Determination of Energy Level Alignment within an Energy Cascade

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Determination of Energy Level Alignment within an Energy Cascade Organic Solar Cell James Endres,† István Pelczer,‡ Barry P. Rand,†,§ and Antoine Kahn*,† †

Department of Electrical Engineering, ‡Department of Chemistry, and §Andlinger Center for Energy and the Environment, Princeton University, Princeton, New Jersey 08544, United States S Supporting Information *

ABSTRACT: The interfacial band alignment among boron subnaphthalocyanine chloride (SubNc), boron subphthalocyanine chloride (SubPc), and α-sexithiophene (α-6T) is explored using ultraviolet, inverse, and X-ray photoemission spectroscopies (UPS, IPES, and XPS, respectively). With these tools, the ionization energy (IE) and electron affinity (EA) for each material are determined. Layer-by-layer deposition of SubPc and SubNc on α6T as well as SubPc on SubNc, combined with UPS and IPES, allows for the direct determination of the energy level alignment at the interfaces of interest. A small dipole is found at the α-6T/ SubNc/SubPc interface, expanding the donor-LUMO to acceptorHOMO gap and explaining the large open circuit voltage obtained with these devices. However, there is a small electron barrier between SubNc and SubPc, which may limit the efficiency of electron extraction in the current device configuration. Excess chlorine may be responsible for the high IE and EA found for SubNc and could potentially be remedied with improved synthetic methods or further purification.

1. INTRODUCTION The efficiency of organic photovoltaic (OPV) cells has increased significantly over the past decade, in large part because of the introduction of the bulk heterojunction architecture based on the fullerene acceptor.1 This structure has a sizable donor/acceptor interfacial volume, which ensures efficient exciton harvesting while still allowing for a sufficiently thick layer to have sufficient absorption. However, the need for the bulk heterojunction architecture is due in part to the weaknesses of the fullerene absorption. Although fullerenes are excellent electron acceptors and have high electron mobility,2,3 they have relatively poor absorption within the solar spectrum and their high electron affinity significantly limits the maximal achievable open circuit voltage (VOC).4 One device architecture that has received attention recently and results in an increased absorption bandwidth consists of an energy transfer cascade. In such a device, an absorbing layer that is spatially separated from the photocurrent-generating donor/acceptor interface can still contribute to photocurrent via energy transfer to either the donor5 or acceptor6 material. Using this structure, efficiencies as high as 8.4% have been demonstrated using the non-fullerene acceptor alternatives boron subphthalocyanine chloride (SubPc) and boron subnaphthalocyanine chloride (SubNc) in a planar heterojunction device when paired with an α-sexithiophene (α-6T) donor.6 This high efficiency is due to the complementary absorption spectra of each material as well as long-range Förster exciton transfer from the larger bandgap material SubPc into © XXXX American Chemical Society

the smaller bandgap material SubNc, allowing these excitons to then efficiently dissociate at the α-6T/SubNc interface. However, to improve our understanding of the high efficiency of exciton dissociation and carrier extraction as well as the large VOC achieved with the SubNc, SubPc, and α-6T energy cascade solar cells, it is important to understand the molecular level alignment at each of these material interfaces. In this study, ultraviolet, inverse, and X-ray photoemission spectroscopies (UPS, IPES, and XPS, respectively) are used to measure the highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO), and the core levels for each material, respectively, as well as determine their ionization energy (IE) and electron affinity (EA) values. The HOMO and LUMO level alignments at the α-6T/SubNc, α-6T/SubPc, and SubNc/SubPc heterojunctions are determined, and a complete picture of the electronic structure of the α-6T/SubNc/SubPc solar cell is established.

2. EXPERIMENTAL SECTION UPS, XPS, and IPES measurements were performed in a single ultrahigh vacuum (UHV) chamber with a base pressure of 10−10 Torr. UPS photons were generated with a helium discharge lamp (21.22 eV for He I and 40.81 eV for He II), and the kinetic energy of the resulting photoelectrons was measured with a double-pass cylindrical mirror analyzer (CMA). Satellites due to the He I′ and He III excitations were removed from the final spectra. Nonmonochromatic Received: October 1, 2015 Revised: January 6, 2016

A

DOI: 10.1021/acs.chemmater.5b03857 Chem. Mater. XXXX, XXX, XXX−XXX

Article

Chemistry of Materials X-rays were generated for XPS using an aluminum anode (1486.6 eV), and the photoelectron kinetic energy was, again, measured with a CMA. IPES was performed in the isochromat mode as described previously.7 The electron kinetic energy is converted to binding energy on the basis of a calibration of the Fermi level energy using the Fermi step (UPS and IPES) and the 4f 7/2 core level peak of argon ion sputtered gold (XPS, 84.0 eV). The total resolutions for UPS, XPS, and IPES are approximately 0.15, 0.8, and 0.45 eV, respectively. The organic materials SubPc (Lumtec), SubNc (Lumtec), and α-6T (Sigma-Aldrich) were purified via thermal gradient sublimation. Each material was deposited through thermal evaporation in a UHV chamber with a base pressure of 10−9 Torr. The deposition rate was monitored via a quartz crystal microbalance and was maintained between 0.1 and 0.4 Å/s. Thicknesses for SubPc and SubNc growth conditions were subsequently calibrated with ellipsometry and atomic force microscopy (AFM). All materials were deposited on heavily doped p-type silicon substrates with a native oxide, which were cleaned via sonication in acetone and 2-propanol before being loaded into the vacuum chamber. After deposition, samples were transferred under UHV to the analysis chamber where UPS was used to measure the work function (Φ) and HOMO level from which IE was determined. The core levels of the constituent atoms were then measured via XPS. Finally, IPES was used to measure the LUMO level and, in conjunction with the UPS measurement of Φ, determine EA. For the evaluation of IE and EA, the HOMO and LUMO edges were determined via a conventional linear extrapolation of the leading edge of the HOMO and LUMO features, respectively. Additional UPS measurements were performed after XPS and after IPES measurements to detect any degradation due to X-ray or electron bombardment as well as track changes in the work function. No significant degradation due to these measurement techniques was observed, but minor changes in Φ did occur. To examine the energy level alignment at the interface between α6T and SubPc, 10 nm of α-6T was deposited on silicon (p+ with native oxide), measured with UPS, XPS, and IPES, before 0.5 nm of SubPc (∼1 monolayer) was deposited over the α-6T, and UPS, XPS, and IPES measurements were repeated. Additional measurements were taken after depositing a total of 1.5, 2.5, and 5 nm of SubPc over α-6T. The α-6T/SubNc and SubNc/SubPc interfaces were examined in a similar manner, looking at total thicknesses of 1, 3, and 8 nm of SubPc over 10 nm of α-6T and 0.5, 1.5, and 2.5 nm of SubPc over 8 nm SubNc on 6 nm of α-6T, respectively. The SubNc purity was investigated using 1H and 13C nuclear magnetic resonance (NMR) experiments on an 800 MHz NMR spectrometer equipped with a cryoprobe (see the Supporting Information). For these, the SubNc sample was dissolved in toluene-d8, which was used as the internal chemical shift reference, as well. All experiments were conducted at 295 K. Extensive resolution enhancement was applied using 25% nonuniform sampling (NUS) for the two-dimensional experiments. All other experimental details will be presented in a follow-up paper on NMR analysis.

Figure 1. Combined UPS and IPES spectra and the extracted ionization energies and electron affinities for 7.5 nm SubPc (green), 15 nm SubNc (red), and 15 nm α-6T (blue) on silicon (p+ with a native oxide). The energy scale is referenced to the vacuum level (Evac).

and IPES.11 Using the procedure described in the Experimental Section, these data yield a transport gap of ∼2.7 eV, which is considerably closer to what we find for α-6T. Varene et al. measured a similar transport gap value of 2.9 eV for a single monolayer α-6T on Au using two-photon photoemission.16 This value also uses the difference between the HOMO− LUMO peak centers, yet the proximity of the α-6T to the Au interface would result in enhanced polarization and screening of the photohole and extra electron, reducing the observed HOMO−LUMO gap, possibly explaining the lower peak-topeak value they find. The UPS and IPES spectra for SubPc and SubNc appear similar at first glance (Figure 1), but their differences are quickly distinguished. SubPc on silicon has a higher IE of 5.57 eV as opposed to 5.35 eV for SubNc, while SubPc has an EA (3.26 eV) lower than that of SubNc (3.47 eV). This results in a transport gap of approximately 2.3 eV for SubPc and 1.9 eV for SubNc. The IE value found for SubPc on silicon is comparable to those found previously with UPS on a range of substrates (5.55−5.7 eV).17−19 Until this point, UPS has not been performed on SubNc to the best of our knowledge. However, photoelectron yield spectroscopy (PYS) has measured an IE of 5.3 eV for SubNc,20 which is consistent with what we find. Previous investigations of the SubPc and SubNc LUMO levels have almost entirely been limited to cyclic voltammetry (CV), theoretical modeling, and estimations of EA using IE and the optical gap.21 None of these methods alone are ideal for determining the actual electron affinity or transport gap of a material. CV measurements of a material are performed in an environment that is very different from the solid films used in devices, making it difficult to account for intermolecular structure, disorder, or defects. The optical gap corresponds to the formation of a bound electron−hole pair, which underestimates the transport gap by the exciton binding energy.11 Using these methods, the EA values published have ranged widely from 3.35 to 3.6 eV for both SubPc and SubNc.20,22−25 In contrast, the combination of UPS and IPES allows one to directly probe the IE, EA, and transport gap at the surface of a thin film or stack of films of material, similar to the environment within an actual device. Using these techniques, we find that the transport gaps for SubPc and SubNc are ∼0.3 and ∼0.2 eV larger than the measured optical gaps of 2.0 and

3. RESULTS AND DISCUSSION From the UPS and IPES measurements taken for thick layers (7−15 nm) of each material on silicon (Figure 1), IE and EA values for each material were determined. α-6T was found to have an IE of 4.85 eV and an EA of 2.04 eV, giving a transport gap of ∼2.8 eV. Previous studies have presented many different energy level values for α-6T, which are found to be highly dependent on the substrate on which α-6T is deposited and the molecular ordering exhibited.8−14 The IE we find is consistent with previous values found for α-6T grown on silicon, and the higher intensity of HOMO−1 (centered around 6 eV) compared with that of the HOMO indicates the α-6T molecules are largely standing perpendicular to the surface.8,15 Fewer data are available to compare with the unoccupied α-6T levels; Hill et al. reported a center LUMO to center HOMO gap of 4.2 eV for α-6T on Au through the combination of UPS B

DOI: 10.1021/acs.chemmater.5b03857 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

Figure 2. XPS C 1s spectra for (a) 7.5 nm SubPc and (b) 15 nm SubNc. The blue line represents the C 1s peak for carbon atoms bonded to other carbon atoms within the macrocycle, while the green line represents those bonded to nitrogen. The solid blue and green peaks represent the π → π* shake-up peaks associated with the main C−C and C−N peaks, respectively. Higher-order shake-up peaks are indicated with gray lines. Gray squares represent the experimental data; the red line indicates the sum of the peaks, and the black line indicates the Shirley background. Both samples were deposited on p+ silicon substrates with a native oxide.

∼0.5 eV.28 Moreover, we also observe a 0.2 eV larger EA for SubNc than for SubPc. It is interesting to note that the same LUMO offset was also observed with CV measurements by Rubio et al.22 One possible explanation for the deeper than expected HOMO and LUMO levels for SubNc could be unintentional chlorination of the peripheral benzene rings. It is clear from the Cl 2p XPS spectra for both SubPc and SubNc films (Figure 3) that the SubNc sample has two distinct chlorine peaks while SubPc has one. The first Cl 2p peak for SubNc is centered at a 199.3 eV binding energy, matching the position of the single peak observed for SubPc. This peak is associated with the axial chlorine bonded to boron. The second peak is centered at a 200.5 eV BE, which is consistent with chlorine bonded to a benzene ring.29 The area of the second peak is ∼1.5 times that of the first peak, indicating that we have on average 1.5 additional chlorine atoms beyond the single axial chlorine per molecule. Chlorination of the SubNc macrocycle during synthesis is difficult to prevent entirely and is well-known to reduce chemical yields.28,30,31 1H and 13C NMR experiments with our purified material confirm that the SubNc molecule itself is partially chlorinated, likely in multiple configurations (see Figure S2). A detailed NMR analysis of the exact compounds involved will be presented in a follow-up paper. With partial chlorination of peripheral carbon rings, one would expect the HOMO and LUMO levels to shift toward a higher binding energy as has been demonstrated for halogenated SubPcs.23 By introducing one or two additional chlorine atoms per molecule, one would also expect the C3v symmetry of the SubNc molecule to be broken, causing the LUMO and LUMO+1 levels to lose their degeneracy and separate.28,32 Either of these effects could be responsible for the offset seen in the SubNc LUMO when compared with that of SubPc. Regardless of whether this is the case, it is clear that the IE and EA values we present here are for a partially chlorinated SubNc, rather than those for the pure SubNc molecule. Having an accurate measurement of ionization energy and electron affinity for each material in a device is an important step toward understanding its operation. However, these values alone do not provide the complete picture for energy level alignment within a heterostructure. One must also consider any

1.7 eV, respectively.6 The fact that the transport gap is larger than the optical gap is expected, and the measured differences can be considered an estimate of the exciton binding energy for each material. The SubPc and SubNc C 1s core level spectra (Figure 2) exhibit the C−C and C−N components in expected ratios of ∼3:1 and ∼5:1, respectively. Other components that appear at higher binding energies have been associated with π → π* shake-up satellites, whereby the photoemitted electron undergoes energy losses to excitations of electrons across the gap.18,26 Another point of interest relates to the Cl 2p spectra (Figure 3) and the different HOMO and LUMO level positions for

Figure 3. XPS Cl 2p spectra for (a) 7.5 nm SubPc and (b) 15 nm SubNc. The blue line represents the Cl 2p peak associated with the axial chlorine bonded to boron. The green line in panel b indicates excess chlorine detected in SubNc. Gray squares represent the experimental data; the red line indicates the sum of the peaks, and the black line indicates the Shirley background. Both samples were deposited on p+ silicon substrates with a native oxide.

SubPc and SubNc. The additional outer benzene rings, characteristic of SubNc, are expected to destabilize the HOMO level, lowering the ionization energy of SubNc compared to that of SubPc, while the LUMO level, and thus EA, is expected to be mostly constant between these two molecules.6,22,27,28 An IE difference of 0.22 eV is indeed detected with UPS, yet not as large as the expected value of C

DOI: 10.1021/acs.chemmater.5b03857 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

Figure 4. UPS and XPS spectra for 10 nm α-6T and 0.5, 1.5, 2.5, and 5 nm of SubPc deposited on α-6T. (a) UPS spectra SECO and HOMO regions including the removal of the normalized α-6T signal (dashed blue), resulting in the pure SubPc signal (red). (b) XPS spectra of the sulfur 2p peak. (c) XPS spectra of the chlorine 2p peak. The Φ given in panel a is calculated from the onset using the He I photon energy (21.22 eV).

band bending or vacuum level shifts that may occur due to dipoles or charge transfer at the interfaces.33 For this purpose, UPS, XPS, and IPES measurements were performed before and after depositing incrementally thicker layers of SubPc and SubNc on α-6T and SubPc on SubNc. Figure 4a shows the secondary electron cutoff (SECO) and HOMO regions measured with UPS for 10 nm of α-6T and for 0.5, 1.5, 2.5, and 5 nm of SubPc deposited over the original α6T layer. The x-axis represents binding energy with respect to the Fermi level. Even though UPS is very surface sensitive, with a probing depth of