Delayed Triplet-State Formation through Hybrid Charge Transfer

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Delayed Triplet-State Formation Through Hybrid Charge Transfer Exciton at Copper phthalocyanine/GaAs Heterojunction Heeseon Lim, Hyuksang Kwon, Sang Kyu Kim, and Jeong Won Kim J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02111 • Publication Date (Web): 15 Sep 2017 Downloaded from http://pubs.acs.org on September 17, 2017

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The Journal of Physical Chemistry Letters

Delayed Triplet-State Formation Through Hybrid Charge Transfer Exciton at Copper Phthalocyanine/GaAs Heterojunction Heeseon Lim,†, ‡ Hyuksang Kwon,‡ Sang Kyu Kim,†

Jeong Won Kim*,‡

†

Department of Chemistry, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Korea

‡

Korea Research Institute of Standards and Science, 267 Gajeong-ro, Yuseong-gu, Daejeon

34113, Korea Corresponding Author *[email protected]

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ABSTRACT Light absorption in organic molecules on an inorganic substrate and subsequent electron transfer to the substrate create so-called hybrid charge transfer exciton (HCTE).

The

relaxation process of the HCTE states largely determines charge separation efficiency or optoelectronic device performance. Here, the study on energy and time-dispersive behavior of photoelectrons at the hybrid interface of copper phthalocyanine (CuPc)/p-GaAs(001) upon light excitation of GaAs reveals a clear pathway for HCTE relaxation and delayed triplet formation. According to the ground-state energy level alignment at the interface, CuPc/pGaAs(001) shows initially fast hole injection from GaAs to CuPc. Thus, the electrons in GaAs and holes in CuPc form an unusual HCTE state manifold. Subsequent electron transfer from GaAs to CuPc generates the formation of the triplet state in CuPc with a few ps delay. Such two-step charge transfer causes delayed triplet state formation without singlet excitation and subsequent intersystem crossing within the CuPc molecules.

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Organic/inorganic hybrid systems1-4 have attracted considerable attention as a promising way to simultaneously meet the demands of high efficiency, low cost, and environmental compatibility by incorporating the strengths of both materials.5 For optoelectronic devices based on an organic/inorganic hybrid system such as light-emitting diodes, solar cells, and photodetectors, a fundamental understanding of light absorption/emission and charge carrier motion at the hybrid interface is critical to improve their efficiency. In highly-efficient dyesensitized solar cells, for instance, the electron transfer from photoexcited dye molecules to an inorganic semiconductor is sufficiently fast to compete effectively against recombination loss processes.6 However, such electron transfer may produce a bound electron-hole pair state at the heterointerface before complete dissociation. Recently, the presence of a hybrid charge transfer exciton (HCTE) at ZnO/4,4’-bis(N-carbazolyl)-1,1’-biphenyl and ZnO/polymer heterojunctions has been suggested by optical and device measurements.7,8 The growing interest in the HCTE has also been demonstrated by theoretical and experimental investigations at 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA)/WS2 organic/twodimensional transition metal dichalcogenide hybrid heterojunction.9 The hybrid “FrenkelWannier exciton” at the organic/inorganic hybrid interface10,11 has electron-hole bound pair states, similarly to the well-known character of charge-transfer (CT) states at organic solar cell interfaces.12 The ground-state bound CT pair needs extra energy to be separated, which limits photovoltaic performance. Most hybrid interfaces consist of photo-excited organic donor and inorganic acceptor materials. However, converse configurations such as an inorganic light absorber and an organic counterpart have seen little studied. There have only been a few reports on fast energy transfer from nanostructured inorganic semiconductors to organic or polymer materials.13,14 In another example of a charge transfer system, copper phthalocyanine (CuPc) on GaAs(001) shows resonant hole transfer from GaAs to CuPc15 whereas C60 on GaAs(001) 3 ACS Paragon Plus Environment


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shows efficient electron transfer via a space charge field.16 These results could be well predicted by the initial energy level alignment at each hybrid interface. On the other hand, the possibility of the HCTE state formation at the interface of organic molecules on GaAs has not been questioned. Moreover, by controlling the doping type of GaAs and the intrinsic doping type of organic molecules (either n-type C60 or p-type CuPc), it is possible to switch the direction of the electron-hole pair after charge transfer at the interface. This advantage of the charge transfer scheme over energy transfer can be exploited to achieve charge separation or light emission by selectively pumping a low bandgap material at one side. However, little work demonstrating this mechanism has been reported to date. Here we compare four different combinations of organic/inorganic interfaces regarding the short-lived HCTE state. For CuPc/p,n-GaAs(001) and C60/p,n-GaAs(001) interfaces (see sample preparation in Figure S1), their interfacial electronic structure is investigated by the combination of ultraviolet photoelectron spectroscopy (UPS) and inverse photoemission spectroscopy (IPES), followed by prediction of the interface energy level alignment, and directions of the space charge field and charge transfer. Using time-resolved two-photon photoemission spectroscopy (TR-2PPE), the energy-dispersive and dynamic behavior of charge transfer and separation upon GaAs excitation is studied. While C60/p-GaAs(001) shows electron injection or charge separation from GaAs to C60 layers,16 CuPc/p-GaAs(001) shows hole injection to CuPc. In particular, on CuPc/p-GaAs(001), we observe an intriguing phenomenon of roughly 2 ps delayed triplet formation in CuPc. This must be mediated by the high-energy HCTE state of a hole in CuPc and an electron in GaAs, which is initially generated by the fast hole injection from GaAs(001) to CuPc. As a result, triplet state can be formed without initial pumping of CuPc to a singlet state and subsequent intersystem crossing. The mechanism for the sequential formation of HCTE and delayed triplet states


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upon sub-bandgap excitation will provide a strategy for a new hybrid optoelectronic device design.

Figure 1. UPS and IPES spectra of (a) occupied states and (b) unoccupied states for bare pGaAs(001)

(red),

C60/p-GaAs(001)

(blue) and

CuPc/p-GaAs(001)

(green)

hybrid

heterojunctions. Dotted and full lines at (a) indicate the spectra at 1 ML and 5 ML coverage of C60 and CuPc on p-GaAs(001), respectively. Red dotted vertical lines denoted by EF, indicate the Fermi level position. The bold vertical bars indicate the energy onsets of frontier occupied and unoccupied states below and above EF.

The ground-state electron energy level is a good indication of energy overlap and charge transfer direction upon excitation. To compare energy level alignment at C60/p-GaAs(001) and CuPc/p-GaAs(001), UPS for the valence band and IPES for the conduction band were carried out as shown in Figure 1a and b, respectively. The valence band maximum (VBM) of bare p-GaAs(001) appears clearly at ~0.6 eV below EF by removing the As-capping layer and



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other contamination.16,17 After the deposition of organic molecules of C60 and CuPc on the clean p-GaAs(001) surface, the onsets of the highest occupied molecular orbital (HOMO) levels are observed at ~1.64 eV for C60 and at ~0.42 eV for CuPc below EF, respectively, which are constant irrespective of their coverage. The cleanliness of the initial surface and the organic film thickness are confirmed by XPS measurement in Figures S2 and S3, respectively. Complementary to UPS, IPES measurement was carried out to construct their entire electronic structure by adding unoccupied states in Figure 1b. We identify the positions of the conduction band minimum (CBM) of p-GaAs(001), and the lowest unoccupied molecular orbital (LUMO) levels of C60 and CuPc at ~0.8 eV, ~0.46 eV, and ~1.4 eV above EF, respectively. The transport band gaps determined from the UPS and IPES measurements are ~ 1.4 eV for p-GaAs(001),18 ~2.1 eV for C60,19 and ~1.82 eV for CuPc,20 respectively. Those values are the basic parameters for band alignment at the heterojunction interfaces hereafter.

Figure 2. Two-dimensional TR-2PPE traces of 1ML CuPc/p-GaAs(001) hybrid heterojunction on (a) short and linear time scale and (b) long and logarithmic time scale. The pump (hv1) and probe energies (hv2) are 1.41 eV and 4.22 eV, respectively. (c) Electron energy distribution with different time delays. Inset: magnified spectra in the region of high 6 ACS Paragon Plus Environment

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kinetic energies. (d) Two-dimensional TR-2PPE trace of 10 ML CuPc/p-GaAs(001) hybrid heterojunction on short time scale.

To confirm the carrier dynamics at the hybrid heterojunction, we carried out TR-2PPE spectroscopy of 1 ML CuPc on the p-GaAs(001) using pump energy (hv1) of 1.41 eV and probe energy (hv2) of 4.22 eV, respectively. The pump energy delivers neither an opportunity for CuPc excitation nor GaAs hot electrons (see Figure S4). Figure 2a and b show the pseudo two-dimensional TR-2PPE plot for the 1 ML CuPc/p-GaAs(001) hybrid heterojunction on different time scales. Photoexcited electrons show very fast initial decay within ~1 ps. It is surmised that 1 ML CuPc on p-GaAs(001) attenuates the electron emission because there is no electron transfer channel from GaAs CBM to CuPc LUMO with respect to energy level alignment. However, electron signals with low kinetic energy increase again within ~100 ps. Figure 2c clearly shows that the low kinetic-energy population increases monotonically beyond 0.5 ps, while the electrons with high kinetic energy decay at an early stage in the inset. To verify this unique behavior attributed to the hybrid interface, we carried out the same measurement for 10 ML CuPc on p-GaAs(001) in Figure 2d. The figure shows no clear electron dynamics, which is completely different from the case of the 1ML CuPc/pGaAs(001) hybrid interface. As the CuPc film has a larger band gap (~1.82 eV)20 than the pump energy (hv1) of 1.41 eV, the top CuPc molecules are insensitive to the direct excitation. Given that only the GaAs under the CuPc is excited, the excited electrons should be transported through the CuPc film. However, such a long range of electron penetration does not occur. As a result, it is clear that the unique dynamics of electron decay and regeneration at CuPc/p-GaAs(001) is localized to the hybrid interface.



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Figure 3. (a) Normalized and background-subtracted TR-2PPE spectra of 1ML CuPc/pGaAs(001) hybrid heterojunction at different pump-probe time delays (∆t = 0.5, 3, 80, and 500 ps) with the secondary electron background being marked by a black dotted line. Inset: pseudo two-dimensional TR-2PPE trace. (b) Decomposition of TR-2PPE spectra at the four different time delays. Each spectrum is fitted with three different Gaussian functions (red, green, and blue) at 1.03, 0.42, and 0.17 eV. (d) Intensity changes of the three Gaussian components denoted by T1, HCTE, and HCTE*, respectively, as a function of time delay.

To clarify the carrier dynamics in the CuPc/p-GaAs(001) hybrid system, we removed the secondary electron background after normalizing the TR-2PPE spectra at each time delay as shown in Figure 3a. In the inset of the pseudo two-dimensional TR-2PPE trace, both the decaying component around 1.0 eV and the rising component around 0.2 eV are clearly displayed. To discriminate energy-dependent dynamics, we decompose the spectra with three Gaussian functions at the high, middle, and low kinetic energies, respectively. Figure 3b shows the relative intensity change of the three Gaussians at four different time delays. At a 8 ACS Paragon Plus Environment

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pump-probe delay of 0.5 ps, a broad high-energy peak at 1.03 eV is clearly observed even though its total intensity is the lowest. Since the CuPc LUMO level is above the GaAs CBM, rapid electron transfer from photo-excited p-GaAs(001) to CuPc molecular film is not allowed. Thus the fast decrease of the high-energy peak intensity must have another pathway of decay dynamics. In the CuPc/p-GaAs(001) hybrid heterojunction, it has already been reported that hole injection from photoexcited p-GaAs(001) to CuPc is energetically favorable and very fast.15 Because the “hot” charge transfer exciton has a large delocalized size, it results in a population decrease of total electrons at the interface.21,22 Therefore, we assign the high-energy peak (~1.03 eV) generated in the sub-ps range to a “hot” HCTE state with a hole in CuPc and an electron in GaAs. During the next several ps (~3 ps), the highenergy peak almost vanishes while the middle-energy peak of ~0.42 eV emerges. To quantify each time constant, we fit each intensity change by exponential functions in Figure 3c. The time constants obtained from the fitting are summarized in Table 1. The rising time (τ ~1.7 ps) of the middle-energy peak is comparable to the decay time (~1.9 ps) of the high-energy peak. Thus, the cooling of “hot” HCTE generates “relaxed” HCTE states at the lower energy region. Lastly, the low-energy peak at 0.17 eV evolves. The rising time of the low-energy peak (τ~1.9 ps) is also the same as the decay time of HCTE*. Then its intensity remains constant within the current measurement time window (~500 ps). In the CuPc molecular film, it has been verified by TR-2PPE and transient absorption studies that intersystem crossing (S1→T1) occurs within sub-ps and triplets are the dominant exciton in a vapor deposited CuPc film with a very long lifetime of 8.6 ± 0.6 ns.23,24 For the present case, since there is no direct excitation within the CuPc molecule, the photo-excited electrons in GaAs should be transferred to CuPc from the HCTE state to form the T1 state in CuPc. Thus the low energy peak is attributed to the delayed triplet state through “hot” HCTE states across the interface. However, the “relaxed” HCTE does not contribute to the formation of the delayed T1 state 9 ACS Paragon Plus Environment


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because the population of triplet electrons is almost constant during the relaxation process of the HCTE state (τ = ~31, 430 ps).

Table 1. Decay or rising time constants of excited carriers marked in Figure 3c for CuPc/pGaAs(001) hybrid heterojunction.

Fitted values, 10-12 s

τ ∗ (decay) CuPc/p-GaAs(001) 1.9, 38

τ (decay)

τ (rise)

τ (rise)

31, 430

1.7

1.9

τ ∗ is the decay time of “hot” hybrid charge transfer exciton (HCTE) states, τ the decay time of “relaxed” HCTE states, τ the rising time of “relaxed” HCTE states, and τ the rising time for triplet formation.

In organic/organic systems such as C60 or tetracene with a CuPc interface, similar behavior has been observed through the interfacial charge transfer state.24,25 However, delayed T1 state formation through the HCTE state in an organic/inorganic hybrid heterojunction system is unprecedented. In the present hybrid system, the holes generated in p-GaAs(001) are transferred to the CuPc molecular film in very fast time of ~120 ± 30 fs,15 followed by electron movement toward the surface along with downward band bending in Figure S4b. The transferred hole in the thin CuPc film at the interfacial region is bound within a molecule, because the orientation of CuPc is parallel to the GaAs(001) surface,26 which results in weak coupling between lateral molecules. Thus, if electron moves close to the interfacial region, the electrons and holes could have a strong coulombic interaction. Consequently, this process causes the HCTE states in an organic/inorganic hybrid heterojunction system. The mechanism of the delayed T1 formation is illustrated in Figure 4. To clearly verify the HCTE formation mechanism, we compare the carrier dynamics at two different pump energy densities of 15.9 and ~3.3 µJ/cm2 using TR-2PPE spectroscopy. After pump laser-on, the 10 ACS Paragon Plus Environment

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pump beam induces stronger band flattening due to the surface photovoltaic (SPV) effect,27 as observed in Figure 4 (solid lines). The SPV is typical result from non-equilibrium electron density population induced by laser illumination in space charge layers in p-GaAs(001).27-29 According to a previous study on CuPc/p-GaAs(001) using time-resolved second harmonic generation (TR-SHG) measurement,15 at a low energy density below 14.5 µJ/cm2, excited electrons and holes move in opposite directions along the space charge field (red thin arrows in Figure 4a). This condition does not contribute to the hole injection to CuPc. In Figure S6, the delayed T1 formation is not observable at low pump energy density (~3.3 µJ/cm2) but it is only observed at a high energy density (~15.9 µJ/cm2), where hole injection overwhelms band flattening. Therefore, the hole injection from GaAs to CuPc and the presence of the “hot” HCTE state plays a crucial role in the formation of the CuPc T1 state.

Figure 4. Comparison of charge carrier dynamics in (a) CuPc/p-GaAs(001) and (b) C60/pGaAs(001) organic/inorganic hybrid heterojunctions as derived from UPS, IPES, and TR2PPE measurements. Brown solid lines in VBM and CBM of p-GaAs(001) show the band



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bending variation by the surface photovoltaic effect according to the pump energy density (15.9 µJ/cm2 and 3.3 µJ/cm2).

Figure 4 presents a comparison of carrier dynamics proposed by UPS, IPES, and TR-2PPE results at two kinds of organic/inorganic hybrid heterojunctions. According to interfacial energy level alignments, each system shows completely different dynamical behaviors of photo-excited carriers. Although the carrier dynamics at C60/p-GaAs(001) hybrid heterojunction have been studied before,16 we verified the previous results that describe ultrafast electron transfer (~0.19 ps) from the CBM of p-GaAs(001) and the LUMO+1 of C60 due to the resonant energy level and the drift of electrons in the CBM of p-GaAs(001) toward the LUMO of C60 within a few ps, as shown in Figure S7. Consistently with the type II energy level alignment between p-GaAs(001) and n-type C60, efficient electron transfer from p-GaAs(001) to the C60 occurs without any time delay. Thus, C60/p-GaAs(001) is regarded as an efficient charge separation system. the On n-GaAs(001) surface in Figure S8, however, the opposite space charge field forms a barrier to the electron transfer to molecules and only hole transfer to CuPc is possible. On the other hand, in the case of the p-type CuPc/p-GaAs(001) hybrid heterojunction, a unique and unusual hybrid state at the interface is identified. The HCTE state formation at an organic/inorganic heterojunction has been theoretically and experimentally predicted as a hybrid “Frenkel-Wannier exciton” bound to the heterojunction.30,31 As the HCTE formation is strongly dependent on carrier transfer between energetically resonant states at the two sides of the junction,32 the energy resonance between the VBM of p-GaAs(001) and the HOMO of CuPc induces strong electronic coupling, and thus generates a new hybrid state at the interface through ultrafast hole injection. The charge transfer state generally consists of a 12 ACS Paragon Plus Environment

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manifold of vibronic states generated at the donor/acceptor interface.33 Thus, it is noted that HCTE* and HCTE states do not mean two distinct states but are categorized as “hot” exciton state and “relaxed” exciton state depending on the kinetic energy of photoelectrons, respectively. During the course of the “hot” HCTE state relaxation at the interface, one of the highly probable pathways is the formation of the T1 state. The time or probability of T1 formation is entirely dependent on the formation of “hot” HCTE states. Another competitive path is cooling of the “hot” HCTE states to “relaxed” HCTE states. The lifetime of “relaxed” HCTE states (τ = ~31, 430 ps) is comparable with the lifetime of band edge electrons of bare p-GaAs(001). Therefore, the “relaxed” HCTE cannot contribute to the T1 formation but rather it undergoes recombination. To emit luminescence from an organic material, a high energy above S1 excitation is generally needed because direct excitation to a T1 is forbidden by symmetry. Thus, to form a singlet in CuPc, a high energy photon of ~1.82 eV is necessary. However, it is shown that the GaAs excitation with photon energy of 1.4 eV, which is slightly above the T1 energy of CuPc, can be utilized to form triplet states from organic materials. Our findings present a new possible way of T1 formation even without passing through singlet formation and subsequent intersystem crossing. This observation may provide motivation for developing new types of hybrid optoelectronic devices where low-energy charge excitation is needed. Regardless of whether initial excitation is achieved by optical pumping or electrical activation, optoelectronic devices can have low energy consumption.32 However, the pathways of HCTE relaxation should be avoided to achieve high yield of T1 formation or phosphorescence. In conclusion, we have investigated the energy level alignments and carrier dynamics at the CuPc/p-GaAs(001) organic/inorganic hybrid heterojunction. To reveal the dependence of carrier dynamics on interfacial energy level alignment, we controlled the doping type of GaAs(001) and the organic counterpart using C60. On CuPc/p-GaAs(001), photo-excited 13 ACS Paragon Plus Environment


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electrons are transferred from the p-GaAs(001) to the surface region along the direction of band bending after resonant hole injection toward organic molecules occurs on a few hundred fs time scale. This rearrangement of the interface charge population causes a manifold of HCTE states within 1 ps. During the course of HCTE state relaxation, further electron injection from the GaAs to the CuPc T1 state takes place. The T1 formation at the hybrid interface is not mediated by the S1 state of the CuPc molecule but through the HCTE state with excitation energy of 1.4 eV, far below the S1 energy. This is a remarkable advantage for low-power optoelectronic devices since sub-bandgap excitation is enough for operation. However, as only hot HCTE states contribute to the T1 formation, a strategy to utilize relaxed HCTE states before recombination for higher efficiency of T1 formation is required.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected]

ACKNOWLEDGMENT We acknowledge support from the National Research Foundation (NRF) (grant No. 2014R1A2A2A01007296, 2017R1A2B4012086, and 2015R1A2A1A01004470).

ASSOCIATED CONTENT Supporting Information


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Sample preparation, core level spectra and their intensity changes with film thickness, carrier dynamics of bare p-GaAs(001), laser energy-density dependence of the lowest energy peak, carrier dynamics at C60/p-GaAs(001) hybrid heterojunction, and carrier dynamics of hybrid heterojunction at n-GaAs(001).

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