ARTICLE pubs.acs.org/JPCC
Highly Photoactive Titanyl Phthalocyanine Polymorphs as Textured Donor Layers in Organic Solar Cells Diogenes Placencia, Weining Wang,† Jeremy Gantz, Judith L. Jenkins, and Neal R. Armstrong* Department of Chemistry & Biochemistry, University of Arizona, Tucson, Arizona 85721, United States
bS Supporting Information ABSTRACT: We present a comparison of the photovoltaic activity of organic solar cells (OPVs) based on vacuum-deposited and solvent-annealed titanyl phthalocyanine (TiOPc) donor layers with C60 as the electron acceptor, where the TiOPc donor layer exists in three different polymorphic forms: TiOPc included the “as-deposited” form, with a Q-band absorbance spectrum reminiscent of the phase I polymorph, and films subjected to solvent annealing which systematically increased the fraction of either the phase II (α-phase) or the phase III (γ-TiOPc) polymorphs. As-deposited TiOPc/C60 heterojunctions showed open-circuit photopotentials (VOC) of ca. 0.65 V, with estimated AM 1.5G efficiencies of ca. 1.4%. Partial conversion of these thin films to their phase II or phase III polymorphs significantly enhanced the short-circuit photocurrent (JSC), as a result of (i) texturing of the TiOPc layers (ca. 100 nm length scales) and (ii) enhancements in near-IR absorptivity/photoelectrical activity. All TiOPc/C60 heterojunctions, characterized by UV photoelectron spectroscopy (UPS), showed that estimated EHOMOPc ELUMOC60 energy differences, which set the upper limit for VOC, are nearly identical for each TiOPc polymorph. Incident and absorbed photon current efficiency measurements (IPCE, APCE) are consistent with previous studies that showed a majority of the photoactivity in these higher order polymorphs deriving from excitonic states created at λmax ≈ 680 and 844 nm for both the phase II and the phase III polymorphs. The near-IR absorbance features (844 nm) in these Pc polymorphs are believed to have substantial charge-transfer (CT) character, leading to enhanced probabilities for photoinduced electron transfer (PIET). APCE measurements of TiOPc/C60 OPVs, however, show that higher photocurrent yields per absorbed photon arise from the higher energy (680 nm) excitonic state. When C70 is used as the electron acceptor, with its higher electron affinity, APCE was increased throughout the visible and near-IR region, now showing a nearly constant APCE across the entire Q-band spectrum of the TiOPc polymorphs, consistent with increased driving force (ELUMOPc ELUMOC60) for PIET involving the lowest energy CT excitonic state. The highest estimated AM 1.5G efficiencies for these textured TiOPc/C60 heterojunctions, with the TiOPc film partially converted to phase II or phase III polymorphs, are 4.5% and 3.5%, respectively.
’ INTRODUCTION Solution-processed and vacuum-deposited organic solar cells (OPVs) continue to see increased efficiencies as the sophistication in material design and processing improves.1 12 There are clear challenges in increasing the interfacial contact area between donor and acceptor phases in the active layer of these OPVs so as to increase the photocurrent yield, overcoming problems associated with small exciton α 3 LD products, where LD is the exciton diffusion length and α absorptivity.13 15 Increasing near-IR absorptivity (above 800 nm) is also critical, which can be accomplished by appropriate choice of low-band-gap polymer, doping of a polymer host with near-IR-absorbing small molecules or semiconductor nanocrystals, or tailoring of the processing conditions to achieve near-IR absorbing polymorphs in either the donor or the acceptor components of the OPV.8,16 19 OPVs based on vacuum-deposited small-molecule donors (e.g., phthalocyanines (Pc), pentacenes, and oligothiophenes paired with fullerene or perylenebisimide (PTCDI) electron acceptors) have achieved efficiencies nearing 5% with opencircuit photopotentials (VOC) between 0.4 and 1.0 V.4,5,20,21 r 2011 American Chemical Society
Many of the trivalent and tetravalent metal Pcs have shown excellent photoconductivity, resulting from absorption of much of the available visible and near-IR regions, leading to sizable photocurrents (JSC) in OPVs based on these donors.7 11 Nanotexturing of a Pc donor layer has been demonstrated by sophisticated modifications to complex vacuum deposition processes, which greatly enhances the interfacial contact area between the donor and the acceptor, thereby enhancing the short-circuit photocurrent (JSC).22,23 Similar nanotexturing and interdigitation of the donor and acceptor layers has been recently achieved using a solution-processed benzoporphyrin donor layer which, subsequent to deposition, is converted to a textured crystalline dye layer.24 This allows for “vectoral charge transport” to be maintained, leads to the significant enhancement of JSC,13 and may retain the contact selectivity seen in planar heterojunction OPVs, where donor and acceptor layers exhibit large energy barriers that keep holes and electrons confined to opposite sides Received: June 3, 2011 Published: August 18, 2011 18873
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The Journal of Physical Chemistry C of the OPV.25 It therefore appears that if an appropriate level of interfacial contact between the donor and the acceptor layers in OPVs is created, while retaining the features of vectoral charge transport, efficiencies in small-molecule OPVs might eventually be competitive with those seen in “bulk heterojunction” polymer/small molecule composite OPVs. Constructing these textured thin films, however, is extremely challenging, especially since complete mixing of the donor and acceptor layers can enhance surface recombination at both upper and lower contacts and may also lower the shunt resistance (RP) described in simple equivalent circuit models of OPVs.26 28 Interlayer films are often used to selectively harvest holes and electrons at opposing contacts to mitigate these problems.26 We have recently shown that solvent annealing of as-deposited titanyl phthalocyanine (TiOPc) and chloroindium phthalocyanine (ClInPc) films can easily lead to texturing, enhanced Pc/C60 interfacial contact areas, and significant enhancements in JSC.7 9 It is well established that the solvent-annealing process textures these Pc films at the same time that their near-IR absorptivity and photoelectrical activity are significantly enhanced.29 39 In this article we establish what appears to be the upper limits to photocurrent enhancement with these solvent-annealing protocols. Several investigators have explored the types of excitonic states created in polymorphs of TiOPc, since the near-IRabsorbing forms of this Pc are attractive as xerographic photoreceptors and charge generation layers. Mizuguchi et al.,40 Popovic and co-workers,41 Kobayashi and co-workers,42 and several other groups have concluded that close Pc Pc contact, which is seen in several trivalent and tetravalent Pcs with sizable internal dipoles along the oxo metal or halo metal bonds, leads to significant charge-transfer (CT) character in the excitonic states associated with their near-IR absorbance bands. Bredas and co-workers further posited from modeling studies that the phase II polymorph of TiOPc, with its extremely close Pc Pc contacts, should also show high charge (hole) mobilities.43 In explorations of various small-molecule and polymer/smallmolecule blended OPVs it has become increasingly clear that the energy differences between the transport LUMO of the donor (TiOPc in these studies) and the transport LUMO of the acceptor (e.g., C60) determines the efficiency of photoinduced electron transfer (PIET).44,45 It is expected that as the electron affinity of the acceptor is increased, relative to the lowest excited state energy of the Pc aggregate, the probability of dissociation of CT excitonic states would increase, and as shown below, we find that OPVs based on these TiOPc polymorphs are a good test of that hypothesis. In this paper we report a thorough study of the phase transformations of TiOPc thin films as donor layers in TiOPc/ C60 heterojunction OPVs using solvents that partially transform these films to the phase II or phase III polymorphs. OPV efficiencies are enhanced relative to our earlier studies of TiOPc/C60 heterojunctions.8 The degree of texturing and phase transformation to either phase II or phase III polymorphs is controlled by the length of exposure of the as-deposited TiOPc films to various solvent vapors. Electron and atomic force microscopies (SEM, AFM) show that varying degrees of film roughening are produced by the solvent-annealing process. Short-circuit photocurrents (JSC) in OPVs based on these higher order polymorphs increases by more than a factor of 2 times in concert with a broadening of the absorption and photocurrent spectral response from 600 to 900 nm. The offsets in frontier orbital energies within these systems, EHOMOTiOPc ELUMOC60,
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estimated from UV photoelectron spectroscopy (UPS) studies, are not significantly impacted by these solvent-annealing procedures,8,9 and the open-circuit photopotential (VOC) is nearly identical for all TiOPc devices. The incident photon current efficiency (IPCE) in OPVs based on solvent-annealed TiOPc films is enhanced by a factor of more than 2 times over the entire Q-band region versus OPVs based on as-deposited Pc films, while the absorbed photon current efficiency (APCE) suggests that the internal efficiency for photocurrent production is higher for the higher energy excitonic states (LUMO + 1) in these Pc films when using C60 as the electron acceptor. Switching to C70 as the electron acceptor, with its higher electron affinity (EA) and higher ELUMOTiOPc ELUMOC70 offset, enhances the photocurrent response for the lowest energy absorption features (LUMO), providing a more constant response in APCE from 600 to 900 nm. Using IPCE spectra, corrected for the solar irradiance in the AM1.5G spectrum, we estimate OPV efficiencies of 4.5% and 3.5% for TiOPc films partially converted to the phase II and phase III polymorphs, respectively. From these studies it appears that even higher OPV efficiencies may be obtainable for more textured and stabilized donor layers. The studies presented in this paper provide guidance in the creation of solution-processable forms of TiOPc (s-TiOPc and related chromophores) for use in emerging OPV platforms.46,47
’ EXPERIMENTAL SECTION ITO Thin-Film Pretreatment. Commercial indium tin oxide-coated glass (Colorado Concept Coating LLC) with an approximate thickness of 150 nm and a sheet resistance of ∼15 ohms per square (measured using a four-point probe conductivity apparatus) was used for all experiments. Substrates of 1 in. 1 in. size were rinsed with absolute ethanol and then patterned with a positive photoresist (Rohm and Haas), followed by the necessary exposure, etching, heating, and photoresist removal. Each patterned ITO substrate was etched with aqua regia at a 3:1 ratio at 120 °C for ca. 30 s. Prior to use each substrate was cleaned and activated by the following approach: (1) scrubbing with a microfiber cloth (Peca Products) and 10% Triton X-100 (Alfa Aesar) for at least 20 s, (2) sonication in 10% Triton X-100 for 15 min, (3) rinsing with nanopure water, (4) sonication in nanopure water for 5 min, (5) rinsing with 200 proof ethanol (Decon Laboratories Inc.), and (6) sonication in 200 proof ethanol for 15 min. Device Fabrication. Prior to being introduced into the vacuum system, cleaned ITO substrates were removed from ethanol and dried under a stream of nitrogen, followed by an RF-generated plasma cleaning (Harrick, model PDC-32G) for 10 min at 10.5 W and an operating pressure of 400 mTorr in compressed dry air. Titanyl phthalocyanine and C60 (Aldrich, MER Corp.) were purified via vacuum sublimation at least three times, while bathocuproine (BCP, Aldrich) was sublimed twice prior to use. Aluminum top contacts (Alfa Aesar) were deposited with 99.999%-rated material. All organic materials were thoroughly degassed prior to deposition. System base pressure was below 8.0 10 7 Torr prior to fabrication. Thin films were deposited at a rate of approximately 1 Å s 1, measured with a 10 MHz quartz crystal microbalance (QCM-Newark) and a frequency monitor (Agilent, model 53131A). Solvent annealing on different thicknesses of as-deposited TiOPc were carried out by placing the thin films faced up in closed containers (Nalgene 4 oz., 18874
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Figure 1. (A) Visible spectra of four of the most studied polymorphs of TiOPc: the as-deposited and phase I (β-phase) (which give nearly identical spectra), phase II (α-phase), and phase III (γ-phase) TiOPc. (B) Visible spectra of 1 monolayer of TiOPc on an air-plasma-cleaned ITO before (1) and after (2) chloroform solvent annealing. X-ray crystal structures of the monoclinic phase I (C, along the b axis) and the triclinic phase II polymorph (D, along the b axis). The convex pairs and concave pairs are highlighted.
125 mL polypropylene) with either methanol (1 h) or chloroform (3 h) in a small glass vial, as described elsewhere.8,9 Top contacts were deposited via thermal evaporation and measured with a 6 MHz QCM (Tangydine) and a frequency monitor (Inficon, model 758-500-G1). Current voltage (J/V) and incident photon current efficiency (IPCE) measurements were obtained for at least six devices per substrate, with a 10 15% variation in photocurrent produced and almost no detectable difference in photopotential; the bestperforming devices are shown in this publication. The geometry of the masking system was provided for a device active area of 0.125 cm2. Devices were further apertured during data acquisition to ensure that only this area was illuminated. J/V measurements were carried out in a nitrogen-filled glovebox (MBraun, Model LabMaster) with water and oxygen levels 2 times) are seen with increasing fraction of phase II or phase III-like polymorphs, where 60% phase X showed the highest photocurrents. These increases in JSC are coupled with changes in both open-circuit potentials and dark-current values (C and D, see Table 1).
Linear J/V curves for OPVs with varying degrees of conversion from as-deposited TiOPc films to either phase II (Figure 3A) or phase III-like (Figure 3B) polymorphs are shown in Figure 3, while Figure 3C and 3D show the semilog plots of only the dark current. The OPV device parameters are summarized in Table 1 with further information presented in the Supporting Information. The reverse saturation current, J0, which helps to control VOC,44,63 66 was ca. 3.8 10 8 mA/cm2 for the OPVs based on as-deposited TiOPc, consistent with formation of a conformal Pc film with low pinhole density and a large energetic barrier to dark charge injection into the C60 phase.64 Studies in progress which will be reported elsewhere show that these types of conformal films, with good device performance, are only obtained with properly oxygen- or air-plasma-etched ITO surfaces and that contamination or chemical modification which disrupts the wetting of the Pc/ITO interface can critically impact device performance. As the percentage of either the phase II or the phase III-like polymorph increases, the photocurrent increases but only up to the point where ca. 60% of the as-deposited TiOPc thin film was converted to either the phase II or the phase III-like polymorph. Above that conversion percentage JSC begins to decline slightly. It is hypothesized that at 60% of phase X the combination
between high surface area at the donor/acceptor interface and low pinhole density throughout the device is optimized. J0 increases by nearly 100 times after any solvent-annealing pretreatment, which may arise from (i) adventitious doping of these films, (ii) an increase in pinhole density in the donor layer, and/ or (iii) an increase in probability for dark charge transfer to the acceptor phase as a result of the increase in interfacial contact area.65,66 Small decreases in VOC were observed for phase II or phase III-like devices versus as-deposited TiOPc devices, where the biggest changes were observed at about 40% of phase X. An exception was noted in the 20% phase III-like device, where an abnormally low photocurrent and open-circuit potential was seen. These devices were created and characterized several times in order to confirm that the data shown is representative of the best-performing devices and reproducible. IPCE and APCE Measurements. Absorbance, incident photon current efficiency (IPCE), and absorbed photon current efficiency (APCE) spectra are useful in identifying which optical transitions in the Q-band region lead to enhanced OPV performance as a way of estimating internal (absorbed light) efficiencies for each spectral region and absolute OPV efficiency under AM1.5 illumination conditions. Figure 4A and 4B show the absorbance spectra of thin films of as-deposited TiOPc during 18878
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Table 1. Device Performance Parameters for the TiOPc/C60 OPVs Shown in Figure 3 as a Function of the Percentage of Either Phase II or Phase III-Like Polymorph within the Donor Layer type phase II polymorph
phase III polymorph
a
phase (%)
VOC [V]a
JSC [mA/cm2]b
FFc
J0 [mA/cm2]d
JSC-AM1.5G [ mA/cm2]e
ηAM1.5Gf
0
0.65
6.8
0.45
3.8 10
8
4.9
1.4
20 40
0.59 0.58
11.5 14.9
0.51 0.49
1.2 10 4.6 10
6
9.0 12.3
2.7 3.5
60
0.60
16.7
0.57
4.4 10
6
12.8
4.4
80
0.59
15.5
0.59
2.0 10
6
13.0
4.5
100
0.62
14.0
0.55
1.2 10
6
11.6
3.9
0
0.65
6.8
0.45
3.8 10
8
4.9
1.4
20
0.50
6.5
0.38
1.6 10
6
3.9
0.74
7.5
2.0
10.5 9.3
3.5 2.9
7.7
2.3
6
40
0.54
11.3
0.48
3.7 10
5
60 80
0.55 0.55
13.3 11.7
0.61 0.56
1.9 10 9.0 10
6
100
0.53
0.56
1.5 10
7
b
11.1 c
7
d
Open-circuit photopotential. Short-circuit photocurrent. Fill factor. Reverse saturation current (estimated from lowest dark current at 0 V; log plots). e Predicted photocurrent from IPCE measurements based on AM 1.5G spectral irradiance. f Predicted power conversion efficiency (using measured open-circuit photovoltage, fill factor, and calculated short-circuit photocurrent. Under AM 1.5G illumination.
conversion to phase II and phase III-like films, respectively. The as-deposited TiOPc thin films show a clear peak at ca. 738 nm, which is diminished in favor of an absorbance feature at ca. 854 nm after nearly full conversion to the phase II polymorph. Even for the as-deposited thin film there is still significant tailing of the absorbance up to 1050 nm. For ca. 60% conversion of these thin films to the phase II polymorph the absorbance spectra confirm the appreciable remaining concentration of the asdeposited species. The differences between these spectra and those for 1 2 monolayer coverage TiOPc films (Figure 1A) are striking. Using methanol as the annealing solvent promotes formation of the phase III polymorph, although complete conversion is not seen to the same extent as for chloroform-annealed films. The near-IR absorbance (CT exciton) feature is not as sharp as for the phase II polymorph, and the concentrations of residual asdeposited absorbance features in the fully annealed thin film are appreciable. IPCE measurements for properly apertured OPVs8,67 based on as-deposited TiOPc thin films and mixtures of the higher order polymorphs and as-deposited TiOPc are shown in Figure 4C and 4D. The IPCE spectrum for as-deposited TiOPc/C60 OPVs is not as sharp as the absorbance spectrum and shows only a small response beyond ca. 850 nm.7 9 As the percentage of phase X increases, the overall photocurrent increases dramatically even at low conversion percentages along with greatly enhanced responses in the 800 850 nm region. The separation between optical transitions previously associated with excitation to the LUMO (ca. 844 nm) and LUMO+1 level (ca. 686 nm) is clearly seen, with some overlapping photoactivity from the as-deposited material. IPCE values were largest when phase II or phase III-like polymorphs made up ca. 60% of the total Pc in the film. The maximum values for IPCE in these studies (ca. 0.6 at ca. 850 nm) exceed those seen in our previous studies of TiOPc-based OPVs (0.3), which we attribute to better control of the texturing of the Pc layer and enhanced interfacial contact at the Pc/C60 interface. Using IPCE spectra, corrected for the solar irradiance in the AM1.5G spectrum,8,67 we estimate OPV efficiencies of ca. 1.5% for OPVs based on as-deposited TiOPc and 4.5% and 3.5% for TiOPc films partially converted to the phase II and phase III polymorphs, respectively. These estimated efficiencies could
clearly be as high as ca. 6% if the degree of texturing were further enhanced without sacrificing other important OPV device characteristics. APCE data were normalized to the maximum value observed for 60% phase II and are shown in Figure 4E and 4F. These data confirm the trends seen in the IPCE data inasmuch as overall APCE increases with the increased conversion of the TiOPc film to phase II or phase III-like polymorphs, i.e., texturing of the Pc/ C60 interface increases the internal efficiency for photocurrent production. APCE responses also suggest that the higher energy absorbance features near 650 nm lead to greater photocurrent yields versus the absorbance features near 850 900 nm.9,68 Previous studies of excited state lifetimes and xerographic response, where the TiOPc polymorph is used to photoinject holes into a host polymer, have shown that the near-IR absorbance feature correlated with formation of the CT excitonic state is more photoelectrically active versus the absorbance feature centered around 650 nm.32 34,40,41,44,60 62,69,70 These earlier studies suggested that the excitonic states formed at either 650 or 850 nm show 1 100 ps lifetimes and both radiative and nonradiative decay channels, which finally populate the lowest energy CT excitonic state. Estimating the energy of this state is therefore a critical component of determining which polymorphic state is likely to generate more charge carriers in the OPV. UV Photoelectron Spectroscopy Investigations of TiOPc Polymorphs. Figure 5 shows the low-kinetic energy (KE; Figure 5A, 5C, and 5E) and high-KE (Figure 5B, 5D, and 5F) regions in UPS spectra for all three TiOPc polymorphs. The thin films were deposited on clean Au substrates (also shown), while C60 was systematically added to form the heterojunction. Au was chosen as a substrate for these experiments since it is easier to maintain electronic equilibrium with the photoelectron spectrometer; however, equivalent results have been obtained using ITO as the substrate. In the case of both phase II and phase III-like thin films, the shifts in the low-KE edge, versus the as-deposited TiOPc film, were significant, consistent with shifts in the local vacuum level as a result of the change in TiOPc crystal structure, as expected from molecules with a strong internal dipole, and has been observed in several previous UPS studies of TiOPc films on ordered substrates.56,71 73 Calculating the ionization potential (IP) for 18879
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Figure 4. Absorbance and incident-photon and absorbed-photon current efficiency (IPCE, APCE) measurements for as-deposited/phase X (where X = II or III-like) diodes. (A and B) As-deposited to phase X transition, which shows an increase in the near-IR region ca. 844 nm. (C and D) As-deposited/ phase X IPCE measurements showing the increase in photoactivity as the percentage of phase X increases up to 60% and decreasing thereafter. As the fraction of phase X increases, strong response within the 650 700 nm region is observed. As-deposited/phase X APCE measurements (E and F) normalized to the highest value within the 60% phase II diode. Results show that the transition within 650 700 nm is that which has the highest APCE value relative to every other region within the spectra.
all three systems yields values of 5.2, 5.3, and 5.4 eV (all (0.1 eV) for as-deposited, phase III-like, and phase II polymorphs, respectively. Subsequent addition of C60 to form TiOPc/C60 heterojunctions additionally shifts the low-KE edge energies, suggesting further small changes in the local vacuum level.7 9,48,56 Proposed band-edge offsets are shown in Figure 6 for the phase II TiOPc/ C60 heterojunction and in the Supporting Information for the
other two heterojunctions. Using inverse photoemission spectroscopy (IPES) studies to estimate the transport LUMO energy for C60,74,75 for each TiOPc/C60 heterojunction the EHOMOTiOPc ELUMOC60 gaps range from 1.1 to 1.3 eV. The observed VOC for these heterojunctions is significantly smaller than these energy offsets. We have recently shown that VOC in comparable heterojunctions based on chloro indium Pc (ClInPc)/C60, with low values of reverse saturation current, can be as high as 0.8 V, but 18880
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Figure 5. Low- and high-kinetic energy edge regions in the UPS data for (A,B) as-deposited, (C,D) phase II, and (E,F) phase III-like TiOPc films with varying amounts of subsequently deposited C60.
these values are still 0.4 V less than the maximum predicted by these energy offsets. Having established these energy offsets, we can now estimate the differences in driving force for exciton dissociation for each of the excited states in the as-deposited and phase II TiOPc films (shown schematically in Figure 7). Lacking IPES data for TiOPc films we use previous IPES studies of Pcs and optical data for TiOPc to estimate transport LUMO energies.74 For as-deposited TiOPc (or its nearly equivalent phase I polymorph) we estimate that the peak of the Q-band absorbance at 738 nm creates an excitonic state with an approximate energy offset versus
ELUMOC60 of ca. 0.6 eV. For the Q-band features for the phase II polymorph of TiOPc, the Frenkel-like excitonic feature at 686 nm, and the CT excitonic feature at 844 nm, these energy offsets with respect to ELUMOC60 are estimated to be ca. 0.5 and 0.2 eV, respectively. Estimates of the excess free energy needed to drive exciton dissociation and charge carrier formation at the Pc/ C60 interface have been made by Perez et al.44,45 using a Marcuslike approach to predicting relative rates of charge transfer as an exponential function of ELUMODONOR ELUMOACCEPTOR, and using this approach we would expect that charge generation efficiency for the excitonic features in phase II TiOPc films, 18881
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The Journal of Physical Chemistry C at 686 and 844 nm, would differ by a factor of ca. 2 times in favor of the higher energy feature. This assumption is also coupled with the expectation that radiative and radiationless energy transfer from the higher energy excitonic state is slower than electron transfer to the acceptor, given the short lifetimes of these higher energy excitonic states. This leads to the conclusion that some energy transfer to the lower energy excitonic state, from which either luminescence or electron transfer events
Figure 6. Band-edge offsets proposed for the TiOPc phase II/C60 heterojunction from UPS studies here and other IPES studies of C60 films (ref 75 and references therein).
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can occur, is taking place.32,34,51 These estimates also do not account for the fact that contact of the C60 acceptor phase is not likely to be uniform across the entire donor/acceptor interfacial region in these textured films, where as-deposited TiOPc is added on top of the textured film prior to deposition of the acceptor layer. From the APCE data for the fully converted phase II TiOPcbased OPVs, it is clear that the higher energy (Frenkel) excitonic state is more efficient in photocurrent formation than the lower energy CT excitonic features by a factor of almost 2 times.44,45 The photocurrent yield clearly does not match the expected differences based on absorbance spectral changes alone, and it appears that excitonic features at ca. 900 nm and above may have insufficient energy to form mobile charge carriers at the Pc/C60 interface. When the electron acceptor in these OPVs was switched from C60 to C70, with all other deposition parameters held constant, for both as-deposited and phase II TiOPc films (data shown in Supporting Information), both the maximum values in IPCE and APCE are increased by additional factors of ca. 1.3 tmies, and normalized APCE plots now show more nearly constant internal efficiencies from 650 to 900 nm, suggesting that the increased electron affinity of C70 (ca. 0.1 0.2 eV higher than C60)45,76 makes the ELUMODONOR ELUMOACCEPTOR offset sufficient to produce photocurrent for excitons formed at all significant Q-band absorbance wavelengths with larger increases seen in the near-IR transition of phase II TiOPc.
Figure 7. Hypothetical band-edge offsets at the TiOPc/C60 heterojunction, taking into account the specific optical transitions that take place in the TiOPc film. As-deposited TiOPc has its Q band ca. 738 nm (LUMO) (2) with a low rate of exciton dissociation (K1 leading to ca. 20% maximum for IPCE). Phase II and phase III-like TiOPc are dominated by transitions with lower and higher relative excess free energies for PIET compared to asdeposited TiOPc (1 and 3). From IPCE measurements, the rates of exciton dissociation for both transitions (K2, K3) are predicted to be significantly higher leading to a maximum IPCE of ca. 60% due to both texturing of the Pc/C60 interface and in the case of the 686 nm feature a higher energetic driving force for PIET. Changing the electron acceptor to C70, with it higher electron affinity, makes the internal efficiencies for PIET nearly equal across the entire Q-band spectral response, i.e., electron transfer rates exceed rates for other energy loss processes. 18882
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’ CONCLUSIONS We have shown that the polymorphic structure and texturing of TiOPc donor layers in TiOPc/C60 OPVs plays a significant role in determining short-circuit photocurrent yield and overall OPV performance. Systematically increasing the fraction of either the phase II (α-phase) or the phase III (γ-TiOPc) polymorphs increased the photocurrent yield in TiOPc/C60 OPVs by over 2 times and increased OPV efficiencies from ca. 1.4% to 4.5%. Interestingly, it appears that CT excitons formed at ca. 900 nm and above in these Pc films do not possess sufficient energy to dissociate efficiently into charge carriers using an electron acceptor like C60 and that careful attention must be paid to the choice of electron acceptor in future studies using these near-IR-absorbing Pc polymorphs, perhaps at the cost of a smaller VOC.10,45 It would also appear that further texturing of the Pc layers, while retaining sufficient wetting of the hole-harvesting contact to keep RP high, may further increase device efficiency to ca. 6%. Such texturing by solvent annealing alone is challenging and will likely require electron-blocking, hole-selective interlayers on the hole-harvesting contact to ensure that contact with the acceptor layer and surface recombination does not dominate device performance.26 28 These studies also play a significant role in the design of solution-processable forms of this TiOPc (s-TiOPc),46,47 where it has been shown that the near-IR CT excitonic features can also be formed, for both planar (s-TiOPc/ C60) and blended heterojunction (s-TiOPc/PCBM) OPVs, and where the near-IR CT exciton band contributes disproportionally to the photocurrent response.47 ’ ASSOCIATED CONTENT
bS
Supporting Information. Additional experimental details, SEM micrographs, band edge offsets from UPS data, and APCE data for TiOPc/C70 heterojunctions. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Present Addresses †
Department of Physics, Seton Hall University, South Orange, New Jersey 07079, United States. E-mail:
[email protected].
’ ACKNOWLEDGMENT We acknowledge funding for this project from the NSF Science and Technology Center-Materials and Devices for Information Technology – DMR-0120967 (salary support for D.P.), the Office of Naval Research (salary support for W.W.), and as part of the Center for Interface Science: Solar Electric Materials, an Energy Frontier Research Center funded the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award Number DE-SC0001084 (salary support for J.L.J., J.G. and N.R.A.). ’ REFERENCES (1) Park, S. H.; Roy, A.; Beaupre, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Nat. Photonics 2009, 3, 297–U295.
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(2) Chen, H. Y.; Hou, J. H.; Zhang, S. Q.; Liang, Y. Y.; Yang, G. W.; Yang, Y.; Yu, L. P.; Wu, Y.; Li, G. Nat. Photonics 2009, 3, 649–653. (3) Dennler, G.; Scharber, M. C.; Brabec, C. J. Adv. Mater. 2009, 21, 1323–1338. (4) Wynands, D.; Mannig, B.; Riede, M.; Leo, K.; Brier, E.; Reinold, E.; Bauerle, P. J. Appl. Phys. 2009, 106, 5. (5) Riede, M.; Mueller, T.; Tress, W.; Schueppel, R.; Leo, K. Nanotechnology 2008, 19, 12. (6) Walzer, K.; M., B.; Pfeiffer., M.; Leo, K. Chem. Rev. 2007, 107, 1233–1271. (7) Brumbach, M.; Placencia, D.; Armstrong, N. R. J. Phys. Chem. C 2008, 112, 3142–3151. (8) Placencia, D.; Wang, W.; Shallcross, R. C.; Nebesny, K. W.; Brumbach, M.; Armstrong, N. R. Adv. Funct. Mater. 2009, 19, 1913–1921. (9) Wang, W.; Placencia, D.; Armstrong, N. R. Org. Electron. 2011, 12, 383–393. (10) Heremans, P.; Cheyns, D.; Rand, B. P. Acc. Chem. Res. 2009, 42, 1740–1747. (11) Vasseur, K.; Rand, B. P.; Cheyns, D.; Froyen, L.; Heremans, P. Chem. Mater. 2010, 23, 886–895. (12) Hains, A. W.; Liang, Z. Q.; Woodhouse, M. A.; Gregg, B. A. Chem. Rev. 2010, 110, 6689–6735. (13) Gunes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. Rev. 2007, 107, 1324–1338. (14) Li, L. Q.; Tang, Q. X.; Li, H. X.; Yang, X. D.; Hu, W. P.; Song, Y. B.; Shuai, Z. G.; Xu, W.; Liu, Y. Q.; Zhu, D. B. Adv. Mater. 2007, 19, 2613–+. (15) Forrest, S. R. MRS Bull. 2005, 30, 28–32. (16) Dang, X. D.; Tamayo, A. B.; Seo, J.; Hoven, C. V.; Walker, B.; Nguyen, T. Q. Adv. Funct. Mater. 2010, 20, 3314–3321. (17) Walker, B.; Kim, C.; Nguyen, T. Q. Chem. Mater. 2011, 23, 470–482. (18) Habas, S. E.; Platt, H. A. S.; van Hest, M.; Ginley, D. S. Chem. Rev. 2010, 110, 6571–6594. (19) Dayal, S.; Reese, M. O.; Ferguson, A. J.; Ginley, D. S.; Rumbles, G.; Kopidakis, N. Adv. Funct. Mater. 2010, 20, 2629–2635. (20) Chauhan, V.; Hatton, R.; Sullivan, P.; Jones, T.; Wan Cho, S.; Piper, L.; deMasi, A.; Smith, K. J. Mater. Chem. 2010, 20, 1173–1178. (21) Pegg, L. J.; Schumann, S.; Hatton, R. A. ACS Nano 2010, 4, 5671–5678. (22) Li, N.; Forrest, S. R. Appl. Phys. Lett. 2009, 95, 3. (23) Yang, F.; Forrest, S. R. ACS Nano 2008, 2, 1022–1032. (24) Matsuo, Y.; Sato, Y.; Niinomi, T.; Soga, I.; Tanaka, H.; Nakamura, E. J. Am. Chem. Soc. 2009, 131, 16048–+. (25) Tang, C. W. Appl. Phys. Lett. 1986, 48, 183–185. (26) Ratcliff, E. L.; Zacher, B.; Armstrong, N. R. J. Phys. Chem. Lett. 2011, 1337–1350. (27) Wagenpfahl, A.; Deibel, C.; Dyakonov, V. IEEE J. Sel. Top. Quantum Electron. 2010, 16, 1759–1763. (28) Wagenpfahl, A.; Rauh, D.; Binder, M.; Deibel, C.; Dyakonov, V. Phys. Rev. B 2010, 82, 115306. (29) Coppede, N.; Castriota, M.; Cazzanelli, E.; Forti, S.; Tarabella, G.; Toccoli, T.; Walzer, K.; Iannotta, S. J. Phys. Chem. C 2010, 114, 7038–7044. (30) Dienel, T.; Forker, R.; Leo, K.; Fritz, T. J. Phys. Chem. C 2007, 111, 14593–14596. (31) Coppede, N.; Toccoli, T.; Pallaoro, A.; Siviero, F.; Walzer, K.; Castriota, M.; Cazzanelli, E.; Iannotta, S. J. Phys. Chem. A 2007, 111, 12550–12558. (32) Yamaguchi, S.; Sasaki, Y. Chem. Phys. Lett. 2000, 323, 35–42. (33) Gulbinas, V. Chem. Phys. 2000, 261, 469–479. (34) Yamaguchi, S.; Sasaki, Y. J. Phys. Chem. B 1999, 103, 6835–6838. (35) Mascaro, D. J.; Thompson, M. E.; Smith, H. I.; Bulovic, V. Org. Electron. 2005, 6, 211–220. (36) Del Cano, T.; Parra, V.; Rodriguez-Mendez, M. L.; Aroca, R. F.; De Saja, J. A. Appl. Surf. Sci. 2005, 246, 327–333. (37) Del Cano, T.; Duff, J.; Aroca, R. Appl. Spectrosc. 2002, 56, 744–750. 18883
dx.doi.org/10.1021/jp2052085 |J. Phys. Chem. C 2011, 115, 18873–18884
The Journal of Physical Chemistry C (38) Conboy, J. C.; Olson, E. J. C.; Adams, D. M.; Kerimo, J.; Zaban, A.; Gregg, B. A.; Barbara, P. F. J. Phys. Chem. B 1998, 102, 4516–4525. (39) Adams, D. M.; Kerimo, J.; Olson, E. J. C.; Zaban, A.; Gregg, B. A.; Barbara, P. F. J. Am. Chem. Soc. 1997, 119, 10608–10619. (40) Mizuguchi, J.; Rihs, G.; Karfunkel, H. R. J. Phys. Chem. 1995, 99, 16217–16227. (41) Popovic, Z. D.; Khan, M. I.; Atherton, S. J.; Hor, A. M.; Goodman, J. L. J. Phys. Chem. B 1998, 102, 657–663. (42) Nakai, K.; Ishii, K.; Kobayashi, N.; Yonehara, H.; Pac, C. J. Phys. Chem. B 2003, 107, 9749–9755. (43) Norton, J. E.; Bredas, J. L. J. Chem. Phys. 2008, 128, 7. (44) Perez, M. D.; Borek, C.; Forrest, S. R.; Thompson, M. E. J. Am. Chem. Soc. 2009, 131, 9281–9286. (45) Rand, B. P.; Burk, D. P.; Forrest, S. R. Phys. Rev. B 2007, 75, 11537. (46) Mayukh, M.; Sema, C. M.; Roberts, J. M.; McGrath, D. V. J. Org. Chem. 2010, 75, 7893–7896. (47) Mayukh, M.; Placencia, D.; Sema, C. M.; Armstrong, N. R.; McGrath, D. V. Manuscript in preparation. (48) Cahen, D.; Kahn, A. Adv. Mater. 2003, 15, 271–277. (49) Schlaf, R.; Parkinson, B. A.; Lee, P. A.; Nebesny, K. W.; Armstrong, N. R. J. Phys. Chem. B 1999, 103, 2984–2992. (50) Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K. Adv. Mater. 1999, 11, 605–+. (51) Saito, T.; Sisk, W.; Kobayashi, T.; Suzuki, S.; Iwayanagi, T. J. Phys. Chem. 1993, 97, 8026–8031. (52) Oka, K.; Okada, O.; Nukada, K. Jpn. J. Appl. Phys., Part 1 1992, 31, 2181–2184. (53) Tsushima, M.; Motojima, Y.; Ikeda, N.; Yonehara, H.; Etori, H.; Pac, C.; Ohno, T. J. Phys. Chem. A 2002, 106, 2256–2264. (54) Yonehara, H.; Ogawa, K.; Etori, H.; Pac, C. Langmuir 2002, 18, 7557–7563. (55) Kera, S.; Abduaini, A.; Aoki, M.; Okudaira, K. K.; Ueno, N.; Harada, Y.; Shirota, Y.; Tsuzuki, T. Thin Solid Films 1998, 327, 278–282. (56) Kera, S.; Yabuuchi, Y.; Yamane, H.; Setoyama, H.; Okudaira, K. K.; Kahn, A.; Ueno, N. Phys. Rev. B 2004, 70. (57) Walzer, K.; Toccoli, T.; Pallaoro, A.; Pallaoro, A.; Verucchi, R.; Fritz, T.; Leo, K.; Boschetti, A.; Iannotta, S. Surf. Sci. 2004, 573, 346–358. (58) Brinkmann, M.; Wittmann, J. C.; Barthel, M.; Hanack, M.; Chaumont, C. Chem. Mater. 2002, 14, 904–914. (59) Kuwabara, T.; Sugiyama, H.; Kuzuba, M.; Yamaguchi, T.; Takahashi, K. Org. Electron. 2010, 11, 1136 1140. (60) Wang, Y.; Hang, D. J. Adv. Mater. 2010, 22, 1521–1525. (61) Zhang, X. R.; Wang, Y. F.; Ma, Y.; Ye, Y. C.; Wang, Y.; Wu, K. Langmuir 2006, 22, 344–348. (62) Nitschke, C.; O’Flaherty, S. M.; Kroll, M.; Blau, W. J. J. Phys. Chem. B 2004, 108, 1287–1295. (63) Giebink, N. C.; Wiederrecht, G. P.; Wasielewski, M. R.; Forrest, S. R. Phys. Rev. B 2010, 82, 155305. (64) Potscavage, W. J.; Yoo, S.; Kippelen, B. Appl. Phys. Lett. 2008, 93, 3. (65) Vandewal, K.; Tvingstedt, K.; Gadisa, A.; Inganas, O.; Manca, J. V. Phys. Rev. B 2010, 81, 8. (66) Vandewal, K.; Tvingstedt, K.; Gadisa, A.; Inganas, O.; Manca, J. V. Nat. Mater. 2009, 8, 904–909. (67) Shrotriya, V.; Li, G.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Adv. Funct. Mater. 2006, 16, 2016–2023. (68) Placencia, D.; Wang, W. N.; Shallcross, R. C.; Nebesny, K. W.; Brumbach, M.; Armstrong, N. R. Adv. Funct. Mater. 2009, 19, 1913–1921. (69) Gulbinas, V.; Jakubenas, R.; Pakalnis, S.; Undzenas, A. J. Chem. Phys. 1997, 107, 4927–4933. (70) Gulbinas, V.; Jakubenas, R.; Pakalnis, S.; Undzenas, A. Adv. Mater. Opt. Electron. 1996, 6, 412–414. (71) Kera, S.; Yamane, H.; Fukagawa, H.; Hanatani, T.; Okudaira, K. K.; Seki, K.; Ueno, N. J. Electron Spectrosc. Relat. Phenom. 2007, 156, 135–138. (72) Fukagawa, H.; Kataoka, T.; Hosourni, S.; Kera, S.; Ueno, N. J. Electron Spectrosc. Relat. Phenom. 2007, 156, 37.
ARTICLE
(73) Fukagawa, H., Yamane, H., Kera, S.,. Okudaira, K. K., Ueno, N. Phys. Rev. B 2006, 73, 041302. (74) Zahn, D. R. T.; Gavrila, G. N.; Gorgoi, M. Chem. Phys. 2006, 325, 99–112. (75) Hwang, J.; Wan, A.; Kahn, A. Mater. Sci. Eng.: R 2009, 64, 1–31. (76) Scharber, M. C.; Wuhlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A. J.; Brabec, C. L. Adv. Mater. 2006, 18, 789–+.
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