C60 Heterojunctions: Band ... - ACS Publications

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J. Phys. Chem. C 2008, 112, 3142-3151

Titanyl Phthalocyanine/C60 Heterojunctions: Band-Edge Offsets and Photovoltaic Device Performance Michael Brumbach, Diogenes Placencia, and Neal R. Armstrong* Department of Chemistry, UniVersity of Arizona, Tucson, Arizona 85721 ReceiVed: September 8, 2007; In Final Form: NoVember 12, 2007

Planar heterojunction organic photovoltaic devices have been created using oxo-titanium phthalocyanine (TiOPc) as the donor layer and fullerene (C60) as the acceptor layer, with comparisons to devices based on copper phthalocyanine (CuPc) as the donor. TiOPc/C60 and CuPc/C60 heterojunctions were first characterized by a combination of UV-photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) to estimate the frontier orbital energy offset (EDHOMO - EALUMO), which is related to the open-circuit photopotential (VOC). A small interface dipole effect was seen at the TiOPc/C60 interface (eD ≈ 0.02 eV), whereas a significant interface dipole was observed for the CuPc/C60 interface (eD ≈ 0.3 eV). On the basis of the work presented here and previously reported electrochemical and UPS/XPS studies, we estimate an EDHOMO - EALUMO energy offset of ca. 1.1 eV for the TiOPc/C60 heterojunction and 0.7 eV for the CuPc/C60 heterojunction. Maximum VOC values observed at room temperature for corresponding planar heterojunction photovoltaic devices were 0.3-0.4 V lower than the energy offset potentials, even at high light intensities, where the maximum VOC, at room temperature, was achieved. TiOPc/C60 heterojunctions offer higher VOC values than CuPc/C60 heterojunctions, but with a lower intrinsic driving force for exciton dissociation (photoinduced charge transfer).

Introduction Organic photovoltaic cells (OPVs) based on heterojunctions of two or more organic thin films, with different electron affinities (EA) and ionization potentials (IP), have recently shown a significant improvement in overall efficiencies for both small molecule (vacuum deposited),1-10 and blended-heterojunction polymer-based devices.11-25 Further enhancements in efficiency of these devices require (i) extension of the absorbance spectra of the hole and electron transport materials to the near-IR region to better overlap with the AM 1.5 solar spectrum; (ii) enhanced exciton diffusion lengths (in excess of 10-20 nm, which are “typical” in both small molecule and polymer systems); (iii) exciton dissociation probabilities at the donor/acceptor interface approaching 100%; and (iv) enhanced hole and electron mobilities in their respective transport layers (mobilities in excess of 1 cm2/volt‚sec are desired). These improvements should be accompanied, where possible, while retaining significant offsets in frontier orbital energies (highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels) between the donor and acceptor layers so as to maximize the open circuit photopotential (VOC) in these cells. Several investigators have recently shown for bulk heterojunction (BHJ) devices that VOC should be linearly related to the frontier orbital energy offsets between the HOMO of the donor and the LUMO of the acceptor (EDHOMO EALUMO).2,8-10,17,19 A correction term may be needed, of approximately 0.3 V for BHJ polymer devices, to account for voltage losses in the device due to large diode quality factors (n), high reverse saturation currents (Jo), low field-dependent mobilities of charge carriers, and voltage losses at the collection electrodes.17-27 If the donor and acceptor layers are doped with * To whom correspondence should be addressed: [email protected].

strong electron acceptors and electron donors, respectively, one can anticipate that these voltages losses can be minimized (i.e., the top and bottom electrode contacts become “pinned” to the condensed phase redox potentials of these doped layers).30,32,33 Rand et al. have recently discussed these same relationships for planar heterojunction devices, suggesting that the upper limit to VOC is EDHOMO - EALUMO, minus the energy needed to dissociate the bound electron-hole pair at the donor/acceptor interface immediately after its formation via photoinduced charge transfer.2,22 The energy required to separate these newly formed charges appears to be critically dependent upon the dielectric constant of the organic layers and their initial separation distance.2,9,10,22 At low temperatures, however, certain planar heterojunction OPVs appear to achieve a VOC which is close to the value predicted by the frontier orbital offsets. For single layer devices, Malliaras et al. suggested that VOC is related to a built-in potential (VBI directly relatable to EDHOMO EALUMO) modified by a term to account for differences in the mobility of either electron (e) or hole (h) charge carriers and the density of photogenerated holes and electrons at the anode and cathode.22,26 For BHJ photovoltaic devices, Blom and coworkers have used continuity equations, including terms for drift and diffusion, to estimate VOC, based on the dissociation probability of an exciton into free carriers, the generation rate of excitons, and a recombination constant for diffusioncontrolled bimolecular recombination.22-24 Variations in VOC within a given materials system could be associated with small changes in recombination processes which arise due to fielddependent currents. There are several factors which can control EDHOMO EALUMO at a donor/acceptor interface, such as the formation of interface dipoles, and charge redistribution across the interface,27-44 which may further limit the attainable VOC and JSC for planar or blended heterojunction OPVs. These factors are considered in this publication, for new heterojunctions based on oxo-titanyl

10.1021/jp0772171 CCC: $40.75 © 2008 American Chemical Society Published on Web 02/06/2008

Titanyl Phthalocyanine/C60 Heterojunctions phthalocyanine and fullerene (TiOPc/C60), and established heterojunctions based on copper phthalocyanine and C60 (CuPc/ C60). Phthalocyanines are often chosen as the electron donor and principal light absorber in planar heterojunction vacuumdeposited OPVs.1-11,16,27,45-49 Copper phthalocyanine and zinc phthalocyanine have often been used as donor materials for OPV investigations, where they have been interfaced with electron acceptors such as perylene-bisimides or C60.1-12,16,46,47 Our previous explorations of photoelectrochemical cells based on di-, tri-, and tetravalent metal phthalocyanines, where an electrolyte serves as the top contact, showed that thin TiOPc films exhibited the highest photoactivity.48,49 TiOPc is known to be a good photoconductor and is routinely studied as a prototypical electrophotographic photoreceptor.48-64 Several known crystalline polymorphs of TiOPc (e.g., amorphous TiOPc, Phase I, Phase II, and the Y Phase) are known to extend the Q-band absorbance well into the near-IR region; these dramatic shifts in Q-band maximum and shape arise as a result of changes in the overlap of transition dipoles in adjacent Pc cores, and some charge-transfer character in the optical transition itself. It should be noted that there appears to be some disagreement in the naming of these phases and their respective Q-band spectral shapes. There are wide variations in the photoelectrical sensitivity of TiOPc films, depending upon which crystal polymorph is dominant.53-64 We know of only one report where TiOPc has been used as a donor layer in an OPV device (the Pc layer was doped with C60), using a perylene-bisimide electron acceptor layer to complete the heterojunction.45 Planar heterojunction cells have recently been reported which are based on the amorphous phase of chloroaluminum phthalocyanine (ClAlPc) interfaced to C60, showing a better near-IR response than CuPc-based OPVs. ClAlPc is another of the nonplanar Pcs which can show extensively red-shifted Q-band spectra.48-64 Previous photoelectron spectroscopy studies39-43 suggest that TiOPc has a higher IP than CuPc or ZnPc, which is predicted to result in higher values of VOC in comparable Pc/C60 OPVs. Consideration of IPs alone or of first oxidation/reduction potentials, however, may not be sufficient to describe the heterojunctions formed between a Pc and an electron acceptor such as C60. There are now several recent reports of frontier orbital offset characterization of planar heterojunctions of TiOPc with various other dyes,39-43 where IP values for dyes like TiOPc are strongly dependent upon the packing architecture in the Pc film, especially where the orientation of adjacent oxotitanium groups is critical. In addition, Knupfer and co-workers have explored the heterojunctions formed between CuPc and C60, using UVphotoelectron spectroscopy/X-ray photoelectron spectroscopy (UPS/XPS) to characterize the frontier orbital offsets.34 They confirmed the lower IP value for CuPc, noted a substantial interface dipole (ca. 0.5 eV) regardless of which organic layer was deposited first, and rationalized the equilibrium achieved between the two organic materials in terms of the “charge neutralization level” concept recently proposed by Kahn and co-workers.37 In this work we explore the frontier orbital offsets, interfacial dipoles, and charge redistribution effects observed at the organic heterojunctions of identically prepared planar heterojunctions of TiOPc/C60 and CuPc/C60. We next introduce the device properties of simple TiOPc/C60 OPVs and compare them with the established device properties of CuPc/C60 OPVs. For the CuPc/C60 heterojunction, EDHOMO - EALUMO values between ca. 0.7 and 0.9 eV were estimated from our UPS results or other

J. Phys. Chem. C, Vol. 112, No. 8, 2008 3143 recent UPS studies,34 with an interface dipole contribution (eD ) 0.3 eV) and little evidence of further charge redistribution at this heterojunction. For the TiOPc/C60 heterojunction, a value for EDHOMO - EALUMO ) ca. 1.1 eV was observed, with a small interface dipole (eD ) 0.02 eV), and shifts in the core-level photoemission spectra which suggest significant charge redistribution in both organic layers beyond the Pc/C60 heterojunction interface. A statistically viable population of OPV devices was characterized using an established multilayer architecture: ITO/ Pc20nm/C6040nm/BCP10nm/Al (BCP ) bathocuproine, an “exciton blocking layer”; ITO ) tin-doped indium oxide), keeping the deposition conditions, film morphologies, etc. as nearly identical as possible.2-7,12,16,27 The current/voltage (J/V) behavior of both TiOPc/C60 and CuPc/C60 devices, deposited on O2-plasmacleaned ITO, can be modeled with a single-diode equivalent circuit.12,13,27 The VOC values for Pc/C60 planar heterojunction devices were increased by an amount proportional to the increase in EDHOMO - EALUMO for CuPc versus TiOPc, with comparable short-circuit photocurrents and fill factors, which leads to a significant increase in the power conversion efficiency in these OPVs. Experimental Methods Commercial ITO was obtained from Colorado Concept Coating, LLC, with a sheet resistance of ca. 15 Ω/0. The films were determined to have a thickness of 100 ( 10 nm with an rms roughness between 1 and 3 nm. X-ray diffraction showed the films to be polycrystalline with a preferred orientation. XPS revealed a concentration ratio of Sn/In of 0.11 ( 0.01. Substrates were vigorously cleaned with a lens cleaning cloth or polishing pad, using diluted Triton X-100, followed by successive ultrasonication in a dilute solution of Triton X-100 followed by 18 MΩ Millipore water, and, last, pure ethanol for at least 10 min in each solution. Samples were dried in a stream of nitrogen and then etched via oxygen plasma cleaning (ca. 10-3 Torr O2, Harrick, model PDC-32G, 60 W) for 15 min. Copper phthalocyanine (CuPc), titanyl phthalocyanine (TiOPc), and bathocuproine (BCP) were obtained from Aldrich. Fullerene (C60) was obtained from MER (Tucson). All chemicals were purified by multiple entrainer sublimation prior to use.27 A Spectral Instruments SI400 Series CCD array UV-vis spectrophotometer was used for absorbance measurements. All atomic force microscopy images were taken with a NanoScope III system (Digital Instruments, Santa Barbara, CA). Images were taken in air in tapping mode with TESP7 oxide-sharpened silicon nitride tips (Digital Instruments). Roughness values were calculated with NanoScope III software (either version 4.22 or 4.43r8). XPS studies were conducted with a Kratos Axis-Ultra X-ray photoelectron spectrometer equipped with a monochromatic Al KR source at 1486.6 eV. For all the data presented here, the analyzed spot size was 300 × 700 µm. Base pressure in the analysis chamber was typically less than 5 × 10-9 Torr. UPS experiments were performed in the Kratos system with He I (21.2 eV) used as the excitation source. A -5 V bias was applied to the sample to allow for collection of the lowest kinetic energy electrons. Sputter-cleaned gold with a work function of 5.1 eV was routinely analyzed for Fermi level calibration. A UHV organic deposition system attached to the Kratos system allowed for the deposition of organic thin films and in vacuo characterization. Organic films were deposited under conditions identical to those used for photovoltaic fabrication.27 In this work, more than 20 devices from several separate deposition runs have been used for characterizing the perfor-

3144 J. Phys. Chem. C, Vol. 112, No. 8, 2008 mance of a specific device type. Representative J/V curves have been selected from the accumulation of data to give JSC, FF (fill factor), and VOC values matching the averaged values obtained from the respective sets of data. Device areas ranged from 0.00785 to 0.0628 cm2. Organic thin films were deposited via vacuum deposition from Knudsen-type cells at a base pressure less than 10-6 to 10-7 Torr. A 10 MHz quartz crystal microbalance (QCM) and a frequency counter (HP 5384A) were used to monitor the rate of deposition. All organic sources were prebaked at ca. 70% of their sublimation temperature for 30 min prior to film growth. Deposition rates of 2, 1.5, and 3 Å/s were used for Pc, C60, and BCP films, respectively. Higher rates of deposition (up to 4 Å/s) for Pc and C60 films did not drastically affect the OPV performance. Low deposition rates (less than 1 Å/s) resulted in devices with poor performance. Vacuum-deposited 100 nm thick Al top contacts were used for all devices, deposited at a rate of 0.9 to 3 Å/s, at a base pressure less than 9 × 10-6 Torr. A 6 MHz QCM and an Inficon deposition monitor were used to monitor Al deposition. Occasionally, TiOPc films were solvent annealed in order to provide a change in morphology and a shift in the near-IR absorbance band (see below) as described in several previous papers.55-64 These experiments involved exposing the freshly deposited TiOPc film to atmospheric pressures of a solvent such as chloroform, for several minutes. The absorbance spectra for these films are discussed here. Device data for these films will be reported in publications to follow. OPV testing was performed in either (1) a N2-filled Vacuum Atmospheres glovebox (model HE-493/MO-5 or (2) an MBraun Lab Master double glovebox, both with less than 1 ppm O2 and H2O). These gloveboxes were connected to our vacuumdeposition apparatus so that OPVs could be transferred to the test chamber without the exposure of the devices to the atmosphere. Current-voltage measurements were made with a Keithley 2400 source meter, and data was recorded by LabView, version 5.1 (National Instruments). Typical scans were from 1.5 to -1.5 V with a step size of 10-20 mV. No difference in J/V behavior was observed as a function of scan direction or scan rate. A CUDA Products Corp. light source with a 250 W quartz halogen lamp (model I-250) was used as the light source. This light was filtered with diffuse and IR filters to limit heating of the OPVs. The spectral output ranged from ca. 900-400 nm, with a maximum at ca. 650 nm. The light intensity arriving at the OPV device was controlled by changing the distance between the light source and the device. In earlier studies, the power output of the light source was routinely checked with a calibrated photodiode. In later studies, the power output of the light source was calibrated with an Apogee PYR-S pyranometer, which was used to estimate incident white light power. We have noted about a 20-30% discrepancy in these power values, and it is possible that the power conversion efficiencies reported here for TiOPc-based OPVs are lower than those that would be seen with a calibrated AM 1.5 light source. Results and Discussion Thin Film Absorbance Spectra. The absorbance spectra of typical CuPc and TiOPc films used in these studies are shown in Figure 1, along with the AM 1.5 solar spectrum.27,65 The absorbance spectra and photoelectrical activity of the different polymorphs of TiOPc have been extensively investigated, and at least nine polymorphs of TiOPc have been identified which are accessible by postdeposition treatments such as thermal and vapor-phase annealing.27,48-64 Vacuum deposition of TiOPc thin

Brumbach et al.

Figure 1. Absorbance spectra for thin films of TiOPc and CuPc, at the thicknesses used in the OPV devices reported here, relative to the AM 1.5 solar spectrum. The peak absorbance of R-phase (“Phase I”) TiOPc (red) in the as-deposited TiOPc films used in this study is redshifted from the spectra seen for the CuPc films (blue). Solvent annealing of the as-deposited TiOPc films shifts the peak absorbance further toward the near-IR region due to partial conversion to the “Phase II” polymorph.27,48-64

films typically leads to either the amorphous phase (a-TiOPc) or the crystalline “Phase I,” with a principle Q-band absorbance which peaks at ca. 730-740 nm (Figure 1).52,57 A “Phase II” TiOPc film can be obtained by solvent vapor annealing of the Phase I films or by careful control of temperature, rate of deposition, and/or substrate selection or preparation, with an extensively broadened and red-shifted absorbance (also shown in Figure 1).48,49 It is important to note that absorbance for “Phase II” films is not observed below the Q-band transition, but rather the energy of this transition has been altered by virtue of changes in the morphology of the Pc, specifically, the positioning of molecules within one molecular diameter of each other.48-64 Photogeneration of carriers has also been investigated for various phases of TiOPc films, with some interesting differences noted between these different polymorphs.53-64 Various polymorphs of CuPc are also known. The R-phase and β-phase polymorphs are most often encountered in vacuumdeposited thin films.46 To keep the CuPc and TiOPc films of a consistent thickness and to allow for reproducible film morphology and OPV device performance, only the as-deposited materials are the focus of the work reported here. It can be seen that for TiOPc and CuPc films of comparable thickness (ca. 28 nm, see also Figure 8) that the TiOPc films have only a slightly better overlap with the AM1.5 solar spectrum versus the CuPc films.27,65 Future studies will focus on OPVs created from TiOPc films which have been converted to the more near-IR absorbing phase(s), using light sources with a spectral distribution which better matches those absorbance bands. Photoelectron Spectroscopic Characterization of Pc/C60 Interfaces. For UPS/XPS studies, C60 layers (ca. 400 Å) were deposited on clean silver surfaces to give a C60 film thick enough to provide the IP and work function of bulk C60 without charge redistribution effects from the C60/silver interface.27-34 Ultrathin phthalocyanine films were then deposited sequentially for determination of work function, IP, and changes in frontier orbital energies for each Pc thickness. Previously described protocols were used to monitor the formation of interface dipoles (shifts in the low kinetic energy (KE) edge of the UVphotoemission spectra), and charge redistribution effects (seen primarily as shifts in core-level photoemission bands) after heterojunction formation.27-31 Figure 2 shows UPS data for comparable TiOPc and CuPc deposition sequences. Photoemis-

Titanyl Phthalocyanine/C60 Heterojunctions

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Figure 2. UPS spectra are shown for (A) TiOPc deposited onto C60 and for (B) CuPc deposited onto C60, in small thickness increments. The HOMO photoemission peak for each phthalocyanine (peak 2) develops as the phthalocyanine thickness increases, while the HOMO photoemission peak for C60 (peak 1) becomes attenuated. The high KE cut-offs for each HOMO peak (used to find IP for each material) are roughly indicated by dashed lines. The spectrum for the thickest CuPc film has been artificially shifted on the BE axis to compensate for charging effects.

Figure 3. XPS core level photoemission data for TiOPc (left: A, C, and E) and CuPc (right: B, D, and F) deposited in small thickness increments onto 400 Å C60 films. The deposition sequence for both phthalocyanines was 2, 6, 14, 30, 62, and 126 Å. A and B are the C(1s) lines, C is the Ti(2p) line, D is the Cu(2p3/2) line, and E and F are the N(1s) photoemission lines. The core levels for the thickest CuPc film have been shifted on the BE scale to align with the rest of the data (see text).

sion from the HOMO peak for C60 is clearly observable until a Pc thickness of ca. 14 Å is reached. The spectra indicate a significant interface dipole formation for the CuPc/C60 interface, but only a small dipole for the TiOPc/C60 interface. There is very little change in spectral line shape observed between the 62 Å thickness and 126 Å thickness phthalocyanine layers indicating that the respective materials have achieved nearly bulk characteristics at those thicknesses as observed by XPS and UPS. XPS core level emission spectra are shown in Figure 3 and were used to monitor charge redistribution effects at the C60/ TiOPc and C60/CuPc interfaces. We used protocols developed previously to monitor shifts of core level photoemission lines, in both the Pc and C60 layers, as the top Pc layers were deposited.27-33 Similar results were obtained whether the N(1s) or Ti(2p) (or Cu(2p) for CuPc) peaks were used; however, the shift in the N(1s) signal was the most reliable for observation of charge redistribution in the Pc layers, as it appeared with good signal-to-noise ratio even at low Pc coverages. The low concentrations of titanium and oxygen, in TiOPc, and copper, in CuPc, make it difficult to accurately identify BE changes relative to initial coverages of those species on C60. The C(1s) spectra is a combination of emission from Pc and C60; however, the spectral envelope can be fit to deconvolve the components arising from each molecular species. The deposition sequence of Pc on top of C60 was selected to aid in the identification of the BE of initial C60 film such that BE shifts of the C(1s) component from C60 could be accurately determined. Shifts of

C(1s) from the Pc films are less accurate since identification of the initial binding energy (BE) of the prominent peak (phthalocyanines show several emissions from two electronically different forms of carbon as well as a shakeup satellite66) is difficult to accurately determine due to the complicated fitting process. Nevertheless, Molodtsova and Knupfer have recently performed both deposition sequences for CuPc/C60 heterojunctions and have concluded that the energy level alignments are the same regardless of deposition sequence.34 In this work, charging was noted for the thickest layers of CuPc (126 Å) during PES analyses; however, charging was not observed during examination of TiOPc layers. The spectra for 126 Å CuPc in Figures 2b, 3b,d, and 3f have been included to show the spectral line shape for a thick layer of the phthalocyanine (corrected on the BE axis for charging), but were not used for charge redistribution measurements. The IPs of C60 films were found to be 6.4 eV, consistent with previously reported values.67-73 Optical band gaps were estimated from thin film absorbance data and were determined to be 2.0, 1.57, and 1.64 eV for C60, TiOPc, and CuPc, respectively, also generally consistent with reported literature spectroscopic or electrochemical values.27-34,67-76 The comparisons of EDHOMO - EALUMO values are, of necessity, based on assumed energy levels for the transport LUMO for the Pc films. Charge transport energy gaps for the phthalocyanines could be estimated from either electrochemical data (the difference between their first oxidation potential and first reduction potential) or direct measurement directly by inverse

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Figure 4. The binding energy shifts of photoemission core level spectra for the incrementally deposited Pc films (TiOPc in A, CuPc in C) of Figures 2 and 3 on 400 Å C60 films are shown. Also shown are the changes in HOMO position and work function for the respective deposition sequences (TiOPc in B, CuPc in D). For TiOPc films (left column), the shifts of N(1s) and Ti(2p) are similar, especially at greater thickness, indicating charge redistribution within the TiOPc, while the overall shift of the C(1s) associated with C60 is about half as much, indicating some charge redistribution within the C60 layer as well. The binding energy shifts of core levels for CuPc (N(1s) and Cu(2p)) and C(1s) from C60 are seen only for the thicker films and are due to charging effects rather than charge redistribution. Charging decreased the apparent work function at CuPc thicknesses greater than 14 Å. The change in HOMO position and work function up to that thickness is therefore best described as an interfacial dipole for the CuPc/C60 interface.

Figure 5. Proposed band-edge offsets for TiOPc/C60 and CuPc/C60 heterojunctions. For TiOPc/C60 heterojunctions, a small interfacial dipole shift of 0.02 eV was determined along with charge redistribution in both the C60 and TiOPc layers, 0.1 and 0.2 eV, respectively. The IPs, 6.4 eV for C60 and 5.2 eV for TiOPc, lead to a HOMO/LUMO offset of EDHOMO - EALUMO ) 1.12 eV at the interface. For the CuPc/ C60 heterojunction, a significant interfacial dipole, 0.3 eV, is observed with no charge redistribution resulting in EDHOMO - EALUMO ) 0.7 eV.

photoemission spectroscopic (IPES) data if available, although significant assumptions are often required.68-75 IPES data for TiOPc is not available; however, Zahn and co-workers recently calculated a transport gap of 2.2 ( 0.2 eV for CuPc (ca. 0.35 eV larger than the optical gap).75 Description of the TiOPc/C60 Interface. The relative BE shifts in the core levels, changes in the HOMO, and changes in work function with TiOPc deposition are summarized in Figures 4 and 5. Core level binding energy shifts are observable within the first several monolayers of TiOPc grown on C60. The N(1s) and Ti(2p) core level photoemission bands exhibit similar BE shifts as a function of TiOPc coverage. The charge redistribution near the TiOPc/C60 interface was estimated to be 0.2 eV in the Pc layer (Figure 5) and ca. 0.1 eV in the C60 layer; these charge redistributions were estimated from the shift in the C(1s) signal

for C60, up to 60 Å thickness of TiOPc where this C(1s) peak disappeared underneath the TiOPc C(1s) emission. From the small shifts in the low KE edge of the UPS data with the first TiOPc deposition, we conclude that only a small interfacial dipole exists at the TiOPc/C60 junction, eD ) 0.02 eV. Thicker layers of TiOPc gave an average IP (obtained from the high KE edge of the HOMO peak) of 5.2 eV.27-33 From the offsets in frontier orbital energies for thick films of TiOPc and C60, we estimate that EDHOMO - EALUMO ) 1.12 eV. The major uncertainty in band-edge offset energies is in EDLUMO - EALUMO where the use of optical band gaps, rather than the transport gaps inferred from IPES data,75 may underestimate this gap by up to 0.35 eV. Description of the CuPc/C60 Interface. No changes in BE were observed for any of the core levels of C60 or CuPc up to ca. 30 Å thickness CuPc (Figure 4). At this thickness all core levels were observed to shift to higher binding energy by exactly the same amount, consistent with charging effects. Repeat experiments revealed similar effects even when thinner C60 films were used as a substrate for CuPc deposition. The work function for bulk CuPc films was, therefore, estimated from ca. 14 Å CuPc films. The IP was not affected by charging, and a value of 4.8 eV was determined for CuPc films. No charge redistribution was observed in the XPS spectra for the deposition of the initial layers of CuPc on C60; however, changes in IP and effective work function were induced by a significant interfacial dipole, eD ) 0.3 eV. Molodtsova and Knupfer observed an interface dipole of ca. 0.5 eV for both the CuPc/C60 and C60/ CuPc interfaces,34 and Kahn and co-workers have noted a similar interface dipole for heterojunctions based on CuPc and perylene dyes with comparable electron affinities to C60.77-79 Similar work function values for CuPc and IPs ranging up to 5.1 eV have been observed, although IP values between 4.8 and 5.0 are more often reported.39,74,75 In Figure 5 we show band-edge offsets from our UPS/XPS data, and a value for EDHOMO EALUMO was calculated to be 0.7 eV for the CuPc/C60 heterojunction. Using the IP values and interface dipoles determined



TABLE 1: Photovoltaic Performance for Pc/C60 Photovoltaic Devices on Oxygen-Plasma-Cleaned ITO for a PINCIDENT of ca. 100 ( 10 mW/cm2 area (mm2) avg CuPca avg TiOPca TiOPc (15 nm) TiOPc (20 nm) TiOPc (28 nm)

3.3-4.3 0.785-6.28 3.94 3.14 2.27

JSC (mA/cm2)

VOC (mV)

4.3 ( 0.3 450 ( 10 4.1 ( 0.5 600 ( 20 5.1 610 4.0 600 3.4 640

FF

PMAX (mW/cm2)

0.57 ( 0.01 0.55 ( 0.05 0.59 0.51 0.46

1.0 ( 0.1 1.4 ( 0.2 1.8 1.2 1.0

a Average of performance characteristics for devices with 20 nm phthalocyanine layers.

from the work of ref 34 the value for EDHOMO - EALUMO was 0.9 eV. The same limitations for estimating EDLUMO - EALUMO exist in the CuPc/C60 system as in the TIOPc/C60 heterojunction. The enhancements in EDHOMO - EALUMO for TiOPc/C60 versus CuPc/C60 heterojunctions are consistent with the recent work of Kera et al. who created CuPc/TiOPc heterojunctions on HOPG substrates, showing a greater IP for TiOPc relative to CuPc.39 These differences in frontier orbital energies are predicted to lead to enhanced values of VOC for the TiOPc-based OPVs; however, it should be noted that the driving force for exciton dissociation, dictated by EDLUMO - EALUMO, is predicted to be smaller in TiOPc/C60 heterojunctions, based on either their optical gap or transport gap energies (assuming transport gaps in CuPc films, determined by IPES, are comparable to those for TiOPc).75 If Marcus relationships control the rates of photoinduced charge separation in these heterojunctions,2,10,17 we might therefore expect an increase in VOC accompanied by a lower probability for photocurrent production. In addition, the charge redistribution effects at the TiOPc/C60 interface appear to add small energy barriers to exciton dissociation (Figure 5), which, if they persist for the heterojunction under illumination, would appear to provide for enhanced rates of recombination at the D/A interface. TiOPc/C60 Photovoltaic Device Performance. All OPV devices were deposited on ITO, which had been detergent/ solvent/O2-plasma cleaned as described in the Experimental Methods.27 The surface preparation of ITO substrates can be critical in determining device behavior, especially for OPVs based on extremely thin light absorbing/hole transport layers such as the phthalocyanines.80,81 Oxygen-plasma cleaning of the ITO substrates, which leads to a relatively carbon-free surface with a high effective work function and good uniformity in its electrical activity,80-87 was carried out immediately prior to device formation for all the OPVs discussed here. For the TiOPc/ C60-based OPVs discussed here, we have explored a minimum of 20-50 devices of comparable area, giving comparable device performance. Table 1 summarizes averaged photovoltaic performance characteristics for CuPc/C60 and TiOPc/C60 devices. There is a range of behavior that can be observed for Pc/C60 solar cells, as has also been observed by reports in the literature, and J/V curves have been selected to most closely represent the characteristics of averaged device performance. Variability in device performance can often be attributed to small changes in the Pc layer thickness, as will be discussed below. Figure 6A shows the J/V behavior for a representative TiOPc/ C60 device in the dark and at light intensities of ca. 15, 35, 100, and 150 mW/cm2. Figure 7 shows the illumination dependence of VOC, JSC, FF, and power conversion efficiency (η) of a typical TiOPc/C60 device, where the Pc layer thickness is thin, ca. 15 nm. VOC increases with light intensity in a logarithmic fashion, as expected from the diode equation (eq 1) solved for VOC (eq 2) and indicated by the dotted line where VOC has been

Figure 6. (A) The J/V characteristics for a TiOPc/C60 OPV device on oxygen-plasma-cleaned ITO are shown at varying light intensities (ca. 0, 15, 35, 100, and 150 mW/cm2). Light intensity values are shown in the figure next to the respective J/V curves. (B) The J/V responses for a similar device at ca. 150 mW/cm2 and at the highest light intensity available with our illumination source (>1 W/cm2, unfiltered) are shown. At 150 mW/cm2, JSC ) 7.9 mA/cm2; VOC ) 0.61 V; FF ) 0.45; RS ) 1.2 Ω‚cm2; and RPL ) 265 Ω‚cm2. At the highest attainable light intensity, JSC ) 62 mA/cm2; VOC ) 0.66 V; FF ) 0.42; RS ) 1.0 Ω‚cm2; and RPL ) 48 Ω‚cm2.

calculated using Jo values from the dark diode, Jph ) JSC for the illuminated diodes, and a constant value of 2 for the diode quality factor (no resistance effects were included in the calculation).2-7,12,13,22 JSC increased in a nearly linear fashion with light intensity as also expected. FF typically exceeded 0.5 at all light intensities and decreased only slightly at higher light intensities; consequently, the power conversion efficiency approached 2% at light intensities of ca. 150 mW/cm2. Identically prepared TiOPc/C60 devices always exhibited a VOC value in excess of 0.6 V for light intensities above ca. 80 mW/cm2. To produce maximum photocurrents and photopotentials from these devices we maximized the incident light at the sample, producing light intensities in excess of 1 W/cm2. Under these conditions (Figure 6B) VOC values as high as ca. 0.66 V were observed with JSC values in excess of 60 mA/cm2.

( (

J ) Jo exp VOC )

) )

V - 1 -Jph nokBT/e

( )

nokBT Jph ln +1 e Jo

(1)

(2)

The J/V performance for TiOPc/C60 devices is quite sensitive to TiOPc film thickness, as shown in Figure 8A and Table 1. Figure 8B shows the visible absorbance spectra for the three OPV devices (obtained in transmission at a location adjacent to the device being tested) showing the thickness variations of the TiOPc layers for the three devices and confirming that these different thicknesses were exclusively R-phase TiOPc. The

3148 J. Phys. Chem. C, Vol. 112, No. 8, 2008

Figure 7. The open circuit potential (VOC, in A), fill factor (FF, in A), short circuit current density (JSC, in B), and efficiency (η, in B) for a TiOPc/C60 photovoltaic (TiOPc layer thickness ) 15 nm) on oxygenplasma-cleaned ITO are shown as a function of light intensity. VOC increases in a nearly logarithmic fashion as indicated by the dotted line (and open squares) where values of VOC have been calculated using the ideal diode equation (see text). Following expected behavior, JSC shows a nearly linear correlation with illumination intensity as indicated by the fitted line.

device with the thinnest TiOPc layer gave the highest FF and JSC values. The optimal TiOPc thickness was found to be 1520 nm, which is likely to be approximately 3 times the exciton diffusion length, and which is similar to the optimal thicknesses determined in other Pc-based planar heterojunction OPVs.1-8 As the TiOPc layer thickness was increased, FF and JSC decreased while VOC was observed to increase. The photocurrent action spectrum, also shown in Figure 8B, closely approximated the Pc absorption of the OPV, for all three Pc thicknesses, indicating that filter effects were not significant for even the thickest TiOPc films. The TiOPc layer is the primary lightabsorbing and charge-creating component in the Q-band spectral region, and the C60 layer appears to add some small photocurrent contribution below ca. 500 nm. The decrease in FF and JSC values with increasing TiOPc thickness can be understood as arising from low exciton diffusion lengths in these Pc filmss excitons formed at distances larger than 3 times the exciton diffusion length do not aid in photocurrent generation, and the photocurrent is decreased by the presence of the additional material.1 It is proposed here that the effect is not solely due to a series resistance, but is coupled to increased recombination of photogenerated carriers since the Coulombic attraction for separated charges is high.1,22 The increase in VOC with increasing Pc thickness has been routinely observed within our own systems and among those in the literature and appears to be a result of increased recombination of photogenerated carriers, which will be the focus of a future work.27,88 Figure S1 in the Supporting Information section summarizes the OPV behavior for similar CuPc/C60 and TiOPc/C60 OPVs. The dependence of VOC on light intensity is comparable for both types of devices where the TiOPc/C60-based devices always had a VOC ca. 0.2 V higher than the VOC in CuPc/C60-based devices. VOC values for TiOPc/C60 devices nearly always exceeded 0.6 V under standard 100 mW/cm2 illumination. This observation is consistent with the differences in offsets in frontier orbital energies EDHOMO - EALUMO as indicated from our UPS studies

Brumbach et al. or electrochemical comparisons of the first oxidation of the Pc versus the first reduction of C60 (Table 2).2,27,34,75 The shortcircuit photocurrents were comparable in both device types; hence, the overall efficiencies of the TiOPc-based OPVs were enhanced relative to the CuPc-based devices. While most organic solar cells exhibit J/V behavior that departs from behavior which can be described by the ideal diode equation and a one diode model, the J/V response for OPVs with the thinnest Pc thicknesses on O2-plasma-etched ITO could be modeled with a single-diode equivalent circuit (eq 3 and Figure 8C).2,7,12,13,19,22-24,27 Equation 3 is a modification of eq 1 to account for the finite shunt (parallel) resistance (RP) and series resistance (RS) and an additional shunt resistance for the changes in RP that occur under illumination (RPL). For most organic photovoltaic devices the separation of “geminate” electron-hole pairs, owing to the low dielectric constant of these materials, is field-dependent, which gives rise to the voltage dependence of the photocurrent response at potentials negative of VOC.4,22-27 This voltage dependence of the photocurrent response can be modeled by inclusion of an additional parallel resistance term, RPL, in eq 3,12 which is substantially lower than RP measured in the dark. Although there may be no real change in shunt pathways of these devices during illumination, comparison of RPL between identically prepared devices provides for some measure of the differences in efficiency of separation and collection of charge carriers under illumination.4,12,22-25

( (

J ) Jo exp

) )

V - JRsA V - JRsA V - JRsA + - Jph -1 + nokBT/e RPA RPLA (3)

A typical TiOPc/C60 device in the dark showed Jo ≈ 4 × 10-8 A/cm2 and no ≈ 2. RS values for these cells were typically less than ca. 0.5 Ω‚cm2. RP was ca. 2 MΩ‚cm2 in the dark, and RPL was typically ca. 600 Ω‚cm2 at illumination intensities of ca. 100 mW/cm2. The reduction of shunt resistance upon illumination lowers FF by ca. 16% and VOC by ca. 10 mV from the predicted ideal J/V curve.27 For thin CuPc/C60 devices (Figure S1, Supporting Information) the J/V behavior was also fit with a single-diode model, and we find on average that Jo ≈ 1 × 10-5 A/cm2 and no ≈ 2 ( 0.3. RS values for these cells were typically ca. 1-3 Ω‚cm2; RP was ca. 10,000 Ω‚cm2 in the dark, and RPL was ca. 1000 Ω‚cm2 under ca. 100 mW/cm2 illumination; i.e., losses of power near VOC are comparable for both CuPc-based and TiOPc-based devices. Malliaras et al. defined a term known as the “compensation potential” at which the dark current and photocurrent are equal.26 They suggested the following relationship between the “compensation potential” (VC) and the built-in potential (VBI) for a single organic layer device26

VC ) VBI -

kT µege2 + µhgh1 ln e µege1 + µhgh2

(4)

where µ represents the mobility of either the electron (e) or hole (h) charge carriers and g represents the density of photogenerated holes and electrons at the anode, 1, and cathode, 2. Under ideal conditions or low temperatures, VC should be equal to VBI. For a single organic layer device, VBI would be given by the difference in work functions of the contacting electrodes; however, for the bilayer devices under consideration here, VBI is be approximated by the frontier orbital offsets (EDHOMO - EALUMO). Figure 8D shows the J/V data for another typical TiOPc/C60 OPV and the difference between photocurrent and dark current



Figure 8. (A) J/V response for TiOPc/C60 OPVs as a function of Pc thickness, on oxygen-plasma-cleaned ITO (ca. 100 mW/cm2). The highest JSC and FF values are obtained for devices with the thinnest TiOPc layers, while the highest VOC value was obtained for the device with the thickest TiOPc layer. (B) Absorbance spectra (solid lines) of the three devices in A along with a typical photocurrent action spectrum (dotted line) confirming that the photocurrent response closely tracks the Q-band absorbance of the Pc layer. (C) The J/V behavior of the OPV device with the thinnest TiOPc layer in A (red line, 15 nm) was fit using an equivalent circuit model with only one diode (circles, eq 3; see text). The change in shunt resistance upon illumination alone is sufficient to model the experimental curve. The “ideal” response of the dark diode offset only by a constant photocurrent (JSC) and without series or shunt resistances (RS, RP, and RPL) is also shown as the solid black line in C. This clearly illustrates that RPL has a large effect on decreasing FF in these bilayer OPV devices. (D) The “compensation potential” (VC) is determined where the difference between the photocurrent and dark current intersects the potential axis.25,26 VC is larger than VOC, as it is not affected by differential charge carrier mobilities in the respective organic thin films and, thus, more closely approximates the built-in potential (VBI).

TABLE 2: Table of O/O′ Heterojunction Potentials for CuPc/C60 and TiOPc/C60a VOC (open circuit, mV) offset potential (mV) E (electrochemical, mV)

CuPc

TiOPc

450 ( 10 700 1190

600 ( 20 1120 1390

aV OC is the open circuit potential obtained from device characterization at ca. 100 mW/cm2. The offset potential was obtained from interface characterization using photoelectron spectroscopy. The electrochemical potential was obtained from estimates of the oxidation potentials of CuPc and TiOPc respective to the reduction potential of C60.

response, which intersects the bias potential axis at VC ) ca. 0.72 V and which is generally always ca. 0.1 V larger than the maximum VOC for these devices. VC values determined for our CuPc/C60 devices were also larger than the observed maximum VOC by ca. 0.1 V. As mentioned in the Introduction, the deviation of VOC from EDHOMO - EALUMO arises from a combination of factors including the differences in mobilities of holes and electrons in their respective films and differences in hole and electron densities on either side of the heterojunction, which could be influenced by charge injection at the contacting electrodes, traps, and/or recombination processes. Low hole mobility is likely to be one critical component causing an asymmetry of carrier concentration profiles in the device, thereby reducing the observed compensation potential from the offset potential. The EXBE must also be subtracted from EDHOMO - EALUMO to approximate VC. Additionally, the state of the ITO electrode on device behavior is well documented and is likely to play a role as voltage drops at the contacting electrodes will also lead to deviations of VOC from ideality. For TiOPc OPVs based on thicker films or OPVs where the ITO pretreatments have been modified, single-diode equivalent circuit models do not succeed in fitting the entire J/V response

under illumination. An additional diode term is needed to adequately model the OPV response since photogenerated carrier recombination becomes a significant contributor to decreased optimal photovoltaic performance.27 From those studies we have quantified photogenerated carrier recombination as a function of applied potential and have concluded that the rates of recombination in such thin Pc/C60 films are very sensitive to slight changes of the Pc (partially manifested in the dielectric constant) which may affect the propensity for separated charges to remain separated and travel toward their respective collecting electrodes.27 These studies are the focus of a future work.88 Conclusions TiOPc/C60 planar heterojunction photovoltaics have been introduced which have the potential for higher VOC values than CuPc/C60 or pentacene/C60 devices and may also provide for enhancing near-IR absorption through various accessible phases of TiOPc.2,12 The VOC values for TiOPc-based OPVs was found to be ca. 0.2 V higher than those for similar CuPc-based devices. The trend in VOC values for the two Pc systems is correlated to the differences in IPs of the phthalocyanines, which manifests itself as a greater EDHOMO - EALUMO offset in the TiOPc (which has a higher IP). UPS/XPS studies showed additional differences in interfacial dipoles and charge redistribution for the two systems studied. While it is unclear exactly how these differences affect device behavior, it has been observed that trivalent phthalocyanines tend to give interfaces with smaller dipoles and larger charge redistribution affects than systems with divalent phthalocyanines such as CuPc.29-46 Additionally, AlClPc has recently been shown to give a greater VOC in AlPcCl/C60 OPVs versus those routinely observed for CuPc.7 JSC values were comparable in TiOPc/C60 or CuPc/C60 devices; however, power conversion efficiencies were enhanced in our TiOPc-based devices due to an increased VOC. The observed lowering of the driving force for exciton dissociation and charge redistribution

3150 J. Phys. Chem. C, Vol. 112, No. 8, 2008 effects at the TiOPc/C60 interface may be a limitation to further enhancements in photocurrent production in R-phase TiOPc OPVs. Enhanced photocurrent production has been recently observed, however, in TiOPc-based devices, when phase changes are induced to enhance their near-IR absorption, and the device properties of these modified materials will be the subject of future reports.27,88 Acknowledgment. This work was supported in part by the National Science Foundation (NRA: CHE-0517963), the Office of Naval Research, and the NSF Science and Technology Center-Materials and Devices for Information Technology DMR-0120967. We gratefully acknowledge frequent discussions of OPV behavior and modeling of that behavior with Seunghyup Yoo, Benoit Domercq, and Bernard Kippelen, Georgia Institute of Technology. Supporting Information Available: Direct comparisons of comparably created CuPc/C60 and TiOPc/C60 OPVs. This information is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Forrest, S. R. Mater. Res. Soc. Bull. 2005, 30, 28-32. (2) Rand, B. P.; Burk, D. P.; Forrest, S. R. Phys. ReV. B: Condens. Matter Mater. Phys. 2007, 75, 115327. (3) Xue, J. G.; Forrest, S. R. J. Appl. Phys. 2004, 95, 1869-1877. (4) Peumans, P.; Forrest, S. R. Chem. Phys. Lett. 2004, 398, 27. (5) Xue, J. G.; Uchida, S.; Rand, B. P.; Forrest, S. R. Appl. Phys. Lett. 2004, 84, 3013-3015. (6) Rand, B. P.; Xue, J.; Uchida, S.; Forrest, S. R. J. Appl. Phys. 2005, 98, 124902. (7) Bailey-Salzman, R. F.; Rand, B. P.; Forrest, S. R. Appl. Phys. Lett. 2007, 91, 013508. (8) Tang, C. W. Appl. Phys. Lett. 1986, 48, 183. (9) Gledhill, S. E.; Scott, B.; Gregg, B. A. J. Mater. Res. 2005, 20, 3167. (10) Gregg, B. A. J. Phys. Chem. B 2003, 107, 4688. (11) Chu, C. W.; Shrotriya, V.; Li, G.; Yang, Y. Appl. Phys. Lett. 2006, 88, 153504. (12) Yoo, S.; Domercq, B.; Kippelen, B. J. Appl. Phys. 2005, 97, 1037061. (13) Green, M. A. Solar cells: operating principles, technology, and system applications; Prentice Hall: Englewood Cliffs, NJ, 1982; pp 62102. (14) Mayer, A. C.; Lloyd, M. T.; Herman, D. J.; Kasen, T. G.; Malliaras, G. G. Appl. Phys. Lett. 2004, 85, 6272. (15) Kim, J. Y.; Bard, A. J. Chem. Phys. Lett. 2004, 383, 11. (16) Armstrong, N. R.; Carter, C.; Donley, C.; Simmonds, A.; Lee, P.; Brumbach, M.; Kippelen, B.; Domercq, B.; Yoo, S. Thin Solid Films 2003, 445, 342. (17) Gunes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. ReV. 2007, 107, 1324. (18) Yang, C. Y.; Hu, J. G.; Heeger, A. J. J. Am. Chem. Soc. 2006, 128, 12007. (19) Scharber, M. C.; Wuhlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A. J.; Brabec, C. L. AdV. Mater. 2006, 18, 789. (20) Shrotriya, V.; Li, G.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. AdV. Funct. Mater. 2006, 16, 2016-2023. (21) Ma, W. L.; Yang, C. Y.; Gong, X.; Lee, K.; Heeger, A. J. AdV. Funct. Mater. 2005, 15, 1617. (22) Blom, P. W. M.; Mihailetchi, V. D.; Koster, L. J. A.; Markov, D. E. AdV. Mater. 2007, 19, 1551-1566. (23) Koster, L. J. A.; Smits, E. C. P.; Mihailetchi, V. D.; Blom, P. W. M. Phys. ReV. B: Condens. Matter Mater. Phys. 2005, 72, 085205. (24) Koster, L. J. A.; Mihailetchi, V. D.; Blom, P. W. M. Appl. Phys. Lett. 2006, 88, 093511. (25) Marsh, R. A.; Groves, C.; Greenham, N. C. J. Appl. Phys. 2007, 101, 083509. (26) Malliaras, G. G.; Salem, J. R.; Brock, P. J.; Scott, J. C. J. Appl. Phys. 1998, 84, 1583. (27) Brumbach, M. Ph.D. Dissertation, University of Arizona, 2007. Alloway, D. Ph.D. Dissertation, University of Arizona, 2007. (28) Alloway, D.; Hofmann, H.; Smith, D. L.; Gruhn, N. E.; Graham, A. L.; Colorado, R.; Wysocki, V. H.; Lee, T. R.; Lee, P. A.; Armstrong, N. R. J. Phys. Chem. B 2003, 107, 11690.

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C60 Heterojunctions: Band ... - ACS Publications

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