Enhanced photovoltaic performance and stability of planar perovskite solar cells by introducing dithizone

Enhanced photovoltaic performance and stability of planar perovskite solar cells by introducing dithizone

Solar Energy Materials & Solar Cells xxx (xxxx) xxx Contents lists available at ScienceDirect Solar Energy Materials and Solar Cells journal homepag...

1MB Sizes 2 Downloads 91 Views

Solar Energy Materials & Solar Cells xxx (xxxx) xxx

Contents lists available at ScienceDirect

Solar Energy Materials and Solar Cells journal homepage: http://www.elsevier.com/locate/solmat

Enhanced photovoltaic performance and stability of planar perovskite solar cells by introducing dithizone Shina Li a, b, Ruixin Ma c, Xing zhao a, Jiahui Guo a, d, Yuchun Zhang a, Chenchen Wang a, d, He Ren a, d, Yong Yan a, d, * a

CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing, 100190, China Tianjin Research Institute for Water Transport Engineering, M. O. T, Tianjin, 300000, China c University of Science and Technology Beijing, Beijing, 100083, China d University of Chinese Academy of Sciences, Beijing, 100049, China b

A R T I C L E I N F O

A B S T R A C T

Keywords: Planar heterojunction Perovskite solar cells Dithizone Chelating agents Two-step spin-coating method

In the two-step spin-coating method, the crystallization and morphology of PbI2 film are essential for producing highly efficient and stable planar heterojunction (PHJ) perovskite solar cells. In this work, the dithizone (DTZ) molecules were introduced into PbI2 precursor to improve the performance of perovskite films. We found that adding DTZ was an effective method to retard the crystallization of PbI2 film and consequently, produced a highquality perovskite film with pinhole-free, smoother, and fewer defects surface. Most importantly, the presence of residual DTZ in wet PbI2 film also assisted DMSO to slow down the growth of perovskite grains. By tuning the concentration of DTZ, the power conversion efficiency of the best performed cell has increased to 20.66% with negligible photocurrent hysteresis. Meanwhile, the best DTZ device offer an excellent stability, which retained 97% of the initial PCE after storage in the dark for approximately 24 days. We expect this controlled crystalli­ zation method could be further explored and provides a useful strategy to improve the performance of perovskite solar cells.

1. Introduction In recent years, organic–inorganic hybrid perovskite solar cells have attracted widespread attentions because of the advantages such as rapid increase of power conversion efficiency (PCE), cost-effective processing methods, strong light absorption, and low nonradiative carrier recom­ bination rates [1–3]. So far, the reported highest PCE has reached 24.02% [3], close to the single crystal silicon devices [4]. In a typical perovskite solar cell, it is composed by three important components, specifically, electron transport layer, light-absorbing layer, and hole transport layer [5]. The key element is perovskite layer in which the film morphology, domain size, crystalline structure, materials composition, as well as surface coverage are particularly important for achieving a high PCE [6–8]. Currently, there are many techniques in fabricating perovskite layer such as one-step method [9], two-step method [10,11], vapor deposition depostion [12], and several others [13,14]. In these techniques, the two-step technique is found to be a promising method in

fabricating a well-covered perovskite film, especially in fabricating a large-area cell [5]. In this two-step spin-coating method, PbI2 layer was firstly deposited onto the ETL layer (TiO2 [15], SnO2 [10,16], ZnO [17], and so on [18–20]). The methylammonium iodide (MAI) and/or for­ mamidinium iodide (FAI) was subsequently spin-coated, diffused into, and reacted with PbI2 to produce perovskites. In the diffusion step, the crystallinity of PbI2 precursor film was found to significantly affect the kinetics of ammoniums intercalation into PbI2 layer. Therefore, the film morphology, crystallinity, and composition of the final perovskite layer are largely determined by the property of PbI2 film. Usually, PbI2 tends to form a flat and compact structure, which is unfavorable for the complete reaction with upcoming ammoniums. In order to solve this problem, several strategies have been developed to reduce the crystal­ linity of the PbI2 film. For example, adding a high boiling solvent, DMSO, was found to be an effective method to retard the crystallinity of the initial PbI2 film [21]. Solvent coordination and anti-solvent extrac­ tion (SCAE) strategy was also employed to produce low crystalline PbI2

* Corresponding author. CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing, 100190, China. E-mail address: [email protected] (Y. Yan). https://doi.org/10.1016/j.solmat.2019.110290 Received 25 July 2019; Received in revised form 5 November 2019; Accepted 8 November 2019 0927-0248/© 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Shina Li, Solar Energy Materials & Solar Cells, https://doi.org/10.1016/j.solmat.2019.110290

S. Li et al.

Solar Energy Materials and Solar Cells xxx (xxxx) xxx

[6]. In addition, polymer P(VDF-TrFE) was incorporated into PbI2 so­ lution which could also retard the crystallinity of PbI2 film [22]. Tech­ nologically, we have previously shown an unannealed PbI2 film could improve the reaction between PbI2 and ammoniums, producing a better perovskite layer with improved efficiency [23]. In addition, by modu­ lating the quantity of remaining DMSO solvent in wet PbI2 film, we could obtain densely packed large grains and smooth surface because of the coordination between O and Pb. However, the O donor in DMSO was easy to escape from PbI2 framework and then caused insufficient perovskite grain growth. Dithizone (DTZ) is one of commonly used and effective chelating reagent for Pb cations because it consists of nitrogen and sulfur donor atoms (Fig. 1a) [24–27]. Incorporation of DTZ was expected to effec­ tively retard the crystallinity of PbI2 film, assist DMSO to slow down the perovskite crystallization process, and improve the diffusion of ammo­ niums. Consequently, in this work, DTZ was firstly used as processing additive for the PbI2 precursor solution in the two-step method. At an optimal concentration of DTZ in PbI2 precursor solution (DTZ-1, 0.59 μM, 1.3 M of Pb2þ), pervoskite with better crystallinity, dense and smooth surface (RMS ¼ 15.1 nm) was produced where the charge recombination rate was reduced, carrier lifetime was improved, and finally, the photovoltaic performance was enhanced (6.14%). With DTZ, the best device has demonstrated a power conversion efficiency of 20.66%.

peaks belonging to perovskite has on the contrary, decreased. These XRD diffraction peak intensities suggested the improved crystallinity of perovskites with 0.59 μM DTZ. With DTZ, the crystallization process of PbI2 was retarded and consequently, improved the diffusion of ammo­ niums and completed the reaction between PbI2 and FAI/MABr/MACl. In addition, the remaining DTZ in the wet PbI2 film also assisted DMSO to coordinate Pb cations strongly and consequently, slow down the perovskite crystallization process, favored the diffusion of ammoniums, and completed the reaction between FAI/MACl/MABr and PbI2. How­ ever, as more DTZ added, this reaction has been inhibited and incom­ plete conversion of perovskites was observed. This can be confirmed by calculating the relative intensity of perovskites (see peak at approxi­ mately 14.1� ) and remaining PbI2 phase (peak at approximately 12.7� ) in the layer. The peak intensity ratio between 14.1� and 12.7� has increased to the maximum value of 8.7 for DTZ-1 but rapidly decreased to 5.7 with more DTZ (DTZ-2). The quality of perovskite films was then characterized by using UV–vis absorption spectroscopy (Fig. 2c). Comparing to a film without DTZ molecules, perovskite films with DTZ displayed stronger absorption intensity at the wavelength region of 400–500 nm. The enhanced ab­ sorption demonstrated a better crystallinity of perovskite film in which DTZs coordinated with Pb cations and subsequently, retarded the crys­ tallization of PbI2 layer and slowed down the perovskite nucleation process at the surface of PbI2 layer. This is favorable for the diffusion of ammoniums and the complete reaction between FAI/MACl/MABr and PbI2. In addition, the band gap has slightly increased with the increase of DTZ concentration. This characteristic, in consistent with XRD mea­ surements, is because more DTZ additive prevented the complete con­ version of perovskites and therefore, the composition difference has led to the change of band gap. The quality of perovskite films was further examined by using Scanning electron microscopy (SEM) and Atomic force microscopy (AFM) (Fig. 3). Perovskite film with DTZ chelating molecules (DTZ-1, Fig. 3b and d) has shown bigger grain size and smoother surface comparing to the pristine perovskite film (DTZ-0, Fig. 3a and c). There were two color phases in these perovskite films where the white and black phases should be designated to PbI2 and perovskite phases, respectively [10,23]. With the increase of DTZ, the concentration of PbI2 phase was relatively lower which is consistent with the XRD measure­ ments (Fig. 2b). In addition, the surface roughness (RMS) of DTZ-1 perovskite film (RMS ¼ 15.1 nm) was relatively lower than that of pristine perovskite films (RMS ¼ 16.6 nm), indicating a smoother sur­ face of DTZ-1 perovskite film. The smooth surface with DTZ was more promising to provide high performance perovskite solar cells with less hysteresis [28]. Next, the transient photoluminescence (PL) spectra of perovskites were recorded on the soda-lime glass substrate (Fig. 4a). In contrast to a film without DTZ additive, the PL intensity has enhanced by adding DTZ

2. Results and discussion To confirm the chelating reaction between DTZ and Pb cations (Fig. 1a), the UV–vis absorption spectrum was recorded. As shown in Fig. 1b, the pure DTZ molecules dissolved in a cosolvent of DMF/DMSO (32.0 μM) displayed two characteristic peaks around 455 nm and 620 nm respectively (Fig. 1b, blue curve). Adding Pb cations (1.5 mM), the solution color transferred from green to red, where the peaks at 455 nm and 620 nm were disappeared and a new peak appeared at around 500 nm (Fig. 1b, red curve), indicating the formation of Pb-DTZ complex. It must note that with trace amount of DTZ (5.9 nM, 100 times diluted of DTZ-1), the characteristic peaks are hard to detect (Fig. 1b, black curve). The deposition of perovskite was subsequently proceeded according to a new two-step spin coating procedure in which the PbI2 layer was not heated (Fig. 2a). The crystallinity of perovskite was characterized by using X-ray diffraction (XRD, see Fig. 2b). Apparently, two diffraction peaks appeared at approximately 14.1� and 28.5� which could be ascribed to the (110) and (220) plane of FAMAPbBrI3, demonstrating the formation of perovskites. Interestingly, as the increase of DTZ concen­ tration in the PbI2 precursor solution from 0 (absence of DTZ) to 0.59 μM (DTZ-1), the diffraction peak intensity has significantly enhanced. Further increase of the DTZ concentration to 1.18 μM (DTZ-2), these

Fig. 1. (a) Schematic chemical chelating between DTZ molecules and Pb cation. (b) UV–vis absorption spectra of pure DTZ molecules (32.0 μM, blue curve), PbI2 and DTZ complexes (red curve, 1.5 mM Pb2þ and 32.0 μM DTZ; black curve, 13.0 mM of Pb2þ, 5.9 nM DTZ, 100 times dilution of DTZ-1) in DMF/DMSO (95%/ 5%) cosolvents. 2

S. Li et al.

Solar Energy Materials and Solar Cells xxx (xxxx) xxx

Fig. 2. (a) Scheme of two-step spin coating procedure for the preparation of perovskite film. Different from previous method, DTZ was added to chelate Pb cations. (b) XRD and (c) UV–vis absorption spectra of perovskite films fabricated with different amount of DTZ molecules. DTZ-0, no chelating molecules. DTZ-0.5, DTZ-1, and DTZ-2 correspond to 0.30 μM, 0.59 μM, and 1.18 μM DTZ respectively.

where ε and ε0 are the dielectric constants of perovskite and the vacuum permittivity, respectively. L is the thickness of the perovskite layer and e is the elementary charge [29,30]. Since L is the same for all perovskite films, the difference of charge trap density depends only on the VTFL. In the dark I–V curves (Fig. 4b), distinct values of VTFL have been observed in a device with DTZ (0.33 V, DTZ-1) and a device without DTZ (0.59 V, DTZ-0). The lower VTFL was in DTZ-1 device indicated that there was lower trap density, in consistent with PL measurement (Fig. 4a). This lower trap density was mainly attributed to the less surface defects and the better crystallinity of perovskite layer. The charge transport/transfer and the charge recombination char­ acteristics between different interfaces were subsequently studied by using electrochemical impedance spectroscopy (EIS, Fig. 4c). These resulting EIS curves were fitted using the equivalent circuit in the inset of Fig. 4c. In these EIS curves, there are two semicircles where the first one is ascribed to the charge transport resistance (Rct) at the ETL/ perovskite interface while the second arc at low-frequency represents the charge recombination resistance (Rrec) [30–32]. The fitting curves were superimposed with the experimental measurements and deduced a RS of 24.03 Ω with DTZ molecules (DTZ-1) and a RS of 28.21 Ω at the absence of DTZ (DTZ-0). This lower RS indicated a higher charge transfer rate which could be attributed to good crystallinity, high phase purity, and smooth surface of the DTZ-1 perovskite film. Meanwhile, the Nyquist plots apparently showed that the DTZ-1 device has a lower Rct of 200.9 Ω comparing to a value of 478.9 Ω for DTZ-0 perovskite. This demonstrated that the charge transfer inside the DTZ-1 perovskite layer was more efficient due to a better quality perovskite film. In order to gain further insight of the charge recombination process in these perovskite devices, the carrier recombination rate in the PSCs was evaluated by performing the open-circuit voltage (Voc) decay measurements. During the Voc decay measurement, the device was firstly placed under a constant light illumination (AM 1.5 G) for 20 s to reach a photo steady state. The light was then switched off and the decay of voltage transient was monitored. Since the device was in an opencircuit condition, the voltage decay reflected the recombination behavior of charge carriers. Fig. 4d showed the Voc decay curves of two PSCs with 0.59 μM DTZ (DTZ-1) and without DTZ (DTZ-0). Apparently, the DTZ-1 device has shown a longer Voc decay time comparing to the DTZ-0 device, indicating that the DTZ-1 device has a lower charge

Fig. 3. SEM and AFM images of perovskite films at the absence of DTZ (a) and (c) and with 0.59 μM DTZ (b) and (d). The scale bars are 1 μm.

molecules, suggesting reduced surface defect and enhanced crystallinity in these DTZ perovskites. The strongest PL intensity was found in the perovskite film with 0.59 μM DTZ (DTZ-1), in consistent with XRD measurement. In order to reveal the charge trap density in these films, the dark current–voltage (I–V) characteristics of perovskite-only devices were recorded by using a device structure of ITO/perovskite/Au (Fig. 4b, the inset show the scheme of device). The dark I–V curves consists of three regions: the Ohmic region, trap filling limited region, and the space-charge-limited-current region. At low bias, the linear correlation reveals an ohmic-type response [29]. When the bias voltage is above the kink point (defined as the trap filled limit voltage (VTFL)), the current has increased nonlinearly, indicating the traps completely filled. The defect density can be calculated by using the equation � Ndefects ¼ 2εε0 VTFL eL2 (1)

3

S. Li et al.

Solar Energy Materials and Solar Cells xxx (xxxx) xxx

Fig. 4. (a) Steady-state PL spectra of perovskite film with different amount of DTZ molecules. (b) Current–voltage characteristics of ITO/perovskite/Au devices. The inset is the device scheme. (c) EIS curves of DTZ-0 and DTZ-1 PSCs. The symbols are experimental data, the solid curves are fitting data and the corresponding electronic circuit is shown on top inset. (d) Open-circuit voltage decay DTZ-0 and DTZ-1 PSCs.

recombination rate and longer carrier lifetime. This is attributed to the decreased trap density in the DTZ-1 perovskite film. The typical cross-sectional SEM image was shown in Fig. 5a. The thickness of perovskite layer was about 400 nm which was the most suitable thickness for both carrier transport and photon capture [33]. The current–voltage (J-V) characteristics of the best performed PSC based on DTZ-1 perovskite film was shown in Fig. 5b and the detail of performance were summarized in Table S1. The PCE of the DTZ-1 cell has reached as high as 20.66% (reverse scan) where the short-circuit current (JSC) was 23.60 mA/cm2, the open-circuit voltage (VOC) was 1.14 V, and the fill factor (FF) was 77.07%. Comparing the best per­ formed cells of DTZ-0 and DTZ-1 (see Table S1), the device with DTZ-1 exhibited higher short-circuit current (JSC), which increased from 23.28 to 23.60 mA/cm2 due to the better absorption and higher charge transportation of perovskite films. In addition, the open-circuit voltage (VOC) of device with DTZ-1 was obviously higher than DTZ-0 device because of fewer surface defect found in the DTZ-1 perovskite film. Besides the JSC and VOC, the fill factor (FF) was also a parameter for the high efficiency perovskite solar cells. The FF of DTZ-1 device was improved from 76.66% to 77.07% which is related to the surface reflection, series resistance, and shunt resistance [34]. In Fig. 5b, it was found that with forward scan, negligible hysteresis (0.32%) has observed and the PCE of 20.34% was measured (Jsc, 23.69 mA/cm2, Voc, 1.13 V, and FF, 76.16%). The external quantum efficiency (EQE) and the integrated Jsc (from EQE curve) of PSC with and without DTZ chelating molecules were subsequently tested and compared (Fig. 5c). The EQE response of both devices displayed a significant contribution at wave­ lengths between 400 and 800 nm. The maximum EQE appeared at approximately 518 nm which was in consistent with the absorption spectra (Fig. 2c). The integrated Jsc were 21.78 and 22.7 mA/cm2 for DTZ-0 and DTZ-1 devices respectively, which was close to the Jsc value obtained by J–V sweep measurements, confirming the accuracy of de­ vice efficiency characterizations. The statistical PCE of 32 DTZ-0 and DTZ-1 devices were summarized in the Fig. 5d. Normal distributions were found and with DTZ additive. The average PCE of PSC has enhanced from 18.25 � 0.77% (DTZ-0) to 19.37 � 0.54% (DTZ-1). In

addition, the enhancement of performance by adding DTZ was further confirmed in the devices with large active area (1.1 cm2). The J-V characteristics of the devices based on DTZ-0 and DTZ-1 perovskite film were shown in Fig. 5e. The DTZ-0 device show a good efficiency (~8% by average, Fig. S1) with the highest PCE of 12.22%. With DTZ, the average PCE has increased to ~10% (Fig. S1, statistics based on 15 devices) and the PCE of best performed device has reached 14.69% at reverse scan. A slight hysteresis was found between the forward and reverse scans where a PCE of 13.38% was obtained at forward scan (Fig. 5f). To study the stability of the DTZ PSC, the steady power output close to the maximum power point was measured at the voltage of 0.89 V under simulated AM 1.5 G illumination (100 mW/cm2) in ambient condition (Fig. 6 a). As shown in Fig. 6 a, the steady power output was measured where an average PCE of 19.92% and JSC of 22.42 mA/cm2 were obtained for the DTZ-1 PSC. Meanwhile, long-term stability at the ambient environment is a key factor of perovskite solar cells for practical applications [35]. The PCEs of PSCs as a function of storage time were displayed in Fig. 6b. After approximately 24 days at the absence of light, the DTZ-1 solar cell preserved 97% of the initial PCE while the DTZ-0 solar cell has degraded to approximately 82%. Such high ambient air stability could be attributed to the high crystallinity and low surface defects of perovskite layers. 3. Conclusion In summary, we have demonstrated an effective method to improve the PCE of PSC by adding DTZ into of PbI2 precursor solution. The DTZ could retard the crystallization of PbI2, enhanced the reaction between ammoniums and PbI2 and consequently, improved the perovskites quality. In addition, the remaining DTZ in wet PbI2 film also assisted DMSO to reduce the nucleation process of perovskite. Based on these improved perovskite layers, the average PCE has enhanced from 18.25 � 0.77% at the absence of DTZ to 19.37 � 0.54% with 0.59 μM DTZ molecules. The PCE of the best performed device has reached 20.66% and no noticeable hysteresis has been found. In addition, the 4

S. Li et al.

Solar Energy Materials and Solar Cells xxx (xxxx) xxx

Fig. 5. (a) The cross-sectional SEM image of perovskite solar cell with the optimized concentration of DTZ molecules. (b) J–V curves of the best performed PSC. (c) EQE curves and the integrated current densities of DTZ-0 and DTZ-1 PSCs. (d) Statistical histograms of DTZ-0 and DTZ-1 PSCs. (e) The J-V characteristics of large area (S ¼ 1.1 cm2) PSC based on DTZ-0 and DTZ-1 perovskite films. (f) J–V curves of the best performed PSC with an area of 1.1 cm2.

Fig. 6. (a) Current density (left y-axis) and PCE (right y-axis) transients of DTZ-1 solar cells. Note: the average values (JSC ¼ 22.42 mA/cm2 and PCE ¼ 19.93%) are calculated by counting the data between 100 to 300 s. (b) Comparison of long-term stability performance of DTZ-0 and DTZ-1 PSCs. Both devices were stored in the dark where the ambient humidity is approximately 10%–30% and the temperature is approximately 10–25 � C.

stabilized power output of the device with 0.59 μM DTZ (DTZ-1) pre­ sented an average PCE of 19.92%. After 24 days, its’ PCE has preserved approximately 97%. Looking forward, we expect this approach could open up new opportunities for large-scale perovskite solar cells via commercial printing fabrications.

performed the data analysis. All authors wrote the manuscript. Y.Y. supervised the project. Competing financial interests The authors declare no competing financial interests.

Author contributions S.L. carried out the experiments and, with X.Z., J.G, Y.Z., C.W., H.R. 5

S. Li et al.

Solar Energy Materials and Solar Cells xxx (xxxx) xxx

Declaration of competing interest

[16] M. Abuhelaiqa, S. Paek, Y. Lee, K.T. Cho, S. Heo, E. Oveisi, A.J. Huckaba, H. Kanda, H. Kim, Y. Zhang, R. Humphry-Baker, S. Kinge, A.M. Asiri, M.K. Nazeeruddin, Stable perovskite solar cells using tin acetylacetonate based electron transporting layers, Energy Environ. Sci. 12 (2019) 1910–1917. [17] M. Yang, J. Li, J.H. Li, Z.T. Yuan, J.J. Zou, G. Lei, L. Zhao, X.B. Wang, B.H. Dong, S. M. Wang, High efficient and long-time stable planar heterojunction perovskite solar cells with doctor-bladed carbon electrode, J. Power Sources 424 (2019) 61–67. [18] J.S. Feng, Z. Yang, D. Yang, X.D. Ren, X.J. Zhu, Z.W. Jin, W. Zi, Q.B. Wei, S.Z. Liu, E-beam evaporated Nb2O5 as an effective electron transport layer for large flexible perovskite solar cells, Nano Energy 36 (2017) 1–8. [19] Y.H. Chiang, C.K. Shih, A.S. Sie, M.H. Li, C.C. Peng, P.S. Shen, Y.P. Wang, T.F. Guo, P. Chen, Highly stable perovskite solar cells with all-inorganic selective contacts from microwave-synthesized oxide nanoparticles, J. Mater. Chem. 5 (2017) 25485–25493. [20] K. Mahmood, B.S. Swain, A.R. Kirmani, A. Amassian, Highly efficient perovskite solar cells based on a nanostructured WO3–TiO2 core–shell electron transporting material, J. Mater. Chem. 3 (2015) 9051–9057. [21] Y.Z. Wu, A. Islam, X.D. Yang, C.J. Qin, J. Liu, K. Zhang, W.Q. Peng, L.Y. Han, Retarding the crystallization of PbI2 for highly reproducible planar-structured perovskite solar cells via sequential deposition, Energy Environ. Sci. 7 (2014) 2934–2938. [22] C.Y. Sun, Y.P. Guo, B.J. Fang, J.M. Yang, B. Qin, H.A. Duan, Y.J. Chen, H. Li, H. Z. Liu, Enhanced photovoltaic performance of perovskite solar cells using polymer P(VDF-TrFE) as a processed additive, J. Phys. Chem. C 120 (2016) 12980–12988. [23] S.N. Li, H. Ren, Y. Yan, Boosting efficiency of planar heterojunction perovskite solar cells to 21.2% by a facile two-step deposition strategy, Appl. Surf. Sci. 484 (2019) 1191–1197. [24] B. Salih, A.D. Denizli, C. Kavaklı, R. Say, E. Piskin, Adsorption of heavy metal ions onto dithizone-anchored poly (EGDMA-HEMA) microbeads, Talanta 46 (1998) 1205–1213. [25] H.A. Omar, Adsorption of 60Co on natural and dithizone-modified chitin, Radiochemistry 55 (2013) 101–107. [26] M.E. Mahmoud, M.M. Osman, M.E. Amer, Selective pre-concentration and solid phase extraction of mercury(II) from natural water by silica gel-loaded dithizone phases, Anal. Chim. Acta 415 (2000) 33–40. [27] S.S.H. Davarani, N. Sheijooni-Fumani, A.M. Najarian, M. Tabatabaei, S. Vahidi, Preconcentration of lead in sugar samples by solid phase extraction and its determination by flame atomic absorption spectrometry, Am. J. Anal. Chem. 2 (2011) 626–631. [28] Y.C. Shao, Y.J. Fang, T. Li, Q. Wang, Q.F. Dong, Y.H. Deng, Y.B. Yuan, H.T. Wei, M. Y. Wang, A. Gruverman, J. Shielda, J.S. Huang, Grain boundary dominated ion migration in polycrystalline organic-inorganic halide perovskite films, Energy Environ. Sci. 9 (2016) 1752–1759. [29] Y.F. Dong, Y.J. Fang, Y.C. Shao, P. Mulligan, J. Qiu, L. Cao, J.S. Huang, Electronhole diffusion lengths >175 μm in solution-grown CH3NH3PbI3 single crystals, Science 347 (2015) 967–969. [30] H. Lu, W. Tian, B. Gu, Y. Zhu, L. Li, TiO2 electron transport bilayer for highly efficient planar perovskite solar cell, Small 13 (2017) 1701535. [31] H.S. Kim, J.W. Lee, N. Yantara, P.P. Boix, S.A. Kulkarni, S. Mhaisalkar, M. Gr€ atzel, N.G. Park, High efficiency solid-state sensitized solar cell-based on submicrometer rutile TiO2 nanorod and CH3NH3PbI3 perovskite sensitizer, Nano Lett. 3 (2013) 2412–2417. [32] Y.Y. Zhu, K.M. Deng, H.X. Sun, B.K. Gu, H. Lu, F.R. Cao, J. Xiong, L. Li, TiO2 phase junction electron transport layer boosts efficiency of planar perovskite solar cells, Adv. Sci. 5 (2018) 1700614. [33] D.Y. Liu, M.K. Gangishetty, T.L. Kelly, Effect of CH3NH3PbI3 thickness on device efficiency in planar heterojunction perovskite solar cells, J. Mater. Chem. 2 (2014) 19873. [34] M.D. Xiao, F.Z. Huang, W.C. Huang, Y. Dkhissi, Y. Zhu, J. Etheridge, A. GrayWeale, U. Bach, Y.B. Cheng, L. Spiccia, A fast deposition-crystallization procedure for highly efficient lead Iodide perovskite thin-film solar cells Angew, Chem. Int. Ed. 53 (2014) 9898–9903. [35] S. Bai, P.M. Da, C. Li, Z.P. Wang, Z.C. Yuan, F. Fu, M. Kawecki, X.J. Liu, N. Sakai, J. T.W. Wang, S. Huettner, S. Buecheler, M. Fahlman, F. Gao, H.J. Snaith, Planar perovskite solar cells with long-term stability using ionic liquid additives, Nature 571 (2019) 245–250.

There are no conflicts to declare. Acknowledgements This work was supported by the Chinese Academy of Sciences. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.solmat.2019.110290. References [1] K.T. Akihiro Kojima, Yasuo Shirai, Tsutomu Miyasaka, Organometal halide perovskites as visible-light sensitizers for photovoltaic cells, J. Am. Chem. Soc. 131 (2009) 6050–6051. [2] N.G. Park, Perovskite solar cells: an emerging photovoltaic technology, Mater. Today 18 (2015) 65–72. [3] M. Kim, G.H. Kim, T.K. Lee, I.W. Choi, H.W. Choi, Y. Jo, Y.J. Yoon, J.W. Kim, J. Lee, D. Huh, H. Lee, S.K. Kwak, J.Y. Kim, D.S. Kim, Methylammonium Chloride Induces Intermediate Phase Stabilization for Efficient Perovskite Solar cells,Joule, 2019, https://doi.org/10.1016/j.joule.2019.06.014. [4] K.H. Kim, C.S. Park, J.D. Lee, J.Y. Lim, J.M. Yeon, I.H. Kim, E.J. Lee, Y.H. Cho, Record high efficiency of screen-printed silicon aluminum back surface field solar cell: 20.29%, Jpn. J. Appl. Phys. 56 (2017), 08MB25. [5] J. Burschka, N. Pellet, S.J. Moon, R. Humphry-Baker, P. Gao, M.K. Nazeeruddin, M. Gratzel, Sequential deposition as a route to high-performance perovskitesensitized solar cells, Nature 499 (2013) 316–319. [6] J.T. Zhang, G.M. Zhai, W.H. Gao, C.F. Zhang, Z.M. Shao, F.H. Mei, J.B. Zhang, Y. Z. Yang, X.G. Liu, B.S. Xu, Accelerated formation and improved performance of CH3NH3PbI3-based perovskite solar cells via solvent coordination and anti-solvent extraction, J. Mater. Chem. 5 (2017) 4190–4198. [7] Y. Zhao, K. Zhu, Organic-inorganic hybrid lead halide perovskites for optoelectronic and electronic applications, Chem. Soc. Rev. 45 (2016) 655–689. [8] H. Back, J. Kim, G. Kim, T.K. Kim, H. Kang, J. Kong, S.H. Lee, K. Lee, Interfacial modification of hole transport layers for efficient large-area perovskite solar cells achieved via blade-coating, Sol. Energy Mater. Sol. Cells 144 (2016) 309–315. [9] D.B. Yang, T. Sano, Y. Yaguchi, H. Sun, H. Sasabe, J. Kido, Achieving 20% efficiency for low-temperature-processed inverted perovskite solar cells, Adv. Funct. Mater. 29 (2018) 1807556. [10] Q. Jiang, Z. Chu, P.Y. Wang, X.L. Yang, H. Liu, Y. Wang, Z.G. Yin, J.L. Wu, X. W. Zhang, J.B. You, Planar-structure perovskite solar cells with efficiency beyond 21%, Adv. Mater. 29 (2017) 1703852. [11] W.Q. Wu, X.D. Wang, X. Han, Z. Yang, G.Y. Gao, Y.F. Zhang, J.F. Hu, Y.W. Tan, A. L. Pan, C.F. Pan, Flexible photodetector arrays based on patterned CH3NH3PbI3xClx perovskite film for real-time photosensing and imaging, Adv. Mater. 31 (2019) 1805913. [12] J. Jang, G. Choe, S. Yim, Effective control of chlorine contents in MAPbI(3- x)Clx perovskite solar cells using a single-source vapor deposition and anion-exchange technique, ACS Appl. Mater. Interfaces 11 (2019) 20073–20081. [13] W.Q. Wu, Q. Wang, Y.J. Fang, Y.C. Shao, S. Tang, Y.H. Deng, H.D. Lu, Y. Liu, T. Li, Z.B. Yang, A. Gruverman, J.S. Huang, Molecular doping enabled scalable blading of efficient hole-transport-layer-free perovskite solar cells, Nat. Commun. 9 (2018) 1625. [14] H. Chen, F. Ye, W.T. Tang, J.J. He, M.S. Yin, Y.B. Wang, F.X. Xie, E. Bi, X.D. Yang, M. Grӓtzel, L.Y. Han, A solvent- and vacuum-free route to large-area perovskite films for efficient solar modules, Nature 550 (2017) 92–95. [15] H.S. Kim, J.Y. Seo, S. Akin, E. Simon, M. Fleischer, S.M. Zakeeruddin, M. Grӓtzel, A. Hagfeldt, Power output stabilizing feature in perovskite solar cells at operating condition: selective contact-dependent charge recombination dynamics, Nano Energy 61 (2019) 126–131.

6