High Crystallization of Perovskite Film by a Fast Electric Current

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High Crystallization of Perovskite Film by a Fast Electric Current Annealing Process Wei Luo, Cuncun Wu, Weihai Sun, Xuan Guo, Lixin Xiao, and Zhijian Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07775 • Publication Date (Web): 26 Jul 2017 Downloaded from http://pubs.acs.org on July 27, 2017

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ACS Applied Materials & Interfaces

High Crystallization of Perovskite Film by a Fast Electric Current Annealing Process Wei Luo a, Cuncun Wu a, Weihai Sun a, Xuan Guo a, Lixin Xiao a, b, c, *, Zhijian Chen a, b, c, *

a

State Key Laboratory for Mesoscopic Physics and Department of Physics, Peking

University, Beijing,100871, PR China. b

Co-Innovation Center for Micro/Nano Optoelectronic Materials and Devices,

Chongqing University of Arts and Sciences, Yongchuan, Chongqing, 402160, PR China c

New Display Device and System Integration Collaborative Innovation Center of the

West Coast of the Taiwan Strait, Fuzhou, 350002, PR China

KEYWORDS: electric current annealing, crystallization, grain, perovskite film, perovskite solar cell

ABSTRACT High efficiency organic–inorganic hybrid perovskite solar cells have experienced rapid development and attracted significant attention in recent years. The crystal growth as an important factor would significantly influence the quality of perovskite films and ultimately the device performance, which usually requires thermal annealing for 10 min or more. Herein, we demonstrate a new method to get high

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crystallization of perovskite film by electric current annealing for just 5 s. Contrast to conventional thermal annealing, a homogenous perovskite film was formed with larger grains and fewer pinholes, leading to a better performance of device with higher open circuit voltage and fill factor. An average power conversion efficiency of 17.02% with electric current annealing was obtained, which is higher than that of devices with conventional thermal annealing process (16.05%). This facile electric current annealing process with less energy loss and time consumption shows great potential in the industrial mass production of photovoltaic devices.


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1. INTRODUCTION Since first reported by Miyasaka in 20091, organic-inorganic hybrid perovskite solar cells (PSCs) have drawn tremendous attention due to their low-cost, simple fabrication and unique photovoltaic properties including outstanding light harvesting, high charge carrier mobility and long diffusion length2-7. After plenty of studies examining a variety of perovskite compositions8,9 and device architectures10,11, remarkable performance progress of PSCs has been achieved with certified power conversion efficiency (PCE) now exceeding 22%12. The crystallization and morphology of perovskite film have a great effect on the performance of PSCs. Several certified methods have been adopted to fabricate homogenous and dense perovskite films with high device performance, including the two-step sequential deposition13, the vapor co-deposition14, the anti-solvent assisted crystallization15, the gas-assisted method16, the gas-pump drying method17 and so on. For all of these methods, post annealing is needed to accelerate the nucleation and crystallization of perovskite. The most common post annealing process for CH3NH3PbI3 (MAPbI3) perovskite film is thermal annealing at 100 °C for 10 min16-19. Meanwhile, a few studies were focused on the annealing engineering by varying the temperature and time of thermal annealing. Eperon et al. investigated the effect of the annealing temperature on perovskite film quality over the temperature range of 90 °C to 170 °C.20 Liu et al. reported that a higher temperature would produce perovskite film with larger grain size and better photovoltaic performance with a PCE of 12.9% prepared at 200 °C for 6 min.21 Besides, several alternative annealing processes were also used to replace the conventional thermal annealing process. Li et al. introduced laser irradiation as an approach of rapid crystallization of MAPbI3 film, effectively increasing the open circuit voltage (Voc) of planar PSCs.22 Cao et al. reported a fast and controllable crystallization of MAPbI3 film by microwave irradiation, improving the photovoltaic performance with a PCE of 14.91%.23 In this work, we demonstrate a new electric current annealing process for fast crystallization of perovskite film by simply applying a direct voltage on the opposite



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sides of the FTO/SnO2/MAPbI3 films. The MAPbI3 film was annealed quickly by the joule heat generated in the FTO layer, through which the current mainly flowed. For an applied voltage of 30 V, the annealing time was shortened to just 5 s. The fast crystallization process introduced by a large electric current annealing effectively increased the grain size and crystallinity of perovskite, leading to a better device performance with higher Voc and fill factor (FF). For the device structure of FTO/SnO2/MAPbI3/spiro-OMeTAD/Au, an average PCE of 17.02% with electric current annealing was obtained, which is higher than that of the devices with conventional thermal annealing process (16.05%). Furthermore, the electric current annealing process provides heat to the film intensively and uniformly with less energy loss and time consumption, which is more appropriate for the industrial mass production in the future.

2. EXPERIMENTAL SECTION 2.1.

Materials 25 mm × 25 mm × 2.2 mm fluorine tin oxide (FTO) coated glass substrates with

sheet resistance of 14 Ω/sq and FTO thickness of 400 nm were purchased from Wuhan Jinge Solar Energy Technology Co., Ltd. SnO2 colloid precursor (tin(IV) oxide, 15% in H2O colloidal dispersion), anhydrous N, N-dimethylformamide (DMF), anhydrous dimethyl sulfoxide (DMSO) and chlorobenzene were obtained from Alfa Aesar. PbI2 (99.9985%) and spiro-OMeTAD were purchased from Xi’an Polymer Light Technology Co., Ltd. Lithium bis(trifluoromethylsulphonyl)imide (Li-TFSI) and 4-tert-butylpyridine (tBP) were obtained from Aldrich. All these commercially available materials were used as received without any further purification. Methylammonium iodide (CH3NH3I) was synthesized according to the procedure reported previously.24

2.2.

Device fabrication The FTO coated glass substrates were cleaned ultrasonically in detergent,


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deionized water, acetone and isopropyl alcohol sequentially and ultraviolet-ozone treated for 10 min. The SnO2 colloid precursor was ultrasonically diluted by H2O (1:6 volume ratio) for 30 min. The final solution was spin-coated onto glass/FTO substrates at 3,000 rpm for 30 s, then baked on a hot plate at 150 °C for 30 min. The perovskite precursor solution (1.25 M) was prepared by mixing the PbI2, CH3NH3I and DMSO (1:1:1 molar ratio) in DMF and then heated at 60 °C for complete dissolution. After cooling down, the perovskite precursor solution was spin-coated onto the cooled SnO2/FTO substrate at 1,000 rpm for 1 s and 4,000 rpm for 8 s. The coated film was quickly moved to a small sample chamber which pumped immediately for 40 s (below 20 Pa). A brown and transparent film was obtained. 17 For the conventional thermal annealing method, the gas-pump dried film was baked on a hot plate at 100 °C for 10 min. For our new electric current annealing method, two strip electrodes with length of 25 mm were clipped onto the opposite sides of the gas-pump dried film (as seen in Figure 1), and subsequently applied with a direct voltage of 30 V for 5 s, 20 V for 15 s or 10 V for 60 s. The gas-pump dried film was annealed by the joule heat of current flowing through the FTO layer, and turned from brown into black.

Figure 1. Schematic procedure of the electric current annealing process

The hole transport layer was spin-coated with a Spiro-OMeTAD solution at 4,000 rpm. for 40 s, where 1mL Spiro-OMeTAD/chlorobenzene (72.3 mg/mL) solution was employed with the addition of 20 µL Li-TFSI/acetonitrile (520 mg/mL) and 30 µL tBP.



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So far, all processes were performed in ambient atmosphere. Finally, an 80 nm Au back electrode was deposited by thermal evaporation at a pressure of 1×10-3 Pa. The active area was 0.09 cm2.

2.3.

Characterization The current density-voltage (J-V) curves were measured with a scan rate of 20

mV/s under 100 mW/cm2 AM 1.5G simulated illumination using a Keithley 2611 Semiconductor Characterization System with a Newport solar simulator as a light source. Incident photon-to-current conversion efficiency (IPCE) spectrum was observed using a lock-in amplifier (Model SR830 DSP) coupled with a 1/4 m monochromator (Crowntech M24-s) and 150 W tungsten lamp (Crowntech). Scanning electron microscope (SEM) images were collected with a Hitachi S-4800. The X-ray diffraction (XRD) patterns of MAPbI3 films on the glass/FTO/SnO2 substrates were obtained by using a D/MAX-2000 X-ray diffractometer with monochromatic Cu Kα irradiation (l ¼ 1.5418 Å) at a scan rate of 6 °/min. The absorption spectrum of MAPbI3 films on the glass/FTO/SnO2 substrates were measured by an Avates avaspec-2048-2-USB2 fiber optic spectrometer with a 10 W Philips halogen tungsten lamp as the light source. The temperature of MAPbI3/SnO2/FTO/glass substrate was measured by a TES 1310 TYPE-K thermometer. Time-resolved photoluminescence measurements were carried out by a Delta flex Ultrafast Lifetime Spectrometer.

3. RESULTS AND DISCUSSION When we applied a voltage of 30 V, 20 V, 10 V on the two opposite sides of the FTO/SnO2/MAPbI3 films, the corresponding circuit current was 2.2 A, 1.4 A, 0.7 A, respectively, which fits with the formula I=U/R, where R is the sheet resistance of FTO film (14 Ω/sq). To determine the proper electric current annealing time, time-resolved absorption spectrum were observed with a fiber optic spectrometer. As seen in Figure 2, when a voltage of 30 V was applied for 2 s, the absorption of the film is almost the same as that with conventional thermal annealing (100 °C for 10


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min), which indicates that as little as 2 s is enough to form perovskite crystal for the electric current annealing process. The conversion time for 20 V and 10 V are 5 s and 16 s, respectively. Figure S1 shows the time-dependent absorption spectrum of the MAPbI3 film on a SnO2/FTO/glass substrate without any anneal treatment. The film was very hard to turn into black even in a few minutes. Finally, to further remove the solvent and get better crystallization of perovskite, we increased the electric current annealing time to 5 s, 15 s, 60 s for 30 V, 20 V, 10 V voltage applied, respectively.

Figure 2. The time-dependent absorption spectrum of MAPbI3/SnO2/FTO/glass with different electric current annealing voltages: (a) 30 V; (b) 20 V; (c) 10 V

The time-dependent temperature of perovskite/SnO2/FTO/glass substrate was also measured using an electrothermometer during the electric current annealing process (Figure S2). When 30 V voltage was applied for 5 s, the temperature of substrate reached to 112 °C, which is higher than that in conventional thermal annealing process. The final temperature of substrates with 20 V and 10 V treatment was 123 °C and 105 °C. Considering the transmission of heat between the layers and the substrate,



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the real temperature of perovskite layer would be even higher due to its extremely short distance to the FTO layer where a great deal of heat is generated. Figure 3 shows the XRD patterns of the MAPbI3 films with conventional thermal annealing and electric current annealing. All of the patterns exhibit the main peak at 14.1 ° and 28.4 °, corresponding to the (110) and (220) planes of the MAPbI3 crystal.25 It is remarkable that the MAPbI3 films with 30 V electric current annealing displayed a higher intensity and a smaller full width (0.12 °) at half maximum (FWHM) of the (110) diffraction peak (Figure S3) than that with conventional thermal annealing (0.16 °). We estimated the crystal size (D) of perovskite using the formula D=λ/ (FWHM*cos θ), here λ=1.5418 Å is the wavelength of the Cu Kα irradiation and 2θ=14.1 ° is the position of the (110) peak. The calculated crystal size was 74.2 nm for the perovskite film treated with 30V electric current annealing, which is larger than that treated with conventional thermal annealing (55.7 nm). It suggests that the crystallinity of the MAPbI3 film was enhanced with larger electric current annealing26, which is in accordance with previous studies that higher temperature is helpful to get better crystallization of perovskite film.27 By the way, the calculated crystal size is far smaller than the grain size measured by SEM image (600nm, as seen below), indicating that the perovskite grain is not composed of one single crystal.


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Figure 3. XRD patterns of MAPbI3 films on SnO2/FTO/glass substrates based on conventional thermal annealing and electric current annealing with different voltages

To get direct observations of the perovskite films with electric current annealing process, the morphology of MAPbI3 film based on SnO2/FTO substrate was observed by SEM (Figure 4). The perovskite film with conventional thermal annealing has small grains with diameter about 300 nm, along with some pinholes between the grain boundaries. But for the film annealed with a voltage of 20 V or 30 V, larger perovskite grains without pinholes were observed. The size of perovskite grains increased to 600 nm or more, which further proved that fast annealing with a larger current heating process improves the crystallization of perovskite and film quality.

Figure 4. SEM images of MAPbI3 films based on electric current annealing with different voltages: (a) 30 V, (b) 20 V, (c) 10 V and (d) conventional thermal annealing

A time-resolved photoluminescence (TRPL) technique was employed to check trap state density in the perovskite films annealed with different conditions (Figure S4). The spectra of the perovskite films on the SnO2/FTO/glass monitored at the peak emission (770 nm) could be fitted to biexponential decays with a fast component τ1



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and a slow component τ2, while the fitted data are listed in Table S1. The fast decay component could be assigned to the interfacial charge separation property, while the slow component related to the nonradiative recombination caused by the trap state of perovskite film.28 The slower τ2 of fluorescence decay for the 30 V current annealed perovskite film suggesting the reduced trap state density in this fast and high crystallization process. In order to further investigate the feasible of electric current annealing assisted crystallization,

we

prepared

devices

with

the

structure

of

FTO/SnO2/MAPbI3/spiro-OMeTAD/Au. The average photovoltaic parameters of Voc, Jsc, FF and PCE from 24 devices for each group were shown in Figure 5 and summarized in Table 1. The reference devices with conventional thermal annealing showed an average PCE of 16.02%, along with a Voc of 1.06 V, Jsc of 21.50 mA/cm2 and FF of 0.70. In contrast, when current annealed with a voltage of 20 V or 30 V, higher Voc and FF were obtained with PCE increasing to 16.97% and 17.02%, respectively. The best device performance leads to a Voc of 1.10 V, Jsc of 22.27 mA/cm2, FF of 0.75 and a total PCE of 18.25%. We attribute the better performance of electric current annealing devices to the better perovskite film quality. Larger grains and fewer pinholes result in reduced surface defect and charge leakage, leading to improved FF and Voc of devices.29 However, it is worth noting that the devices annealed with a small voltage of 10 V showed inferior performance with all parameters decreasing. Due to the slow heating rate, the solvent was unable to evaporate completely, thus retarding the crystal growth of perovskite.


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Figure 5. The average photovoltaic parameters: (a) Voc, (b)Jsc, (c)FF, (d)PCE of devices based on convention thermal annealing and electric current annealing with different voltages from 24 devices for each group

Table 1. The best and average photovoltaic parameters of devices based on conventional thermal annealing (TA) and electric current annealing (ECA) with different voltages Jsc (mA/cm2)

Voc (V)

FF

PCE (%)

Champion ECA

22.27

1.10

0.75

18.25

Average ECA-30 V

21.58

1.08

0.72

17.02 ± 0.57

Average ECA-20 V

21.72

1.07

0.73

16.97 ± 0.48

Average ECA-10 V

21.09

1.04

0.68

15.12 ± 0.59

Average TA-100 °C

21.50

1.06

0.70

16.05 ± 0.38

It is well known that hysteresis exists commonly in the J-V curves of the planar structure PSCs, which affects the estimate of actual performance to some extent.30 Figure 6a shows the typical J-V curve of the device based on 30 V electric current



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annealing process under both forward and reverse directions with a scanning rate of 20 mV/s. As we can see, almost no hysteresis was observed, which is attributed to both the excellent transport ability of SnO2 31 and the improved quality of perovskite film. Integration of IPCE spectrum in the range of solar emission yields AM 1.5 photocurrents of 20.38 mA/cm2 (Figure 6b), within a 7% mismatch compared with the measured Jsc value. The steady state photocurrent measured at the maximum power point with bias voltage of 0.90 V gives a stabilized PCE of 17.48% (Figure 6c), which is very close to the PCE value of J-V measurement, further demonstrating the actual and stable performance of devices with electric current annealing process.

Figure 6. The performance of the typical device based on 30 V electric current annealing process: (a) J-V curves under different scanning directions with a scanning rate of 20 mV/s; (b) IPCE spectrum along with the corresponding integrated Jsc; (c) steady state photocurrent output and PCE measured at the maximum power point (bias voltage = 0.90 V)

Figure S5 shows the long-term stability of PSCs based on conventional thermal


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annealing and electric current annealing treatment for 40 days. All devices retain above 80% of their initial performance for 40 days when stored in the dark at room temperature in a dry box (humidity < 20%). Compared with the conventional thermal annealing treated devices, the 20V electric current annealing treated devices show a little better stability due to the fewer pinholes and cracks of the perovskite film. We also applied the electric current annealing method to the fabrication of SnO2 layer, which usually requires a post thermal annealing at 150 °C for 30 min. We spin-coated the SnO2 colloid precursor on the FTO substrates without any annealing process followed. After the perovskite precursor was spin-coated and gas-pump dried, we carried out a one-step electric current annealing process for both the electron transport layer (ETL) and the perovskite layer by applying a voltage of 20 V for 20 s on the two opposite sides of perovskite/SnO2/FTO films, then followed with the deposition of hole transport layer and anode layer. As seen in Figure S6, the device with one-step electric current annealing process also showed a high PCE of 17.16%, along with a Voc of 1.07 V, Jsc of 21.98 mA/cm2 and FF of 0.73. The one-step electric current annealing process greatly simplifies the procedure of PSCs without performance reduction, which is a good choice for the mass industrial production of PSCs in the future.

4. CONCLUSIONS In summary, we demonstrate a new electric current annealing method to assist the crystallization of perovskite by simply applying a voltage on the opposite sides of the FTO/SnO2/coated perovskite film. The quick heated process introduced by the large current effectively improve the grain size and crystallinity of the perovskite film, thus leading to the increases of Voc and FF. The average PCE consequently improved to 17.02% from 16.05% with the conventional thermal annealing. What’s more, the annealing time is significantly shortened to 5 s and the heating area is relatively concentrated to the only film, which will greatly decrease the energy consumption and loss. The electric current annealing method is also friendly to the industrial flow



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production, showing a bright future in the application of PSCs.

ASSOCIATED CONTENT Supporting Information Further information relating to the biexponential decay fitted carrier lifetime and TRPL spectra of MAPbI3/SnO2/FTO films treated with convention thermal annealing and electric current annealing with different voltages, the J-V curves and steady state photocurrent output and PCE with one-step electric current annealing based device.

AUTHOR INFORMATION Corresponding Author *Email: [email protected], [email protected]

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Key Basic Research and Development Program of China (Grant No.2016YFB041003) and the National Natural Science Foundation of China (11574009, U1605244, 61575005).


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Controllable Crystallization of Perovskite Films by Microwave Irradiation Process.

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High Crystallization of Perovskite Film by a Fast Electric Current

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