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Fast and Controllable Crystallization of Perovskite Films by Microwave Irradiation Process Qipeng Cao, Songwang Yang, Qianqian Gao, Lei Lei, Yu Yu, Jun Shao, and Yan Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01558 • Publication Date (Web): 10 Mar 2016 Downloaded from http://pubs.acs.org on March 17, 2016

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ACS Applied Materials & Interfaces

Fast and Controllable Crystallization of Perovskite Films by Microwave Irradiation Process Qipeng Cao,a,b Songwang Yang,*,a Qianqian Gao, a,b Lei Lei, a,b Yu Yu,a,b Jun Shao,a,b Yan Liu*,a a

CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics,

Chinese Academy of Sciences, 588 Heshuo Road, Shanghai, 201899, P. R. China. b

University of Chinese Academy of Sciences, Beijing 100039, P. R. China.

*S.W.Y.: e-mail, [email protected]. *Y.L.: e-mail, [email protected].

ABSTRACT: The crystal growth process significantly influences the properties of organic−inorganic halide perovskite films along with the performance of solar cell devices. In this letter, we adopted the microwave irradiation to treat perovskite films through a one-step deposition method for several minutes at a fixed output power. It is found that the specific microwave irradiation process can evaporate the solvent directly and heat perovskite film quickly. In comparison with the conventional thermal annealing process, microwave irradiation process assisted fast and controllable crystallization of perovskite films with less energy-loss and timeconsumption and therefore resulted in the enhancement in the photovoltaic performance of the corresponding solar cells. KEYWORDS: microwave irradiation, perovskite film, solvent evaporation, fast crystallization, controllable grain, solar cell

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INTRODUCTION Since the first introduction in the mesoscopic solar cells as light harvesting materials in 2009,1 Organic−inorganic halide perovskite materials (eg.CH3NH3PbI3) have raised global attention for their excellent properties: cheap, tunable direct band gap,2 excellent absorption coefficient,3 extremely long-range balanced electron and hole transport lengths,4 and easy process from solution to thin films at low temperature.5, 6 In less than 7 years, perovskite solar cells (PSCs) have received an unprecedented and excellent rise in efficiency, which has exceeded 20%,7 and the device stability has also been improved by varying the composition8,

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of the perovskite

materials and altering the architecture10, 11 of perovskite solar cells. It is believed that the performance of perovskite solar cells strongly depends on the morphology12, 13 and composition2, 8of perovskite films, which highlights the work done by the deposition engineering and annealing engineering of perovskite films. Through all these years, the deposition engineering of perovskite films has been developed rapidly to get homogeneous perovskite films, including one-step solution method,5 two-step solution method6 and solid−gas interaction method.14 Meanwhile, the annealing engineering of perovskite films has also been investigated a lot based on conventional thermal annealing system. At first, the research work was normally focused on varying the temperature and time of the thermal annealing process to remove solvent and make perovskite films well crystallized without additional PbI2. The most common annealing process for CH3NH3PbI3 films13 is thermal annealing at 100 oC for 10 min. Subsequently, several alternative thermal annealing processes were also developed to further improve the quality of the perovskite films, including high temperature thermal annealing process15 and multi-step thermal annealing process.16 Very recently, some post-annealing


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methods were developed to reconstruct the uncontrollable morphology of perovskite film, such as hot-pressing process17 and solvent annealing process.18 Herein, we introduce a novel annealing method, adopting microwave irradiation to anneal perovskite films. Microwave irradiation process is a non-contact, rapid, and selective heating method, which is already applied in organic photovoltaic.19, 20 We systematically investigated the crystallization behavior of perovskite films under microwave irradiation as well as the corresponding photovoltaic performance. It is found that perovskite films treated by microwave irradiation process can get fast and controllable crystallization with less energy-loss and timeconsumption, which combines the advantages of thermal annealing process and post-annealing process. Moreover, the photovoltaic performance of the corresponding solar cells was also enhanced.

EXPERIMENTAL SECTION Anhydrous N,N-dimethylformamide (DMF) was obtained from distilling DMF (SCRC, 99.5%) at 153 oC. As-prepared perovskite powder. The perovskite precursor solution was prepared as reported by Ahn et al.5 461 mg of PbI2 (Sigma-Aldrich, 99%), 159 mg of CH3NH3I (TCI, 98%), and 78 mg of N,N-dimethylsulfoxide (DMSO, Sigma-Aldrich, 99.9%) (molar ratio 1:1:1) were mixed in 600 mg of anhydrous DMF solution with stirring at room temperature for 1 h. Then diethyl ether was added to the solution, and the precipitation was filtered, which was subsequently used for microwave irradiation experiment. The as-prepared perovskite powder might contain PbI2, CH3NH3I, DMSO, DMF and diethyl ether.


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PbI2-CH3NH3I adduct. 461 mg of PbI2 (Sigma-Aldrich, 99%), and 159 mg of CH3NH3I (TCI, 98%), (molar ratio 1:1) were mixed in 600 mg of anhydrous DMF solution with stirring at room temperature for 1 h. Then diethyl ether was added to the solution. The precipitation was filtered, and dried in vacuum oven for 1 h. It was used for the subsequent microwave irradiation experiment. PbI2-CH3NH3I adduct should contain PbI2 and CH3NH3I, without DMSO, DMF or diethyl ether. Solar Cell Fabrication. FTO glasses (Pilkington, 8Ω/sq) were ultrasonic cleaned successively in cleaning regent, deionized water, acetone and ethanol in an ultrasonic bath for 30 min. Then, the TiO2 blocking layer (bl-TiO2) was deposited on the substrates by sol-gel method. The sol was prepared by mixing solution A (titanium tetraisopropoxide, ethanol, acetylacetone) and solution B (ethanol, HCl, H2O).The sol was spin-coated onto the substrate at 3000 rpm for 20 s, which was further sintered at 510 oC for 30 min. The substrates were further treated with diluted 40 mM TiCl4 (Aladdin, 99%) solution at 70 oC for 40 min, cleaned with deionized water and then sintered at 510 oC for 30 min. The mesoporous TiO2 (mp-TiO2) was deposited on the bl-TiO2 by spin-coating a diluted TiO2 paste (20 nm TiO2, 3%wt) at 3000 rpm for 20 s, which was further sintered at 510 oC for 30 min. The perovskite precursor solution was prepared as reported by Ahn et al.5 461 mg of PbI2 (Sigma-Aldrich, 99%), 159 mg of CH3NH3I (TCI, 98%), and 78 mg of DMSO (Sigma-Aldrich, 99.9%) (molar ratio 1:1:1) was mixed in 600 mg of anhydrous DMF solution at room temperature with stirring for 1 h. The perovskite precursor solution was spin-coated on the mp-TiO2 layer at 5000 rpm for 20 s and 0.5 ml of diethyl ether was dripped on the rotating substrate at the 6th second. And then they were treated with different annealing processes. For the microwave irradiation process, the samples were put on a Teflon shelf, and then placed in the microwave oven (G80w23csl-A6, Galanz) for serval minutes (1-5


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min) at an output power of 160W. For conventional thermal annealing process, the sample was placed on a hotplate at 100 oC for 10 min. 15uL of hole transport material (HTM) solution consisted of 72.3 mg of spiro-MeOTAD (Merck), 17.6 µL of 4-tert-butyl pyridine (SigmaAldrich) and 21.8 µL of lithium bis(trifl uoromethanesulfonyl)imide (Li-TFSI) solution (520 mg of Li-TSFI (J&K scientific Ltd.) in 1 ml of acetonitrile (Merck)) in 1ml of chlorobenzene, was spin-coated on top of perovskite film at 4000 rpm for 30 s. Thermal evaporation (DZS-50, SKY Technology Development Co., Ltd.) was used to deposit a 120 nm of Ag film as a counter electrode at a constant evaporation rate of 0.2 nm/s. The device fabrication was conducted in dry room with the humidity level of less than 1% except for the evaporation of Ag electrode. Characterization. The temperature of perovskite films was measured with infrared radiation thermometer (AS0388, AS ONE). In order to avoid the transmission of the infrared ray, the perovskite films were formed on the 3M black friction tape, which was stuck on the FTO Glass. X-ray diffraction (XRD) measurements were performed with Ultima IV X-ray diffractometer using Cu Kα radiation under operation condition of 40 kV and 40 mA at room temperature in the 2θ range of 5-60°, with a scanning speed of 4° min-1. The scanning electron microscope (SEM) images of the perovskite films were obtained using FEI Magellan 400. Ultraviolet-visible absorption spectroscopy (UV-Vis) was recorded on a Shimadzu UV-2550PC spectrometer. Steady photoluminescence spectroscopy (PL) was recorded on a Horiba-Ltd FluoroMax-4 device with an excitation wavelength of 457 nm. All the perovskite samples were deposited onto the films of meso-TiO2 and bl-TiO2 on FTO glass as described above. The current density–voltage (J-V) curves were measured under standard AM 1.5G illumination of 100 mW cm-2 with a Keithley-2420 source meter in combination with a Sol3A class AAA solar simulator IEC/JIS/ASTM equipped with

a 450 W Xenon lamp. The exact light intensities of the


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measurements were determined using a calibrated silicon solar cell (Oriel-91150) as reference. The ‫ܬ‬-ܸ curves were obtained by applying an external voltage bias with a scan rate of 40 mV s-1. Incident photon-to-current conversion efficiency (IPCE) curves were measured from 300−900 nm using a Xe source. And the signal was recorded as a function of the wavelength using a SM250 system (Bunkoh-keiki, Japan) and calibrated using a standard Si photodiode (S13371010BQ).

RESULTS AND DISCUSSION As we all know, microwave irradiation21,

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process needs a specific material to have

appropriate dipolar polarization and ionic conductivity to absorb microwave energy. Thus, in order to clarify the annealing mechanism of perovskite films under microwave irradiation, as a preliminary experiment, two kinds of the perovskite precursor powders were evaluated. One contains solvent residue, the other without. If the precursor powder contains solvent residue, here we called it as-prepared perovskite powder, which might contain PbI2, CH3NH3I, DMSO, DMF and diethyl ether. If there was no solvent residue, we called it PbI2-CH3NH3I adduct. Both of them were treated by 800W microwave irradiation for several minutes, and then characterized by X-ray diffraction (XRD) measurement. When the microwave irradiation time increasing from 0 min to 2 min, the as-prepared perovskite powder with the solvents turned from white (Figure 1a) to yellow (Figure 1b) and finally turned into black (Figure 1c). The black one was identified to be CH3NH3PbI3 by XRD measurements (Figure S1a), indicating CH3NH3PbI3 was formed under microwave irradiation. It demonstrates that microwave irradiation has the ability to anneal perovskite films. However, Figure 1 (d, e) shows that the PbI2-CH3NH3I adduct didn’t change color under microwave irradiation, indicating CH3NH3PbI3 was not formed. The result was


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further confirmed by XRD measurements seen in Figure S1b, and there was no detectable peaks ascribed to CH3NH3PbI3. It implies that the solvent residue in the as-prepared perovskite powder plays an important role in annealing perovskite films under microwave irradiation. If there was no solvent residue, perovskite can’t be formed. The solvents may absorb the microwave energy and then transfer it to the perovskite powders, and thus they are heated. To identify the role of the exact solvent on perovskite films under microwave irradiation process, PbI2-CH3NH3I adducts added with different solvents were investigated, including DMF, DMSO and diethyl ether, respectively. As can be seen in Figure 1e, PbI2-CH3NH3I adduct added with diethyl ether didn’t change color under microwave irradiation, indicating diethyl ether could not anneal perovskite films under microwave irradiation. Meanwhile, PbI2-CH3NH3I adduct added with DMF or DMSO turned from yellow to black (Figure 1f, g), indicating that DMF or DMSO could absorb microwave energy and then anneal the PbI2-CH3NH3I adduct. The solvent (DMF, DMSO) can convert microwave energy to heat,22 and then it evaporates quickly which may facilitate the formation of perovskite materials. On the other hand, the solvent may also directly anneal the PbI2-CH3NH3I adduct by conducting heat to it, and help the formation and crystallization of the corresponding perovskite materials. In addition, according to the literature,23 TiO2 and FTO on the substrates could also convert microwave energy and then conduct heat to perovskite film. And considering that the residual solvent in the as-prepared perovskite film would be little and evaporate quickly under microwave irradiation, the heating effect of the solvent may be neglected in contrast with that of the mesoTiO2 and bl-TiO2 coated FTO substrates. The heating effect of the substrates was investigated by measuring the temperature of perovskite films soon after treated by microwave irradiation process. As shown in Figure 2a, the temperature of perovskite films rises gradually from 70 oC to


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124 oC when the time of microwave irradiation increases from 1 min to 4 min, and then it rises slowly to the maximum temperature (~130 oC) as the time gets longer. On the basis of the above investigation, a model in Figure 2b was proposed to illustrate the annealing mechanism of perovskite films under microwave irradiation. Firstly, the solvent (DMF, DMSO) in the as-prepared perovskite films can directly absorb microwave energy and then evaporate quickly. The evaporation of the solvents may also help the crystallization of the perovskite films. Secondly, TiO2 and FTO on the substrates could convert microwave energy into heat, and then conduct it to perovskite films quickly. Therefore, we believe that perovskite films would get rapid and controllable crystallization with less energy-loss and time-consumption under appropriate microwave irradiation treatment. Figure 2c illustrates the preparation procedures of perovskite films under microwave irradiation process and conventional thermal annealing process. First, the as-prepared perovskite films were formed based on the study of Ahn, et al.5 After that, the samples were transferred onto a Teflon shelf (Figure S2), and the samples were made erect to accommodate more samples in the Teflon shelf. The Teflon shelf loaded with the samples was subsequently placed in the microwave oven (160 W) for several minutes (1-5 min). The Teflon shelf (microwave-transparent material) was used to avoid the direct contact between the FTO glass and the base plate of the microwave oven, which could break down the FTO glass due to uneven heating. For comparison, the as-prepared perovskite films based on the same method were also annealed on a hotplate at 100 oC for 10 min.13 The thicknesses of these different perovskite films were almost the same due to the same spin-coating technique. Figure 3 shows the XRD patterns of perovskite films deposited on meso-TiO2 and bl-TiO2 coated FTO substrates treated by microwave irradiation for 1, 2, 3, 4, 5 min and thermal


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annealing at 100oC for 10 min, respectively. As illustrated, all the perovskite films exhibit strong diffraction peaks at 14.1°, 28.4° and 31.9°,which can be indexed to the (110), (220) and (310) planes of the CH3NH3PbI3 crystal (β-phase).24,

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The pure CH3NH3PbI3 crystal phase of

perovskite films formed under microwave irradiation process for 1 min, indicating that the time of solvent evaporation is less than 1 min. And this direct and fast evaporation of the solvent accelerates the crystallization of the perovskite film. Moreover, with the increase of microwave irradiation time from 1 min to 3 min, the full widths at half maximum (FWHM) of the diffraction peaks gets narrower, which indicates the crystallinity of the perovskite film is getting higher. However, when the time of the microwave irradiation process is longer than 3 min, the crystallinity of perovskite films descends and an additional peak at 12.6° related to PbI2 (001)26 can be observed in the XRD pattern, indicating the partially decomposition of the perovskite film. This is ascribed to the exorbitant temperature (~126 oC) of the samples resulting from improper microwave irradiation time. At this exorbitant temperature, the perovskite films start to decompose,27 which also leads to the descent of crystallinity. Overall, it is obvious that the optimum microwave irradiation treating time is 3 min, which could achieve the highest crystallinity of the pure perovskite films. Scanning electron microscopy (SEM) was used to investigate the influence of the microwave irradiation treatment on the morphology of perovskite film formed on the meso-TiO2 and bl-TiO2 coated FTO substrates. As can be seen in Figure 4, homogeneous perovskite coverage was formed under microwave irradiation process, and the mean grain sizes under different microwave irradiation time were estimated according to the statistical method, involving the counts of the number and each grain size of the grains in a given area. As shown in Figure 5, the grain size of perovskite films showed the trend of linear increase when the time of microwave


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irradiation treatment varied from 1 min to 5 min, growing from ~160 nm to ~422 nm. It is obvious that the grain size of perovskite film can be controlled by altering the time of microwave irradiation treatment. That is attributed to the specific microwave dielectric heating manner which can control the heating rate and temperature of the perovskite film instantaneously. As the growth rate of grains is sensitive to the heating rate and temperature, we can control the crystallization of perovskite film by precisely altering the power and time of microwave irradiation. Additionally, Figure 4d and 4e show that some grain boundaries shrank, which looked like cracks between the grains. This phenomenon is in accordance with the partially decomposition of perovskite films seen in the XRD patterns (Figure 3). Furthermore, as shown in Figure 5, if we plot the mean grain size and the microwave irradiation time, it is linear fitting except one point at 1 min. Perovskite film formed with microwave irradiation process for 1 min got larger grain size (~160 nm) than that deduced from the linear fitting (~90 nm). This exceptional larger grain size can be explained by the solvent evaporation effect at the initial stage under microwave irradiation. The solvent (DMF, DMSO) in the as-prepared perovskite film can directly absorb microwave and then evaporate quickly, which accelerates the crystallization of the perovskite films. Moreover, the grains in the perovskite film formed with microwave irradiation treatment looked not as dense as that with thermal annealing method at 100 oC for 10 min. This morphological difference is also ascribed to the solvent evaporation effect under microwave irradiation. The fast evaporation of the solvent under microwave irradiation leads to the volumetric growth of perovskite grains. In contrast, the conventional thermal annealing process transfers energy into the as-prepared perovskite film from bottom to up, which retards the evaporation of the solvent and makes the grains grow gradually from bottom to up.


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In order to understand the difference between the perovskite films under different annealing processes, we further characterized the films by using ultraviolet-visible (UV-vis) and steady state photoluminescence (PL) spectroscopy. Figure 6a shows the UV-vis absorption spectra of perovskite films annealed by microwave irradiation for 1, 2, 3, 4, 5 min and thermal annealing at 100oC for 10 min, respectively. All the samples exhibit a typical absorption spectrum of the perovskite material (CH3NH3PbI3),28, 29 with little difference in their absorption intensities. It is believed that the difference of the films in absorbance can be attributed to their different coverages on the substrates at the same thickness. The UV-vis absorption spectra demonstrate that the perovskite films annealed by microwave irradiation process possess a homogeneous converge, which is in agreement with the results of SEM images. The steady state PL spectra for perovskite layers treated by microwave irradiation for 1, 2, 3, 4, 5 min and thermal annealing at 100oC for 10 min were measured, respectively. As shown in Figure 6b, all the samples show typical sharp PL peaks centered at ~765 nm, corresponding to the excitation wavelength of 457 nm, which is in agreement with the previously reported data.18, 28

It is found that perovskite film under microwave irradiation process for 3 min has the lowest

PL intensity. Generally speaking, the PL emission is related to the recombination channel, such as the bandgap and trap state.30, 31 When increasing the time of microwave irradiation treatment from 1 min to 3 min, the crystallinity gets higher and the grain size gets larger, resulting in the lower defect density along with the decreased PL intensity. And interestingly, when the time of microwave irradiation treatment is longer than 3 min, the PL intensity increases even though the grain continues to grow up, revealing a relatively higher trap density around the band-edge. It can also be verified by the XRD and SEM results, in which the crystallinity descends and the cracks emerge when microwave irradiation time is longer than 3 min.


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We adopted the most common configuration of perovskite solar cells32 ( FTO/bl-TiO2/mesoTiO2/CH3NH3PbI3/spiro-OMeTAD/Ag) to evaluate the influence of different annealing processes on device performance. Solar cells fabricated from the perovskite films annealed by microwave irradiation for 1, 2, 3, 4, 5 min and thermal annealing at 100oC for 10 min were measured under standard illumination condition (AM 1.5G, 100mW/cm2), respectively. Figure 7a shows J–V curves (reverse scan) of the best performance of the devices, and the detailed photovoltaic parameters can be found in Table 1 and Figure S3. It is obvious that the device treated by microwave irradiation for 3 min has the highest short-circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF) and a power conversion efficiency (PCE) of 14.91%. As the time of microwave irradiation increases from 1 min to 3 min, the grain size gets larger and the crystallinity gets higher, which leads to the increase of the Jsc, Voc, FF and PCE of the devices., However, when further increasing microwave irradiation time from 3 min to 5 min, CH3NH3PbI3 partially decomposes to PbI2 and CH3NH3I, leading to the descent of crystallinity along with the emergence of cracks. Though the grains continue to grow, the Jsc, Voc, FF and PCE of the devices decrease. We believe that by reducing the output power of microwave irradiation, the heating rate and maximum temperature of perovskite films could be adjusted. The photovoltaic performance of the devices would be improved if the morphology of perovskite films is more desirable. Furthermore, as shown in Figure 7b, the steady-state current measurement of the device based on microwave irradiation process for 3 min gives a stabilized output power of 13.4%, which is close to the PCE value by J–V measurement. Meanwhile, the forward and backward scans of the device with microwave irradiation process for 3 min can be found in Figure S4, which shows a little hysteresis.


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As compared with the devices based on conventional thermal annealing process, the devices with the optimum microwave irradiation time show a little superiority. Perovskite film treated by microwave irradiation for 3 min shows almost the same grain size as that with thermal annealing process at 100oC for 10 min (~250 nm). However, the crystallinity of perovskite film treated by microwave irradiation for 3 min is slightly higher than that with the thermal annealing process (Figure 3). Furthermore, as can be seen in Figure S5, the less homogeneous perovskite coverage was obtained by thermal annealing process at 110 oC for 3 min, which was ascribed to the insufficient heating time and uneven heat conduction. Thus, it leads to the reduced Jsc and PCE values of the corresponding solar cell (Figure S6). Undoubtedly, the PCE of the devices based on the optimum microwave irradiation time is a little higher than that treated by the conventional thermal annealing process. Additionally, the microwave irradiation process only takes about 3 min, which also saves a lot of time and energy. To further investigate the photovoltaic performance of the solar cells, the monochromatic incident photon-to-current conversion efficiency (IPCE) curves of perovskite solar cells under different annealing processes are shown in Figure 7c. The photocurrent generation onset is around 800 nm, which is related to the optical absorption of the perovskite (CH3NH3PbI3).The IPCE curve measures the light response of photovoltaic devices, which is related to the shortcircuit current density (Jsc). As expected, the perovskite film under microwave irradiation process for 3 min gets the highest IPCE value over the whole wavelength region from 400–800 nm, which is in accordance with the trend of the J–V measurement, where the device based on it gets the maximum Jsc value. This is attributed to its higher crystallinity and lower defect density, which results in efficient charge extraction. The integration of IPCE curve at the optimum time


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of microwave irradiation process gives a calculated photocurrent value of 20.10 mA/cm2 (Figure 7d), which is close to the measured Jsc value from the J-V curves.

CONCLUSIONS In summary, we adopted a microwave irradiation method to anneal perovskite films for the first time and investigated its mechanism and effect on the crystallization and morphology of perovskite films and the corresponding photovoltaic performance of the solar cells. As the solvent (DMF, DMSO) in the as-prepared perovskite films and the TiO2 and FTO on the substrates can absorb microwave energy and convert it into heat, our microwave irradiation process evaporates the solvent directly and heats perovskite film quickly, leading to the fast and controllable crystallization of perovskite films and the corresponding increase in photovoltaic performance. Furthermore, we believe that a more desirable perovskite film with excellent photovoltaic performance would be achieved by optimizing the output power of microwave irradiation. In addition, our microwave irradiation process is an ideal method with less energyloss and time-consumption. Thus, we are convinced that this microwave irradiation process would be applied in both the lab and stream-line manufacture of perovskite solar cells in the near future.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org.


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XRD patterns of the as-prepared perovskite and PbI2-CH3NH3I adduct powders, the picture of the samples on the Teflon shelf, the box charts exhibiting the statistical features of the photovoltaic parameters of solar cells, J-V characteristics of forward scan and backward scan of the best cell, the SEM image of the perovskite film treated by thermal annealing at 110oC for 3 min and J-V curve of the corresponding solar cell (PDF)

ACKNOWLEDGMENT This work was financially supported by the National High Technology Research and Development Program of China (Grant No. 2014AA052002), Shanghai Municipal Sciences and Technology Commission (Grant No. 12DZ1203900) and the Shanghai High & New Technology's Industrialization Major Program (Grant No. 2013-2). REFERENCES (1) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050-6051. (2) Noh, J. H.; Im, S. H.; Heo, J. H.; Mandal, T. N.; Seok, S. I. Chemical Management for Colorful, Efficient, and Stable Inorganic-organic Hybrid Nanostructured Solar Cells. Nano Lett. 2013, 13, 1764-1769. (3) Lin, Q. Q.; Armin, A.; Nagiri, R. C. R.; Burn, P. L.; Meredith, P. Electro-optics of Perovskite Solar Cells. Nat. Photonics 2015, 9, 106-112. (4) Xing, G.; Mathews, N.; Sun, S.; Lim, S. S.; Lam, Y. M.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C. Long-range Balanced Electron- and Hole-transport Lengths in Organic-inorganic CH3NH3 PbI3. Science 2013, 342, 344-347.


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(12) Eperon, G. E.; Burlakov, V. M.; Docampo, P.; Goriely, A.; Snaith, H. J. Morphological Control for High Performance, Solution-Processed Planar Heterojunction Perovskite Solar Cells. Adv. Funct. Mater. 2014, 24, 151-157. (13) Zhao, Y.; Zhu, K. Solution Chemistry Engineering toward High-Efficiency Perovskite Solar Cells. J. Phys. Chem. Lett. 2014, 5, 4175-4186. (14) Pang, S.; Zhou, Y.; Wang, Z.; Yang, M.; Krause, A. R.; Zhou, Z.; Zhu, K.; Padture, N. P.; Cui, G. Transformative Evolution of Organolead Triiodide Perovskite Thin Films from Strong Room-Temperature Solid-Gas Interaction between HPbI3-CH3NH2 Precursor Pair. J. Am. Chem. Soc. 2016, 138, 750-753. (15) Saliba, M.; Tan, K. W.; Sai, H.; Moore, D. T.; Scott, T.; Zhang, W.; Estroff, L. A.; Wiesner, U.; Snaith, H. J. Influence of Thermal Processing Protocol upon the Crystallization and Photovoltaic Performance of Organic–Inorganic Lead Trihalide Perovskites. J. Phys. Chem. C 2014, 118, 17171-17177. (16) Hsu, H.-L.; Chen, C.-P.; Chang, J.-Y.; Yu, Y.-Y.; Shen, Y.-K. Two-step Thermal Annealing Improves the Morphology of Spin-coated Films for Highly Efficient Perovskite Hybrid Photovoltaics. Nanoscale 2014, 6, 10281-10288. (17) Xiao, J.; Yang, Y.; Xu, X.; Shi, J.; Zhu, L.; Lv, S.; Wu, H.; Luo, Y.; Li, D.; Meng, Q. Pressure-assisted CH3NH3PbI3 Morphology Reconstruction to Improve the High Performance of Perovskite Solar Cells. J. Mater. Chem. A 2015, 3, 5289-5293. (18) Liu, J.; Gao, C.; He, X.; Ye, Q.; Ouyang, L.; Zhuang, D.; Liao, C.; Mei, J.; Lau, W. Improved Crystallization of Perovskite Films by Optimized Solvent Annealing for High Efficiency Solar Cell. ACS Appl. Mater. Interfaces 2015, 7, 24008-24015.


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Figure 1. The as-prepared perovskite powders treated by microwave irradiation for (a) 0, (b) 1, (c) 2 min; and the PbI2-CH3NH3I adduct powders treated by microwave irradiation for (d) 0, (e) 2 min; and the PbI2-CH3NH3I adduct powders added with (f) diethyl ether, (g) DMF and (h) DMSO treated by microwave irradiation for 2 min. The output power of microwave irradiation is 800W.


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Figure 2. (a) Time dependence of temperature of perovskite film under 160W microwave irradiation, (b) annealing mechanism model for the perovskite film under microwave irradiation process, (c) schematic diagram of fabrication procedures of perovskite films under microwave irradiation process and conventional thermal annealing process.

Figure 3. XRD patterns of perovskite films treated by 160W microwave irradiation for 1, 2, 3, 4, 5 min and thermal annealing at 100 oC for 10 min.


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Figure 4. Top-view SEM images of perovskite films treated by 160W microwave irradiation for (a) 1 min, (b) 2 min, (c) 3 min, (d) 4 min, (e) 5 min, and (f) thermal annealing at 100 oC for 10 min.


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Figure 5. Time dependence of mean grain size of perovskite film under 160W microwave irradiation.

Figure 6.(a) UV-Vis absorption spectra and (b) steady state PL spectra of perovskite films treated by 160W microwave irradiation for 1, 2, 3, 4, 5 min and thermal annealing at 100 oC for 10 min.


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Figure 7. (a) J-V curves of the solar cells fabricated from perovskite films annealed by microwave irradiation for 1, 2, 3, 4, 5 min and thermal annealing at 100 oC for 10 min, under standard illumination condition (AM 1.5G, 100mW/cm2). The scan rate is 40 mV/s. (b) Steadystate current measured at a maximum power point (0.814 V) along with the stabilized power output of perovskite solar cell annealed by microwave irradiation process for 3 min. (c) IPCE curves of perovskite solar cells annealed by microwave irradiation for 1, 2, 3, 4, 5 min and thermal annealing at 100 oC for 10 min, (d) IPCE curve along with the corresponding integration current density of the perovskite solar cell annealed by microwave irradiation for 3 min.


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Table 1. Photovoltaic Parameters of Devices with Different Annealing Processes. Four Cells were Fabricated for Each Type of Devices Cell

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

PCEbest (%)

MW-1 min

0.99±0.02

18.30±0.42

42±1

7.67 ± 0.38

8.16

MW-2 min

1.06±0.01

19.07±0.32

63±1

12.64 ± 0.36

12.91

MW-3 min

1.06±0.01

20.72±0.19

66±1

14.47 ± 0.39

14.91

MW-4 min

1.05±0.01

20.24±0.34

61±1

12.99 ± 0.42

13.40

MW-5 min

1.05±0.01

19.11±0.26

61±1

12.33 ± 0.37

12.79

TA-10 min

1.05±0.01

20.69±0.31

64±1

14.02 ± 0.28

14.33


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Table of Contents/Abstract Graphic


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Fast and Controllable Crystallization of Perovskite ... - ACS Publications

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