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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Effects of Surface Passivation on Trap States, Band Bending and Photoinduced Charge Transfer in P3HT-TiO Hybrid Inverse Opals 2

Christopher S. Kerr, Artem Kryukovskiy, and Jennifer I. L. Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04931 • Publication Date (Web): 12 Jul 2018 Downloaded from http://pubs.acs.org on July 13, 2018

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The Journal of Physical Chemistry

Effects of Surface Passivation on Trap States, Band Bending and Photoinduced Charge Transfer in P3HT-TiO2 Hybrid Inverse Opals Christopher S. Kerr, Artem Kryukovskiy, Jennifer I. L. Chen* Department of Chemistry, York University, 4700 Keele Street Toronto, Ontario Canada, M3J 1P3 Corresponding Author [email protected]

ABSTRACT

We investigate the effect of a common TiO2 passivation reagent, TiCl4, on the photoinduced charge transfer of poly(3-hexylthiophene) (P3HT) to TiO2 in the inverse opal structure. Treating the inorganic oxide framework with TiCl4 leads to an increase in the size of the TiO2 nanoparticles, a thickening of the inverse opal framework and a decrease in the trapstate photoluminescence. These changes lead to different energy alignment at the interface. In comparison to the unpassivated P3HT/TiO2 inverse opal, we measured a larger polaron yield, by as high as 9-fold, and significantly shorter and more uniformly-distributed polaron lifetimes in TiCl4-treated samples. We show that downward band bending in the polymer can be 1 ACS Paragon Plus Environment

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circumvented by tuning the trap states on the metal oxide using TiCl4, thereby eliminating the energetic barrier for photoelectron injection from the polymer to the metal oxide. The findings suggest a way to overcome a potential factor that has plagued the performance of metal oxidepolymer hybrid photovoltaics.

INTRODUCTION The use of inorganic acceptors in bulk heterojunction (BHJ) solar cells presents some advantages over conventional fullerene-based organic acceptors. It allows for the synthetic control over the morphology and crystallinity,1,2 provides higher dielectric screening that reduces exciton binding energies,3-6 enhances stability of the donor-acceptor micro-phase structure,4,7,8 and permits the assembly or fabrication of an ordered acceptor structure on the electrode.2,9 Metal oxides such as TiO2 are suitable acceptors, however photovoltaic devices comprising oxide nanoparticles and polymers have historically shown low efficiencies.10 Studies have shown that energy alignment at the interface is an important factor,10-14 and by using small molecular surface modifiers such as benzoic acids, the interfacial dipole can induce band edge shifts that can facilitate charge carrier generation.15 A recent study suggests a key reason for the limited photocurrent in metal oxide-polymeric BHJ photovoltaic devices may be due to the downward band bending (decreasing energy moving away from the interface) in the polymer.16 The band bending results from the spontaneous electron transfer from P3HT to TiO2 upon contact. This charge transfer occurs when the Fermi level of TiO2 is lower than the hole polaron level (ECT+) of the polymer and there exists trap states above the Fermi level of the oxide. The equilibration of the Fermi level of TiO2 with ECT+ of P3HT leads to a downward band bending of 0.7 eV in P3HT that prohibits the injection of the photoexcited electrons. It was suggested that favorable

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interfacial energy alignment can be achieved by raising the Fermi level of the metal oxide to be higher than the electron polaron level (ECT-) of the polymer. Alternatively, downward band bending may be mitigated by tuning the trap states of the metal oxide. Trap states are intermediate energy states that exist between the valence and conduction bands of a semiconductor. They originate from structural defects produced during heating, calcination and cooling of crystal samples. Shallow trap states are found at energy levels closer to either the conduction or valence bands of the semiconductor, whereas deep trap states are found near halfway within the band gap.17 In metal oxides, trap states for holes arise from oxygen vacancies and surface hydroxyl groups, while trap states for electrons arise from undercoordinated surface metal ions.18 Different types of trap states may be associated with specific crystal faces. For TiO2, a common passivating agent for reducing trap states on nanocrystals is TiCl4. Treating mesoporous TiO2 used in dye-sensitized solar cells with 5 – 200 mM of aqueous TiCl4 decreases recombination, increases surface area and enhances charge transport, as evident by changes in the charge carrier mobility and trap-state photoluminescence.19-22 Additionally, an upward shift in the Fermi level of TiO2 upon TiCl4 treatment has been reported.23 The application of TiCl4, however, has not been investigated in hybrid polymer/TiO2 BHJ photovoltaics. Infiltration of polymer in conventional mesoporous TiO2 films may be impeded by this methodology, however it can be overcome in a macroporous structure such as the inverse opal. Previously we studied the photoinduced charge transfer in P3HT/TiO2 inverse opals and found that structures with the highest interfacial area produce the greatest yield of polarons.9 The macroporous structure is amenable to surface modifications and can be readily coated or filled with polymer. Herein we employ TiCl4 treatment on P3HT/TiO2 inverse opals and investigate its 3 ACS Paragon Plus Environment


effect on the structure, physical properties, and photoinduced charge transfer process. We employ ultraviolet photoelectron spectroscopy to characterize the energy levels, and find that the origin of a thermodynamically more favorable photoelectron injection upon TiCl4 treatment is explained by the band bending phenomenon as governed by trap states on the metal oxide. EXPERIMENTAL Co-assembly of polystyrene/TiO2 opal films. The co-assembly method was adapted from Hatton et al.24 Microscope slides, cleaned with piranha solution and thoroughly rinsed, were inserted in shell vials containing a mixture of 0.21 mL of 148.8±3.8 nm polystyrene spheres (4.22 wt%), 30 μL of a 10 vol % ammonium dihydroxo-bis(lactato) titanate (IV) (TiBALDH, Sigma-Aldrich) and 5.76 mL of milli-Q H2O. The evaporation-induced self-assembly was carried out in an oven at 60 oC in the presence of a 0.01 vol% of NH4OH solution. After two and a half days, the deposited films were calcined at 450 oC for 4 hours at a ramp rate of 1 oC/min. The number of layers in each opal film (~16 layers) was deduced by modeling the Fabry–Perot fringes in the optical spectra. A total of 6 films were examined for each TiCl4 condition. Passivation of TiO2 inverse opals using TiCl4. Aqueous TiCl4 (Sigma-Aldrich) solutions were prepared based on a procedure adapted from Lee et al.19 In a nitrogen-purged glove bag, a stock solution of 2 M was prepared by combining 0.22 mL of TiCl4 with 0.78 mL of frozen water, and subsequently removed from the glove bag and warmed to room temperature. Dilute TiCl4 solutions (0.025 M and 0.05 M) were prepared from the stock solution. Inverse opals were coated with 60 µL of the respective TiCl4 solutions and placed in a water-saturated chamber at 70 oC for 30 minutes. The films were rinsed with MQ H2O water and calcined again at 450 oC


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for 30 minutes (ramp rate of 5 oC/min). The passivation and calcination processes were repeated for twice passivated sample at 0.05 M TiCl4. Preparation of P3HT/TiO2 inverse opals. Regioregular P3HT (Mw: 50,000-70,000, RR = 9194%, Rieke Metals) was weighed out in a glovebox and dissolved by stirring in dichlorobenzene at 70 oC for 3 hrs to yield 7 mg/mL solution. Inverse opal samples (1.3 cm x 1.3 cm in dimension) were spin-coated with 80 µL of P3HT solution at 2000 rpm/min for 1.5 minutes in the glovebox. Samples were stored in the dark in the glovebox under nitrogen atmosphere. Scanning electron microscopy. A FEI Quanta 3D dual-beam scanning electron microscope equipped with an Everhart-Thornley detector (ETD) operated under high vacuum, 25 kV accelerating voltage and a specimen current of -0.52 pA was used to obtain the images. Image analysis (~60 measurements for wall thickness determination) was performed using ImageJ software. Samples were sputtered with Au prior to imaging. Optical characterizations. A Perkin Elmer Lambda 950 UV-Vis-NIR spectrophotometer equipped with a 150-mm integrating sphere was used to measure the total reflectance and absorptance of the samples. Photoluminescence (PL) of TiO2 was measured using a pumped excitation source (395 ± 25 nm) from Lumencor Spectra X light engine, combined with a monochromator, a photodiode detector and lock-in amplifier for spectral analysis of the emitted light. The PL spectra were corrected with the responsivity of the photodiode and the background of glass substrate. Photoinduced absorption spectroscopy. The PIA setup is based on a pump-probe system using a blue light emitting diode (Luxeon Star Rebel) with 460 ± 20 nm band pass filter as the pump beam and an Oriel APEX 100-W quartz tungsten halogen lamp as the probe beam. The pump 5 ACS Paragon Plus Environment


LED was powered by a Tektronix DC power supply and modulated with by a Tektronix arbitrary function generator (AFG2021), at 200 Hz for spectral collection. The intensity of the LED was 139 W/m2 and an aluminum mask was used to limit the sampling area to 0.79 cm2 (1 cm diameter). A silicon/indium gallium arsenide photodiode (Si/InGaAs) detector was used in combination with an Acton monochromator for spectral collection from 600 to 1700 nm at 10nm intervals. Stanford Research System low-noise current pre-amplifier (SR570) and lock-in amplifier (SR830) were used to detect the change in probe beam transmittance (ΔT). A Keithley 2000 digital multi-meter was used to measure the constant voltage of the photodiode (T). The phase of the lock-in amplifier was set such that scattered pump light appeared as a positive value in the X-channel. All polaron signals were corrected by the photoluminescence and expressed as the fractional change (ΔT/T) of the probe beam. For modulation frequency-dependent studies, a 900-nm long-pass filter was used with the probe beam and an InGaAs trans-impedance amplified photodetector (Thorlabs PDA10CS) was used. The pump frequency was modulated between 10 and 1000 Hz. All PIA measurements were performed under dynamic vacuum at room temperature. Photoelectron spectroscopy. Ultraviolet photoelectron spectroscopy (UPS) was conducted with a helium discharge lamp (He I, hν = 21.22 eV) under constant analyzer energy (CAE) mode with a pass energy of 1.0 eV and an acquisition time of 1 min and 18 s. Signal was measured between 0 and 21.22 eV with a step size of 0.02 eV under -5 V bias. A reference sputtered Ag film was used to calibrate the Fermi level. A model developed by Henrich and co-workers25 was used to correct the inelastic electron background in the valence band and HOMO regions of the spectra. The HOMO region of the polymer is obtained by subtracting corresponding TiO2 spectrum. Experimental error for the determination of binding energy is ~0.04 eV. 6 ACS Paragon Plus Environment

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RESULTS AND DISCUSSIONS Scheme 1 depicts the experimental approach. We fabricated the TiO2 inverse opals by coassembling polystyrene spheres with a water-soluble TiO2 precursor (ammonium dihydroxobis(lactato) titanate, abbreviated as TiBALDH). The evaporation-induced self-assembly process results in a face-centered cubic array of polystyrene spheres with the voids filled by the condensed TiBALDH complexes. Subsequent calcination removed the template spheres and crystalized the TiO2. Films produced by the co-assembly method show superior structural integrity and less cracking over large areas compared to those obtained from conventional twostep templating and inversion procedure employing titanium alkoxides.9 The calcined inverse opals were then subjected to different concentrations of TiCl4, followed by additional heating. They were spin-coated with P3HT to obtain hybrid structure with a thin coating of polymer throughout the 3D macroporous network. Scheme 1. Fabrication of P3HT-coated TiO2 inverse opals with different TiCl4 treatments.

Figure 1 shows the SEM images of the TiO2 inverse opals with different TiCl4 treatment: the unpassivated inverse opal exhibits smooth framework with few point defects (Fig. 1a and b). The inverse opal framework thickens with increasing TiCl4 concentration as additional TiO2 is deposited (Fig. 1c – f). We did not observe further change (structurally or optically) when the concentration of TiCl4 was increased beyond 0.05 M, and thus we repeated the passivation with 0.05 M TiCl4 (referred to as the 0.05 M twice-passivated sample) to achieve additional change.



The thickness of the inverse opal walls ranges from 17.1±0.4 nm to 20.2±0.5 nm, 28.4±0.8 nm and 29.1±0.5 nm with increasing TiCl4 treatment; the framework also appears rougher for 0.05 M-treated samples. Lee and co-workers reported that thick framework facilitates electron transport and reduces recombination in TiO2 inverse opals.26 In all samples we only detect anatase phase in XRD (Fig. S1), with an increase of nanocrystal size from 5.1 nm to 6.6 nm for 0 M and 0.05 M TiCl4-treated samples, respectively. Note that the TiO2 nanocrystals obtained from the TiBALDH precursor are substantially smaller compared to those obtained from alkoxide precursor (5.1 nm vs ~20 nm27).

Figure 1. SEM images of TiO2 inverse opals with different TiCl4 treatments: 0 M (a and b), 0.025 M (c), 0.05 M (d), 0.05 M twice (e). Cross-sectional view of sample treated with 0.05 M TiCl4 twice shown in (f).


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Figure 2a-c shows the absorptance, reflectance and photoluminescence spectra of the TiO2 inverse opals with different TiCl4 passivation. The onset of the TiO2 absorption (Fig. 2a) shifts to longer wavelengths as additional TiO2 is deposited with increasing TiCl4 treatment. In addition to the increase in the amount of absorbing material, the redshift is also attributed to quantum confinement effect, which has been observed for TiO2 particles below 10 nm.28 In thick films where absorbance is not limited by the amount of TiO2 available, we find a decrease in the electronic bandgap from 3.36 eV to 3.29 eV with increasing TiCl4 treatment (Fig. S2), consistent with changes in the crystal size. The small periodicity of the inverse opal (d111 = 103 nm) does not yield photonic properties at the transparent absorption window of TiO2 (i.e. wavelengths longer than 380 nm). However, the reflectance spectra in Fig. 2b show weak peaks at 250 – 300 nm that correspond to the Bragg (photonic stop band) reflection (Fig. S3). This feature grows in intensity and shifts to longer wavelengths with increasing TiCl4 treatment, in line with the increase in the refractive index of the inverse opal. We estimate that the overall volume fraction of TiO2 increased from 12 % for unpassivated inverse opal to 18 % for 0.05 M twice-passivated inverse opal. Figure 2c shows the photoluminescence (PL) spectra of the various TiO2 inverse opals: a significant decrease in PL intensity and a blue shift in the peak wavelength are observed upon TiCl4 treatment. Red PL (580 – 640 nm) in TiO2 upon bandgap excitation has been attributed to the recombination of trapped electrons on undercoordinated surface Ti ions with holes in the valence band.29 On the other hand, green PL (centered at 520 nm) has been attributed to the recombination of trapped holes on oxygen vacancies or hydroxyl groups with photoexcited electrons in the conduction band.18,30,31 The changes in TiO2 PL suggest that the TiCl4 treatment converts the surface from being rich in undercoordinated Ti ions to hydroxyl groups. 9 ACS Paragon Plus Environment


Furthermore, the decrease in PL intensity suggests that the hydrolyzed TiO2 nanocrystals from TiCl4 solution exhibit lower population of trap states compared to nanocrystals derived from TiBALDH. The nature and density of trap states can strongly alter the energy alignment with the polymer at the interface.

Figure 2. Representative absorptance (a), reflectance (b) and photoluminescence (c) spectra of TiO2 inverse opals with different TiCl4 treatments: 0 M (black), 0.025 M (green), 0.05 M (red) and 0.05 M twice (blue). Absorptance spectra of P3HT/TiO2 inverse opals (d) with different TiCl4 treatments. The absorptance spectra of the P3HT/TiO2 inverse opals are shown in Figure 2d; the loading of P3HT on the inverse opals is similar. Typical vibronic coupling in P3HT is observed with the excitonic bandwidths ranging between 90.2 meV and 85.0 meV (Fig. S4) based on the


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weakly interacting H-aggregate model.32 The findings contrast our previous study where the inverse opal framework obtained from alkoxide precursor is rough and significantly disrupts the H-aggregate formation to yield excitonic bandwidth of 18 meV.9 In the co-assembled inverse opals, the small TiO2 nanocrystals and smooth framework allowed for ordered packing of P3HT chains. On the other hand, more unaggregated and disordered P3HT is observed with increasing TiCl4 treatment (Table S1), also evident by the increase in absorption near 400 nm for the twicepassivated sample. This finding aligns with previous report that surface roughness affects the packing of P3HT on inorganic nanoparticles.33 Next, we employ quasi-steady state transient photoinduced absorption (PIA) spectroscopy to probe the polaron population and lifetime in P3HT/TiO2 inverse opals with different TiCl4 treatments. Figure 3a shows representative PIA spectra of the inverse opals, in which the phase-sensitive lock-in amplifier outputs the transmitted source signal into two channels, X and Y, where X is in-phase with the pump beam and Y is phase-shifted by 90 degrees. The absorption feature of the P3HT P—P* polaron transition is observed at 990 nm,34 and additional features such as electro-absorption due to Stark effect35,36 and bleaching due to the depletion of ground state P3HT are observed at 750 nm and 600 nm, respectively. The polaron absorption intensity increases dramatically with increasing TiCl4 treatment. The total magnitude of polaron signal R (=(X2+Y2)-1/2) increases from 71 ppm for the unpassivated inverse opal to 148, 369 and 642 ppm for 0.025 M, 0.05 M and 0.05M-twice passivated samples, corresponding to enhancements of 2.1-, 5.3- and 9.2-fold. The enhancement is significantly higher than the increase in the volume fraction of TiO2 in the structure.



Figure 3. (a) PIA spectra of P3HT/TiO2 inverse opals with different TiCl4 treatments: 0 M (black), 0.025 M (green), 0.05 M (red) and 0.05 M twice (blue). Data in X-channel are filled and Y-channel are open symbols, respectively. (b) Pump modulation dependence of the P3HT P-P* polaron transition for P3HT/TiO2 inverse opals with different TiCl4 treatments. Solid curves are fits to the dispersive recombination model. The polaron lifetime and its distribution are found to change with TiCl4 treatment. Figure 3b shows the pump modulation frequency-dependence of the normalized polaron absorption of the different inverse opals. The polaron signal of the unpassivated P3HT/TiO2 inverse opal rapidly decays with increasing pump modulation frequency, in contrast to that of 0.05 M and twice-passivated samples. The data are fitted to a dispersive recombination model:37


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where ω is the frequency, (dT/T)o is the polaron absorption at 0 Hz, τ is the mean polaron lifetime and γ is a factor indicating the degree of dispersity of the lifetimes. The lifetimes can be dispersive due to inhomogeneities of the film and a variety of factors related to charge recombination. Values of γ can range between 0 and 1, with 1 indicating a narrower and more uniform distribution of lifetimes. Table 1 summarizes the lifetime (τ) and dispersion factor (γ) of the P3HT/TiO2 inverse opals. Significant variations in the lifetime of independent trials of the unpassivated inverse opals were observed, giving an average lifetime of 110 ± 24 ms. In contrast, the polaron lifetime shortens to 57 ± 23 ms, 12.4 ± 2.2 ms, and 9.4 ± 1.2 ms, for 0.025 M, 0.05 M, and 0.05 M-twice passivated samples, respectively. Changes in the polaron lifetime distribution are also observed: the dispersion factor is low for unpassivated sample (γ = 0.79) but reaches nearly 1 for 0.05 M TiCl4-treated inverse opal. The trends observed in PIA, namely the increased signal, lower lifetime and more uniform lifetime dispersion upon passivation, are in line with previous reports Table 1. Polaron signal, lifetime and dispersion factor. Six independent samples were measured for each TiCl4 condition. Errors are standard deviations of the mean. 0M Total polaron signal (R)

0.025 M

70.5 ± 4.9 ppm 148 ± 24 ppm

0.05 M

0.05 M x2

369 ± 82 ppm

642 ± 198 ppm

Lifetime (τ)

110 ± 24 ms

57 ± 23 ms

12.4 ± 2.2 ms

9.4 ± 1.2 ms

Dispersion factor (γ)

0.79 ± 0.04

0.89 ± 0.05

0.98 ± 0.02

0.98 ± 0.02



that investigate the role of ligands on quantum dots in CdSe4- and CdS38-polymer blend films, in which alkylamine ligands were found to increase crystallite size and reduce trap state density. Charge transfer dynamics from the polymer donor to the metal oxide acceptor can be influenced by band bending. Band bending of the polymer arises from the alignment of its charge transfer state (ECT+ or ECT-) with the Fermi level (EF) of the metal oxide upon contact. Downward band bending of P3HT (decreasing energy away from the interface) in contact with mesoporous TiO2 has been found to result from the spontaneous oxidation of P3HT.16 This occurs when the EF of TiO2 is below the hole polaronic state (ECT+) of P3HT with the presence of empty trap states above the EF of the metal oxide. To examine the effect of TiCl4 treatment on the energy alignment in P3HT/TiO2, we employed ultraviolet photoelectron spectroscopy (UPS). Figure 4a and b show the UPS spectra of the different TiO2 inverse opals at the secondary cutoff (SEC) and valence band regions, respectively. We observe a shift of SEC to lower binding energy, from 17.79 eV to 17.42 eV for unpassivated to 0.05 M twice-passivated TiO2, respectively. On the other hand, the onset of the valence band (at ~3.5 eV binding energy) does not change for the different TiO2 inverse opals (Fig. 4b). The unpassivated TiO2 therefore exhibits the largest bandwidth in the UPS spectrum (between electrons with the highest and lowest kinectic energy). Using the relationship that EVB = hν – Φ for a semiconductor, where hν is the source energy and Φ is the width of the spectrum, EVB is the energy of the valence band maximum with respect to vacuum level, we deduce a general lowering of EVB with increasing TiCl4 treatment. The energy of the conduction band (ECB) is then obtained from the optical bandgap. The energy levels of the valence and conduction bands of the TiCl4-treated inverse opals are close to the reported values of anatase; however, the energy levels of the unpassivated TiO2 obtained from the water-soluble precursor are unexpectedly higher by ~0.3 eV. 14 ACS Paragon Plus Environment

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Figure 4. UPS spectra of TiO2 and P3HT/TiO2 inverse opals with different TiCl4 treatments: 0 M (red), 0.025 M (green), 0.05 M (blue) and 0.05 M twice (orange). (a) Secondary cut-off region of TiO2 samples; (b) valence band region of TiO2 samples; (c) HOMO region of P3HT and (d) magnified HOMO onset region of P3HT. The difference in binding energies between 0 M and 0.05 M-twice passivated samples are highlighted by dashed grey lines in a and d. Fermi edge of a sputtered Ag film was used to calibrate zero binding energy. Spectra are offset vertically for clarity. Figure 4 c and d show the HOMO regions of P3HT, which are obtained after subtracting the respective TiO2 signals from the composite. No significant shifts in the SEC were observed upon incorporating the thin polymer coating suggesting negligible vacuum level offset. The spectra of HOMO (or occupied band states) of P3HT are in line with previous reports,39 but a difference in the binding energy of the P3HT HOMO is observed depending on the nature of the TiO2. Figure 4d shows that the binding energy of P3HT HOMO shifts higher (by 0.18 eV) with increasing TiCl4 treatment, indicating different degrees of band bending in P3HT at the interface of different TiO2 inverse opals. That is, the HOMO of P3HT at the interface is furthest away 15 ACS Paragon Plus Environment


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from EVB of the unpassivated TiO2. Table 2 summarizes the energy levels of TiO2 and P3HT determined from UPS. With neat P3HT having an ionization potential of 4.60 eV (Fig. S5), close to previously reported value,40,41 we measured a downward band bending of 0.43 eV for P3HT in contact with the unpassivated TiO2. Upon treating TiO2 with 0.025 M TiCl4, less downward bending is observed; for samples treated with more TiCl4, negligible bending is observed. The results show that changes of the TiO2 nanoparticles directly influence the energy alignment at the interface and alter the band bending of the polymer dramatically. Table 2. Summary of energy levels obtained from UPS.

TiO2

P3HT/TiO2

Neat P3HT

TiCl4 (M)

SEC (eV)

0 0.025 0.05 0.05x2 0 0.025 0.05 0.05x2

17.79 17.53 17.39 17.42 17.81 17.47 17.43 17.43 17.34

VBM binding energy (eV) 3.55 3.46 3.48 3.48

EVBa (eV)

Optical bandgapb (eV)

ECBa,c (eV)

-6.98 -7.15 -7.31 -7.28

3.36 3.34 3.31 3.29

-3.62 -3.81 -4.00 -3.99

HOMO binding energy (eV)

EHOMOa (eV)

0.74 0.81 0.93 0.92 0.68

-4.17 -4.50 -4.74 -4.72 -4.60

a

With respect to vacuum level From Tauc plot (Fig. S2) c ECB = EVB + Eopt. bandgap b

We now discuss how the different energy alignments arise. Band bending in TiO2 is not considered because the nanoparticles are smaller than the width of the space charge layer.42,43 In the unpassivated TiO2, PL data suggest the presence of electron traps located at 0.7 – 1.6 eV below the conduction band. These defect states are unfilled without illumination, and the majority are above the Fermi level of TiO2. The scenario of the unpassivated TiO2 with P3HT is then what was proposed by Haring et al.16 – equilibration upon contact leads to downward band bending in the polymer (Fig. 5a). In the 0.025 M TiCl4-treated sample, the overall PL peak 16 ACS Paragon Plus Environment

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wavelength is intermediate (at 550 nm). Although the red PL is significantly suppressed, the overall PL may have contributions from both electron and hole traps (red and green PL, respectively). The hole traps (filled with electrons) are located at 0.7 – 1.4 eV above the valence band. Because these hole trap states are located below both EF of TiO2 and ECT- of P3HT, they do not yield any interfacial charge transfer with P3HT. On the other hand, the reduction in the density of empty trap states above the EF of TiO2 limits spontaneous electron transfer from P3HT to TiO2. Hole trap states arising from oxygen vacancies in addition would raise the Fermi level of TiO2 to be closer to ECT+ of the polymer. Hence both factors lead to less band bending (0.10 eV) in 0.025 M TiCl4-treated inverse opal (Fig. 5b) compared to untreated TiO2. With increasing TiCl4 concentration to 0.05 M, the PL of TiO2 blue shifts to 520 nm suggesting further reduction in the density of electron trap states. If no unoccupied trap states are available above EF of TiO2, then there is no electron transfer from P3HT to TiO2. The samples treated with 0.05 M TiCl4 therefore show negligible band bending (Fig. 5c). The energy of P3HT HOMO on 0.05 M TiCl4treated TiO2 is in fact slightly lower (-0.12 eV) than pristine P3HT. It is unlikely that electron transfer from TiO2 to P3HT can occur to yield upward band bending because the Fermi energy of TiO2 remains lower than ECT- of P3HT. This difference in energy of P3HT may be due to differences in polymer ordering, with higher interchain ordering giving a lower ionization potential.41 Because polymer ordering on the inverse opal structure is lower than on the flat substrate as evident from the excitonic bandwidth, the energy of the bulk P3HT HOMO on the inverse opal structure is expected to be lower than -4.6 eV.



Figure 5. Energy band diagrams of TiO2 and P3HT before and after contact for samples treated with 0 M (a), 0.025 M (b) and 0.05 M (c) TiCl4. Electron trap states above Ef (black unfilled before contact or filled after contact) and hole trap states below Ef of TiO2 (filled grey) are shown. Spontaneous electron transfer (red arrow) and the resultant band bending decreases in magnitude with increasing TiCl4 treatment. Taken together all the factors involved, we rationalize the dramatic enhancement in the polaron signal upon TiCl4 treatment of P3HT/TiO2 inverse opals as follow. In the unpassivated sample, downward band bending of P3HT arising from the high density of defect states on the small and less crystalline TiO2 nanoparticles offers thermodynamically unfavorable energetics for the injection of photoexcited electron from the polymer to TiO2. Charge transfer upon illumination is impeded in unpassivated P3HT-TiO2 inverse opal. On the other hand, the reduction of trap state density in the larger and more crystalline nanoparticles of the passivated inverse opals leads to negligible downward bending of the HOMO and LUMO of P3HT. The absence of energetic barrier for photoinduced electron injection leads to high polaron yields in TiCl4-treated inverse opals. The effect of reducing trap state density is also evident by the low polaron lifetime upon TiCl4 treatment. These factors add to the effect of increasing the amount


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and surface area of TiO2 brought on by the TiCl4 treatment. Our findings demonstrate the importance of surface properties of metal oxides and energy alignment in hybrid photovoltaics. CONCLUSION We studied the effect of TiCl4 passivation treatment on the photoinduced charge transfer of P3HT to TiO2. Changes in the structural properties and trap state density of the metal oxide resulted in different yields, lifetimes and dispersions of polarons generated in P3HT/TiO2 inverse opals upon photoexcitation. While band bending in P3HT has been reported previously, we demonstrate the link between trap states, interfacial energy alignment and photoinduced charge transfer – a key process in solar energy conversion. The different energy alignment scenarios at the polymer-metal oxide interface, from downward to negligible band bending, validate the ability to modify the energetics of the interface of the same materials via surface modification – more specifically trap state engineering. The inverse opal with its large and accessible surface area could serve as a model structure for evaluating the photoinduced properties of other conjugated polymers at the interface with various inorganic acceptors. Supporting Information. Additional optical characterizations and modelling, XRD patterns and UPS data. This material is available free of charge via the Internet at http://pubs.acs.org. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGEMENTS



This work is supported by Natural Science and Engineering Research Council of Canada (Discovery grant), Canada Foundation for Innovation (JELF grant) and Province of Ontario (ORF). CK thanks Province of Ontario for an Ontario Graduate Scholarship. The authors thank N. Tulsiram for technical assistance, Dr. Soheilnia for XRD, Dr. Sodhi (Surface Interface Ontario at University of Toronto) for photoelectron spectroscopy and M. Jacklewicz for assistance with the electron microscope.


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