Charge-Transfer Hybridization Simultaneously

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Energy Conversion and Storage; Plasmonics and Optoelectronics

Local Excitation/Charge Transfer Hybridization Simultaneously Promotes Charge Generation and Reduces Non-Radiative Voltage Loss in Non-Fullerene Organic Solar Cells Guangchao Han, and Yuanping Yi J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b00928 • Publication Date (Web): 15 May 2019 Downloaded from http://pubs.acs.org on May 16, 2019

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The Journal of Physical Chemistry Letters

Guangchao Han,† and Yuanping Yi*,†,‡ †Beijing

National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡University of Chinese Academy Sciences, Beijing 100049, China ABSTRACT: High power conversion efficiencies in state-of-the-art non-fullerene organic solar cells (NF OSCs) call for elucidation of the underlying working mechanisms of both high photocurrent densities and low non-radiative voltage losses under small energy offsets. Here, to address this fundamental issue, we have assessed the nature of interfacial charge-transfer (CT) states in a representative small-molecule NF OSC (DRTB-T:IT-4F) by time-dependent density functional theory calculations. The calculated results point to that the CT states can borrow considerable oscillator strengths from the energyclose local-excitation (LE) states or be fully hybridized with these LE states by molecular aggregation at the donor-acceptor interfaces. The LE/CT hybridization can promote charge generation by direct population of thermalized CT or LE/CT states under illumination. At the same time, the increased oscillator strengths of the lowest CT state will improve the luminescence quantum efficiencies and thus reduce non-radiative voltage losses. Our work suggests that it is crucial to tune the LE/CT hybridization by optimization of the donor and acceptor molecular and interfacial structures for further improving the NF OSC performance.

(TOC)

Organic solar cells (OSCs) have achieved remarkable progress in the last three years, mainly due to the rapid development of non-fullerene (NF) acceptors.1-4 Now the power conversion efficiencies (PCEs) of NF OSCs have reached 16% for single-junction devices,5,6 which outperform the best fullerene-based OSCs (11-12%).7-9 In these NF OSCs, charge generation is very efficient even under small charge-transfer (CT) driving force (ΔECT < 0.1 eV).10-17 We recall that ΔECT = Eg – ECT, and ≈ (ΔEHOMO or ΔELUMO) – ΔEb under the first-order approxation,18 where Eg is the lowest optical gap of either the D or A material, and ECT is the energy of the CT exciton, ΔEHOMO or ΔELUMO is the HOMO or LUMO energy difference between electron donor (D) and electron acceptor (A), ΔEb is the difference of the

binding energies of a local-excitation (LE) exciton and of a CT exciton. Moreover, the non-radiative (NR) voltage loss ΔVnr is quite low due to high electroluminescent external quantum efficiency [EQEEL ~ 10-5-10-4, ΔVnr = (kBT/q) *ln(EQEEL-1)19 ~ 0.2-0.3 V, where kB is the Boltzmann constant, T is the temperature, and q is the elementary charge].10,15,20-22 As a result, high photocurrent and low voltage loss can coexist [ΔVloss = Eg/q – VOC = ΔECT/q + ΔVr + ΔVnr ≤ 0.6 V, where VOC is the open-circuit voltage and ΔVr (≈ 0.3 V) is the radiative voltage loss that is unavoidable for any type of solar cells]. In contrast, for traditional fullerenebased OSCs, charge generation usually becomes inefficient under small ΔECT23-25 and low EQEEL (~ 10-9-10-6) yields large ΔVnr (~ 0.35-0.55 V).26,27 These striking phenomena

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thus stimulate us to elucidate the new working mechanisms in the state-of-the-art NF OSCs. It has been well recognized that the CT states at D-A interfaces mediate the processes of exciton dissociation (ED), charge separation (CS), and charge recombination (CR).28,29 Therefore, assessing the nature and energetics of the interfacial CT states is pivotal to understand the fundamental mechanisms of charge generation and recombination as well as voltage loss.30-36 Experimentally, the energy (ECT) and reorganization energy (λ) of the lowest CT state (CT1) can be obtained by fitting the absorption and emission spectra of D-A blends in terms of Marcus37 or Marcus-Levich-Jortner (MLJ)38 theory,24,39-44 and the CT1state lifetime can be determined by ultrafast transient absorption spectroscopy.45-49 It should be noted that when the CT1 state is very close to the LE state, a three-state vibronic model would be imperative.35 Theoretically, firstprinciples calculations, e.g. via (time-dependent) density functional theory [(TD)DFT], can address these issues as well.28,50-53 Moreover, by analyzing the electron-hole density distributions in D-A complexes, charge delocalization and separation can be understood at the molecular level.46,54-57 Most of previous theoretical studied focused on fullerenebased systems (such as pentacene-C60,54,55,58,59 DTDCTBC60,56,60 P3HT-PC61BM,46,57,61-65 and PCPDTBT-PC61BM64,66). Relatively, the high-performance NF systems are rarely calculated, which lags far behind the rapid development of NF acceptors. Generally, high-performance NF OSCs comprise of a wide band gap small-molecule (SM) or polymer donor and a narrow band gap SM acceptor (e.g., ITIC67 and its derivatives68-71) with a small ΔEHOMO (~ 0.2 eV or less) to achieve complementary sunlight absorption and high VOC at the same time. The schematic diagram of frontier molecular orbitals energy levels for such NF OSCs is displayed in Figure 1a. Here, it should be mentioned that the LUMO+1 level of the NF acceptor is usually lying below the donor LUMO level.72 Thus the excitons on donor can be dissociated into high-lying CT states and further separated into free charge carriers through a hot process (see Figure 1b).60,62,73,74 However, owing to the small ΔEHOMO, most of the excitons on acceptor will be dissociated into the CT1 state, which then undergoes a cold CS process,75-77 if geminate CR is suppressed.78-80

Figure 1. Schematic diagrams of energy level alignments of (a) frontier molecular orbitals and of (b) excited states for a high-performance non-fullerene organic solar cell that consists of a wide band gap small-molecule or polymer donor and a narrow band gap SM acceptor with a small ΔEHOMO. The black dashed lines represent the vibrational states of relevant electronic states. D: donor; A: acceptor; S0: ground state; S1D and S1A: the first singlet excited states on D and A; CT: charge-transfer states; CS: charge-separation

states; hν1, hν2, and hν3: vertical excitation energies of S1D, S1A, and CT1 (the lowest CT state); k*ED and kED: the rates for exciton dissociation at the D-A interface; k*CS and kCS: the rates for hot CS and cold CS; kr and knr: radiative and nonradiative decay rates of the CT1 state; kCR: the total charge recombination rate. Recently, Hou et al. fabricated NF OSC devices using a wide band gap molecule DRTB-T as donor (Figure S1a)81 and IT-4F (tetra-fluorinated ITIC, Figure S1b) as acceptor,68 and achieved a PCE of 11.24%, among the highest values of all-small-molecule OSCs.82 Same to other high-performance NF OSCs, the DRTB-T:IT-4F based OSCs also have small driving force for hole transfer (Figure S1c). The experimental measured ΔEHOMO between DRTB-T and IT-4F is only ~ 0.15 eV. Considering the Eg of IT-4F (~ 1.52 eV) and VOC (~ 0.92 V), a low ΔVloss of ~ 0.6 V is obtained. If the ΔVr is set to a typical value of 0.3 V, the ΔVnr will be less than 0.3 V due to the presence of ΔECT. Here, taking DRTB-T/IT4F complexes as representative D-A interfaces, we have investigated the nature and energetics of interfacial excited states to elucidate the underlying charge generation mechanisms in high-efficiency NF OSCs by means of highlevel DFT/TDDFT calculations. The D-A interface geometries and molecular aggregation have important impact on the interfacial energetics and electronic processes.28,50-52,54-57,62,83-98 Molecular dynamics simulations have shown that the DRTB-T and IT-4F molecules are inclined to interact with each other by endgroup π-π stacking due to the large steric hindrance of side chains on the central core.90,91,99,100 According to such packing mode, we construct the structures of the (DRTB-T)(IT-4F) complex and the dimers of DRTB-T and IT-4F [2(DRTB-T) and 2(IT-4F)] (see Figure 2 and S2). To reveal the influence of molecular aggregation on charge delocalization and initial CS process, the 2(DRTB-T)-(IT-4F) and (DRTB-T)-2(IT-4F) complexes are constructed by using two donor and one acceptor molecules or one donor and two acceptor molecules. The geometries of all the complexes and dimers are then optimized by DFT at the ωB97XD/6-31G** level. It should be noted that ωB97XD is a range-separated (RS) functional with dispersion correction included, which can reliably describe weak intermolecular interaction.101 Based on the optimized geometries, the excited states are calculated by TDDFT at the ωB97XD/6-31G** level, with the RS parameter (ω) of the functional optimally tuned under the dielectric environment that is considered via the polarizable continuum model (PCM). The dielectric constant εr is set to 4.0, a medium value for organic semiconductors.83 Such tuned RS functionals have been demonstrated to be able to provide reasonable description of the nature and energetics of excited states both in the condensed phase and at the DA interface.56,57,59,83,102-106 All the DFT and TDDFT calculations were carried out by using the Gaussian 16 package.107


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The Journal of Physical Chemistry Letters

Figure 2. DFT-ωB97XD/6-31G** optimized geometries of (a) the DRTB-T and IT-4F molecules, (b) the 2(DRTB-T) and 2(IT4F) dimers, and (c) the (DRTB-T)-(IT-4F), 2(DRTB-T)-(IT-4F), and (DRTB-T)-2(IT-4F) complexes. All alkyl chains are replaced with methyl groups to reduce the computational costs.

The calculated ΔEHOMO between DRTB-T and IT-4F or between 2(DRTB-T) and 2(IT-4F) is 0.24 or 0.22 eV, which is consistent with the small value measured by experiment. The calculated excitation energies of the first singlet excited state (S1) for DRTB-T (2.09 eV) and 2(DRTB-T) (2.07 eV) are very close to the first absorption peak energy measured for the spin-coated DRTB-T film (~ 2.12 eV). And the first absorption peak energy of the IT-4F film (~ 1.73 eV) is between the calculated S1 excitation energies of IT-4F (1.79 eV) and 2(IT-4F) (1.71 eV). This further verifies the computation methods chosen here. The oscillator strengths (f), main electronic transition configurations, and relevant frontier molecular orbitals for S1 are listed in Table S1. The excited states of the three D-A complexes can be classified into LE (local excitation on the D or A region), CT (electron transfer from the D to A region), LE/CT (hybridized), or CS type. The excitation energies are shown in Figure 3. The f and main electronic transition configurations for each excited state along with relevant frontier molecular orbitals can be found in Table S2. To better understand the mechanisms of charge generation, the electron-hole density distributions for these states were also depicted in Figure 4 and S3.108,109

Figure 3. Vertical excitation energies calculated by TDDFT at the PCM-tuned-ωB97XD/6-31G** level for (a) (DRTB-T)-(IT4F), (b) 2(DRTB-T)-(IT-4F) and (c) (DRTB-T)-2(IT-4F). The calculated absorption spectra for single molecules, dimers, and D-A complexes are included inset.

Figure 4. Electron-hole density maps (red: electron; blue: hole) for local-excitation, CT, hybridized (brown), and CS (green) states: (a) (DRTB-T)-(IT-4F), (b) 2(DRTB-T)-(IT-4F), and (c) (DRTB-T)-2(IT-4F). The distance between electron and hole centers for each state is included inset. For (DRTB-T)-(IT-4F), the first LE state (S1A, mainly localized on IT-4F) is at 1.8 eV (f = 2.44), which is only 0.09 eV higher than CT1 (1.71 eV, f = 0.88). This indicates a very small ΔECT for ED, corresponding to a small value of ΔEHOMO. Apart from CT1, the second and third CT states (CT2: 1.96 eV, f =0.02; CT3: 2.04 eV, f = 0.23) are also lying below the second LE state (S1D: 2.05 eV, f = 2.19, mainly localized on DRTB-T), which implies that the DRTB-T excitons can be dissociated into the high-lying CT states. Interestingly, owing to the close energies of S1A and CT1 (S1D and CT3), S1A and S1D have a CT fraction (which leads to a smaller f) while CT1 and CT3 have a LE fraction (which leads to a noticeable f, especially for CT1). Importantly, owing to the incorporation of a degree of LE component, the two CT states can be populated directly by photoexcitation.65,110

Interestingly, molecular aggregation of DRTB-T or IT-4F at the D-A interface can render the formation of fully hybridized LE/CT states: nearly degenerate S1D/CT3 and S1D/CT3’ for 2(DRTB-T)-(IT-4F) and S1A/CT1 and S2A/CT1 for (DRTB-T)-2(IT-4F). As can be seen, the endgroup π-π stacking can decrease the S1 energy to some extent, ~ 0.02 eV for DRTB-T and ~ 0.08 eV for IT-4F. This makes the energy difference between S1D and CT3 (or between S2A and CT1) almost vanished, thus inducing the full hybridization between them. It is worth to note that for the S2A/CT1 hybridized state of (DRTB-T)-2(IT-4F), the electron is distributed over the two IT-4F molecules while the hole over the whole complex. By combining with the CS states in 2(DRTB-T)-(IT-4F) and (DRTB-T)-2(IT-4F), we attempt to unveil the mechanisms of charge generation in the NF OSC devices. From energetics, the DRTB-T excitons can be dissociated into the high-lying CT2 or CT3 states in (DRTB-T)-2(IT-4F) or transferred to the high-lying hybridized S1D/CT3 or S1D/CT3’ states and then to the CT2 state in 2(DRTB-T)-(IT-



4F), which will be further separated into free charge carriers through a fast hot process. By contrast, the IT-4F excitons can be only dissociated into the lowest CT1 state in 2(DRTB-T)-(IT-4F) or transferred to the low-lying hybridized states in (DRTB-T)-2(IT-4F), which will undergo a slow cold CS process before geminate CR. This is consistent with the coexistence of both hot and cold charge separation observed experimentally in the highperformance NF OSCs (as depicted in Figure 1b). In the case of (DRTB-T)-2(IT-4F), there are also two low-hybridization ‘bright’ states on the IT-4F region (S4A: 1.9 eV, f = 0.44; S5A: 1.94 eV, f = 0.6), which can be directly separated by a hot process. Furthermore, the electron in the high-lying CT2 and CT3 states is delocalized, which will facilitate CS. As known, the adiabatic energy of an excited state is equal to the vertical excitation energy minus or the vertical emission energy plus the relaxation energy λ of the excited or the ground state. Considering a medium value of 0.3 eV for λ (generally in the range 0.2-0.4 eV for D-A systems),18,24,39 the adiabic energy of CT1 will be ~ 1.4 eV. According to the Marcus Levich-Jortner (MLJ) formalism,38 the NR decay rate for the CT1 state (knr) is estimated to be ~ 3.5 × 109 s-1 (by using the C=C stretching frequency ωeff = 1500 cm-1 and Huang-Rhys factor Seff = 1 for the effective vibration mode, and the electronic coupling between the CT and ground states V = 200 cm-1). The radiative decay rate (kr) is estimated to be ~ 2.9 × 107 s-1 according to the Einstein coefficient relation (by using the vertical emission energy of ~ 1.1 eV and the calculated transition dipole moment μ = 11.5 D). Thus the total CR rate (kCR) that is mainly determined by knr. To ensure efficient cold CS, the rate of the first CS step (kCS) should be much higher than knr. Since the CS state in (DRTB-T)-2(IT-4F) is lower than that in 2(DRTBT)-(IT-4F) (~ 0.1 eV), the first CS step would be driven by electron transfer rather than hole transfer, with the driving force of ~ -0.1 eV according to the calculations (Figure 3). Under the same values of λ, ωeff, Seff and V as the NR decay, the estimated kCS would be ~ 2.3 × 1011 s-1, which is about two orders of magnitude higher than the kCR. This indicates that the cold CS process in the DRTB-T:IT-4F blends can be efficient compared to geminate CR.78-80 We note that recently CS via thermalized (vibrational) CT1 state has been observed by time-delayed-collectionfield experiments.75,76,111,112 Energetically, the thermalized CT1 state is more favorable to be separated (Figure 5a). Due to the Franck-Condon shift, the thermalized CT1 or LE/CT1 state in the DRTB-T:IT-4F blend can be produced directly by light absorption, rather than by thermal activation. The large f will result in a great number of thermalized CT1 or LE/CT1 states at the D-A interface that can undergo efficient separation. At this stage, we can discuss the crucial role of LE/CT1 hybridization in reducing the NR voltage loss (ΔVnr).20,35,113As pointed out above, ΔVnr = (kBT/q)*ln(EQEEL1), and EQEEL = pe*kr/(knr + pe*kr),114 where pe is the outcoupling efficiency (taking the common value of 0.2 for organic light-emitting diodes). Based on the kr and knr values calculated above, the ΔVnr for the DRTB-T:IT-4F device is estimated to be only ~ 0.16 V at room temperature. As seen in Figure 5b, ΔVnr has strong dependence on ECT and μ. On one hand, as ECT increases, knr will be decreased according to the energy-gap law;115 consequently ΔVnr is reduced

gradually.27,116 On the other hand, the increase of hybridization will enhance μ or f, thus leading to a higher kr and a lower ΔVnr. For comparison between fullerene and NF acceptors, we have also calculated the μ of CT1 for a (DRTBT)-(PC71BM) complex. Because of the large energy difference and negligible hybridization between S1A and CT1 states (Figure S4), the calculated μ is much smaller (~ 0.9 D). Therefore, the LE/CT1 hybridization is critical to achieve a low ΔVnr for NF OSCs with a small ΔECT.

Figure 5. (a) Schematic diagram of charge separation and recombination processes of the CT1 or LE/CT1 state in the DRTB-T:IT-4F blend (1 or 4: vertical absorption or emission; 2 or 3: charge separation via thermalized or relaxed state; 5: nonradiative recombination), (b) Calculated knr, kr, and nonradiative voltage loss (ΔVnr) as a function of the CT1 energy (ECT).

Finally, it is important to note that when the ECT is low (~1.0 eV), the knr will be comparable to the kCS (Figure 5b), which can result in severe recombination and thus inefficient cold CS as well as large ΔVnr. Besides increasing ECT, reducing λ can be another effective strategy to reduce knr, especially in low-Eg and low-ΔECT systems. Once the NR recombination is suppressed, a certain LE/CT hybridization can further promote charge generation and reduce ΔVnr. In summary, we have investigated the nature and energetics of interfacial excited states in model D-A complexes with a wide band gap SM donor of DRTB-T and a narrow band gap SM acceptor of IT-4F by means of TDDFT calculations. The main conclusions we can draw from the calculated results are: (i) The CT states will borrow some oscillator strength from the energy-close LE states. Especially, molecular aggregation at the D-A interface will lead to the formation of fully hybridized LE/CT states. (ii) Energetically, the high-lying CT and hybridized states can be separated into free charge carriers by a hot process, while the separation of CT1 and LE/CT1 states undergoes a cold process. Such cold CS process should be efficient compared to the geminate CR. This is consistent with the experimental observations that hot and cold charge generation can coexist in the state-of-the-art NF OSCs with small ΔEHOMO. (iii) Photoexcitation can directly generate a large number of thermalized CT1 or LE/CT1 states at the D-A interface due to the increased oscillator strength; these thermalized states are energetically favorable to be separated into free charge carriers. (iv) The large oscillator strength for CT1 or LE/CT1 will also be beneficial to improve the luminescence quantum efficiency and thus result in a low non-radiative voltage loss.


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The Journal of Physical Chemistry Letters These results provide the rationalization of both high charge generation efficiencies and low NR voltage losses in small-ΔECT NF OSCs.35 The optimal trade-off between high photocurrent densities and low voltage losses can be obtained by tuning the LE/CT hybridization at the D-A interface toward high-performance NF OSCs.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxx. Chemical structures of DRTB-T and IT-4F, side views of optimized geometries of DRTB-T and IT-4F dimers and complexes, detailed excited-state properties for DRTB-T and IT-4F, as well as their dimers and complexes.

*E-mail: [email protected] The authors declare no competing financial interest.

The work is supported by the National Natural Science Foundation of China (Grant No. 91833305, 51773208, 51803216), Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB12020200), and Ministry of Science and Technology of China (Grant No 2017YFA0204502).

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