Planar Perovskite Solar Cells with High Efficiency and Fill Factor

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Planar Perovskite Solar Cells with High Efficiency and Fill Factor Obtained Using Two-Steps Growth Process Fengjiu Yang, Jiewei Liu, Xiaofan Wang, Kenya Tanaka, Keisuke Shinokita, Yuhei Miyauchi, Atsushi Wakamiya, and Kazunari Matsuda ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02948 • Publication Date (Web): 08 Apr 2019 Downloaded from http://pubs.acs.org on April 9, 2019

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ACS Applied Materials & Interfaces

Planar Perovskite Solar Cells with High Efficiency and Fill Factor Obtained Using Two-Steps Growth Process

Fengjiu Yang†, Jiewei Liu‡, Xiaofan Wang†, Kenya Tanaka†, Keisuke Shinokita†, Yuhei Miyauchi†, Atsushi Wakamiya*‡, and Kazunari Matsuda*†

†Institute

‡Institute

of Advanced Energy, Kyoto University, Uji, Kyoto 611-0011, Japan

for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan

Corresponding Authors E-mail: [email protected] E-mail: [email protected]

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Abstract Hybrid organic–inorganic perovskite solar cells (PSCs) have been regarded as the most promising next-generation photovoltaics (PVs), because of their potential for low-cost fabrication and advances in their development. Superior quality of the photoactive perovskite layer is a main factor for further increasing the PV performance of the organic–inorganic perovskite solar cells (PSCs). Herein, we successfully obtained perovskite with a high crystallinity and large grain size by utilizing excess PbI2 and second solution growth process (SSGP) technique, and demonstrated a superior PV performance of normal-architecture planar PSCs. The SSGP-PSCs with the highest fill factor (FF) reported thus far (83.4%) as we known were obtained without sacrificing other parameters. Moreover, a high efficiency of 21.3% (21.6%) with a high FF of 80.0% (81.2%) in forward (reverse) scan was achieved. The un-encapsulated SSGP-PSCs showed robust continuous light soaking and thermal stability under harsh characterization conditions. Additionally, we achieved a high efficiency of 20.1% with a negligible hysteresis on the large active area SSGP-PSCs ( 1 cm2). The optical properties, efficient carrier extraction, and reduction of recombination loss of the SSGP perovskite significantly contribute to the high PV performance and robust stability of SSGP-PSCs. 2 ACS Paragon Plus Environment

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Keywords: perovskite solar cells, second solution growth process, excessive PbI2, photovoltaic performance, stability, large active area



INTRODUCTION The rapid progress of hybrid organic–inorganic perovskite solar cells (PSCs) in recent years has attracted considerable attention in the research community of photovoltaics (PVs),1–7 and the power conversion efficiency (PCE) of PSCs has been increased beyond 23.0%.8 The rapid development of PSCs is assisted by the superior properties of organic– inorganic perovskite as a photoactive layer, such as its large absorption coefficient, high carrier mobility, long carrier diffusion length,9,10 and tunable band gap.11–13 The realization of the excellent PV performance of PSCs is mainly determined by the quality of the perovskite layer and its interfaces, because the initial PV processes of charge-carrier generation and charge-carrier transport occur in the perovskite and its interfaces.14 Therefore, considerable endeavors have been directed toward improving the quality of the perovskite layer, e.g., the vacuum-assisted solution deposition process,15,16 solution process secondary growth,5,17 and adjusting the composition of the perovskite precursor.11,13,18 Among these approaches, the utilization of excess PbI219–26 could improve the crystallinity20 and increase the grain size of perovskite crystals.20,22–24 Excess PbI2 has also been demonstrated that a proper residual PbI2 could passivate the grain boundary of the perovskite and its interfaces,19,21,23,27,28 and be regarded as efficient electron-blocking layer, 4 ACS Paragon Plus Environment

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accelerating the hole injection, and decreasing the carrier recombination.19 In contrast, the dubious amount of residual PbI2 resulting from non-stoichiometry or an excessive annealing temperature19,20,23,24 seriously deteriorates the quality of perovskite and causes intrinsic instability under light illumination.24,26 The PV performance of PSCs fabricated using excess PbI2 remains in a deficient and dissatisfying stage, as the PCE is still lower than 22.0%, even though current density (JSC) has been enhanced at the expense of the open-circuit voltage (VOC) or fill factor (FF).22,23,27 Thus, it is necessary to develop an effective approach for achieving both a high PCE and robust stability of PSCs by using excess PbI2 for perovskite fabrication. In this case, achieving an FF higher than 80% becomes the critical challenge, because the FF is usually increased at expense of JSC or VOC.29,30 In this study, we investigated an effective approach for realizing superior PV performance of normal-architecture planar PSCs utilizing the combination of excess PbI2 and the second solution growth process (SSGP) of perovskite layer. The SSGP perovskite was systematically examined by using X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), photoluminescence (PL) mapping/spectroscopy and photoelectron yield spectroscopy, which revealed the significant improvement of the quality of the perovskite layer due to the removal of excess PbI2, and the reduction of trap states. We 5 ACS Paragon Plus Environment


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successfully achieved a high PCE of 21.3% in forward scan without sacrificing the VOC of 1.14 V and FF of 80.0%, as well as a sustained PCE of 70% even after 250 h under continuous light soaking at high humidity ( 50%) and 75% of initial efficiency even after 270 h at high temperature (80 C) . The detailed mechanism of the superior PV performance in the SSGP PSCs is discussed.

EXPERIMENTAL SECTION Materials. All chemicals were directly used as obtained without further refinement. The ITO substrate (10 E/ was ordered from GEOMATEC Co., Ltd. An Alfa Aesar SnO2 nanoparticle (15 wt%) H2O colloidal dispersion with a particle size of 10–15 nm was used. Lead (II) iodide (PbI2,

99.99%),

Lead

(II)

bromide

(PbBr2,

99%),

methylammonium

bromide

(MABr, >98.0%) formamidinium iodide (FAI, >98.0%), cesium iodide (CsI, 99.9%), methylammonium chloride (MACl, >98.0%), formamidinium bromide (FABr, >98.0%), and formamidium chloride (FACl, >98.0%) were purchased from Tokyo Chemical Industry Co., Ltd. The solvent chemicals of N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), chlorobenzene, toluene and isopropanol (IPA) were dried before use. ( (H < 1 µm in size. The surface roughness of the as-prepared and SSGP perovskites was characterized by AFM, as shown in Figure S2. The surface roughness significantly decreased after the SSGP, indicating that the surface of the perovskite becomes more flat, as confirmed by the cross-sectional SEM images in Figure S3. Figure 2d shows X-ray diffraction (XRD) patterns of the as-prepared and SSGP perovskites obtained using FABr, FAI, and MACl, which are denoted by FABr-, FAI-, and MACl-SSGP, respectively. The XRD peaks of the as-prepared perovskite—indicated as asterisks—are due to Pb(I1-xBrx)2 crystals, and the other peaks are the diffraction peaks of Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3 perovskite. The XRD peak intensity of Pb(I1-xBrx)2 14 ACS Paragon Plus Environment

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decreases significantly for the FAI-SSGP perovskite and almost disappears for the FABrand MACl-SSGP perovskite. The composition of Pb(I1-xBrx)2 is evaluated as Pb(I0.82Br0.18)2 using the Vegard’s law according to the diffraction angle, where the typical diffraction peaks of PbI2 and PbBr2 are located at 12.7°24 and 14.4°11, respectively. The normalized relative XRD intensities of Pb(I0.82Br0.18)2 and the full-width of half maximum (FWHM) for the asprepared and SSGP perovskites are plotted in Figure S4. The average particle size of Pb(I0.82Br0.18)2 is evaluated as approximately 180 nm according to the FWHM using Scherrer’s formula, which is also consistent with the SEM images in Figure 2b. The magnified XRD peaks of the perovskites are shown in the inset of Figure 2d. The main XRD peak of the as-prepared perovskite of Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3 occurs at 14.49 , and the XRD peaks of the FAI- and MACl-SSGP perovskites show higher-angle shifts than that of the as-prepared perovskite owing to the compositional changes, for instance, Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3-MAPb(IxBryClz)3 in the MACl-SSGP perovskite. These results strongly suggest that a perovskite layer with different composition was formed on the surface of the first-stage perovskite layer via the SSGP. The as-prepared and SSGP perovskites were characterized using optical spectroscopy and microscopy to evaluate their optical properties. Figure 3a shows the ensemble-averaged PL 15 ACS Paragon Plus Environment


spectra of the as-prepared and SSGP perovskite films. The PL peaks of the SSGP perovskite exhibit blue-shifts compared with that of the as-prepared perovskite, except for the FAISSGP perovskite. The PL intensity and FWHM values significantly increase and decrease by the SSGP (Figure S5), respectively, indicating that the quality of perovskite films was significantly enhanced by the elimination of trap states in the SSGP. The optical absorption spectra of the as-prepared and SSGP perovskites are shown in Figure 3a. The absorption edges in the FABr- and MACl-SSGP perovskites are slightly shifted to a shorter wavelength compared with that of the as-prepared perovskite, except for the FAI-SSGP perovskite, which exhibits a slight increase of the bandgap.13,17,27 The blue-shifted trend and the value of the absorption edge are well consistent with the PL spectra in Figure 3a. The bandgaps of the asprepared and FABr-, FAI- and MACl-SSGP perovskites are evaluated as 1.60, 1.62, 1.60 and 1.61 eV, respectively. The optical properties of the as-prepared perovskite before and after spiro-OMeTAD and SSGP are depicted in Figure S6. Figure 3b shows the PL decay profiles of the as-prepared and SSGP perovskites. Notably, the PL decays of the SSGP perovskites are longer than that of the as-prepared perovskite. The decay profile of the perovskite films comprises a fast-component due to the surface recombination and a slow-component due to the bulk radiative recombination of electron16 ACS Paragon Plus Environment

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(b) 400

Pero FABr FAI MACl

300

0.6 0.4

100

0.2 600 700 800 Wavelength (nm)

0.0 900

100 10-1

10-3 0

20

5

20

0

12

5

8

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0

3.5x104 3.0x10

15 10 5 X Position ( m) MACl

20

Y Position ( m)

24

Y Position ( m)

0(d)

16

(f)

Standard

In

Intensity (a. u.) 3.500E+04 3.000E+04 2.500E+04

12

2.000E+04

8

1.500E+04 10000

4

5000

20

0

10 15 5 X Position ( m)

20

Pero

500 FWHM: 3.85 nm MACl

Counts

2.0x104 4

1.0x104 5.0x103 745

900

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300 600 Decay Time (ns)

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10-2

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(a)

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Normalized PL intensity (a. u.)

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400 FWHM: 4.1 nm 300 200 100

750

755 760 765 Wavelength (nm)

0 740 745 750 755 760 765 770 Wavelength (nm)

770

Figure 3. (a) Optical absorption and PL spectra of the as-prepared and SSGP perovskites. (b) Time-resolved PL profile and analyzed results for the as-prepared and SSGP perovskites. (c, d) Integrated PL intensity mapping of the as-prepared and MACl-SSGP perovskites, respectively. (e) Distribution of the PL intensity as a function of the peak wavelength, analyzed from 5,476 spectra. (f) Distributions of PL spectral positions for the as-prepared and SSGP perovskites.

hole pairs,35 as summarized in Table S1. The amplitude ratio and decay time of the fastcomponent significantly decrease and increase, respectively, which indicates that the surface 17 ACS Paragon Plus Environment


recombination loss was remarkably eliminated because of the improvement of the surface quality of the perovskite by the SSGP. Moreover, the lifetime of the slow-decay component remarkably changed from 310 ns of the as-prepared perovskite to 410 ns for the MAClSSGP perovskite. The longer decay times of the slow-component indicate that the SSGP not only formed the second perovskite layer on the as-prepared perovskite, but also improved the quality of the bulk part of perovskite layer. These experimental results on SSGP perovskite indicate the large charge-carrier diffusion length9 and high PV performance of SSGP PSCs.20,32,36 Figure 3c and d and Figure S7 show the integrated PL intensity mapping of the as-prepared and SSGP perovskite films. The PL image of the as-prepared perovskite shows a spatially homogeneous intensity distribution, while that of the SSGP perovskite shows large fluctuations. In contrast, the average PL intensities of the SSGP perovskite are significantly improved compared with that of the as-prepared perovskite, which is consistent with the PL spectra in Figure 3a. A large number of micro-PL spectra (5476 spectra) for the as-prepared and SSGP perovskite were analyzed. Figure 3e shows the distribution of the PL intensity as a function of the peak wavelength in the as-prepared and MACl-SSGP perovskites according to the analysis of 5,476 spectra. The PL intensity of MACl-SSGP perovskite is more broadly 18 ACS Paragon Plus Environment

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by the experimental results of SEM and XRD in Figures 2a–d. And, we found the similar PL intensity enhancement in the FAI- and FABr SSGP perovskite in Figure S7. The histogram distribution of the PL spectral position is shown in the Figure 3f and Figure S8. The SSGP perovskite films exhibit relative broader spectral position compared with that of the standard perovskite film due the reduction of the density of Pb(I0.82Br0.18)2. Figure 4a shows the results of photoelectron yield spectroscopy for the as-prepared and SSGP perovskites. The increase of the photoelectron yield signals due to the valence band (VB) electrons systematically shift to the higher-energy side in the order of -5.46, -5.42, 5.41 and -5.39 eV for the as-prepared, FABr-, FAI- and MACl-SSGP perovskites, respectively. The inset of Figure 4a shows the energy position of the top of the VB for the as-prepared, FABr, FAI and MACl perovskites from the vacuum level. The bandgap values for energy alignments in Figure 4b in the PSCs with as-prepared, FABr, FAI and MACl perovskites are evaluated according to the optical absorption and PL spectra of Figure 3a. The top of the VB is lowered, and the bottom of conduction band (CB) becomes shallow in the order of the as-prepared, FABr, FAI, and MACl perovskites. Interestingly, the energy alignment between the perovskite and hole transport layer is optimized by lowering the top of the VB, which enhances the electron and hole extraction efficiency from the perovskite to 20 ACS Paragon Plus Environment

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the electron and hole transport layer of SnO2 and spiro-OMeTAD, respectively. The shallow CB bottom of the SSGP perovskite films indicates an efficient electron blocking layer, enriches hole in the interface of perovskite and hole transport layer, which reduces the recombination loss. This is also confirmed by the results of impedance spectroscopy for the PSCs, as shown in Figure S18. Figures 5a–d show the PV performance of the PSCs using the as-prepared perovskite (standard-PSCs) and SSGP PSCs using MACl treated perovskite (MACl-SSGP PSCs). The PSCs were characterized with two scanning directions as following: the forward from the short-circuit current (JSC) to the open-circuit voltage (VOC) with a scanning rate of 50 mV/s; the reverse direction with same scanning rate from the VOC to JSC. The PV performance of the PSCs was significantly enhanced by the SSGP, as shown in Figure 5a and Table S2. The JSC and FF increased from 21.2 to 23.1 mA/cm2 and from 75.3% to 83.4%, respectively, in the MACl-SSGP PSCs under reverse scan condition. The PV performances of the standard and MACl-SSGP PSCs under forward scan were also shown in the Table S2, indicating the negligible hysteresis of both devices. Consequently, the PCE was significantly improved from 18.4% to 21.5% in the SSGP PSCs. The low PV performance of the standard PSCs19,21,24,26,34 is attributes to the too excess Pb(I0.82Br0.18)2 with high bandgap of 2.3 eV, 21 ACS Paragon Plus Environment


(b) Standard PSC MACl PSC

12

21.5%

JSC:

21.2

23.1 mA/cm2

VOC:

75.3 1.15

83.4% 1.12 V

6 FF:

0.0

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(d)

0.4 0.6 0.8 Voltage (V)

1.0

1.2

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0 17

(e)

18

19 20 PCE (%)

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20 15

Standard PSC J 0.95 V PCE 18.2% MACl PSC J 1.0 V PCE 21.3%

10 5 0 0

(f)

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0 100 150 200 250 300 Time (s)

100 1.0

Forward Reverse Dark Forward Dark Reverse

18 12

JSC

VOC

FF

PCE

6 Forward 23.3 1.14 80.0 21.3 Reverse 23.3

1.15 81.2 21.6

0 0.0

0.2

0.4 0.6 0.8 Voltage (V)

1.0

1.2

SSGP PSCs PCE Standard PSCs PCE

0.8

60

0.6

40

0.4 SSGP Humidity Standard Humidity

0.2 0.0 0

1.0

80

50

100 150 Time (h)

200

20

Humidity (%) Normalized PCE

0

Standard PSCs MACl-PSCs

12

PCE: 18.4

Counts

18

(c)

PCE (%)

18

Current density (mA/cm2)

24

Normalized PCE


(a)


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

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0 250

SSGP PSCs Standard PSCs

0.8 0.6 0.4 0.2 0.0 0

50 100 150 200 250 300 Time (h)

Figure 5. (a) PV performance of the standard and SSGP PSCs, measured under the scanning condition (scanning rate: 50 mV/s). The black (red) square symbol-line shows the data under forward (reverse) scan. (b) SPO of standard and SSGP PSCs. (c) Statistical distribution of the PCE, where 20 and 36 devices of standard and MACl-SSGP PSCs, respectively, were tested. (d) J–V curves of the highest-performance MACl-PSCs. (e) Time-evolutions of the normalized PCE of standard and SSGP PSCs. The PCE stability of the standard and SSGP PSCs was checked under continuous AM 1.5 light soaking at air condition ( 25.0 C). The time-evolution of the humidity is also shown. (f) Time-evolutions of the thermal stability of standard and SSGP PSCs was characterized at annealing temperature of 80 C in inert condition without any encapsulations.

because the as-prepared perovskite with a high density Pb(I0.82Br0.18)2 reduced the light absorption, and caused recombination loss at Pb(I0.82Br0.18)2. The obtained FF in the SSGP PSCs is the highest value in normal architecture PSCs with a high VOC37,38 and comparable with some inverted architectures as summarized in Table S3, and is very close to the Shockley-Queisser limit ( 90%) at room temperature.2,27,39 22 ACS Paragon Plus Environment

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The external quantum efficiency (EQE) of the standard and SSGP PSCs was characterized using incident-photon-to-current-efficiency spectroscopy (IPCE). Figure S9 shows the EQE spectra of the standard and MACl-SSGP PSCs. The EQE reached to an average value of 88.4% in the range from 400 to 730 nm with an integrated current density of 22.8 mA/cm2 for the MACl-SSGP PSC, which is significantly higher than that of the standard PSC (an average EQE of 84.9 % in the range from 400 to 730 nm with 21.0 mA/cm2). The EQE spectrum exhibits the rise of signal at a wavelength slightly shorter than

800 nm due to the larger

band gap of the perovskite after the SSGP17. The EQE of the SSGP PSC increases at 400 nm compared with that of the standard PSC owing to the reduction of Pb(I0.82Br0.18)2. We statistically evaluated the PV performance of the standard and SSGP PSCs using a large amount of devices (20 and 36 devices, respectively), as shown in Figure 5b and Figure S10. The PCEs of the SSGP PSCs were significantly higher than those of the standard PSCs, accompanied with a narrower distribution. All the PV parameters of the SSGP PSCs except for the VOC were significantly higher than those of the standard PSCs. The stabilized power output (SPO) with a bias voltage under the maximum-power-point (MPP) was extracted from the J–V curves, as shown in Figure 5c. The SPO of 21.3% for the SSGP-PSC is higher than that of 18.2% for the standard PSCs and exhibits better stability. The excellent performance 23 ACS Paragon Plus Environment


of the SSGP-PSCs might stem from the enhancement of the quality of perovskite layer and its interface between the perovskite and hole-transport layer, as well as the optimized band alignment. The highest PV performance of the MACl-SSGP PSCs in our study is shown in Figure 5d. The SSGP PSC exhibits an excellent PCE value of 21.3% (21.6%) with negligible hysteresis under forward (reverse) scan. This PCE is significantly higher than those of many invertedarchitecture PSCs with a high FF over 80.0% and comparable to those of normal-architecture PSCs with a high FF as shown in Table S3. The PV performances of the SSGP-PSCs using FABr, FAI and FACl are also shown in Figure S11. The FABr-, FAI- and FACl-SSGP PSCs exhibit little lower PV performance compared with MACl-SSGP PSCs (PCEs of 20.9%, 20.4% and 21.4%, respectively, and FFs of 77.1%, 77.7% and 79.1%, respectively.), which these results are coherent with above observations. The FABr-SSGP PSC exhibits a higher VOC (1.17 V) than in the FAI-SSGP PSC (1.14 V), as shown in Figures S10a and d. The PV performance of the MACl-SSGP PSCs fabricated under various growth conditions is presented in Tables S3–7, and the effects of spiro-OMeTAD are shown in Figure S12. According to previously reported results,30,38,40–42 the improved FF is accompanied by the reduction of VOC; however, the high PV performance of the SSGP PSCs in this study was 24 ACS Paragon Plus Environment

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achieved by improving of the FF without sacrificing the VOC. Note that the thickness of spiroOMeTAD passivation layer is also quite important. We found the PV performance and the hysteresis of SSGP PSCs become seriously worse and larger, when the concentration of spiro-OMeTAD is higher than 2 mg/mL. We fabricated large active area standard and SSGP PSCs using the optimized fabrication conditions. Figure S13a shows the PV performances of large-area standard and MACl-SSGP PSCs (1.0 cm2). The significantly higher PCE (19.9%) of the SSGP PSCs compared with that of the standard PSCs (17.8%) is realized in the large active area devices. The obtained FF is 74.5% in reverse scan, which is comparable to the record in previously reported results for a large active area PSCs ( 1 cm2)15,43–45 and significantly higher than the value for standard PSC (72.0%). The forward scan, SPO and statistical PV parameters of standard and SSGP PSCs are shown in Figure S13b and Table S8-9, indicating the superior performance of the SSGP PSCs even with a large active area. Characterization of continuous light soaking and high temperature stability of the standard and SSGP PSCs without any encapsulations were performed under harsh conditions with a high humidity of

50% and a high temperature of 80

. Figure 5e and Figure S14c show

the time evolutions of the normalized PCE and FF of the standard and SSGP PSCs. The 25 ACS Paragon Plus Environment


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SSGP PSC exhibits sustainable high PCE and FF values of 70.0% and 88.1%, respectively, even after 250 h continuous light soaking, which are significantly higher than those of the standard PSC (40.3% and 57.3%, respectively). The results for the VOC and JSC in the stability testing are shown in Figure S14 a-b. Figure 5e and Figure S14d-f show the thermal stability of the standard and SSGP PSCs at high temperature of 80 C in inert condition. The SSGP PSCs exhibited significantly better thermal stability, keeping

75% of the initial PCE

compared with the standard PSCs even after 270 h. The higher thermal stability of the SSGP PSCs is confirmed by the lower concentration of pin-holes at the interfaces of the perovskite and hole-transport layer compared with those in the standard PSCs, as shown in the crosssectional SEM images in Figure S15. Thus, the higher stability of the SSGP PSCs for continuous light soaking and high temperature conditions might be owing to the improvement of the quality of perovskite layer and interfaces by the SSGP. To confirm this, the aging tests monitored by XRD and PL spectroscopy were performed on the standard and SSGP perovskites fabricated after 10 d under ambient conditions, as shown in Figures S16 and 17, where the humidity conditions are summarized in Table S10. The XRD patterns for the as-prepared and SSGP perovskites show an increase of the peak at 13.0° from that of Pb(I0.82Br0.18)2; however, the degree of increase is reduced for the SSGP 26 ACS Paragon Plus Environment

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perovskite than for the as-prepared perovskite, as shown in Figure S4. The degree of the reduction of PL intensities and spectral broadening are larger in the as-prepared perovskite. These experimental results as show in the Supporting information (Figure S4, Figure S7, Figure S17, Figure S18 and Supplementary Note S1) strongly indicate that the improvement of the PV stability of the SSGP PSC is due to the robustness of the perovskite layer fabricated via the SSGP. Thus, the SSGP perovskite with higher crystallinity, flat surface, robust stability, and reduction of recombination loss, significantly improves the PV performance and stability of PSCs.

CONCLUSIONS We successfully achieved superior PV performance PSCs by combining the techniques of excess PbI2 and the SSGP. The excess PbI2 increases the crystallinity and grain size of the perovskite layer during the perovskite growth process. The SSGP allows the removal of excess Pb(I0.82Br0.18)2 and enhances the crystallinity of the perovskite, accompanied by the reduction of surface trap states. The SSGP PSCs achieve a high FF of 82.0% (83.4%) without sacrificing the JSC and VOC under forward (reverse) scan. The SSGP PSCs exhibits a high PCE of 21.3% with a negligible hysteresis, and maintain a significant better continuous light 27 ACS Paragon Plus Environment


soaking stability and thermal stability under harsh conditions. Moreover, we achieve a superior PCE of 19.9% with a negligible hysteresis for MACl SSGP PSCs on a large active area of 1 cm2. In addition to the improvement of the crystalline quality, the SSGP perovskite layer significantly promoted the charge-carrier extraction to carrier-transport layer and reduction of the recombination loss at the interfaces, resulting in high PV performance and stability of the SSGP PSCs. Our findings can be widely utilized and will provide a greatly impact on the development of PSCs.

ASSOCIATED CONTENT Supporting information Supporting Information is available from the ACS Publication website or from the author.

Additional data on perovskite film and device characterization using JV, EQE, PL, PL mapping, PL spectra distribution, continuous light soaking, thermal treatment and other additional results.

AUTHOR INFORMATION 28 ACS Paragon Plus Environment

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Corresponding Authors E-mail: [email protected] E-mail: [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS This work was supported by JSPS KAKENHI (Grant Numbers JP22740195, JP25400324, JP24681031, JP16H00911, JP15K13337, JP15H05408, JP23340085, JP25610074, and JP16H06331), the Canon Foundation and the Asahi glass foundation Center of Innovation Program from Japan, Exploratory Research for Advanced Technology (ERATO, JPMJER1302), the Center of Innovation Program (COI), and Advanced Low carbon Technology Research and Development Program (ALCA, JPMJAL1603) from the Japan Science and Technology Agency (JST), New Energy and Industrial Technology Development Organization (NEDO), and research program of the Institute of Materials and Systems for Sustainability (IMaSS), Nagoya University. F.Y. was financially supported by 29 ACS Paragon Plus Environment


China Scholarship Council (CSC). The purified PbI2 (L0279) sample and the substrates were kindly gifted by Tokyo Chemical Industry Co., Ltd. (TCI) and Oike Co., Ltd., respectively. We sincerely thanked Prof. Y. Shimakawa and Dr. T. Saito (Kyoto Univ.) for XRD characterization supporting, and T. Nakamura (Kyoto Univ.) for AC3 measurements instruction.

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Planar Perovskite Solar Cells with High Efficiency and Fill Factor

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