Perovskite Solar Cells with >19% Efficiency Achieved by Advanced

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Perovskite Solar Cells with >19% Efficiency Achieved by Advanced Three-Step Method Using Additional HC(NH2)2I-NaI Spin-Coating Yuji Okamoto, Masatomo Sumiya, and Yoshikazu Suzuki ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01978 • Publication Date (Web): 06 Feb 2019 Downloaded from http://pubs.acs.org on February 6, 2019

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ACS Applied Energy Materials

Perovskite Solar Cells with >19% Efficiency Achieved by Advanced Three-Step Method Using Additional HC(NH2)2I-NaI Spin-Coating

Yuji Okamoto†, Masatomo Sumiya‡, and Yoshikazu Suzuki*,§ †Graduate

School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai,

Tsukuba, Ibaraki 305-8573, Japan. ‡Widegap

Materials group, National Institute for Materials Science (NIMS), 1-1 Namiki,

Tsukuba, Ibaraki 305-0044, Japan. §Faculty

of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki

305-8573, Japan.

ABSTRACT Recently, a doping of sodium ion (Na+) into the perovskite solar cells has attracted much attention. Suppose that the Na+ ions substitute the A-site cation of the perovskite structure, the Na+ doping is expected to enlarge a bandgap of perovskite layer. Additionally, the Na+ doping may increase the grain size and decrease the trap density. We recently reported a new 3-step method using additional spin-coating of a HC(NH2)2I (FAI) solution on a 2-step prepared CH3NH3PbI3 (MAPbI3) layer containing an unreacted PbI2, which produced a multiple bandgap

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structure by forming a FA rich layer with a smaller band gap near the electron transport layer. Here, we attempted to form the top layer with larger bandgap by adding a small amount of NaI into the FAI solution for the 3rd step and investigated the effect of NaI post-treatment on the perovskite layer. The NaI addition slightly enlarged the bandgap of surface of the perovskite layer. Additionally, it accelerated the grain growth and effectively decreased grain boundaries, while the Na+ tended to increase non-radiative recombination. As a result, the NaI addition enhanced the power conversion efficiency from 18.12% (FAI only) to 19.07% (with NaI addition) without degrading stability in air. The appropriate amount of NaI addition on the FAI solution is beneficial to boost the photovoltaic performance.

KEYWORDS perovskite solar cells, perovskite layer, additional FAI spin-coating, Na+ ion, multiple bandgap structure

1. INTRODUCTION At the initial stage of research on perovskite solar cells, CH3NH3PbI3 (MAPbI3) and CH3NH3PbBr3 (MAPbBr3) were used for a light absorbing layer.1 Since the perovskite structure is composed of 3 sites (ABX3), the compositional engineering has been widely studied. In 2012, Lee et al.2 reported a power conversion efficiency (PCE) of 10.9% by mixing Cl−. After that, the A-site substitution of CH3NH3+ (MA+) by formamidinium (HC(NH2)2+, FA+) or Cs+ ion attracted much attention. FAPbI3 and CsPbI3 have better thermal stability than MAPbI3,3,4 and a bandgap of α-FAPbI3 is ~1.43-1.48 eV4–6 which is more suitable for the solar cell application. However,


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δ-FAPbI3 and δ-CsPbI3 (non-perovskite structure and unsuitable for a solar cell application) are stable at the room temperature,7,8 so it is difficult to achieve a high PCE with pure FAPbI3 or CsPbI3. Recently, it was revealed that the MA+ or Br− incorporation is effective to stabilize the α-FAPbI3 and α-CsPbI3.7–9 Therefore, the mixed composition of those cations and anions have been studied, and it demonstrated the larger PCE with keeping better stability than those of pure FAPbI3-, MAPbI3- or CsPbI3 based perovskite solar cells.10–12 Besides the compositional engineering, various preparation methods of perovskite layer were also developed, such as a sequential deposition method,13 a solvent vapor assisting method,14,15 and an anti-solvent dropping method.16,17 Consequently, the maximum PCE of perovskite solar cells currently reached to 23.7%.18 In order to keep the perovskite structure, Goldschmidt tolerance factor [𝑡 = (𝑟𝐴 + 𝑟𝐼) / 2( 𝑟𝑃𝑏 + 𝑟𝐼)] must be within ~ 0.8-1.1.19 The ionic radii are in the order of FA+> MA+> Cs+, and the other monovalent cations (Rb+, K+, Na+, Li+) are too small to keep the perovskite structure. However, Saliba et al.20 reported the PCE of >21% with high stability by doping appropriate amount of Rb+ into the CsMAFA system perovskite crystal. After that, K+ and Na+ doping have been studied since they are much cheaper than Rb.21–26 The Na+ has particularly smaller ionic radius (1.39 Å for A site with 12 coordination) 27 than those of Cs+ (1.88 Å), Rb+ (1.72 Å) and K+ (1.64 Å), so it is still not clear whether Na+ can be incorporated into the A-site or it is just incorporated into the interstitial sites. Still, as for the effects of Na+ and K+ doping, it is reported that the appropriate amount of Na+ and K+ doping is effective to increase the grain size of perovskite particles, to improve the crystallinity, and to decrease the trap density, resulting in the improved PCE.21–26



The compositional engineering is also effective to control a band alignment of the perovskite layer. By forming a compositional gradation (FA/MA/Cs cations or I/Br anions) in the perovskite layer, a band gradient structure can be obtained.28-30 Since the band gradient structure is a driving force for the carrier transportation, it is effective to suppress the carrier recombination and to improve the photovoltaic performance. The band gradient structures in the literatures were usually formed at the top side of the perovskite layer by post-treatments.28-30 However, we recently reported a formation of multiple bandgap structure at the bottom side using a 3-step method, i.e. by an additional spin-coating of MAI or FAI solution on a 2-step prepared MAPbI3 layer containing an unreacted PbI2 next to the TiO2 mesoporous layer.31,32 The additional FAI spin-coating converted MAPbI3 into FAxMA1-xPbI3 and also converted the unreacted PbI2 into a FA rich FAyMA1-yPbI3 layer (y > x) with a smaller bandgap at the bottom side, which improved the PCE. Suppose that Na+ ions can be incorporated into the A-site of perovskite structure, the Na+ doping is expected to increase the band gap, since the bandgap of "NaPbI3" is reported to be larger than those of FAPbI3 and MAPbI3.33,34 This hypothesis brought us an idea that an addition of small amount of NaI into the 3rd step FAI solution may form the band gradients at the top and bottom sides simultaneously. It will produce a larger bandgap area only in the vicinity of the surface with keeping the FA rich layer with the smaller bandgap at the bottom side as shown in Figure 1. The larger bandgap layer at the top side will further improve the PCE as the reported studies.28-30 Furthermore, the Na+ doping was carried out by mixing NaI into a perovskite precursor solution in the previous studies.24–26 On the other hand, since our case is post-treatment, it will give new insights about the effects of Na+ post-treatment. In this study, the FAI solution with NaI was spin-coated on the 2-step prepared perovskite layer for the band structure


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controlling and enhancement of photovoltaic performance. The effect of NaI addition was also investigated.

Figure 1. Schematic illustration of expected perovskite layer structure by the additional spincoating of FAI solution with trace NaI.

2. EXPERIMENTAL SECTION 2.1. Preparation of TiO2 compact and mesoporous layers Transparent conductive oxide (TCO, Type-0052, 10 Ω/sq., Geomatec) substrates were patterned by etching with Zn powder (>96.0%, Tokyo Chemical Industry) and 1 M HCl (Wako Pure Chemical Industry). Then, the substrates were cleaned by an ultrasonication in ethanol for 5 min and dried in air. The TiO2 compact layers were prepared by spray-pyrolysis method; a solution of titanium diisopropoxide bis(acetylacetonate) (TAA, 75wt% in isopropanol, SigmaAldrich) mixed in ethanol at the volume ratio of TAA:ethanol=2:25 was sprayed on the patterned substrates with heating at 500 °C. TiO2 mesoporous layers were then prepared by spin-coating a TiO2 paste (18NR-T, Dyesol), diluted in ethanol at the weight ratio of TiO2 paste:ethanol=1:5, on the substrates at 4000 rpm for 25 s and by annealing at 500 °C for 1 h. Then, the substrates were



exposed to ultraviolet light for 1 night in order to improve the contact with the PbI2 solution in the next section.

2.2. Preparation of perovskite layer, hole transport layer and metal electrode The perovskite layers and the hole transport layers (HTLs) were prepared in air with humidity of ~34%. The TiO2-coated substrates were pre-heated at 60 °C. A 1 M solution of PbI2 (>98.0%, Tokyo Chemical Industry) in DMF (99.5%, Nacalai tesque) was also pre-heated at 60 °C. The PbI2 solution was spin-coated on the substrates at 3000 rpm for 20 s (1st step). After annealing at 60 °C for 10 min, the substrates were dipped in a 10 mg/mL solution of MAI (98%, Wako Pure Chemical Industry) dissolved in isopropanol (99.5%, Nacalai tesque) for 20 s, followed by rinsing with isopropanol and by annealing at 60 °C for 10 min (2nd step). Then, 25 µL of 0.19 M FAI (32.4 mg/mL, ≥98%, Sigma-Aldrich) solution in isopropanol was dropped on the prepared MAPbI3 films. After the waiting time for 10 s, the substrates were spun at 4000 rpm for 25 s (3rd step). As for the NaI addition, the NaI was added in the FAI solution to be four different concentrations (1.5, 3, 6, 12 mM), and the solutions were used for the 3rd step spincoating. After the annealing at 60 °C for 10 min, the substrates were rinsed with isopropanol to eliminate excess FAI and NaI on the surface, and then annealed at 60 °C for 10 min again. The HTLs with the thickness of ~200 nm were prepared by spin-coating a spiro-MeOTAD solution at 4000 rpm for 35 s. The spiro-MeOTAD solution in 1 mL chlorobenzene (99%, Nacalai tesque) was composed of 73 mg spiro-MeOTAD (N2,N2,N2′,N2′,N7,N7,N7′,N7′-octakis(4methoxyphenyl)-9,9′-spirobi[9H-fluorene]-2,2′,7,7′-tetramine, 99%, Sigma-Aldrich), 28.8 μL 4tert-butylpyridine (TBP, 96.0%, Sigma-Aldrich) and 17 μL solution of [520 mg/mL lithium


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bis(trifluoromethylsulphonyl)imide salt (>98.0%, Tokyo Chemical Industry) in acetonitrile (99.5%, Wako Pure Chemical Industry)]. Finally, Ag electrodes with the thickness of ~50 nm were deposited on the HTLs with a thermal evaporator under the pressure of 3×10-5 Torr. Hereinafter, we denote the samples with spin-coating of “FAI solution” as “FAI only” and those with “FAI + NaI (1.5, 3, 6 and 12 mM) solutions” as “+NaI:1.5mM”, “+NaI:3mM”, “+NaI:6mM”, and “+NaI:12mM”.

2.3 Characterizations Crystal structures of the prepared perovskite layers on ETLs were characterized by X-ray diffraction (XRD, Multiflex, Cu-Kα, 40 kV and 40 mA, Rigaku). X-ray photoelectron spectroscopy (XPS) measurements were carried out using a JPS 9010 TR (JEOL) with an Al-Kα X-ray source (1486.6 eV). The top-surface and cross-section of the perovskite layers on ETLs were observed by scanning electron microscopy (SEM, JSM-6500F, JEOL). Atomic force microscopy (AFM, E-sweep, High-technologies) was used to measure the surface roughness in a dynamic force mode. The optical absorbance spectra were measured by ultraviolet-visible absorption spectroscopy (UV-Vis, UV-1280, Shimadzu). Steady-state photoluminescence (PL) was measured using a Ramascope System1000 (Renishaw) with an excitation laser of wavelength 325 nm (IK5651R-G, Kimmon). Current density-voltage (J-V) characteristics were measured using a solar simulator (XES-40S1, SAN-EI Electric), which was calibrated to AM 1.5, 100 mW/cm2 with a standard silicon photodiode (BS-520BK, Bunkoukeiki). The J-V curves were measured by forward (0V → 1.1V) and reverse (1.1V → 0V) scans. The voltage step and delay time were 20 mV and 50 ms, respectively. The device size was 1.25×1.25 cm2, and the



active area was limited to 0.084 cm2 by using a black mask. External quantum efficiency (EQE) was measured using a hyper monolight system (SM-250, Bunkoh-Keiki).

3. RESULTS AND DISCUSSION Figure 2(a) shows XRD patterns of the perovskite layers prepared on the ETLs by the 2step method and the 3-step method using additional FAI spin-coating. For the 2-step prepared perovskite layer, strong peaks were confirmed at 2 = ~14.1°, 28.4°, 31.8° and 35.3°, and a peak at 28.4° was split. These results indicate that the 2-step prepared perovskite layer is composed of tetragonal MAPbI3.35 Relatively large peaks of unreacted PbI2 at ~12.6° and ~39.5° were also confirmed. On the other hand, after the FAI additional spin-coating, the peaks of unreacted PbI2 were significantly reduced by the conversion into a perovskite phase. In addition, the peak at ~28.4° did not split and it shifted to a smaller angle, suggesting the phase transformation from tetragonal to cubic and the lattice expansion. Since FAPbI3 has a cubic structure and the ionic radius of FA+ (2.53 Å) is reported to be larger than that of MA+ (2.17 Å),20 these results suggest the change of MAPbI3 into FAxMA1-xPbI3, which is in good agreement with our previous report.32 Since the amount of peak shift is proportional to the ratio of FA+ in FAxMA1-xPbI3 following to the Vegard's law,36–38 the x value of FAxMA1-xPbI3 was roughly estimated by calculating the difference of lattice constant a for the 2-step and 3-step prepared perovskite layers and by comparing it with the reported value. ã = a /

2 was used for the 2-step prepared perovskite

layer (tetragonal MAPbI3) to compare with the lattice constant a of cubic FAxMA1-xPbI3. The ã for the 2-step prepared perovskite layer (tetragonal MAPbI3) was calculated from the peak


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position of 220 reflection, and the lattice constant a for the 3-step FAI spin-coated perovskite layer (cubic FAxMA1-xPbI3) was calculated from the peak position of 200 reflection. The 2-step and FAI spin-coated perovskite layers had ã = 6.277 Å and a = 6.333 Å, respectively, and the difference was 0.056 Å. Slimi et al.36 reported the relationship between the lattice constant a and the composition x in FAxMA1-xPbI3. By using their relationship, x value of our FAxMA1-xPbI3 layer was estimated to be ~0.4, i.e., FA0.4MA0.6PbI3. Figure 2(b) shows XRD patterns of the prepared perovskite layers on ETLs with additional spin-coating (3-step) of the FAI solutions with and without NaI addition. For all the samples, unreacted PbI2 peaks at ~12.6° were not obviously observed, which indicates that the NaI addition does not suppress the conversion of unreacted PbI2. For all the samples with NaI addition, the peaks corresponding to FA0.4MA0.6PbI3 were confirmed at the almost same position as FAI only spin-coated perovskite layer. The obvious peak shift was not confirmed even from the zoomed range at 13°-15° for the 100 peak and 27°-31.5° for the 200 peak, probably due to the small concentration of NaI. No impurity phase was confirmed under the detection limit of this XRD analysis. The XRD study clarifies that the NaI addition does not significantly change the crystal structure. The full width at half maximum (FWHM) calculated from the 100 peaks are shown in Figure S1. The FWHM tended to decrease with increasing the NaI addition amount, which implies the increases of grain size and the improvement of crystallinity.



Figures 2. XRD patterns of the perovskite layers (a) prerpaed by 2-step method and with additional FAI spin-coating (3-step) and (b) prerpared by 3-step method using additional spincoating of the FAI solutions with and without NaI addition.


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Figure 3 shows surface XPS spectra of narrow scans for C 1s, Pb 4f, I 3d, and Na 1s of the additionally FAI spin-coated perovskite layer without and with NaI addition of 3 or 12 mM. The C 1s peaks at ~286.5 eV may correspond to a C-C bond from organic contamination and that at ~288.5 eV may correspond to a C-N bond from FA or MA part. The C 1s peaks slightly shifted to larger energy compared with some reported positions,39,40 possibly due to the surface charging, but the peak positions were comparable in the three samples. There were also no significant differences in peak position and intensity for the peaks of Pb 4f (~138.8 eV and ~143.6 eV) and I 3d (~619.7 eV and ~631.2 eV)26 in all the samples. The +NaI:12mM sample showed a Na 1s peak at ~1072.5 eV, while the FAI only spincoated perovskite layer did not show a peak, which indicates the existence of Na on the perovskite layer of +NaI:12mM sample. To check the homogeneity of Na distribution on the perovskite layer, surface XPS spectra were collected at two different points in the +NaI:12mM sample as shown in Figure S2. The peak intensities of C 1s, Pb 4f, and I 3d were comparable between the two points, but the Na 1s peaks showed different peak intensity. This difference suggests that the Na distribution on the perovskite layer was not uniform in +NaI:12mM. Although the +NaI:3mM sample did not have an apparent Na 1s peak (detectable in +NaI:12mM) due to its small Na amount, the graph seems to have a small inflection at ~10721074 eV compared with that of FAI only spin-coated sample, which implies the existence of Na even in the +NaI:3mM sample. The Pb:I atomic ratio measured from XPS was estimated to be ~1:4 in all the sample. The Pb:I atomic ratio of ~1:4 was somewhat different from the theoretical Pb:I ratio of 1:3 in FAxMA1-xPbI3, which may be attributed to some excess I coming from the additional spin-coating process. The atomic % of Pb:I:Na of the +NaI:12mM sample were



estimated to be 77.5:20.4:2.1(%). Considering the chemical formula of FAxMA1-xPbI3, the Na atomic % in the perovskite layer must be less than 1%.

Figures 3. The surface XPS spectra of narrow scan for (a) C 1s, (b) Pb 4f, (c) I 3d, and (d) Na 1s of the additionally FAI spin-coated perovskite layer without and with NaI addition of 3 or 12 mM.

Figure 4 shows surface SEM images of the prepared perovskite layers on the ETLs. The size of perovskite particles for the FAI only spin-coated sample was ~0.5-1 μm, and the particles partially fused with the surrounded particles. This phenomenon is in good agreement with our previous report.32 As for the 2-step method, a grain growth is reported to proceed through Ostwald ripening growth model.41,42 Small MAPbI3 nuclei are firstly formed by the reaction of


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MAI and PbI2, and then, they are dissolved in the MAI solution and redeposited on other large MAPbI3 particles as following equation. CH3NH3PbI3 + I ―

→ ←

CH3NH3+ + PbI24 ―

(eq.1)

Therefore, the additional spin-coating dissolved the surface of 2-step prepared MAPbI3 particles, and the particles partially connected each other (i.e., liquid-phase assisted grain growth). However, there are still spatial gaps among the particles in the FAI only spin-coated perovskite layer. With increasing the NaI addition amount, the fusion of particles was gradually accelerated, and the spatial gaps and grain boundaries tended to disappear, which resulted in the increase of grain size. Although some pin-holes were still observed, the gaps among the perovskite particles almost disappeared in the +NaI:12mM sample, and the grain size became larger than that of the FAI only spin-coated sample. The decrease of FWHM in the XRD pattern (Figure S1) with increasing the NaI addition amount is attributable to this grain growth. For the 1-step method, the increase of grain size by the NaI addition into the precursor solution of the perovskite layer was reported by Bag et al.,24 where possible mechanisms were proposed as (1) promotion of nucleation of perovskite particles or (2) enhancement of grain boundary mobility by the addition of small Na+ ion. For our 3-step method, however, the first mechanism (promotion of nucleation) is not applicable because the additional spin-coating is a post-treatment. The second one (enhancement of grain boundary mobility) may be possible, but through the SEM observation (Figure 4), we think that another one, i.e., the liquid-phase assisted grain growth accelerated by the NaI addition, can be a most probable mechanism.



Figure 4. Surface SEM images of the prepared perovskite layers on the ETLs.

Figure 5 shows cross-sectional SEM images of the prepared perovskite layers on ETLs. For all the samples, the thicknesses of TiO2 compact layer, TiO2 mesoporous/perovskite layer, and perovskite capping layer were ~30 nm, ~150 nm and ~400 nm, respectively. As for the perovskite capping layer, the vertical grain boundary was hardly seen in the +NaI:12mM sample due to the accelerated fusion of perovskite particles. In addition, the shape of perovskite capping layer was macroscopically comparable among all the samples, but the microscopic surface 14 Environment ACS Paragon Plus

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texture tended to decrease with increasing the amount of NaI addition. Figure S3 shows AFM images of the perovskite layers for the +NaI:3mM and +NaI:12mM samples. The roughness of root mean square (RMS) of +NaI:12mM (74.9 nm) was similar to that of +NaI:3mM (73.9 nm), but a surface area of +NaI:12mM (116 μm2) was smaller than that of +NaI:3mM (120 μm2) in the measured area of 10×10 μm2. These results indicate that the macroscopic shapes of perovskite capping layers were similar, but microscopically, the perovskite particles in +NaI:12mM had flatter surface than +NaI:3mM, which is in good agreement with the results of cross-sectional SEM observation.

Figure 5. Cross-sectional SEM images of the prepared perovskite layers on ETLs.

Figure 6 shows absorbance spectra of the prepared perovskite layers on ETLs. The TCO glass was used as the substrates, and the absorbance of TCO glass and ETL were subtracted from the total absorbance spectra. The bandgaps were estimated by converting the absorbance spectra



into Tauc plots (Figure S4) as summarized in Table 1. The light was irradiated from the TCO glass side. All the samples showed absorption edges at the comparable wavelength of ~800 nm which corresponds to the bandgap value of ~1.55 eV, but the +NaI:12 mM sample had a slightly larger bandgap. This result suggests that a part of FA+/MA+ ions might be substituted by Na+ ions, since the increase of the bandgap can be induced by the possible incorporation of Na+ into the A site (FA+, MA+). In the wavelength range over the band gap (λ < ~800 nm), the absorbance spectra of +NaI:1.5-6mM samples were slightly larger than that of FAI only spin-coated sample, which is beneficial to produce larger JSC. On the other hand, the absorbance decreased when the NaI addition amount increased up to 12 mM. The increase of absorbance is attributable to the increased grain size by the NaI addition (Figure 4), while the decrease of absorbance for the +NaI:12mM sample is attributable to the decrease of microscopic surface texture of the perovskite layer as seen in Figure 5, which resulted in less light scattering at the top surface side.

Figure 6. Absorbance spectra of the prepared perovskite layers on ETLs.


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Table 1. Bandgaps of the prepared perovskite layers on ETLs estimated from the band edge of the absorbance spectra and Tauc plot.

Samples

Bandgap (eV)

FAI only

1.546

+NaI:1.5mM

1.546

+NaI:3mM

1.546

+NaI:6mM

1.546

+NaI:12mM

1.550

Figures 7(a) and 7(b) show normalized steady-state photoluminescence (PL) spectra of the prepared perovskite layers on ETLs measured from the top side (from perovskite layer side) and back side (from substrate side). A commercial slide glass plate (No. 10005000, Marienfeld) was used as the substrate for this measurement to avoid the absorption of the laser by the TCO layer. The wavelength of the excitation laser was 325 nm, and the penetration depth into the (FA,MA)PbI3 layer was estimated to be ~20-25 nm using the inverses of their absorption coefficients at the 325 nm (α=~4-5×107 m-1).43 The PL spectra were measured at five points for each sample, and we confirmed that the PL peak positions did not depend on the measurement point as shown in Figure S5. When measuring from the top side (Figure 7(a)), the PL peak top position of band to band transition for the FAI only spin-coated perovskite layer was ~792 nm. As for the NaI added samples, the peak position was almost the same as that of FAI only spin-coated perovskite layer up to +NaI:3mM, but small peak shift to shorter wavelength were confirmed for the +NaI:6mM



and +NaI:12mM, indicating the increase of bandgap by the NaI addition. This tendency is in good agreement with the bandgaps estimated from the absorbance edge. These results also imply the substitution of FA+ and MA+ with Na+. When measuring from the back side (Figure 7(b)), the PL peak top position of FAI only spin-coated sample shifted to longer wavelength of ~798 nm than that of top side. This indicates the formation of the FA rich layer with the smaller bandgap near the ETL due to the conversion of PbI2 by the additional FAI spin-coating.32 With the NaI addition, all the samples showed the peak top at the comparable position to that of FAI only spin-coated sample. These results imply that the Na+ did not substantially reach near the ETL due to the small addition amount. As a result, the perovskite layer which consisted of the FA rich layer with the smaller bandgap near the ETL and the Na+ doped layer with the larger bandgap at the top surface was achieved as shown in Figure 7(c). Hence, this 3-step method is effective to form the band gradients at the both top and bottom sides of perovskite layer just by a simple spin-coating method. Figure 7(d) shows relative PL intensities of the prepared perovskite layers divided by that of FAI only spin-coated perovskite layer. The perovskite layer was prepared on a quartz glass (MTWKS100PP01, MONOTECH Co., Ltd.) without ETL to avoid a quenching of the PL intensity. The relative PL intensity of +NaI:1.5mM was comparable to that of FAI only spin-coated sample, but the relative intensity tended to decrease by increasing the addition amount of NaI. The decrease of relative PL intensity suggests an increase of non-radiative recombination related to trap states or defects.44 DeQuilettes et al.45 reported that the non-radiative recombination frequently occurred at the grain boundary in the perovskite layer. In this study, the relative PL intensity decreased with increasing the addition amount of NaI (Figure 7(d)) despite the


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decrease of grain boundary, which implies that the doped Na+ ions act as trap states or defects causing the non-radiative recombination. Chang et al.25 and Zhao et al.26 reported that the appropriate amount of NaI or KI addition into the perovskite precursor solution increased the PL intensity and carrier lifetime by reducing the non-radiative recombination induced by the trap states or defects. However, too much NaI or KI addition decreased the PL intensity and carrier lifetime, possibly due to an excess Na+ or K+ cations.21,25 In our case, as is discussed for the surface XPS at the two different points (Figure S2), the peak intensity of Na was dependent to the measurement point (i.e., heterogeneous Na+ distribution particularly for +NaI:12mM). However, all the PL spectra at five different points were identical (Figure S5). Hence, it is deduced that further excess Na+ ions were not incorporated within the (Na)FAxMA1-xPbI3 top layer (Figure 7(c)) but heterogeneously remained on the top layer, and these excess Na+ on the surface possibly caused the non-radiative recombination.



Figures 7. Steady-state photoluminescence (PL) spectra of the prepared perovskite layers on ETLs measured from the (a) top side (from perovskite layer side) and (b) back side (from substrate side). (c) Expected band structure of the additionally FAI spin-coated perovskite layer with NaI addition. (d) Relative PL intensity of perovskite layers prepared on quartz glasses divided by that of FAI only spin-coated perovskite layer.

The current density-voltage (J-V) characteristics of the prepared perovskite solar cells measured with a reverse scan are summarized in Table 2, and the J-V curves are shown in Figure 8(a). The values in Table 2 are the average of 15 devices. The box plots of the parameters are shown in Figure 8(b). The FAI only spin-coated sample demonstrated JSC: 23.85


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mA/cm2, VOC: 0.985 V, FF: 0.771 and PCE: 18.12%. As for the NaI added samples, the averaged JSC values of +NaI:1.5, 3, and 6mM (24.09, 24.16, and 23.74 mA/cm2) were similar to that of FAI only spin-coated sample, but slight increases were confirmed from the box plots for +NaI:1.5mM and +NaI:3mM. By increasing the amount of NaI addition up to 12 mM, JSC decreased to 23.11 mA/cm2. Figure S6 shows EQE spectra of the FAI only, +NaI:3 and 12mM samples. The shape of EQE spectra was comparable in all the samples, but the +NaI:3mM sample had the slightly higher EQE spectrum and integrated current density (22.8 mA/cm2) than the other samples (FAI only: 22.3 mA/cm2, +NaI:12mM: 22.1 mA/cm2). The tendency was similar to that of JSC obtained from J-V measurement, but the values were slightly smaller possibly due to the differences of measurement method. The slight increase of JSC for +NaI:1.5 and 3mM is due to the enhanced light absorption as shown in Figure 6 by the increased perovskite grain size, while the decrease of JSC for +NaI:12mM is attributable to the decreased light absorption by the decrease of microscopic surface texture (Figure 5). The increase of nonradiative recombination (Figure 7(d)) could also cause the decrease of JSC, since Chang et al.25 also confirmed the decrease of JSC by the doping of too much Na+. The VOC was also improved for the +NaI:1.5, 3, and 6mM (1.000, 1.005, and 1.004 V) compared with that of FAI only spin-coated sample, but the NaI:12mM showed smaller VOC (0.996 V) than the other NaI added samples. Since the reduction of carrier recombination increases the VOC, the increase of VOC for +NaI:1.5-6mM is mainly attributable to the decrease of trap sites or defects (via grain growth), whereas the decrease of VOC for +NaI:12mM is mainly attributable to the increase of trap sites or defects (via excess Na+ heterogeneous dispersion on the surface). The multiple bandgap structure shown in Figure 7(c) may also contribute to the enhanced VOC by improving the carrier transportation. However, this effect should be minor in



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this study, since the +NaI:12mM sample which had the largest bandgap difference (hence the highest expected VOC) showed smaller VOC than the other NaI added samples. The FF also increased to 0.781, 0.785, and 0.783 for the +NaI:1.5, 3 and 6mM, respectively, and it decreased to 0.774 for the +NaI:12mM. This improved FF is due to the decrease of grain boundary by the NaI addition. As a result, higher averaged PCEs of 18.82% (+NaI:1.5mM), 19.07% (+NaI:3mM), 18.66% (+NaI:6mM) and a smaller PCE of 17.83% (+NaI:12mM) than that of FAI only spin-coated sample were obtained. The larger scattering of J-V characteristics for the +NaI:12mM (Figure 8(b)) is ascribed to the heterogeneous distribution of excess Na+ on the top layer. From these results, it was found that the appropriate amount of NaI addition into the FAI solution is beneficial to boost the photovoltaic performance of the 2-step and 3-step prepared perovskite solar cells. In this study, the NaI addition amount of 3 mM demonstrated the best photovoltaic performance.

Table 2. The current-voltage (J-V) characteristics of the prepared perovskite solar cells. The values are average of 15 devices.

Samples

JSC (mA/cm2)

VOC (V)

FF

PCE (%)

FAI only

23.85

0.985

0.771

18.12

+NaI:1.5mM

24.09

1.000

0.781

18.82

+NaI:3mM

24.16

1.005

0.785

19.07

+NaI:6mM

23.74

1.004

0.783

18.66

+NaI:12mM

23.11

0.996

0.774

17.83


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Figures 8. (a) J-V curves of the prepared solar cells measured with a back scan. (b) Box plots of JSC, VOC, FF and PCE values for the prepared devices. 15 devices were prepared for each condition. In the box plots, center dots and horizontal lines represent average and median values.

Figure 9(a) shows normalized PCEs as a function of time. After the J-V measurement, the devices were stored in dark at humidity of ~50%, and these processes were repeated. The FAI only spin-coated sample gradually degraded after the fabrication, and its PCE decreased to ~80%



of the original value after ~350 h. Similarly, +NaI:1.5mM and 3mM samples showed comparable degrading speed as the FAI only spin-coated sample, and their PCEs also decreased to ~80% of the original value after ~350 h. On the other hand, the PCEs of +NaI:6mM and 12mM samples decreased up to ~70% after 350 h, which indicates the poorer stability in air. Figure 9(b) shows XRD patterns of the prepared devices just after the fabrication and after 350 h. The slight increases of PbI2 peak intensity were observed in all the devices after 350 h due to the decomposition of perovskite into PbI2. Particularly, the +NaI:12mM showed the larger increase of PbI2 peak intensity compared with the other samples, which is in good agreement in the poor stability (Figure 9(a)). Although the obvious peak shift by the NaI addition was not confirmed in the XRD patterns (Figures 2 and 9(b)), the small Na+ ions might destabilize the perovskite structure, which possibly causes the poor stability in air with larger NaI addition amount. These results indicate that only the appropriate amount of NaI addition into the 3-step FAI solution can enhance the photovoltaic performance without degrading the stability in air.

Figures 9. (a) Normalized PCEs as a function of time. The devices were stored in the dark with humidity of ~50% after the measuring, and this process was repeated. (b) XRD patterns of the prepared devices just after the fabrication and after 350 h. 24 Environment ACS Paragon Plus

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4. CONCLUSIONS In conclusion, we have demonstrated the effects of NaI addition into the additional FAI spin-coating solution and the enhancement of photovoltaic performance. The NaI addition accelerated the grain growth of the perovskite layer and effectively decreased the grain boundary. The increase of grain size slightly improved the light absorption, while +NaI:12 mM showed decreased absorbance. The PL peak positions suggested that the NaI addition into the FAI spincoating solution could form the perovskite layer consisted of Na+ doped layer with the larger bandgap at the top side and FA rich layer with the smaller bandgap at the bottom side. On the other hand, the PL intensity decreased by the NaI addition in spite of the decreased grain boundary, indicating the Na+ possibly produced the defects or trap states and increased the nonradiative recombination. As a result, the PCE was enhanced from 18.12% (FAI only spincoating) to 19.07% (+NaI:3mM) without decrease of stability in air, while too much NaI addition decreased the photovoltaic performance to 17.83% (+NaI:12mM). The appropriate amount of NaI addition into the additional FAI spin-coating solution is beneficial to boost the photovoltaic performance. Toward future applications, the spin-coating based 3-step method developed in this study can be further optimized for a large-area production e.g. by using FAI-NaI vapor treatment or FAI-NaI dip-coating.

ASSOCIATED CONTENTS Supporting Information



FWHM for 100 peaks in XRD (Figure S1), surface XPS spectra measured at another point in +NaI:12mM sample (Figure S2), AFM images of the perovskite layers for +NaI:3mM and 12mM (Figure S3), Tauc plots for the prepared perovskite layers (Figures S4), normalized PL spectra measured at five points for +Na:12mM (Figure S5) and EQE spectra of FAI only, +NaI:3mM and +NaI:12mM (Figure S6).

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We thank the Kato Foundation for Promotion of Science for financial support (KS-2826). We also thank Dr. Takeshi Yasuda at NIMS for the help of EQE measurement. 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.


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Perovskite Solar Cells with >19% Efficiency Achieved by Advanced

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