TiO2 Hybrid

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Photoinduced Charge Transfer in Poly(3-hexylthiophene)/ TiO Hybrid Inverse Opals: Photonic vs Interfacial Effects 2

Nicholas Tulsiram, Christopher Kerr, and Jennifer I. L. Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09113 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 14, 2017

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The Journal of Physical Chemistry

Photoinduced

Charge

hexylthiophene)/TiO2

Transfer Hybrid

in

Inverse

Poly(3Opals:

Photonic vs Interfacial Effects Nicholas Tulsiram, Christopher Kerr and Jennifer I. L. Chen* Department of Chemistry, York University, 4700 Keele Street Toronto, Ontario Canada, M3J 1P3 Corresponding Author [email protected]

ABSTRACT

We study the effects of tailored light-matter interactions on charge transfer in conjugated polymer films. We use inverse opal structures of titania as the electron acceptor and the model polymer poly(3-hexylthiophene) (P3HT) as the electron donor. We systematically tune the periodicity of the inverse opal to study how the photophysical properties of the conformally coated P3HT are affected by the photonic stop band and band-edge light localization to observe both suppression and enhancement of absorption. Surprisingly, we observe changes in the vibronic coupling in the P3HT absorption spectra in the inverse opal structures as compared to control films. We determine that the polymer in the inverse opals show more J-aggregate like

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behavior with exciton bandwidths of 18 meV, compared to that of 124 meV for P3HT on planar mesoporous TiO2 film. We also study charge transfer at the polymer/inorganic interface by photoinduced absorption spectroscopy. We find that the polaron signal depends on the excitation wavelength, periodicity of the inverse opal, and the interfacial area. The inverse opal structures exhibit significantly increased charge generation compared to the control films, and we determine that photonic effects of the lattice, while observable, play a secondary role in this enhancement relative to the increased surface area.

INTRODUCTION Organic photovoltaics (OPVs) comprising solution-processable conjugated polymers are a low-cost alternative to conventional inorganic photovoltaics.1 They offer advantages including the use of inexpensive materials, large-scale fabrication methods and a variety of design possibilities that capitalizes on their low weight, transparency and flexibility.2,3,4 A promising type of OPV is the bulk heterojunction (BHJ) solar cell which employs an active layer comprising a phase-separated mixture of donor and acceptor materials.5 The morphology of the active layer plays an important role in the production and transport of free charge carriers.6,7 As internal quantum efficiency decreases with film thickness, charges are extracted most effectively in active layers of less than 100 nm.8 The corollary of the limitation in film thickness is a tradeoff in the amount of light that can be absorbed. Hence methods to enhance the absorption of light via photon management9,10 are highly sought-after. Light management strategies utilizing gratings11, antireflection or scattering structures9, plasmonics12,13 and waveguides14 have been explored for OPV. These structures may require topdown fabrication that is unattractive for large-scale production. Colloidal photonic crystals are


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cost-effective for large area implementation and have been explored in Si15, dyesensitized16,17,18,19,20,21, perovskite22,23, and quantum dot24 solar cells. Enhancements on the absorption and photoactivity have been achieved by integrating the inverse opals as a dielectric mirror in a bi-layer structure17,21, or via a reduction of group velocity of light at the photonic band edge also known as slow photons.21,25,26,27 The latter phenomenon, though sensitive to structural disorder28 and provide limited bandwidth of enhancement, has been successfully adapted for enhancing the absorption of semiconductor materials for photocatalysis and solar fuel applications.29,30 It is of interest to investigate the feasibility of exploiting slow photons for enhancing absorption in polymeric photoactive layer. While the most common acceptor materials are based on fullerene-derivatives, inorganic acceptors such as metal oxides31,32,33 and chalcogenides34,35,36,37 can serve as efficient electron acceptors when incorporated with conjugated polymers. They can be synthesized in different sizes and morphologies and offer several advantages over organic acceptors, including the ability to tune the microstructure of the acceptor phase, decreased Coulombic binding energies of excitons due to higher dielectric constants, increased active layer absorption and enhanced electron mobilities.35,38,39,40 Inorganic acceptors have enabled the systematic study of charge carrier dynamics41, interfacial modification42 and nanoscale phase separation33 which are otherwise difficult to elucidate in randomly mixed organic blends. In addition, the morphology of the inorganic-organic composite would not solely depend on processing conditions as compared to organic blends6,7, where the metastable phase-separated domains could change over time.43 Although metal oxides such as titanium dioxide (TiO2) and zinc oxide (ZnO) can be patterned or structured prior to the incorporation of the organic material44,45,46, achieving effective polymer infiltration and high interfacial contact have been challenging in mesoporous


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and nanorod films.47 Hence we aim to examine the structural and interfacial benefits of the macroporous inverse opal in addition to the photonic effects. The inverse opal structure offers some advantages and disadvantages in polymer-based photovoltaics. The structure could yield a bicontinuous framework48 that provides high interfacial area between two materials, such as the donor and acceptor, and a junction thickness of few to ten nanometers – on the order of the exciton diffusion length that is important for charge generation.5,49 On the other hand, charge transport may be impeded by the film thickness and the 3D morphology; this drawback may be overcome by incorporating conductive electrode50 in a core-shell inverse opal structure. Hence the synthetic flexibility of the 3D macroporous photonic structure could be exploited in applications where charge transport across micron-thick film is feasible. While many factors govern the overall photovoltaic performance, the ability to probe polarons in conjugated polymers directly by transient absorption spectroscopy provides the opportunity to investigate photoinduced charge transfer exclusively. Herein we report a systematic study on the effects of the macroporous structure and photonic properties on a model polymer P3HT. We find that the absorptance of the polymer is modified near the energy of the photonic stop band in addition to spectral changes that suggest disorder in the packing of the regioregular polymer on the 3D surface. The polaron generation efficiency is a convoluted result of structural, interfacial and photonic factors. EXPERIMENTAL Synthesis of polystyrene spheres. Polystyrene spheres with diameters of 148.8±3.8, 182.6±5.6, 233±6.3, 275.8±4.1, 334±5.5 and 393.9±10.1 nm were synthesized via surfactant-free emulsion


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polymerization. A three-necked round-bottom flask was cleaned thoroughly and filled with 100 mL of Milli-Q water and 15 mL of anhydrous ethanol and heated in an oil bath at 75.8°C. While stirring, the solution was purged with nitrogen for 1 hour. After purging, approximately 5.05 g of styrene monomer (Fisher), 100 to 400 µL of styrene sulfonate and 0.06 g of initiator were added to the solution. The initiators consisted of either 4,4′-azobis(4-cyanovaleric acid) or ammonium persulfate, depending on the desired sphere size. The size of the sphere was controlled by the amount of styrene sulfonate added. The mixture was left to stir overnight under nitrogen atmosphere then cooled to room temperature and filtered with glass wool. All chemicals were obtained from Sigma Aldrich unless otherwise noted. Deposition of opal template. Opal templates were deposited onto glass substrates via evaporation induced self-assembly. Glass substrates were cut, rinsed with water and cleaned using piranha solutions of 3:1 sulfuric acid, hydrogen peroxide. Glass slides were rinsed with copious amounts of water, dried and then suspended vertically in shell vials containing 6 mL solutions of ethanol with 3.3 wt% of polystyrene spheres. The solution with the suspended slide was placed in an oven at 55°C for 2 days to yield the opal template. The overall thickness of the films was kept at ~2-2.5 µm (unless otherwise specified) by controlling the number of layers deposited from ~20 for the smallest template sphere size to ~13 for the largest template sphere size. The number of layers for all templates was deduced from modelling the Fabry-Perot fringes in the optical spectra. Fabrication of TiO2 inverse opal. TiO2 inverse opals were fabricated by infiltrating opal templates with titanium (IV) butoxide (Ti(OBut)4. Opal templates were suspended vertically in a solutions of ethanol containing 0.4 to 1.1 vol% of Ti(OBut)4 in a desiccator under vacuum. The rate of evaporation was controlled using a needle valve. The Ti(OBut)4 filled the voids present in


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the opal template as the ethanol solution evaporated, where the titanium precursor hydrolyzed to yield amorphous TiO2. The inverse opals were calcined at 450°C for 4 hours to burn off the opal template and crystallize the TiO2. Preparation of Mesoporous TiO2 Films. A reference mesoporous TiO2 (meso-TiO2) film was prepared via sol-gel method.51 First, Ti(OBut)4 was added to a vial containing hydrochloric acid while stirring. A second solution containing 0.32 g Pluronic P123 (BASF) dissolved in 3.75 mL of 1-butanol was prepared and then added to the acidic Ti(OBut)4. The solution was left to stir for 2 hours where it was then spin coated onto cleaned glass substrates for 20 seconds at 2400 rpm. The coated glass substrates were placed in a humidity chamber for 2 days. The films were then calcined at 450°C for 4 hours to yield a transparent, uniform film of meso-TiO2. Incorporation of P3HT. Solutions of ~16 mg/mL P3HT (Mw = 50000 – 70000, RR = 91 – 94%, Rieke Metals) were prepared by dissolving the polymer in chlorobenzene and stirring for 3 hours at 70°C. Materials were weighed out in a glovebox under argon atmosphere and air-free techniques were employed for transferring solvents into sealed vials. Films of mesoporous TiO2 and inverse opals (approximately 1.3 cm by 1.3 cm) were spin-coated with 100 and 50 µL of the polymer solution respectively, at 1000 rpm for 2 minutes. A solution of 15 mg/mL of PCBM (Nano-C) was mixed with P3HT for an additional 3 hours before spin-coating at 1000 rpm for 2 minutes. The samples were handled in air in the dark and stored under vacuum. Photoinduced Absorption Spectroscopy. A quartz tungsten halogen (QTH) light source (Newport Oriel Apex Monochromator Illuminator 70528) with a 550 nm longpass filter was used as the probe beam, and light-emitting diodes (LED) (Luxeon Star Rebel, 700 mA) modulated by a function generator (Tektronix AFG2021) were used as the pump beam. Blue (470 nm) and green (530 nm) LEDs were used in combination with 460 and 525 nm bandpass filters,


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respectively to excite the samples at 200 Hz. A silicon/indium gallium arsenide (Si/InGaAs) dual band photodiode (Thorlabs DSD2) was used to detect the monochromated (Princeton Instruments, Acton SP2150) light from the probe beam. Spectra were obtained from 600 to 1700 nm at 10 nm intervals with 60 averages taken at each wavelength. A lock-in amplifier (Stanford Research Systems SR830), in phase with the LED pump light, was used to extract the fractional changes (ΔT) from the probe beam signal, which were reported normalized to probe beam transmission values (T). To perform pump modulation frequency dependence studies, an InGaAs transimpedence amplified detector (Thorlabs, PDA10CS) was used instead of the dual band Si/InGaAs detector. The 900 nm longpass was used in the QTH light source while all spectra were monitored at 1000 nm. Spectra were taken by modulating the pump beam at different frequencies ranging from 10 to 103 or 104 Hz with 60 averages taken at 30 points. Samples were measured under dynamic vacuum. Quantification of P3HT. After PIA spectroscopy, P3HT was dissolved from each film via heating in 8 mL of toluene at 80 oC. The films were then removed from the vials and the dissolved P3HT-toluene solutions were left to evaporate. The recovered P3HT was dissolved again in 4 mL of toluene, and the absorbance was measured using an Agilent 8453 UV-vis spectrophotometer. PIA spectra were normalized to the absorbance peak of P3HT solutions. Characterization of inverse opals. Scanning electron microscopy (SEM) images were taken using a FEI Quanta 3D dual-beam FEG FIB operating at voltages ranging from 10 to 20 kV. Top-view images were obtained at 0.30 Torr at working distances of 7 to 10 mm using a low vacuum secondary electron detector. Cross-sectional images were obtained at high vacuum at 20 kV after sputtering the sample with Au. Optical characterization of the films was carried out using a Perkin Elmer Lambda 950 UV/VIS/NIR Spectrometer equipped with a 150 mm


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integrating sphere. Reflectance (R) spectra (diffuse plus specular) were measured by placing the samples at the back port of the integrating sphere. Absorptance of the samples was obtained from 1 – Ttotal where Ttotal is the light scattered and reflected by the sample placed in the center of the integrating sphere. Alternatively absorptance was obtained through 1 – T – R where R is the total reflectance and T is the diffuse transmittance measured by placing the sample at the front port of the integrating sphere.

RESULTS AND DISCUSSIONS Photonic P3HT/TiO2 inverse opals (denoted as i-P3HT/TiO2-o) were fabricated by a templating method to first yield anatase inverse TiO2 opals (denoted as i-TiO2-o) and subsequently spin-coating the inorganic structure with P3HT (Scheme 1). Polystyrene spheres of 148 to 393 nm were used as templates to yield inverse opals with photonic stop bands at wavelengths from the UV to the visible, thus allowing the tuning of photonic effects across the absorption of P3HT. Figure 1 shows the SEM images of the i-TiO2-o and i-P3HT/TiO2-o, in which the highly ordered open structure of the framework can be seen. The TiO2 nanocrystals in inverse opals templated by both small and large spheres are in the anatase phase, as confirmed by Raman spectroscopy (Fig. 1 inset). Upon coating with P3HT, the inverse opal remains porous without any polymer overlayer and hence maintains the dielectric contrast necessary to produce photonic properties. Scheme 1. Fabrication of P3HT-coated TiO2 inverse opal.


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Figure 1. SEM images of the inverse opals: (a) top-view of i-TiO2-o templated with 149 nm spheres, and (b) cross-sectional view and top-view (inset) of i-P3HT/TiO2-o templated with 183 nm spheres. The inset in (a) shows the Raman peaks of anatase at 401, 516 and 637 cm-1. UV-vis spectrophotometer equipped with an integrating sphere was used to obtain reflectance measurements to determine the photonic stop band position of the inverse opals. Figure 2a shows the reflectance spectra of i-TiO2-o where the stop band reflections arising from the constructive interference and Bragg diffraction of the (111) planes of the colloidal crystals are observed. The series of inverse opals has stop bands near 300, 330, 360, 400, 440 and 540 nm (herein the prefix of i-TiO2-o and i-P3HT/TiO2-o indicates the photonic stop band position of the uncoated inverse opal). The larger inverse opals (400-, 440- and 540-i-TiO2-o) display welldefined stop band reflectance peaks, while the intensity of the 360-i-TiO2-o stop band is reduced because of its overlap with the absorption of TiO2. Part of the stop band of 330-i-TiO2-o can be observed as a small bump in the spectrum as it is almost completely attenuated by the UV absorption of TiO2. The photonic properties are further confirmed by their respective peak shifts when immersed in water which leads to an overall increase in the effective refractive index (Fig. S1).


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Figure 2. Reflectance spectra of i-TiO2-o (a) and i-P3HT/TiO2-o (b). Absorptance spectra of iP3HT/TiO2-o (c) and of P3HT on selected inverse opals (d) by subtracting the absorptance of i-TiO2-o from i-P3HT/TiO2-o and normalizing at 565 nm. The legend indicates the stop band positions. Inset in (c) shows the photograph of 300-, 440- and 540-i-P3HT/TiO2-o where the purple and green reflectivity for the latter two can be visually discerned.

Figure 2b shows the reflectance spectra of the inverse opals after spin coating with P3HT. Small red shifts (5 – 9 nm) in the positions of the stop bands were observed upon coating with P3HT (Fig. S2), suggesting a thin conformal layer of the polymer on the inorganic framework instead of a complete filling of the voids. The shift suggests an increase in the refractive index of the wall from ~1.19-1.33 to 1.22-1.48 with P3HT. The composition of the wall for the different inverse opals estimated from the modified Bragg’s equation are shown in Table S1. The filling fraction of TiO2 is 0.15-0.16 and is consistent with previous reports that structures obtained from alkoxide precursors undergo shrinkage upon calcination to yield nanoparticle framework that is


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typically 10-50% of metal oxide in the wall.52 The fraction of P3HT, however, is difficult to deduce as the refractive index dispersion of the partially disordered P3HT is undetermined and expected to differ from both regioregular and regiorandom P3HT (see discussions below). For simplicity the prefix used to denote the composite inverse opal is unchanged from that of the uncoated TiO2 inverse opal. The absorption of P3HT causes a decrease in the reflectance from 400 to 650 nm across all nanocomposites. The 300-i-P3HT/TiO2-o, which does not exhibit photonic properties >350nm, displays the lowest amount of random scattering. The stop bands of 360-, 400- and 440-i-P3HT/TiO2-o are detectable but with reduced intensities in comparison to the uncoated inverse opals. The stop band of 440-i-P3HT/TiO2-o is suppressed significantly, from 40% of reflectivity in the uncoated i-TiO2-o to 15% of the inorganic-polymer composite, because of the overlap of its energy with the blue edge of the P3HT absorption. Furthermore the absorption of the P3HT leads to the almost complete suppression of the stop band of 540-iP3HT/TiO2-o, where no reflectance peak is present. Highly absorbing materials have previously been shown to suppress photonic properties53; however, the reflectance of 540-i-P3HT/TiO2-o between 400 and 600 nm is higher (~9%) compared to other inverse opals suggesting some existence of stop band reflection near this region. The photonic effects on the absorption of i-P3HT/TiO2-o are seen in Fig. 2c (see Fig. S3 for absorptance of uncoated i-TiO2-o). The absorptance spectra were measured by placing the sample in the center of the integrating sphere (i.e. Absorptance = 1 – Ttotal where Ttotal is light captured by the integrating sphere). The effect of the stop band reflection translates to a dip in the absorptance spectrum for most inverse opals. For example, the dips at 366, 398 and 445 nm in Fig. 2c correspond to the stop bands of the 360-, 400- and 440-i-P3HT/TiO2-o, respectively. For 540-i-P3HT/TiO2-o, no well-defined absorptance dip exists as the stop band is significantly


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suppressed. To elucidate the photonic effects on P3HT absorptance, we subtracted the absorptance of respective i-TiO2-o from i-P3HT/TiO2-o and normalized the spectra at 565 nm (Fig. 2d). The absorption of P3HT on inverse opals that exhibit photonic effects (i.e. 360-, 400and 440-i-P3HT/TiO2-o) is clearly modified when compared to 300-i-P3HT/TiO2-o, a reference sample that has the same macroporous structure and similar P3HT ordering but in which the stopband is suppressed by the absorption from titania. Figure 2d shows the suppression of absorptance by the stop band, as well as enhancements near the band edges. At energies above and below the photonic stop band, photonic crystals localize the electromagnetic field of light on the low and high dielectric material, respectively. The tuning of the photonic stop band position concomitantly allows us to tune the energy of light localization and slow photons that occur at energies near the red edge of the stop band.21,25 Marginal increases in absorptance at wavelengths redshifted from the absorptance dips of the stop band are observed in 360- and 400-iP3HT/TiO2-o. The changes in the spectra of i-P3HT-TiO2-o qualitatively agree with the FDTD calculations (Fig. S4). Note that thin film effects, such as Fabry-Perot interference, are not observed experimentally because of the scattering nature of the samples. The photonic properties of i-P3HT/TiO2-o are further confirmed via angle-dependent measurements – an increase in the angle of incident light results in a blue-shift of the stop band for 360-, 400-, 440- and 540-iP3HT/TiO2-o (Fig. S5). The changes in the absorptance spectra of i-P3HT/TiO2-o demonstrate the influence of photonic properties on the absorption of the polymer-inorganic composite. The photographs of the visually distinctive films are shown in Fig. 2c inset. Unexpectedly, the absorbance of i-P3HT/TiO2-o exhibit very different vibronic structure than typical P3HT films. Figure 3a shows the absorbance (log(1/Ttotal)) spectra of a blended film of P3HT with phenyl-C61-butyric acid methyl ester (P3HT:PCBM), a bilayer film of P3HT on


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mesoporous anatase TiO2 (denoted as P3HT/meso-TiO2, Fig. S6), neat P3HT film on glass, and a 300-i-P3HT/TiO2-o that has no photonic properties near the absorbing range of P3HT. The 300i-P3HT/TiO2-o shows a more prominent A1 intrachain absorption at 605 nm than A2 interchain absorption at 556 nm7, and higher relative absorption at shorter wavelengths (

TiO2 Hybrid

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