Anti-solvent dependent device performance in CH₃NH₃PbI₃ solar cells: the role of intermediate phase content in the as-prepared thin films

Abstract

For high-performance perovskite solar cells, one-step solution deposition with anti-solvent washing is reported as an excellent processing method. In this paper, we employ attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), to quantitatively evaluate the relative contents of an intermediate phase in the as-prepared films, altered by anti-solvents, i.e. dichloromethane (DCM), toluene (TL), and diethyl ether (DE). Based on the joint measurements of scanning electron microscopy (SEM), X-ray diffraction (XRD), photoluminescence (PL), femtosecond transient absorption (fs-TA), current–voltage (J–V), and impedance spectroscopy, we reveal that a high content of the intermediate phase endowed with the anti-solvent DE is beneficial to film quality and device efficiency. A high coverage and pin-hole free CH₃NH₃PbI₃ film with a long PL lifetime attained via DE washing is utilized to fabricate a CH₃NH₃PbI₃ solar cell exhibiting over 17% power conversion efficiency. It is found that the formation of a high content intermediate phase in the as-prepared thin film is crucial to finally produce a dense and compact perovskite film with the aid of thermal annealing.

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Introduction

Organic–inorganic hybrid perovskite solar cells have gone through a rapid development in the past five years.^1–10 Continuous efforts on solvent engineering,^11–13 composition engineering,^6,14–16 and interface engineering^5,17–19 have led to the highest certified power conversion efficiency (PCE) of 22.1% for a small-area, single-junction perovskite solar cell.²⁰ In a typical perovskite solar cell, a several-hundred-nanometer-thick perovskite film is sandwiched between an electron-transporting material (ETM) and a hole-transporting material (HTM). For an efficient device, the preparation of a smooth, compact, and uniform perovskite film is highly pertinent to avoid charge recombination. In this regard, various processing methods to afford high-quality perovskite films have been reported, such as one-step deposition,^21–24 sequential deposition,^3,25–30 and vapor deposition.^4,31,32

So far high-efficiency perovskite solar cells have always been made via an anti-solvent washing based one-step deposition method.^6,8–10 In 2014, Jeon et al.¹¹ demonstrated that an intermediate phase CH₃NH₃I–PbI₂–DMSO, a Lewis acid–base adduct,³³ can be formed in the as-prepared thin film with TL as an anti-solvent. The formation of the intermediate phase is crucial to slow down the rapid reaction between CH₃NH₃I and PbI₂ in the process of spin coating. During subsequent thermal annealing intramolecular exchange between DMSO and CH₃NH₃I in the solid containing the intermediate phase leads to a high-quality perovskite film.^7,34 Later Jung et al.³⁵ put forward that anti-solvents featuring large dipole moments (i.e. chlorobenzene and o-dichlorobenzene) can induce a transformation directly to a crystalline perovskite phase bypassing the intermediate phase. However, the presence of an intermediate phase in the as-prepared film using chlorobenzene was still observed.⁸ Other anti-solvents i.e. DE,^36,37 hexane,³⁸ chloroform,³⁹ and iodobenzene⁴⁰ were also reported for the fabrication of perovskite solar cells, and the device performance varied significantly.

Both perovskite phase and intermediate phase are likely to coexist in the as-prepared films. Therefore, we assume that the content of an intermediate phase may have a critical influence on the subsequent crystallization of perovskite thin films. In our preliminary experiments, we also observed a remarkable difference in the colors of the as-prepared films (Fig. 1a) when DCM, TL, and DE were used as anti-solvents, suggesting different ratios of perovskite and intermediate phases. In this paper, we will scrutinize the anti-solvent dependent device performance of CH₃NH₃PbI₃-based solar cells, by taking a close look at the impact of intermediate phase content on the morphology, crystallinity, photophysical and electrical behaviors of thin films.

Fig. 1 (a) Photographs of the as-prepared thin films on titania, made with different anti-solvents. (b) Vertically shifted ATR-FTIR spectra of the as-prepared films. The spectral regions of the S=O and NH₃⁺ vibrational bands are enlarged. The tangents of the vibrational band onset are displayed as the background line. The heights of the vibrational bands are obtained through subtracting the background intensity at the peak position (A_background) from the peak intensity (A_Peak). (c) Comparison of the S=O vibrational bands in pure DMSO and the as-prepared film.

Results and discussion

As detailed in the Experimental section, we first fabricated compact titania on fluorine-doped tin oxide (FTO) by a hydrothermal method, followed by the deposition of mesoporous titania and thermal doping of LiTFSI.⁴¹ Afterwards, CH₃NH₃PbI₃ thin films were deposited on titania via one-step spin coating with the aid of anti-solvent washing. Fig. 1a compares the as-prepared films with diverse colors, which are likely associated with different ratios of perovskite to intermediate phases (CH₃NH₃I–PbI₂–DMSO).

In order to quantify the relative contents of the intermediate phase, we carried out ATR-FTIR measurements. As shown in Fig. 1b, the vibrational bands centered at 3188 ± 3 cm⁻¹, can be assigned to NH₃⁺ in CH₃NH₃I of both perovskite and intermediate phases. However, the bands peaked at 1015 cm⁻¹ due to S=O are only from the intermediate phase. Note that the presence of the intermediate phase in these films can be identified with the bands embodying reduced S=O bond strength, i.e. 1015 cm⁻¹ for CH₃NH₃I–PbI₂–DMSO versus 1043 cm⁻¹ for pure DMSO (Fig. 1c).³⁶ In terms of the Beer–Lambert law, the peak intensity of NH₃⁺ is proportional to the total concentration of the perovskite and intermediate phases, while the peak intensity of S=O is proportional to the concentration of the intermediate phase. Hence, the relative content of the intermediate phase could be determined via the intensity ratio of S=O and NH₃⁺ peaks, as tabulated in Table 1. As listed in Table 1, the intensity ratios of S=O and NH₃⁺ peaks increase in the order DCM < TL < DE, indicating the increased content of the intermediate phase in the as-prepared thin films.

Table 1 Background and peak intensities of the S=O and NH₃⁺ vibrational bands from Fig. 1b

Anti-solvent	[mOD]	[mOD]	[mOD]	[mOD]
a ΔA^ν(NH3+) and ΔA^ν(S=O) were obtained via equations and .
DCM	41	51	58	86	2.8
TL	19	28	43	79	4.0
DE	5	12	27	70	6.1

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SEM measurements were further conducted to study the effect of anti-solvents on the morphology of CH₃NH₃PbI₃. Fig. 2 gives the SEM images of CH₃NH₃PbI₃ films after 10 min annealing at 100 °C. A porous film with a wide size distribution of round pinholes was obtained with DCM as the anti-solvent (Fig. 2a and b). In contrast, the TL-treated film (Fig. 2c and d) possesses numerous irregular pinholes with smaller sizes. However, a dense, compact, and pinhole-free film (Fig. 2e and f) fabricated via DE washing features a granular structure. The grain sizes are in the range from a few hundred nanometers to several micrometers. It can be concluded that the morphologies of CH₃NH₃PbI₃ thin films are strongly influenced by anti-solvents.

Fig. 2 SEM images of annealed CH₃NH₃PbI₃ thin films on titania made with different anti-solvents: (a and b) DCM, (c and d) TL, and (e and f) DE.

The crystallinity of CH₃NH₃PbI₃ films on titania obtained via anti-solvent washing was also surveyed by means of XRD measurements. As depicted in Fig. 3a, the diffraction peaks are assigned to the lattice planes (110), (112), (211), (202), (220), (310), (224), and (411) respectively. The intensity ratios of the (110) to (310) diffraction peaks were calculated to be 2.0 for DE, 1.2 for TL, and 1.3 for DCM suggesting the influence of anti-solvents on crystal orientation. To understand the anti-solvent dependent crystallinity of CH₃NH₃PbI₃ films, we took a closer look at the optimal orientation of the (110) plane illustrated in Fig. 3b. The improvement of the crystal quality can be deduced from the increase of the diffraction intensity, in the order DCM < TL < DE. The 2θ peaks of CH₃NH₃PbI₃ films with DCM, TL, and DE as anti-solvents are centered at 14.28°, 14.15°, and 14.07°, respectively. The Δ(2θ) values of the full width at half maxima (FWHM) obtained via Gaussian fitting are 0.35°, 0.27°, and 0.22°, respectively. The comparable 2θ and reduced Δ(2θ) values have jointly proven that the crystallite size becomes large in the order DCM < TL < DE. Resorting to the Scherrer equation, the mean crystallite sizes were estimated to be 23 nm for DCM, 29 nm for TL, and 36 nm for DE. The increase of crystallite sizes is probably related to the improved content of the intermediate phase in the as-prepared thin films. These crystallites aggregate to form a polycrystalline⁴² perovskite film with large grains as shown in SEM images (Fig. 2).

Fig. 3 (a) XRD patterns of CH₃NH₃PbI₃ films on titania made with different anti-solvents. The intensities were normalized with respect to the (110) lattice plane. (b) Enlarged XRD diffraction peaks of the (110) plane. The intensities of the (110) peak were corrected with respect to FTO intensities. The grey lines were obtained via Gaussian fitting.

Scheme 1 summarizes the formation mechanism of CH₃NH₃PbI₃ films when different anti-solvents were applied. When DCM and TL were used as the anti-solvents, DMSO was probably over-extracted and thereby a high content perovskite phase was formed in the as-prepared thin films with more pinholes. In the case of DE, characteristic of the electron-donating oxygen atom, a high content of the intermediate phase can effectively retard the rapid nucleation of the perovskite phase during spin coating, and produce a homogeneous distribution of the intermediate phase in the as-prepared films. In the subsequent annealing stage, the intermediate phase gradually transforms into the perovskite phase through the intramolecular exchange between DMSO and CH₃NH₃I. We believe that in the solid-phase thermal chemical reaction to perovskite, the nucleation rate is slower than the growth rate, leading to a uniform and compact film with good crystallinity.

Scheme 1 Schematic illustration for the formation process of CH₃NH₃PbI₃ thin films made with different anti-solvents.

Next, we recorded PL traces of CH₃NH₃PbI₃ films on alumina presented in Fig. 4. PL traces feature a fast decay and a slow decay, which can be attributed to the surface component and bulk component related charge recombinations, respectively.⁴³ Table S1† displays the kinetic parameters obtained through fitting the PL decays with a two exponential decay function. The time constants of the surface component and bulk component (4.5 ns and 42 ns for DCM; 5.1 ns and 57 ns for TL; 7.6 ns and 72 ns for DE) increase in the sequence DCM < TL < DE, suggesting the decreased trap-assisted recombination rate and trap density.^44,45 A considerable long amplitude-averaged PL lifetime (62 ns) for the DE-treated film was obtained due to the improved film quality.

Fig. 4 Normalized PL decay traces of CH₃NH₃PbI₃ films on alumina. Excitation wavelength: 482 nm; probe wavelength: 760 nm.

fs-TA was further employed to monitor the trap-assisted charge recombination in the early stage right after photoexcitation. Extensive Auger recombination was avoided through the power-dependent measurements shown in Fig. S1.† A short pass filter (750 nm) was also placed in the pathway of probe lights before the sample to avert the distortion of spectral features and dynamics, in virtue of the excitation of the band edge or tail states.⁴⁵ The fs-TA spectra (Fig. 5a–c) of CH₃NH₃PbI₃ films on alumina were measured with a 490 nm pump light. Two obvious spectral features centered at around 650 nm and 760 nm were assigned to the photo-induced excited state absorption (ESA) and photo-bleaching (PB) bands, respectively. A dual-conduction band model⁴⁶ and dual excited state model⁴⁷ have been proposed to explain the spectral feature in the fs-TA spectra. It is well known that the 760 nm centered PB band is mainly attributed to the direct band transition induced band filling.⁴⁸ The red-shifting of the PB band and the narrowing of FWHM with time delay can also be explained well with the band-filling model.

Fig. 5 (a–c) fs-TA spectra of CH₃NH₃PbI₃ films on alumina made with different anti-solvents: (a) DCM, (b) TL, and (c) DE. Pump wavelength: 490 nm; power fluence: 10 μJ cm⁻². (d) Kinetic traces probed at 760 nm. The solid grey lines are fittings viaeqn (1).

On account of the relatively low exciton binding energy of about a few millielectronvolts at room temperature,⁴⁹ the species in the early stage after photoexcitation are predominantly free charges instead of bound excitons.⁵⁰ Hence, the recovery kinetics of the PB band observed in our fs-TA spectra is mainly ascribed to the recombination of the photo-induced electrons and holes. The recovery kinetics at 760 nm (Fig. 5d) can be fitted using two exponential functions, including a rise exponential function and a decay exponential function convoluted with a Gaussian instrument response function (IRF),

ΔA = IRF ⊗ [(−A₁)exp(−t/τ_rise) + A₂ exp(−t/τ_decay)], (1)

where A₁ and A₂ are both positive and the fitting parameters as listed in Table S1.† The time constant (0.5–0.6 ps) of the rising process could be attributed to the hot-hole cooling.^46,51,52 For the decay process, the time constants are 1.5 ns for DCM, 2.7 ns for TL, and 5.8 ns for DE, which match well with those of the surface components derived from PL decay measurements. The increase of the decay time constant indicates the reduction of the trap-assisted mono-molecule recombination rate.

Subsequently, we fabricated perovskite solar cells to investigate the anti-solvent dependent device performance. The device structure, J–V curves, steady-state PCE output and incident-photon-to-current conversion efficiency (IPCE) spectra are shown in Fig. 6. The J–V curves were measured under an irradiance of 100 mW cm⁻² stimulated AM1.5G sunlight and the parameters are tabulated in Table 2. The CH₃NH₃PbI₃ solar cell with DE as the anti-solvent has a highest open-circuit photovoltage (V_OC) of 1.11 V, a highest short-circuit photocurrent (J_SC) of 20.95 mA cm⁻², and a highest fill factor (FF) of 74.1%, generating the best performance with an average PCE of 17.2%, compared with those of TL (12.6%) and DCM (6.5%) treated cells. The tendency of PCEs obtained from the J–V curves is in good agreement with that of the steady-state PCE output. The values of V_OC, J_SC, and FF vary with anti-solvents, in the order DCM < TL < DE, so do the IPCE heights in the spectral range from 450 to 700 nm.

Fig. 6 (a) Device structure: FTO/TiO₂/CH₃NH₃PbI₃/Spiro-OMeTAD/Au. (b) J–V curves measured under simulated AM1.5G solar light (solid lines) and in the dark (dashed lines). Scan rate was 10 mV s⁻¹. (c) Steady-state photocurrent measured at the maximum power point (0.93 V for DE, 0.85 V for TL, and 0.59 V for DCM) and stabilized power output. (d) IPCE spectra.

Table 2 Photovoltaic parameters under 100 mW cm⁻² stimulated AM1.5G sunlight^a

Solvent	J SC [mA cm^−2]	V OC [V]	FF [%]	PCE [%]
a Each value is the average of the data taken from 18 devices.
DCM	15.02 ± 1.75	0.94 ± 0.08	45.5 ± 6.9	6.5 ± 1.8
TL	18.49 ± 0.92	1.08 ± 0.02	63.4 ± 3.2	12.6 ± 1.4
DE	20.95 ± 0.85	1.11 ± 0.02	74.1 ± 3.5	17.2 ± 1.4

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To unlock the anti-solvent dependent IPCE heights (86% for DE, 78% for TL, and 62% for DCM at 490 nm) of these perovskite solar cells, light-harvesting efficiency (Fig. S2†) was monitored through measuring the UV-vis absorption spectra of films on titania. All samples show almost 100% of light-harvesting efficiency at 490 nm. The PL spectra with or without an electron and hole extraction layer were further measured to reveal anti-solvent dependent charge dissociation.⁵³ With the assumption that no electron can be injected into alumina from CH₃NH₃PbI₃ due to a higher conduction band of alumina relative to that of CH₃NH₃PbI₃, the PL of Al₂O₃/CH₃NH₃PbI₃ mainly originates from the bulk recombination of photogenerated electrons and holes. Significant PL quenching can be probed for TiO₂/CH₃NH₃PbI₃/Spiro, signifying charge transfer occurring at the energy-offset TiO₂/CH₃NH₃PbI₃ and CH₃NH₃PbI₃/Spiro interfaces. Hence, the charge-dissociation efficiency can be roughly estimated from the PL quenching efficiency, in the order DCM < TL < DE (Fig. 7). Highly efficient charge dissociation can be realized with DE as the anti-solvent.

Fig. 7 Steady-state PL of Al₂O₃/CH₃NH₃PbI₃ (red) and TiO₂/CH₃NH₃PbI₃/Spiro (blue) fabricated with different anti-solvents: (a) DCM, (b) TL, and (c) DE. The PL intensity (I) was normalized with respect to the PL maximum of the corresponding Al₂O₃/CH₃NH₃PbI₃ film (I_max,a).

Finally, we measured the impedance spectroscopies of CH₃NH₃PbI₃ solar cells. The impedance spectra in Fig. 8a exhibit non-standard semicircles superposed by two semicircles. Generally, the semicircle at high frequency is related to hole transportation, whereas the semicircle at low frequency is related to the bulk charge recombination of CH₃NH₃PbI₃ or the interface charge recombination between CH₃NH₃PbI₃ and titania.^54,55 Recombination resistances were obtained via fitting the impedance spectroscopies with a simplified equivalent circuit (Fig. 8a). As presented in Fig. 8b, the recombination resistance decreases mono-exponentially as a function of forward potential bias. At a given bias, recombination resistance increases in the order DCM < TL < DE, which is well consistent with the V_OC tendency.

Fig. 8 (a) Nyquist plots in the dark at a forward bias of 1.0 V, for CH₃NH₃PbI₃ solar cells made with different anti-solvents. The inset is a simplified equivalent circuit for impedance spectroscopy analyses. (b) Plots of recombination resistance derived from impedance spectroscopy versus forward potential bias.

Conclusions

To summarize, we have used DCM, TL, and DE as anti-solvents in the one-step deposition of CH₃NH₃PbI₃ to tune the thin film quality and photovoltaic performance. The relative contents of intermediate and perovskite phases that coexisted in the as-prepared films have been determined by ATR-FTIR. It is found that a high content of an intermediate phase in the as-prepared film is favorable for the eventual formation of the CH₃NH₃PbI₃ film with a large grain size and low defects in the subsequent thermal annealing. The CH₃NH₃PbI₃ film made with DE as the anti-solvent has a long PL lifetime owing to the reduced trap-assisted recombination. Moreover, in a TiO₂/CH₃NH₃PbI₃/Spiro-OMeTAD solar cell the perovskite light absorption layer injects charges quantitatively to charge extraction layers and attenuates charge recombination effectively, leading to high device efficiency. Our work has revealed the intrinsic role of the intermediate phase in controlling the film quality of a solution processed organic–inorganic hybrid CH₃NH₃PbI₃.

Experimental section

Deposition of electron-transporting layer

An etched fluorine-doped tin oxide (FTO) conducting glass was ultrasonically cleaned with detergent, deionized water, acetone, and ethanol in turns for 10 min. The nitrogen dried FTO was further treated with UV-ozone for 20 min, immersed in a 40 mM TiCl₄ aqueous solution at 70 °C for 90 min, rinsed with deionized water, and annealed at 500 °C for 30 min, so as to be coated with a compact TiO₂ layer. Next, a mesoporous TiO₂ layer was deposited on the compact TiO₂ layer by spin coating for 30 s at 3000 rpm, using the commercial TiO₂ paste (18NRT, Dyesol) diluted with anhydrous ethanol at a weight ratio of 1/3.5. After drying at 100 °C for 10 min, the TiO₂ film was annealed at 500 °C for 30 min under dry air flow to remove organic components. Finally, the film was treated with 20 mM TiCl₄ aqueous solution at 90 °C for 10 min, cleaned with deionized water, and annealed at 500 °C for 30 min. The thickness of the mesoporous titania layer is 200 ± 50 nm. Li-doping of TiO₂ was carried out by spin coating 0.1 M LiTFSI in acetonitrile at 3000 rpm for 30 s with a ramp of 1000 rpm s⁻¹, which was followed by another annealing step at 450 °C for 30 min according to the reported method.⁴¹

Deposition of MAPbI₃ perovskite layer

1.45 M of PbI₂ (TCI, 99.99%) and MAI (Xi'an Polymer Light Technology Corp., 99.5%) were dissolved in DMSO under stirring at 70 °C for 60 min. After filtering with a PTFE syringe filter (pore diameter: 0.22 μm), the solution was kept at 70 °C as the TiO₂ electrode. A CH₃NH₃PbI₃ film was deposited by a consecutive two-step spin-coating process at 1000 rpm for 10 s and 5000 rpm for 30 s on the TiO₂ electrode. At the time of 20 s prior to the program end, 0.8 mL of DE, TL, or DCM was dripped onto the spinning substrate. The films were then annealed at 100 °C for 10 min in a nitrogen-filled glove box.

Deposition of hole-transporting layer and gold electrode

The filtered 60 mM Spiro-OMeTAD (Xi'an Polymer Light Technology Corp., 99.9%) in chlorobenzene was spin coated atop the annealed MAPbI₃ films at 6000 rpm for 30 s. The molar ratios of the dopants are 0.5 for Li-TFSI, 0.1 for FK209, and 3.3 for TBP with respect to Spiro-OMeTAD. Finally, 60 nm-thick Au was thermally evaporated on top of the device under a vacuum of 1 × 10⁻⁴ Pa at a deposition rate of 0.01 nm s⁻¹ for the first 5 nm, 0.03 nm s⁻¹ for the next 15 nm, and 0.05 nm s⁻¹ for the last 40 nm.

Deposition of mesoporous Al₂O₃

Mesoporous alumina films for control measurements were deposited by spin coating of a diluted colloidal dispersion (5 wt%) of Al₂O₃ nanoparticles (Sigma-Aldrich) in isopropanol for 60 s at 6000 rpm, onto cleaned FTO described above. The films were dried at 150 °C for 10 min, and further annealed at 500 °C for 30 min under dry air flow. The thickness of the alumina layer was controlled to be almost the same as that of titania.

ATR-FTIR, SEM, XRD, UV-vis, and PL measurements

ATR-FTIR spectra were collected with a Perkin Elmer FT-IR/NIR spectrometer (Frontier). Scanning electron microscopic images were recorded with a Hitachi S-4800 Instrument (Japan). The X-ray diffraction patterns were measured with a Bruker D8 FOCUS Powder X-ray Diffractometer (XRD) using Cu Kα radiation (λ = 0.15405 nm). UV-vis absorption spectra were recorded on a Perkin Elmer Lambda 900 spectrophotometer. Steady-state PL and time-resolved PL measurements were performed on a LifeSpec-II fluorescence spectrometer. The EPL485 laser diode was used to supply excitation light.

EQE, J–V, and impedance spectroscopy measurements

The incident photon-to-electron conversion efficiency (IPCE) measurement was performed using a system combining a 150 W xenon lamp, a Zolix Omni-λ300 monochromator, a Keithley 2400 source meter and a Hamamatsu S1337-1010BQ silicon diode calibrated in National Institute of Metrology, China. The data were collected with a wavelength sampling interval of 10 nm and a current sampling time of 1 s under the full computer control. The cell performance evaluated by the current–voltage (J–V) measurements under simulated solar light (a model LS1000-4S-AM1.5G-1000W solar simulator, Solar Light Company, USA) with a light intensity of 100 mW cm⁻², tested with a PMA2144 pyranometer and a calibrated PMA 2100 dose control system. The effective area was set to 0.04 cm² (0.2 cm × 0.2 cm) by using a non-reflective metal mask. Impedance spectroscopies were recorded on an IM6ex electrochemical workstation in the dark by applying a 20 mV voltage perturbation over a series of forward bias, with the frequency range from 1 Hz to 1 MHz. The measured impedance spectra were further fitted using the Z-view software (v2.80, Scribner Associates Inc.).

Femtosecond transient absorption measurements

For femtosecond optical spectroscopy, the laser source was a Spectra Physics Spitfire regenerative amplifier (130 fs, 1 kHz, 800 nm) seeded by a mode locked Spectra Physics Tsunami Ti: sapphire laser. The 800 nm laser was divided into two parts with a 9/1 beam splitter. The pump pulses were generated by a Light Conversion TOPAS-C optical parametric amplifier with the main part of the 800 nm laser, and then modulated with a phase-locked chopper. The white-light supercontinuum (440–1700 nm) probe pulse was generated by focusing the minor portion of the 800 nm laser onto a 4 mm-thick sapphire plate. The fluctuation of both pump and probe beams was cancelled out by measuring the unexcited sample with a reference pulse, an almost equal beam with the probe pulse. A 750 nm SPF has been placed on the probe path before the sample to avoid band-edge or tail-state pumping. The optical density changes at selected wavelengths were measured with the CDP2022i UV-vis multichannel detector with a pair of photodidode arrays. The fs-TA data collected with the ExciPRO software (CDP Corp.) and group-velocity dispersion of the white-light continuum have been corrected with the Surface Xplorer software (version 2.3) before kinetic analysis. More detailed information about the fs-TA system has been described in our previous article.⁵⁶ All the films used for optical study were encapsulated under an inert atmosphere.

Acknowledgements

The National 973 Program (2015CB932204) and the Key Technology R&D Program (BE2014147-1) of Science and Technology Department of Jiangsu Province have supported this work. We are grateful to Prof. Nam-Gyu Park and Dr Jin-Wook Lee for fruitful discussion on cell fabrication.

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Footnote

† Electronic supplementary information (ESI) available: Details of film fabrication and characterization. See DOI: 10.1039/c7se00125h

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