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Abstract
A simple method is presented to determine the thickness, refractive index and extinction coefficient of a film with the resolutions of ±1 nm, ±2 × 10−3, and 6 × 10−5, respectively. The method requires only ultraviolet-visible-near-infrared spectroscopy measurement of the transmittance of films with thicknesses of a few micrometers. A very small extinction coefficient of intermolecular charge transfer (CT) absorption of an exciplex-forming organic film is measured in the sub-bandgap wavelength region. This is to demonstrate that the CT absorption with extinction coefficient in the range of 10−3 to 10−5 can be measured indeed using the method, which is in the same resolution as photothermal deflection spectroscopy and Fourier transform photocurrent spectroscopy. The simplicity and feasibility of the proposed approach is expected to promote active study of intermolecular CT absorption in exciplex-forming films.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Excited-state charge-transfer (CT) complexes (exciplexes) are generated in organic semiconductors when one of two different molecules is excited optically and partial charge transfer between the two molecules takes place [1–5]. A new photoluminescence (PL) band is observable when a mixture of the two molecules is optically excited, and indicates that the two molecules both participate in the exciplex emission process [2,4,6–9]. However, no new absorption band was observed in the optical absorption process, and there is no evidence of simultaneous participation of the two molecules forming the exciplex. Thus, the exciplex does not appear to engage in the absorption process [2,6,9].
Despite the common assumption that intermolecular CT absorption forming exciplexes does not occur, CT absorptions with very low intensities have been detected in photothermal deflection spectroscopy (PDS) and Fourier transform photocurrent spectroscopy (FTPS) studies of organic photovoltaic (OPV) materials [10–14]. Due to the importance of CT absorption in the device physics of OPVs, various mixed films of polymers with [6,6]-phenyl C61 butyric acid methyl ester have been investigated for intermolecular CT absorption. Recently, Kim et al. reported a simple method to determine the extinction coefficients for an intermolecular CT absorption band in a mixed film of electron donor and acceptor molecules of 4,4′,4′′-tris(3-methyl-phenylphenylamino)triphenylamine (m-MTDATA) and (1,3,5-triazine-2,4,6-triyl)tris(benzene-3,1-diyl))tris(diphenylphosphine oxide) (PO-T2T), which are exploited for organic light-emitting diodes [15]; here, ultraviolet-visible-near-infrared (UV-Vis-NIR) spectroscopy and spectroscopic ellipsometry were utilized for the measurements.
In the present study, we propose a simpler method than the previous one to measure CT absorption with very small extinction coefficients. Considering the equipment requirements for PDS, FTPS, and spectroscopic ellipsometry, we just used a UV-Vis-NIR spectrophotometer, thus also demonstrating the versatility of our method. Using the proposed method, the extinction coefficient can be measured to 5 × 10−5, and the refractive index and thickness of exciplex-forming films can be attained.
2. Experiment
Mixed films of TAPC (di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane) and PO-T2T (1,3,5-triazine-2,4,6-triyl)tris(benzene-3,1-diyl))tris(diphenylphosphine oxide), with a 1:1 molar ratio, were used in this study. TAPC and PO-T2T, purchased from Shine Materials Technology (Kaohsiung City, Taiwan) and Nichem Fine Technology (Jhubei City, Taiwan), respectively, were co-evaporated on precleaned fused silica substrates under vacuum at a base pressure of < 5×10−7 Torr. Steady-state PL spectra were obtained with a spectrofluorometer (Photon Technology International, Inc., Birmingham, NJ, USA), equipped with an incorporated monochromator (Acton Research Co., Acton, MA, USA). Absorbance was measured using a Cary 5000 UV-Vis-NIR spectrophotometer with a wavelength interval of 0.1 nm, an integration time of 0.1 s for each wavelength point, and a spectral band width of 0.5 nm. The resolution of absorbance is ±1.9 × 10−4 in the measurement conditions. A surface profilometer (Alpha Step IQ; KLA Tenco, Milpitas, CA, USA) was employed for measuring film thickness.
3. Result and discussion
3.1 Photoluminescence and absorbance
The chemical structures of TAPC and PO-T2T molecules are shown in g. 1(a). The 1:1 mixed film shows exciplex emission, manifested in featureless red-shifted emission compared to molecular emissions [16,17] [g. 1(b)]. For the purpose of extracting extinction coefficients for intermolecular CT absorption, we measured the absorbance of an TAPC neat film and an TAPC:PO-T2T mixed film (thickness: 1.0, 1.5, and 2.0 µm) deposited on fused silica substrates; the film thicknesses were measured by a surface profilometer. The absorbances of TAPC and TAPC:PO-T2T films are shown in Figs. 2(a) and 2(b), respectively. Absorbance was measured with respect to the normal incidence of light on the fused silica substrate, and the baseline for the absorbance measurement was set to that of air. Films with thicknesses of around 1 µm induced thin-film interference, resulting in oscillation of the absorbance. The absorbance of the film on the fused silica substrate is equal to $- \log ({1 - R - A} )$, where R and A correspond to thereflectivity and absorptivity of the film on the fused silica substrate, respectively [15,18,19].
Fig. 1. (a) Molecular structure of 1,1-bis[4-[N,N-di(p-tolyl)amino]phenyl]cyclohexane (TAPC) and (1,3,5-triazine-2,4,6-triyl)tris(benzene-3,1-diyl))tris(diphenylphosphine oxide) (PO-T2 T). (b) Steady-state photoluminescence spectra of the TAPC solution with an excitation wavelength of 330 nm, PO-T2 T solution [16,17] and TAPC:PO-T2 T mixed film (m.r. 1:1) with an excitation wavelength of 280 nm.
Fig. 2. (a) Absorbances of the TAPC neat films with thicknesses of 1.0, 1.5, and 2.0 µm. (b) Absorbances of the TAPC:PO-T2T blended film (m.r. 1:1) with thicknesses of 1.0, 1.5, and 2.0 µm, where the absorbance points are denoted by dark cyan, navy, and magenta points, at which destructive, constructive, and middle interference, respectively, takes place for the transmitted waves. The maximum absorbance limit of the equipment is around 4.
Optical bandgaps of PO-T2T and TAPC in neat films are 4.0 eV [15,17] and 3.5 eV, respectively, from onsets of their absorption spectra. The absorbance of the TAPC:PO-T2T film was higher than that of the TAPC film in the sub-bandgap wavelength region. This indicates that a new absorption band is generated with a very small extinction coefficient when TAPC and PO-T2T are in close proximity.
3.2 Transmissivity
The transmissivity (T) of the film on a fused silica substrate under normal incidence is given by [15]
3.3 Exact film thickness
Thin-film interference can be used to obtain the exact thickness, refractive index, and small extinction coefficient of films in the sub-bandgap wavelength region. First, the refractive index of the film can be obtained based on the absorbance at the absorbance points where the destructive interference of transmitted waves takes place and the extinction coefficients are zero. Second, the exact film thickness can be calculated from the wavelength at the absorbance points where destructive interference occurs. The refractive index can also be obtained from the exact film thickness. Finally, the film’s extinction coefficient can be determined based on the exact thickness and refractive index of the film.
The exact thickness and refractive index of the film are required for calculating the extinction coefficient from the absorbance when the difference between the reflectivity and absorptivity of the film is small. When the film thickness is around 1 µm, the absorptivity becomes comparable to the reflectivity in the sub-bandgap wavelength region, as shown in g. 2(b).
Tfilm can be written as follows, at the absorbance points where the destructive interference of the transmitted waves occurs (ϕ =π, 3π, 5π, 7π, $\cdots $) and the extinction coefficients are zero (k = 0) based on Eq. (3):
Combining Eq. (9) with Eqs. (1) and (2) results in the following equation, where Rfilm is equal to (1 – Tfilm) when the extinction coefficients of the films are zero:Fig. 3. (a) Refractive indices of the TAPC:PO-T2 T films with various thicknesses calculated from the absorbances (closed) at which destructive interference of the transmitted waves occurs using Eq. (11, and refractive indices calculated from the wavelengths (open) of destructive interference based on the exact film thicknesses. (b) Refractive indices of the TAPC:PO-T2 T films with various thicknesses calculated from wavelengths where destructive (dark cyan), constructive (magenta), and middle (navy) interference of the transmitted waves occurs, and their fit line. (c) m of the TAPC:PO-T2 T mixed films with various thicknesses calculated from wavelengths where destructive (dark cyan), constructive (magenta), and middle (navy) interferences of the transmitted waves occurs, and an interpolation line for m of the destructive and constructive interference.
Employing this refractive index, the exact film thickness is given by
where m is the number of wavelengths required to pass through a film with a thickness of 8d; m = 2, 6, 10, 14… for wavelengths with destructive interference. The restriction for m provides a key to solve one equation with two unknowns (m and d), where the approximate thickness of the film derived from surface profilometer measurements provides the restriction for d. The exact film thicknesses of TAPC:PO-T2T films with approximate thicknesses of ∼1.0, ∼1.5, and ∼2.0 µm, as measured by a surface profilometer, were determined to be 1,008, 1,503, and 2,020 nm, respectively.3.4 Refractive index
The refractive index at wavelengths of the destructive interference can be obtained using Eq. (11), based on the exact film thickness in the sub-bandgap wavelength region. n is a continuous function of λ, indicating that m is also a continuous function of λ given a constant film thickness; the refractive index is plotted as dark cyan open points in Figs. 3(a) and 3(b).
We determined the wavelengths at which constructive interference of the transmitted waves occurs as the local minima for the oscillating absorbance, denoted by magenta points in g. 2(b). The refractive indices of the films were obtained using Eq. (11), where m = 4, 8, 12, 16… for the wavelengths of constructive interference, shown as magenta points in g. 3(b).
m was then determined as a function of λ using Eq. (11), as shown in g. 3(c). The interpolation of m allows the wavelengths of the “middle interference” (m = 1, 3, 5, 7…) of the transmitted waves takes place to be determined. Middle interference takes place when the phase difference between the transmitted waves is an odd multiple of π/2. The wavelengths where middle interference of transmitted waves occur are denoted by navy points in Figs. 3(c) and 2(b). The refractive indices at the wavelengths of middle interference, derived from Eq. (11), are plotted as navy points in g. 3(b).
3.5 Extinction coefficient
Given the exact thicknesses and refractive indices of the films at the wavelengths of destructive, constructive, and middle interference, the extinction coefficient can be calculated from the absorbance. The substitution of Eqs. (2)–(8) into Eq. (1) provides an analytical solution for the extinction coefficient, with Eqs. (12), (13), and (14) used for destructive, constructive, and middle interference, respectively, considering that ϕ in Eq. (8) is equal to mπ/2, according to Eq. (11).
The calculated extinction coefficients are shown in g. 4(a). The similar onsets of the exciplex PL band and sub-bandgap absorption band indicate that the sub-bandgap absorption band arises from optical absorption to the exciplex state. The fitting for the extracted refractive indices and extinction coefficients was carried out using the Sellmeier equation and multiple Gaussian functions, respectively; the fit lines are shown in Figs. 3(b) and 4(a) as solid lines. The refractive indices and extinction coefficients of the films derived from the fit lines and the exact film thicknesses were used to calculate absorbance using the transfer-matrix method [15]. The calculated absorbances well matched the experimental data, as shown in g. 4(b), demonstrating the utility of the method for extracting very small extinction coefficients from the absorbance.
Fig. 4. (a) Extinction coefficients of the TAPC:PO-T2 T films with various thicknesses for destructive (dark cyan), constructive (magenta), and middle (navy) interference, and their fit line on a linear (up) and a logarithmic scale (down). (b) Experimental absorbances of the TAPC:PO-T2 T films with various thicknesses on the fused silica substrates and calculated absorbances in the sub-bandgap wavelength region.
4. Conclusion
We developed a new and simple method to extract the very small extinction coefficients present in the sub-bandgap wavelength region of organic films. Only transmissivity measurement of the films on the substrate is required to extract the thickness, refractive index and extinction coefficient of a film with the resolutions of ±1 nm, ±2 × 10−3, and 6 × 10−5, respectively, leading to versatile application. Considering the difficulties associated with measuring and analyzing quantitatively the very small intermolecular CT absorption for exciplex-forming films, the method introduced in this study is expected to lead to further advances in the understanding of the direct formation of exciplex from the ground state of a donor-acceptor pair without exciting either the donor or acceptor molecule. This method can also be used to study impurities or defect states in organic and inorganic semiconductors without using elaborated absorption measurement systems.
Funding
National Research Foundation of Korea (2018R1A2A1A05018455, 21A20131912052); Ministry of Trade, Industry and Energy (10079671).
Disclosures
The authors declare no conflicts of interest.
References
1. G. J. Kavarnos and N. J. Turro, “Photosensitization by reversible electron transfer: theories, experimental evidence, and examples,” Chem. Rev. 86(2), 401–449 (1986). [CrossRef]
2. N. J. Turro, Modern Molecular Photochemistry (University Science Books, 1991).
3. I. R. Gould, R. H. Young, L. J. Mueller, A. C. Albrecht, and S. Farid, “Electronic Structures of Exciplexes and Excited Charge-Transfer Complexes,” J. Am. Chem. Soc. 116(18), 8188–8199 (1994). [CrossRef]
4. R. H. Young, A. M. Feinberg, J. P. Dinnocenzo, and S. Farid, “Transition from charge-transfer to largely locally excited exciplexes, from structureless to vibrationally structured emissions,” Photochem. Photobiol. 91(3), 624–636 (2015). [CrossRef]
5. H.-B. Kim, D. Kim, and J.-J. Kim, “Exciplex: Its Nature and Application to OLEDs,” in Highly Efficient OLEDs: Materials Based on Thermally Activated Delayed Fluorescence, H. Yersin, ed. (John Wiley & Sons, 2018), pp. 331–376.
6. A. P. Kulkarni and S. A. Jenekhe, “Blue-green, orange, and white organic light-emitting diodes based on exciplex electroluminescence of an oligoquinoline acceptor and different hole-transport materials,” J. Phys. Chem. C 112(13), 5174–5184 (2008). [CrossRef]
7. Y.-S. Park, K.-H. Kim, and J.-J. Kim, “Efficient triplet harvesting by fluorescent molecules through exciplexes for high efficiency organic light-emitting diodes,” Appl. Phys. Lett. 102(15), 153306 (2013). [CrossRef]
8. K. Goushi, K. Yoshida, K. Sato, and C. Adachi, “Organic light-emitting diodes employing efficient reverse intersystem crossing for triplet-to-singlet state conversion,” Nat. Photonics 6(4), 253–258 (2012). [CrossRef]
9. K.-H. Kim, C.-K. Moon, J. W. Sun, B. Sim, and J.-J. Kim, “Triplet Harvesting by a Conventional Fluorescent Emitter Using Reverse Intersystem Crossing of Host Triplet Exciplex,” Adv. Opt. Mater. 3(7), 895–899 (2015). [CrossRef]
10. L. Goris, K. Haenen, M. Nesládek, P. Wagner, D. Vanderzande, L. De Schepper, J. D’haen, L. Luisen, and J. V. Manca, “Absorption phenomena in organic thin films for solar cell applications investigated by photothermal deflection spectroscopy,” J. Mater. Sci. 40(6), 1413–1418 (2005). [CrossRef]
11. L. Goris, A. Poruba, L. Hod’Ákova, M. Vaněček, K. Haenen, M. Nesládek, P. Wagner, D. Vanderzande, L. De Schepper, and J. V. Manca, “Observation of the subgap optical absorption in polymer-fullerene blend solar cells,” Appl. Phys. Lett. 88(5), 052113 (2006). [CrossRef]
12. K. Vandewal, A. Gadisa, W. D. Oosterbaan, S. Bertho, F. Banishoeib, I. Van Severen, L. Lutsen, T. J. Cleij, D. Vanderzande, and J. V. Manca, “The relation between open-circuit voltage and the onset of photocurrent generation by charge-transfer absorption in polymer: Fullerene bulk heterojunction solar cells,” Adv. Funct. Mater. 18(14), 2064–2070 (2008). [CrossRef]
13. M. Hallermann, I. Kriegel, E. Da Como, J. M. Berger, E. Von Hauff, and J. Feldmann, “Charge transfer excitons in polymer/fullerene blends: The role of morphology and polymer chain conformation,” Adv. Funct. Mater. 19(22), 3662–3668 (2009). [CrossRef]
14. T. M. Clarke and J. R. Durrant, “Charge photogeneration in organic solar cells,” Chem. Rev. 110(11), 6736–6767 (2010). [CrossRef]
15. H.-B. Kim and J.-J. Kim, “A simple method to measure intermolecular charge-transfer absorption of organic films,” Org. Electron. 62, 511–515 (2018). [CrossRef]
16. W. Y. Hung, G. C. Fang, S. W. Lin, S. H. Cheng, K. T. Wong, T. Y. Kuo, and P. T. Chou, “The first tandem, all-exciplex-based WOLED,” Sci. Rep. 4(1), 5161 (2015). [CrossRef]
17. J.-H. Lee, S.-H. Cheng, S.-J. Yoo, H. Shin, J.-H. Chang, C.-I. Wu, K.-T. Wong, and J.-J. Kim, “An exciplex forming host for highly efficient blue organic light emitting diodes with low driving voltage,” Adv. Funct. Mater. 25(3), 361–366 (2015). [CrossRef]
18. Grant R. Fowles, Introduction to Modern Optics (Courier Corporation, 1975).
19. K. J. Pascoe, Relectivity and Transmissivity Through Layered Lossy Media: A User-Friendly Approach (Wright Patterson Air Force Base, Ohio, USA, 2001).
