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Vacuum co-deposited CH3NH3PbI3 films by controlling vapor pressure for efficient planar perovskite solar cells
Solar Energy 181 (2019) 339–344
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Vacuum co-deposited CH3NH3PbI3 films by controlling vapor pressure for efficient planar perovskite solar cells
T
V. Arivazhagana, Jiangsheng Xiea, Zhengrui Yanga, Pengjie Hanga, M. Manonmani Parvathia, ⁎ Ke Xiaob, Can Cuib, Deren Yanga, Xuegong Yua, a b
State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China Department of Physics, Zhejiang Science and Technology University, Hangzhou 310027, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Perovskite solar cells Co-evaporation Micrometer grains Vapor pressure Stable output
Physical vacuum deposition (PVD) of organo-metal halide thin film has advantages of uniform and large scale deposition for emerging perovskite solar cell technology. Evaporation of organic halides such as CH3NH3I (MAI) is crucial for obtaining phase pure perovskite thin films. However, controlling the evaporation rate of MAI remains difficult owing to its “vapor-gas” like nature upon sublimation. Here we report, highly crystalline CH3NH3PbI3 (MAPbI3) films with micrometer grains can be prepared by gradually controlling the MAI vapor pressure rather than evaporation rate in co-evaporation method. When integrating an optimal film into a planar heterojunction solar cell, the best device achieved a power conversion efficiency of 15.74% by deploying TiO2 as an electron transport layer. This strategy alleviates the strict calibration of MAI evaporation rate and led to obtain phase pure MAPbI3 films for high performance solar cells.
1. Introduction
substrate temperature have been reported (Chen et al., 2014). On the other hand, controlling the vapor pressure of MAI has been reported as an alternative method to form perovskite films (Hsiao et al., 2016; Zhao et al., 2016). However, sequential deposition is well studied among vacuum deposition methods to obtain high quality films, wherein metal halide is first deposited followed by organic halide sublimation (Chen et al., 2014; Longo et al., 2018; Yang et al., 2015). For instance, a sequential deposition method was reported for high performance solar cells by controlling the partial MAI vapor pressure for about 2 h. Vacuum co-deposition of perovskite films by controlling organic halide vapor pressure has advantages such as reduced processing time with improved device performance, however, not much reported. A few reports based on vapor pressure controlled growth of perovskite films have been reported. Nicolas Tetreault et al (Teuscher et al., 2015) studied various factor on the stochiometric control of MAPbI3 using codeposition. Unlike sequential deposition, systematic investigation on gradual change in organic halide vapor pressure is crucial in co-deposition method with respect to PbI2 evaporation rate. Therefore, research efforts on the evaporation conditions in order to obtain the phase pure as well as large grain perovskite thin films using co-deposition method is needed. Here we employ a vacuum co-deposition method to produce high quality MAPbI3 films through controlling the MAI vapor pressure. We
Perovskite solar cells (PSCs) are stepping ahead successfully towards emerging scalable photovoltaic technology thank to its rapid improvement in the contest of converting sun light into electricity (Burschka et al., 2013; Grancini et al., 2017). Most of the works on perovskite solar cells are based on solution processing techniques (Chiang et al., 2017; Kulkarni et al., 2014; Sidhik et al., 2017) while not much work on physical vapor deposition (PVD) methods (Fan et al., 2016; Momblona et al., 2016; Patel et al., 2017). In fact, PVD methods are preferred for the industrialization of optoelectronic devices including photovoltaic technology owing to its uniform and large scale fabrication. However, for vacuum deposition of organo-metal halide perovskite films, one of the main challenges is to control the evaporation rate of organic halides such as CH3NH3I (MAI) precisely (Ono et al., 2016). Literatures shows that MAI sublimates like a “vapor-gas” in vacuum owing to small molecular weight, making it difficult to control the evaporation rate using conventional quartz microbalance sensor (Gao et al., 2015; Hsiao et al., 2016; Liu et al., 2013). We also noted that, the evaporation rate of MAI is much higher (> twofold) than its actual rate during co– evaporation due to an influence from metal halide’s vapor such as PbI2. In order to make the MAI deposition conditions convenient, additional controllable parameters such as source and
⁎
Corresponding author. E-mail address: [email protected] (X. Yu).
https://doi.org/10.1016/j.solener.2019.02.012 Received 21 September 2018; Received in revised form 25 January 2019; Accepted 7 February 2019 0038-092X/ © 2019 Published by Elsevier Ltd on behalf of International Solar Energy Society.
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demonstrate that highly crystalline MAPbI3 films with micrometer grains can be obtained by gradually controlling the vapor pressure by adjusting MAI source temperature with constant PbI2 evaporation rate. The best solar cell device fabricated using optimum vapor pressure controlled deposition of MAPbI3 films achieved the power conversion efficiency (PCE) and stable output (SOP) of 15.74% and 13.72%, respectively, deploying TiO2 as an electron transport layer (ETL). This strategy alleviates the strict calibration of MAI evaporation rate and can be beneficial for the large scale vacuum co-deposition of perovskite films with phase pure and micrometer grains.
surface morphology of the MAPbI3 films prepared with gradual change in vapor pressure was imaged using scanning electron microscopy (SEM) and is shown Fig. 2(a–d). The surface morphology of the M1 film (Fig. 2a) shows fully covered and uniform small grains due to poor formation of perovskite phase. An agglomerated coverage with cubic shaped grains was observed for M2 (Fig. 2b). Increasing vapor pressure further for M3 led to the formation of highly dense and uniform morphology with the majority of grains observed in micrometer sizes of 0.5–1 µm (Fig. 2c). There are no pin holes or incomplete surface coverage observed during the scan across the several micrometer range (Fig. S2). It has been widely reported that micrometer sized grains with smooth surface morphology are the promising features for high performance perovskite solar cells (Gamliel et al., 2015; Nie et al., 2015). The smooth surface morphology with micrometer sized grains attributes to the best stoichiometric composition of vapor pressure controlled growth of perovskite film. The high vapor deposited M4 film consists of larger and small grains as shown in Fig. 2d. A close look at the M4 film surface shows that those small grains are mostly occupied at the grain boundaries. The possible growth mechanism of the perovskite films at different vapor pressure is shown in Fig. 2e. The formation of perovskite phase is limited by the low vapor pressure for M1 sample, which resulting PbI2 rich domains. The PbI2 domains reduce for M2 with increasing MAI vapor pressure. For M3 film, all the evaporated PbI2 reacted with MAI vapor and thus forming highly crystalline perovskite grains. Further increasing MAI vapor lead to perovskite surface morphology with small grains at the grain boundaries. In order to investigate the crystalline phase quality of the perovskite films, X-ray diffraction was carried out and shown in Fig. 3a. The films M1 and M2 show relatively low intensity due to poor crystallinity. The film M1 shows a dominant PbI2 peak at 2θ = 12.45° which decreases with increasing vapor pressure for M2 as shown clearly in Fig. S3. It is obvious that the film, M3, prepared at high vapor pressure shows the phase pure orientation of MAPbI3 in consistent with surface morphology. The dominant Bragg's reflection at 2θ = 14.08° and 28.32° are corresponding to (1 1 0) and (2 2 0) plane of tetragonal MAPbI3 (Zeng et al., 2017) and are highly crystalline as evident from the intensity and full width half maximum of the peaks. In addition to that a minor peak at 2θ = 43.08° was observed corresponding to (3 1 4) crystallographic plane. There is no change in peak positions observed with increasing the vapor pressure for M4, however, (1 1 0) plane intensity became identical to (2 2 0) plane which could be attributed to the selective preferential orientation of the grains in consistent with Fig. 2d. Further crystallographic studies required in order to investigate an insight about the evolution of MAPbI3 crystallographic orientation at different planes in relation with stoichiometric composition. From the structural and morphological studies, it is obvious that MAI with a source temperature of 175 °C (total vapor pressure of 4.5 ± 0.3 × 10−3 pa) gives the best stoichiometric composition with PbI2 to obtain a phase pure and high quality MAPbI3 films. The optical properties of the MAPbI3 films were done in order to investigate the absorption and charge transport properties. Fig. 3b shows the optical absorption spectra of MAPbI3 thin films prepared at different vapor pressures. The film M1 shows absorption onset at high energy region at around 495 nm in correlation with the exciton absorption of PbI2. The PbI2 absorption peak decreases with an evolution of a MAPbI3 peak at 780 nm for M2 film. Further increasing vapor pressure, the film M3 and M4 display a distinct MAPbI3 absorption peak at 780 nm with an improved absorption in visible region as a result of enhanced crystallinity in consistent with XRD pattern. Fig. 3c shows steady state photoluminescence spectra of MAPbI3 films deposited on glass substrates. All the films show an emission peak centered at around 770 nm is assigned to band to band transition and is well in agreement with previous reports for MAPbI3 films based on solution methods (Nie et al., 2015; Yang et al., 2017). The PL spectrum of M1 film is shown in Fig. S4 to see the poor signal at 780 nm clearly. The intensity of the MAPbI3 increases with increasing vapor pressure until M3 and decreases
2. Experimental The two source materials of MAI and PbI2 were placed in an organic source (silica crucible) and inorganic source (boron nitride crucible), respectively. An organic source was controlled with temperature while the inorganic source was controlled by applying current to the source. TiO2 ETL was deposited on patterned FTO substrates according to the literature (Liu et al., 2013) and loaded above the sources with TiO2 surface facing down. A dedicated PVD setup consist of both organic and inorganic sources is shown in Fig. S1. The chamber was evacuated to the base pressure of 1.5 ± 0.5 × 10−4 pa. Then, MAI source was heated gradually to the desirable temperatures such as 125 °C, 150 °C, 175 °C and 190 °C. Meanwhile, the PbI2 source was heated for the optimal evaporation rate of 1.1 ± 0.1 Å/s. The working total vapor pressure was stabilized to 7.5 ± 0.3 × 10−4 pa, 1.5 ± 0.3 × 10−3 pa, 4.5 ± 0.3 × 10−3 pa and 7.5 ± 0.3 × 10−3 pa for the MAI source temperatures of 125 °C, 150 °C, 175 °C and 190 °C, respectively. In this study, vapor pressure (i.e working pressure during co-evaporation) is the main parameter complemented with source temperature. The rate of evaporation of PbI2 was monitored using an in-situ quartz microbalance sensor. After the vapor pressure stabilized, the source shutters and substrate baffle were opened to expose dual source vapor onto the substrates. The substrate holder was kept at ambient temperature and rotated with 600 rpm in order to ensure the uniform deposition on the substrates. The deposition was carried out until quartz sensor showed 125 ± 5 nm thickness of PbI2 and the time was about 17 ± 1 min. After deposition, the samples were post annealed at 100 °C for 30 min in order to improve the crystallinity and interface contact between the MAPbI3 and TiO2. The solar cell devices were fabricated with the hole transporting layer of Spiro-OMeTAD by spin coating as described in our earlier reports (Xie et al., 2017a) followed by 120 nm silver electrodes with a mask for the active area of 10 mm2. Hereafter, for simplicity, the films and devices based on 7.5 ± 0.3 × 10−4 pa (125 °C), 1.5 ± 0.3 × 10−3 pa (150 °C), 4.5 ± 0.3 × 10−3 pa (175 °C) and 7.5 ± 0.3 × 10−3 pa (190 °C) vapor pressures will be referred as M1, M2, M3 and M4 throughout the article. The materials, methods, characterization techniques used for this study is described in the supporting information. 3. Results and discussions The MAPbI3 thin films were prepared on glass substrates as a function of total vapor pressure with constant PbI2 evaporation rate. Fig. 1a shows the schematic of the co-evaporation method used in this work and Fig. 1b shows photographs of the prepared perovskite films, i.e M1, M2, M3 and M4. The film M1 appears to be yellowish in color, which indicates an insufficient MAI to form perovskite. Further increasing the vapor pressures, a reddish brown1 and block colors were observed for M2 and M3 films, respectively. The slightly grey in color (not clearly visible in photograph) was observed for the film prepared at highest vapor pressure (M4) indicating excess MAI in the film. The 1 For interpretation of color in Fig. 1, the reader is referred to the web version of this article.
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Fig. 1. (a) Schematic of the vapor pressured controlled co-deposition method, (b) photographs of the MAPbI3 thin films prepared at different vapor pressures.
Fig. 2. SEM images of MAPbI3 thin films prepared at different vapor pressures, (a) M1, (b) M2, (c) M3 and (d) M4. (e) The schematic of the formation of perovskite film under different vapor pressures.
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Fig. 3. Structural and optical characterization of the films with different vapor pressure. (a) XRD pattern of films, (b) absorption spectra of the films, (c) steady- state PL spectra and (d) normalized time resolved PL spectra of the films deposited on glass substartes.
fabricated solar cell employing M3 film. The stacked layers of glass/ FTO/c-TiO2/MAPbI3/Spiro-OMeTAD/Ag can be clearly seen. A corresponding sketched band diagram of the device is shown in Fig. 4b. Fig. 5a shows the reverse scan J-V characteristics of solar cells based on M1, M2, M3 and M4 films, and the measured photovoltaic parameters are listed in Table 1. The device based on M1 shows the least PCE of 4.23% and Voc of 0.95 V due to insufficient perovskite phase. Further increasing the vapor pressure, the M2 device performance was improved to a certain extent and yields the PCE of 10.51%. As anticipated from the structural and optical characterization, the device based on M3 film exhibits improved solar cell characteristics with Voc = 1.08 V, Jsc = 21.76 mA/cm2, FF = 66.80% and PCE = 15.74%. The PCE of the M4 film based device decreases to 11.79% as result of excess MAI. The drop in PCE for M4 compared to M3 device is mainly associated with Jsc and FF. The forward and reverse scan of the champion device based on M3 is shown in Fig. S7. The observed hysteresis in Fig. S7 is mainly associated with TiO2 ETL as reported elsewhere (Patel et al., 2017). Fig. 5b shows an external quantum efficiency (EQE) of the devices as a function of wavelength. The device based on M3 exhibits the highest EQE of about 80% in the range of 450–650 nm. We note a step like feature in the EQE spectra which attributes to an optical interference from the layer stack. Fig. 5c shows the stable output at maximum power point tracking, Vmp, for 50 s. The devices based on M1, M2, M3 and M4 were biased at 0.64, 0.66, 0.83 and 0.71 V, respectively. The stabilized
for M4. The high PL intensity of M3 indicates the phase pure quality of the film in accordance with SEM and XRD analysis, which is preferable for optoelectronic devices. Fig. 3d shows the normalized time resolved photoluminescence (TRPL) spectra of MAPbI3 films measured in a 770 nm emission window. The exiton life time of the perovskite films were determined by fitting exponential equation. The life time was calculated as τ = 5, 7, 31 and 15 ns for M1, M2, M3 and M4, respectively. It is well known that the intensity of the PL signal drops when depositing on ETL due to built-in electric field formed between perovskite and ETL which results in mobile carrier extraction (Singh et al., 2018; Xie et al., 2018). Upon depositing the M3 film on TiO2/FTO, the PL intensity of MAPbI3 dropped due to quenching effect which infers an efficient charge extraction into TiO2 as shown in Fig. S5. Fig. S6 shows the TRPL decay of MAPbI3 on TiO2/FTO and glass substrates. For M3/ TiO2, the fast decay was calculated as τ1 = 2.2 ns (using bi-exponential equation) which is close to solution processed perovskite film on TiO2 (Xie et al., 2017b). we attribute this fast charge carrier transfer is due to the micrometer sized grains that promote charge carriers from Perovskite to TiO2 without choosing grain boundaries, and thus reducing surface recombination. The solar cells were fabricated with different vapor pressures grown MAPbI3 absorber and its performance such as open-circuit voltage (Voc), short-circuit current (Jsc), fill factor (FF) and power conversion efficiency (PCE) were tested. Fig. 4a shows a cross sectional view of the 342
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Fig. 4. (a) Cross-sectional SEM view of the solar cell device with M3 film, (b) band diagram of the solar cell device.
but still not close to PCE due to the anomalous hysteresis effect widely observed in TiO2 ETL based solar cells (Snaith et al., 2014). The overall improvement in the J-V characteristics of M3 film based device can be correlated to the better surface morphology and crystallinity of the perovskite films. One of the advantages of the vacuum deposition method is the feasibility to prepare perovskite films with different thicknesses of
power output (SOP) for M1, M2, M3 and M4 were obtained as 2.9, 7.1, 13.72 and 8.1%, respectively. We note that SOP is not much reported on vacuum deposited MAPbI3/TiO2 based devices. Patel et al. (Patel et al., 2017) reported that vacuum deposited MAPbI3/TiO2 based device with the PCE of 15.58% showed significantly lower SOP of < 6% where surface passivation of TiO2 with PCBM was suggested to obtain the SOP identical to PCE. In our work, the SOP is significantly higher
Fig. 5. (a) J-V curves of the devices biased under reverse scan, (b) external quantum efficiency spectra (c) stable output at maximum power point and (d) J-V curves of the solar cell devices as a function of MAPbI3 thicknesses. 343
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Table 1 Solar cell parameters of the devices biased in the reverse direction. Device
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
M1 M2 M3 M4
0.95 1.00 1.08 1.03
8.86 17.50 21.76 20.08
49.98 59.73 66.80 57.11
4.23 10.51 15.74 11.79
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identical stoichiometric composition, which is desirable for different optoelectronic devices. To further investigate the J-V characteristics of the device as a function of thicknesses, we prepared the MAPbI3 films with the different thicknesses range from 290, 350, 430, 550 and 700 ± 10 nm. The total thickness were obtained by varying the equivalent thickness of PbI2 (100, 150, 200 and 250 ± 5 nm) monitored using quartz microbalance sensor during co-deposition. The optimized vapor pressure of 4.5 ± 0.3 × 10−3 pa was fixed constant as used for optimal M3 film. The J-V curves of the devices with different MAPbI3 film thicknesses is shown in Fig. 4d. The device with 290 nm MAPbI3 thickness delivers the PCE of 13.8% which increases further to 15.74% with increasing thickness to 350 nm. The increase in PCE is mainly attributed to the increase in Jsc and Voc due to more photoexcited charge carriers. Further increasing the thickness to 430 nm, the device performance decreases as a result of decrease in Voc and FF. The similar trend continues further increasing the thicknesses to 550 and 700 nm due to internal recombination of charge carriers within the perovskite absorber. Hence, we conclude that 350 ± 10 nm is optimum for vapor pressure controlled growth of Perovskite thin films investigated in this research work. Finally, the stability of the device based on M3 was tested over a period of 15 days stored under ambient condition (∼45% humidity, ∼28 °C temperature) and is shown Fig. S8 together with solution processed MAPbI3 film based device. Both the devices retain ∼84% of its initial efficiency. 4. Conclusions In summary, we have systematically investigated vacuum co-deposition of phase pure MAPbI3 thin films through controlling a gradual change in vapor pressure. This study alleviated a strict calibration of MAI evaporation rate using conventional quartz microbalance sensor to obtain perovskite films. As a consequence, highly crystalline MAPbI3 thin films with micrometer size grains were obtained at the vapor pressure of 4.5 ± 0.3 × 10−3 pa. The solar cell device constructed using an optimal vapor pressure deposited perovskite film (M3) showed enhanced solar cell characteristics with PCE and SOP of 15.74% and 13.76%, respectively. The strategy used in this work to prepare perovskite films can be highly beneficial for uniform and large scale vacuum deposition for scalable perovskite photovoltaic technology. Replacing solution processed charge transport layers by vapor deposition is the future direction of this work. Acknowledgements The authors would like to thank for financial support from National Natural Science Foundation of China (No. 61721005) and Zhejiang Province Science and Technology Plan (No. 2018C01047). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solener.2019.02.012. References Burschka, J., Pellet, N., Moon, S.J., Humphry-Baker, R., Gao, P., Nazeeruddin, M.K.,
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Vacuum co-deposited CH3NH3PbI3 films by controlling vapor pressure for efficient planar perovskite solar cells
T
V. Arivazhagana, Jiangsheng Xiea, Zhengrui Yanga, Pengjie Hanga, M. Manonmani Parvathia, ⁎ Ke Xiaob, Can Cuib, Deren Yanga, Xuegong Yua, a b
State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China Department of Physics, Zhejiang Science and Technology University, Hangzhou 310027, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Perovskite solar cells Co-evaporation Micrometer grains Vapor pressure Stable output
Physical vacuum deposition (PVD) of organo-metal halide thin film has advantages of uniform and large scale deposition for emerging perovskite solar cell technology. Evaporation of organic halides such as CH3NH3I (MAI) is crucial for obtaining phase pure perovskite thin films. However, controlling the evaporation rate of MAI remains difficult owing to its “vapor-gas” like nature upon sublimation. Here we report, highly crystalline CH3NH3PbI3 (MAPbI3) films with micrometer grains can be prepared by gradually controlling the MAI vapor pressure rather than evaporation rate in co-evaporation method. When integrating an optimal film into a planar heterojunction solar cell, the best device achieved a power conversion efficiency of 15.74% by deploying TiO2 as an electron transport layer. This strategy alleviates the strict calibration of MAI evaporation rate and led to obtain phase pure MAPbI3 films for high performance solar cells.
1. Introduction
substrate temperature have been reported (Chen et al., 2014). On the other hand, controlling the vapor pressure of MAI has been reported as an alternative method to form perovskite films (Hsiao et al., 2016; Zhao et al., 2016). However, sequential deposition is well studied among vacuum deposition methods to obtain high quality films, wherein metal halide is first deposited followed by organic halide sublimation (Chen et al., 2014; Longo et al., 2018; Yang et al., 2015). For instance, a sequential deposition method was reported for high performance solar cells by controlling the partial MAI vapor pressure for about 2 h. Vacuum co-deposition of perovskite films by controlling organic halide vapor pressure has advantages such as reduced processing time with improved device performance, however, not much reported. A few reports based on vapor pressure controlled growth of perovskite films have been reported. Nicolas Tetreault et al (Teuscher et al., 2015) studied various factor on the stochiometric control of MAPbI3 using codeposition. Unlike sequential deposition, systematic investigation on gradual change in organic halide vapor pressure is crucial in co-deposition method with respect to PbI2 evaporation rate. Therefore, research efforts on the evaporation conditions in order to obtain the phase pure as well as large grain perovskite thin films using co-deposition method is needed. Here we employ a vacuum co-deposition method to produce high quality MAPbI3 films through controlling the MAI vapor pressure. We
Perovskite solar cells (PSCs) are stepping ahead successfully towards emerging scalable photovoltaic technology thank to its rapid improvement in the contest of converting sun light into electricity (Burschka et al., 2013; Grancini et al., 2017). Most of the works on perovskite solar cells are based on solution processing techniques (Chiang et al., 2017; Kulkarni et al., 2014; Sidhik et al., 2017) while not much work on physical vapor deposition (PVD) methods (Fan et al., 2016; Momblona et al., 2016; Patel et al., 2017). In fact, PVD methods are preferred for the industrialization of optoelectronic devices including photovoltaic technology owing to its uniform and large scale fabrication. However, for vacuum deposition of organo-metal halide perovskite films, one of the main challenges is to control the evaporation rate of organic halides such as CH3NH3I (MAI) precisely (Ono et al., 2016). Literatures shows that MAI sublimates like a “vapor-gas” in vacuum owing to small molecular weight, making it difficult to control the evaporation rate using conventional quartz microbalance sensor (Gao et al., 2015; Hsiao et al., 2016; Liu et al., 2013). We also noted that, the evaporation rate of MAI is much higher (> twofold) than its actual rate during co– evaporation due to an influence from metal halide’s vapor such as PbI2. In order to make the MAI deposition conditions convenient, additional controllable parameters such as source and
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Corresponding author. E-mail address: [email protected] (X. Yu).
https://doi.org/10.1016/j.solener.2019.02.012 Received 21 September 2018; Received in revised form 25 January 2019; Accepted 7 February 2019 0038-092X/ © 2019 Published by Elsevier Ltd on behalf of International Solar Energy Society.
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demonstrate that highly crystalline MAPbI3 films with micrometer grains can be obtained by gradually controlling the vapor pressure by adjusting MAI source temperature with constant PbI2 evaporation rate. The best solar cell device fabricated using optimum vapor pressure controlled deposition of MAPbI3 films achieved the power conversion efficiency (PCE) and stable output (SOP) of 15.74% and 13.72%, respectively, deploying TiO2 as an electron transport layer (ETL). This strategy alleviates the strict calibration of MAI evaporation rate and can be beneficial for the large scale vacuum co-deposition of perovskite films with phase pure and micrometer grains.
surface morphology of the MAPbI3 films prepared with gradual change in vapor pressure was imaged using scanning electron microscopy (SEM) and is shown Fig. 2(a–d). The surface morphology of the M1 film (Fig. 2a) shows fully covered and uniform small grains due to poor formation of perovskite phase. An agglomerated coverage with cubic shaped grains was observed for M2 (Fig. 2b). Increasing vapor pressure further for M3 led to the formation of highly dense and uniform morphology with the majority of grains observed in micrometer sizes of 0.5–1 µm (Fig. 2c). There are no pin holes or incomplete surface coverage observed during the scan across the several micrometer range (Fig. S2). It has been widely reported that micrometer sized grains with smooth surface morphology are the promising features for high performance perovskite solar cells (Gamliel et al., 2015; Nie et al., 2015). The smooth surface morphology with micrometer sized grains attributes to the best stoichiometric composition of vapor pressure controlled growth of perovskite film. The high vapor deposited M4 film consists of larger and small grains as shown in Fig. 2d. A close look at the M4 film surface shows that those small grains are mostly occupied at the grain boundaries. The possible growth mechanism of the perovskite films at different vapor pressure is shown in Fig. 2e. The formation of perovskite phase is limited by the low vapor pressure for M1 sample, which resulting PbI2 rich domains. The PbI2 domains reduce for M2 with increasing MAI vapor pressure. For M3 film, all the evaporated PbI2 reacted with MAI vapor and thus forming highly crystalline perovskite grains. Further increasing MAI vapor lead to perovskite surface morphology with small grains at the grain boundaries. In order to investigate the crystalline phase quality of the perovskite films, X-ray diffraction was carried out and shown in Fig. 3a. The films M1 and M2 show relatively low intensity due to poor crystallinity. The film M1 shows a dominant PbI2 peak at 2θ = 12.45° which decreases with increasing vapor pressure for M2 as shown clearly in Fig. S3. It is obvious that the film, M3, prepared at high vapor pressure shows the phase pure orientation of MAPbI3 in consistent with surface morphology. The dominant Bragg's reflection at 2θ = 14.08° and 28.32° are corresponding to (1 1 0) and (2 2 0) plane of tetragonal MAPbI3 (Zeng et al., 2017) and are highly crystalline as evident from the intensity and full width half maximum of the peaks. In addition to that a minor peak at 2θ = 43.08° was observed corresponding to (3 1 4) crystallographic plane. There is no change in peak positions observed with increasing the vapor pressure for M4, however, (1 1 0) plane intensity became identical to (2 2 0) plane which could be attributed to the selective preferential orientation of the grains in consistent with Fig. 2d. Further crystallographic studies required in order to investigate an insight about the evolution of MAPbI3 crystallographic orientation at different planes in relation with stoichiometric composition. From the structural and morphological studies, it is obvious that MAI with a source temperature of 175 °C (total vapor pressure of 4.5 ± 0.3 × 10−3 pa) gives the best stoichiometric composition with PbI2 to obtain a phase pure and high quality MAPbI3 films. The optical properties of the MAPbI3 films were done in order to investigate the absorption and charge transport properties. Fig. 3b shows the optical absorption spectra of MAPbI3 thin films prepared at different vapor pressures. The film M1 shows absorption onset at high energy region at around 495 nm in correlation with the exciton absorption of PbI2. The PbI2 absorption peak decreases with an evolution of a MAPbI3 peak at 780 nm for M2 film. Further increasing vapor pressure, the film M3 and M4 display a distinct MAPbI3 absorption peak at 780 nm with an improved absorption in visible region as a result of enhanced crystallinity in consistent with XRD pattern. Fig. 3c shows steady state photoluminescence spectra of MAPbI3 films deposited on glass substrates. All the films show an emission peak centered at around 770 nm is assigned to band to band transition and is well in agreement with previous reports for MAPbI3 films based on solution methods (Nie et al., 2015; Yang et al., 2017). The PL spectrum of M1 film is shown in Fig. S4 to see the poor signal at 780 nm clearly. The intensity of the MAPbI3 increases with increasing vapor pressure until M3 and decreases
2. Experimental The two source materials of MAI and PbI2 were placed in an organic source (silica crucible) and inorganic source (boron nitride crucible), respectively. An organic source was controlled with temperature while the inorganic source was controlled by applying current to the source. TiO2 ETL was deposited on patterned FTO substrates according to the literature (Liu et al., 2013) and loaded above the sources with TiO2 surface facing down. A dedicated PVD setup consist of both organic and inorganic sources is shown in Fig. S1. The chamber was evacuated to the base pressure of 1.5 ± 0.5 × 10−4 pa. Then, MAI source was heated gradually to the desirable temperatures such as 125 °C, 150 °C, 175 °C and 190 °C. Meanwhile, the PbI2 source was heated for the optimal evaporation rate of 1.1 ± 0.1 Å/s. The working total vapor pressure was stabilized to 7.5 ± 0.3 × 10−4 pa, 1.5 ± 0.3 × 10−3 pa, 4.5 ± 0.3 × 10−3 pa and 7.5 ± 0.3 × 10−3 pa for the MAI source temperatures of 125 °C, 150 °C, 175 °C and 190 °C, respectively. In this study, vapor pressure (i.e working pressure during co-evaporation) is the main parameter complemented with source temperature. The rate of evaporation of PbI2 was monitored using an in-situ quartz microbalance sensor. After the vapor pressure stabilized, the source shutters and substrate baffle were opened to expose dual source vapor onto the substrates. The substrate holder was kept at ambient temperature and rotated with 600 rpm in order to ensure the uniform deposition on the substrates. The deposition was carried out until quartz sensor showed 125 ± 5 nm thickness of PbI2 and the time was about 17 ± 1 min. After deposition, the samples were post annealed at 100 °C for 30 min in order to improve the crystallinity and interface contact between the MAPbI3 and TiO2. The solar cell devices were fabricated with the hole transporting layer of Spiro-OMeTAD by spin coating as described in our earlier reports (Xie et al., 2017a) followed by 120 nm silver electrodes with a mask for the active area of 10 mm2. Hereafter, for simplicity, the films and devices based on 7.5 ± 0.3 × 10−4 pa (125 °C), 1.5 ± 0.3 × 10−3 pa (150 °C), 4.5 ± 0.3 × 10−3 pa (175 °C) and 7.5 ± 0.3 × 10−3 pa (190 °C) vapor pressures will be referred as M1, M2, M3 and M4 throughout the article. The materials, methods, characterization techniques used for this study is described in the supporting information. 3. Results and discussions The MAPbI3 thin films were prepared on glass substrates as a function of total vapor pressure with constant PbI2 evaporation rate. Fig. 1a shows the schematic of the co-evaporation method used in this work and Fig. 1b shows photographs of the prepared perovskite films, i.e M1, M2, M3 and M4. The film M1 appears to be yellowish in color, which indicates an insufficient MAI to form perovskite. Further increasing the vapor pressures, a reddish brown1 and block colors were observed for M2 and M3 films, respectively. The slightly grey in color (not clearly visible in photograph) was observed for the film prepared at highest vapor pressure (M4) indicating excess MAI in the film. The 1 For interpretation of color in Fig. 1, the reader is referred to the web version of this article.
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Fig. 1. (a) Schematic of the vapor pressured controlled co-deposition method, (b) photographs of the MAPbI3 thin films prepared at different vapor pressures.
Fig. 2. SEM images of MAPbI3 thin films prepared at different vapor pressures, (a) M1, (b) M2, (c) M3 and (d) M4. (e) The schematic of the formation of perovskite film under different vapor pressures.
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Fig. 3. Structural and optical characterization of the films with different vapor pressure. (a) XRD pattern of films, (b) absorption spectra of the films, (c) steady- state PL spectra and (d) normalized time resolved PL spectra of the films deposited on glass substartes.
fabricated solar cell employing M3 film. The stacked layers of glass/ FTO/c-TiO2/MAPbI3/Spiro-OMeTAD/Ag can be clearly seen. A corresponding sketched band diagram of the device is shown in Fig. 4b. Fig. 5a shows the reverse scan J-V characteristics of solar cells based on M1, M2, M3 and M4 films, and the measured photovoltaic parameters are listed in Table 1. The device based on M1 shows the least PCE of 4.23% and Voc of 0.95 V due to insufficient perovskite phase. Further increasing the vapor pressure, the M2 device performance was improved to a certain extent and yields the PCE of 10.51%. As anticipated from the structural and optical characterization, the device based on M3 film exhibits improved solar cell characteristics with Voc = 1.08 V, Jsc = 21.76 mA/cm2, FF = 66.80% and PCE = 15.74%. The PCE of the M4 film based device decreases to 11.79% as result of excess MAI. The drop in PCE for M4 compared to M3 device is mainly associated with Jsc and FF. The forward and reverse scan of the champion device based on M3 is shown in Fig. S7. The observed hysteresis in Fig. S7 is mainly associated with TiO2 ETL as reported elsewhere (Patel et al., 2017). Fig. 5b shows an external quantum efficiency (EQE) of the devices as a function of wavelength. The device based on M3 exhibits the highest EQE of about 80% in the range of 450–650 nm. We note a step like feature in the EQE spectra which attributes to an optical interference from the layer stack. Fig. 5c shows the stable output at maximum power point tracking, Vmp, for 50 s. The devices based on M1, M2, M3 and M4 were biased at 0.64, 0.66, 0.83 and 0.71 V, respectively. The stabilized
for M4. The high PL intensity of M3 indicates the phase pure quality of the film in accordance with SEM and XRD analysis, which is preferable for optoelectronic devices. Fig. 3d shows the normalized time resolved photoluminescence (TRPL) spectra of MAPbI3 films measured in a 770 nm emission window. The exiton life time of the perovskite films were determined by fitting exponential equation. The life time was calculated as τ = 5, 7, 31 and 15 ns for M1, M2, M3 and M4, respectively. It is well known that the intensity of the PL signal drops when depositing on ETL due to built-in electric field formed between perovskite and ETL which results in mobile carrier extraction (Singh et al., 2018; Xie et al., 2018). Upon depositing the M3 film on TiO2/FTO, the PL intensity of MAPbI3 dropped due to quenching effect which infers an efficient charge extraction into TiO2 as shown in Fig. S5. Fig. S6 shows the TRPL decay of MAPbI3 on TiO2/FTO and glass substrates. For M3/ TiO2, the fast decay was calculated as τ1 = 2.2 ns (using bi-exponential equation) which is close to solution processed perovskite film on TiO2 (Xie et al., 2017b). we attribute this fast charge carrier transfer is due to the micrometer sized grains that promote charge carriers from Perovskite to TiO2 without choosing grain boundaries, and thus reducing surface recombination. The solar cells were fabricated with different vapor pressures grown MAPbI3 absorber and its performance such as open-circuit voltage (Voc), short-circuit current (Jsc), fill factor (FF) and power conversion efficiency (PCE) were tested. Fig. 4a shows a cross sectional view of the 342
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Fig. 4. (a) Cross-sectional SEM view of the solar cell device with M3 film, (b) band diagram of the solar cell device.
but still not close to PCE due to the anomalous hysteresis effect widely observed in TiO2 ETL based solar cells (Snaith et al., 2014). The overall improvement in the J-V characteristics of M3 film based device can be correlated to the better surface morphology and crystallinity of the perovskite films. One of the advantages of the vacuum deposition method is the feasibility to prepare perovskite films with different thicknesses of
power output (SOP) for M1, M2, M3 and M4 were obtained as 2.9, 7.1, 13.72 and 8.1%, respectively. We note that SOP is not much reported on vacuum deposited MAPbI3/TiO2 based devices. Patel et al. (Patel et al., 2017) reported that vacuum deposited MAPbI3/TiO2 based device with the PCE of 15.58% showed significantly lower SOP of < 6% where surface passivation of TiO2 with PCBM was suggested to obtain the SOP identical to PCE. In our work, the SOP is significantly higher
Fig. 5. (a) J-V curves of the devices biased under reverse scan, (b) external quantum efficiency spectra (c) stable output at maximum power point and (d) J-V curves of the solar cell devices as a function of MAPbI3 thicknesses. 343
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Table 1 Solar cell parameters of the devices biased in the reverse direction. Device
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
M1 M2 M3 M4
0.95 1.00 1.08 1.03
8.86 17.50 21.76 20.08
49.98 59.73 66.80 57.11
4.23 10.51 15.74 11.79
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identical stoichiometric composition, which is desirable for different optoelectronic devices. To further investigate the J-V characteristics of the device as a function of thicknesses, we prepared the MAPbI3 films with the different thicknesses range from 290, 350, 430, 550 and 700 ± 10 nm. The total thickness were obtained by varying the equivalent thickness of PbI2 (100, 150, 200 and 250 ± 5 nm) monitored using quartz microbalance sensor during co-deposition. The optimized vapor pressure of 4.5 ± 0.3 × 10−3 pa was fixed constant as used for optimal M3 film. The J-V curves of the devices with different MAPbI3 film thicknesses is shown in Fig. 4d. The device with 290 nm MAPbI3 thickness delivers the PCE of 13.8% which increases further to 15.74% with increasing thickness to 350 nm. The increase in PCE is mainly attributed to the increase in Jsc and Voc due to more photoexcited charge carriers. Further increasing the thickness to 430 nm, the device performance decreases as a result of decrease in Voc and FF. The similar trend continues further increasing the thicknesses to 550 and 700 nm due to internal recombination of charge carriers within the perovskite absorber. Hence, we conclude that 350 ± 10 nm is optimum for vapor pressure controlled growth of Perovskite thin films investigated in this research work. Finally, the stability of the device based on M3 was tested over a period of 15 days stored under ambient condition (∼45% humidity, ∼28 °C temperature) and is shown Fig. S8 together with solution processed MAPbI3 film based device. Both the devices retain ∼84% of its initial efficiency. 4. Conclusions In summary, we have systematically investigated vacuum co-deposition of phase pure MAPbI3 thin films through controlling a gradual change in vapor pressure. This study alleviated a strict calibration of MAI evaporation rate using conventional quartz microbalance sensor to obtain perovskite films. As a consequence, highly crystalline MAPbI3 thin films with micrometer size grains were obtained at the vapor pressure of 4.5 ± 0.3 × 10−3 pa. The solar cell device constructed using an optimal vapor pressure deposited perovskite film (M3) showed enhanced solar cell characteristics with PCE and SOP of 15.74% and 13.76%, respectively. The strategy used in this work to prepare perovskite films can be highly beneficial for uniform and large scale vacuum deposition for scalable perovskite photovoltaic technology. Replacing solution processed charge transport layers by vapor deposition is the future direction of this work. Acknowledgements The authors would like to thank for financial support from National Natural Science Foundation of China (No. 61721005) and Zhejiang Province Science and Technology Plan (No. 2018C01047). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solener.2019.02.012. References Burschka, J., Pellet, N., Moon, S.J., Humphry-Baker, R., Gao, P., Nazeeruddin, M.K.,
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