Single-Crystal-like Perovskite for High-Performance Solar Cells Using

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Single-crystal-like perovskite for high-performance solar cells using effective merged annealing method Yanliang Liu, Insoo Shin, In-Wook Hwang, Seungmin Kim, Jihoon Lee, Mi-Sun Yang, Yun Kyung Jung, Jae-Won Jang, Jung Hyun Jeong, Sung Heum Park, and Kwang Ho Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16541 • Publication Date (Web): 27 Mar 2017 Downloaded from http://pubs.acs.org on March 27, 2017

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ACS Applied Materials & Interfaces

Single-crystal-like perovskite for highperformance solar cells using effective merged annealing method Yanliang Liua,b, Insoo Shina,b, In-Wook Hwangc, Seungmin Kima, Jihoon Leea, Mi-Sun Yanga, Yun Kyung Jungd, Jae-Won Janga, Jung Hyun Jeonga*, Sung Heum Parka,b* and Kwang Ho Kim b* a

Department of Physics, Pukyong National University, Busan, 608-737, South Korea

b

Hybrid Interface Materials Global Frontier Research Group, Pusan National University, Busan 608-

737, South Korea c

Advanced Photonics Research Institute, Gwangju Institute of Science and Technology, Gwangju

500-712, South Korea d

Department of Medical Engineering, Inje University, Gyeongsangnam-Do, 621749, South Korea.

KEYWORDS : Single-crystal-like perovskite grain, merged annealing, solution-process, thermal stability, inverted planar perovskite solar cells. 1

ACS Paragon Plus Environment


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ABSTRACT

We report a simple, low cost, and quite effective method for achieving single-crystallike CH3NH3PbI3 perovskite leading to a significant enhancement in the performance and stability of inverted planar perovskite solar cells (IPSCs). By employing a merged annealing method during the fabrication of an IPSC for preparing the perovskite CH3NH3PbI3 film, we remarkably increase the crystallinity of the CH3NH3PbI3 film and enhance the device performance

and

stability.

An

IPSC

with

the

indium

tin

oxide/poly(3,4-

ethylenedioxythiophene)-poly(styrenesulfonate)/CH3NH3PbI3 (active layer)/ [6,6]-phenylC61-butyric acid methyl ester/Al structure was fabricated using the merged annealing method and exhibited significantly enhanced performance with a high power conversion efficiency of 18.27% and a fill factor of 81.34%. Moreover, since two separate annealing processes are merged in the proposed annealing method, the fabrication step becomes much simpler and easier, leading to a reduction in fabrication costs.

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1. Introduction

Interest in organic–inorganic halide perovskite solar cells as next-generation energy sources has recently increased owing to their promising advantages such as a high efficiency, high carrier mobilities, small exciton binding energies, long electron–hole diffusion lengths, and long lifetimes.1-8 In particular, inverted (p-i-n) planar perovskite solar cells (IPSCs) fabricated using [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as the top chargetransport layer provide additional advantages in terms of their simple solution-based fabrication, low-temperature processing, and high flexibility.9,10 Moreover, they exhibit a low degree of photocurrent hysteresis, which is difficult to achieve in other types of perovskite cells.11-13 Although IPSC performance has steadily improved as the power conversion efficiency (PCE ≡ ηe) has approached 16%,14 further improvements in the device performance along with a reduction in the cost of solar electricity are still needed for IPSCs to be able to compete with other solar-cell technologies. It is well known that the device performance of IPSCs is very sensitive to the quality of the active perovskite layer, which is related to the crystallinity, the extent of coverage, and the sizes of the grains in the layer.15-18 This sensitivity has motivated researchers to realize a high-quality perovskite film with smooth, homogeneous, and pinhole-free grains and high degrees of coverage.19-24 A number of researchers have reported reasonable approaches for enhancing the quality of the film using various techniques such as control of the thermal annealing temperature,19 the use of a perovskite precursor solution at non-stoichiometric 3



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ratios,20 the addition of chemical additives to the precursor solution,21,22 modification of the fabrication atmosphere,23 and solvent-inducing techniques.24 However, despite remarkable enhancements in the film quality by the use of these methods for improving the efficiency, they inevitably require an additional process, additional materials, or additional time to achieve the improvement.25-27 As a consequence, these methods cause an increase in the cost of solar electricity for commercialization. Therefore, the development of a reasonable fabrication method for producing a high efficiency and reducing the fabrication cost is crucial for commercialisation of perovskite solar cells. In this work, we report a low cost, highly effective method for improving the device efficiency along with a reduction in the fabricating cost. By merging separate annealing processes for the fabrication of IPSCs, we achieve single-crystal-like CH3NH3PbI3 active layers and thus attain a marked improvement in the performance of solar devices based on these films. When this method was incorporated in the one-step fabrication of an IPSC, the resulting device exhibited a high PCE of ~18.27% with significantly enhanced fill factor (FF). Moreover, since two separate annealing processes are merged, the process step becomes much simpler and easier, leading to a reduction in the fabrication costs.

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2. Experimental

2.1. Material preparation MAI was synthesized according to the method previously described.28 24 mL methylamine (33 wt% in ethanol) and 10 mL HI (57 wt% in water) were mixed in a flask under N2 atmosphere and reacted at 0 °C for 2 h under stirring. The resulting solution was dried at 50 °C by rotary evaporation and then a white powder of MAI was obtained. The dried MAI powder was dissolved in ethanol, followed by sedimentation in diethyl ether during stirring. This process was performed repeatedly three times and then the MAI powder with high purity was recovered and dried for 24 h at 60 °C in a vacuum oven.

2.2. Device fabrication The perovskite solar cells with device structure of ITO coated glass substrate/PEDOT:PSS/ CH3NH3PbI3 active layer/[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM)/Al were fabricated. The PEDOT:PSS (Baytron PH) was spin-coated on cleaned ITO substrate (4500 rpm, 40 s) to form a 40 nm thick film. The substrate was dried for 10 min at 140 °C in air and then transferred into N2-filled glove box. A 45 wt% perovskite precursor solution with molar ratio of 1:1:1 (MAI:PbI2:DMSO) in DMF was spin-coated on PEDOT:PSS layer (5000 rpm, 50 s) with instant chlorobenzene treatment during spinning.29 In CA process, the film is directly annealed at 100 °C for 2 min to remove solvents inside and form complete perovskite film. Then, the PC61BM (20 mg/mL in chlorobenzene) was spin-coated on the complete perovskite film (1000 rpm, 40 s). In the novel MA process, a PC61BM film is primarily 5



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deposited on perovskite precursor film at 1000 rpm for 40 s. The following heating at 100 °C for 10 min is applied to form complete perovskite film with PC61BM film (electron transport layer). Finally, an Al cathode was deposited by thermal evaporation in a vacuum of about 5×10-7 Torr.

2.3. Film characterization The absorbance and XRD spectra of the perovskite films were measured using a Varian 5E UV/Vis/NIR spectrophotometer and an X’Pert-MPD (Philips, Netherland), respectively. The film morphologies were obtained by AFM (BRUKER, Icon-PT-PLUS), SEM (S-2700, Hitachi, Japan) and fluorescence microscopy (Carl Zeiss Axio A10). Steady-state PL spectra were measured using a F-7000 ﬂuorescence spectrometer (Hitachi, Japan) by photoexciting the samples at 470 nm. Time-resolved PL measurements were recorded using a timecorrelated single photon counting equipment (PicoQuant, Germany) with a 470 nm picosecond pulsed laser source (PicoQuant, LDH-P-C-470) and laser drivers (PicoQuant, PDL 800-D). The PL was collected at a wavelength of 760 nm using an imaging triple grating monochromator/spectrograph (Princeton Instruments, Acton SP2300, USA).

2.4. Photovoltaic characterization All measurements were performed in ambient air and the devices were encapsulated by a transparent plastic tape before measurements. The J-V curves of the devices were recorded with a sweep time of 0.7 s using a Keithley 2400 Source Measure Unit. The light intensity of the solar simulator was calibrated by a standard silicon solar cell. The performance of the 6


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perovskite solar cells was measured by forward (-0.1 to 1.1V forward bias) or reverse (1.1 to -0.1V) scans by using an Air Mass 1.5 Global (AM 1.5 G) solar simulator with an irradiation intensity of 1000 Wm-2. An aperture was used on the top of the active cell (3.8 mm2) of the device to eliminate the extrinsic effects such as wave guiding, shadow effects, cross-talk, etc.

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3. Results and discussion

In the one-step fabrication process for IPSCs, a solution of CH3NH3I (methylammonium iodide or MAI) and PbI2 is spin-coated onto the hole-transport layer and annealed on a hot plate to form the perovskite phase via evaporation of the residual dimethylformamide (DMF) and dimethylsulfoxide (DMSO) solvent in the perovskite precursor film. Similarly, the PC61BM solution is subsequently deposited onto the complete perovskite film and annealed again to form the complete film. Because the perovskite is insoluble in the PC61BM solution dissolved in the organic solvent, the properties of the perovskite film remain unchanged during the PC61BM deposition process. Considering the orthogonal solubility of perovskite in the PC61BM solution, we expected that each separate annealing process for the complete perovskite film and PC61BM layer described above can be effectively merged. The PC61BM solution is dropped before film annealing onto the incomplete perovskite film, spin-coated, and finally annealed with the perovskite film. Since the residual DMF and DMSO solvent inside the perovskite film must pass through the PC61BM top layer to evaporate, the deposited PC61BM serves as a shield to retain the residual DMF and DMSO solvent within the perovskite film and provides sufficient time for it to self-assemble, leading to highly crystalline perovskite grains. In addition, both the perovskite film and PC61BM are not rigidly solid before annealing; it is expected that contact between PC61BM and the perovskite film will improve via infiltration of PC61BM into the perovskite surface. In IPSCs, PC61BM plays a critical role as the top charge-transport

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layer and facilitates electron transport to the cathode and interface engineering.30 Thus, an improvement in the interface between the perovskite active layer and the PC61BM layer leads to a high-performance IPSC.31 On this basis, we designed a novel processing technique for controlling the morphology of perovskite films by merging two annealing processes for the simultaneous formation of complete perovskite and PC61BM layers during the film preparation process. Figure 1a shows the processing technique designed in this study and compares it with the conventional fabrication process. A solution of MAI, PbI2, and DMSO with a molar ratio of 1:1:1 dissolved in DMF was spin-coated onto a poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) layer. Process (i) in Figure 1a shows the conventional annealing (CA) method for fabricating complete perovskite films. In this case, the perovskite film is annealed directly on a hot plate after completion of the spinning process, and the DMF solvent is quickly evaporated.

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Figure 1. (a) Schematics of the conventional thermal annealing (CA) and merged annealing (MA) fabrication processes. (b) Photographs of the colour evolution of perovskite films fabricated by the CA and MA methods after various annealing times. XRD patterns of the CA and MA perovskite films after annealing at 100 °C for (c) 2 min and (d) 10 min. 10


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In contrast, in process (ii), the PC61BM solution is deposited onto the perovskite film after completion of the spinning process without an annealing process. Instead, after completion of the spinning process for PC61BM, the film is annealed at once on the hot plate, as shown in Figure 1a. In this case, the PC61BM and perovskite are not completely in the film phase until the film is annealed; this process is called the merged annealing (MA) method in this work. Figure 1b shows the colour evolution of films fabricated by the CA and MA methods. During annealing at 100 °C, the CA film changes its colour from reddish brown to dark brown immediately after applying heat, whereas the MA film changes its colour to dark brown slowly, indicating slow conversion to the complete perovskite phase. The corresponding ultraviolet (UV)–visible absorption spectra of the perovskite film at different stages are shown in supporting information Figure S1. Figures 1c and d show the X-ray diffraction (XRD) spectra of perovskite films fabricated by the CA and MA methods after annealing at 100 °C for 2 and 10 min, respectively. Even after annealing at 100 °C for 2 min, the MA film still exhibits a clear peak around 9 ° in contrast to the CA film with no peak. This peak was previously assigned to MAI-PbI2-DMSO and naturally observed in the precursor of the perovskite.29 Therefore, this clearly indicates that the MA method significantly delays the evaporation of the residual solvents in the perovskite precursor film owing to the supernatant PC61BM on top of it, which can promote crystal growth and create larger perovskite grains, as reported for DMF solvent annealing techniques.32 The effects of the prolonged evaporation of the solvent on the growth of the perovskite film grains are also observed for the MA film with increasing covering time (see 11



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the Figure S2 in the supporting information).33,34 Figure 1d shows the XRD spectrum for both films annealed for 10 min. Both films do not exhibit a peak around 9.0 °, and there are two dominant peaks at 14.2 ° and 28.5 °, which respectively correspond to the (110) and (120) planes of CH3NH3PbI3.35 However, the peak intensities of the MA film are much higher than those of the CA film, clearly indicating that the MA film has a much higher crystallinity. The morphologies of the CH3NH3PbI3 films fabricated using the CA and MA processes were inspected by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Figures 2a and b show the top-view SEM and AFM images (insets) of the CA and MA films, respectively. Obviously, the MA film exhibits a significantly enhanced crystallinity and enlarged grains (1.0-2.5 µm) compared to the CA perovskite film. Remarkably, singlecrystalline-like perovskite grains are observed in the cross-sectional SEM image of the MA film. As shown in Figures 2c and 2d, in contrast to the CA film with unclear and differently sized small grains, the MA film exhibits uniform single-crystalline-like perovskite grains, and most grains approach the top PC61BM/Al layer from the bottom electrode. This can lead to efficient charge extraction at the electrodes via the straightforward transport channels in the crystalline perovskite with remarkably reduced grain boundaries in the longitudinal direction.

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Figure 2. SEM and AFM (inset) images of perovskite films fabricated by (a) CA and (b) MA. Cross-sectional SEM images of perovskite films fabricated by (c) CA and (d) MA.

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For further inspection, we intentionally fabricated a perovskite film using various shield materials such as a glass sheet, plastic tape, a small molecular material of Spiro-MeOTAD and P-DTS(FBTTh2)2, and P3HT polymer. Figures 3a–d show SEM images of the surface morphologies of films fabricated with these shield materials. As expected, enlarged and highly crystalline perovskite grains are clearly observed for the films fabricated using shielding materials. In addition, the use of an additional glass sheet on top of the PC61BM layer provides an enhanced grain size, as shown in Figures 3e–f. These results indicate that one of the main reasons for achieving highly crystalline grains by the MA method is the shielding effect of PC61BM. However, the grain sizes and shapes of the films are different. Moreover, the use of P3HT as a shield material on top of the perovskite film before annealing does not provide highly crystalline grains (see Figure 3g), whereas the additional use of glass on top of the P3HT polymer leads to enlarged grains, as shown in Figure 3h. Although the reason is not clear at this moment, we infer that the molecular weight, sizes of the pinholes in the covered film, degree of penetration of the solvent through the covered film, and interaction with the shielding materials might influence the film morphology. Further studies are needed to clarify the working mechanism of this shielding effect. Interestingly, we can effectively control the grain size by simply changing the PC61BM-to-P3HT ratio of the shield layer. As shown in Figures 3i–l, the grain size of the perovskite film gradually shrinks as the amount of P3HT in the P3HT:PC61BM blend increases.

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Figure 3. SEM images of the surface morphologies of films fabricated using various shield materials of (a) glass, (b) plastic tape, (c) a small molecule material, (d) a hole-transporting Spiro-MeOTAD material, (e) PC61BM /glass, (f) PC61BM, (g) P3HT, (h) P3HT/glass, and mixtures of PC61BM/P3HT with blending ratios of (i) 9:1, (j) 8:2, (k) 7:3, and (l) 1:1.

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In contrast to the highly crystalline grains in the MA film, the horizontal grain boundaries in the MA film are significantly wider and deeper than those in the CA film. In this case, it is expected that the PC61BM molecules can penetrate the perovskite film through grain boundaries, which can improve the interfacial contact and enhance the electron transport between the CH3NH3PbI3 and PC61BM films. Huang et al. also reported that such infiltration of PC61BM into the perovskite boundary leads to a higher-quality perovskite film and suppresses the hysteresis of the device.36,37 In order to verify our expectation, we designed a reasonable photoluminescence (PL) measurement experiment. Because of the orthogonal solubility of the perovskite in the PC61BM solvent, we can selectively remove PC61BM from the perovskite/PC61BM film by rinsing with an organic solvent such as chlorobenzene (CB), dichlorobenzene (DCB), and chloroform (CF). Therefore, we removed PC61BM and deposited a luminescent polymer onto the rinsed perovskite/PC61BM film. The left side of Figure 4a shows AFM images of the perovskite surface after rinsing the PC61BM component. As expected, enlarged grains as well as deep and clear grain boundaries are observed. The right side of Figure 4a shows a fluorescence microscopy image of the luminescent polymer/perovskite film. Distinct green PL surrounds the perovskite grains, whereas almost no PL from the grains is obtained. Moreover, the shape of the grains in the fluorescence microscopy image is very consistent with the SEM and AFM images. Because the thickness of the luminescent polymer on the grains is much thinner than that of the polymer at the grain boundaries, most of PL from the grain is quenched by charge transfer to the perovskite film. Therefore, the PL signals indicate 16


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the grain shape of the perovskite in the MA film. From this result, it is evident that the PC61BM molecules can easily infiltrate grain boundaries, and it is even better than the largemolecule luminescent polymer.

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Figure 4. (a) Schematic of the PL measurement using a luminescent polymer. The left side shows an AFM image of the perovskite surface after rinsing the PC61BM component. The right side shows a fluorescence microscopy image of the luminescent polymer/perovskite film. (b) PL intensities and (c) time-resolved PL spectra of the CA and MA perovskite films with and without PC61BM rinsing.

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For further investigation of the effect of this infiltration, we measured the steady-state and time-resolved PL behaviours for the CA and MA perovskite films. Figure 4b shows the PL intensities of the perovskite/PC61BM and PC61BM-rinsed perovskite films fabricated by the CA and MA methods. Both of the perovskite/PC61BM films exhibit a much lower PL intensity than that of the PC61BM-rinsed perovskite films owing to PL quenching, which originates from charge transfer from the perovskite to PC61BM, as expected. However, the MA perovskite/PC61BM film exhibits a lower PL intensity compared to those of the CA films. Moreover, even the PC61BM-rinsed perovskite film fabricated by MA exhibits a red-shifted maximum peak and a lower PL intensity than those of the corresponding CA film. Although the reason for the red shift is not clear at this stage, the lower PL intensity of the MA film obviously indicates that the infiltrated PC61BM is hardly removed by rinsing and remains inside the perovskite prepared by the MA method. This tendency is also observed in the timeresolved PL spectra in Figure 4c. The perovskite/PC61BM and PC61BM-rinsed perovskite films fabricated by MA exhibit a faster decay compared to the films fabricated by CA. The PL decay originates from exciton dissociation at the perovskite/PC61BM interface. Since the infiltrated PC61BM in MA perovskite significantly enhances the interfacial area and the distance between the perovskite and PC61BM leads to a faster decay, it is concluded that PC61BM deeply infiltrates the perovskite and is hardly removed even after rinsing, resulting in residual PC61BM remaining at the grain boundaries. One of the outstanding advantages of the highly crystalline MA film is to provide an improvement in the thermal stability. Figure 5a shows the XRD spectra of perovskite films 19



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fabricated by the CA and MA methods after annealing at 150 °C for 5 min. The CA film exhibits a small peak around 13.0 °, whereas the spectrum of the MA film does not change from that of the pure film in Figure 1d with no peak around 13.0 °. This peak is assigned to the pure PbI2 peak and results from the degradation of CH3NH3PbI3.18 Figure 5b shows the corresponding absorbance spectra of the CA and MA films. As expected, the absorbance of the CA film changes after annealing, whereas the MA film has a constant absorbance. For further comparison, the CA and MA perovskite films were heated at a temperature of 200 °C. From Figure 5c, the CA perovskite film gradually decomposes into a yellow PbI2 film. However, the MA perovskite film colour still remains dark brown (the perovskite material colour). This is similar to a previous report in which polymethyl methacrylate (PMMA)functionalised carbon nanotubes can mitigate the thermal degradation of a perovskite film.38 These results indicate that MA technology can significantly improve the thermal stability of perovskite films and inhibit the generation of charge traps. In the perovskite film solution fabrication process, a thermal annealing treatment is essential for the complete perovskite phase via removal of the residual organic solvent. However, as previously studied, the perovskite film is thermally unstable,39 particularly the surfaces and grain boundaries.36 M. Huang et al. explained that perovskite decomposes to PbI2 at a relatively low thermal annealing temperature of 105 °C.40 Therefore, we believe that the MA method can be promising candidate for providing high performance and thermal stability for perovskite solar cells. The device stability test shown in Figure S3 in supporting information provides evidence in favour of our expectation. 20


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Figure 5. (a) XRD patterns and (b) absorbance spectra of the CA and MA perovskite films for various thermal annealing times. (c) The colour evolution of the CA and MA perovskite films for various annealing times at 200 °C.

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Using the MA method, we directly fabricated a one-step-processable IPSC and investigated its device performance. Figure 6a shows the IPSC architecture. When the IPSC was irradiated, the generated free holes were collected at the ITO electrode via the PEDOT:PSS layer, whereas the generated free electrons were collected at Al electrode via the PC61BM layer. The photoactive perovskite layer was prepared using the CA and MA methods. The current-density–voltage (J–V) characteristics of the devices measured under standard AM 1.5 G irradiation of 100 mW/cm2 are shown in Figure 6b, and their photovoltaic parameters are listed in Table 1. The conventionally fabricated device exhibited a reasonably high efficiency of 11.93%, a short-circuit current (Jsc) of 18.07 mA/cm2, a fill factor (FF) of 71.60%, and an open-circuit voltage (Voc) of 0.92 V. These values are comparable to those reported for IPSCs fabricated by the one-step process.41 However, the performance of the devices fabricated using the MA method was markedly better. When the perovskite layer was subjected to the MA process during device fabrication, the resulting device exhibited better performance, with Jsc increasing from 18.07 to 22.41 mA/cm2, Voc increasing from 0.92 to 1.00 V, and the FF increasing from 71.60% to 81.34%. In particular, the MA devices exhibit significantly enhanced FF compared to that of the CA devices, which originates from the enhanced film quality, leading to a lower device resistance (see the electrochemical impedance spectroscopy (EIS) results of the device shown in Figure S4 in the supporting information) and a reduction in the number of grain boundaries on the pathway of carrier transport (see the cross-sectional SEM images in Figure 2d).42-46 Consequently, the PCE improved from 11.93% to 18.27%. The distribution of the PCE is plotted in Figure 6c. 22


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Considering that this IPSC device was fabricated at a lower cost through a one-step process in a printable manner, a PCE of 18.27% is quite high and comparable to those of perovskite solar cells fabricated by significantly more complex and costlier methods such as two-step processes as well as those involving thermal or vacuum evaporation.

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Figure 6. (a) Configuration of the perovskite device. (b) J–V curves of the device. (c) Perovskite device efficiency distributions. (d) Incident-photon-to-current efficiency (IPCE) spectra of the CA and MA perovskite devices. (e) CA device and (f) MA device hysteresis in the J–V curves by scanning from negative to positive bias and from positive to negative bias. 24


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Table 1. Photovoltaic parameters of the devices fabricated using different fabrication processes.

Sample

Jsc [mA/cm2]

CA

18.07

MA

22.41

Voc [V]

FF [%]

PCE(max) [%]

PCE(average) [%]

0.92

71.60

11.93

10.82 ± 1.11

1.00

81.34

18.27

17.43 ± 0.84

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Similar improvements were observed during the measurements of the incident-photon-tocurrent efficiency (IPCE) shown in Figure 6d. The enhancements in the IPCE for the MA devices corresponded to increases in the device photocurrent. This is in agreement with the data shown in Figure 6b. Therefore, it was concluded that MA improves the quality of the perovskite active layer and hence the performance of the resulting solar device. One of the reasons for the enhancement in the photocurrent of the device fabricated by MA originates from the enhancement in the absorbance. Figure S5 shows the UV–visible absorption spectra measured in the reflection mode of both devices. The MA device exhibits a higher absorption over the entire wavelength range, with a high absorbance over ~ 80 % in the 400 – 750 nm range. Because the morphology of the Al electrode projected the morphology of the perovskite, as shown in Figure S6, we ascribe this enhancement in the absorption to enhanced diffraction of the incident light at the curved surface of the electrode, which leads to a longer optical path in the perovskite active film and improves the lightharvesting properties of the perovskite device. One noteworthy phenomenon observed during this study was that the J–V curve of the MA device did not exhibit hysteresis, even after the repeated application of a bias voltage, which is difficult to achieve in other types of perovskite solar cells.11-13 Figures 6e and f show the J–V characteristics of devices fabricated by the CA and MA methods, respectively, under forward and reverse biases. In contrast to the CA device in Figure 6e, no difference was observed in the J–V characteristics of the MA device when the applied bias was increased from -0.1 V to +1.1 V or decreased from +1.1 V to -0.1 V, indicating that no 26


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photocurrent hysteresis was present. This is in sharp contrast to other types of perovskite solar cells and highlights the advantage of using the MA method to fabricate IPSCs.

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4. Conclusions

In summary, we developed a simple and novel technology for fabricating high-quality single-crystal-like perovskite films. By applying effective MA technology to prepare CH3NH3PbI3 films, we could improve the perovskite grain size to 2.5 µm, as confirmed by UV–visible absorption; SEM, AFM, and fluorescence microscopy images; and XRD spectra, which significantly enhances the charge transport properties in the perovskite film. Furthermore, small PC61BM molecules infiltrated the perovskite film through the grain boundaries and surrounded the perovskite grains, which improved the thermal stability of CH3NH3PbI3 during the annealing process and enhanced the electron transport between CH3NH3PbI3 and PC61BM. When we used the MA process, the fabricated perovskite devices exhibited a PCE of 18.27%, which was almost 55% higher than that of the conventional CA device. Moreover, the MA device did not exhibit any hysteresis, which is difficult to achieve in other perovskite solar cells. Therefore, we believe that the proposed MA technology should lead to the development of high-performance and low-cost solution-processable perovskite solar cells.

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ASSOCIATED CONTENT

Supporting information. Additional data for absorbance of films (Figure S1), SEM images of the MA film with increasing covering time (Figure S2), device stability test results (Figure S3), Nyquist curves of devices (Figure S4), absorbance of devices (Figure S5) and morphological change of MA film (Figure S6) are provided in supporting information. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Authors * E-mail : [email protected] (J.H.Jeong), [email protected](S.H.Park), [email protected](K.H.Kim)

ACKNOWLEDGMENT This research was mainly supported by the Global Frontier Program through the Global Frontier Hybrid Interface Materials (GFHIM) of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2013M3A6 B1078874). This research was also supported by the NRF Grant funded by the Korea Government (CD20160979).

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Single-Crystal-like Perovskite for High-Performance Solar Cells Using

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