Zinc ion as effective film morphology controller in perovskite solar cells

Abstract

Over the past several years, organic–inorganic hybrid perovskite solar cells have developed rapidly. One of the hot topics with perovskite solar cells is how to achieve a high quality perovskite thin film. A Lewis acid–base adduct is effective for controlling perovskite morphology during the film preparation process. Zinc ion (Zn²⁺) is a stronger Lewis acid than lead ion (Pb²⁺) and it has a high ability to coordinate with methylamine groups (CH₃NH₃⁺), so it is doped into perovskite precursors to substitute for Pb²⁺ to influence crystallization during the annealing process. In this work, by adjusting zinc ion's concentration, high quality perovskite films with larger grain sizes and fewer pinholes were obtained. Finally, the average power conversion efficiency (PCE) of solar cells based on optimal concentration reached to 16.3% (with an open circuit voltage of 1.06 V, short circuit current density of 21.98 mA cm⁻², and fill factor of 70%), resulting in a 34.7% improvement compared to a pristine one. Our work highlights the metal-ion additive as an effective method to control perovskite film quality.

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Introduction

As a representative of third-generation solar cells, organic–inorganic hybrid perovskite solar cells are experiencing great development, with the latest certified power conversion efficiency reaching 22.7%.^1–6 Perovskite materials have many wonderful features such as high light absorption coefficient, tunable band gaps, high electron mobility, and low cost manufacturing.^7–9 These characteristics make them very suitable as light absorbing layers in solar cells.

A good quality perovskite absorber layer is the key point for improving photovoltaic properties of perovskite solar cells.¹⁰ Many research groups have already done a great deal of work in this area. Some have used physical methods. For example, vacuum-assisted annealing and vapor assistance technology have been used to synthesize high quality perovskite film.^11,12 Other groups have used chemical means. For example, Lewis acid–base adducts and polymers were added into a perovskite system.^13–15 As an important part of these chemical methods, the addition of metal ions has proven to be useful in the crystallization of perovskites. For example, the addition of Na⁺ and Al³⁺ can enlarge the grain size and improve the quality of perovskite films; the adulteration of In³⁺ can distinctly change the crystal orientations of a perovskite layer.^16–18 Besides, Cu⁺, Cu²⁺, Ag⁺, and Fe²⁺ have been introduced to a perovskite system to tune the band-gap and improve the crystallization of perovskite films.^19–21

Zinc(II) is a member of the transition elements, and Zn²⁺ is a stronger Lewis acid than Pb²⁺; it has a medium ionic radius (r = 74 pm), the same valence as the Pb²⁺ ion, and amphoteric ability. Zn has higher chemical activity than lead atoms plus the ability to coordinate with a CH₃NH₃ group,^22–24 which means Zn²⁺ not only consumingly attracts anions (such as I⁻ and Cl⁻) but also a cation (CH₃NH₃⁺) in this system. Therefore, Zn²⁺ is supposed to influence the film morphology. On the other hand, according to the Goldschmidt rule , the tolerance factor of CH₃NH₃ZnI₃ is 1.05.²⁵ So, this means that Zn²⁺ may have the capacity to form a CH₃NH₃Pb_1−xZn_xI_3−yCl_y perovskite structure in this system; therefore, the energy state might be changed.

Derived from these concepts, the transition element zinc was introduced into a CH₃NH₃PbI_3−xCl_x perovskite system and how it impacts perovskite thin film was checked. It was discovered that a small quantity of Zn²⁺ addition can effectively affect the crystallization of a perovskite thin film with little contribution to the material energy level. During preparation of this manuscript, we noticed a just accepted reference on ZnCl₂ doping to influence film morphology and energy state.²⁶ Quite different from the above reference, here we additionally discuss in detail the mechanism of film morphology changes by a small amount of addition of Zn²⁺, which is ascribed to the Lewis acid–base coordination mechanism. In this paper, we first give a reasonable explanation for the stronger interaction between an inorganic cage and organic groups, which was discovered in this work but has not been explained before.¹⁸ Additionally, we discuss the difference of a specific mechanism between zinc ions and other typical ions in the perovskite system. By means of adjusting the concentration of Zn²⁺ in a perovskite precursor, perovskite solar cells produced by the optimal concentration of Zn²⁺ show evident improvements in electrical or optical properties. Especially, by achieving a high quality perovskite film with larger grain sizes and smaller pinholes, target cells achieved 34.7% improvement in average PCE, which is up to 16.3% (AM 1.5, 1 sun). The current–voltage curves of doped samples show lesser hysteresis than pristine ones.

Results and discussion

To understand the impact that Zn²⁺ has on the perovskite absorption layer, we added different ratios of zinc chloride (ZnCl₂, from 0.05% to 1%) into a prepared precursor to substitute PbCl₂ and synthesized a series of perovskite thin films. In Fig. 1(a–f), it is seen that morphologies of a CH₃NH₃Pb_1−xZn_xI_3−yCl_y perovskite layer (on top of a c-TiO₂ layer), with different concentrations of ZnCl₂, are variable under a scanning electron microscope. The impact of Cl⁻ on the perovskite film is confirmed.^27–29 Because the dose of Cl⁻ is invariable, we mainly discuss the change caused by the addition of Zn²⁺. The formulation showing how we prepared the perovskite precursor is given in Table S2.† The resulting zinc ion in the prepared film has been checked, which is shown in Table S3.† As shown in Fig. 1(a–c), the grain sizes gradually became bigger and the pinholes were reduced with increasing concentration of Zn²⁺. When the concentration was increased beyond 0.2% (Fig. 1d–f), it not only made the grain size large, but also affected the general coverage of perovskite film on the TiO₂ compact layer. Therefore, it is confirmed that Zn²⁺ has a strong effect on the crystallization of CH₃NH₃Pb_1−xZn_xI_3−yCl_y perovskite films. These positive improvements in the quality of perovskite films with larger grain size and lesser pinholes may lead to high light absorbance and photovoltaic performance of the perovskite solar cells.

Fig. 1 Top view SEM of perovskite films with different concentrations of ZnCl₂ from 0% to 1% on the TiO₂ compact layer (a) 0%, (b) 0.05%, (c) 0.1%, (d) 0.2%, (e) 0.5%, and (f) 1%.

X-ray diffraction (XRD) spectra and the enlarged picture of diffraction peaks (110) and (220) are displayed in Fig. 2 and S1,† respectively. As the doped concentration increased up to 1%, the intensities of corresponding perovskite (110) and (220) peaks were substantially enhanced compared to the control perovskite film and there no impurity peak appeared in the pattern. So, we think that Zn²⁺ may form a perovskite structure with CH₃NH₃⁺ and I⁻ in this system, or the impurity peaks caused by the Zn²⁺ are too weak to be detected by XRD. These evidences indicate that Zn²⁺ can increase the crystallinity of (hh0) planes during the growth of perovskite. An enlarged picture and detailed information of (110) and (220) peaks are shown in Tables 1 and S1, † where it is noticed that the location of (110) and (220) diffraction peaks gradually shift to a high degree. This means that the d-space of the doped perovskite lattice became tight. By simple consideration, Zn²⁺ has a smaller ion radius and higher chemical activity than Pb²⁺. If Zn substitutes for Pb, it may explain this change caused by the addition of Zn²⁺. On the other hand, in Table 1 notice that the full width at half maximum (FWHM) data of the (110) peak gradually become small along with increasing Zn²⁺ concentration. Therefore, the grain size that was calculated from the Scherrer equation (shown in Table 1) gradually becomes larger from 92 nm with pure perovskite to 105 nm of the 1% doped sample. This result is consistent with the increased grain size in SEM pictures, and also means the smaller microstrain in the perovskite lattice is due to the addition of Zn²⁺.^30,31

Fig. 2 (a) X-ray diffraction spectra pattern of perovskite film with a series concentration of Zn²⁺ ion, (b) enlarged pictures of the (110) peak.

Table 1 The detail information of the XRD (110) diffraction peak

Concentration	FWHM (°)	Peak location (°)	Grain size (nm)	Intensity (a.u.)
0%	0.088	14.209	92.03	20 684
0.05%	0.088	14.225	92.21	24 121
0.1%	0.086	14.228	95.28	24 906
0.2%	0.083	14.232	97.58	34 476
0.5%	0.082	14.253	98.77	36 386
1%	0.077	14.258	105.18	48 217

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To further research the existence of Zn²⁺ in the perovskite absorber layer, we used energy dispersive X-ray spectroscopy (EDS) to confirm element mapping of the 0.1% doped perovskite film. The EDS system was used and determined the specific location of each kind of elements in the film. The EDS mapping patterns are shown in the ESI Fig. S3(a–d).† From the element mapping we can easily observe that Zn²⁺ (yellow dots) is uniformly distributed in the seed of the CH₃NH₃PbI_3−xCl_x perovskite film, and it does not aggregate at the grain boundaries.

To determine the binding energy difference between Pb and I before and after addition of Zn, we employed ESCALAB 250 XI (Thermo Scientific) to record X-ray photoelectron different Zn²⁺ concentrations. Binding energies of Pb 4f and I 3d are displayed in Fig. 3(a and b), which are diminished along with spectroscopy (XPS) data of a series of perovskite films with the addition of Zn²⁺. Specifically, binding energy difference between Pb 4f_7/2 and I 3d_5/2 is a decrease by 0.1 eV in the Zn²⁺–0.1% sample and 0.17 eV in the Zn²⁺–0.5% sample when compared to a pristine one. This result is in line with our previous speculation: Zn has a stronger chemical interaction with I than Pb, which can whittle the columbic interaction both between extranuclear and core electrons in both Pb²⁺ and I⁻.³²

Fig. 3 XPS core spectra and Fourier transform infrared spectroscopy (a) Pb 4f, (b) I 3d, (c) ATR-FTIR spectra of the films: the stretching vibration peak of NH₃⁺ in three types of film; (d) schematic diagram of Zn²⁺ ion doped into perovskite film.

Attenuated total refraction Fourier transform infrared spectroscopy (ATR-FTIR) was further employed to research the interaction between the metal ion and the organic group (methylamine). As shown in Fig. 3(c), the stretching vibration of symmetrical NH₃⁺ in CH₃NH₃I appears at 3086 cm⁻¹, meanwhile it shifts to 3122 cm⁻¹ in pure perovskite and a doped sample because there is chemical coordination between the CH₃NH₃I group and Pb or Zn to form a perovskite structure. Furthermore, it is obvious that the intensity of the stretching vibration of symmetrical NH₃⁺ was increased by the doping of Zn²⁺ which is because of a stronger interaction between inorganic metal and the organic group.¹⁸ We thoroughly researched the strong interaction between zinc and the organic group (CH₃NH₃) from an electron orbit coordination aspect, which is ascribed to the attraction between zinc 4s, 3d empty orbits and the lone pair electrons in nitrogen.^24,33 This coordination function can be demonstrated as the below eqn (a),³⁴ and it finally interacts with anions and forms the special zinc-based perovskite structure in Fig. 3(d). So, Zn²⁺ has a higher coordination interaction with CH₃NH₃⁺ (N element) than Pb²⁺, which lead to the electron cloud between Zn²⁺ ion and CH₃NH₃⁺ being closer to the metal ion. So, it is determined that there is strong reaction between Zn²⁺ and the organic group, which is also supposed to affect the crystallization of perovskite film.

Zn + :NH₃CH₃ = Zn·NH₃CH₃ (a)

If the Zn²⁺ existed outside the lattice it will only make the d-space become dilated because of the interaction between the Zn²⁺ with anions and CH₃NH₃⁺. That is inconsistent with the XRD results in Fig. 2(a) and Table 1. Therefore, the location of Zn²⁺ is in the crystalline lattice, and the addition of it makes the perovskite lattice become tight for the smaller ion radius and the higher chemical interactions. These evidences are in accord with the law in previous reports about perovskite elements substitution.^35–37

The mechanism of zinc ion affecting the morphology of a perovskite system can be expressed as follows: the ZnCl₂ doped perovskite system includes different ratios of ZnCl₂/PbCl₂ and CH₃NH₃I compared to PbCl₂ and CH₃NH₃I in a traditional perovskite system. After annealing, the Cl⁻ is released from the perovskite system by the formation of CH₃NH₃Cl gas and finally turns into a CH₃NH₃Pb_1−xZn_xI_3−yCl_y perovskite structure. During the growth of perovskite crystals, this small number of Zn²⁺ substitutes in the location of Pb in the perovskite framework; thus, the crystallization process has been changed. As shown in Fig. 3(d), we think it is the higher chemical interaction between Zn²⁺ and anions, along with a strong coordination function with organic groups, which makes the doped system tend to grow into bigger grain sizes. Therefore, the doped perovskite films majorly have bigger grain sizes. We thought this is possibly also because of the stronger chemical interaction, so the over doped perovskite system tends to accumulate into bigger grain seeds on the previous small grain seeds and affects the general coverage (Fig. 4).

Fig. 4 (a) The UV-vis absorption spectra of a series concentration of perovskite films, (b) stead-state PL spectrum on SiO₂ glass.

Different from Mg and the other second group elements (Sr, Ba, and Ca),^38,39 they also can enter into a perovskite lattice to substitute Pb and have higher chemical interactions with anions (I⁻, Cl⁻). Their different effects on perovskite morphology mostly result from their different ion radai. Some elements like Sn, which has a very close structure like Pb, can largely substitute in the location of Pb and change the morphology band gap of a perovskite layer.⁴⁰ But it is not reported to have stronger coordination interactions with organic groups (CH₃NH₃) compared to zinc ion to influence the crystalline morphology of perovskite.³³

This is different from the small monovalent ions (Na⁺, Li⁺),^16,41 which occupy the interstitial sites and enhance the crystallization of perovskite. Some small metal ions (Al³⁺),¹⁷ which only extract at the perovskite grain boundary, repair the defect at the grain boundary to improve the quality of a perovskite film. But, Zn²⁺ can easily implant a perovskite lattice and enlarge the grain size.

Beside the research of morphology of the Zn²⁺ doped film, we also investigate the optical properties of these perovskite films. From Fig. 4(a), it is easily to find that when the concentration below 0.2%, the addition of Zn²⁺ can improve the absorption of perovskite films. We attribute the improvement to the bigger grain sizes and lesser pinholes by the addition of Zn²⁺. When the concentration beyond 0.2% (including it), the absorbance is rapidly decreases because of the poor coverage. The PL intensity of the perovskite film with the addition of Zn²⁺ ion increased 50% compare to the pristine sample. The normalized PL are shown in Fig. S2(a),† which exhibits narrower PL peak in doped perovskite film. From the Fig. S2(b, c),† it is found out that these little concentration addition of Zn²⁺ have negligible contribution to the energy level of perovskite films. Base on the previous reports, these changes are caused by the decrease of the trap density due to the improvement of crystal quality.^42,43

Solar cells fabricated by the pristine precursor have the short circuit current density (J_sc) of 19.92 ± 1.21 mA cm⁻² and open circuit voltage (V_oc) of 1.0 ± 0.04 V. Meanwhile, perovskite solar cells with the addition of Zn²⁺ (0.1%) achieve remarkable improvement, with the J_sc of 21.98 ± 0.82 mA cm⁻² and V_oc of 1.06 ± 0.02 V (Table 2). At the same time, we find that the average FF increased from 61% to 70%. Thanks to improvements such as J_sc, V_oc, and FF, the average PCE increased from 12.1% of control samples to 16.3% of 0.1% doped ones. From Fig. 5(b), an IPCE graph of two kinds of concentrations shows that the addition of Zn²⁺ can obviously increase the optical-energy to electric-energy conversion efficiency of the PSCs. Once the addition concentration increases up to 0.2%, then PCE rapidly decreases due to the poor coverage of film. This decrease of PCE is mainly caused by poor coverage of the film, and the high recombination because of the direct contact of hole transport layer (HTL) and electron transport layer (ETL). Hysteresis in perovskite solar cells could be caused by ion migration, defect states, and electron extraction speed in the interface.^42–44 The J–V hysteresis is suppressed by the doped Zn²⁺, which is shown in Fig. S4(a and b). † As a result, doped samples show a lesser hysteresis index than pristine samples, which is ascribed to reduced trap states and bigger grain size from the doped Zn²⁺.

Table 2 The photovoltaic conversion parameters of a series of perovskite solar cells with different Zn²⁺ concentrations

Concentration	J sc (mA cm^−2)	V oc (V)	FF	PCE (%)
0%	19.92 ± 1.21	1.00 ± 0.04	0.61 ± 0.05	12.1 ± 1.1
0.05%	20.61 ± 1.13	1.04 ± 0.03	0.67 ± 0.04	14.4 ± 0.6
0.1%	21.98 ± 0.82	1.06 ± 0.02	0.70 ± 0.03	16.3 ± 0.6
0.2%	20.86 ± 1.26	1.01 ± 0.04	0.63 ± 0.04	13.3 ± 0.8
0.5%	19.64 ± 0.93	0.93 ± 0.07	0.58 ± 0.06	10.6 ± 1.2
1%	18.37 ± 0.85	0.85 ± 0.12	0.56 ± 0.08	8.7 ± 2.1

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Fig. 5 (a) J–V characteristics inset: the cross section of 0.1% doped perovskite solar cell, (b) IPCE spectrum.

To further understand the behavior of photo carriers in perovskite solar cells, we researched the electrochemical impedance spectra (EIS), transient photo-current, and photo-voltage decay based on full solar cells. EIS is a useful method to investigate the bulk and interfacial photo carriers' transport and recombination rule of solar cells. Nyquist plots of perovskite solar cells with two concentrations of Zn²⁺ at a bias voltage of −0.8 V are shown in Fig. 6(a). The inset of Fig. 6(a) shows Nyquist plots in the high frequency range. The equivalent circuit is shown in the top-right corner of Fig. 6(a). There are two semicircles in each Nyquist plot; the left one is related to the charge transport resistance (R_ct), which is mainly ascribed to charge extraction and separation at the interface between HTL or ETL and the perovskite layer. The right one is related to the photo carrier recombination resistance (R_rec) in the PSCs system; the starting point's real part represents the series resistance of the solar cells.^45–48 As shown in Fig. 6(a and b), it is clear that the doping of Zn²⁺ into the perovskite layer can increase the recombination resistance; it is due to the enhanced quality of perovskite film which can decrease the recombination coefficient. Fig. 6(c) shows that the addition of Zn²⁺ decreases the series resistance, which means the transport photo carriers could be enhanced in doped good quality perovskite solar cells. From Fig. 6(d), the R_ct of the doped sample is obviously decreased, which means smaller transport resistance between HTL or ETL and the perovskite layer. This is because the good quality of a perovskite film is useful for the extraction and transport of a photo carrier. These results are consistent with perovskite solar cells' performance, and further illustrate that the addition of Zn²⁺ can improve the quality of perovskite films, therefore improving their electrical performances.

Fig. 6 EIS and fast transient decay of the PSCs. (a) Nyquist plots of PSCs with different concentrations of Zn²⁺ at a bias voltage of −0.8 V. Inset: Nyquist plots in the high frequency range. (b) PSCs' recombination resistance, (c) series resistance, (d) charge transport resistance. (e) Normalized transient photo-voltage curves and (f) normalized transient photo-current curves.

Transient photo-current (TPC) and photo-voltage (TPV) decay can further unravel the series process (transfer, recombination) of photo carriers in the scale of time. The curve of transient photo-voltage records the process of carriers' vanishing. The carrier's lifetime is defined as the time when the photo-voltage value decreased to 1/e of the initial intensity.^44,48 As demonstrated in Fig. 6(e), pristine perovskite solar cells have a lifetime of 1.14 ms. Under the same conditions, these optimal addition cells exhibit three times longer life time (4.45 ms) than the control one. The longer life time in doped perovskite solar cells is due to suppressed trap state density in large grains, which can suppress the recombination in solar cells. Transient photo-current decay is performed at the short circuit current condition, which is deemed as the carrier's transfer program from the perovskite layer to the counter electrodes. TPC curves of two kind of perovskite solar cells are shown in Fig. 6(f); it is obvious that TPC attenuates faster in the Zn²⁺ doped sample, which means the charge transport ability in Zn²⁺ doped solar cells (10.74 μs) is stronger than the control one (12.91 μs). We attribute this rapid charge transport process to high quality film, which is useful for the transport of a photo carrier from a perovskite layer into HTL and ETL.

Conclusions

We introduced a kind of stronger Lewis acid (ZnCl₂) into a perovskite precursor to substitute for pristine PbCl₂ and further discussed the mechanism of changes caused by the addition of Zn²⁺. By adjusting the concentration of Zn²⁺ ion, the target film has better quality with lesser trap state density and bigger grain size. Perovskite solar cells produced by suitable addition of Zn²⁺ have obvious improvement in photovoltaic conversion property, with the average PCE also improved from 12.1% to 16.3%. The study highlights that metal ion doping in a perovskite system is useful to improve the quality of the perovskite film. Further research to find other metal ions, polymers, or other molecules that will improve quality of perovskite film or tune the perovskite materials' band-gap should be encouraged.

Experimental sections

Preparation of TiO₂ precursor and CH₃NH₃I powder

The TiO₂ precursor was prepared by a previously reported method.⁴⁹ Methyl ammonium iodide (CH₃NH₃I) was synthesized following the process report.² First, 30 mL methylamine (Aladdin, 33 wt% in water) and 32 mL hydriodic acid (Aladdin, 55–58 wt%) in water were stirred in an ice bath for 2 hours. Then, the mother liquor was evaporated at 50 °C until there was no water in the bottle. Finally, white powder CH₃NH₃I was obtained by recrystallization with diethyl ether and ethanol (three times) and then drying at 60 °C in vacuum for 24 h.

Device fabrication

F-doped SnO₂ glass (FTO Nippo, 14 Ω □⁻¹) was cleaned with a neutral detergent, deionized water, ethanol, and then their surface was modified with ozone-ultraviolet treatment. TiO₂ compact layer was covered on the substrates by the sol–gel method at a speed of 4000 rpm for 25 s, after which it was annealed at 450 °C for 30 min. Then it was treated with 40 mM aqueous TiCl₄ solution. Finally, TiO₂ blocking layers were sintered at 500 °C for 30 min to ensure a compact surface.

About 38 wt% (0.8 M (PbCl₂ + ZnCl₂) and 2.4 M CH₃NH₃I) were mixed in dimethyl formamide (DMF); this perovskite precursor with x mol% zinc chloride (ZnCl₂, the x is the ratio of Zn relative to entire metal cations) was spin-coated on the surface of the TiO₂ compact layer at a speed of 2800 rpm for 30 s. To assure a stable perovskite phase, these wet films were annealed on a hot plate by a method that was reported previously.⁵⁰ As the hole transport layer, spiro-MeOTAD's chlorobenzene solution (72.3 mg mL⁻¹) was added with 28.5 μL TBP and 17.5 μL Li-TFSI acetonitrile solution (520 mg mL⁻¹) was spin-coated at a speed of 3000 rpm for 30 s on the top face of the perovskite layer.^51,52 Finally, a 100 nm thick sliver counter electrode was evaporated on the surface of HTL under high vacuum.

Characterizations and measurements

The surface appearance, cross section images, and atomic energy dispersive X-ray spectroscopy (EDS) mapping of the perovskite films were taken by a Hitachi SU-70 scanning electron microscope (SEM). X-ray diffraction patterns were detected by a Bruker D8 Advance X-ray diffractometer using Cu-Kα radiation . Attenuated total refraction Fourier transform infrared spectroscopy (ATR-FTIR) spectra were acquired by a Micro-FTIR (Cary 660). X-ray photoelectron spectroscopy (XPS) plots were recorded by an ESCALAB 250XI (Thermo Scientific). The steady-state photoluminescence spectra were achieved by a Cary Eclipse Fluorescence Spectrophotometer (Agilent) with the excitation wavelength of 532 nm. The ultraviolet and visible (UV-vis) absorption spectra were tested by a UV-vis spectrophotometer (Cary).

Current–Voltage (I–V) data were recorded by a 4200 system source meter (Keithley) with a sunlight simulator (Newport, 91192A, AM 1.5, 1 sun). The intensity of the light was calibrated by a commercial standard Si cell (Newport). A perovskite solar cell's active area was defined by a black plastic aperture (0.07 cm²). Electrochemical impedance spectroscopy (EIS) and fast intensity transient decay were recorded by an electrochemical workstation (Zahner, Germany). EIS was tested from 10⁶ Hz to 0.1 Hz with 10 mV perturbation bias in the dark.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by National Natural Science Foundation of China (No. 11374168), Zhejiang Provincial Natural Science Foundation of China (LY18F040004) and K. C. Wang Magna Fund in Ningbo University of China.

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Footnote

† Electronic supplementary information (ESI) available: Enlarged picture of the (220) diffraction peak, normalized PL spectra, EDS mapping pattern, J–V curves measured by reverse scan and forward scan condition. See DOI: 10.1039/c8se00059j

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