Lewis AcidâBase Interaction-Induced Porous PbI2 Film for Efficient

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Lewis Acid-Base Interaction Induced Porous PbI2 Film for Efficient Planar Perovskite Solar Cells Kai Sun, Ziyang Hu, Baihui Shen, Chunyan Lu, Like Huang, Jing Zhang, Jianjun Zhang, and Yuejin Zhu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00160 • Publication Date (Web): 24 Apr 2018 Downloaded from http://pubs.acs.org on April 24, 2018

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ACS Applied Energy Materials

Lewis Acid-Base Interaction Induced Porous PbI2 Film for Efficient Planar Perovskite Solar Cells Kai Sun,† Ziyang Hu,*,† Baihui Shen,† Chunyan Lu,† Like Huang,‡ Jing Zhang,† Jianjun Zhang,‡ Yuejin Zhu*,† †

Department of Microelectronic Science and Engineering, Ningbo Collabrative Innovation Center

of Nonlinear Hazard System of Ocean and Atmosphere, Ningbo University, Ningbo 315211, China ‡

College of Electronic Information and Optical Engineering, Nankai University, Tianjin 300071,

China ABSTRACT: The quality of perovskite film is an important factor influencing the photovoltaic properties of solar cells, which is greatly affected by deposition methods. Here, Lewis acid-base interaction induced porous PbI2 film is obtained by 4-tert-butylpyridine (TBP) vapor treatment method. X-ray diffraction, Raman spectroscopy, and Fourier transform infrared characteristics indicate that a new complex including both TBP and PbI2 molecules was formed by Lewis acid-base reaction. A continuous, uniform, and PbI2-free perovskite film with large grains was obtained from the porous PbI2 reacted completely with CH3NH3I. As a result, a promising power conversion efficiency of ~18% is achieved in planar-heterojunction perovskite solar cells. Furthermore, via controlling the vapor treatment time, the porosity and thickness of PbI2 film can be readily adjusted and the unexpected mesoporous perovskite film was first fabricated. Our work demonstrates the preparation of porous PbI2 films by Lewis base vapor treatment for high efficient planar perovskite solar cells. KEYWORDS: Lewis acid-base interaction, porous PbI2, perovskite solar cells, 4-tert-butylpyridine, vapor treatment 1. INTRODUCTION Metal halide perovskite materials have been found to pose excellent photoelectric properties and have been widely used in solar cells in recent years.1−3 Many researchers predict the potential of its commercial applications due to the rapid development of perovskite solar cells(PeSCs). High PeSCs efficiency from 9.7% in 2012 to 22.1% in 2017 have been achieved successively,1−3 which is comparable to that of commercial bulk silicon solar cells. The impressive photovoltaic performance is attributed to their long diffusion length,4 high carrier mobility,5 1

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suitable optical bandgaps,6 and strong absorption of light.7-10 Manipulating the morphology and photoelectric properties of perovskite films are crucial to achieve high-performance devices. Although high-quality perovskite films can be prepared by vapor-assisted methods,11, 12 solution routes are more advantageous and competitive for large-scale production.13, 14 Currently, various solution processes have been developed to improve the crystallinity, uniformity, coverage, and morphology of the perovskite layer, such as a one-step approach,15, 16 and two-step approach.2, 8, 17-18

For the two-step method, PbI2 layer was first deposited, followed by the conversion to

perovskite in CH3NH3I (MAI) solution. Since the deposition processes and the control strategy of PbI2 layer are versatile and flexible, uniform and full coverage PbI2 layer could be easily obtained, which tends to improve the quality of the perovskite layer. The main problem for the two-step method is the incomplete conversion of PbI2, which is mainly induced by the high-quality and compact PbI2 crystals due to its easy crystallization. The compact PbI2 is always formed due to the self-organization of [PbI6]4- units and growth of PbI2 crystals during annealing.19 The compact PbI2 crystals decrease the accessibility of MAI to the underlying PbI2 and hinder further conversion.20-22 To fully convert the PbI2 layer about hundreds of nanometer in thickness to MAPbI3 usually requires more than 1 h, which becomes more serious for planar device fabrication.23-24 Long-term conversion duration would lead to randomly dispersed large crystals due to the Ostwald ripening effect and even the dissolution or peeling-off of the perovskite into the solution. It is difficult for researchers to use the two-step method to prepare PeSCs with high efficiency compared to the one-step prepared PeSCs.25-26 To address this challenge, the addition of a solvent (eg. 4-tert-butylpyridine, dimethyl sulfoxide, H2O or hydrochloric acid) to react with PbI2 has been reported by some researchers.27-30 Besides, solvent engineering of the PbI2 solution,23, 31 annealing-driven inter diffusion process,32 and small crystal size of the synthesized PbI233 have been reported to solve this problem, and obtain better quality of perovskite films. Apart from these, porous PbI2 films were formed by solvent annealing PbI2 wet film,34 which significantly facilitates the CH3NH3PbI3 (MAPbI3) perovskite formation. By means of all these methods, the crystallinity of PbI2 layer was weaken and an adjustable morphology was obtained, which ensures better conversion from PbI2 to MAPbI3, and precise control of the final morphology of the MAPbI3 films, so the device performance and stability can be promoted. 2


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Here, we developed the preparation of porous PbI2 films by Lewis base vapor treated method to achieve high-quality MAPbI3 films using a two-step deposition method. By a simple 4-tert-butylpyridine (TBP) vapor treatment, a porous film composed of PbI2 complex by in situ Lewis acid-base interaction was obtained. This novel porous PbI2 complex could accelerate the reaction between PbI2 and CH3NH3I (MAI), and then a continuous, uniform, and PbI2-free perovskite film with large grains was formed. Furthermore, the porosity and thickness of the PbI2 porous film can be controlled by the vapor treatment time. Also, the mesoporous structure of perovskite films dependent of the porosity and thickness of PbI2 film were first observed. 2. EXPERIMENTAL DETAILS Fabrication of Solar Cells. The perovskite solar cells were fabricated on FTO coated glass substrates. First, the etched FTO were cleaned by deionized water, ethanol and acetone. After drying, the substrates were treated by ozone-ultraviolet for 15 min. A TiO2 compact (c-TiO2) layer was spin-coated on the substrates using the sol-gel solution with a spin speed and time of 4000 rpm/25 s. The ~40 nm c-TiO2 layer was then annealed at 450 ºC for 30 min. After that, the substrates were treated in a aqueous solution of TiCl4 (0.04 M) at 70 °C for 30 min, then rinsed with deionized water and dried at 120 °C for 15 min. To prepare the traditional PbI2 film, 462 mg PbI2 was first dissolved into 1 ml anhydrous dimethyl formamide (DMF) and then were spin-coated on c-TiO2/ FTO substrates at 4000 rpm for 30 s, finally drying at 70 °C for 30 min in a glove box. After that, CH3NH3I was spin-coated on top of the PbI2 film from a 30 μL isopropanol solution with 20 mg/ml at spin rates of 4000 rpm for 30 s and dried at 70 °C for 30 min to form CH3NH3PbI3 film. Preparation of the porous PbI2 film was conducted by TBP vapor treatment with different time( 5, 10, 30 min). The traditional PbI2 sample was placed in a closed container and 20 μl of TBP solvent was added dropwise before annealing. After thermal treated at 70 °C for 30 min, CH3NH3I solution was spin-coated on top of the treated PbI2 film and dried at 70 °C for 30 min to form the dense CH3NH3PbI3 films. After cooling, a hole transport layer (HTL) solution was spin-coated at 3500 rpm for 40 s, in which 1 mL spiro-OMeTAD/chlorobenzene (72.3 mg/mL) solution was employed with addition of 18 µL Li-TFSI/acetonitrile (520 mg/mL), and 29 µL TBP. Lastly, silver electrodes were thermally evaporated on top of the HTL under a pressure of 5×10−6 Torr at a rate of 0.3 nm/s. Characterization. The UV-Vis absorption spectra of PbI2 and perovskite film were recorded 3



using the VARIAN Cary 5000 UV-Vis-NIR spectrophotometer. X ray diffraction (XRD) pattern (2θ scan) was obtained by the Bruker AXS D8 Advance X-ray diffractometer using the Cu- Kα radiation (λ= 1.54050). The top view and thickness of the deposited PbI2 and perovskite films were confirmed by the Hitachi SU-70 scanning electron microscope (SEM). The Fourier transform infrared (FTIR) spectrum was recorded on the Bruker Tensor 27 infrared spectrometer. Raman spectroscopy measurements were carried on a Renishaw inVia Raman spectrometer. The steady-state photoluminescence spectra (PL) measurement was obtained using the Edinburgh instrument FLS 920 fluorescence spectrometer with the excitation of 532 nm wavelength. The FV1200 laser scanning confocal microscope with laser wavelength of 457 nm was used to measure PL life. The current density-voltage (J-V) measurements were conducted under simulated AM 1.5G sunlight of 100 mW/cm2 using an AM 1.5G type filter (Newport, 81904, USA). The light intensity was adjusted by using a standard Si cell. J-V curves were recorded with a Keithley model 4200 digital source meter at room temperature in the ambient air. The effective area of the cell was defined to be 0.1 cm2 by a metal mask. Electrochemical impedance spectra were measured with an electrochemical workstation (Zennium, Germany) under dark at a bias voltage of 0.8 V in 0.1-106 Hz and 5 mV as the sine perturbation bias. 3. RESULTS AND DISCUSSION The schematics of the TBP vapor treated PbI2 for formation of MAPbI3 film are shown in Figure S1. The fabrication of perovskite films mainly contains two stages, namely, porous PbI2 film development induced by TBP vapor treatment and perovskite film formation facilitated by ligand exchange reaction. The as-prepared PbI2 film with light yellow color was placed in a closed container for 5, 10, and 30 min, respectively. We found that the film was bleached to a dusky yellow within only 1 min in a TBP vapor atmosphere. The speed of discoloration is positively correlated with the treated time. After annealing at 70 oC, the treated sample was cooled to room temperature. When the CH3NH3I solution was dropped on the top of the sample, the color was immediately changed from yellowish to dark-brown. A key finding of our work is that the vapor treatment of TBP greatly facilitates the room temperature conversion of PbI2 to the perovskite in a short time. Also, we find that the quality and morphology of the final perovskite film are greatly affected by the vapor treatment time. To investigate how TBP affect PbI2 films, X-ray diffraction (XRD) analysis and UV-vis absorption 4


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spectroscopy (Figure 1) were first examined. In previous literatures, PbI2 was dissolved in DMF by spin coating on the substrate, usually in the form of 2H polytype, and the crystals were preferentially grown along the C axis.20 As shown in Figure 1(a), the as-prepared film shows strong XRD peaks at 12.61° (2θ), and additional peaks at 25.86°, 38.65° that are assigned to [001], [002], and [003] lattice planes of the 2H polytype PbI2, respectively.22 These peaks were also observed in the XRD spectra of the vapor treated films, while the intensity of the peaks is decreased along with the treated time. The peak at 38.65°even disappears when the treated time is reached to 10 min. These results indicate that vapor treatment results in low PbI2 crystallinity. Furthermore, increasing with the treated time, some new diffraction peaks appear and gradually become strong, as shown in Figure S2. Even after annealing at 70 °C for 30 min, these new peaks still remain. This result means that TBP vapor molecules and PbI2 molecules have a strong interaction, and produce the stable intermediate complex.35

(a)

(b)

(c)

Figure 1. (a) XRD patterns of the PbI2 films prepared from the vapor treatment with different time. “*” , “Δ” and “#” denote the PbI2 phase, FTO phase and PbI2 complex, respectively, (b) UV−vis absorpƟon spectra of PbI2 films, (c) the calculated band gap of PbI2 and PbI2 complex according to the absorption coefficient. In contrast to as-prepared PbI2 film, the treated PbI2 film was cloudy and roughness after TBP vapor treatment (Figure S3). The UV-vis absorption spectra of the treated PbI2 film after annealing become weaken compared to the as-prepared film in Figure 1(b). The weaken absorption of the treated PbI2 films was due to the formation of PbI2 (TBP) complex.35 It should be noted that the absorption spectrum of PbI2 complex without annealing is different with the PbI2 films. We found that the interaction between PbI2 and TBP resulted in a variation of optical bandgap (Eg). For as-prepared PbI2, the calculated Eg is 2.29 eV, while the Eg of the treated film is 5



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increased to 2.50 eV in Figure 1(c). The broaden Eg (~ 0.21 eV) for the treated film contributing to the reduced absorption and color change. Such optical bandgap shift was explained by the changed PbI2 electronic band structure due to the interaction of host layered material with the guest nitrogen-containing TBP molecules.36 In order to further illustrate the role of TBP vapor in the PbI2 crystallization process, we analyzed the interaction between the PbI2 and TBP in the complex by Fourier transform infrared spectroscopy (FTIR) and Raman spectrum. In Figure 2(a), a characteristic peak of TBP at 1640 cm−1 is identified. The vibrations down shifts to 1604 cm−1 after interact with PbI2, which indicates that bond strength between PbI2 and TBP due to Lewis acid-base interaction. The interaction is also supported by their UV-vis absorption spectra in above section. In Figure 2(b), the Raman spectrum of TBP, PbI2, and PbI2 complex are presented, respectively. The Raman spectrum of PbI2 complex situated at 75, 99, and 115 cm-1 were noticed, which practically retains the PbI2 crystal signature. However, compared with the Raman spectra of TBP, the PbI2 complex appearance is accompanied by several Raman peaks shift to higher frequencies, especially the 996 cm-1 is the (N–C) breathing mode and shift to 1010 cm–1. The up-shift of the Raman peak is considered as an evidence for the presence in the intercalated compound of chemisorbed form of TBP.37 This shows that the interaction between nitrogen and lead ions in the ring is stronger. We can explain that it forms a Lewis adduct of PbI2∙2TBP through Lewis acid-base reaction,38 where PbI2 acts as Lewis acid, and TBP acts as Lewis base. In the PbI2∙2TBP, the nitrogen-donor is contributed from TBP because it contains a lone-pair electron on nitrogen.

(a)

(b)

Figure 2. (a) Fourier transform infrared (FTIR) spectra of TBP, and PbI2 complex,(b) Raman spectra of PbI2 powder, TBP, and PbI2 complex. Taken together, we can conclude that TBP vapor can weaken the PbI2 crystallinity by forming 6


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intermediate complex. SEM examination of the treated PbI2 films shows that the surface of as-prepared PbI2 film is extremely dense and homogeneous as shown in Figure 3(a). However, after vapor treatment the surface morphology was dramatically changed. The dense plain morphology disappears and porous PbI2 film emerges in Figure 3(b-d). It is noticed that the size of voids within porous PbI2 film and the thickness of PbI2 layer were simultaneous increased with the vapor treated time. We can infer that TBP molecules reacted with PbI2 and induce the volume expansion for PbI2 film. For the benzene ring structure of TBP, the steric effect is evident, which contributes to the volume expansion occurred, especially in the vertical direction of the PbI2 film. When the expanded film was annealing, some excess TBP molecules were escaped, leaving the porous structure. Along with the treated time, the vapor pressure of TBP will rise in the confined container, more and more TBP molecules react with PbI2 and induce the continuous volume expansion of PbI2film. Therefore, the size of voids and thickness of the porous PbI2 layer are increased when the treated time is prolonged. It is surprised that the thickness of porous PbI2 layer is reached to ~6 µm when the treated time is 30 min, while as-prepared PbI2 layer is only ~250 nm as shown in Figure S4.

1µm

1µm

10 µm

10 µm

(a)

(b)

1µm

1µm

10 µm

10 µm (c)

(d) 7



Figure 3. Plane-view SEM images of porous PbI2 films formed by the vapor treatment method. (a) 0 min; (b) 5 min, (c) 10 min, (d) 30 min.

When the CH3NH3I solution was drop-coated on the top of PbI2 film, the distinctive morphology of MAPbI3 perovskite film was expected. The color change is very evidence as shown in Figure S1(c), when the CH3NH3I solution was first dropped on the top of PbI2 film. As expected, the morphology of MAPbI3 films depends on their starting PbI2 films in the two-step method. For the MAPbI3 perovskite film fabricated by as-prepared PbI2, rough surface with small grains, voids, and cracks are observed in Figure 4(a). Notably, the MAPbI3 film fabricated by 5 min vapor treated sample exhibits a dense and homogeneous film with large grains in Figure 4(b). For 10 and 30 min vapor treated samples, we are surprised that the mesoporous perovskite films were obtained. These mesoporous perovskite films were composed of small particles connected with each other. Increasing with the treated time, larger voids and smaller particles are presented as shown in Figure 4(c-d). The reasons of formation different MAPbI3 films can be explained as following. The porous PbI2 film is more advantageous compared to that of the compact one because a greatly enlarged surface area is available for the subsequent reaction with CH3NH3I solution, which assures the full conversion of PbI2 into CH3NH3PbI3. Furthermore, the PbI2∙2TBP complexes more facilitate to react with CH3NH3I by ligand exchange, which results in large grains as illustrated in In Figure 4(b). Considering that the excessive volume expansion of the vapor treated PbI2 film, the perovskite formation by the CH3NH3I and PbI2 reaction is insufficient to fill the voids of the porous structure, so the mesoporous perovskite is obtained. In other words, the quality of the desired perovskite can be controlled by adjusting the thickness of the vapor treated PbI2 or the treatment time. Therefore, the introduction of TBP is beneficial to forming TBP complex thin films. Via annealing, partial TBP molecules escaped from the complex, thus, the porous structure was formed. The porous film with enlarged surface area is more available for the subsequent reaction with CH3NH3I solution. Hence, the porous structure efficiently improves the transformation from PbI2 to CH3NH3PbI3. In Figure 5, the possible schematics of the TBP-promoted formation of CH3NH3PbI3 film are illustrated.

8


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(a)

(b)

(c)

(d)

Figure 4. Top-view SEM images of porous MAPbI3 formed by vapor treatment method (a) 0 min, (b) 5 min, (c) 10 min, (d) 30 min. Scale bars represent 1 μm.

Figure 5. Schematics of the TBP-promoted formation of CH3NH3PbI3 film.

To evaluate the electronic quality of the perovskite films formed from the porous PbI2 films, we 9



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compare the photoelectric properties of all perovskite films prepared by XRD and UV-vis absorption, photoluminescence. In Figure 6(a), XRD studies were performed to monitor crystalline transformations. As-prepared MAPbI3 shows an obvious peak at 14.05°=2θ, which is characteristic of the MAPbI3 [001] lattice distance in the tetragonal crystal structure, with further peaks at 19.98°,23.48°, 24.48°, 28.42°, 31.80°, 40.56°, and 43.13° corresponding to the reflections from [200], [211], [202], [220], [310], [224], and [314] of the perovskite.39 The presence of residual PbI2 follows from small XRD peaks at 2θ = 12.72° without vapor treatment. This is a typical problem assigned to incomplete conversion of PbI2 in thin films. However, the diffraction peaks of PbI2 are disappeared in the treated samples, which means that PbI2 has been fully reacted with CH3NH3I solution. Compared with 5 min treated sample, the 10 min treated sample shows lower intensity of XRD patterns due to the formed small perovskite grains. In other words, the problem of residual PbI2 in the process of preparing perovskite thin films can be resolved by two-step method using vapor treatment. The UV-vis absorption spectra of the perovskite films based on as-prepared PbI2 and treated PbI2 are shown in Figure 6(b). There was a very notable increase in the whole absorption of the perovskite film prepared from the treated PbI2, while the perovskite film prepared from as-prepared PbI2 have a relatively poor absorption. These results also demonstrated that the treated PbI2 was beneficial for the full conversion of PbI2 to MAPbI3 perovskite. It is also noted that the mesoporous perovskite shows the strongest absorption due to the light-trapping effect for the mesoporous structure.40

(a)

(b)

Figure 6. (a) XRD patterns of perovskite films, “*” symbols, and “Δ” symbols denote the PbI2 phase and FTO phase, respectively; “#” symbol denotes the possible complex TBP-perovskite 10


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phase. (b) UV–vis absorption spectra of perovskite films deposited from different PbI2 films.

Based on these perovskite films as photoactive layers, we fabricated planar PeSCs with a structure of FTO/c-TiO2/MAPbI3/spiro-MeOTAD/Ag. The current–voltage (J–V) curves of typical PeSCs based on different PbI2 are presented in Figure 7(a). In Table S1, the average values based on the as-prepared devices exhibited a PCE of 13.3 ± 1.0%, open-circuit voltage (Voc) of 1.03 ± 0.03 V, short-circuit current (Jsc) of 22.2 ± 0.5 mA/cm2, and fill factor (FF) of 58.3 ± 1.5%. When the perovskite layer was prepared from the 5 min treated sample, the resulting device exhibited better performance, with Voc increasing to 1.10 ± 0.02V, Jsc increasing to 23.2 ± 0.3 mA/cm2, and the FF increasing to 69.2 ± 1%. Consequently, the PCE improved to 17.7 ± 0.8%. The statistical results of Jsc, Voc, FF, PCE for the planar perovskite solar cells are shown in Figure S5(a, b). The enhanced Voc and FF can be explained by the reduced charge carrier recombination in the bulk of the material as well as at the contacts due to the high-quality of the MAPbI3 film with fewer defects and trap sites. Moreover, the MAPbI3 film based on the treated PbI2 was of high quality, the hysteresis of the device would be suppressed, which agrees with the J–V curves of the typical PeSC measured in different scan directions. Due to the full conversion of PbI2 to MAPbI3, more visible light would be absorbed. The residual PbI2 was eliminated (as confirmed by XRD) so that transport and collection of photo-generated electrons would be improved greatly. Thus, the Jsc of the PeSC based on the treated PbI2 increased obviously. However, the solar cells based on the mesoporous perovskite films show low photovoltaic properties compared with that of high-quality perovskite films. Considering that the mesoporous structure of perovskite, the experimental conditions of the device fabrication may be not identical to that of the device based on the planar perovskite film. Especially, the mesoporous perovskite with small grains would retard the carrier transport and collection. Therefore, in order to obtain better device performance for the mesoporous perovskite, the fabrication process should be further optimized. In addition, we found that the mesoporous perovskite films with special optoelectronic properties(eg. absorption, photoluminescence spectra) are different from the dense and uniform perovskite films. Photodetectors based on such mesoporous perovskite films may be exploited in the future.41 Further studies of these properties are beyond our scope in this paper. The stability of the devices with and without vapor treatment was investigated. The tested devices without 11



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encapsulation were stored in air environment (humidity: ~30%) and tested at every 24 h for 15 days as shown in Figure S6(a-d). Obviously, the PeSC based on 5 min treated sample showed good long-term stability. According to the literature,42 the unreacted PbI2 in the MAPbI3 perovskite film plays a negative role in the long-term stability of the devices. Moreover, a continuous, uniform perovskite film with large grains is able to prevent moisture penetrate into the internal perovskite layer, alleviating the decomposition process.

(a)

(b)

(c)

Figure 7. (a) Representative current–voltage (J–V) curves and the PeSCs based on different PbI2 samples, (b) steady state photoluminescence spectra detected at a peak 532 nm excitation wavelength, (c) Nyquist plot of PeSC devices.

Generally, for high efficiency devices, effective charge transport, low charge recombination and long carrier lifetime are needed. A high quality perovskite film may play a key role in achieving these factors for the PeSCs.43 The detailed cross-sectional SEM structures of representative solar cells made from 0 and 5 min TBP treatment are depicted in Figure 8(a, b). As depicted, for the treated sample, the perovskite layer is homogeneous with large crystals which connect closely from bottom to top, suggesting low density of grain boundaries. The no treated sample does not show distinctive large grains connected each other. Kelvin probe force microscopy (KPFM) is a powerful technique not only to examine surface topography but also the corresponding contact potential differences. The KPFM topographies of MAPbI3 formed by vapor treatment for 0 min and 5 min are shown in Figure 9(a). The root mean square as an indicator of roughness of the MAPbI3 film is 25 nm for 0 min treatment and 84 nm for 5 min treatment, respectively. The increased rougher surface is attributed to larger grain size. It appears that the surface of both perovskite films is uniform over a large area (3 × 3 μm2) with clear grain boundaries in Figure 9(b). Obviously, the film made from 5 min treatment consists of larger grains as also illustrated by SEM. 12


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Measurement of the surface potential spectra of MAPbI3 films (Figure 9(c)) reveals an increase of mean values of contact potential difference (CPD) by ~200 mV in the treated MAPbI3 film compared the no treated MAPbI3 film. It is known that the conduction band of MAPbI3 perovskite is more negative than that of n-type TiO2, which forms an energy barrier for electron injection from MAPbI3 to n-TiO2 during charge transport process.44 The decrease of electron quasi-Fermi level near conduction band edge of the treated MAPbI3 facilitates improved energy level alignment with adjacent TiO2 for electron transfer at the MAPbI3/n-TiO2 interface. KPFM measurements provide a new insight into the mechanism that both morphology and electronic properties of MAPbI3 film exert effects on device performance.

Ag Sprio-OMeTAD Perovskite c-TiO2 FTO (a)

(b)

Figure 8. Cross-sectional SEM images of solar cells based on MAPbI3 formed by vapor treatment method (a) 0 min, (b) 5 min. Scale bars represent 500 nm.

(a)

(b) 13


(c)


Figure 9. KPFM measurements of topography(a), deflection (b), and corresponding contact potential differences (CPDs) images (c) of MAPbI3 formed by vapor treatment method for 0 min(bottom), 5 min(top).

To further study the electron transport of the perovskite films based on the as-prepared and treated PbI2 and the internal charge transfer dynamics in the devices, the steady-state/time-resolved photoluminescence (PL) properties and electrochemical impedance spectroscopy (EIS) were investigated. In comparison to the untreated sample of glass/MAPbI3, the treated sample of glass/MAPbI3 presented a higher steady-state PL intensity and a little redshift in Figure 7(b). These indicate that the radiation fraction of the sample increases, thus reducing the non-radiative composite fraction, indicating a reduction in defects in the perovskite film.45 In other words, the MAPbI3 films prepared from the treated PbI2 were of high quality with fewer defects and trap sites. The residual PbI2 in the interface of glass/perovskite may block the charge transport of its higher conduction band over perovskite.46 The time-resolved PL decay (TRPL) curves of the perovskite films based on the as-prepared and treated PbI2 are shown in Figure S7. The as-prepared perovskite film exhibits weak fluorescence intensity and short PL lifetime. The carrier lifetime of the MAPbI3 film prepared from the treated sample was 18.3 ns, which was much longer than that of the MAPbI3 film prepared from as-prepared 9.7 ns. The detailed fitting parameters are presented in Table S2. This reveals that the treated PbI2 perovskite film contains less bulk defects or traps that act as carrier recombination centers.47 This result was in accordance with the steady-state PL spectra. Hence, the high efficiency of the PeSCs based on the 5 min treated PbI2 was achieved. The electrochemical impedance spectroscopies (EIS) and the corresponding Nyquist plots are shown in Figure 7(c). The semicircle corresponding to high frequency is ascribed to the charge transfer resistance, the size of the diameter is indicative of the resistance.48 The charge transfer resistance was reduced in the devices based on vapor treated samples by comparison of the diameter. The low value implies smooth carrier transport at the interface between the treated perovskite and electron transport layer. The residual PbI2 will directly result in the deterioration in charge-recombination. Therefore, there is no residual PbI2 is present in the perovskite film based on the treated PbI2, leading to high Jsc and FF. 4. Conclusion 14


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In summary, we have developed the preparation of porous PbI2 films by Lewis base vapor method to achieve high-quality MAPbI3 perovskite films via a two-step deposition method. By TBP vapor treatment, an in situ Lewis acid-base interaction induced porous PbI2 film was obtained. This novel porous PbI2 could accelerate the reaction between PbI2 and MAI, and then a continuous, uniform, and PbI2-free perovskite film with large grains was formed. Furthermore, the size of the voids and thickness of porous PbI2 film can be controlled by the vapor treatment time. The influence of the porous PbI2 on the formation morphology, chemical composition, crystallization, absorbance, and carrier transportation properties of the perovskite films was systemically investigated. Based on this strategy, the higher PCEs of the porous PbI2-based PeSCs with better performance were achieved. This simple, low-cost preparation of high-quality perovskite film method will be conducive to large-scale commercialization of PeSCs. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: ×××. Schematics of the porous PbI2 for the formation of CH3NH3PbI3 film, XRD patterns of the treated PbI2 films before and after annealing, photographs of the PbI2 films, cross-sectional SEM images of the PbI2 films, stability of unsealed perovskite devices AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]. Notes The authors declare no competing financial interest. Acknowledgments This work was supported by the Natural Science Foundation of Zhejiang Province (Grant No. LY18F040003), the National Science Foundation of China (Grant No. 11304170), the Foundation of Zhejiang Educational Commission (Grant No. Y201737090), and the Natural Science Foundation of Ningbo City (Grant No. 2017A610018). The author Z. Hu would like to thank the sponsored by K.C. Wong Magna Fund in Ningbo University. References 15



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Lewis AcidâBase Interaction-Induced Porous PbI2 Film for Efficient

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Lewis AcidâBase Interaction-Induced Porous PbI2 Film for Efficient