Excitation of Charge Transfer States and Low-Driving Force Triplet

Letter pubs.acs.org/JPCL

Excitation of Charge Transfer States and Low-Driving Force Triplet Exciton Dissociation at Planar Donor/Acceptor Interfaces Fortunato Piersimoni,*,† David Cheyns,‡ Koen Vandewal,†,§ Jean V. Manca,† and Barry P. Rand‡ †

imec-IMOMEC, vzw, and Institute for Materials Research, Hasselt University, Wetenschapspark 1, B-3590 Diepenbeek, Belgium imec, Kapeldreef 75, B-3001 Leuven, Belgium

‡

S Supporting Information *

ABSTRACT: Here, we investigate charge transfer at archetypal planar heterojunction solar cells based upon phthalocyanines as donors and C60 or a perylene derivative as acceptors. We demonstrate the ability to measure photocurrent from direct charge transfer state excitation despite the intrinsically small interface area for bilayer systems. We then discuss the implications of triplet versus singlet excitons in these systems, and find that neither the low triplet energy nor low mobility of perylene acceptors with respect to C60 are responsible for reduced performance, but rather the low exciton diffusion length and unfavorable absorption profile. Furthermore, we show that triplet and singlet excitons from the phthalocyanine donors are able to dissociate with equal efficiency, even though the driving force is 0.5 eV less, and in fact only about twice the background thermal energy. Therefore, hot charge transfer states are not required, and efficient exciton dissociation is driven by an internal electric field at the heterojunction from either an interface dipole or beneficial polarization effects. SECTION: Energy Conversion and Storage; Energy and Charge Transport

T

tetracarboxylic bisbenzimidazole (PTCBI) as an acceptor. More recently, C60 is overwhelmingly used as an acceptor, resulting in a higher photocurrent. In this respect, there remain some fundamental and yet unanswered questions concerning these archetypal DA heterojunctions, with regards to why the use of PTCBI as an acceptor leads to lower donor EQE compared to C60. It is unclear, for example, whether this is solely the result of the longer exciton diffusion length of C60 with respect to PTCBI.24 Additional losses on the pathway from excitons to free charge carriers are geminate recombination, i.e., decay via CTS due to a slow dissociation into free charge carriers, or intersystem crossing, forming triplet states on the donor and/or acceptor material, with subsequent decay to the ground state. Here, we investigate the CTS and triplet states of PHJ solar cells composed of either CuPc or metal-free phthalocyanine (H2Pc) as donors and either PTCBI or C60 as acceptors. We show that the CuPc triplet state is active in photocurrent production, but that this does not result in a substantial loss pathway despite the fact that the CTS energy (ECT) is centered 0.04 eV less than the triplet energy (ET). Furthermore, the lowlying ET of PTCBI is shown to have a minor effect on the recombination of CTS. Rather the low exciton diffusion length

he increasing research effort devoted to organic solar cells has resulted in efficiencies now exceeding 10%,1 largely driven by the advent of new materials and the improvement of bulk heterojunction morphology to provide an advantageous trade-off between charge transport and exciton diffusion. Intrinsic to all efficient organic solar cells is the presence of an electron donor/electron acceptor (DA) heterojunction, whereby an offset in frontier molecular orbitals provides an energetic driving force capable of overcoming the exciton binding energy. Nevertheless, owing to the low dielectric environment and the localized nature of excited molecules in organic semiconductors, immediately after charge transfer, the geminate pair is not considered to be separated free charge carriers, but rather still bound by a Coulomb potential in the form of a charge transfer state (CTS) that exists across molecules at the DA interface.2−5 The CTS energy therefore normally correlates with the energy difference between the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor.6 Despite the fact that high external quantum efficiencies (EQEs) have been reported in certain DA systems, the exact mechanisms involved in the separation of CTS are still the topic of ongoing experimental and theoretical research.2−5,7−22 The first DA-based solar cell was introduced by Tang,23 and consisted of a planar heterojunction (PHJ) of copper phthalocyanine (CuPc) as a donor and 3,4,9,10-perylene © 2012 American Chemical Society

Received: June 15, 2012 Accepted: July 20, 2012 Published: July 20, 2012 2064

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(for current density versus voltage characteristics measured under 1 sun simulated AM1.5G conditions, see the Supporting Information). In all EQE spectra, the constituent D and A signals can be clearly distinguished, although in the case of PTCBI as an acceptor, there is considerable overlap of the CuPc and H2Pc signal with that of PTCBI, whereas the use of C60 results in very complementary absorption of the two materials. The results of transfer-matrix-based EQE simulations27 are plotted in Figure 1b for the various devices. For ease of comparison, we have assumed that the DA interface is the only exciton quenching interface, and that the charge transfer and charge collection efficiencies are unity. With these assumptions, we note that the exciton diffusion lengths, LD, of CuPc (LD = 8 nm) and H2Pc (LD = 7 nm) do not need to be altered when using C60 versus PTCBI as acceptors. This indicates that the reduction in phthalocyanine signal when paired with PTCBI originates from optical interference effects owing to the overlapping absorption within the PTCBI layer. Then, the major differences in performance are explained by considering that PTCBI has a lower LD = 4 nm compared with C60 which possesses LD = 20−25 nm, and are not therefore due to differences in charge transfer efficiency. In order to measure the EQE in the low energy, subgap region, we use the sensitive FTPS technique, which has been successfully applied numerous times to the study of CTS for bulk heterojunction solar cells,6,25,28 devices that exhibit an intrinsically large DA interfacial volume for the direct excitation of CTS. Surprisingly, and despite the considerably reduced DA interfacial volume, the CTS can be effectively measured through FTPS on PHJ devices as well, as shown in Figure 2. The FTPS spectra of DA devices with C60 are shown in Figure 2a, whereas FTPS spectra from solar cells utilizing PTCBI are shown in Figure 2b. One observation is that all FTPS spectra of the DA devices present a broad EQE component significantly red-shifted from that of the pure constituent layers. These broad features are assigned to CTS, and fitting this feature to a Gaussian shape6 provides the ECT values given in Table 1 for the various PHJs. The energies of the CTS are between 1.04 and 1.08 eV for all DA interfaces studied here, which is expected from the fact that the HOMO and LUMO energies of these donors and acceptors are all energetically similar, as given in Table 1. Another feature that emerges from the CuPc FTPS spectrum in Figure 2c is a transition at 1.12 eV that is embedded within the broad CTS band for the CuPc-based PHJs. This transition also prominently features in the FTPS spectra of the CuPc/C60 and CuPc/PTCBI devices, along with two additional peaks at 1.21 and 1.34 eV. In order to verify that these transitions originate from the CuPc film itself and not from the DA interface, we performed a sensitive PDS measurement to look at the absorption of a CuPc film. This spectrum is shown in Figure 2c, and indeed confirms that the transitions at 1.12, 1.21, and 1.34 eV are absorptive transitions intrinsic to the CuPc film. These low-energy transitions are similar to those observed by Fenukhin et al.36 via the constant photocurrent method, which they attributed to triplet absorption. Although normally spin-forbidden, triplet absorption in CuPc is weakly allowed owing to the paramagnetic Cu 2+ ion that splits the phthalocyanine triplet state into two states: a doublet and a quartet. The quartet is lower in energy, and the transitions to the ground state are forbidden, while they are allowed for the doublet state.37,38 In contrast, H2Pc, which lacks an efficient

and unfavorable absorption profile of PTCBI are responsible for the decreased EQE of this acceptor material as compared to C60. The PHJ devices used in this work were produced on indium−tin−oxide (ITO)-coated glass substrates (Kintec). Substrates were cleaned using detergent, acetone, and isopropanol, followed by a 15 min ultraviolet/O3 treatment. All organic materials (purified once with vacuum thermal gradient sublimation) and Ag top contacts used in this study are deposited in a high-vacuum evaporation chamber with a base pressure of 1 × 10−7 Torr. The PHJ device structures are as follows, with thicknesses in parentheses given in nanometers: ITO/donor(30)/acceptor(50)/bathocuproine(BCP, 10)/ Ag(150). For the single layer devices, the same donor or acceptor layer was employed, sandwiched between ITO and BCP/Ag electrodes. For the EQE measurements, light from Xe and quartz halogen lamps is coupled into a monochromator, and their intensities are calibrated with a Si-photodiode. For the Fourier transform photocurrent spectroscopy (FTPS)25 experiments, photovoltaic devices are used as an external detector of an Fourier transform infrared (FTIR), equipped with a quartz beamsplitter and quartz halogen lamp. Photothermal deflection spectroscopy (PDS)26 is performed using Fluorinert FC as the deflection medium and a 633 nm CW HeNe laser as the probe beam, as described previously. Optical constants are measured on highly doped n++-type Si substrates with thermally grown SiO2 using spectroscopic ellipsometry (SOPRA, ges5). The regression was performed assuming an isotropic medium. In Figure 1a we present the EQE spectra of the PHJ solar cells composed of the various DA pairs studied in this work, along with their extinction coefficients, k, shown in Figure 1c

Figure 1. (a) Measured, and (b) simulated EQE spectra for bilayer DA devices based on CuPc and H2Pc as donors and C60 and PTCBI as acceptors. (c) The extinction coefficient, k, as measured by spectroscopic ellipsometry, for thin films of the active materials used in this work. 2065

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the thermal energy (0.026 eV). Moreover, when we compare the EQE signal of the low energy, singlet Q-band of CuPc to that of H2Pc, we note that they are of the same order of magnitude, indicating that CuPc triplet excitons can contribute to photocurrent with similar efficiency as H2Pc singlet excitons, despite the fact that H2Pc singlet excitons allow for significantly more excess energy (≈0.5 eV) at the DA interface. The absorption39,40 and emission40 of CTS in coevaporated phthalocyanine:C60 films have been observed previously, and shown to be in the same energy range as that demonstrated here for bilayers, at approximately 0.8−1.1 eV. In fact, Akaike et al. proposed that this ground state CT interaction should also exist at a planar phthalocyanine/C60 heterojunction,39 as we confirm here via FTPS measurements. It is important to note that the CTS observed for the various DA interfaces are distributed over a broad energy range, the value given in Table 1 corresponding to the center of a Gaussian fit to the FTPS spectra minus a value accounting for the reorganization energy, estimated from the width of the fit.6 Therefore, even though H2Pc singlet excitons possess noticeably more excess energy with respect to CuPc triplet excitons, there still exists a considerable density of CTS at energies below the CuPc triplet level. In the case of PTCBI as an acceptor, we showed that CTS energies are comparable to that of C60. However, whereas ET of C60 has an energy of 1.56 eV, the triplet state of PTCBI lies at a much lower 0.86 eV (cf. Table 1). More importantly, the CTS for the various interfaces possess an energetic distribution almost entirely above ET of PTCBI, and therefore is a potential loss pathway for CTS, perhaps partially preventing the formation of charge separated states. In fact, the formation of PTCBI triplets in coevaporated films with both CuPc and C60 has previously been demonstrated,35 and therefore the presence of such a loss mechanism could be expected. Furthermore, back recombination of geminate pairs mediated via the donor triplet state has been observed in polymer-based bulk heterojunction systems.10,11,14 However, in the PHJ cells studied here, we found through our EQE simulations presented in Figure 1b that thin film optical interference accounts for the lower EQE from H2Pc and CuPc in devices utilizing PTCBI as an acceptor, without the need to consider additional geminate recombination mediated by the low PTCBI ET or with respect to the lower electron mobility of PTCBI versus C60. The final point concerns the efficient photocurrent production at the CuPc/C60 interface despite the small difference between ET and ECT. A few mechanisms are possible for converting the CTS into free photogenerated charges that

Figure 2. EQE spectra measured with FTPS for devices based upon (a) C60 and DA interfaces with C60 as acceptor, (b) PTCBI and DA interfaces with PTCBI as acceptor, and (c) CuPc and H2Pc. Also shown in panel c is an absorption spectrum of CuPc measured with PDS.

mechanism for intersystem crossing, presents no absorption in this energy range despite a similar ET (see Table 1). We note that the intensity of the triplet absorption feature of CuPc in going from the CuPc-only FTPS spectrum (Figure 2c) to the CuPc/C60 and CuPc/PTCBI spectra in Figure 2a,b, respectively, rises commensurately with the intensity of the CuPc absorption features owing to singlet absorption, at energies greater than 1.6 eV. Excitons that reach the DA interface are overwhelmingly of triplet character due to the high intersystem crossing yield of CuPc, according to past work.13 This observation at least confirms that CuPc triplet excitons are able to contribute efficiently to photocurrent despite the fact that ET is only 0.04−0.05 (±0.02) eV greater than the CTS energy, a difference that only amounts to approximately twice

Table 1. The Relevant Characteristics for the Materials (IP: Ionization Potential or HOMO; EA: Electron Affinity or LUMO; ES: Singlet Energy; ET: Triplet Energy) and DA Interfaces (ECT: Charge Transfer State Energy; VOC: Open-Circuit Voltage) Employed in This Work ECT {VOC (V)e}(eV)f material CuPc H2Pc C60 PTCBI

IP (eV)a 29−31

5.0−5.2 5.0−5.2 29 6.2−6.330,33 6.2 31

EA (eV)b 29−31

2.7−3.2 2.7−3.2 29 3.6−4.030,33 3.6 31

ES (eV)c

ET (eV)d

with C60

with PTCBI

1.6 1.6 1.8 1.5

1.12 1.1 32 1.56 34 0.86 35

1.08{0.52} 1.06{0.48}

1.07{0.44} 1.04{0.46}

Measured with ultraviolet photoemission spectroscopy with an error of ±0.1 eV. bMeasured with inverse photoemission spectroscopy with an error of ±0.5 eV. cMeasured from the absorption edge of a thin film. dMeasured from the absorption spectra with a Gaussian width of 0.04 eV. eMeasured at room temperature under 100 mW/cm2 AM1.5G simulated solar illumination. fMeasured from the EQE spectra with an error of ±0.02 eV for CuPc and ±0.01 eV for H2Pc. a

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do not take into account the formation of hot CTS, which appear to not play a role at the DA interfaces considered here. They involve entropic increase aided by an internal electric field: one, that interface dipoles are formed at the DA interface, and the other, that of a “gap bending” effect whereby polarization effects are different comparing the bulk of a thin film with that at the DA interface.17,39,41 The formation of an interfacial dipole at the CuPc/C60 interface has been measured and found to be quite significant, in the range of 0.3−0.5 eV,30,39,42,43 while the magnitude of the interface dipole at the CuPc/PTCBI interface is considerably smaller, approximately 0.1 eV.44 The second possibility, that of a gap bending effect, has been described experimentally for the CuPc/C60 system39 and theoretically for the H2Pc/PTCBI system,17 and could provide a sufficient internal field at the DA interface to form the basis by which these heterojunctions are able to function, despite the low driving forces involved. Finally, an additional consideration is the influence of the symmetry of the acceptor on the gap bending around the interface: the spherical shaped C60 molecule has no quadrupole moment, while PTCBI does.41 In summary, we have evaluated the charge transfer mechanisms at archetypal PHJ solar cells based upon CuPc or H2Pc as donors and C60 or PTCBI as acceptors. Via FTPS, we were able to measure the photocurrent due to direct CTS excitation for bilayer cells despite the intrinsically small interface area for such devices. We found that the low triplet energy of PTCBI with respect the CTS energy does not by itself account for the lower efficiencies observed when utilizing this acceptor, but is rather dominated by the low exciton diffusion length of PTCBI and its overlapping absorption with the phthalocyanine donors. Furthermore, we showed that triplet excitons on CuPc are able to dissociate with an efficiency comparable to that of H2Pc singlet excitons, even though the driving force is 0.5 eV less, and in fact only about twice the background thermal energy. This implies that hot CTSs are not needed for photocurrent generation in these devices, but that efficient exciton dissociation is rather explained by virtue of an internal electric field due to either the presence of an appreciable interface dipole or beneficial polarization effects at the DA interface.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The current density versus voltage characteristics for the various DA heterojunctions studied are given in Figure S1. This information is available free of charge via the Internet at http:// pubs.acs.org.

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Letter

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address §

Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the project ORGANEXT (EMR. INT4-1.2.-2009-04/054) under the program INTERREG IV-A Euregio Maas-Rijn for financial support of this work. 2067

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