C60-Fullerene

J. Phys. Chem. C 2007, 111, 15415-15426

15415

Understanding and Improving Solid-State Polymer/C60-Fullerene Bulk-Heterojunction Solar Cells Using Ternary Porphyrin Blends Paul C. Dastoor,*,† Christopher R. McNeill,† Holger Frohne,† Christopher J. Foster,† Benjamin Dean,† Christopher J. Fell,† Warwick J. Belcher,‡ Wayne M. Campbell,‡ David L. Officer,‡ Iain M. Blake,| Pall Thordarson,§,| Maxwell J. Crossley,| Noel S. Hush,| and Jeffrey R. Reimers*,| School of Mathematical and Physical Sciences, UniVersity of Newcastle, Callaghan, NSW 2308, Australia, Nanomaterials Research Centre and MacDiarmid Institute for AdVanced Materials and Nanotechnology, Massey UniVersity, Palmerston North, New Zealand, The Australian Key Centre for Microscopy and Microanalysis, and School of Chemistry, The UniVersity of Sydney, NSW 2006, Australia ReceiVed: June 22, 2007

Solid-state heterojunction solar cells formed from binary blends of conjugated polymers and fullerenes provide a promising class of organic devices. We demonstrate that ternary blends incorporating porphyrins can be of similar morphology to the binary mixture and retain full device functionality. They allow, for the first time, the construction of efficient devices containing less than 10% polymer. By analyzing the absorption spectra as a function of concentration, we determine the proportion of light absorbed by each individual component in both binary and ternary mixtures. This analysis reveals that the majority of the light is absorbed by the fullerene in 1:4 polymer/C60-fullerene blends, with over 50% of the photocurrent produced under AM 1.5 conditions occurring subsequent to C60-fullerene absorption. This result provides for a consistent understanding of the origin of primary charge separation in general polymer/C60-fullerene blends, polymer/C70-fullerene blends, and polymer-fullerene dyad molecules. Porphyrins are demonstrated to add broad-band character to the device and may be used to tune for particular wavelengths; they also are shown to initiate primary charge separation through electron-transfer to the fullerene. Finally, addition of the porphyrin is shown to increase the internal quantum efficiency following polymer absorption from ca. 60 to 80%.

1. Introduction Molecular-based solar cells utilize heterogeneous junction regions that dissociate excitons (absorbed optical energy) to produce separated electrical charges. These ‘heterojunction’ regions may be well-ordered regions at surfaces or boundaries between layers, or they may be single layer ‘bulk-heterojunctions’ (BHJs), intimately mixed two component regions that incorporate an electron donor and an electron acceptor.1-5 The morphology of the BHJ must allow for efficient charge separation across the junction and for efficient charge transportation without recombination through bicontinuous networks.5-10 In this way, balance must be achieved between the processes of charge generation, which requires donor and acceptor to be closer than an exciton diffusion length, and charge collection, which requires separate charge-transport paths. Bulkheterojunction solar cells, formed by combining conducting polymers (the electron donor that forms the hole-transport network) and fullerenes (the electron acceptor that forms the electron-transport network7), have now been demonstrated to achieve 6% efficiency in the conversion of solar AM 1.5 radiation into electrical energy11 and have promise for use in new solar energy technologies. A significant advantage that they * To whom correspondence should be addressed. Email: P.C.D.: [email protected], J.R.R.: [email protected]; telephone: +61 2 4921 5426; fax: +61 2 4921 6907. † University of Newcastle. ‡ Massey University. § The Australian Key Centre for Microscopy and Microanalysis. | The University of Sydney.

present over alternative technologies is their simplicity of manufacture, with the active junctions being produced spontaneously by self-assembly during spin coating rather than by laborious engineering. Although morphology is known to be critical to the function of binary polymer/fullerene blends, nanoscale and molecularlevel descriptions of the key structures and junction geometries are only now being determined.6 For the case of poly[2methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEHPPV) and related polymers, the optimal polymer/fullerene ratio is known5,6,12 to be on the order of 1:4, with the general increase in efficiency with increasing fullerene concentration being curtailed by phase separation. At low fullerene concentrations, the majority of the incident light is absorbed by the polymer; primary charge separation involving electron-transfer from polymer to fullerene then occurs efficiently on the 45 fs time scale,1,13-15 producing the charge carriers that constitute the electric current. This process has been assumed to dominate charge generation in films of high fullerene concentration as well, especially so for C60-based fullerenes.2,5 However, for both intra- and intermolecular charge separation involving polymer/ fullerene dyads in solution,16-19 hole-transfer from photoexcited fullerenes is found to be the dominant process; this can arise from both directly photoexcited fullerenes and fullerenes excited following photochemical energy transfer from the polymer. It has been apparent since binary polymer/fullerene films were first produced that primary charge separation by way of holetransfer following fullerene excitation does occur,15 and this feature has recently been exploited using C70-based fullerenes

10.1021/jp0748664 CCC: $37.00 © 2007 American Chemical Society Published on Web 10/03/2007

15416 J. Phys. Chem. C, Vol. 111, No. 42, 2007 to enhance spectral sensitivity.10,20 In addition, it is known that pure fullerene films even undergo primary charge separation.21-24 Nevertheless, most modern discussion of the nature of C60-based fullerene/polymer photovoltaics focuses only on primary charge separation following light absorption solely by the polymer.5,6,25-28 One property that has hindered elucidation of the actual relative importance of the polymer and the fullerene as initiators of primary charge-separation processes has been the phase separation that can occur in fullerene/polymer blends, a feature that has so far prevented the preparation of working devices at polymer concentrations of less than 20%. In this work, we investigate the operation of polymer/fullerene BHJ solar cells through the production of ternary mixtures formed by simply adding porphyrins to the blend. Porphyrins have long attracted attention as potential light absorbers for photovoltaic applications because porphyrinic compounds are intimately involved in light absorption, exciton transport, primary charge separation, and charge-transport processes in natural photosynthesis.29,30 There have been many attempts to produce porphyrin-containing artificial donor-acceptor systems to mimic the early steps of photosynthesis, either for solar applications or as chemical energy for artificial light-driven proton pumps.31 They may act directly as light harvesters, initiators of primary charge separation, and/or charge carriers,32 as well as indirectly through changing the morphology or otherwise influencing the performance of the native processes. Porphyrins have been incorporated into both electrochemical photovoltaic cells and photovoltaic devices.33-41 Early electrochemical cells made with porphyrins typically consisted of thin films of porphyrin deposited onto a conducting or semiconducting electrode that is placed in a redox solution with a counter electrode.42,43 Upon photoexcitation, through one of the transparent electrodes, electron-transfer occurs from porphyrin to the electrode at the porphyrin/electrode interface. The porphyrin film is then regenerated by hole-transfer to the redox species in solution that is then reduced at the counter electrode. Because of the low diffusion length of excitons in porphyrin thin films, typically less than 10 nm,44 photoexcitations created away from the electrode/porphyrin interface do not efficiently contribute to photocurrent, and the overall devices perform poorly. Gra¨tzeltype cells45 have been used to overcome this problem, achieving power conversion efficiencies of up to 6% under AM 1.5 solar radiation (incident intensity 100 mW cm-2).46 Improvements in efficiency are also being brought about by the use of large arrays of porphyrins acting as light-harvesting antennas and assembled with C60, which acts as the reaction center.32 In particular, quantum efficiencies of up to 15% and a power conversion efficiency of 0.32% (input power 6.2 mW) have been achieved using supramolecular assemblies of dendritic arrays of porphyrins with C60 attached to nanostructured SnO2 electrodes.47 Similarly, Hasobe et al. have also demonstrated high quantum efficiencies (up to 28%) in devices comprised of quaternary self-organized clusters of porphyrins, C60, and gold nanoparticles attached to SnO2 electrodes.48 Although a drawback of photoelectrochemical devices is their use of a liquid electrolyte that limits their practical use, recent work on dyesensitized solar cells, in which the liquid electrolyte is replaced with a hole-transport material, has shown promising power conversion efficiencies of over 4%.37,40,41 Attempts to incorporate porphyrins into polymer solar-cell devices have not achieved power conversion efficiencies as high as electrochemical cells. Similar to the early electrochemical devices, solid-state devices consisting of a vacuum-deposited film of porphyrin sandwiched between metal electrodes29 or two-

Dastoor et al. layer organic p-n junction devices sandwiched between metal electrodes49 show low efficiencies because of the low exciton diffusion length in the evaporated film. The high efficiencies exhibited by copper phthalocyanine/C60 p-n junction devices50 can largely be attributed to the longer exciton diffusion length in phthalocyanine films as compared to porphyrin films, which is reported to be as high as 68 nm.51 However, success in creating two-component BHJ films through the co-deposition of copper phthalocyanine (CuPc) and C60 with postproduction annealing52 indicates that efficient solid-state devices incorporating porphyrins indeed may be feasible. Other examples of binary blends involving porphyrins have also been reported, designed for narrow-band detection at the violet end of the visible spectrum.53,54 The active film thickness in these devices is typically small (20-60 nm), and no n-type material is present in these layers for efficient electron-transport, so device efficiencies are quite low.53 Ternary mixtures involving porphyrins retain the ease of manufacturability of standard polymer-blend solar cells. Herein, we construct solar cells using commercially available indiumtin-oxide (ITO) based anodes, ternary polymer/fullerene/porphyrin blends, and deposited calcium/silver cathodes, and we investigate the device morphology, device functionality, and device spectroscopy. No attempt has been made to optimize the efficiency of these devices. The addition of porphyrins to the blend could lead to improvements in the device efficiency because porphyrins act as photosensitizers, molecules that absorb light in a spectral range that is not properly covered by the original binary blend,55,56 thus facilitating light conversion into electrical current. As such, this ternary device could produce either a broad-band polymer-based photovoltaic cell or a cell tailored for a specific spectral response. They could also lead to enhanced performance via the introduction of new exciton transport, primary charge separation, and/or charge-transport mechanisms, as well as influence the native morphology and photophysics. More significantly, however, ternary blends allow for greater variability in the chemical composition, and we exploit this to investigate the basic mechanisms of polymer/ fullerene operation and morphology formation. This results in the first detailed map of the components absorbing the light (in both binary and ternary blends) and determination of the probability with which this energy is converted into short-circuit current. 2. Methods a. Materials. MEH-PPV was supplied by American Dye Source (Baue d’Urfe´, Quebec, Canada) and was purified before use. The C60 derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) was supplied by the Hummelen group at the University of Groningen. b. Preparation of Photovoltaic Devices. Films of poly(3,4ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS, Baytron P from Bayer AG) were deposited on to precleaned, patterned ITO substrates by spin coating from an aqueous solution to give a thickness of 80 nm. Blended films of MEHPPV and PCBM were spin-coated from a chlorobenzene solution with a weight ratio of 1:4 (MEH-PPV/PCBM), yielding films of ∼110 nm thick. Blended films of PCBM, MEH-PPV, and the porphyrin [2,3,12,13-tetracyano-5,10,15,20-tetrakis(3,5-ditert-butylphenyl)porphyrinato]copper(II) [Cu(CN)4P] were similarly spin-coated from a chlorobenzene solution containing all three materials with a weight ratio of 1:8:1 (polymer/fullerene/ porphyrin) to give a film thickness of ∼120 nm. Finally, the top metal electrode with a total thickness of ∼150 nm was

Polymer-Fullerene-Porphyrin Bulk-Heterojunction Solar Cells deposited by thermal evaporation of ∼50 nm of calcium followed by ∼100 nm of silver. Because of the patterned ITO substrate, six individually addressable devices were fabricated from each prepared film, with the active area of each device (determined by the overlap between the ITO and the metal) approximately 4 mm2. c. Photovoltaic Measurements. A Solux quartz halogen lamp (4700 Kelvin, 10° beam spread) was employed for white-light illumination, delivering a power density of 80 mW/cm2 as measured by a calibrated silicon photodiode (Newport 818-SL head and 840 meter). Current/voltage (IV) curves were taken using a Keithley 2400 source measure unit. Quantum efficiency spectra were collected using a lock-in amplifier (Ithaco Dynatrac) to measure the short-circuit current signal from the devices when illuminated by chopped light from a tungsten halogen lamp passed through a monochromator (Oriel Cornerstone 130). A preamplifier was employed to hold the devices at virtual short-circuit and act as a current-to-voltage converter. The power incident upon the device during quantum efficiency measurements was determined using the Newport calibrated photodiode, with typical power densities (L) of 0.1-1 mW/ cm2 and quantum efficiency calculated as EQE ) 1240ISC/λL. Internal quantum efficiencies (IQEs) were calculated by determining the amount of light absorbed by the blend films from the UV-Vis measurements of the films on fused silica. As light is reflected form the back metal electrode in the device geometry, absorption was calculated assuming two passes of the film, with variations in film thickness between the device films and films used for optical transmittance measurements corrected for. To correct for the light absorbed/reflected/scattered by the PEDOT:PSS coated ITO glass substrate, transmittance measurements of the device substrate were also performed. d. Cyclic Voltammetry. The electrochemistry measurements on MEH-PPV, PCBM, and Cu(CN)4P were obtained using a BAS 100 electrochemical analyzer. Samples were dissolved in dry deoxygenated tetrahydrofuran with 0.1 M tetrabutylammonium perchlorate as the supporting electrolyte, and measurements were taken at room temperature with a scan rate of 100 mV/s. The working electrode was glassy carbon, the auxiliary electrode was platinum, and the reference electrode was Ag/ AgCl/NaCl. e. Spectroscopy. UV-Vis measurements were carried out using a Cary-1E spectrophotometer. For UV-Vis measurements on films, a thickness of ∼100 nm was used on a substrate of fused silica. Diffuse reflectance measurements were made on a Cary-5 spectrometer. The light absorbed by the molecular region in functional devices was determined using the matrix method formalism to simulate the electric field and absorption at every point in the device structure.57 Details of the method used are provided in the Supporting Information. f. Atomic Force Microscopy (AFM). The AFM measurements were performed in noncontact mode using a Nanonics SPM 100 system. The thickness of the films used to determine the optical constants of the component layers was measured using a Nanoscope AFM. 3. Results Chemical structures of the major materials used in this study are presented in Figure 1. MEH-PPV (1) was used as the p-type conjugated polymer in conjunction with PCBM (2), whereas Cu(CN)4P (3) was, unless otherwise specified, employed as the porphyrin. Blend films were prepared either by dissolving 1 part of MEH-PPV to 4 parts of PCBM by weight in chlorobenzene or 1 part of MEH-PPV to 4, 6, 8, or 9 parts of PCBM

J. Phys. Chem. C, Vol. 111, No. 42, 2007 15417

Figure 1. Chemical structures of MEH-PPV (1), PCBM (2), and Cu(CN)4P (3).

Figure 2. Noncontact AFM image of a 1:8:1 MEH-PPV/PCBM/ Cu(CN)4P blend film.

to 1 part of Cu(CN)4P by weight in chlorobenzene and spincoating. Chlorobenzene was chosen as the solvent because it is the optimum solvent for the preparation of PPV/PCBM solar cells.9 The weight ratio of 1:8:1 MEH-PPV/PCBM/Cu(CN)4P was found to produce optimum performance; interestingly, this contains the same net fraction of PCBM (80%) as is known to provide optimal performance in binary MEH-PPV/PCBM mixtures.7 a. Device morphology. As has been borne out by a number of studies,5-10 efficient charge generation and collection in the polymer/methanofullerene system occurs only when the blend does not phase-separate. In particular, devices spin-coated from chlorobenzene that show nearly 3 times the efficiency of toluene devices have a smoother morphology as revealed by AFM, indicating a more intimate mixing of polymer and methanofullerene9,10 without distinct domains.6 To demonstrate that the addition of porphyrin to the polymer/methanofullerene system preserves this prerequisite morphology, AFM measurements were performed. Figure 2 shows a 2 × 2 µm noncontact AFM image of the surface of a 1:8:1 by weight MEH-PPV/PCBM/ Cu(CN)4P blend film. This figure shows that the surface of the ternary blend film is uniform in height to within 1 nm over an area of 500 × 500 nm, which is similar to previously published AFM images of functional binary 1:4 polymer/methanofullerene films.7,9 Thus, the morphology of the ternary blend film is at least as smooth as that of the binary blend film and does not show nanoscopic, microscopic, or macroscopic phase-separated domains. Most significantly, this morphology is maintained at a polymer concentration of only half of the minimum previously demonstrated concentration. b. Device operation. Figure 3 depicts the external quantum efficiency (EQE) spectra from devices of 1:4 blends of MEHPPV/PCBM and 1:8:1 blends of MEH-PPV/PCBM/Cu(CN)4P

15418 J. Phys. Chem. C, Vol. 111, No. 42, 2007

Dastoor et al.

Figure 4. Current/volatge (IV) characteristics of ITO/PEDOT:PSS/ MEH-PPV/PCBM/Ca/Ag (solid line) and ITO/PEDOT:PSS/MEH-PPV/ PCBM/Cu(CN)4P/Ca/Ag (dotted line) solar cells under ∼80 mW/cm2 white light illumination.

Figure 3. Spectral dependence of EQE and IQE obtained using raw and corrected active-layer absorption data for ITO/BLEND/Ca/Ag photovoltaic devices using blends of MEH-PPV/PCBM/ Cu(CN)4P of 2:8:0 (---) and 1:8:1 (s).

sandwiched between ITO/PEDOT:PSS and Ca/Ag electrodes. This efficiency depicts, under short-circuit conditions, the fraction of harvested charge carriers per incident photon. For both devices, the peak quantum efficiency is around 30- 35%. These results demonstrate that the ternary blends afford functional photovoltaic devices, which is surprising given that it has been well demonstrated that small changes to binary component mixtures and sample preparation conditions can lead to large changes in morphology that inhibit photocurrent generation; these devices remain fully functional at polymer concentrations well below the minimum concentration demonstrated for functional binary devices. Even though the concentration of the highly absorbent polymer is halved in the ternary blend, the device characteristics show only slight changes. Photosensitization through the porphyrin Q-band region at 630680 nm (see Section 4c) is clearly seen to add to the current. By renormalizing the EQE for the fraction of light absorbed by the device, the IQE representing the fraction of absorbed photons converted to electrical current is obtained, and this is also shown for both devices in Figure 3. Because of the presence of the metal electrode, ITO layer, etc., the actual amount of light absorbed in the active layer of the device is not readily measurable, however, adding uncertainty to derivations of the IQE. We take two approaches, a naive one in which the metal is regarded as a perfect reflector, thus realizing two passes of the light through the layer, and a complex approach in which the optical properties of the entire device are simultaneously modeled,57 as described in detail in the Supporting Information. In Figure 3 and henceforth, these results are labeled “raw” to indicate use of the raw active-layer absorption and “corrected” to indicate the results obtained from the extensive modeling. Through comparison of the results thus obtained, we estimate error bounds for the quantities that we shall later extract from the IQE data; in particular, Figure 3 presents qualitatively similar

raw and corrected IQE curves. The major difference is that the correction attributes much more absorption to the device layer than the simple model perceives, thus reducing the IQEs. Changes in the IQE with composition are very significant because they directly reflect the effects of changing morphology and changing operational mechanisms. Focusing on the corrected IQE data only, Figure 3 shows that, in the region of the porphyrin Q-band at 650 nm, the IQE in the ternary blend exceeds 20%, double that observed for the standard MEH-PPV/PCBM mixture. This increased IQE indicates that light absorbed by the porphyrin Q-band is more efficiently converted into current than the light of similar energy absorbed by the other device components. If the porphyrin merely harvested the light energy and transferred this by exciton transport to the polymer or fullerene for subsequent primary charge separation, then the IQE for light absorbed by the porphyrin could only be less than that for direct polymer or fullerene absorption. This result thus indicates that excited states of the porphyrin initiate primary charge separation. In addition, across the entire 500-750 nm region, the IQE is found to increase significantly in the ternary blend, indicating that the presence of the porphyrin has made the whole device more efficient. It is, however, unclear as to whether this increase is because of improved morphology, additional primary chargeseparation processes, or porphyrin-based improved chargetransport. Figure 4 shows the IV curves of the binary and ternary blend devices under ∼80 mW cm-2 white light illumination that is similar to AM 1.5 solar radiation. This figure shows that, although the ternary film has significantly less polymer content, the short-circuit current produced is comparable to that of the control binary-blend device. This is remarkable given the (apparent) relatively small contribution from direct porphyrin absorption to overall current [see Figure 3] and is a consequence of the higher broad-band IQE of the ternary film relative to the binary film [see Figure 3]. However, the figure also indicates that the open-circuit output voltage, and hence the power efficiency of the device under optimal load conditions, is reduced by ca. 10% in the ternary mixture as compared to the binary one. This indicates that the ternary blend contains higher internal resistance, and hence it is clear that the porphyrins do not act to improve charge-transport mechanisms. Hence, they must improve the broad-band IQE either by improving the morphology for standard MEH-PPV/fullerene primary charge-

Polymer-Fullerene-Porphyrin Bulk-Heterojunction Solar Cells


Figure 6. Vis-Near-Infrared absorption spectra of a pure MEH-PPV film (10:0:0), a PCBM film (0:10:0), and MEH-PPV/PCBM/Cu(CN)4P blended by weight films. All films are approximately 100 nm thick assembled on quartz.

Figure 5. UV-Vis absorption spectra of the materials in (a) chlorobenzene solution, (b) pure film, and (c-d) MEH-PPV/PCBM/Cu(CN)4P blended films by weight; Panel e shows the component absorption from each film that is attributed to PCBM rescaled to equal intensity at 335 nm. All films are approximately 100 nm thick assembled on quartz. Panel b shows observed results for pure MEH-PPV and PCBM films as well as some deconvolved components (Comp) from ternary blend films.

separation mechanisms and/or through new primary chargeseparation mechanisms that directly involve charge-transport from the photoexcited species. c. Device Spectroscopy. Figure 5a presents the UV-Vis absorption profiles of dilute solutions of the component polymer, fullerene, and porphyrin materials dissolved in chlorobenzene. The absorption profile of MEH-PPV covers the 400-550 nm region, peaking at 501 nm, whereas the absorption spectrum of Cu(CN)4P consists of a characteristic Soret peak at 442 nm and a weaker Q-band at 637 nm, with smaller Q-bands present at 730, 597, and 551 nm. The absorption spectrum of PCBM consists of two intense UV transitions at 331 and 287 nm with

a broad and weak visible band commencing with a weak shoulder at 705 nm leading to a peak at 495 nm. Figure 5b presents the corresponding spectra for pure onecomponent polymer and fullerene films of ca. 100 nm thickness on quartz, as well as deconvolved spectra for the porphyrin and polymer components of the blended films (see later). The deconvolved spectrum for the porphyrin is slightly red-shifted and slightly broadened on the high-energy side of the band as compared to the spectrum in dilute solution; the Soret and major Q-band maxima shift to 461 and 651 nm, respectively. Such changes could arise from either the solvation and inhomogeneity in the thin film58 or from “T”-shaped dimer formation59 within the film. The pure-film polymer absorption band is somewhat broader than the solution spectrum, and an additional very flat tail is observed that extends through the infrared region. Further details of this tail are provided in Figure 6 for the 500-2000 nm region; its origin has been assigned to singlet exciton formation rather than to, perhaps, absorption from charged polarons produced by the illumination.60-62 To verify that this band does not arise from light scattering, the scattered intensity was measured and was found to account for only ca. 1% of the nontransmitted light. Most pure polymers used in thin-film photovoltaics possess a similar spectral feature and do produce photocurrent when irradiated.5,15 However, the addition of as little as 1% of C60 can eliminate the tail,15 indicating that it is very sensitive to the morphological structure of the polymer. The PCBM spectra show considerable changes in going from dilute solution to the film, in close analogy to the spectra of C60 itself.63 The two intense UV peaks are clearly split into three at 337, 266, and 218 nm, which is indicative of significant exciton coupling acting between neighboring PCBM molecules.63-65 A broad shoulder at ca. 450 nm is also apparent; this has also been assigned to states resulting from exciton coupling of the UV bands63-65 rather than from intensification of the native molecular 495 nm band. In addition, an intense tail is found at low energy, with sometimes resolvable peaks63 at ca. 620 and 705 nm. Of these, the weak band at 620 nm is known as the lowest-energy transition of pure C60 films and arises through intermolecular interactions.65 The lower-energy band centered at 705 nm (1.76 eV) is not found in pure C60 films (absorption limit66 of 1.72 eV) and arises as a consequences of the chemical linkages formed to the C60 moiety within PCBM. As indicated in Figure 6, absorption by PCBM in films extends all the way to 2000 nm (0.6 eV).


Figure 7. Energy levels with respect to the vacuum level for the materials in the porphyrin blend device, as obtained from measured electrochemical potentials. The HOMO of PCBM (dotted) is quoted from other sources2 because measurement was outside our solvent window; the value for ITO is taken from ref 2, for PEDOT:PSS from ref 53, and for Ca from ref 2.

The observed absorption spectra for 2:8:0 and 1:8:1 blends of MEH-PPV/PCBM/Cu(CN)4P are shown in Figure 5, panels c and d, respectively. In general, these spectra closely resemble the sum of the individual components; this description is quantified in Section 4a. However, a new band appears in the spectra at wavelengths above 700 nm. Figure 6 shows this band in detail; it is centered at ca. 1100 nm (1.2 eV). In principle, this band contains significant information concerning the mechanism of the photovoltaic device operation, but because its assignment is uncertain and because Figure 3 shows that the wavelength region above 700 nm does not significantly contribute to the photocurrent, discussion is deferred to the Supporting Information. d. Device Energetics. Cyclic voltammetry is used to measure the oxidation and reduction potentials of MEH-PPV, PCBM, and Cu(CN)4P and the results, expressed as energy levels with respect to the vacuum, are shown in Figure 7. The observed energies for MEH-PPV and PCBM are in excellent agreement with literature values.2 Processes of primary charge separation involving either electron- or hole-transfer between the polymer and the fullerene require only 1.3 eV of energy, as is well established.1 This value is much less than the optical band gaps of the polymer and the porphyrin and so indicates that primary charge separation is exothermic. Electron-transport is facilitated by the PCBM phase whereas hole-transport is facilitated by the polymer.7 Experiments show that the hole mobility of PPV/ PCBM blends is 2 orders of magnitude higher than that for

Dastoor et al. pristine films of PPV,67 suggesting that hole-transport may also involve the PCBM.68 However, as shown in Figure 7, it costs 1 eV in energy to adiabatically transfer a hole from the polymer to the fullerene so that the fullerene cannot directly contribute to hole-transport; hence, the effect of the PCBM must be primarily morphological. Primary charge separation involving only the polymer requires 1.9 eV of available energy and is a known process,1 but that involving only the fullerene requires 2.3 eV and is not observed other than through high-energy photodetachment.69 The energy levels of the Cu(CN)4P porphyrin are much lower lying than those for most porphyrins. This feature arises from the electron-withdrawing nature of the cyano substituents acting to oppose oxidation and favor reduction. The solution electrochemistry reveals that the lowest unoccupied orbital (LUMO) energy for Cu(CN)4P is 0.1 eV lower than that for PCBM, suggesting that the porphyrin can oxidize the PCBM anion. Because this would inhibit electron-transport to the electrodes, it is clear from the observation of significant photocurrent that this process does not occur in the blended films. Primary chargeseparation processes involving the porphyrin and the fullerene are moved to high energies of ca. 2.0 eV because of the stabilization of the porphyrin highest-occupied molecular orbital (HOMO). This is energy neutral for the hole-transfer process from the 650 nm Q-band of the porphyrin but is endothermic for electron-transfer from the 750 nm band of the fullerene. Porphyrin-to-fullerene primary charge separation is a wellestablished process in dyad molecules,70 but the electronwithdrawing groups in Cu(CN)4P could inhibit it; alternatively, electron-transfer from excited polymers to porphyrins is, in general, not observed because of endothermicity,71 but this process may become allowed for Cu(CN)4P. As the IQE data shown in Figure 3 clearly indicate that excited-state porphyrin does initiate primary charge separation, it is clear from the energy-level diagrams that the only allowed process for porphyrin-induced primary charge separation is porphyrin-tofullerene electron-transfer. e. Use of Alternative Porphyrins. Ternary blend devices have also been fabricated with other porphyrins. Figure 8 presents the uncorrected raw IQE spectra of devices fabricated with zinc metalated, free-base and acid salt porphyrins, fabricated with a weight ratio of 1:4:1 polymer/fullerene/porphyrin by weight. Contributions from the Q-bands of each of these porphyrins at wavelengths above 600 nm can clearly be seen. Thus, the process of charge generation and collection from porphyrins blended into the conventional polymer/fullerene

Figure 8. The IQEs of three devices fabricated with three different porphyrins: (A) diprotonated bistriflate salt of 5,10,15,20-tetraxylyl-21H,23Hporphine (black line), (B) 5,10,15,20-tetraxylyl-21H,23H-porphine (dashed line), and (C) 5,10,15,20-tetraxylylporphyrinatozinc(II) (dotted line).



TABLE 1: Deduced Key Operational Parameters from Spectral Deconvolution for MEH-PPV/PCBM/Cu(CN4)P Blended Polymer Films corrected film absorptiona solution polymer band shapeb

raw film absorption

polymer film band shape with damped tailb

solution polymer band shapeb

polymer film band shape with damped tailb

property

2:8:0

1:8:1

2:8:0

1:8:1

2:8:0

1:8:1

2:8:0

1:8:1

% IQE after MEH-PPV absorptionc % IQE after PCBM absorptiond @ 800 nm % IQE after PCBM absorptiond @ 400 nm % IQE after Cu(CN)4P absorptionc AM 1.5 current (mA cm-2) % AM 1.5 current arising from MEH-PPV absorption % AM 1.5 current arising from PCBM absorption % AM 1.5 current arising from Cu(CN)4P absorption

64

82

56

76

71

101e

66

99

-1

1

-1

1

4

8

3

7

30

32

29

30

40

42

38

38

30

30

40

42

1.2 44

1.3 27

1.2 49

1.3 32

2.4 38

2.7 25

2.4 46

2.7 31

56

58

51

51

62

60

54

50

15

18

15

19

a On the basis of IQE data from Figure 3 obtained using either the raw single-phase active-layer absorption spectrum or that as corrected for the optical properties of the whole device. b Sample source used to estimate the absorption band shape of the polymer in the films; these interpretations span the range of feasible representations. c Assumed to be independent of wavelength. d Assumed to be linearly dependent upon wavelength. e Exceeds maximum possible value of 100% because of uncertainties in analysis.

system is generic and does not depend upon the particular form of the porphyrin. There has been no attempt to optimize the performance of these devices, however, and under slightly different conditions these porphyrins induced macroscopic phase separation to essentially eliminate device functionality. Nevertheless, ternary mixtures involving porphyrins may lead to a new path for overall optimization of performance of fullerene/ polymer photovoltaic devices. 4. Quantitative Analysis of Device Function a. Deconvolution of the Absorption Spectra into Individual Components. Through studying the spectral changes as a function of concentration, it is possible to apportion the observed absorbance into contributions from each individual component. To do this, we assume that (i) the spectral bandshapes of the porphyrin and polymer in the largely PCBM films do not change as a function of concentration, (ii) that the band shape for the fullerene part is identical in the 1:8:1 and 2:8:0 MEH-PPV/ PCBM/Cu(CN)4P blends, and (iii) that only PCBM absorbs at 335 nm. It is clear from Figure 5 that the porphyrin and polymer do absorb slightly at 335 nm, but the errors that use of assumption (iii) introduces are insignificant. These assumptions allow the observed spectra for the 1:8:1 and 2:8:0 blends to be normalized at 335 nmm, and then one is subtracted from the other to remove the absorption from the fullerene to present the porphyrin-minus-polymer difference spectrum (not shown). This difference spectrum is then deconvoluted into components describing the porphyrin and the polymer in the largely PCBM film. Because no unique solution for this process is possible, we introduce spectroscopic models in which the component spectra in the film are described as perturbations of the spectra observed in dilute solution and the spectrum of the pure polymer film. Using the assumption that only small perturbations are feasible, the available solution space for the deconvolution becomes restricted, and we present two possible deconvolutions that appear to span the available space. It is immediately clear that the large tail to long wavelengths observed in the spectrum of the pure polymer film (shown in

Figure 5b) is absent in the component blends; indeed, as little as 1% PCBM in polymer is sufficient to eliminate this tail.15 However, the very simple assumption that the spectrum of the polymer in 10-20% concentration in PCBM is identical to that found in solution (see Figure 5a) leads to a realistic deconvolution; the spectrum for the porphyrin in the polymer is obtained as the porphyrin-minus-polymer difference spectrum plus a scaled contribution from the solution polymer spectrum. This approach forms our first spectroscopic deconvolution model and embodies just one adjustable parameter. The results from this approach are not shown explicitly in the figure because it leads to an unrealistic feature, namely, that the fitted porphyrin absorption at 530 nm is slightly negative, whereas from Figure 5a it should be slightly positive. In our second spectral model, additional parameters were introduced to correct the shortcoming in the first model, as evidenced at 530 nm. The polymer spectrum in the pure film in the 530-570 nm region (the red side of the absorption maximum) is slightly broader than that in solution, and it is plausible that it is also broader in the PCBM film. In this model we assume that the spectrum in PCBM is the same as that observed in pure film at wavelengths below an optimized value of 510 nm and that the spectrum to higher wavelengths is the pure-film spectrum dampened by a Gaussian weighting, optimized with a standard deviation of 50 nm. The relative band intensities are again scaled, so this model involves three adjustable parameters. Shown in Figure 5b are the components attributed to the polymer and the porphyrin obtained using the second model. The fitted porphyrin-component absorption at 530 nm is slightly more intense than that observed in solution (see Figure 5a); this is a nearly equivalent but opposite error to that produced by the first model at that wavelength. Also, in the region of the band maximum of 500 nm through the major absorptive region up to 400 nm, one model assumes that the spectrum in PCBM blends is equivalent to that in solution, whereas the other assumes it is equivalent to that in pure polymer film. Hence, these two models are assumed to span the available realistic


Dastoor et al. and ternary blend films, no light absorption is attributed to the polymer for incident light wavelengths greater than 580 nm, a region that, from Figure 3, is clearly seen to be photovoltaically active. Thus, it is clear that the fullerene in C60-fullerene/polymer solar cells not only absorbs most of the light, but it also initiates a significant fraction of the primary charge separation. Interestingly, although the fact that this process occurs is well-known72 and is utilized to provide enhancements through the use of C70 rather than C60 fullerenes,10,20 its importance for C60-fullerenes/ polymer blends has not previously been recognized. c. The IQE for Light Absorbed by Each Blend Component. The observed IQE shown in Figure 3 arise in the simplest approximation as an absorption-weighted average of more basic quantities associated with photocurrent generation following light absorption by each of the three blend components, that is, via eq 1,

Figure 9. The total percentage of incident light absorbed by MEHPPV/PCBM/Cu(CN)4P blended films resolved into contributions from each individual component. All films are approximately 100 nm thick assembled on quartz.

solution space. Although Figure 5. shows only the results for the damped-film deconvolution, Table 1 gives the results obtained using both procedures so as to quantify the associated degree of uncertainty. The most significant conclusion to be drawn from Figure 5b is that deconvolution produces component spectra that are in very good agreement with expectations based on the observed spectra of the pure polymer and porphyrin. The component of absorption attributed to the fullerene is determined as the total absorption for each blend less the proportionate contributions from polymer and fullerene. Figure 5, panels c and d, shows the total observed spectra and their (damped-film based) partitioned components for the 2:8:0 and 1:8:1 films, respectively, whereas Figure 5e compares the (normalized) PCBM components for these films as well as for 1:3:1 and 1:6:1 films analyzed by the same procedure. The inferred PCBM components for each film are very similar to each other and to the spectrum of the pure PCBM film, indicating again that the component separation procedure has produced physically realistic results. Results not shown for the solution-spectrum based deconvolution are very similar to these. Because the spectrum of pure C60 films is known to be dominated by exciton coupling,63-65 a slight contraction of the bandwidth, as depicted in Figure 5d, is expected as the fullerene becomes diluted with other components. b. The Fraction of Light Absorbed by Each Blend Component. The most striking aspect of Figure 5, panels c and d, is that light absorption over the entire spectral range is dominated by the fullerene, with the polymer contributing only slightly. This observation is contrary to the normal assumptions made for C60-fullerene/polymer solar cells that absorption and primary charge separation are processes dominated by the polymer.2,5,6,25-28 Quantitatively, the actual percentage of the incident light absorbed by each component of the 2:8:0 and 1:8:1 blends are shown in Figure 9. For the optimized binary blend film, the PCBM absorbs the majority of the light at all wavelengths apart from the narrow band of 500-540 nm associated with the peak in the polymer absorption response. This figure also shows that, for the optimized ternary blend, the PCBM again absorbs the majority of the light across the entire spectral range probed except for just at the peak of the porphyrin Q-band absorption at 640 nm. For both the binary

IQE ) fpolymerIQEpolymer + ffullereneIQEfullerene + fporphyrinIQEporphyrin fpolymer + ffullerene + fporphyrin (1) where fpolymer, ffullerene, and fporphyrin are the fractions of light absorbed by the polymer, fullerene, and porphyrin shown in Figure 9. As has been previously argued, it is clear that primary charge separation is directly activated from excited states of all three components; however, the quantities IQEpolymer, etc. in this equation reflect not only direct primary charge-separation processes but also the net effects of any other process such as exciton transfer between blend components prior to primary charge separation. For the 2:8:0 binary mixture, it is clear that photocurrent from all of the light absorbed in the 600-800 nm range arises through hole-transfer (either direct or via internal PCBM charge separation) from excited PCBM to polymer and that the IQE for this process decreases nearly linearly with increasing wavelength. We thus assume that IQEfullerene has this same linear dependence over the entire 375-800 nm range depicted in Figure 3. For IQEpolymer and IQEporphyrin, we introduce the simplest assumption that these functions are excitationenergy independent. The observed data from Figure 3 are then fitted by over-determined linear least-squares minimization to determine values for the four parameters that specify the partial IQE responses. The resultant parameters are given in Table 1, and fitted spectra and spectral components are given in Figure 10. Results are presented for both the 2:8:0 and 1:8:1 blends, using IQE values obtained using either the raw device-layer absorption or the as-corrected absorption for the actual device optics. Very good fits are obtained to the raw IQE data, whereas the peaks in the corrected IQE curves are slightly shifted from those produced after IQE correction. Most significantly, however, Figure 10 shows that the qualitative nature of the results are not affected by the method used to determine the IQEs, whereas Table 1 shows that the primary quantitative effect of detailed modeling of the device absorption is to reduce the IQE after MEH-PPV absorption from 65-100% to 55-80% and to reduce the IQE after PCBM absorption from ca. 40% to 30%. These results indicate that the very simplistic model (eq 1) used to depict the IQEs for the various processes is adequate. In particular, the use of the same IQE values for current generation following light absorption by the porphyrin Q- and Soret bands appears qualitatively reasonable. Although Figure 9 shows results from only the damped-film representation of the polymer absorption profile, Table 1 gives numerical results for both it and the alternate solution repre-



Figure 10. The observed IQEs for MEH-PPV/PCBM/Cu(CN)4P blended films shown in Figure 3 based on raw and corrected active-layer absorption data are fitted using eq 1 as the sum of individual contributions following light absorption by the MEH-PPV, PCBM, and porphyrin components; the parameters extracted from these fits are given in Table 1.

Figure 11. On the basis of the observed adsorptivity and IQE (using raw and corrected active-layer absorption data) for MEH-PPV/PCBM/ Cu(CN)4P blended films, the expected short-circuit photocurrent for AM 1.5 excitation is shown and partitioned into components arising from light absorbed by each of the blend components; the components are taken from the fit to the IQE shown in Figure 10, whereas the current arising as the sum of these components is compared to that as directly obtained using the full observed IQE.

sentation. Numerically, the values obtained by both approaches typically agree to within 5% for the raw IQE data and within 20% for the corrected IQE data, indicating the stability of the numerical analysis to the additional detail of the method. The addition of porphyrin to the binary blend leads to photocurrent following light absorption by the porphyrin at an IQE of 30% (40%) for the corrected (raw) data. In both the binary and ternary blends, the photocurrent generation efficiency following light absorption by the polymer is very similar, near zero at 800 nm and ca. 40% at 400 nm, but the photocurrent generation efficiency following light absorption by the polymer increases from ca. 60% (70%) to 80% (100%) for the corrected (raw) data. Hence, it is clear that the increase in the IQE seen in Figure 3 for the ternary blend is because of an enhancement of the processes that commence with polymer absorption. The possibility that this enhancement arises primarily from polymerto-porphyrin exciton transfer followed by primary charge separation is eliminated because IQEporphyrin is less than IQEpolymer. An active role for the porphyrin in the process could occur via the opening of a new primary charge-transfer mechanism involving excited-polymer to porphyrin electrontransfer. Such a mechanism would naively not be expected to produce overall IQEs near 100%, however, because porphyrin anions are localized species that likely are subject to charge recombination processes before the charge is transferred to the PCBM. Alternatively, the porphyrin may have no direct involvement in the process but may simply improve the

efficiency of the charge collection following polymer-tofullerene electron-transfer through induced morphological changes. d. Contributions to the Photocurrent from Light Absorbed by Each Component Under Simulated AM 1.5 Conditions. Because the absorption of the device and the IQE for photocurrent production are known, it is possible to simulate the shortcircuit photocurrent response following illumination with AM 1.5 radiation, and the results are given in Figure 11 (spectral dependence of current) and Table 1 (total current). For the 2:8:0 and 1:8:1 blends, the simulated total currents are 1.2 (2.4) and 1.3 (2.7) mA cm-2, respectively, using the corrected (raw) IQE data, which are near the observed short-circuit currents of 2.2 mA cm-2 measured for each blend under 80 mW cm-2 whitelight excitation (see Figure 4). Agreement between these quantities is reliant not only on all aspects of the device analysis but also on the match of the white-light source used to the AM 1.5 spectrum. Although the light source used has a similar emission profile to AM 1.5 radiation, it is 2 orders of magnitude weaker in intensity; hence, these results support the notion that photocurrent generation in these devices is roughly proportional to illumination intensity. Also shown in Figure 11 is the current partitioned into contributions from light absorbed on each of the components of the blended films, with the percentage contributions of each component given in Table 1. For the binary blend, the analysis yields a polymer/fullerene current ratio of between 38:62 and 49:51, depending on the assumptions used in the analysis.

15424 J. Phys. Chem. C, Vol. 111, No. 42, 2007 Hence, it is clear that the major fraction of the photocurrent in C60-fullerene/MEH-PPV polymer solar cells is generated subsequent to light absorption by the fullerene. For the 1:8:1 ternary blend, the contribution from the fullerene changes little, whereas that for the polymer decreases to ca. two-thirds its value. As the concentration of polymer is halved in the ternary blend, a greater reduction in this current was anticipated, but the improvement in IQEpolymer noted earlier increases the yield. Light absorbed by the porphyrin produces ca. 15-20% of the total current, with both the Q- and Soret bands contributing equally. The unexpected effectiveness of the weakly absorbing Q-band arises because this transition is in a region of poor absorbance and high photon flux, thus providing broad-band character to the photocurrent generation. 5. Conclusions Although heterojunction solar cells made using fullerene/ polymer blends may lead to economic, efficient plastic devices, development is hindered by the poorly understood and uncontrollable nature of the morphological processes that dominate device performance. We have shown that ternary mixtures involving the addition of porphyrins to the blend form a general class of materials that typically maintain basic functionality, offer insights into basic mechanistic processes, could lead to more efficient devices, and allow broad-band absorption capacity and/or responses to specifically desired illumination wavelengths to be built in to devices. Through variation of the blend composition, a method has been developed for the partitioning of light absorbed by both binary and ternary blends into individual contributions from each of the blend components. This analysis reveals that, at almost all excitation wavelengths, more light is absorbed by the PCBM C60-fullerene in the 1:4 polymer/fullerene blends than is absorbed by the polymer. Further, the spectrum of the polymer component is found to be similar to that of the polymer in dilute solution, that it does not possess the extended long-wavelength contribution typical of pure polymer films. Thus, the fullerene absorbs light at lower energy than does the polymer, indicating that it acts to facilitate primary charge separation via holetransfer to the polymer. The observed sharp increase in IQE for the porphyrin Q-band is interpreted as indicating that primary charge separation by electron-transfer to fullerene also occurs. Because primary charge separation by electron-transfer from excited-state polymer to fullerene is well-known, this indicates that all three components may initiate primary charge separation. The observed IQE was fitted to a four-parameter model, thus allowing a deconvolution of the contributions to the photocurrent generated by each blend component. These parameters were wavelength-independent IQE’s following polymer and porphyrin absorption and a linearly dependent IQE following fullerene absorption. Although the method used to determine the extent of absorption by the active layer in the functioning device quantitatively affected the analysis, all primary qualitative features remained invariant. Solving the electric field and absorption at every point in the multilayered devices led to deduced IQEs for light absorbed by the fullerene ranging from near 0% at 800 nm to 30% at 400 nm, whereas the broad-band IQE following porphyrin absorption was 30%. In the binary mixture, the IQE from the polymer was larger than those at 56-64%, but the addition of porphyrin to the mixture increased this to ca. 76-82%. The use of different porphyrins revealed that the charge generation and collection processes due to porphyrins blended into conventional polymer/fullerene systems is generic and does not appear to depend upon the particular form of the porphyrin.

Dastoor et al. Finally, the observed photocurrent in PCBM C60-fullerene/ MEH-PPV polymer blends was partitioned into contributions arising from light absorbed on each of the blend components. This approach revealed that more current was generated following C60-fullerene absorption than is generated following polymer absorption. Furthermore, replacing half of the polymer in the blend with porphyrin resulted in a slight increase in shortcircuit current under simulated AM 1.5 conditions, despite the rather dilute nature of the polymer and the loss of significant photocurrent because of the reduced polymer absorption. This relative enhancement arose because the porphyrin both increased the IQE for photocurrent generation by the polymer and provided additional broad-band absorption with primary charge separation; in 1:8:1 ternary mixtures of MEH-PPV/PCBM/ Cu(CN)4P under AM1.5 conditions, the short-circuit photocurrent was found to arise 27-32% from polymer absorption, 51-58% from fullerene absorption, and 15-18% from porphyrin absorption. In binary polymer/fullerene blends with low to moderate fullerene concentration the polymer is known to dominate the light absorption, producing primary charge separation by way of electron-transfer to the fullerene;1,13-15 the same processes have been assumed to apply to the 1:4 blends used in optimally efficient binary-blend solar cells.2,5 This mechanism appears inconsistent with modern discoveries of rapid polymer-tofullerene exciton transport and primary charge-separation mechanisms dominated by excited-state fullerene-to-polymer holetransfer observed in MEH-PPV trimer and tetramer dyads with fullerenes.16-18 By demonstrating that fullerenes contribute to primary charge separation in binary and ternary mixtures and by reducing the polymer concentration in fully functional devices to low levels, we have rationalized the results from these previous experiments. This work demonstrates that both polymer and fullerene absorb light and contribute to primary charge separation, with the fullerene contribution being the most important because of its high concentration and despite its intrinsically low efficiency. Ternary blends involving simple porphyrins, in principle, open a new path for the improvement of fullerene/polymer solarcell device efficiency. This improvement may be achieved not just through composition variations but also by modification of the chemical structure of the porphyrin to alter the size and position of its characteristic absorption peaks. Porphyrins with stronger absorbing Q-bands will, in particular, address the problem of low EQEs from the porphyrin Q-bands observed here. Chemical tailoring will also allow for the synthesis of porphyrins to better complement the absorption profile of the polymer, allowing for coverage into the red and infrared end of the solar spectrum. Indeed, by using porphyrin blends with a mixture of porphyrins with different Q-bands it may be possible to fabricate organic solar cells whose spectral response is matched to the solar spectrum. Acknowledgment. We thank the Australian Research Council, the New Zealand Foundation for Research, Science and Technology New Economy Research Fund, and the MacDiarmid Institute for Advanced Materials and Nanotechnology for funding this work. C.R.M. gratefully acknowledges the University of Newcastle for the provision of a research scholarship. We thank Dr. Alison Funston (The University of Melbourne) for helpful discussions. Supporting Information Available: Details of the method used to determine the amount of light absorbed by the active layer in functioning devices,57,73-76 along with key results

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C60-Fullerene

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