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Preparation of planar CH3NH3PbI3 thin films with controlled size using 1-ethyl-2-pyrrolidone as solvent
Journal of Alloys and Compounds 671 (2016) 11e16
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Preparation of planar CH3NH3PbI3 thin films with controlled size using 1-ethyl-2-pyrrolidone as solvent Qiuyan Hao a, Yixia Chu a, Xuerong Zheng b, Zhenya Liu a, Liming Liang a, Jiakun Qi c, Xin Zhang a, Gang Liu d, Hui Liu a, *, Hongjian Chen a, Caichi Liu a, ** a
Engineering Laboratory of Functional Optoelectronic Crystalline Materials of Hebei Province, School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300132, PR China Key Laboratory of Advanced Ceramics and Machining Technology of Ministry of Education, School of Materials Science and Engineering, Tianjin University, Tianjin 300072, PR China c State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, PR China d School of Chemical Engineering, Hebei University of Technology, Tianjin 300132, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 December 2015 Received in revised form 7 February 2016 Accepted 8 February 2016 Available online 11 February 2016
Recently, planar perovskite solar cells based on CH3NH3PbI3 have attracted many researcher's interest due to their unique advantages such as simple cell architecture, easy fabrication and potential multijunction construction comparing to the initial mesoporous structure. However, the preparation of planar perovskite films with high quality is still in challenge. In this paper, we developed a vapor-assisted solution process using a novel and green solvent of 1-Ethyl-2-pyrrolidone (NEP) instead of the traditional N, N-dimethylformamide (DMF) to construct a high-quality perovskite CH3NH3PbI3 thin film with pure phase, high compactness, small surface roughness and controlled size. The phase evolution and growth mechanism of the perovskite films are also discussed. Utilizing the NEP of low volatility and moderate boiling point as solvent, we dried the PbI2-NEP precursor films at different temperature under vacuum and then obtained PbI2 thin films with different crystalline degree from amorphous to highly crystalline. The perovskite films with crystal size ranged from hundreds of nanometers to several micrometers can be prepared by reacting the PbI2 films of different crystalline degree with CH3NH3I vapor. Moreover, planar-structured solar cells combining the perovskite film with TiO2 and spiro-OMeTAD as the electron and holes transporting layer achieves a power conversion efficiency of 10.2%. © 2016 Elsevier B.V. All rights reserved.
Keywords: Perovskite 1-Ethyl-2-pyrrolidone Controlled size Planar structure
1. Introduction Organic-inorganic perovskite material such as CH3NH3PbI3 is one of the most competitive light-absorbing materials for thin film solar cells due to its low-cost, easy fabrication and unique optical and electrical properties such as appropriate and tunable direct bandgap, high absorption coefficient, efficient ambipolar transport and long carrier diffusion length [1e3]. The perovskite solar cell (PSC) has been reported to achieve relatively high power conversion efficiency (PCE) of 20.1% [4] since the first report of PSC with the PCE of 3.8% during the past 6 years [5]. Although the initial architecture of PSC device adopts mesoporous structure which
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H. Liu), [email protected] (C. Liu). http://dx.doi.org/10.1016/j.jallcom.2016.02.079 0925-8388/© 2016 Elsevier B.V. All rights reserved.
originates from the traditional dye-sensitized solar cells, the planar architecture PSC has also been confirmed soon [6,7]. PSC with planar structure has many unique advantages such as simple cell architecture, easy fabrication and potential multijunction construction et al., which attracts many researcher's interest. Nevertheless, the preparation of planar perovskite films with high quality is still in challenge. Vacuum evaporation process is successfully used to deposit the perfect planar perovskite films with pure phase, full surface coverage and uniform crystal size [8e10]. But this vapor deposition need high vacuum and leads to an abundant waste of raw materials which will increase the cost of large scale fabrication. Meanwhile, vapor technology will also produce toxic PbI2 vapor which is harmful to the humanity. Solution technology such as one-step [11,12] or two-step [13e15] sequential solution deposition process is also introduced to prepare the planar perovskite films. However, both the two solution technology have their disadvantages: a relatively low surface
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coverage and broaden size distribution for one-step solution deposition, and incomplete conversion of PbI2 and uncontrolled perovskite crystal sizes as well as surface morphology. Considering the benefits and drawbacks of the vapor and two-step sequential solution deposition technology, Yang's group developed a facile vapor assisted solution process (VASP) to fabricate planar CH3NH3PbI3 thin films of high quality with full surface coverage, small surface roughness, and grain size up to microscale, achieving the high PCE of 12.1% [16]. This simple method provides a way for high reproducibility of films and devices with high quality. However, efficient controlling the crystal size of pure perovskite films is still in challenge by using VASP. According to Han's report, retarding the crystallization of PbI2 using DMSO as a solvent to form amorphous structure can impact the growth of perovskite crystal grain and lead to high quality perovskite films with uniform particles size by a two-step sequential solution deposition process [17]. We assume that tuning the crystalline degree of PbI2 phase may impact the growth of perovskite films and efficiently control the perovskite crystal size for sequential deposition. Nevertheless, the commonly used DMF solvent has a high saturated vapor pressure and volatility which makes it easy to crystallize PbI2 after spincoating the PbI2-DMF solution, so it is not easy to control the crystalline degree of PbI2. Other commonly used solvents which can dissolve PbI2 such as gamma-butyrolactone (GBL) dimethylsulfoxide (DMSO) and 1-Methyl-2-pyrrolidinone (NMP) sometimes have high boiling point and low volatility, which makes it need high temperature to dry PbI2 precursor solution films, leading to control the crystallization of PbI2 hard. Therefore, finding a solvent of low volatility and moderate boiling point which can also dissolve PbI2 is necessary. 1-Ethyl-2-pyrrolidone (NEP) is an appropriate candidate with moderate boiling point (82e83 , 101.3 KPa) and low volatility which can efficiently dissolve PbI2 (>1 M) at room temperature. In addition, NEP is non-toxic for humanity which gives it more benefits comparing to the toxic DMF or DMSO for the future scalable production. To our best knowledge, no researches have been reported to use NEP to prepare perovskite thin film. In this paper, we developed a vapor-assisted solution process using a novel and green NEP as solvent instead of DMF to construct a high-quality perovskite CH3NH3PbI3 thin film with pure phase, high compactness, small surface roughness and controlled size. The phase transformation and growth mechanism of the perovskite films are also discussed. The PbI2 films with different crystalline can be obtained by drying the PbI2-NEP precursor films at different temperature under vacuum. And then the crystal size of the perovskite films ranged from hundreds of nanometers to several micrometers can be prepared by using the PbI2 films with different crystalline degree to react with the CH3NH3I vapor. Moreover, planarstructured solar cells combining this perovskite film with TiO2 and spiro-OMeTAD as the electron and holes elective transporting layer achieves the PCE of 10.2%. 2. Experimental section CH3NH3I Synthesis. CH3NH3I (MAI) was synthesized using the approach described in the literature [16]. Typically, 24 mL methylamine (33 wt% in absolute methanol, Aladdin, China) and 10 mL of hydroiodic acid (58 wt% in water, Aladdin, China) were in a 250 mL round-bottom flask at 0 C for 2 h with stirring. The precipitate was collected by evaporating the solvents on a rotary evaporator at 50 C. The product CH3NH3I was washed and precipitated with the addition of the absolute ethanol and diethyl ether for three times, respectively. And the solid was collected and dried at 60 C in a vacuum oven for 24 h. CH3NH3PbI3 Thin Film Fabrication. Glass substrates were
ultrasonic cleaning with deionized water, acetone and ethanol for 30 min, respectively, and dried with clean N2 for further using. The low-toxicity and low-volatility solvent of NEP was used to dissolve PbI2 powders. PbI2 thin films were prepared by dip-coating a 0.3 M PbI2-NEP solution on an ITO or FTO glass substrate, and vacuum drying at desired temperature for 20 min in a vacuum oven. CH3NH3I powder was spread out around the PbI2 coated substrates with a petridish covering on the top, and heated with N2 protection at 150 C for 0 h, 0.5 h, 1 h, 2 h and 4 h in an oven, respectively. After cooling down, the thin film was washed with isopropanol and dried in air. Solar Cell Device Fabrication. The FTO glass substrates were spin-coated with 0.3 M titanium diisopropoxide bis(acetylacetonate) (Sigma) at 3, 000 r.p.m for 30 s subsequently, following by sintering at 550 C for 20 min in air. The substrate was immersed in 50 mM TiCl4 (Sigma) aqueous solution for 30 min at 70 C and washed with distilled water and ethanol, followed by annealing at 550 C for 20 min in air to form a compacting TiO2 layer. The PbI2NEP solution was dip-coating on the compacting TiO2 layer, and vacuum dried at 70 C for 20 min in a vacuum oven. The PbI2 coated substrates were on the top of CH3NH3I powder in a glass culture dish, and heated at 150 C for desired time in a vacuum oven filled with N2 atmosphere. The as-prepared substrates were washed with isopropanol and dried in air, and then deposited by spin coating a hole transport layer (HTL) solution at 2000 rpm for 30 s, where a spiro-OMeTAD-chlorobenzene (Sigma) solution was employed with addition of 50 mL Li-bis(trifluoromethanesulfonyl) imide (LiTFSI, Sigma)/acetonitrile (170 mg/1 mL) and 20 mL tertbutylpyridine (tBP, Sigma). Finally, the counter electrode was deposited by thermal evaporation of silver under a pressure of 5 105 Torr. The final active area was 0.5 cm2. Characterization. X-ray diffraction (XRD) was detected by Rigaku D/Max 2500 V/PC X-ray powder diffractometer with CuKa radiation. FESEM morphology and Energy Dispersive X-Ray Fluorescence (EDX) analyses were observed by Hatchi s-4800 field emission scanning electron microscope. Absorption spectra were recorded by UV-3600 UVevis absorption spectrometer with optics intergrating sphere. Surface morphology and roughness were characterized with a Multimode nano scope IV atomic force microscopy (AFM). The thickness of thin film is measured by Dektak 150. Power conversion efficiencies were done using a San-Ei Electric PV cell tester with scan rate of 20 mV s1 and Xenon Lamp Solar Simulator equipped with an AM 1.5 filter. 3. Results and discussion Fig. 1a presents the XRD pattern of the synthesized CH3NH3PbI3 film synthesized at 150 C for 2 h on ITO substrate. The strong and sharp diffraction peaks at 2q ¼ 14.19 ,24.59 , 28.33 , 28.50 and 31.97 are corresponding to (110), (211), (004), (220) and (310) planes of the orthorhombic crystal structure of perovskite CH3NH3PbI3 crystal with high crystallinity, which is consistent with the reference [18]. No other diffraction peaks except ITO from the glass substrate are observed, which means the complete transformation from PbI2 to CH3NH3PbI3. Fig. 1b shows the UVevis absorption spectra of the perovskite thin films. The film exhibited a strong and broad range of light absorption from 350 to 760 nm, which completely cover the entire visible light spectra. The film quality of the perovskite is further evaluated by SEM and AFM, shown in Fig. 1c and d. The surface of the synthesized perovskite film is smooth, compact and continuous, and the average size is ~400 nm (Fig. 1c). Further investigating the surface roughness of the perovskite film shown in Fig. 1d, which can be calculated to be 32.8 nm in the range of 10 mm 10 mm. The roughness of the film is much smaller than other perovskite prepared by solution process
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Fig. 1. (a) XRD pattern, (b) UVevis absorption spectrum, (c) SEM and (d) AFM images of perovskite CH3NH3PbI3 thin films synthesized at 150 C for 2 h.
[19]. In addition, the thickness of the perovskite thin film is about ~250 nm measured by step profiler. In order to understand the phase transformation and growth mechanism of perovskite CH3NH3PbI3 thin films using this process. The XRD patterns of four samples with different annealing time of 0 h, 0.5 h, 1 h and 2 h were investigated by annealing PbI2 thin films in the presence of CH3NH3I at 150 C., shown in Fig. 2. It can be clearly observe that the thin film is completely composed of pure PbI2 phase at 0 h. While the annealing time increase to 0.5 h and 1 h, the PbI2 phase gradually transform to CH3NH3PbI3 phase. Finally, the PbI2 phase completely disappears and only pervoskite CH3NH3PbI3 phase exists in the XRD pattern when the annealing time increase to 2 h, shown in Fig. 2. The result agrees well with Yang's report [16]. In addition, the morphology evolution of the
Fig. 2. XRD patterns of the thin films by annealing PbI2 films in the presence of CH3NH3PbI3 for 0 h, 0.5 h, 1 h and 2 h.
perovskite thin films with annealing time increasing are also discussed in Fig. 3. Fig. 3a shows the morphology of the thin film annealed at 0 h. It can be seen that the initial PbI2 thin film are composed of irregular plate-like nanocrystals with average diameter of ~230 nm and thickness of ~45 nm, as well as scattered voids among adjacent nanocrystals. The morphology of PbI2 thin film using NEP as solvent is different from the product using traditional DMF that is polygonal [16]. As the annealing time increases to 1 h (Fig. 3b), plates-like PbI2 nanocrystals almost transform to pebbleslike CH3NH3PbI3 with an average size of ~300 nm. Moreover, the scattered voids disappear which leads to a compact thin film. The increase of the nanocrystals size and thin-films compactness may be attribute to the intercalation of CH3NH3I into the PbI2 crystallites leading to crystalline lattice volume expansion [18]. Further prolonging the annealing time to 2 h (Fig. 3c), the average size of the perovskite nanocrystals increase to ~400 nm. As the reaction time reaches to 4 h, some perovskite nanocrystals with grain size up to microscale meters are observed. Meanwhile, the size distribution of the perovskite films become increasing and some voids reappear again which may be attribute to the Ostwald Ripening [20]. As is well known that PbI2 can be dissolved in many polar solvents such as DMF,DMSO,GBL and NMP [21e24] et al.. However, to our best knowledge, NEP has not been reported as an efficient solvent to dissolve PbI2 (>1 M, at room temperature). NEP is a more green and safe solvent comparing to common solvent such as DMF, which is attributed to its low toxicity and low volatility. On the other hand, NEP has a low saturated vapor pressure and volatility, which could be a benefit to maintain solvent- PbI2 complexes stable in the films, and then efficiently control the crystallinity of PbI2 by drying the PbI2 films at different temperature. The XRD patterns of the four PbI2 films synthesized with different drying temperature of 50 C, 70 C, 90 C and 120 C for 20 min under vacuum are shown in Fig. 4. As is shown in Fig. 4, no characteristic diffraction peak of PbI2 phase but ITO is present at 50 C, indicating its amorphous
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Fig. 3. SEM images of the thin films by annealing PbI2 films in the presence of CH3NH3PbI3 for (a) 0 h, (b) 0.5 h, (c) 2 h and (d) 4 h.
Fig. 4. XRD patterns of the PbI2 thin films drying at different temperature of 50 C, 70 C, 90 C and 120 C for 20 min under vacuum.
feature. Further increasing drying temperature to 70 C, a strong diffraction peak at 12.6 appears in the XRD pattern, corresponding to the (001) latter plane of crystallized PbI2. With the drying temperature increasing to 90 C and 120 C, the diffraction peak of (001) latter plane become more and more strong and sharp, indicating the crystalline degree of PbI2 become better. In addition, the color of PbI2 films changed from yellowish to golden yellow with the drying temperature increasing (inset of Fig. 4) also confirmed the enhancement of PbI2 crystallinity, which is according to Han's research [17]. The four PbI2 thin films mentioned above were used to prepare perovskite films by evaporating the CH3NH3I powder at 150 C for 2 h under N2 protection. Fig. 5aed presents the SEM images of the perovskite films prepared by the PbI2 film with drying temperature of 50 C, 70 C, 90 C and 120 C, respectively. The amorphous PbI2 drying at 50 C leads to large perovskite CH3NH3PbI3 with grain size up to 1e2 mm, and many gaps appear in the thin film which may be
attributed to the growth of large crystalline grain, shown in Fig. 5a. The average size of perovskite grain prepared by 70 C and 90 C dried PbI2 films decrease from ~400 nm to ~230 nm, shown in Fig. 5b and c. No obvious grain size difference are observed between the perovskite samples by using 90 C and 120 C dried PbI2 films (Fig. 5d). The surface roughness of the four perovskite films calculated by AFM (shown in Fig. 6) are 136 nm, 32.8 nm, 34.3 nm and 26.3 nm, respectively. As is well known, the crystallized PbI2 can react with CH3NH3I efficiently as the ordered crystal structure already exists, only requiring the intercalation of CH3NH3I into the lattice to form CH3NH3PbI3 [16]. However, only a few researches has reported the amorphous PbI2 can generate perovskite crystals [17,25]. Han's group [17] confirmed amorphous PbI2 using DMSO as solvent can easily transform to crystalline perovskite CH3NH3PbI3 with higher transformational speed than crystallized PbI2, and which may be attributed to the high activity of amorphous PbI2 phase. We also conclude the crystalline degree of PbI2 phase will impact the growth of perovskite phase that PbI2 with low crystalline degree will be easily transform to perovskite CH3NH3PbI3 and grow more large grain size, which is consistent with our results shown in Figs. 4 and 5. The perovskite CH3NH3PbI3 thin film prepared by the PbI2 film with drying temperature of 70 C were used for solar cell fabrication. The structure of the planar-structure solar cell is as following: the FTO glass substrate is coated with a compact layer of TiO2 as an electron-transporting layer, followed by the CH3NH3PbI3 layer as a light-absorbing layer, spiro-OMeTAD is employed as a holetransporting layer, and the silver is evaporated as the back contact of the device. The current densityevoltage (JeV) curves of the device presents an obvious hysteresis between the forward scan and reversed scan (shown in Fig. 7). For the forward scan (JSC / VOC), it exhibits an open circuit voltage (VOC) of 0.98 eV, a short circuit current density (JSC) of 16.5 mA/cm2 and a filling factor (FF) of 43.5%, resulting in a power conversion efficiency (PCE) of 7.0% for an efficient area of 0.5 cm2 under standard AM 1.5 sunlight. However, for the reverse scan (VOC / JSC), an increase of FF up to 63.3% and PCE of 10.2% are observed in Fig. 7. The value of hysteresis for the planar perovskite cells is similar to Snaith's and Park's report
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Fig. 5. SEM images of the CH3NH3PbI3 thin films by annealing PbI2 films with different drying temperature of (a) 50 C, (b) 70 C, (c) 90 C and (d) 120 C at 150 C for 2 h under N2 protection.
Fig. 6. AFM images of the CH3NH3PbI3 thin films by annealing PbI2 films with different drying temperature of (a) 50 C, (b) 70 C, (c) 90 C and (d) 120 C at 150 C for 2 h under N2 protection.
[26,27]. However, the reason for the hysteresis is still confused and needed more study in the future.
4. Conclusions In summary, we developed a novel and green NEP as solvent instead of the common DMF to dissolve PbI2 and then adopted
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Fig. 7. Current densityevoltage (JeV) characteristics of the solar cell based on the asprepared perovskite films under AM 1.5G illumination.
vapor-assisted solution process to successfully prepare high-quality planar perovskite films with pure phase, high compactness, low surface roughness and uniform crystalline grain with average size of ~400 nm. The time evolution study on perovskite films indicated the PbI2 can be completely transformed to CH3NH3PbI3 by the intercalation of CH3NH3I after the reaction time of 2 h. In addition, just efficiently controlling the crystalline degree of PbI2 films by drying the PbI2-NEP precursor films at different temperature, we can successfully tune the perovskite crystal size from hundreds of nanometers to several micrometers. Using the perovskite films with excellent quality as a light-absorbing to construct a planar architecture, the PCE of 10.2% with an area of 0.5 cm2 were obtained. Acknowledgment The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (No.51402085) and Nature Science Foundation of Hebei Province (E2015202295). References €tzel, The light and shade of perovskite solar cells, Nat. Mater. 13 (2014) [1] M. Gra 838e842. [2] M.A. Green, A. Ho-Baillie, H.J. Snaith, The emergence of perovskite solar cells, Nat. Photon. 8 (2014) 506e514. [3] S.D. Stranks, H.J. Snaith, Metal-halide perovskites for photovoltaic and lightemitting devices, Nat. Nanotech. 10 (2015) 391e402. [4] M.A. Green, K. Emery, Y. Hishikawa, W. Warta, E.D. Dunlop, Solar Cell Efficiency Tables (Version 45), Prog. Photovoltaics 23 (2015) 1e9. [5] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal halide perovskites as visible-light sensitizers for photovoltaic cells, J. Am. Chem. Soc. 131 (2009)
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Preparation of planar CH3NH3PbI3 thin films with controlled size using 1-ethyl-2-pyrrolidone as solvent Qiuyan Hao a, Yixia Chu a, Xuerong Zheng b, Zhenya Liu a, Liming Liang a, Jiakun Qi c, Xin Zhang a, Gang Liu d, Hui Liu a, *, Hongjian Chen a, Caichi Liu a, ** a
Engineering Laboratory of Functional Optoelectronic Crystalline Materials of Hebei Province, School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300132, PR China Key Laboratory of Advanced Ceramics and Machining Technology of Ministry of Education, School of Materials Science and Engineering, Tianjin University, Tianjin 300072, PR China c State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, PR China d School of Chemical Engineering, Hebei University of Technology, Tianjin 300132, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 December 2015 Received in revised form 7 February 2016 Accepted 8 February 2016 Available online 11 February 2016
Recently, planar perovskite solar cells based on CH3NH3PbI3 have attracted many researcher's interest due to their unique advantages such as simple cell architecture, easy fabrication and potential multijunction construction comparing to the initial mesoporous structure. However, the preparation of planar perovskite films with high quality is still in challenge. In this paper, we developed a vapor-assisted solution process using a novel and green solvent of 1-Ethyl-2-pyrrolidone (NEP) instead of the traditional N, N-dimethylformamide (DMF) to construct a high-quality perovskite CH3NH3PbI3 thin film with pure phase, high compactness, small surface roughness and controlled size. The phase evolution and growth mechanism of the perovskite films are also discussed. Utilizing the NEP of low volatility and moderate boiling point as solvent, we dried the PbI2-NEP precursor films at different temperature under vacuum and then obtained PbI2 thin films with different crystalline degree from amorphous to highly crystalline. The perovskite films with crystal size ranged from hundreds of nanometers to several micrometers can be prepared by reacting the PbI2 films of different crystalline degree with CH3NH3I vapor. Moreover, planar-structured solar cells combining the perovskite film with TiO2 and spiro-OMeTAD as the electron and holes transporting layer achieves a power conversion efficiency of 10.2%. © 2016 Elsevier B.V. All rights reserved.
Keywords: Perovskite 1-Ethyl-2-pyrrolidone Controlled size Planar structure
1. Introduction Organic-inorganic perovskite material such as CH3NH3PbI3 is one of the most competitive light-absorbing materials for thin film solar cells due to its low-cost, easy fabrication and unique optical and electrical properties such as appropriate and tunable direct bandgap, high absorption coefficient, efficient ambipolar transport and long carrier diffusion length [1e3]. The perovskite solar cell (PSC) has been reported to achieve relatively high power conversion efficiency (PCE) of 20.1% [4] since the first report of PSC with the PCE of 3.8% during the past 6 years [5]. Although the initial architecture of PSC device adopts mesoporous structure which
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H. Liu), [email protected] (C. Liu). http://dx.doi.org/10.1016/j.jallcom.2016.02.079 0925-8388/© 2016 Elsevier B.V. All rights reserved.
originates from the traditional dye-sensitized solar cells, the planar architecture PSC has also been confirmed soon [6,7]. PSC with planar structure has many unique advantages such as simple cell architecture, easy fabrication and potential multijunction construction et al., which attracts many researcher's interest. Nevertheless, the preparation of planar perovskite films with high quality is still in challenge. Vacuum evaporation process is successfully used to deposit the perfect planar perovskite films with pure phase, full surface coverage and uniform crystal size [8e10]. But this vapor deposition need high vacuum and leads to an abundant waste of raw materials which will increase the cost of large scale fabrication. Meanwhile, vapor technology will also produce toxic PbI2 vapor which is harmful to the humanity. Solution technology such as one-step [11,12] or two-step [13e15] sequential solution deposition process is also introduced to prepare the planar perovskite films. However, both the two solution technology have their disadvantages: a relatively low surface
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coverage and broaden size distribution for one-step solution deposition, and incomplete conversion of PbI2 and uncontrolled perovskite crystal sizes as well as surface morphology. Considering the benefits and drawbacks of the vapor and two-step sequential solution deposition technology, Yang's group developed a facile vapor assisted solution process (VASP) to fabricate planar CH3NH3PbI3 thin films of high quality with full surface coverage, small surface roughness, and grain size up to microscale, achieving the high PCE of 12.1% [16]. This simple method provides a way for high reproducibility of films and devices with high quality. However, efficient controlling the crystal size of pure perovskite films is still in challenge by using VASP. According to Han's report, retarding the crystallization of PbI2 using DMSO as a solvent to form amorphous structure can impact the growth of perovskite crystal grain and lead to high quality perovskite films with uniform particles size by a two-step sequential solution deposition process [17]. We assume that tuning the crystalline degree of PbI2 phase may impact the growth of perovskite films and efficiently control the perovskite crystal size for sequential deposition. Nevertheless, the commonly used DMF solvent has a high saturated vapor pressure and volatility which makes it easy to crystallize PbI2 after spincoating the PbI2-DMF solution, so it is not easy to control the crystalline degree of PbI2. Other commonly used solvents which can dissolve PbI2 such as gamma-butyrolactone (GBL) dimethylsulfoxide (DMSO) and 1-Methyl-2-pyrrolidinone (NMP) sometimes have high boiling point and low volatility, which makes it need high temperature to dry PbI2 precursor solution films, leading to control the crystallization of PbI2 hard. Therefore, finding a solvent of low volatility and moderate boiling point which can also dissolve PbI2 is necessary. 1-Ethyl-2-pyrrolidone (NEP) is an appropriate candidate with moderate boiling point (82e83 , 101.3 KPa) and low volatility which can efficiently dissolve PbI2 (>1 M) at room temperature. In addition, NEP is non-toxic for humanity which gives it more benefits comparing to the toxic DMF or DMSO for the future scalable production. To our best knowledge, no researches have been reported to use NEP to prepare perovskite thin film. In this paper, we developed a vapor-assisted solution process using a novel and green NEP as solvent instead of DMF to construct a high-quality perovskite CH3NH3PbI3 thin film with pure phase, high compactness, small surface roughness and controlled size. The phase transformation and growth mechanism of the perovskite films are also discussed. The PbI2 films with different crystalline can be obtained by drying the PbI2-NEP precursor films at different temperature under vacuum. And then the crystal size of the perovskite films ranged from hundreds of nanometers to several micrometers can be prepared by using the PbI2 films with different crystalline degree to react with the CH3NH3I vapor. Moreover, planarstructured solar cells combining this perovskite film with TiO2 and spiro-OMeTAD as the electron and holes elective transporting layer achieves the PCE of 10.2%. 2. Experimental section CH3NH3I Synthesis. CH3NH3I (MAI) was synthesized using the approach described in the literature [16]. Typically, 24 mL methylamine (33 wt% in absolute methanol, Aladdin, China) and 10 mL of hydroiodic acid (58 wt% in water, Aladdin, China) were in a 250 mL round-bottom flask at 0 C for 2 h with stirring. The precipitate was collected by evaporating the solvents on a rotary evaporator at 50 C. The product CH3NH3I was washed and precipitated with the addition of the absolute ethanol and diethyl ether for three times, respectively. And the solid was collected and dried at 60 C in a vacuum oven for 24 h. CH3NH3PbI3 Thin Film Fabrication. Glass substrates were
ultrasonic cleaning with deionized water, acetone and ethanol for 30 min, respectively, and dried with clean N2 for further using. The low-toxicity and low-volatility solvent of NEP was used to dissolve PbI2 powders. PbI2 thin films were prepared by dip-coating a 0.3 M PbI2-NEP solution on an ITO or FTO glass substrate, and vacuum drying at desired temperature for 20 min in a vacuum oven. CH3NH3I powder was spread out around the PbI2 coated substrates with a petridish covering on the top, and heated with N2 protection at 150 C for 0 h, 0.5 h, 1 h, 2 h and 4 h in an oven, respectively. After cooling down, the thin film was washed with isopropanol and dried in air. Solar Cell Device Fabrication. The FTO glass substrates were spin-coated with 0.3 M titanium diisopropoxide bis(acetylacetonate) (Sigma) at 3, 000 r.p.m for 30 s subsequently, following by sintering at 550 C for 20 min in air. The substrate was immersed in 50 mM TiCl4 (Sigma) aqueous solution for 30 min at 70 C and washed with distilled water and ethanol, followed by annealing at 550 C for 20 min in air to form a compacting TiO2 layer. The PbI2NEP solution was dip-coating on the compacting TiO2 layer, and vacuum dried at 70 C for 20 min in a vacuum oven. The PbI2 coated substrates were on the top of CH3NH3I powder in a glass culture dish, and heated at 150 C for desired time in a vacuum oven filled with N2 atmosphere. The as-prepared substrates were washed with isopropanol and dried in air, and then deposited by spin coating a hole transport layer (HTL) solution at 2000 rpm for 30 s, where a spiro-OMeTAD-chlorobenzene (Sigma) solution was employed with addition of 50 mL Li-bis(trifluoromethanesulfonyl) imide (LiTFSI, Sigma)/acetonitrile (170 mg/1 mL) and 20 mL tertbutylpyridine (tBP, Sigma). Finally, the counter electrode was deposited by thermal evaporation of silver under a pressure of 5 105 Torr. The final active area was 0.5 cm2. Characterization. X-ray diffraction (XRD) was detected by Rigaku D/Max 2500 V/PC X-ray powder diffractometer with CuKa radiation. FESEM morphology and Energy Dispersive X-Ray Fluorescence (EDX) analyses were observed by Hatchi s-4800 field emission scanning electron microscope. Absorption spectra were recorded by UV-3600 UVevis absorption spectrometer with optics intergrating sphere. Surface morphology and roughness were characterized with a Multimode nano scope IV atomic force microscopy (AFM). The thickness of thin film is measured by Dektak 150. Power conversion efficiencies were done using a San-Ei Electric PV cell tester with scan rate of 20 mV s1 and Xenon Lamp Solar Simulator equipped with an AM 1.5 filter. 3. Results and discussion Fig. 1a presents the XRD pattern of the synthesized CH3NH3PbI3 film synthesized at 150 C for 2 h on ITO substrate. The strong and sharp diffraction peaks at 2q ¼ 14.19 ,24.59 , 28.33 , 28.50 and 31.97 are corresponding to (110), (211), (004), (220) and (310) planes of the orthorhombic crystal structure of perovskite CH3NH3PbI3 crystal with high crystallinity, which is consistent with the reference [18]. No other diffraction peaks except ITO from the glass substrate are observed, which means the complete transformation from PbI2 to CH3NH3PbI3. Fig. 1b shows the UVevis absorption spectra of the perovskite thin films. The film exhibited a strong and broad range of light absorption from 350 to 760 nm, which completely cover the entire visible light spectra. The film quality of the perovskite is further evaluated by SEM and AFM, shown in Fig. 1c and d. The surface of the synthesized perovskite film is smooth, compact and continuous, and the average size is ~400 nm (Fig. 1c). Further investigating the surface roughness of the perovskite film shown in Fig. 1d, which can be calculated to be 32.8 nm in the range of 10 mm 10 mm. The roughness of the film is much smaller than other perovskite prepared by solution process
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Fig. 1. (a) XRD pattern, (b) UVevis absorption spectrum, (c) SEM and (d) AFM images of perovskite CH3NH3PbI3 thin films synthesized at 150 C for 2 h.
[19]. In addition, the thickness of the perovskite thin film is about ~250 nm measured by step profiler. In order to understand the phase transformation and growth mechanism of perovskite CH3NH3PbI3 thin films using this process. The XRD patterns of four samples with different annealing time of 0 h, 0.5 h, 1 h and 2 h were investigated by annealing PbI2 thin films in the presence of CH3NH3I at 150 C., shown in Fig. 2. It can be clearly observe that the thin film is completely composed of pure PbI2 phase at 0 h. While the annealing time increase to 0.5 h and 1 h, the PbI2 phase gradually transform to CH3NH3PbI3 phase. Finally, the PbI2 phase completely disappears and only pervoskite CH3NH3PbI3 phase exists in the XRD pattern when the annealing time increase to 2 h, shown in Fig. 2. The result agrees well with Yang's report [16]. In addition, the morphology evolution of the
Fig. 2. XRD patterns of the thin films by annealing PbI2 films in the presence of CH3NH3PbI3 for 0 h, 0.5 h, 1 h and 2 h.
perovskite thin films with annealing time increasing are also discussed in Fig. 3. Fig. 3a shows the morphology of the thin film annealed at 0 h. It can be seen that the initial PbI2 thin film are composed of irregular plate-like nanocrystals with average diameter of ~230 nm and thickness of ~45 nm, as well as scattered voids among adjacent nanocrystals. The morphology of PbI2 thin film using NEP as solvent is different from the product using traditional DMF that is polygonal [16]. As the annealing time increases to 1 h (Fig. 3b), plates-like PbI2 nanocrystals almost transform to pebbleslike CH3NH3PbI3 with an average size of ~300 nm. Moreover, the scattered voids disappear which leads to a compact thin film. The increase of the nanocrystals size and thin-films compactness may be attribute to the intercalation of CH3NH3I into the PbI2 crystallites leading to crystalline lattice volume expansion [18]. Further prolonging the annealing time to 2 h (Fig. 3c), the average size of the perovskite nanocrystals increase to ~400 nm. As the reaction time reaches to 4 h, some perovskite nanocrystals with grain size up to microscale meters are observed. Meanwhile, the size distribution of the perovskite films become increasing and some voids reappear again which may be attribute to the Ostwald Ripening [20]. As is well known that PbI2 can be dissolved in many polar solvents such as DMF,DMSO,GBL and NMP [21e24] et al.. However, to our best knowledge, NEP has not been reported as an efficient solvent to dissolve PbI2 (>1 M, at room temperature). NEP is a more green and safe solvent comparing to common solvent such as DMF, which is attributed to its low toxicity and low volatility. On the other hand, NEP has a low saturated vapor pressure and volatility, which could be a benefit to maintain solvent- PbI2 complexes stable in the films, and then efficiently control the crystallinity of PbI2 by drying the PbI2 films at different temperature. The XRD patterns of the four PbI2 films synthesized with different drying temperature of 50 C, 70 C, 90 C and 120 C for 20 min under vacuum are shown in Fig. 4. As is shown in Fig. 4, no characteristic diffraction peak of PbI2 phase but ITO is present at 50 C, indicating its amorphous
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Fig. 3. SEM images of the thin films by annealing PbI2 films in the presence of CH3NH3PbI3 for (a) 0 h, (b) 0.5 h, (c) 2 h and (d) 4 h.
Fig. 4. XRD patterns of the PbI2 thin films drying at different temperature of 50 C, 70 C, 90 C and 120 C for 20 min under vacuum.
feature. Further increasing drying temperature to 70 C, a strong diffraction peak at 12.6 appears in the XRD pattern, corresponding to the (001) latter plane of crystallized PbI2. With the drying temperature increasing to 90 C and 120 C, the diffraction peak of (001) latter plane become more and more strong and sharp, indicating the crystalline degree of PbI2 become better. In addition, the color of PbI2 films changed from yellowish to golden yellow with the drying temperature increasing (inset of Fig. 4) also confirmed the enhancement of PbI2 crystallinity, which is according to Han's research [17]. The four PbI2 thin films mentioned above were used to prepare perovskite films by evaporating the CH3NH3I powder at 150 C for 2 h under N2 protection. Fig. 5aed presents the SEM images of the perovskite films prepared by the PbI2 film with drying temperature of 50 C, 70 C, 90 C and 120 C, respectively. The amorphous PbI2 drying at 50 C leads to large perovskite CH3NH3PbI3 with grain size up to 1e2 mm, and many gaps appear in the thin film which may be
attributed to the growth of large crystalline grain, shown in Fig. 5a. The average size of perovskite grain prepared by 70 C and 90 C dried PbI2 films decrease from ~400 nm to ~230 nm, shown in Fig. 5b and c. No obvious grain size difference are observed between the perovskite samples by using 90 C and 120 C dried PbI2 films (Fig. 5d). The surface roughness of the four perovskite films calculated by AFM (shown in Fig. 6) are 136 nm, 32.8 nm, 34.3 nm and 26.3 nm, respectively. As is well known, the crystallized PbI2 can react with CH3NH3I efficiently as the ordered crystal structure already exists, only requiring the intercalation of CH3NH3I into the lattice to form CH3NH3PbI3 [16]. However, only a few researches has reported the amorphous PbI2 can generate perovskite crystals [17,25]. Han's group [17] confirmed amorphous PbI2 using DMSO as solvent can easily transform to crystalline perovskite CH3NH3PbI3 with higher transformational speed than crystallized PbI2, and which may be attributed to the high activity of amorphous PbI2 phase. We also conclude the crystalline degree of PbI2 phase will impact the growth of perovskite phase that PbI2 with low crystalline degree will be easily transform to perovskite CH3NH3PbI3 and grow more large grain size, which is consistent with our results shown in Figs. 4 and 5. The perovskite CH3NH3PbI3 thin film prepared by the PbI2 film with drying temperature of 70 C were used for solar cell fabrication. The structure of the planar-structure solar cell is as following: the FTO glass substrate is coated with a compact layer of TiO2 as an electron-transporting layer, followed by the CH3NH3PbI3 layer as a light-absorbing layer, spiro-OMeTAD is employed as a holetransporting layer, and the silver is evaporated as the back contact of the device. The current densityevoltage (JeV) curves of the device presents an obvious hysteresis between the forward scan and reversed scan (shown in Fig. 7). For the forward scan (JSC / VOC), it exhibits an open circuit voltage (VOC) of 0.98 eV, a short circuit current density (JSC) of 16.5 mA/cm2 and a filling factor (FF) of 43.5%, resulting in a power conversion efficiency (PCE) of 7.0% for an efficient area of 0.5 cm2 under standard AM 1.5 sunlight. However, for the reverse scan (VOC / JSC), an increase of FF up to 63.3% and PCE of 10.2% are observed in Fig. 7. The value of hysteresis for the planar perovskite cells is similar to Snaith's and Park's report
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Fig. 5. SEM images of the CH3NH3PbI3 thin films by annealing PbI2 films with different drying temperature of (a) 50 C, (b) 70 C, (c) 90 C and (d) 120 C at 150 C for 2 h under N2 protection.
Fig. 6. AFM images of the CH3NH3PbI3 thin films by annealing PbI2 films with different drying temperature of (a) 50 C, (b) 70 C, (c) 90 C and (d) 120 C at 150 C for 2 h under N2 protection.
[26,27]. However, the reason for the hysteresis is still confused and needed more study in the future.
4. Conclusions In summary, we developed a novel and green NEP as solvent instead of the common DMF to dissolve PbI2 and then adopted
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Fig. 7. Current densityevoltage (JeV) characteristics of the solar cell based on the asprepared perovskite films under AM 1.5G illumination.
vapor-assisted solution process to successfully prepare high-quality planar perovskite films with pure phase, high compactness, low surface roughness and uniform crystalline grain with average size of ~400 nm. The time evolution study on perovskite films indicated the PbI2 can be completely transformed to CH3NH3PbI3 by the intercalation of CH3NH3I after the reaction time of 2 h. In addition, just efficiently controlling the crystalline degree of PbI2 films by drying the PbI2-NEP precursor films at different temperature, we can successfully tune the perovskite crystal size from hundreds of nanometers to several micrometers. Using the perovskite films with excellent quality as a light-absorbing to construct a planar architecture, the PCE of 10.2% with an area of 0.5 cm2 were obtained. Acknowledgment The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (No.51402085) and Nature Science Foundation of Hebei Province (E2015202295). References €tzel, The light and shade of perovskite solar cells, Nat. Mater. 13 (2014) [1] M. Gra 838e842. [2] M.A. Green, A. Ho-Baillie, H.J. Snaith, The emergence of perovskite solar cells, Nat. Photon. 8 (2014) 506e514. [3] S.D. Stranks, H.J. Snaith, Metal-halide perovskites for photovoltaic and lightemitting devices, Nat. Nanotech. 10 (2015) 391e402. [4] M.A. Green, K. Emery, Y. Hishikawa, W. Warta, E.D. Dunlop, Solar Cell Efficiency Tables (Version 45), Prog. Photovoltaics 23 (2015) 1e9. [5] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal halide perovskites as visible-light sensitizers for photovoltaic cells, J. Am. Chem. Soc. 131 (2009)
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