Highly Controlled Codeposition Rate of Organolead Halide Perovskite

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Highly controlled co-deposition rate of organolead halide perovskite by laser evaporation method Tetsuhiko Miyadera, Takeshi Sugita, Hitoshi Tampo, Koji Matsubara, and Masayuki Chikamatsu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07837 • Publication Date (Web): 09 Sep 2016 Downloaded from http://pubs.acs.org on September 10, 2016

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ACS Applied Materials & Interfaces

Highly controlled co-deposition rate of organolead halide perovskite by laser evaporation method

Tetsuhiko Miyadera,* Takeshi Sugita, Hitoshi Tampo, Koji Matsubara, Masayuki Chikamatsu

National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan.

Abstract Organolead-halide perovskites can be promising materials for next-generation solar cells because of its high power conversion efficiency. The method of precise fabrication is required because both solution-process and vacuum-process fabrication of the perovskite have problems of controllability and reproducibility. Vacuum deposition process was expected to achieve precise control; however vaporization of amine compound significantly degrades the controllability of deposition rate. Here we achieved the reduction of the vaporization by implementing the laser evaporation system for the co-deposition of perovskite. Locally irradiated continuous-wave lasers on the source materials realized the reduced vaporization of CH3NH3I. The deposition rate was stabilized for several hours by adjusting the duty ratio of modulated laser based on proportional-integral control. Organic-photovoltaic-type perovskite solar cells were fabricated by co-deposition of PbI2 and CH3NH3I. A power-conversion efficiency of

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16.0% with reduced hysteresis was achieved.

Keywords Organolead halide perovskite, CH3NH3PbI3, solar cells, CW-IR laser deposition, co-deposition, PID control, hysteresis

Introduction Organolead halide perovskite solar cells are developing rapidly

1-4

and have attracted

much attention because of their high power conversion efficiencies (PCE), (e.g. 22% 5). A simple process for the fabrication of these solar cells would be of interest from an industrial perspective, such as the simple mixing of lead halide and amine halide to produce perovskite crystals. High-performance solar cells with reduced cost are expected to be achieved with the use of perovskites. In spite of the rapid development of this field, there have still been some difficulties in the fabrication of high-performance devices with sufficient reproducibility. In addition to the large discrepancy in the characteristics of devices from batch to batch, instability of the photoelectrical characteristics including hysteresis, light soaking, and degradation are important issues that can reduce the reliability of the perovskite solar cells. To overcome such issues, the establishment of a highly controlled fabrication method for perovskites is essential. This could be beneficial for the enhancement of device efficiency, as well as for analysis of the fundamental materials properties of perovskite. Numerous reports have been published on the solution-based fabrication process of


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perovskites and such enthusiastic research has resulted in rapid improvement of the device characteristics. However, control of the solution-based fabrication of perovskites is still difficult and development to date has involved much trial and error. On the other hand, dry processes such as vacuum deposition have been considered to be useful for the controlled fabrication of perovskites. Pioneering work had been conducted by Tsutsui’s group 6 with the combination of various types of lead halide and amine halide. Since the report of a high-efficiency perovskite solar cell by Liu et al. 7, which was based on the co-deposition of lead halide and amine halide, the vacuum deposition method has attracted much attention. However, there are difficulties with the vaporization of amine halide, which has a high vaporization pressure. The vaporization of amine halide causes critical problems such as disturbing the controllability of evaporation rate and causing an inflow of the materials to undesirable regions. Despite the vaporization issue, much effort has been made to develop the vacuum-based fabrication of perovskites, such as those reported by Bolink’s group 8-11, Qi’s group 12-13 and others

14,15

. The maximum PCE for a perovskite solar cell based on vacuum

deposition to date is 16.5%, reported by Lin et al.

16

. Further development of the

vacuum deposition method requires more precise control of the evaporation rate by reducing the gas generation of the amine compounds. Precise control of the deposition rate is initially expected as a benefit of vacuum deposition because fine tuning of the composition and the crystal growth rate enables a fundamental understanding of perovskite crystal growth to be developed, which would lead to further enhancement of device performance. In this paper, we demonstrate a novel system of laser evaporation developed for the co-deposition of perovskite. Here, we note that the laser evaporation system in this



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study is different in principle from the pulsed laser deposition system, which employs an intense pulsed UV laser with single target

17

. In the proposed system, the materials

are heated with a continuous-wave (CW) laser and locally heated materials are evaporated. The method for the CW-laser evaporation of organic compounds was originally developed by Koinuma’s group

18,19

and Matsumoto’s group

20, 21

, whereby

controlled evaporation of organic compounds was demonstrated. We have developed the CW-laser evaporation system for the precise control of the co-deposition of perovskite. Controlled co-deposition of lead halide and amine halide with the reduced gas generation was achieved. The evaporation rate was controlled precisely and quickly by tuning the laser parameters. Proportional integral differential (PID) control was integrated and a stable deposition rate was demonstrated for several hours.

Experimental Section The laser evaporation system (Fig. 1) contains two sets of 808-nm CW semiconductor lasers that irradiate CH3NH3I (MAI) and PbI2 powder target. The laser for MAI evaporation was modulated as a 10 Hz square wave to accomplish precise control and the evaporation rate was adjusted by tuning the duty ratio (D) of the square wave. The deposition rate was monitored with a quartz crystal microbalance (QCM), where the controller was connected to a computer for monitoring and control of the deposition rate. The source materials (MAI and PbI2) were used without purification because sufficiently purified materials were purchased from the supplier (Tokyo Chemical Industry Co. Ltd.) 22. MAI has no absorption of 808 nm light; therefore, silicon powder was mixed into the target to absorb the laser energy. The silicon powder is heated first


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by laser irradiation and then the thermal energy is transferred to the source materials for evaporation. X-ray photoelectron spectroscopy measurements confirmed that silicon was not deposited onto the substrate during the co-deposition process. For device fabrication, glass substrates with indium tin oxide (ITO) transparent electrodes were used after cleaning with acetone and ethanol. Several types of HTL were attempted. For example, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)

and

poly(N-9’-heptadecanyl-2,7-carbazole-alt-5,5-(4’7’-di(thien-2-yl)-2’,1’,3’-benzothiadia zole)) (PCDTBT) layers were spin-coated, and a NiOx layer was deposited by magnetron sputtering. The perovskite layer was formed by co-deposition of MAI and PbI2 using the laser evaporation system. The deposition rate of perovskite was 0.45 Å/s, which was calculated by the thickness of the final film and process time. The substrate is not heated or rotated during the deposition. 6,6-phenyl-C61-butyric acid methyl ester (PCBM) was spin-coated as an electron transport layer and then thermally annealed at 70 °C. Finally, bathocuproine (BCP) as hole transport layer and Al electrodes were evaporated under vacuum to complete the solar cell. Simulated AM 1.5 G 1 sun light (Asahi spectra HAL 320) was irradiated for measurement (Keysight B2902A) of the current density-voltage (J-V) characteristics. The voltage was swept from −0.1 V to 1.1 V with the step of 0.02 V. The delay time of voltage step was 0.4 s; namely, the scan speed was 0.05 V/s. External quantum efficiency (EQE) spectra were measured using a xenon lamp that was integrated with a computer-controlled monochromator, and the source power spectrum was calibrated using a silicon photodiode. The morphology of the perovskite film was observed using SEM (Hitachi S-4800) and the crystallinity was analysed using X-ray diffraction (XRD; Rigaku SmartLab).



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Results and discussion To test the controllability of the laser evaporation, the pulse duty ratio was changed stepwise and the response of the deposition rate was analyzed (Fig. 2). The deposition rate showed exponential time evolution with the change of the duty ratio and stabilized after some first-order delay. When the average delay was estimated by curve-fitting (blue broken line), the first-order delay for MAI was within several tens of seconds. The dead-time delay was less than the integration time (1.0 s) of the deposition rate measurement. The small delay time can be one of the evidence for the achievement of highly controlled deposition. The back pressure (P in Fig. 2) increased with the deposition rate. The pressure is typically within the order of 10-4 Pa, which is rather low compared with that for conventional thermal deposition systems. The typical pressure of conventional thermal evaporation system is 10-3 Pa in our group, whereas even the pressure of 10-1 Pa was reported from another group

13

.

Here, we note that both

sensors detect the MAI component, although sensor 2 is geometrically isolated from the MAI source by a partition. The convection of MAI gas still occurs, even with reduced gas generation. The quick response of MAI to laser evaporation was demonstrated, whereas the stabilization time for the conventional thermal deposition method is typically several tens of minutes (section 3 of supporting information). In the present laser evaporation method, only materials at the laser-irradiation area are evaporated, which would explain the very quick response. In contrast, the thermal deposition method requires a long delay time where the thermal energy from the heater is transported through the crucible to the materials and gradually reaches equilibrium. This can result in non-uniform


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temperature or overshooting that causes the deposition rate to be out of control. Moreover, the long delay time sometimes results in an uncontrolled deposition rate, especially in the case of amine compounds due to changes in the evaporation conditions of the crucibles. Such drawbacks can be eliminated with the laser evaporation method. The negligible dead-time delay and small first-order delay can be advantageous for PID control. An in-house developed PID control system was constructed, where the laser power and duty ratio were adjusted based on the deposition rate to maintain a constant evaporation rate during deposition. The time evolution of the deposition rate is shown in Fig. 3, where the rate was set to a certain value. The deposition rate reached the set value with a small range of overshoot. Deposition was started after the rate was stabilized. A constant deposition rate was demonstrated for several hours with reduced gas generation (Fig. 3). The ratio of the deposition rates of MAI and PbI2 was adjusted to tune the perovskite stoichiometry. The uncertain amount of re-evaporation, especially for MAI, makes it difficult to evaluate the absolute rate of MAI supply. Therefore, co-deposition was attempted several times with different deposition ratios, and X-ray diffraction (XRD) patterns were measured to check the stoichiometry. Figures 4(a) and (b) show examples of deposition rate adjustment for perovskite deposited on NiOx and NiOx/PCDTBT, respectively. The optimized deposition rate can be determined when the XRD pattern indicates a single component perovskite. Here we note that the optimized supply ratio of MAI and PbI2 was different among the substrate materials and very sensitive to the substrate surface conditions. This could be due to differences in the re-evaporation of the materials, especially MAI, which is easily evaporated. Such features were first discovered due to the precise control of the deposition rate by laser evaporation. If



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excess PbI2 is supplied, then a diffraction peak of PbI2 was observed. In the case of NiOx/PCDTBT, an unidentified peak (diamond symbol in Fig 4(b)) was observed at 2θ = 11.3° when excess MAI was supplied. This could possibly be attributed to the delta phase of the perovskite. Wide range XRD pattern of the single-component perovskite film is shown in Fig. 4(c), where substrate diffraction peaks are detected (denoted as “*”). Detailed investigation of the film growth with different deposition rates and the influence of the surface condition are in progress, where the preliminary results are described in the section 4 of supporting information. Figure 5 shows the surface morphology and cross-section of the perovskite film observed using scanning electron microscopy (SEM). A relatively smooth surface with a perovskite grain size of approximately 50 nm was observed, where root mean square surface roughness is 5-10 nm. However, the size of the grains in the bulk region was several hundred nanometers. Similar tendency can be observed in ref. 11. The morphologies of perovskite fabricated with laser evaporation method is different from those fabricated with solution process, where typical crystal habit is often observed for the solution-process films. The crystal growth for solution process is dominated by the vaporization of the solvent and solubility for the solvent, where the anti-solvent addition accelerate the crystallization to obtain smooth surface morphologies. On the other hand, the crystallization for vacuum process is generally dominated by the supply rate of the source materials and growth temperature. In this study, relatively small grain and very smooth surface was obtained by laser evaporation especially for the surface region. The peculiar crystal growth may be due to the frequency of the nucleation and the growth speed of the crystal. Such factors can be varied with the amount of re-evaporation of the materials, which may be different between substrate surface and bulk region and also


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influenced by the surface condition of the substrate. The difference in morphology between the surface and bulk regions could be a subsequent research topic, as well as elucidation of the crystal growth mechanism. Perovskite solar cells were fabricated based on the organic photovoltaic (OPV)-type architecture with the co-deposition of PbI2 and MAI using laser evaporation system. Several hole-transport-layer (HTL) materials were attempted, such as PEDOT:PSS, PCDTBT, and NiOx, while PCBM was used as the electron transport material. Firstly, solar cells were fabricated by the co-deposition of PbI2 and MAI on the HTL of PEDOT:PSS/PCDTBT. J-V characteristics with reduced hysteresis were observed with a PCE of 14.19±0.27%, where average and standard deviation was calculated with forward and backward scan of three cells on the same substrate. Co-deposition condition was different among the substrate materials and surface conditions. The growth process of the perovskite on NiOx seems to be rather complicated and tuning of the deposition ratio was difficult. Although the use of NiOx is expected to obtain high Voc considering the energy level alignment, the device with the NiOx HTL had poor efficiencies. Further effort will be required for the direct deposition of the perovskite on NiOx. Next, a combined HTL of NiOx/PCDTBT was attempted. The device provided the best efficiency in this study, where the PCE was 15.7% and 16.0 % for forward and backward scans, respectively (Fig. 6). The average for three cells on the same substrate was 15.57±0.38% (average of forward and backward scans). We also have constructed the deposition system based on thermal evaporation and constructed the device with a PCE of 15.4 % (section 3 in supporting information). Hence, the film quality and device performance can be similar between thermal evaporation and with laser evaporation. The important benefit of laser evaporation can be the controllability of the deposition



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rate, which is crucial to accomplish accurate experiment with sufficient reproducibility. It should be noted that a significant reduction in hysteresis was observed in this study. The use of the OPV-type architecture has been reported to be beneficial for the elimination of hysteresis. In addition, we found that the OPV-type devices with high efficiency, e.g. more than 13%, tended to show reduced hysteresis. Thus, highly controlled perovskite films or interfaces may be important for the reduction of hysteresis. Further quantitative analysis will be required to clarify the essential mechanism of the hysteresis phenomenon. Quantitative experiments using laser evaporation could be promising for fundamental analysis of organolead halide perovskites, where precise control of the deposition conditions is difficult by conventional deposition methods. We have already reported spectroscopic ellipsometry analyses on perovskite films fabricated by laser evaporation 23,24

. Understanding the perovskite growth mechanism under vacuum is an urgent issue

to realize controlled device fabrication. On the other hand, although laser evaporation is useful for quantitative experiments, the relationship between the stoichiometry and device performance has not yet been clarified. The sample that exhibited the best efficiency in this study contained a certain amount of PbI2 component. In terms of the solution process, there has been one report where an excess amount of PbI2 resulted in higher performance 25. Further study is required to clarify this issue from the standpoint of dry processing. Another important point is the energy level alignment between the carrier transport layer and the perovskite. In this study, although the same PCDTBT HTL was used, different Voc were obtained for different underlying layers (PEDOT:PSS and NiOx). The energy level alignment between various HTLs (including the underlying layer) and the perovskite, and its effect on Voc can be complicated and is expected to be


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revealed by photoelectron spectroscopy studies. Finally, from a technical point of view, the detailed mechanism of MAI vaporization in vacuum chamber should be clarified. Precise deposition control by laser evaporation opens a way to reveal the fundamental mechanism of device operation and perovskite formation.

Conclusions A laser evaporation system was developed for the co-deposition of lead halide and amine halide. The vaporization issue with MAI is significantly reduced and significant improvement of deposition rate control was demonstrated. Precise control of the deposition rate enabled quantitative experimentation on the co-deposition of perovskite, which has been difficult to achieve with conventional methods. Based on the proposed method, we have developed a fabrication method for solar cells and achieved a PCE of 16%. The laser evaporation method is thus expected to be a promising method both for fundamental analysis and practical device fabrication.

Supporting Information Supporting Information Available: Details of experiment and analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

Author information Correspondence

and

requests

for

materials

should

be

addressed

to

T.

M.

([email protected]). Author contributions: T. M. designed and constructed the laser evaporation system. T. S. fabricated the perovskite and device. H. T. supported the deposition of NiOx. T. M. conceived the idea and



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designed the experiment with the advice of T. S., H. T., K. M. and M. C. The manuscript was written by T. M. and edited, corrected and approved by all of the contributors. The authors declare no competing financial interests.

Acknowledgements This paper is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO). This work was supported by the Japan Science and Technology Agency (JST) through its funding program for Precursory Research for Embryonic Science and Technology (PRESTO). The authors would like to thank Mr. K. Nisihkawa to help the experiment.

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Figure captions: Fig. 1. Schematic diagram of the laser evaporation system, which consists of the following components: (1) optical fiber, (2) lens, (3) target plate for amine halide, (4) target plate for lead halide, (5) substrate, (6) QCM sensor, (7) shutter.

Fig. 2. Time evolution of the deposition rate of MAI by changing the duty-ratio (denoted as D [%]) of the laser pulse. Both sensor 1 and sensor 2 detected MAI. Data are taken with the interval of 1.3 s and represented as the average of 10 data. The spike peaks are the noise of the sensor. Blue broken line is the fitting results using exponential functions. The change of the back pressure (denoted as P [Pa]) is also shown.

Fig. 3. Time evolution of the deposition rate during the co-deposition of MAI and PbI2 controlled by PID. Data are taken at 1.3 s intervals and the average of 10 data are presented. The spike peaks are sensor noise. Sensor 1 detects MAI while sensor 2 detects both MAI and PbI2. The change of the back pressure (denoted as P [Pa]) is also shown.

Fig. 4. XRD patterns of MAPbI3 (40 nm) fabricated on (a) NiOx (b) NiOx/PCDTBT with different deposition rates. R1 [Å/s] and R2 [Å/s] denote the deposition rates from sensor 1 (MAI) and sensor 2 (MAI + PbI2), respectively. (c) XRD pattern of MAPbI3 (200 nm). The peaks denoted as “*” are diffractions from ITO.

Fig. 5. (a) Surface and (b) cross-sectional SEM image of MAPbI3 fabricated by laser evaporation.

Fig. 6. (a) Schematic diagram of the device structure. (b) J-V characteristics of the device with best



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

PCE. (c) EQE spectra of the same device.

Table caption: Table 1. Solar cell parameters for the typical devices. The FWD denotes forward scan and BWD denote backward scan.


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Table 1. Solar cell parameters for the typical devices. The FWD denotes forward scan and BWD denote backward scan.

PEDOT:PSS/ PCDTBT

NiOx/ PCDTBT

Average Best Average Best

FWD BWD FWD BWD FWD BWD FWD BWD

Jsc

Voc

[mA/cm2]

[V]

18.5 ± 0.3 18.1 ± 0.3 18.4 18.0 19.3 ± 0.3 19.1 ± 0.3 19.5 19.3

FF

1.05 ± 0.01 0.72 ± 0.01 1.05 ± 0.01 0.76 ± 0.01 1.05 0.73 1.05 0.76 1.09 ± 0.01 0.73 ± 0.00 1.09 ± 0.01 0.76 ± 0.00 1.09 0.74 1.09 0.76


PCE [%] 14.0 ± 0.2 14.4 ± 0.05 14.2 14.5 15.4 ± 0.4 15.8 ± 0.3 15.7 16.0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Fig. 1

Thickness monitor (1)

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Thickness monitor

(2)

Sensor 1 (6)

(5)

Sensor 2 (7)

Laser oscillator

(3)

(4)

Vacuum pump


Laser oscillator

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Fig. 2

1.0

Sensor 1 Sensor 2 Fitting

Rate [Å/s]

0.8 0.6 0.4 0.2

P [Pa]

0.0 10-3

D [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


10-4

8

4 0 0

200

400 600 Time [s]

800 1000



Fig. 3 Rate [Å/s]

Sensor 1 Sensor 2

1.5 1.0 0.5

P [Pa]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

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0.0 10-3 10-4 0

2000

4000 6000 Time [s]


8000

Intensity [A. U.]

Fig. 4

(a)

MAPbI3[110] PbI2[001]

R1 : R2

0.35 : 1.0 0.32 : 1.0 0.28 : 1.0

(b)

MAPbI3[110] R1 : R2 0.17 : 1.0 0.15 : 1.0 0.10 : 1.0 0.05 : 1.0

0.22 : 1.0 10

12

(c)

14 16 /2 [°] [110]

Intensity [A. U.]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


Intensity [A. U.]

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18

10

12

14

18

[220] [114] *

*

* *

10

16

/2 [°]

20

30 /2 [°]


40

* 50


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Fig. 5

(a)

(b) Perovskite Polymer ITO


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(a)

Fig. 6 (b)

Al BCP PCBM Perovskite CH3NH3PbI3 PCDTBT NiOｘ ITO Glass

PCBM

PCDTBT

n

20 15 Light (FWD) Light (BWD) Dark

10 5 0 -5

(c)

100

100 80

80

60

60

40

40

20

20

0

400

600 800 Wavelength [nm]

0 1000


Current density [mAcm -2]

0.0 0.2 0.4 0.6 0.8 1.0 1.2 Voltage [V]

EQE [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


Current density [mAcm-2]

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Graphical Abstract


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Highly Controlled Codeposition Rate of Organolead Halide Perovskite

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