt ering ng filmprope filmproperti rties. es. In thi thiss pro proces cess, s, a com commonsolven monsolventt suc such h as water wat er is use used d to lif lift-o t-off ff the ble blend nd film filmss fro from m var variou iouss sub substr strate atess and thefloatingfilmsarethentransferredtonewconductivesubstrates with wit h theselect theselected ed fac facee on thetop forXPS ana analys lysis.The is.The com compos positi ition on of the P3HT:PCBM blend film can be determined from the C/S atomic ratio obtained from XPS. Films spin-coated on various substrates with different procedures are studied. Furthermore, a fullerene fulle rene deriv derivative ative,, [6,6][6,6]-(4-fluo (4-fluoro-phe ro-phenyl)nyl)-C C61-b -but utyr yric ic ac acid id methyl ester (FPCBM), with a fluorine atom attached on the phenyl ring of PCBM (Scheme 1b) molecule, is also blended with P3HT P3H T. The att attach ached ed fluo fluorin rinee doe doess not alt alter er the mol molecu ecular lar properties significantly while providing a label for XPS analysis. Specifi Spe cifical cally ly,, a dir direct ect com compar pariso ison n bas based ed on the XPS res result ultss addresses the advantage of the inverted structure over the regular one in terms of the vertical phase separation. The results are supported suppor ted by corre correspondin sponding g atom atomic ic force microscopy microscopy (AFM) images and device characteristics. Finally, possible mechanisms which induce the vertical phase separation are also discussed.

the spin spin-coa -coated ted film filmss are eva evalua luated ted usin using g C/S ato atomic mic ra ratio tioss obtained from the XPS measurement. O/S atomic ratios can also be use used d to det determ ermine ine the com compos positi ition; on; how howeve everr, the oxy oxygen gen contamination at the sample surfaces makes the results very  unreliable. For samples lifted off in water, the oxygen signals increases significantly. Since carbon is another common surface contam con tamina inant, nt, FPCBM is used as a sub substi stitue tuent nt of PC PCBM BM to evaluate the surface compositions. For samples spin-coated from P3HT:FPCB P3HT :FPCBM M soluti solution, on, surfac surfacee compos compositions itions can be direct directly  ly  determined deter mined from the F/S ratios without concern for the oxygen and an d ca carb rbon on co cont ntam amin inat atio ions ns.. As sh show own n in Fi Figu gure re 1, th thee concentrati concen trations ons obtai obtained ned from vario various us P3HT P3HT:FPC :FPCBM BM sampl samplee surf su rfac aces es ar aree co consi nsist sten entt wi with th th thos osee ac acqu quir ired ed fr from om th thei eirr P3HT:PCBM counterparts. The results indicate that the carbon contamination at the sample surfaces is not as serious as the oxygen and the P3HT to PCBM ratios calculated from the C/S ratio ra tioss are tru trustw stwort orthy hy.. Mor Moreov eover er,, the res result ultss all allow ow us to use FPCBM to examine certain systems where it is hard to use C/S ratios rati os to derive the P3HT to PCBM ratios, such as the polymer– PEDOT:PSS PEDOT :PSS interface. Figure 1a compares the compositions of the films spin-coated on glass substrates under four different procedures, namely fastgrown, slow-grown, fast-grown with annealing, and slow-grown

2. Results and Discussion 2.1. XPS Analysis The PCBM to P3HT weight ratios at the free (air) surfaces (top surface) and the organic/substrate interfaces (bottom surface) of 

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Figure 1.  The compositions at the top and bottom surfaces of the blend films spin-coated on a) glass and b) Cs 2CO3.

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with annealing.[3] After lifting-off the polymer films, the glass substrates are analyzed by XPS and neither S nor F signal is detected, suggesting that the films are lifted-off entirely and no residue is left on the glass substrate. The fast-grown and slowgrown films show similar results, with higher PCBM (FPCBM) concentration at the bottom surfaces for both films. After thermal annealing, the PCBM (FPCBM) concentrations at both sides of  the fast-grown films decrease slightly while the concentrations of  the slow-grown films remain almost invariant. This may indicate that the fast-grown films are less thermodynamically stable. Figure 1b illustrates the results from the films on spin-coated Cs2CO3. During the lift-off process, Cs 2CO3  dissolves in water and the polymer films separate from the substrates instantaneously. The dissolution of Cs 2CO3 is complete and no Cs signal is observed at the Cs2CO3  side of the films. As Figure 1b shows, the PCBM (or FPCBM) concentrations at the polymer/Cs 2CO3 interfaces are higher than those at the polymer/glass interfaces, indicating increased vertical segregation on Cs2CO3  coated ITO substrates. In addition, thermal annealing further enhances this inhomogeneous distribution. The PCBM to P3HT (or FPCBM to P3HT)) ratios at the Cs2CO3 side increase over one- and two-fold P3HT in the slo slow-g w-grow rown n and fas fastt gro grown wn film films, s, res respec pectiv tively ely;; on the contrary, the ratios at the top surfaces only slightly decrease. By decreasing the take-off angle (the angle between the surface of the sample and the detector) of the photoelectrons, a more near-surface composition of the polymer film can be revealed. The XPS probing depth at a take-off angle   u  can   can be estimated as d  sin  sinu , where d  where d  is  is the probing depth at 90  take-off angle. Further XPS studies of the fast-grown P3HT:FPCB P3HT:FPCBM M film with therm thermal al annealing at a lower take-off angle (10 ) detect no F signal at the top surface and extremely weak S signals at the bottom surface. The results show a pure P3HT thin layer at the air side and an almost pure FPCBM thin layer at the Cs2CO3 side. Since, the S 2p and F 1s signals mainly originate from a depth of about 6 nm (the electron electr on mean free paths are around 2 nm) at 90  take-off angle, the probing depth at 10  take-o  take-off ff angle is appro approximat ximately ely 1 nm. Thus, Thu s, the two lay layers ers are est estima imated ted to be 1 nm thick. Because Because P3HT P3H T is a hol hole-c e-cond onduct ucting ing p-t p-type ype sem semico icondu nducto ctorr and PC PCBM BM is an electron-conducting n-type semiconductor, this vertical inhomogeneous distribution is ideal for the previously reported inverted device structure, in which the Cs 2CO3   layer was used as the cathode.[23] From ref.,[23] devices fabricated on annealed Cs2CO3 layer show a much improved performance; consequently, fastgrow gr own n P3 P3HT HT:P :PCB CBM M fil films ms sp spin in-co -coat ated ed on 17 170 0 C-annealed Cs2CO3  layer are also studied. The compositions are similar to those obtained from films spin-coated on non-annealed Cs 2CO3, with only small decrease in the PCBM concentrations at the bottom interfaces. Other factors besides the PCBM concentration should account for the improvement of the device performance using 170 C-annealed Cs2CO3, and details about I–V behavior of  the devices will be discussed. Thee po Th poly lyme mer– r–PE PEDO DOT T:P :PSS SS in inte terf rfac acee is on onee of th thee mo most  st  important interfaces in polymer photovoltaic devices since in a regular device the polymer active layer is usually deposited on an ITO substr substrate ate coate coated d with the PEDO PEDOT T:PSS interfacial interfacial layer layer.. However, the PEDOT:PSS remnant at the bottom surface of the water-lifted film hinders the elemental analysis of this interface. The PEDOT:PSS layer consists of both S and C atoms, which makes mak es it dif difficu ficult lt to der derive ive the com compos positi itions ons in P3H P3HT T:PC :PCBM BM film filmss

from th from thee C/S ra rati tio. o. No None neth thel eles ess, s, th thee co comp mpos osit itio ions ns in P3HT P3H T:FP :FPCBM CBM film filmss can ins instea tead d be est estima imated ted usin using g the F/S ratios and the compositions of the P3HT:PCBM can be inferred owing to the resemblance of the two kinds of films. Figure 2a shows sho ws the normaliz normalized ed S 2p pea peaks ks (co (conta ntaini ining ng S 2p3 2p3/2 /2 and S 2p1/2) obtained from the top surfaces of P3HT:FPCBM films spin-co spi n-coate ated d wit with h dif differ ferent ent pro proced cedure ures. s. The pea peaks ks at aro around und 164 eV binding energy energy are assigned to the thiophene S atoms in P3HT.. Smal P3HT Smalll decrea decrease se in binding energy (about 0.2–0.3 eV) of  the peaks are observed from the annealed samples. The F 1s and

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Figure 2.  XPS spectra of S 2p region obtained from a) top and b) bottom surfaces of the P3HT:FPCBM films spin-coated on PEDOT:PSS. c) The composition of the films at top and bottom surfaces.

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C 1s peaks from the annealed samples show almost identical shift  to lower binding energies (not shown). Thus the phenomenon is unlike unl ikely ly cau caused sed by int intera eracti ctions ons bet betwee ween n P3H P3HTand Tand FPC FPCBM, BM, but is induced by the change of the energy alignment at the sample/ substrate interface. The S 2p peaks measured from the bottom (PEDOT (PE DOT:PS :PSS) S) side of the P3H P3HT T:FP :FPCB CBM M film filmss and fro from m the pristine PEDOT:PSS surface are shown in Figure 2b. There are two broad peaks in the spectrum sp ectrum measured from the PEDOT:PSS PEDOT:PSS surface. surfa ce. The higher (around (around 168 eV) and lower (around (around 164 eV) binding energy peaks correspond to the S atoms in the sulfonate group gro up of PSSand in the thi thioph ophene ene rin ring g of PED PEDOT OT,, res respec pectiv tively ely..[29] Sim imil ilar arly ly,, th thee S 2p sp spec ectr traa fr from om th thee bo bott ttom om si side de of th thee P3HT:FPCB P3HT :FPCBM M films also exhibi exhibitt an additional additional peak at 168 eV eV,, which comes from the remnant of PEDOT:PSS PEDOT:PSS layer on the lifted films. For comparison, all the spectra from the P3HT:FPCBM filmss are normali film normalized zed to the peak at aro around und 164 eV while the spectrum spectr um from the PEDOT:PSS PEDOT:PSS surface is norma normalized lized to the 168eV pea peak. k. In Fig Figure ure 2b, the peaks at 168 eV of the annealed annealed filmss are much lar film larger ger than tho those se of the non non-an -annea nealed led ones, indicating indica ting more PEDO PEDOT T:PSS remained remained at the peeled surfac surfacee as well as a stronger bonding between the P3HT:FPCBM and the PEDOT:PSS layers after annealing. Figure 2c shows the FPCBM to P3HT ratios at both sides of the films estimated using F/S atomic ato mic ratios. ratios. The S 2p peak at 168 eV is not include included d in the calculation. (The lower part of the bars in Figure 2c shows the resultss incor result incorporat porating ing the 168 eV peak.) The result resultss indica indicate te that  the FPCBM concentrations at the PEDOT:PSS side of the films are much higher than at the top surface. In fact, the calculation still includes the S signal from the thiophene ring of PEDOT, which is also at about 164 eV eV.. If this signal can be excluded from the calculation, the FPCBM concentration at the bottom side will be even higher. Moreover, since electrons with higher kinetic energy generally have longer mean free paths, an overlying layer diminishes the intensity of high binding energy peaks more than that of low binding energy peaks.[30] Thus, the high binding energy F 1s signal is reduced more by the remaining PEDOT:PSS layer. However, strong F 1s peaks are still observed at the bottom side si de of th thee P3 P3HT HT:F :FPC PCBM BM fil films ms,, wh whic ich h in indi dica cate tess th that at th thee remain rem aining ing PED PEDOT OT:PS :PSS S lay layer er is thi thin n and dis discon contin tinuou uouss and the attenuation only slightly affects the results at the bottom side. The actual FPCBM concentrations at the bottom side of the films spin-coated on PEDOT:PSS are estimated to be higher than on glass but lower than on Cs2CO3. Similar distribution of PCBM is expected for the P3HT:PCBM films spin-coated on PEDOT:PSS layer. Consistent with the results of Campoy-Quiles et al., [15] our findings suggest that this inhomogeneous PCBM distribution in spin-coated films are not favorable since in a regular device, ITO glas gl asss co coat ated ed wi with th PE PEDO DOT T:P :PSSis SSis us used ed as th thee an anod ode. e. Tab able le 1 li list stss al alll the data shown in Figures 1 and 2c.

2.2. AFM Images

Table 1. The composition at the top and bottom surfaces of the blend films spin-coated on different substrates under various conditions.

 

Fast-grown

Fast-grown with anneal Slow-grown

 

 

Slow-grown with anneal

 

 

Fast-grown

Fast-grown with anneal

 

 

Slow-grown

Slow-grown with anneal

 

 

Fast-grown

Fast-grown with anneal

 

 

Slow-grown

Slow-grown with anneal

 

PCBM/P3HT (wt ratio)

FPCBM/P3HT (wt ratio)

Air Glass Air Glass

0.55 1.23 0.34 0.96

0.48 1.19 0.27 0.73

Air Glass Air Glass Air Cs2CO3 Air Cs2CO3 Air Cs2CO3 Air Cs2CO3 Air Ai PEDOT:PSS Air PEDOT:PSS Air PEDOT:PSS

0.57 1.41 0.57 1.47 0.41 2.22 0.38 7.73 7. 0.60 3.53 0.63 8.53 – – – – – –

0.53 1.55 0.54 1.39 0.34 2.41 0.33 10.88 0.58 3.59 0.53 8.05 0.34 1.77 0.18 4.31 0.37 1.36

– –

0.36 1.58

Air PEDOT:PSS

       

exposed top and bottom surfaces of fast grown films spin-coated on glass substrate. Prior to exposing the polymer network, the top and bottom surfaces show similar smooth films (not shown), whic wh ich h do no nott sh show ow a cl clea earr in indi dica cati tion on of th thee in indi divi vidu dual al compon com ponent ents. s. How Howeve everr, aft after er sele selecti ctivel velyy dis dissol solvin ving g awa awayy the fuller ful lerene ene pha phase se by OT OT,, the dif differ ferenc encee bet betwee ween n the exp exposed osed polyme pol ymerr net networ work k of the top and bottom bottom sur surfac faces es is obv obviou iouss (Fig. 3). The bottom surface clearly shows a much larger surface roughness (R  (R q 6.9 nm, compared compared to R  to  R q 1.0 1.03 3 nm for the top surface), and the phase image shows a much larger contrast, indicating the existence of more fullerene phase, consistent with the XPS results. ¼

¼

2.3. Device Properties The XPS and AFM results suggest the advantage of the inverted device structure structure over the regula regularr one. A concen concentrati tration on distri distribubution with PCBM-rich near the cathode and P3HT-rich adjacent to the anode can be expected to improve the short circuit current  ( J   J SC SC). For fast-grown devices without annealing, the morphology  of the active layer is not optimized, thus both the inverted and regula reg ularr str struct ucture uress sho show w sim simila ilarr per perfor forman mances ces of PC PCE E < 1%, hindering the differences of these two configurations. Annealing the P3HT:PCBM (or P3HT:FPCBM) active layer (at 110 C for 10 min) enhanced enhanced the phase separation and crystallinity crystallinity of the P3HT P3H T cha chains ins,, lea leadin ding g to sig signifi nifican cantt imp improv roveme ement nt in dev device ice performan perfo rmance. ce. Moreov Moreover er,, pre-a pre-anneal nnealing ing increa increases ses the vertic vertical al segregation and the advantage of the inverted structure over the regula reg ularr str struct ucture ure is app appar arent ent und under er the sam samee spin spin-co -coati ating ng 8

AFM was also applied to evaluate the top and bottom surface composition. By rinsing the film surface with 1,8-octanedithiol (OT), which selectively dissolves PCBM, [31] the polymer network  can be exposed, allowing the qualitative evaluation of the relative compositions. Figure 3 shows the height and phase images of the

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Figure 3.  AFM topography (left) and phase (right) images of a) top and b) bottom surfaces of the exposed P3HT networks. The PCBM in the fast grown blend films spin-coate spin-coated d on glass was selectively removed using OT. OT.

parameter parame ters. s. Fo Forr the P3H P3HT T:PC :PCBM BM dev device ices, s, as ill illust ustrat rated ed in 2 Figure 4a, a three-fold increase in   J SC   (2.6 (2.61–7.53 1–7.53 mA cm ) is SC obtained for the inverted device. If the 170 C-annealed Cs2CO3 is incorp inc orpora orated ted in the inv invert erted ed str struct ucture ure,, the dev device ice effi efficien ciency  cy  2 tripled to 2%, with even higher   J SC (9.13 mA cm ). For the SC   (9.13 P3HT:FPCBM devices, as shown in Figure 4b, the improvement  is even more significant, with an almost five-fold increase in J  in J SC SC (1.82 vs. 8.64 8.64 mA cm2) and PCE (0.56 vs. 2.70%). As mentioned earlier,, the annealed Cs2CO3 does not further increase the vertical earlier vertical phase separ separation ation.. The   J SC improv ovem emen entt is du duee to a lo lowe werr SC   impr interfacial resistance contact formed upon Cs 2CO3 annealing.[23] The series resistance for the non-annealed Cs 2CO3  devices and the 170 C-annealed Cs2CO3  devices are only a fraction of the regular ones, indicating the reduced resistivity of the interface. This series resistance reduction can be attributed to an improved interface morphology, since for the inverted devices, the Cs2CO3 side (cathode) is PCBM-rich, while the opposite V 2O5/Al side (anode) is P3HT-rich, and thus charge carrier recombination can be sub substa stanti ntiall allyy red reduce uced. d. The det detail ailed ed dev device ice oper operati ationa onall parameters are summarized in Table 2. The external quantum efficiency (EQE) results obtained from the P3HT:PCBM devices are shown in Figure 5. The inverted 8

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Table 2.  Short-circuit current density ( J ( J and 5.

<sub>SC

PEDOT:PSS Cs2CO3   (RT) Cs2CO3  (170 C) PEDOT:PSS

0.68 0.56 0.52 0.60

Cs CO   (RT) Cs2CO3  (170 C)

0.54 0.56

8

P3HT:FPCBM

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device structure clearly shows a higher EQE over the entire solar spectr spe ctrum, um, wit with h a max maximu imum m of 64% at 514nm, whi which ch cor corres respon ponds ds to the significantly higher   J SC   compared to the regular device SC structure. struct ure. Despite the higher EQE on the non-annealed non-annealed Cs 2CO3 films, the J  the  J SC  is lower due to a higher interfac interface e resistance of the SC pristine Cs2CO3 layer itself. The interesting part is the substantial EQE contribution from the PCBM ( 350 nm), which which is not seen for the regula regularr device structure, structure, even for a slow-g slow-grown rown device.[3] Since the UV–Vis absorption results (not shown here) show no distin dis tinct ct dif differ ferenc ences es bet betwee ween n the reg regula ularr and inv invert erted ed dev device ice structure, this increased EQE can be attributed to the accumulation of the electron acceptor material at the cathode due to selfstra st rati tific ficat atio ion, n, wh which ich ma mayy le lead ad to a mo more re ef effic ficie ient nt ch char arge ge collection. This excess EQE contribution from PCBM validates 

), open-circuit voltage (Voc), PCE, and fill factor (FF) of various photovoltaic devices shown in Figures 4 Jsc [mA cm2]

Voc [V] P3HT:PCBM

Figure 4.   J–V  V  charact  characteris eristics tics unde underr illum illuminat ination ion for prepre-anne annealed aled fastgrown a) P3HT P3HT:PCBM :PCBM and b) P3HT:FPCBM photovoltaic photovoltaic devices with different structure.

   2009

FF [%]

PCE [%]

Rs [V  cm 2]

2.61 7.53 9.13 1.82

0.74 1.69 1.93 0.56

41.48 40.14 40.59 50.77

7.19 2.58 0.98 12.62

7.74 8.64

1.94 2.70

46.43 55.69

3.35 2.25

 



 



 



 



 



 



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ably affect the intermolecular interactions between the PCBM molecules. Ohno et al. suggested an enhanced intermolecular interaction due to the dipole field created by the induced dipole moments in C60  overlayers, which was caused by strong charge transfer from the substrate to the first-layer C60 molecules.[35] It is expected that PCBM and FPCBM should behave similarly to C 60 in this aspect. Indeed, our XPS results show a significant binding energy shift ( 0.5 eV) to lower binding energy energy for Cs after spinspincoating coati ng an ultra ultra-thin -thin PCBM laye layerr abov above, e, suggest suggesting ing substa substantial ntial charge transfer. We have previously demonstrated that the spincoated coa ted Cs2CO3   laye layerr fo form rmss a st stro rong ng di dipo pole le on th thee IT ITO O substrate,[36] thus induced dipole–dipole interaction should also contribute to the accumulation of PCBM at the polymer/Cs 2CO3 interf int erface ace.. Fo Forr the 170 C-ann C-annealed ealed Cs2CO3   layer layer, the surfa surface ce [23] becomes becom es less hydro hydrophilic philic,, thuss onl thu onlyy cha charge rge tra transf nsfer er con con-tributes to the weaker affinity for PCBM accumulation. Glass substrates have a relatively hydrophobic surface before UV-ozone treatment, and are not preferable for PCBM accumulation. From our results, both surface energy and charge transfer play a role in the PCB PCBM M acc accumu umulat lation ion on var various ious sub substr strate ates, s, and fur furthe therr studies are undergoing to elucidate the detailed mechanism of  the vertical phase separation. The most significant PCBM accumulation occurs after preannealing the polymer film on non-annealed Cs2CO3 layer, which is due to the str strong ongest est dip dipole ole–di –dipol polee int intera eracti ction. on. Thi Thiss eve even n stronger PCBM segregation after annealing is because the spincoated coate d film has an interm intermediat ediatee morph morphology ology after spin-c spin-coatin oating, g, and the annealing process provides the driving force for the polymer polym er film to achiev achievee a more thermodynamica thermodynamically lly favor favorable able morphology. It has been suggested that if thermal equilibrium is reached, a bilayer structure should form eventually. [10] If the vertical vertical segr segrega egatio tion n can be con contro trolle lled d to a desi desired red morphology morph ology,, where the catho cathode de is accept acceptoror-enric enriched hed and the anode is donor donor-enric -enriched, hed, efficient charg chargee dissoci dissociation ation via the interpenetr inter penetrating ating network and efficie efficient nt charg chargee trans transport port along the interconnected pathway, as well as efficient charge collection at the int interf erface ace can sub substa stanti ntiall allyy enh enhanc ancee the dev device ice per perfor forma mance nce.. This favorable morphology based on the vertical segregation is depicted in Scheme 2. Nonetheless, Khodabakhsh et al. have shown that changes in surface wettability influences how the subsequently subseq uently deposited organic molec molecules ules assem assemble ble and orient  themse the mselve lves, s, thu thuss aff affect ecting ing the den densit sityy of ava availa ilable ble cha charge rge collection sites in organic solar cells. [37] As a consequence, a PCBM-rich layer at the Cs2CO3 interface is beneficial and would improve the device performance for the inverted configurations. 

8

Figure 5.   EQ EQE E res result ultss ob obtai tained ned fro from m the pre pre-a -anne nneale aled d fas fast-g t-grow rown n P3HT:PCBM devices.

the advantage of the vertical phase separation for the inverted structure. Other processed and treatments like slow-grown film followed by the therma rmall ann anneal ealing ing can fur furthe therr imp improv rovee the lat latera erall pha phaseseseparated morphology and lead to better device performance. Meanwhile, the vertical composition distribution does not vary  significantl signifi cantlyy. As a result result,, the effects rela related ted to the vertical phase separation become obscure. The best regular P3HT:PCBM P3HT:PCBM device fabricated in our lab so far has the efficiency of 4.4%, which is slightly higher than our newly-reported inverted device structure with 4.2% PCE. However, the inverted device still benefited from this vertical phase separation, with a higher EQE maximum (72% compared to 63%) and a higher  higher   J SC  (11.13 13 vs. 10. 10.6 6 mA cm2). SC  (11. Furthermore, the non-negligible optical loss from the PEDOT: PSS layer may also account the improvement in the inverted devices.[32] Further performance improvement of the inverted device dev icess can be exp expect ected ed by opt optimi imizin zing g the ene energy rgy ali alignm gnment ent at the polymer/electrode interfaces and improving the conductivity of  the functional buffer layers.

2.4. Formation of the Vertical Phase Separation The vertical phase separation in the polymer blend is believed to be re rela late ted d to th thee di diff ffer eren ence cess in th thee sur surfa face ce en ener ergy gy of ea each ch component. Since P3HT has a lower surface energy than PCBM, it tends to accumulate at the air surface in order to reduce the overall energy.[10,34] The solvent evaporation process during spincoating coati ng allow allowss the morph morphology ology to reach a thermo thermodynam dynamicall ically  y  favorable state via vertical phase separation. This similar surface enrichm enr ichment ent phe phenom nomeno enon n has als also o bee been n obs observ erved ed in oth other er polymer blend systems, such as the multilayer structure formed in the APFO APFO-3/PC -3/PCBM BM (1:4) film, with surfa surface ce enrich enrichment ment of  [10] APFO-3 and an d en enri richm chmen entt of th thee lo lowe werr sur surfa face ce en ener ergy  gy  component TFB at the surface for TFB/F8BT blend, [33] as well as a partially crystallized wetting layer formed by the polyfluorene (PFO) for PFO/F8BT blend.[34] Due to the hydrophilic nature of the non-annealed Cs2CO3 layer,, PCBM tends to accumulate much stronger on Cs2CO3 than layer on gla glass ss sub substr strate ates. s. PC PCBM BM its itself elf has a ver veryy hig high h den densit sityy of  electrons, thus the resulting induced dipole moments presum-

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Scheme 2.   Sche Schemati maticc of the verti vertical cal phase separation separation of the polymer: polymer: fullerene blend.

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3. Conclusions As a conclusion, we introduced a unique method which enables the investigation of the buried interfaces without altering the polyme pol ymerr film pro proper perties ties.. Det Detail ailed ed XPS ana analys lysis is pro provid vided ed an insight ins ight to the int interf erface acess of the pol polyme ymer/s r/subs ubstra trate, te, rev reveal ealing ing spontaneous vertical stratification upon spin-coating the polymer films, film s, as wel welll as the enrichme enrichment nt of the donor and acc accept eptor or components at the top and bottom surfaces, respectively. This vertical phase separation is attributed to the differences in surface energy and induced dipole–dipole interactions between PCBM and an d th thee su subs bstr trat ates. es. By va vary ryin ing g th thee sur surfa face ce pr prop oper erty ty of th thee substr sub strate ate,, the dist distrib ributi ution on of the don donor or and acc accept eptor or ma mater terial ialss can be manipulated, and the consequent vertical phase separation makes the inverted structure a promising choice for polymer solar cells.

4. Experi Experimental mental The device structures structures of regu regular lar and inve inverted rted polymer polymer sola solarr cells are illustrated in Scheme 1a. The ITO substrates were pre-cleaned and treated with wit h UV UV-oz -ozone one for 15min pri prior or to spi spin-c n-coa oatin ting. g. Th Thee reg regula ularr de devic vicee structure consists of an ITO substrate coated with a PEDOT:PSS interfacial layer as the anode; Ca (20nm) and Al (80 nm) were thermally-evaporated thermally-evaporated as the cathode. For For the inverted structure, 0.2 wt % Cs2CO3  was dissolved in 2-et 2-ethoxy hoxyetha ethanol nol and spin spin-coa -coated ted on ITO substrates substrates at 300 3000 0 rpm to function as the cathode. Some of these Cs2CO3  covered cathodes were annealed at 170 C for 20 min inside the glove box before before depositing the polymer active layer, and the surface property of this cathode layer can be manipula mani pulated ted by therm thermal al anne annealin aling g treat treatment ment.. V2O5   (10 (10 nm nm)) an and d Al (800 nm) were thermally-evaporated thermally-evaporated as the anode for the inverted devices. RR-P3H RRP3HT T and PCBM (o (orr FPC FPCBM BM)) wer weree se separ parate ately ly di disso ssolve lved d in 1, 2 dichlorobenzene dichlorobe nzene (DCB) and blended together in a 1:1 w/w ratio to form a 2 wt % sol soluti ution. on. In ord order er to exc exclu lude de the possib possible le eff effect ect of PCB PCBM M distribu dist ribution tion from slow growth, the activ activee laye layers rs were spin-coated spin-coated at 2000rpm for 90 s, and were completel completelyy drie dried, d, visualized visualized from the redpurplish color after spin coating. Preannealing was carried out inside the glove box at 110 C for 10 min before electrode evaporation. evaporation. Device testing was performed following the rules of ref. [24] and under under simulated AM1.5G irradiatio irrad iation n (100mW cm2) usin using g a xeno xenon-la n-lamp-b mp-base ased d sola solarr sim simulato ulatorr (Oriel 96000 150W Solar Simulator). Thee sa Th samp mple less fo forr XP XPS S an anal alys ysis is we were re pr prep epar ared ed us usin ing g th thee sa same me 8

8

P3HT:PCBM andbare P3HT:FPCBM blend The polymer spin-coa spin -coated ted on glass subs substrate tratessolutions. s or subs substrates trates pre-cofilms pre-coated ated were with PEDOT:PSS or Cs2CO3 buffer layers. The Cs2CO3 was spin-coated on glass at 1000 rpm for 60 s to ensure full coverage over the glass surface while while the PEDOT:PSS layer was spin-coated at the same condition as the device fabric fab ricati ation. on. Th Thee pol polyme ymerr film filmss we were re pre prepar pared ed us using ing fou fourr di diffe fferen rentt procedure proce dures, s, name namely ly fastfast-grow grown n (30 (3000 00 rpm, 60 s), slow slow-gro -grown wn (800rpm, 40 s with solvent annealing), fast-grown with annealing (thermal annealing at 110 C for 10 min) and slow-grown with annealing annealing (thermal annealing 110 C for 10 min) min).. During Durin g the liftlift-off off proc process, ess, the spin spin-coa -coated ted poly polymer mer film was pre-c pre-cut ut into several small pieces and rinsed in water. For samples spin-coated on the glass substrates, due to the different surface energy between the glass substrate and the polymer layer, water delaminates the polymer film from the substrate. For samples spin-coated on Cs 2CO3  or PEDOT:PSS buffer layers, laye rs, the P3HT P3HT:PCB :PCBM M (or P3HT P3HT:FPC :FPCBM) BM) filmsdetach from the subs substrate tratess because of the water-soluble Cs2CO3  or PEDOT:PSS buffer layers. As a result, the small pieces of P3HT P3HT:PCBM :PCBM (or P3HT:FPCBM) P3HT:FPCBM) films were lifted off and floated on the water with the free (air) surface on top. Then the

Because Becau se P3H P3HT T and PCB PCBM M (or FPC FPCBM BM)) are ins insolu oluble ble in wa water ter,, immersing in water does not change the PCBM to P3HT (or FPCBM to P3HT) ratios. In another experiment, the top surfaces of the films were treated carefully with water (without peeling off the organic film from the substrate), but no variations in the PCBM to P3HT ratio were observed before and after the water treatment. Due to adsorption of the hydroxyl groups, the oxygen signals from the surfaces contacted with water were much stronger. However, since the oxygen signals were not used in the calculation, rinsing in water did not affect our results. The XPS measurements were performed inside an Omicron XPS/UPS

F   U L   L   P  A P  E  R

system..Th system Thee ba base se pre press ssurein urein theanal theanalysi ysiss ch cham amberof berof thesyste thesystem m wa wass bet better ter than tha n 10 9 mbar mbar.. A mono monochro chromati maticc Al Ka (14 (1486.6eV) 86.6eV) X-raysource was use used d for excitation and the spectra were collected collected with a pass energy of 50 eV. The atomic ratios were evaluated using the following equation:  I 1 =S 1

n1 n2

¼

(1)

 I 2 =S 2

where I is the the peak area and S  is  is the atom atomic ic sens sensitivi itivity ty facto factorr. The inte integrate grated d areas of the XPS peaks were calculated using XPSPEAK41software and the Shirley Shir ley meth method od was usedto subt subtract ract the back backgrou ground. nd. Theatomic sens sensitivi itivity ty factors facto rs were extracted extracted from the empi empirical rical values reported reported by Wagn Wagner er et al.[38]. Due to different instrumental design, the sensitivity factors of  different systems may not be the same. However, the accuracy of our results can be evaluated using the following method. Since the carbon contamination at the sample surfaces is negligible, the S/Cclose and to F/C atomicmetry. ratios in P3HT-only FPCBM-only should be stoichio stoichiometry . The sensitivity and factors of the S, C,samples and F elements can thus be calculated by comparing the intensities of corresponding XPS peaks. The C/S and F/S atomic ratios calculated using these sensitivity factors are 13 and 17% lower than using values from [38], respectively r espectively.. As a result, the FPCBM to P3HT ratios are 17% lower than the values shown in Table 1, while the PCBM to P3HT ratios are 15–25% lower at the bottom surface and 30–40% lower at the top surface. The ratio differences are acceptable and do not affect any conclusion in this paper. Fast grown P3HT:PCBM films on glass substrates were used for AFM imaging. P3HT network films were prepared by rinsing the P3HT:PCBM film with OT for several seconds. In order to observe the organic/glass interface of the sample, P3HT:PCBM films were peeled off using water and transferred to another glass substrate with the bottom side up before the OT treatment. The AFM images were obtained with tapping mode AFM (Nanoscope IIIa, Veeco Instruments).

Acknowledgements

Z. Xu and L.-M. Chen contributed contributed equally equally to this work. The authors authors gratefully acknowledge financial support from Solarmer Inc., University of  California Discovery Grant, and NSF IGERT: Materials Creation Training Program Prog ram (MCT (MCTP) P) (DGE (DGE-011 -011444 4443) 3) and the Cali Californ fornia ia Nano Nano-Sys -Systems tems Institute. The authors also thank Dr. H.-H. Liao, Dr. S. H. Li, Dr. Y. Yao, W. L. Kwan, and V. Tung for their fruitful discussions.

8

Received: August 30, 2008 Published online: March 13, 2009

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‘‘lifted-off’’ films were transferred to Ag coated Si substrates with selected surface on top.

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