The Cesium doping using the nonstoichiometric precursor for improved CH3NH3PbI3 perovskite films and solar cells in ambient air

Thin Solid Films 690 (2019) 137563

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The Cesium doping using the nonstoichiometric precursor for improved CH3NH3PbI3 perovskite films and solar cells in ambient air

T

Ruiguang Changa, Jingyan Zhanga, Saad Ullaha, Zhili Zhub, Yongsheng Chena, Haizhong Guoa, ⁎ Jinhua Gua, a b

School of Physical Engineering and Material Physics Laboratory, Zheng Zhou University, Zhengzhou 450052, China School of Physical Engineering and International Laboratory for Quantum Functional Materials of Henan, Zheng Zhou University, Zhengzhou 450001, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Organic-inorganic hybrid perovskite solar cells Cesium doping Nonstoichiometric solution Stability

Organic-inorganic hybrid perovskite solar cells have shown great prospect as a low-cost and high efficiency photovoltaic technology. The quality of the perovskite absorber layer is most critical to the performance of the device, and the stability remains one of the challenging issues. In this paper, the Cesium (Cs) doping perovskite film was prepared from nonstoichiometric precursor solution in ambient and humidity-controlled conditions. The results showed that the crystallinity, uniformity, absorption, Photo-luminescence intensity and the thermal stability of these films can be effectively improved compared with the films fabricated from stoichiometric one, which is attributed to the combined effect of the Cs doping and excess methylammonium cations passivation. Finally, the highest efficiency of the perovskite solar cells fabricated from nonstoichiometric solution reached to 14.1%, which is 12.8% higher than that of the control device from stoichiometric solution (12.5%). Furthermore, the device displayed the high stability and efficiency degradation of only 2% occurring over a period of 5 weeks in ambient air without encapsulation. This reported work provides a pathway for further improving the performance of perovskite solar cells with higher stability.

1. Introduction Organic-inorganic hybrid perovskite (ABX3) solar cells have drawn more and more attention in recent years with rapid progress in power conversion efficiencies from 3.8% in 2009 [1] to the present record of 23.3% [2]. Organic-inorganic hybrid perovskite has an ABX3 structure, where A is a monovalent cation such as (MA), and formamidinium and Cs etc. [3–5], B is metal ion (Pb2+, Sn2+) [6] and X is a halogen atom, [7–11]. Among ABX3 materials, methylammonium lead halide perovskites (MAPbI3) are regarded as promising light absorbers due to their excellent properties such as high light absorption [6], long charge carrier diffusion lengths [12] and optimal band gap. However, the high efficiency perovskite solar cells (PSC) depend on the high quality perovskite absorbing layer. Therefore, improving the quality of perovskite absorbing layer remains one of the vital issues. Variations of methods have been developed to improve the quality of perovskite absorbing layer, which can be divided into two major types. One type is inducing the nucleation and growing process for smother and large grain perovskite films with high surface coverage, such as: mixed solvent [13,14], some additives (hypophosphorous acid

⁎

(H3PO2) [15,16], 1,8-diiodooctane (DIO) [17], hydroiodic acid (HI) [18,19], ammonium iodide (NH4I) [20] etc.), and doping (sodium (Na+) [21,22], potassium (K+) [21,22], rubidium (Rb+) [23], Cesium (Cs+), [24–28] etc.). Another is the defect passivation at grain boundaries (GBs) which are proved to be effective recombination centers and have negative effect on device performance [29,30]. It can be obtained by post-treatment methods [31–33], or in situ formation using nonstoichiometric precursor solution [34]. Son et al. [33] have demonstrated that in situ formation of the remnant methylammonium iodide (MAI) could effectively passivate the defects at GBs in MAPbI3 using nonstoichiometric (excess MAI) precursor solution. Among variations of methods, the Cs doping into perovskite particularly drew lots of interest recently due to the Cs-based perovskite exhibiting high thermal stability [35]. It has been suggested that Cs doping could improve the quality and thermal stability of perovskites simultaneously [36]. However, in the reported work, the Cs-doped MAPbI3 was usually prepared from the stoichiometric precursor solution, i.e. the molar ration of (MAI + CsI)/PbI2 = 1:1. In this study, we prepared the Cs-doped MAPbI3 using nonstoichiometric precursor solution, i.e. the molar ration of (MAI + CsI)/PbI2 = (1 + x):1, in which

Corresponding author. E-mail address: [email protected] (J. Gu).

https://doi.org/10.1016/j.tsf.2019.137563 Received 4 December 2018; Received in revised form 10 September 2019; Accepted 10 September 2019 Available online 11 September 2019 0040-6090/ © 2019 Elsevier B.V. All rights reserved.

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A cations were in excess. This composition was expected to have combined effect of Cs-doping and the remnant MAI passivation for further improving the quality of MAPbI3 films. The results suggested that the crystallinity, quality and the thermal stability of those films could be effectively improved compared with films fabricated from stoichiometric one. The efficiency of PSC fabricated was increased by 12.8% compared to the control devices from stoichiometric solution. 2. Experimental details 2.1. Perovskite film preparation

Fig. 1. The schematic structure of perovskite solar cell.

Perovskite films were prepared using one-step method in ambient air with humidity below 25%. For the MAPbI3 films, the precursor solution contained equimolar amounts of MAI and PbI2 in a mixture of anhydrous dimethylformamide (DMF): dimethylsulfoxide (DMSO) (7:3, v/v). For the Cs-doped MAPbI3, the stoichiometric precursor solution contained the equimolar amounts of MAI:CsI and PbI2 in the mixed solvent of DMSO and DMF, while the nonstoichiometric stoichiometric one containing equimolar amounts of MAI and PbI2, and small amounts of CsI additive. The concentration of PbI2 was set as 1.25 M for all the solutions. CsI content in precursors x was the molar ration of CsI/PbI2. Solutions were stirred with 550 rpm at 65 °C for 12 h. The precursor solutions were spin coated on the substrates with 500 rpm for 5 s then spin rate accelerated up to 3500 rpm for 25 s. Chlorobenzene was continuously dripped 17 s prior to the end of the program. Then the films were dried on hot plate at 45 °C, 65 °C and 100 °C for 5 min, 5 min and 10 min respectively.

FlouroMax-4 spectrophotometer with an excitation wavelength of 467 nm. Scanning electron microscope (SEM) images were measured by JSM-6700F (JEOL) under operation voltage of 6.0 kV. The current density-voltage (J-V) curves of the solar cells were measured using a 2400 source meter (Keithley) under one sun illumination (AM 1.5 G, 100 mW/cm2). 3. Results Firstly, the perovskite films with different CsI content from the nonstoichiometric precursor solution were prepared on the FTO glass substrates. Fig. 2a-d presents the SEM images of perovskite films with different CsI content. It can be clearly seen from Fig. 2b that perovskite films have grown with larger grain and no visible pinholes for the 2% CsI. Fig. 2c and Fig. 2d show that as the CsI content increases, the surface of films become rougher and pinholes started to appear. It is noteworthy that our perovskite films prepared in ambient air with controlled humidity below 25% exhibit a larger grain size than that of prepared in nitrogen glove box. The grain size of reported perovskites prepared in nitrogen glove box is usually less than 300 nm [23,28], while the grain size exceeds 500 nm in our sample (Fig. 2b). This might suggest that a little water is beneficial for the perovskite to grow into larger grains. So, the controlled humidity may be an effective method to fabricate the perovskite films with large-sized grains. Fig. 3a shows the absorbance spectra of perovskite films with different CsI content. Owing to the larger optical band gap of CsPbI3 (1.73 eV) than MAPbI3 (1.55 eV) [37,38], the band gap of CsxMA1-xPbI3 perovskite gradually increases with increasing the Cs content [39]. The spectra in Fig. 3a show a blue-shift when CsI is added into the solution, which indicates the band gap increase and proves that Cs is incorporated into the MA matrix. XRD patterns are shown in Fig. 3b, which exhibits the pure perovskite phase with characteristic peaks at ≈ 28° for all four samples. The peak intensity of (220) plane is seen to be increasing with the addition of 2% CsI while it starts to decrease with further increasing the CsI content, which suggests that the crystallinity of perovskite improves when small amount of CsI is added and the crystallinity of perovskite decreases for higher CsI content. Considering the SEM images and the crystallinity of perovskite, the optimized CsI content is 2% in our sample. It was suggested that the excess MAI in precursor solution is able to passivate GBS in MAPbI3 [33]. Similarly, it is predicted that the excess A cations in the nonstoichiometric precursor solution might also be able to passivate GBS in Cs doping perovskite films. To confirm our prediction, Fig. 4 compares SEM images of perovskite films prepared with 2% CsI doping from the stoichiometric and nonstoichiometric precursor solution. It can be seen from Fig. 4 that the grains of two Cs doping perovskites are larger than that of perovskite without Cs doping, and the Cs doping perovskites from the nonstoichiometric solution have larger grains than that from the stoichiometric one. Fig. 5 displays the XRD, absorbance and PL spectra of undoped, CsI doped perovskites samples from stoichiometric solution and nonstoichiometric one labeled as A, B and C respectively. From the XRD, it can be seen that the intensity of (110) plane is significantly enhanced

2.2. Solar cell fabrication In the first step, the fluorine-doped tin oxide (FTO) glass was washed using glass cleaner, and then cleaned ultrasonically for 20 min in deionized water, acetone, alcohol and isopropanol orderly. Finally, the cleaned FTO substrates were treated with Ultraviolet for 10 min. The compact titanium dioxide (TiO2) layer was synthesized using chemical bath deposition. The 2.2 ml solution of cooled TiCl4 (99.9%) was added dropwise to 100 ml of ice-water. To maintain the homogeneity, the solution was then stirred for 30 min. The FTO substrates were immersed into the precursor solution and heated at 70 °C for 50 min in an oven. The resulting samples were taken out and washed with ethanol and water before annealing it at 500 °C for 1 h. After cooling down to the room temperature, the perovskite films were prepared onto compact TiO2 layer. The 2,2′,7,7′-tetrakis(N,N-di-4-methoxyphenylamino)-9,9-spirobifluorene (spiroOMeTAD) solution was prepared by dissolving 72.5 mg spiroOMeTAD powder in 1 ml chlorobenzene, then 17.5 μl of a lithium salt solution (520 mg/ml in acetonitrile) and 28.5 μl 4-tert-butylpyridine were added successively. spiroOMeTAD was spin coated onto perovskite layer at 3000 rpm for 30 s. Then the samples were kept for three hours in air. Finally, silver electrodes were deposited by thermally evaporated on top of the spiroOMeTAD layer. The cell effective area was 0.06 cm2. The structure of the planar PSC was FTO/c-TiO2 layer/Perovskite/ Spiro-OMeTAD/Ag, as illustrated in Fig. 1. The thicknesses of TiO2, Perovskite, Spiro-OMeTAD and Ag are about 80 nm, 500 nm, 250 nm and 100 nm, respectively. 2.3. Characterization X-ray diffraction (XRD) patterns of the perovskite films were measured using X-ray diffractometer (Rigaku D/Max-2400) with a Braggbentano θ:2θ configuration, using Cu Kα beam (λ = 1.54 Å) under operation condition of 40 kV and 30 mA. UV–Vis absorption spectra were obtained using Shimadzu UV-3150 spectrophotometer and steadystate Photo-luminescence (PL) spectra were carried out using 2

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Fig. 2. Top view Scanning electron microscope (SEM) images of perovskite films with different CsI content (a) x = 0%, (b) x = 2%, (c) x = 5% and (d) x = 10%.

Fig. 3. (a) Ultraviolet-visible (UV–vis) absorbance spectra and (b) X-ray diffraction (XRD) patterns of perovskite films with different CsI content, and FTO peaks are marked with *.

Fig. 4. Top view SEM images of perovskite films (a) without CsI doping and with (b) 2% CsI doping from stoichiometric precursor solution, and (c) 2% CsI doping from nonstoichiometric one.

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Fig. 5. (a) XRD patterns, (b) The enlarge comparison of (110) diffraction peak, (c) UV–vis absorbance spectra and (d) Photoluminescence spectra of perovskite without CsI doping (A), with 2% CsI doping from stoichiometric precursor solution (B), and 2% CsI doping from nonstoichiometric one (C).

increased absorption from the visible to the near-infrared, which is due to their higher crystallinity. The absorption below 500 nm of Cs doping films from the nonstoichiometric solution is greater than that from stoichiometric one, which may be attributed to improvement in uniformity of grains. The PL intensity of the films from the nonstoichiometric solution is higher than the other two samples, implying a relatively lower non-radiative recombination loss and lower defect states in films. The decrease in defect states may result from the excess A cations passivation effect at GBs where there are more defect states. In short, this suggests that the excess A cations produce the high quality Cs doping perovskite with fewer defects and higher crystallinity. Furthermore, we investigate the film deposition process and thermal stability of sample A, B and C, as shown in Fig. 6. It can be seen that sample C has turned yellow totally (formed yellow phase), B began to turn yellow and A remained transparent after spin coating at 25 °C. When the heat temperature was increased to 45 °C, sample C has turned black totally (formed black perovskite phase), B began to turn black and A turned yellow. Sample A and B turned black totally till 65 °C. It can be seen that sample C crystallized more quickly and formed the perovskite phase at lower temperature compared with sample A and B. So, we can conclude that that adding CsI in nonstoichiometric precursor solution benefits film crystallization and perovskite phase formation. Sample A started bleaching when the annealing temperature increase to 180 °C and turned yellow at 200 °C. And sample B started bleaching at 200 °C. However, sample C remained black and didn't bleaching noticeably. From the above results we can confirm that the thermal stability of sample C is improved. Good thermal stability of sample C may be attributed to the perovskite phase formation at lower temperature. Fig. 7a-c shows the photocurrent density-voltage (J-V) curves of PSC prepared without CsI doping, with 2% CsI doping from stoichiometric solution and nonstoichiometric one. The photovoltaic parameters are listed in Table 1. Improvement in the performance of both Cs doped PSC was observed. The device prepared with CsI doping from nonstoichiometric solution achieved the highest efficiency (14.1%), with a short-circuit current density (JSC) of 20.62 mA/cm2, an open-circuit

Fig. 6. The images of sample A, B and C during deposition (25 °C, 45 °C and 65 °C) and thermal treatment (180 °C and 200 °C) in ambient air, during deposition the films were dried on hot plate at 25 °C, 45 °C, 65 °C and 100 °C for 5 min, 5 min, 5 min and 10 min, respectively, then the films are thermal treated at 120 °C,150 °C, 180 °C and 200 °C for 10 min, respectively.

when CsI is added. And the peak value is maximal for Cs doping films from the nonstoichiometric solution. We calculated the full width half maxima (FWHM) and found that the FWHM is smallest (0.1335°) for Sample C, which indicates the crystallinity is highest for film from the nonstoichiometric solution. Both Cs doped perovskite films exhibit 4

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Fig. 7. The photocurrent density-voltage curves with both forward and reverse scans for the champion perovskite solar cells prepared (a) without CsI doping, (b) with 2% CsI doping from stoichiometric solution and (c) nonstoichiometric one. (d) Durability of the device C, the device is exposed for 5 weeks in ambient air without encapsulation.

where rA, rB and rX are the radii of the A, B and X ions for ABX3 material. The stable perovskite is with a t value between 0.8 and 1.0, while the calculated t value for MAPbI3 is 1.01 out of a t value rang that is favorable for the perovskite structure. For the perovskite with Cs doping, the t value can be decreased when part of MA+ is replaced by a smaller Cs+ cation, resulting in higher stability of perovskite [41]. This mechanism is also supported by the previous report by Choi et al. [25].

Table 1 Photovoltaic performance parameters of solar cells with different Cs content. Device

Voc(v)

Jsc(mA/cm2)

FF(%)

Efficiency (%)

A

0.99 (0.97 ± 0.01) 1.02 (0.98 ± 0.03) 0.99 (0.99 ± 0.02)

20.69 (18.03 ± 2.27) 19.34 (18.06 ± 2.77) 20.62 (19.10 ± 1.69)

53.46 (54.72 ± 4.82) 63.32 (60.76 ± 2.91) 69.40 (66.70 ± 3.43)

10.9a (9.7 ± 0.9)b 12.5a (11.5 ± 1.4)b 14.1a (12.7 ± 0.8)b

B C

a b

4. Conclusions

Best solar cell performance. Average performance from 9 devices.

In summary, we have demonstrated that the quality and the thermal stability of perovskite films fabricated from nonstoichiometric precursor solution with CsI doping in ambient air and humidity-controlled condition can be effectively improved. The crystallinity, uniformity, absorption and PL intensity of the films fabricated from nonstoichiometric solution with CsI doping were greater than that of the films prepared from stoichiometric one with CsI doping. From the film formation process, it was observed that the excess A cations in nonstoichiometric solution promote the crystallization and the perovskite phase transformation and, thereby, improve their crystallinity and stability. The increase of PL intensity may be attributed to the excess A passivating the defects at GBs in perovskite. The efficiency of solar cell was enhanced to 14.1% from 10.9% when 2% CsI was added in nonstoichiometric solution. And the device displayed the high stability as only 2% degradation occurred over a period of 5 weeks in ambient air without encapsulation. This reported method using nonstoichiometric solution with CsI doping can further enhance the performance and the stability of PSC.

voltage (Voc) of 0.99 V and a fill factor (FF) of 69.4%. In comparison with the device prepared with CsI doping from stoichiometric solution, FF and Jsc of the cell prepared with CsI doping from nonstoichiometric one are higher. This is ascribed to the higher crystallinity and lower defect states in films. The stability of PSC is an important issue affecting their potential application. So, we investigated the stability of our perovskite device prepared with CsI doping from nonstoichiometric solution in ambient air. The devices were stored in ambient air with humidity below 25% without encapsulation for 5 weeks, and then their performance was measured under ambient conditions. Fig. 7d presents the change in J-V curves of PSC prepared with CsI doping from nonstoichiometric solution after 5 weeks. Photovoltaic performance of the device had no obvious change and the efficiency retained 13.8%, while the Voc increased a little after 5 weeks. However, Voc didn't change for the devices prepared without CsI doping and with CsI doping from stoichiometric solution. The increase in Voc for the device C may be linked to the excess A cations in nonstoichiometric solution. Some of the remnant A cations at GBs in perovskite probably migrate to the interface of the device over a period of time and passivate the interface. The high stability of PSC with Cs doping may be related to the change of Goldschmidt tolerance factor (t). The structural stability of perovskite can be empirically predicted by the Goldsmith tolerance factor [40]:

t=

rA + rX 2 (rB + rX )

Acknowledgements This work is financially supported by Natural Science Foundation of Henan Province (162300410254)，and National Natural Science Foundation of China (61574129). References [1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal halide perovskites as visible-light sensitizers for photovoltaic cells, J. Am. Chem. Soc. 131 (2009)

(1) 5

Thin Solid Films 690 (2019) 137563

R. Chang, et al.

10.1002/advs.201700131. [22] K.M. Boopathi, R. Mohan, T.-Y. Huang, W. Budiawan, M.-Y. Lin, C.-H. Lee, K.-C. Ho, C.-W. Chu, Synergistic improvements in stability and performance of lead iodide perovskite solar cells incorporating salt additives, J. Mater. Chem. A 4 (2016) 1591–1597, https://doi.org/10.1039/C5TA10288J. [23] M. Saliba, T. Matsui, K. Domanski, J.-Y. Seo, A. Ummadisingu, S.M. Zakeeruddin, J.P. Correa-Baena, W.R. Tress, A. Abate, A. Hagfeldt, M. Grätzel, Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance, Science 354 (2016) 206–209 http://science.sciencemag.org/content/354/6309/ 206. [24] G. Niu, H. Yu, J. Li, D. Wang, L. Wang, Controlled orientation of perovskite films through mixed cations toward high performance perovskite solar cells, Nano Energy 27 (2016) 87–94, https://doi.org/10.1016/j.nanoen.2016.06.053. [25] H. Choi, J. Jeong, H.-B. Kim, S. Kim, B. Walker, G.-H. Kim, J.Y. Kim, Cesium-doped methylammonium lead iodide perovskite light absorber for hybrid solar cells, Nano Energy 7 (2014) 80–85, https://doi.org/10.1016/j.nanoen.2014.04.017. [26] Y. Fan, H. Qin, W. Ye, M. Liu, F. Huang, D. Zhong, Improving the stability of methylammonium lead iodide perovskite solar cells by cesium doping, Thin Solid Films 667 (2018) 40–47, https://doi.org/10.1016/j.tsf.2018.10.001. [27] M. Saliba, T. Matsui, J.-Y. Seo, K. Domanski, J.-P. Correa-Baena, M.K. Nazeeruddin, S.M. Zakeeruddin, W. Tress, A. Abate, A. Hagfeldt, M. Grätzel, Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency, Energy Environ. Sci. 9 (2016) 1989–1997, https://doi.org/10.1039/ C5EE03874J. [28] T. Singh, T. Miyasaka, Stabilizing the efficiency beyond 20% with a mixed cation perovskite solar cell fabricated in ambient air under controlled humidity, Adv. Energy Mater. 8 (2017) 1700677, , https://doi.org/10.1002/aenm.201700677. [29] T. Gershon, B. Shin, N. Bojarczuk, M. Hopstaken, D.B. Mitzi, S. Guha, The role of sodium as a surfactant and suppressor of non-radiative recombination at internal surfaces in Cu2ZnSnS4, Adv. Energy Mater. 5 (2014) 1400849, , https://doi.org/10. 1002/aenm.201400849. [30] Z. Tong, C. Yan, Z. Su, F. Zeng, J. Yang, Y. Li, L. Jiang, Y. Lai, F. Liu, Effects of potassium doping on solution processed kesterite Cu2ZnSnS4 thin film solar cells, Appl. Phys. Lett. 105 (2014) 223903, , https://doi.org/10.1063/1.4903500. [31] S. Bag, M.F. Durstock, Large perovskite grain growth in low-temperature solutionprocessed planar p-i-n solar cells by sodium addition, ACS Appl. Mater. Interfaces 8 (2016) 5053–5057, https://doi.org/10.1021/acsami.5b11494. [32] S.M. Jain, Z. Qiu, L. Häggman, M. Mirmohades, M.B. Johansson, T. Edvinsson, G. Boschloo, Frustrated Lewis pair-mediated recrystallization of CH3NH3PbI3 for improved optoelectronic quality and high voltage planar perovskite solar cells, Energy Environ. Sci. 9 (2016) 3770–3782, https://doi.org/10.1039/C6EE02544G. [33] B. Wang, Z. Zhang, S. Ye, H. Rao, Z. Bian, C. Huang, Y. Li, Room-temperature watervapor annealing for high-performance planar perovskite solar cells, J. Mater. Chem. A 4 (2016) 17267–17273, https://doi.org/10.1039/C6TA08655A. [34] D.-Y. Son, J.-W. Lee, Y.J. Choi, I.-H. Jang, S. Lee, P.J. Yoo, H. Shin, N. Ahn, M. Choi, D. Kim, N.-G. Park, Self-formed grain boundary healing layer for highly efficient CH3NH3PbI3 perovskite solar cells, Nat. Energy 1 (2016) 16081https://www. nature.com/articles/nenergy201681. [35] M. Kulbak, D. Cahen, G. Hodes, How important is the organic part of lead halide perovskite photovoltaic cells? Efficient CsPbBr3 cells, J. Phys. Chem. Lett. 6 (2015) 2452–2456, https://doi.org/10.1021/acs.jpclett.5b00968. [36] G. Niu, W. Li, J. Li, X. Li, L. Wang, Enhancement of thermal stability for perovskite solar cells through cesium doping, RSC Adv. 7 (2017) 17473–17479, https://doi. org/10.1039/C6RA28501E. [37] R. Marina, E. Giles Filip, J.H. Eperon, H.J. Snaith, F. Giustino, Steric engineering of metal-halide perovskites with tunable optical band gaps, Nat. Commun. 5 (2014) 5757 https://www.nature.com/articles/ncomms6757. [38] G.E. Eperon, S.D. Stranks, C. Menelaou, M.B. Johnston, L.M. Herz, H.J. Snaith, Formamidinium lead trihalide: a broadly tunable perovskite for efficient planar heterojunction solar cells, Energy Environ. Sci. 7 (2014) 982–988 https://pubs.rsc. org/en/content/articlelanding/2014/ee/c3ee43822h#!divAbstract. [39] R.G. Niemann, L. Gouda, J. Hu, S. Tirosh, R. Gottesman, P.J. Camerona, A. Zaban, Cs+ incorporation into CH3NH3PbI3 perovskite: substitution limit and stability enhancement, J. Mater. Chem. A 4 (2016) 17819–17827 https://pubs.rsc.org/en/ content/articlelanding/2016/ta/c6ta05869h#!divAbstract. [40] X. Zhang, X. Ren, B. Liu, R. Munir, X. Zhu, D. Yang, J. Li, Y. Liu, D.-M. Smilgies, R. Li, Z. Yang, S. Liu, Stable high efficiency two-dimensional perovskite solar cell via cesium doping, Energy Environ. Sci. 10 (2017) 2095–2102 https://pubs.rsc.org/ en/content/articlepdf/2017/ee/c7ee01145h. [41] R. Sedighi, F. Tajabadi, N. Taghavinia, Vapor assisted deposition of alkaline doped perovskites: pure phase formation of CsxMA1-xPbI3, Electrochim. Acta 259 (2018) 485–491, https://doi.org/10.1016/j.electacta.2017.10.185.

6050–6051, https://doi.org/10.1021/ja809598r. [2] N.J. Jeon, H. Na, E.H. Jung, T.-Y. Yang, Y.G. Lee, G. Kim, H.-W. Shin, S.I. Seok, J. Lee, J. Seo, A fluorene-terminated hole-transporting material for highly efficient and stable perovskite solar cells, Nat. Energy 3 (2018) 682–689 https://www. nature.com/articles/s41560-018-0200-6. [3] T.M. Koh, K. Fu, Y. Fang, S. Chen, T.C. Sum, N. Mathews, S.G. Mhaisalkar, P.P. Boix, T. Baikie, Formamidinium-containing metal-halide: an alternative material for near-IR absorption perovskite solar cells, J. Phys. Chem. C 118 (2014) 16458–16462, https://doi.org/10.1021/jp411112k. [4] M.M. Lee, J. Teuscher, T. Miyasaka, T.N. Murakami, H.J. Snaith, Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites, Science 338 (2012) 643–647 http://science.sciencemag.org/content/early/2012/10/03/ science.1228604. [5] H.-S. Kim, C.-R. Lee, J.H. Im, K.-B. Lee, T. Moehl, A. Marchioro, S.-J. Moon, R. Humphry-Baker, J.-H. Yum, J.E. Moser, M. Grätzel, N.G. Park, Lead iodide perovskite sensitized all-solid-state submicron thin film mesoscopic solar cell with efficiency exceeding 9%, Sci. Rep. 2 (2012) 591–598 https://www.nature.com/ articles/srep00591?page=4. [6] F. Hao, C.C. Stoumpos, D.H. Cao, R.P.H. Chang, M.G. Kanatzidis, Lead-free solidstate organic–inorganic halide perovskite solar cells, Nat. Photonics 8 (2014) 489–494 https://www.nature.com/articles/nphoton.2014.82. [7] C.H. Hendon, R.X. Yang, L.A. Burton, A. Walsh, Assessment of polyanion (BF4− and PF6−) substitutions in hybrid halide perovskites, J. Mater. Chem. A 3 (2015) 9067–9070, https://doi.org/10.1039/C4TA05284F. [8] J.H. Heo, D.H. Song, S.H. Im, Planar CH3NH3PbBr3 hybrid solar cells with 10.4% power conversion efficiency, fabricated by controlled crystallization in the spincoating process, Adv. Mater. 26 (2014) 8179–8183, https://doi.org/10.1002/adma. 201403140. [9] Q. Jiang, D. Rebollar, J. Gong, E.L. Piacentino, C. Zheng, T. Xu, Pseudohalide-induced moisture tolerance in perovskite CH3NH3Pb(SCN)2I thin films, Angew. Chem. 54 (2015) 7617–7620, https://doi.org/10.1002/ange.201503038. [10] S. Nagane, U. Bansode, O. Game, S. Chhatre, S. Ogale, CH3NH3PbI(3-x)(BF4)x: molecular ion substituted hybrid perovskite, Chem. Commun. 50 (2014) 9741–9744, https://doi.org/10.1039/C4CC04537H. [11] P. Prajongtat, T. Dittrich, Precipitation of CH3NH3PbCl3 in CH3NH3PbI3 and its impact on modulated charge separation, J. Phys. Chem. C 119 (2015) 9926–9933, https://doi.org/10.1021/acs.jpcc.5b01667. [12] S.D. Stranks, G.E. Eperon, G. Grancini, C. Menelaou, M.J.P. Alcocer, T. Leijtens, L.M. Herz, A. Petrozza, H.J. Snaith, Electron-hole diffusion lengths exceeding 1 micrometer in an organometal trihalide perovskite absorber, Science 342 (2013) 341–344 http://science.sciencemag.org/content/342/6156/341. [13] S. Paek, N. Cho, H. Choi, H. Jeong, J.S. Lim, J.-Y. Hwang, J.K. Lee, J. Ko, Improved external quantum efficiency from solution-processed (CH3NH3)PbI3 perovskite/ PC71BM planar heterojunction for high efficiency hybrid solar cells, J. Phys. Chem. C 118 (2014) 25899–25905, https://doi.org/10.1021/jp508162p. [14] Z. Xiao, Q. Dong, C. Bi, Y. Shao, Y. Yuan, J. Huang, Solvent annealing of perovskiteinduced crystal growth for photovoltaic-device efficiency enhancement, Adv. Mater. 26 (2014) 6503–6509, https://doi.org/10.1002/adma.201401685. [15] W. Zhang, S. Pathak, N. Sakai, T. Stergiopoulos, P.K. Nayak, N.K. Noel, A.A. Haghighirad, V.M. Burlakov, D.W. deQuilettes, A. Sadhanala, W. Li, L. Wang, D.S. Ginger, R.H. Friend, H.J. Snaith, Enhanced optoelectronic quality of perovskite thin films with hypophosphorous acid for planar heterojunction solar cells, Nat. Commun. 6 (2015) 10030https://www.nature.com/articles/ncomms10030. [16] Z. Xiao, D. Wang, Q. Dong, Q. Wang, W. Wei, J. Dai, X. Zeng, J. Huang, Unraveling the hidden function of a stabilizer in a precursor in improving hybrid perovskite film morphology for high efficiency solar cells, Energy Environ. Sci. 9 (2016) 867–872, https://doi.org/10.1039/C6EE00183A. [17] Z. Liu, E.-C. Lee, Solvent engineering of the electron transport layer using 1,8diiodooctane for improving the performance of perovskite solar cells, Org. Electron. 24 (2015) 101–105, https://doi.org/10.1016/j.orgel.2015.05.027. [18] J.H. Heo, D.H. Song, H.J. Han, S.Y. Kim, J.H. Kim, D. Kim, H.W. Shin, T.K. Ahn, C. Wolf, T.-W. Lee, S.H. Im, Planar CH3NH3PbI3 perovskite solar cells with constant 17.2% average power conversion efficiency irrespective of the scan rate, Adv. Mater. 27 (2015) 3424–3430, https://doi.org/10.1002/adma.201500048. [19] J.H. Heo, H.J. Han, D. Kim, T.K. Ahn, S.H. Im, Hysteresis-less inverted CH3NH3PbI3 planar perovskite hybrid solar cells with 18.1% power conversion efficiency, Energy Environ. Sci. 8 (2015) 1602–1608, https://doi.org/10.1039/C5EE00120J. [20] H. Si, Q. Liao, Z. Yang Ou, J. Meng, Y. Liu, Z. Zhang, Y. Zhang, Deciphering the NH4PbI3 intermediate phase for simultaneous improvement on nucleation and crystal growth of perovskite, Adv. Funct. Mater. 27 (2017) 1701804, , https://doi. org/10.1002/adfm.201701804. [21] W. Zhao, Z. Yao, F. Yu, D. Yang, S. Liu, Alkali metal doping for improved CH3NH3PbI3 perovskite solar cells, Adv. Sci. 5 (2017) 1700131, , https://doi.org/

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