Planar and textured heterojunction organic photovoltaics based on chloroindium phthalocyanine (ClInPc) versus titanyl phthalocyanine (TiOPc) donor layers

Organic Electronics 12 (2011) 383–393

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Planar and textured heterojunction organic photovoltaics based on chloroindium phthalocyanine (ClInPc) versus titanyl phthalocyanine (TiOPc) donor layers Weining Wang, Diogenes Placencia, Neal R. Armstrong ⇑ Department of Chemistry, University of Arizona, Tucson, Arizona 85721, USA

a r t i c l e

i n f o

Article history: Received 2 July 2010 Received in revised form 15 November 2010 Accepted 18 November 2010 Available online 8 December 2010 Keywords: Phthalocyanine Organic solar cell UV-photoemission Near-IR Heterojunction Interface dipole

a b s t r a c t Planar and textured heterojunction solar cells (OPVs) are reported for vacuum deposited chloroindium phthalocyanine (ClInPc)/C60 heterojunctions, and their response compared to previously explored OPVs based on titanyl phthalocyanine (TiOPc)/C60 heterojunctions. As for TiOPc/C60 OPVs, the photoelectrical activity of ClInPc/C60 OPVs extends well into the near infrared, with good activity out to ca. 900 nm. As-deposited ClInPc films (Phase I) produce open-circuit photopotentials, VOC as high as ca. 0.8 V (Phase I form of ClInPc), ca. 0.15 V larger than previously observed for Phase I TiOPc/C60 OPVs. The offsets in frontier C60 orbital energies (EPc HOMO –ELUMO ) revealed by UV-photoemission studies (UPS) are slightly smaller for ClInPc/C60 versus TiOPc/C60 heterojunctions, and the interface dipole contribution (shift in local vacuum level) to these offsets is in the opposite direction for ClInPc/C60 versus TiOPc/C60 heterojunctions, or missing altogether, suggesting differences in molecular interaction at the Pc/C60 interface. Higher VOC values are correlated with lower reverse saturation currents, Jo, for ClInPc/C60, versus other Pc or pentacene/C60 heterojunctions, suggesting weak intermolecular interactions at the ClInPc/C60 interface and large barriers to dark charge injection. Solvent annealing of the ClInPc films enhances the near-IR response, and textures the Pc film, enhancing the Pc/C60 interfacial contact area and the short-circuit photocurrent, JSC. JSC under AM 1.5 illumination conditions was estimated by integration of the incident photon current efficiency (IPCE) response, to compare relative power conversion efficiencies for the two different device types. The estimated efficiency of Phase I ClInPc/C60 OPVs is ca. 2.6%. The estimated AM 1.5 efficiency of ClInPc/ C60 OPVs with solvent annealed Pc layers is estimated to be ca. 3.3%, arising from the extensive texturing achieved of the Pc layer, which nearly doubles JSC for the Phase II versus Phase I Pc films. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction Organic photovoltaics (OPVs) based on ‘‘Type II’’ donor (D)/acceptor (A) heterojunctions [1], have been under investigation for over two decades, in either planar heterojunction (PHJ) or bulk heterojunction (BHJ) formats [2–20]. OPVs have recently achieved AM 1.5 efficiencies higher than 7% in both polymer BHJ formats [2,21], and in PHJ tan⇑ Corresponding author. Tel.: +1 520 621 8242; fax: +1 520 621 8407. E-mail address: [email protected] (N.R. Armstrong). 1566-1199/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2010.11.015

dem OPVs [22–24]. Improving the efficiency of OPVs requires enhancing the short-circuit current (JSC), utilizing materials with enhanced near-IR absorption, and nanotexturing of the donor/acceptor (D/A) interface, facilitating efficient exciton dissociation [15,18,20,25]. Such enhancements need to occur without sacrificing open-circuit voltage, VOC, which is controlled by offsets in transport frontier orbital energies, EDHOMO –EALUMO [1,26–30], and by the strength of intermolecular interactions at the D/A interface, and ultimately the magnitude of the reverse saturation current, Jo [8,12,31].

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amenable to characterization of their ionization potentials and interface dipole effects using UV-photoemission spectroscopies (UPS) [6]. We demonstrate here that OPVs based on Phase I ClInPc/C60 heterojunctions show values of VOC nearly 0.2 V higher than their Phase I TiOPc/C60 counterparts. Estimated AM 1.5 efficiencies for the Phase I polymorph of ClInPc based OPVs are twice as those seen previously for Phase I TiOPc/C60 heterojunctions. Solvent annealing of the Phase I ClInPc films provides texturing of the Pc/C60 interface on ca. 100 nm length scales that enhances the short-circuit photocurrent, JSC, with only small declines in VOC, leading to estimated AM 1.5 efficiencies for heterojunctions based on the Phase II polymorph of ca. 3.3%. Improved efficiencies for OPVs based on textured Phase II and Phase III TiOPc films are reported elsewhere, but have also shown significant enhancements in efficiencies relative to our earlier reports [52]. The larger values of VOC for OPVs based on ClInPc do not C60 arise from larger EPc HOMO –ELUMO offsets, estimated from UPS studies of ClInPc/C60 versus TiOPc/C60 heterojunctions, since these energy offsets are comparable for both donor C60 layers. We show here that EPc HOMO –ELUMO and the magnitude and sign of the interface dipole at the ClInPc/C60 heterojunction are dependent on the deposition rate of the Pc film, which likely controls the micro-crystallinity of these layers. Previous studies of organic heterojunctions of ClInPc with perylene-based acceptors have shown that interface dipoles seen at the O/O0 heterojunction are strongly dependent upon molecular architecture at this interface [6,18], as predicted by several other investigations [53– 56]. We also observed much lower values of Jo for ClInPc/ C60 heterojunctions, suggesting that apparently weaker intermolecular interactions at this interface are primarily responsible for enhanced values of VOC.

We show here that OPVs based on heterojunctions of C60 with planar and textured layers of the trivalent metal phthalocyanine chloroindium phthalocyanine (ClInPc) meet several of the criteria for good near-IR spectral overlap, high VOC brought about by higher ionization potentials (IP) versus most other small molecule donors, and low values of Jo in the Pc/C60 heterojunction. Efficiencies of these ClInPc/C60 OPVs are higher than those we recently reported on using titanyl phthalocyanine (TiOPc) as the donor layer [14,15]. The tetravalent and trivalent metal phthalocyanines (titanyl and vanadyl phthalocyanines, TiOPc and VOPc, and the halo-aluminum, -gallium and -indium phthalocyanines, e.g. ClAlPc, ClGaPc, ClInPc) have been explored as xerographic photoreceptors and as photoelectrochemical energy conversion platforms, in both their as-deposited Phase I polymorph, or as Phase II or related polymorphs [11,14,32–41]. The ionization potentials (IP) of the tetravalent and trivalent metal Pcs are generally higher than seen in other Pc donors (e.g. CuPc, ZnPc), or pentacene and related polycyclic aromatic hydrocarbons [9,11,14,15,42– 47]. For OPVs based on as-deposited (Phase I polymorph) TiOPc/C60 heterojunctions VOC as high as 0.65 V has been observed [14]. Upon solvent annealing of these Pc films significant nano-texturing occurs during the formation of the Phase II polymorph along with a significant broadening of the Q-band absorbance out to ca. 900 nm. The increase in interfacial area between the Pc and C60 layers, along with the added near-IR absorbance, leads to a near doubling of power conversion efficiency as a result of these polymorphic transitions [15]. In this paper, we discuss planar and textured heterojunction OPVs based on ClInPc/C60 (Fig. 1), showing enhanced near-IR responses as for TiOPc OPVs, but with a larger VOC. ClInPc demonstrates close to the same near-IR absorptivity (a, cm1) in its Phase I and Phase II polymorphs as seen in TiOPc polymorphs. As for TiOPc, ClInPc ohms per square is believed to have a high probability for triplet state formation [48], as has been hypothesized for OPVs based on thin films of other heavy metal phthalocyanines (e.g. SnPc) [49]. The trivalent and tetravalent Pcs, especially ClInPc films, can also be formed as well-ordered, epitaxial thin films, demonstrating layer-by-layer growth on ordered substrates, lending themselves to the formation of well-ordered organic/organic0 heterojunctions [50,51],

2. Experimental methods Commercial ITO was obtained from Colorado Concept Coating, LLC, with a sheet resistance of ca. 15 ohms per square. ITO substrates were cleaned with a micro-fiber cloth using 10% Triton-X100, followed by successive sonications in 10% Triton-X100 for 15 min, nanopure water for 5 min, and pure ethanol for 15 min. Prior to the deposition of organic thin films, substrates were dried under a

Al (100 nm) BCP (10 nm) N

N N

N

Ti

N N

N

O N N

C60 (40 nm) N

N

In Cl

N

N

Pc (20 nm)

N

N N

ITO (100 nm)

Fig. 1. Molecular structure of TiOPc and ClInPc, and OPV device structure, respectively.

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stream of nitrogen and then etched via oxygen plasma cleaning (Harrick PDC-32G) for 15 min. Titanyl phthalocyanine (TiOPc), chloroindium phthalocyanine (ClInPc), and bathocuproine (BCP) were purchased from Sigma–Aldrich. Fullerene (C60) was obtained from MER. All chemicals were purified by multiple entrainer sublimation prior to use [57,58]. Organic thin-films were deposited via vacuum deposition (ca. 1 Å/s) at base pressures of ca. 107 Torr via Knudsen-type sublimation cells, monitored with a 10 MHz quartz crystal microbalance (QCM-Newark), and an Agilent Technologies frequency monitor (Model 53131A). Solventannealed TiOPc and ClInPc films were created by depositing the desired Pc film thickness, transfer of the film under nitrogen to an adjacent glovebox where it was placed in a sealed vessel adjacent to a reservoir of liquid chloroform for up to 5 h [15]. The Pc film was then returned to vacuum and the Pc/C60 OPV device completed. Aluminum top contacts were deposited at pressures no greater than 106 Torr at a deposition rate of 1–3 Å/s, monitored via a 6 MHz QCM (Tangidyne) and Inficon deposition monitor (Model 758500-G1). These aluminum top contacts defined the device area, and we typically made from 9 to 26 devices for each deposition sequence, on ITO/glass substrates with dimensions approximately 2.5  2.5 cm. The region at the center of some of these substrates was left open to allow for absorbance spectra to be measured on the same films for which IPCE data was obtained. Current–voltage (J/V) data was obtained for several OPVs for each ClInPc films composition, with areas of 0.019 cm2. The J/V data presented here are representative of at least nine devices. IPCE data was obtained for devices with an area of 0.125 cm2, apertured so as to minimize ‘‘edge effects’’ in the photocurrent response [15,59]. OPV testing was carried out in a N2-filled double-glovebox (MBrawn Labmaster), with water and oxygen levels at less than 0.1 ppm. Current–voltage measurements were made with a Keithley 2400 source meter, while the data was acquired with in-house software created with Labview ver.8.2 (National Instruments). Scans ranged from 1.00 to 1.50 V using a 20 mV step starting from negative bias. A CUDA products light source with a 250 W quartz-halogen lamp (Model I-250) was used as the illumination source. The light was filtered with a 750 nm cutoff filter, along with a sand-blasted light diffuser (the spectral distribution for this filter set is shown in Supporting information). We adjusted the distance to the OPV device arrays to achieve an output of approximately 100 mW/cm2 as measured with an Apogee PYR-S pyranometer. Incident photon current efficiency (IPCE) measurements were carried out in atmosphere immediately after device fabrication. IPCE data were acquired after characterizing dark/light J/V behavior in the glovebox environment. IPCE measurements were made with a 500 W quartz-halogen lamp with a Mercron power stabilizer (Model TXC-500 120/120). The modulated light (250 Hz) was passed through a Jobin Yvon (Model H10/1200) monochromator. Spectra were acquired at 4 nm steps. Incident power through the monochromator was measured with a calibrated Hamamatsu photodiode acquired from Newport Optics (Model 818-SL). The bias for the OPV was main-

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tained so as to obtain short-circuit current at each excitation wavelength. The current-to-voltage output was input to a EG&G (Model 5209) lock-in amplifier, and output into in-house software created with Labview ver.8.2 (National Instruments). XPS/UPS measurements of Pc/C60 heterojunctions were carried out in a Kratos Axis-Ultra X-ray photoelectron spectrometer using an Al Ka source at 1486.6 eV for XPS measurements and a He(I) excitation source (21.2 eV) for UPS measurements. A 9.00 V bias was applied to the sample to further enhance the collection of lowest kinetic energy electrons [14,50,60]. For XPS data, the spot size was 300  700 lm, while for UPS, spot size ranged between 1 and 3 mm. A UHV organic deposition system attached to the Kratos spectrometer allowed for depositions of organic thin-films under conditions like those used to create OPVs. Thin-films were deposited (about 1 Å/s) on gold substrates (Alfa-Aesar) that were polished and etched with pirahna solution prior to the UPS studies [14,60]. Gold substrates were sputtered clean in vacuum to ensure that their effective work functions were 5.1 eV and to provide for measurement of the Fermi energy [60]. For the data shown in Fig. 6, photoemission from the Au Fermi level occurred at 32.0 eV for sputter-cleaned gold substrates and we made sure that this energy did not change during organic film deposition, which would have indicated charging effects [60]. For exploration of solvent-annealed ClInPc films, after the initial deposition and UPS characterization, the films were removed from vacuum into a nitrogen-filled glovebag and annealed as described above. Samples were then returned to vacuum, followed by subsequent UPS characterization. Absorbance measurements were carried out with an Agilent Technologies UV/visible spectrophotometer (Model 8453) at 1 nm intervals with an integration time of 10 s and a range of 400–1100 nm. The morphology and root-mean-square (rms) roughness of ClInPc films were obtained from images collected using a Digital Instruments atomic force microscope (Dimension 3100). The images were collected in tapping mode with Mikromasch silicon cantilevers (Model TESP-7). Images were obtained in ambient air at a scan rate of 0.5 Hz with image sizes of 2.5  2.5 lm. SEM images were taken on a Hitachi 4800 FE-SEM (15 kV accelerating voltage) on as-prepared samples (i.e. no metallic over coating). 3. Results and discussion 3.1. Characterization of microstructure and thin film absorbance spectra and absorptivities Figs. 1 and 2 show the schematics of the Pc donors, the OPV device configurations, and the field-emission (FESEM) images for ClInPc and TiOPc films of ca. 20 nm thickness, in as-deposited (Phase I) and solvent annealed (Phase II) form, on ITO. Fig. 3 (upper panel) shows the absorbance spectra of device-thickness TiOPc and ClInPc films (Phase I ClInPc films (20 nm), Phase I TiOPc films (18 nm), Phase II ClInPc films (22 nm) and Phase II TiOPc films (20 nm)). SEM and AFM images suggest that the Phase I TiOPc film is conformal, and reflects the structure of the underlying

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Fig. 2. SEM of ClInPc (A and B) and TiOPc (C and D) on ITO, in as-deposited, Phase I polymorph (A and C) and solvent annealed, Phase II polymorph (B and D) for 20 nm film thickness. Scale bar = 200 nm.

ITO [15]. The Phase I ClInPc film shows more texturing and surface roughness (rms roughness = 2.9 ± 0.6 nm) versus the Phase I TiOPc film (rms roughness = 0. 9 ± 0.03 nm, i.e. these Phase I TiOPc films were nearly conformal with the ITO substrate) (see AFM data in Supporting information). After solvent annealing, which causes a change in Pc crystal structure from monoclinic (Phase I) to triclinic (Phase II) [34,38], both Phase II ClInPc and Phase II TiOPc films show greatly increased surface roughness (rms roughness = 5.0 nm for Phase II ClInPc and 6.6 for Phase II TiOPc), which subsequently leads to enhanced interfacial contact area between the Pc donor and C60 acceptor, coupled with greatly enhanced absorptivities in the near-IR, out to wavelengths of ca. 900 nm (we estimate that a  1.5  105 cm1 at the peak absorbance – Fig. 3). It has been proposed that this shift in the TiOPc Q-band absorbance is due to the formation of a charge-transferlike state, owing to the much closer proximity of the opposed oxo-metal dipoles in these dyes, accompanying the shift in their crystal structures from monoclinic (Phase I) to triclinic (Phase II) [34,35,43–45,61–63]. We assume a similar situation may occur for the ClInPc system, although the internal dipole moment along the metal-halogen bond is not as large as along the metal-oxo bond in TiOPc [64– 66]. We also note the similarity in structure of the polymorphic forms of this pigment, relative to those observed for TiOPc and VOPc [67,68]. Although there is substantial roughening of the Pc film, it should be noted that each asperity still has dimensions which exceed the presumed exciton diffusion length (3  LD in Pc films  18–22 nm) [20,69], so the kind of texturing needed to increase external quantum efficiencies (IPCE, see below) to greater than 0.85 (seen in polymer bulk heterojunction devices) [3,21], has not yet been achieved.

3.2. UPS studies of frontier orbital energy offsets for ClInPc/C60 versus TiOPc/C60 heterojunctions Fig. 4 shows the UPS data used to characterize the frontier orbital energy offsets that set the upper limit for open-circuit photopotentials, VOC, for Phase I ClInPc/C60 heterojunctions (upper panel) and Phase II ClInPc/C60 heterojunctions (lower panel). Figs. 5 and 6 show the energy level offsets inferred from these UPS data, and from estimates of LUMO levels from either optical absorbance data, or from inverse photoemission spectroscopic data for related Pcs and C60 [70,71]. The data shown in Fig. 4 were obtained by first depositing ClInPc thin films on clean Au substrates (work function  5.1 eV), followed by the deposition of C60 layers in small thickness increments. Using those UPS data in Fig. 5 and previously described analysis protocols [14,50,72–75], we determined the ionization potential (IP) for both Phase I ClInPc and Phase II ClInPc to be ca. 5.4 eV. A shift of ca. 0.3 eV in the local vacuum level (interface dipole effect) was observed as C60 was deposited onto Phase I ClInPc, while a vacuum level shift of 0.2 eV was observed for Phase II ClInPc/C60 heterojunctions. No difference in the photoemission features for C60 were observed for the last two incremental depositions of C60, indicating that it had reached its bulk electronic properties. The IP of C60 in these heterojunctions was estimated to be ca. 6.4 eV. Those IP values and vacuum level shift derived from the UPS data are used to estimate the frontier orbital energy offsets for the ClInPc/C60 junctions. The band diagrams are then compared with those for the TiOPc/C60 heterojunction in Fig. 5 (Phase I Pc/C60 junctions) and Fig. 6 (Phase II Pc/C60 junctions) [14,15,18]. The LUMO energies were estimated from combinations of absorbance spectroscopy, inverse photoemission spectroscopy (IPES), solution electrochemistry,

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0.40

0.25 0.20 0.15 0.10 0.05

ClInPc/C60 TiOPc/C60

C60

162 A 62 A 30 A 14 A 6A 2A

0.2

30 A 14 A 6A 2A 0.33 eV

15.0

0.1

Phase I

Phase II

C60

(D)

162 A 62 A

C60 162 A 62 A 30 A 14 A 6A 2A ClInPc II 20 A

Phase I

ClInPc 20 A

15.5 16.0 28 29 30 31 32 33 34 35 Kinetic energy (eV)

(C) Intensity (arb. units)

IPCE

14.5

0.3

0.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 400

62 A

ClInPc 20 A

Phase II

0.4

162 A

C60

0.00 0.5

APCE

(B)

Phase I

Intensity (arb. units)

Absorbance (A.U.)

0.30

(A)

Phase II

0.35

30 A 14 A 6A 2A 0.2 eV

ClInPc II 20 A

14.0 14.5 15.0 15.5 16.0 28 29 30 31 32 33 34 35 Kinetic energy (eV)

500

600 700 800 Wavelength (nm)

900

Fig. 3. (Upper panel) Absorbance spectra, (middle panel) incident photon to current conversion efficiency (IPCE) and (bottom panel) absorbed photon to current conversion efficiency (APCE) plots for Phase I TiOPc (18 nm)/C60 (40 nm)/BCP (10 nm) (dark dashed line), Phase I ClInPc (20 nm)/C60 (40 nm)/BCP (10 nm) (dark solid line), Phase II TiOPc (20 nm)/C60 (40 nm)/BCP (10 nm) (red dashed line), Phase II ClInPc (22 nm)/C60 (40 nm)/BCP (10 nm) (red solid line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

and scanning tunneling spectroscopic (STS) data for other Pc systems and for other studies of C60 [9,15,50,70,76– 78]. We note that LUMO energies and electron affinities estimated from IPES data can be much smaller than those estimated from absorbance data in the Pc films, owing to the significant exciton binding energy for donor layers like the Pcs, and there is additional uncertainty in determining the LUMO energy to less than ±0.2 eV in IPES experiments, owing to the resolution of that data [70]. For comparison of otherwise quite similar Pc/C60 heterojunctions the uncertainties in these estimates of LUMO energy, which can be ±0.3 eV, are not of concern. C60 We note that EPc HOMO –ELUMO  1.0 eV for Phase I ClInPc/C60 C60 junction versus Phase I TiOPc/C60 where EPc HOMO –ELUMO  1.3 eV (the absolute values of these offsets are larger if

Fig. 4. He (I) UPS spectra for C60 deposited, in small thickness increments, on Phase I ClInPc at low kinetic energy cutoff edge (A) and at high kinetic energy cutoff region (B); and for C60 deposited, in small thickness increments, on Phase II ClInPc at low kinetic energy cutoff edge (C) and at high kinetic energy cutoff region (D).

we use transport LUMO levels estimated from other IPES data) [70], suggesting that VOC should be larger for Phase I TiOPc/C60 heterojunctions. As shown below, we actually see larger values of VOC in ClInPc/C60 OPVs. In addition to C60 these difference in EPc HOMO –ELUMO the shift in local vacuum level for ClInPc/C60 heterojunction (0.33 eV) is opposite in sign to that observed at the TiOPc/C60 heterojunctions (0.2 eV). Interface dipoles observed for previously explored Pc/C60 heterojunctions have been greater than zero, including Phase I TiOPc/C60 (0.2 eV), CuPc/C60 (0.3 eV) and SubPc/C60 (0.5 eV) [14,15,79]. We have recently observed, however, interface dipoles of 0.3 eV for both ClInPc and ClAlPc films in heterojunctions with electron acceptors such as perylenetetracarboxylicdianhydride (PTCDA), which has a slightly higher electron affinity than C60. These different interactions may be attributable to variations in packing structures and the local density of these trivalent metal Pcs, which impacts on the density of local dipoles from the metalhalogen bond, which in turn is dependent upon deposition rate, and the molecular nature of the donor/acceptor

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Δ=0.33 eV

Vacuum

5.4

6.4

Transport LUMO

Δ=0.2 eV 5.2

6.4

Transport LUMO

3.6-4.1 eV

3.6-4.1 eV HOMO

≥1.0 eV

≥1.3 eV

1.33 eV

1.0 eV

ClInPc I

Vacuum

C60

TiOPc I

LOMO

HOMO

C60

Fig. 5. Frontier orbital offset energies for ClInPc/C60 and TiOPc/C60 heterojunctions. The UPS data used to obtain these energies for ClInPc/C60 junction is summarized in Fig. 4. The UPS data used to obtain the energies for TiOPc/C60 junction is from previously published work [14,15]. For Phase I ClInPc/C60 heterojunction, an inverse interface dipole (0.33 eV) was observed, suggesting a different kind of structure and polarizability the ClInPc I/C60 interface, comparing with Phase I TiOPc/C60 heterojunction with a positive interface dipole of 0.2 eV. After correction for vacuum level shifts for each heterojunction, C60 we conclude that the critical energy differences which form the upper limit for VOC in OPVs, EPc HOMO –ELUMO , is lower for Phase I ClInPc/C60 than for Phase I TiOPc/C60 heterojunctions. The energetic driving force for exciton dissociation, however, is higher for Phase I ClInPc/C60 heterojunctions, versus Phase I TiOPc/C60 heterojunctions, consistent with the higher photocurrent from the Phase I ClInPc/C60 heterojunctions.

Δ=0.2 eV

Vacuum

5.4

6.4

Transport LUMO

Vacuum

5.4

6.3

Transport LUMO

3.6-4.1 eV

3.6-4.1 eV HOMO

LOMO

≥1.3 eV

≥1.1 eV

0.9 eV

1.2 eV

HOMO

ClInPc II

C60

TiOPc II

C60

Fig. 6. Frontier orbital offset energies for Phase II ClInPc/C60 and Phase II TiOPc/C60 heterojunctions. The UPS data used to obtain these energies is summarized in Fig. 4. The UPS data used to obtain the energies for TiOPc/C60 junction is from previously published studies [15]. For Phase II ClInPc/C60 heterojunction, an inverse interface dipole (0.2 eV) was observed, suggesting a different kind of reaction at the ClInPc II/C60 interface, comparing with Phase II TiOPc/C60 heterojunction with no interface dipole. After correction for vacuum level shifts for each heterojunction, we conclude that the critical C60 energy differences which form the upper limit for VOC in OPVs, EPc HOMO –ELUMO , are slightly lower for Phase II ClInPc/C60, versus Phase II TiOPc/C60 heterojunctions. The energetic driving force for exciton dissociation, however, is higher for Phase I ClInPc/C60 heterojunctions, versus Phase I TiOPc/C60 heterojunctions, consistent with the higher photocurrent from the Phase I ClInPc/C60 heterojunctions.

interface formed. Jones and coworkers [80], have recently shown that the interface dipole at ClAlPc/C60 heterojunction is 0.1 eV at low deposition rates of ClAlPc films, and that as the deposition rate of ClAlPc increases, and the molecular density decreases, the interface dipole at the ClAlPc/C60 heterojunction changes from 0.1 to 0.2 eV. We have observed similar differences in UPS-derived interface dipoles, dependent upon ClInPc deposition rate. For ClInPc films deposited at 2–3 Å/s (a factor of 3 larger than used for OPV heterojunction formation) we saw no interface dipole upon formation of the Pc/C60 het-

C60 erojunction, and larger EPc HOMO –ELUMO offsets (see Fig. SI-1, Supporting information). We also explored Phase II ClInPc/C60 heterojunctions (Fig. 6). Similar to Phase I Pc/C60 heterojunction in Fig. 5, C60 we observed lower value of EPc HOMO –ELUMO (P1.1 eV) for the Phase II ClInPc/C60 junction than for Phase II TiOPc/ C60 junction (P1.3 eV). An inverse interface dipole (0.2 eV) is again observed at the ClInPc/C60 heterojunction, while no interface dipole is observed at the TiOPc/ C60 heterojunction. Despite these uncertainties in energy levels, it does appear that for both Phase I and Phase II

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W. Wang et al. / Organic Electronics 12 (2011) 383–393 Table 1 Device parameters for OPVs based on ClInPc/C60 and TiOPc/C60 heterojunctions.

a b c d e

Type of Pc/C60 heterojunction (and Pc film thickness); C60 thickness in all cells = 40 nm

VOC (V)a

JSC (mA/cm2)b

Fill factor

noc

Phase Phase Phase Phase

0.79 0.61 0.68 0.57

11.1 8.1 15.9 15.1

0.58 0.53 0.50 0.53

2.3 2.3 2.3 2.5

I – ClInPc (20 nm) I – TiOPc (18 nm) II – ClInPc (22 nm) II – TiOPc (20 nm)

g (%)d

Jo (mA/cm2)e

C60 EPc HOMO –ELUMO (eV)

2.6 1.3 3.3 3.0

1.0  105 1.7  104 4.8  105 7.1  104

1.0 1.3 1.1 1.3

Open-circuit photovoltage. Short-circuit photocurrent. Ideality factor calculated from fitting the dark J–V. AM 1.5 power conversion efficiency [(VOCJSCFF)/PIN]. Reverse saturation current – estimated from lowest dark current (log plots).

2

Current density (mA/cm )

C60 ClInPc polymorphs the estimated EPc LUMO –ELUMO energy offsets, which control the exciton dissociation efficiency [8,9], are larger for ClInPc/C60 versus TiOPc/C60 heterojunctions. The expectation is that for equivalent thickness and textured Pc films, the short circuit photocurrent for ClInPc OPVs will be larger, which is born out in the J/V data for equivalent devices described below.

3.3. ClInPc/C60 photovoltaic device performance ClInPc/C60 OPVs based on optimized thicknesses of Phase I ClInPc film and Phase II ClInPc donor films are compared to Phase I and Phase II TiOPc/C60 OPVs, created as described previously (device configuration in Fig. 1 and data in Figs. 2 and 3) [15]. Table 1 summarizes the device parameters for these OPVs. We first compare Phase I TiOPc/C60 and ClInPc/C60 OPVs. Fig. 7 shows the dark and illuminated J–V characteristics for ITO/Phase I TiOPc (18 nm)/C60 (40 nm)/BCP (10 nm)/Al and ITO/Phase I ClInPc (20 nm)/C60 (40 nm)/ BCP (10 nm)/Al. Optimum OPV performance was obtained for Pc film thicknesses of ca. 18 nm (TiOPc) and ca. 20 nm (ClInPc). For Phase I TiOPc OPVs JSC = 8.1 mA/cm2; VOC = 0.6 V; FF = 0.53, while for Phase I ClInPc OPVs JSC = 11.1 mA/cm2, VOC = 0.8 V, FF = 0.58. There was a significant increase (almost 0.2 V) in VOC as the donor layer was changed from TiOPc to ClInPc. JSC was also improved for the ClInPc OPVs (Fig. 3 – upper panel), correlating with the enhanced light absorption for these films in the nearIR. We found that we could produce optimum response in ClInPc OPVs with 22 nm Pc layers (versus 20 nm thickness for TiOPc devices – see Table 1). For both Pcs the estimated absorptivity, a = 1.5  105 cm1 at the Q-band absorbance maximum, suggesting that other factors lead to improved performance of the ClInPc OPVs, such as enhanced exciton diffusion lengths, and/or enhanced charge mobilities [81,82]. The estimated AM 1.5 power conversion efficiency, g, was doubled for Phase I ClInPc versus TiOPc OPVs. Comparable values of VOC have recently been reported for ‘‘Phase I’’ ClAlPc/C60 OPVs, where thin molecular interlayers (PTCDA) were used at the ITO interface to ‘‘template’’ the growth of ordered ClAlPc layers, and/or add a hole-selectivity to this interface [83,84]. Li and Forrest have also recently reported OPVs based on textured Phase I ClAlPc layers with a VOC of ca. 0.8 V [16,25].

30 25 20 15 10 5 0 -5 -10 -15

(A)

DARK LIGHT

-1.0

1000 100 10

TiOPc/C60 ClInPc/C60

-0.5

0.0

0.5

1.0

(B) LIGHT

1 0.1 0.01 1E-3 1E-4 1E-5 -1.0

DARK

-0.5

0.0 0.5 Voltage (V)

1.0

1.5

Fig. 7. The J–V characteristics for Phase I TiOPc (18 nm)/C60 (40 nm)/BCP (10 nm) (dashed lines) and for Phase I ClInPc (20 nm)/C60 (40 nm)/BCP (10 nm) (solid lines) in the dark (blue lines) and under illumination (red lines). (A) Linear J–V; (B) semi-log J–V. At 140 mW/cm2 for the TiOPc/C60 solar cell, JSC = 8.12 mA/cm2; VOC = 0.61 V; FF = 0.53; for the ClInPc/C60 solar cell, JSC = 11.11 mA/cm2; VOC = 0.79 V; FF = 0.58. Device area = 0.019 cm2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 8 shows the J–V characteristics for Phase II ClInPc (22 nm)/C60 (40 nm)/BCP (10 nm) and Phase II TiOPc (20 nm)/C60 (40 nm)/BCP (10 nm), in the dark and under illumination. For the Phase II TiOPc OPVs JSC = 15.10 mA/ cm2; VOC = 0.57 V; FF = 0.53, while for Phase II ClInPc OPVs, JSC = 15.86 mA/cm2; VOC = 0.68 V; FF = 0.50. Both devices have comparable JSC and FF, but VOC for Phase II ClInPc solar cell is higher than for Phase II TiOPc solar cell, although lower than seen in the Phase I OPV, still leading, however, to substantially higher efficiency for Phase II ClInPc OPVs.

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30

(A) TiOPc II/C60 ClInPc II/C60

20 10

DARK 2

Current density (mA/cm )

0 -10

LIGHT

-20 -1.0

-0.5

0.0

10000 1000 (B) 100 LIGHT 10 1 0.1 DARK 0.01 1E-3 1E-4 1E-5 -1.0 -0.5 0.0 0.5 Voltage (V)

0.5

1.0

1.0

1.5

Fig. 8. The J–V characteristics for Phase II TiOPc (20 nm)/C60 (40 nm)/BCP (10 nm) (dashed lines) and for Phase II ClInPc (22 nm)/C60 (40 nm)/BCP (10 nm) (solid lines) in the dark (blue lines) and under illumination (red lines). (A) Linear J–V; (B): semi-log J–V. At 140 mW/cm2 for the TiOPc/C60 solar cell, JSC = 15.10 mA/cm2; VOC = 0.57 V; FF = 0.53; for the ClInPc/C60 solar cell, JSC = 15.86 mA/cm2; VOC = 0.68 V; FF = 0.50. Device area = 0.019 cm2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

3.4. Incident photon and absorbed photon current efficiencies (IPCE and APCE) Incident photon current efficiency (IPCE) and absorbed photon current efficiency (APCE) data for OPVs based on Phase I ClInPc films (20 nm), Phase I TiOPc films (18 nm), Phase II ClInPc films (22 nm) and Phase II TiOPc films (20 nm) are shown in Fig. 3 (middle and lower panels, respectively). As described elsewhere [15], IPCE data were obtained for OPVs with a device area of 0.125 cm2, with a metal aperture used to eliminate ‘‘edge effects’’ in the photocurrent response [59]. APCE data was calculated using the IPCE data and the absorbance (transmittance) spectra for the solar cell under test, in a region near the OPV, but away from the opaque contacts. For both the Phase I and Phase II Pc/C60 OPVs ClInPcbased devices outperform the TiOPc-based OPVs throughout the entire Q-band spectral region. The spectral shift in the Q-band is not as large for the Phase I ? Phase II transition for ClInPc versus TiOPc (the absorption peak for Phase II ClInPc is at 820 nm, while the maximum absorbance seen for Phase II TiOPc is at 850 nm), however, the shapes of the IPCE curves are comparable for both Phase II Pc/C60 heterojunctions. For both device types some of the enhanced OPV response can be attributed to this en-

hanced Q-band spectral response and the corresponding integrated increase in IPCE at wavelengths above 750 nm, however, the texturing of the Pc/C60 heterojunction via the solvent-annealing process also accounts for a significant enhancement in JSC and IPCE [15]. Normalizing IPCE by the actual absorbance (transmittance) for these Pc/C60 heterojunctions gives the APCE which reaches its maximum at ca. 620 nm, gradually decreases out to ca. 850– 900 nm for both Phase I and Phase II heterojunctions, and then sharply declines in spectral regions where there is still appreciable optical density. APCE is a reflection of the efficiency of photocurrent production per absorbed photon, and might be expected to be constant at all wavelengths in the Q-band region for either the Phase I or Phase II Pc films. The integration of the IPCE data in Fig. 3 over the spectral irradiance of the filtered light source provides estimates of JSC close to the observed JSC values seen in Figs. 7 and 8. Subsequently integrating the IPCE responses in Fig. 3 over the standard AM 1.5G spectrum, we predict a JSC = 5.7 mA/cm2 for OPVs based on Phase I (20 nm) ClInPc films, while we predict a JSC = 9.7 mA/cm2 for OPVs based on Phase II (22 nm) ClInPc films under such illumination conditions. Using the fill-factors and VOC values obtained from Figs. 7 and 8 and Table 1, we predict an AM 1.5G power conversion efficiency for devices based on Phase I ClInPc films g = 2.6% (comparing with Phase I devices based on TiOPc films, g = 1.3%), while for Phase II ClInPc films g = 3.3% (comparing with Phase I devices based on TiOPc films, g = 3.0%). Examining the IPCE plots of Fig. 3 we conclude that a significant fraction of the increased efficiency of ClInPc/C60 versus TiOPc/C60 OPVs is due to enhanced VOC, not predicted from the estimated valC60 ues of EPc HOMO –ELUMO . As seen in Table 1 VOC in these OPVs has an inverse correlation with the observed reverse saturation current Jo. The Phase I ClInPc/C60 solar cell with the highest VOC (0.8 V) has the lowest Jo (1.0  105 mA/cm2), consistent with expectations from the Shockley equation [12]:

V OC ¼

  J ph no kB T ln þ1 e Jo

where Jph is the photocurrent, no is the ideality (quality) factor, kB is the Boltzmann constant, T is the absolute temperature, and e is the charge of an electron. Several different studies have recently suggested that the strength of intermolecular interaction at the D/A interface plays a key role in determining the magnitude of Jo, and thus the maximum obtainable VOC [8,12,31,85]. Kippelen and coworkers, exploring the temperature dependence of Jo, have proposed that Jo is controlled by the energy barrier to dark charge injection, which favors donor layers with high IPs, such as TiOPc, over donor layers such as pentacene or CuPc [12]. Ingannas and coworkers have suggested that, for polymer bulk heterojunction OPVs, the spectral position of a charge-transfer band, which depends mainly on EDHOMO –EALUMO , can cause exponential changes in Jo, and thus changes in VOC. The theoretical maximum VOC can be obtained by eliminating all non-radiative pathways, resulting in maximum charge-carrier lifetime [31]. In the case of

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ClInPc OPVs we see one of the largest values for VOC seen for small molecule systems, and a value that is ca. 0.2 V below the maximum predicted built-in potential, limited by C60 the size of EPc HOMO –ELUMO , because of these low values for Jo (see Table 1 for summary). Using these values of Jo, ideality factor n, JSC (see Table 1), and the Shockley equation, the difference in VOC between ClInPc/C60 and TiOPc/C60 solar cell are predicted to be 0.19 V for Phase I and 0.11 V for Phase II cells, respectively, consistent with the observed increase in values of VOC for those solar cells. 4. Conclusions ClInPc/C60 heterojunctions are an attractive materials combination as a small molecule OPV. With proper film processing to form the Phase II polymorph, the photoelectrical activity extends well into the near infrared, retaining C60 a reasonably large VOC. As a result of the large EPc HOMO –ELUMO offsets, and the low Jo values for ClInPc/C60 heterojunctions, VOC is larger, and the AM 1.5 efficiency of Phase I ClInPc/C60 OPVs is twice as that of Phase I TiOPc/C60 solar cell (2.6% versus 1.3%). For textured Phase II ClInPc/C60 heterojunctions improved efficiencies arise from both improved absorptivities in the near IR, and the increased Pc/ C60 interfacial area. It is encouraging that IPCE values for our best ClInPc/C60 heterojunctions investigated to date maximize at ca. 0.45, suggesting that much greater improvements in efficiency will be achievable as texturing of the Pc/C60 interface allows feature sizes to approach 3  LD, providing that Jo can simultaneously be kept low (and VOC high), by minimizing intermolecular interactions at the Pc/C60 interface leading to dark charge injection processes [13,86]. It is clear that if IPCE could be doubled to 0.9, through texturing of the Pc/C60 interface to create features comparable in size to 3  LD, keeping VOC and FF constant, power conversion efficiencies near 6% might be achievable for single heterojunction ClInPc/C60 OPVs. Studies are underway to explore texturing approaches which would produce those kinds of efficiencies, and to create side chain modified forms of these tetravalent and trivalent metal Pcs which provide the advantages of near-IR absorbing phases, coupled with solution processability. Acknowledgments This research has been supported by the Office of Naval Research, and the National Science Foundation through the Science and Technology Center, Materials and Devices for Information Technology (DMR-0120967). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.orgel.2010. 11.015. References [1] C.W. Tang, 2-Layer organic photovoltaic cell, Appl. Phys. Lett. 48 (1986) 183–185.

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