Stabilization of α-CsPbI3 in Ambient Room Temperature Conditions by

Article Cite This: Chem. Mater. XXXX, XXX, XXX−XXX

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Stabilization of α‑CsPbI3 in Ambient Room Temperature Conditions by Incorporating Eu into CsPbI3 Ajay Kumar Jena,* Ashish Kulkarni, Yoshitaka Sanehira, Masashi Ikegami, and Tsutomu Miyasaka* Toin University of Yokohama, 1614 Kurogane-cho, Aoba, Yokohama, Kanagawa, 225-5055 Japan

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S Supporting Information *

ABSTRACT: Although inorganic perovskite, CsPbI3, shows superior thermal stability over organic−inorganic hybrid perovskites, stabilization of the photoactive black phase (α-CsPbI3) of CsPbI3 perovskite at room temperature and in ambient conditions has remained a challenge. Herein, we present a method of stabilizing the α-CsPbI3 at lower annealing temperature (85 °C) by incorporation of Eu3+ (EuCl3) into CsPbI3, which prevents the black to the yellow phase (δ-CsPbI3) transformation in ambient air (room temperature) for a reasonably long time (>30 days). Photovoltaic performance of this Eu-stabilized α-CsPbI3, as assessed in planar heterojunction solar cells (FTO/TiO2/CsPbI3:xEu/spiro-OMeTAD/Au), shows a power conversion efficiency above 6% on backward scan (stabilized power output above 4%) for CsPbI3:xEu cells with 5−6 mol % of Eu, while CsPbI3 without Eu, as expected, shows no photovoltaic property at all. However, as the cell stability was found to be affected by composition of organic hole transport material (HTM) (spiro-OMeTAD) and morphology of CsPbI3 film, it is believed that optimization of cell composition and structure with a more suitable HTM will further improve the cell performance, as well as life.

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perovskite structures is cesium (Cs+). Cs forms perovskite structures like CsPbX3 and CsSnX3 (X = Cl, Br, I) and has been considered as a model compound among all-inorganic perovskites. Despite its superior stability, utilization of CsPbI3 in perovskite solar cells has several issues to be solved, and therefore, progress in PCE of these cells has been slow. The imposing challenge with CsPbI3 is stabilization of its black photoactive phase (α-CsPbI3) at room temperature (RT) because CsPbI3 preferably crystallizes in a yellow phase (δCsPbI3) at RT,18 while the black phase is stable only at temperatures above 335 °C.18 A tolerance factor of 0.81 favors formation of an orthorhombic yellow phase (δ-CsPbI3) over cubic black phase (α-CsPbI3) at room temperature. The size of Cs cation is too small to sustain the PbI6 polyhedra in cubic αCsPbI3 at room temperature, and therefore, it readily degrades to the orthorhombic δ-CsPbI3. One of the ways to stabilize αCsPbI3 is to increase the tolerance factor, which can be accomplished by partial substitution of either Cs+ (r = 167 pm) with bigger cations like MA+ (r = 217 pm) and FA+ (r = 253 pm) in the A − site or I− ions (r = 220 pm) with smaller anions like Br− (r = 196 pm) and Cl− (r = 181 pm) in the Xsite. The other strategy to stabilize the cubic structure (more symmetry) is to reduce the crystal dimensions (size-controlled

INTRODUCTION Perovskite solar cells based on inorganic−organic hybrid perovskites (APbX3, A = organic cation, X = halide ion) have voyaged from a power conversion efficiency (PCE) of 3%1 to above 22%2 in just few years. However, despite this rapid rise in efficiency, the new technology is not yet ready for commercialization because of two pressing challenges: inadequate device stability3,4 and toxicity of Pb.5,6 Although some recent studies7−10 have shown impressive results on long-term stability by improving the structural/intrinsic stability of perovskites, a large number of studies3,11−15 have directly or indirectly disclosed and discussed the facts of both material and performance instabilities. Although mixing of different cations (methylammonium-MA, formamidinium-FA, Cs) and anions (I, Br) at certain suitable proportion tends to show improved intrinsic stability of the perovskites,10,16 use of organic cations (e.g., MA, FA) still remains a compelling concern because they are believed to be responsible for poor thermal and environmental stability of these materials. For instance, MAPbI3 degrades easily at temperature as low as 120 °C,16 even at 80 °C if heated for long time. FAPbI3 has a better thermal stability over MAPbI3,17 but phase instability of the former under ambient environment17 still remains a challenge for long-term stability. Therefore, replacement of the organic cations completely with inorganic cations is considered to be a good strategy to improve stability. One such inorganic cation that has been found suitable and successfully incorporated in © XXXX American Chemical Society

Received: May 1, 2018 Revised: September 14, 2018 Published: September 16, 2018 A

DOI: 10.1021/acs.chemmater.8b01808 Chem. Mater. XXXX, XXX, XXX−XXX

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thickness was prepared on the clean FTO by spin-coating (3000 rpm × 30 s) a diluted solution of titanium diisopropoxide bis (acetylacetonate) (i.e., [Ti(acac)2(iPrO)2]) (75 wt %, Aldrich) in ethanol (100 μL Ti (acac) + 1 mL EtOH) 2 times with 10 min of drying at 120 °C between first and second coating. Then, the precursor coated FTO substrates were dried at 120 °C again for 10 min before being sintered at 500 °C for 1 h in a muffle furnace. These substrates were then given UV−ozone treatment for 10 min before the perovskite precursor solution (mix of PbI2, CsI and EuCl3 in DMF and DMSO) was coated on them. A mix solution of 1 M PbI2 (Tokyo Chemical Industry) and 1 M CsI (Tokyo Chemical Industry) was prepared by mixing them in a mixed solvent of DMF and DMSO (3:1 v/v). EuCl3 (Sigma-Aldrich) solution of 0.1 M concentration was prepared in a mixed solvent of DMF and DMSO (3:1 v/v). To incorporate Eu3+ of different concentrations into CsPbI3, desired volume of 0.1 M EuCl3 solution was mixed with the mix solution of PbI2 and CsI. For incorporating EuI2 into CsPbI3, 0.1 M EuI2 (SigmaAldrich) solution was used in a similar manner. The perovskite film was made by spin-coating the precursor solution: 1000 rpm for 10 s followed by 5000 rpm for 25 s, and 500 μL of chlorobenzene was dripped on the spinning substrates for 5 s before the spin-coating stopped. The spin-coating was done in ambient environment, not in a glovebox. The precursor coated substrates were then annealed at 85 °C on a hot plate for 15 min to from CsPbI3:xEu crystals. After the perovskite films were annealed and cooled to room temperature, a 12 wt % solution of 2,2′,7,7′-tetrakis(N,N-di-4-methoxyphenylamino)9,9′-spirobifluorene (spiro-OMeTAD, from Merck) including lithium bis(trifluoromethylsyfonyl) imide salt (Li-TFSI) and tert-butylpyridine (t-BP) as dopant and additive respectively in chlorobenzene was coated at a spinning speed of 4000 rpm for 30 s. The spiro-OMeTAD layer was either aged overnight in dry and dark condition to promote oxidation or not aged before gold contact was thermally evaporated on to the spiro-OMeTAD. Perovskite Film and Device Characterization. X-ray diffraction pattern of perovskite (CsPbI3:xEu) was checked with a Bruker Model D8 Discover XRD machine. Scanning electron microscopy (SEM) (SU8000, HITACHI) was used to check the film morphology. UV−visible absorption spectra of the Eu-doped CsPbI3 films were measured with a UV−vis spectrophotometer from Shimadzu (modelUV 3600). Photoluminescence of the films was measured with a fluorescence lifetime spectrometer from Hamamatsu (model C11367). A laser source of wavelength 465 nm was used for excitation. To confirm presence of Eu in the CsPbI3:xEu films, energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) analysis were done using HITACHI-SU 8000 and ULVAC-PHI5000 Versa Probe II machines, respectively. Photovoltaic characterization of the devices was done by measuring current on voltage scan in forward (−0.1 to 1.2 V) and backward (1.2 to −0.1 V) directions both under light (100 mW cm−2, AM1.5, 1 sun intensity) provided by a solar simulator (Peccell Technologies PECL01) and in the dark. A Keithley source meter (Model 2400) was used for all electrical measurements. The voltage scan speed used to obtain photocurrent density−voltage (J−V) curves was kept at 200 mV/s. The active area of the device was 0.09 cm2. For steady state current measurement, current from the cells was measured for more than 2 min at a bias voltage equal to voltage corresponding to maximum power point (determined form current−voltage (I−V) scan). To note, all the measurements were taken on nonencapsulated cells/films in ambient atmosphere (30−40% humidity) and at room temperature.

stabilization) as reduction in size of crystals always leads to formation of a crystal structure with more symmetry. Hence, suitable foreign dopants or even additives in the precursors of CsPbI3 that can restrict the dimensions of crystals to few nanometers (nanocrystals) can effectively stabilize the cubic CsPbI3. Addition of HI,18 Bi,19 ethylenediammine (EDA),20 or phosphonic acid21 to CsPbI3 has been found to stabilize the black phase at room temperature; as a result, efficiency of CsPbI3 based cells has gone from about 318 to 13%,19 and the stability has been improved reasonably. Eu has been widely used as a dopant in phosphor compounds like ZnS:Eu, SrAl2O4:Eu, KSrPO4:Eu, and LiCaPO4:Eu to increase the luminescent emission and also as an activator in a variety of scintillating materials such as SrI2:Eu, CsI:Eu,22 and CsCl:Eu.22 Eu doping in ferroelectric oxide perovskites like PbTiO3,23 BaTiO3,24 and BaSnO325 has also been studied well in the past, showing successful incorporation of Eu at Pb2+ and Ba2+ crystal sites. Moreover, Eu2+ having almost same size (1.17 Å) of Pb2+ (1.19 Å) has been confirmed to form Eu-based perovskite compounds such as RbEuI3 and CsEuI3.26 Recently, Wang et al.27 found that Eu2+-doping into CH3NH3PbI3−xClx could improve the cell efficiency and stability. Improved optical properties of CsPbCl3 nanocrystals by doping lanthanides28 (Ce3+, Eu3+, Sm3+, Er3+, etc.) have been also reported by Pan et al. In fact, very recently, while we were conducting our present study, Yuan et al. reported about synthesis of Eu-doped CsPbBr3 quantum dots for LED applications.29 Hence, it was our curiosity to know if europium (Eu2+ or Eu3+) could be doped into CsPbI3 and influence the black phase evolution. In this study, we explored inclusion of Eu3+ into CsPbI3 and examined its effect on stabilization of black phase and photovoltaic properties of devices made of CsPbI3 including Eu. As found here, addition of certain amount of EuCl3 into precursor of CsPbI3 stabilizes the black phase at room temperature and in ambient conditions (not processed in dry N2 or Ar environment in the glovebox), and stability of the formed black phase varies with Eu concentration: the greater the Eu content, longer the life of the black phase. Although the methods of fabrication and accompanying layers in the solar cells using Eu-stabilized CsPbI3 are not optimized, a power conversion efficiency of above 6%, as observed for 5−7 mol % Eu, suffices potential improvement in future. Different amounts of Eu3+, starting from 0.5 to 10 mol %, were incorporated into CsPbI3 by mixing appropriate amount of 0.1 M EuCl3 solution to 1 M solution of CsI and PbI2. It must be noted that the composition of the perovskite presented here is nonstoichiometric (CsPbEuxI3, x = 0.001, 0.005, 0.02, 0.04, 0.05, 0.06, 0.08, 0.1) because the stoichiometric composition, although it gave similar results of black phase stabilization, was not stable enough (brown color changing during measurements). This observation is rather interesting and suggests that controlled change in the ratio of CsI and PbI2 may have a significant effect formation and stability of the black phase. Indeed, very recently, Xiang et al. reported that excess of CsI in the mix solution of PbI2 and CsI can form the black phase (α-CsPbI3) along with Cs4PbI6 at 110 °C.30

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RESULTS AND DISCUSSION The CsPbI3:xEu films were annealed at 85 °C for 15 min on a hot plate in an ambient atmosphere with a relatively low humidity (10−20% RH). As observed during the process of annealing, the CsPbI3 film without Eu turned yellow, forming the δ phase of CsPbI3 within 5 min, while the films with 0.5 and 1 mol % Eu remained brown until they were on a hot plate at 85 °C and slowly turned yellow when cooled and stored at

EXPERIMENTAL SECTION

Perovskite Film and Device Fabrication. First, FTO substrates were cleaned with soap solution (Hellmanex) followed by acetone and finally with distilled water. A TiO2 compact layer of ∼50 nm B

DOI: 10.1021/acs.chemmater.8b01808 Chem. Mater. XXXX, XXX, XXX−XXX

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showed no absorption in 500−800 nm wavelength range, while the films with Eu showed an absorption edge above 700 nm, which matches with that of the black phase (formed at 320 °C) but with a slight blue shift. As can be seen in Figure S2b, the PL emission peak (λ = 695 nm) of all the Eu-containing CsPbI3 matched that of the black phase formed at high temperature (320 °C), while the yellow phase of CsPbI3 (without Eu) showed no emission at all in the same wavelength range. There was a clear and significant variation in the intensity of PL emissions for the films with different Eu content, increasing from 4 to 8% Eu, which might be due to increasing conversion of the yellow phase to black phase with increasing Eu content. However, reduction of the intensity for 10-Eu case, although not confirmed so far, is suspected to be due to significantly thinner film (looking transparent) because the solution used in the case of 10 Eu was diluted further (EuCl3 solubility is not high in DMF and DMSO). Thermal analysis of the CsPbI3 powder with 0, 5, and 10 mol % Eu (prepared by grinding PbI2, CsI2, and EuCl3 powder together for more than 30 min and then storing for a week before measurement) demonstrated a clear difference in crystallization temperature of the black phase (Figure S3). Pure CsPbI3 (without Eu) showed only one endothermic peak at around 320 °C, corresponding to formation of the black phase, while CsPbI3 containing 5 and 10% Eu showed two major endothermic peaks (one at around 75 °C and the other above 300 °C) and two minor endothermic peaks (one at around 100 °C and the other at 120 °C). The endothermic peak at 75 °C (for 5 and 10% Eu) corresponds to crystallization of the black phase as the XRD patterns of films with Eu, when annealed at around 85−90 °C, shows diffraction peaks matching with the black phase. The second major crystallization peak above 300 °C (300 °C for 10 Eu and 305 °C for 5 Eu) is reformation of black phase because it was observed that Eu-containing CsPbI3 films, which were brown after annealing at 85 °C, turned light yellow when heated at higher temperature (100−180 °C), and then this yellow color changed to brown at temperatures above 300 °C, forming the black phase again (Figure S4a). As confirmed from the XRD (Figure S4b), the intermediate yellow phase was nothing but the δ-CsPbI3. Crystal changes corresponding to the two minor endothermic peaks at 100 and 120 °C have not yet been investigated thoroughly. XRD measurement at varying temperatures is required to know the crystal/structural changes happening precisely at 100 and 120 °C. EDX (Figure S5 and Table S1) and XPS (Figure S6) analysis confirmed presence of Eu in all the CsPbI3:xEu films. Compositional mapping by EDX shows uniform distribution of Eu over the films (Figure S5). To know the site of location of Eu3+ in CsPbI3 crystals, shift in the XRD peaks was analyzed. As shown in Figure 3 and Figure S7, both the (100) and (200) XRD peaks shifted to lower 2θ with increasing content of Eu, implying lattice expansion. This suggests that Eu3+ probably occupies some interstitial sites instead of replacing Pb2+, which would have otherwise resulted in lattice contraction due to smaller ionic radii (0.95 Å) of Eu3+ than Pb2+ (1.19 Å). Therefore, unlike Bi3+,19 which has been assumed (without direct evidence) to substitute Pb2+ and thus increase the tolerance factor, Eu3+ possibly does not substitute Pb2+. Stabilization of the black phase in this case can be ascribed essentially to reduction in grain/particle size, which was evident from the scanning electron micrographs.

RT. When the Eu content in the precursor was increased further from 1 to 2, 4, 5, 6, 7, 8, and 10 mol %, we found that the formed black phase of CsPbI3 lasted for a longer time. The X-ray diffraction (XRD) patterns of the films with different concentrations of Eu are given in Figure 1. Peaks of (012),

Figure 1. XRD patterns of CsPbI3:xEu (x = 4, 5, 6, 7, 8, and 10 mol %) films.

(112), and (122) planes of the orthorhombic phase (δCsPbI3) disappeared, and the (100), (110), and (200) peaks of the cubic phase (α-CsPbI3) emerged, demonstrating clearly that the brown color of the films containing Eu was due to formation of the black phase of CsPbI3. This confirmed that Eu was stabilizing the α-CsPbI3 at RT and in ambient conditions and was increasing its life with increasing Eu concentration. Figure S1 shows the photographs of CsPbI3 films with different amounts of Eu prepared and stored at room temperature and in the dark for one month. As can be seen in Figure S1, the brown color of films containing Eu lasted for longer time, while the black film obtained by heating CsPbI3 (without Eu) at 320 °C became yellow in just 1−2 days. And, it was observed that films with higher concentration of Eu were stable for longer time. For instance, as can be seen in Figure 2, there was no

Figure 2. XRD patterns of CsPbI3 film with 10 mol % of EuCl3 on day 1 and day 30. The film was stored in dry (10−20% RH) and dark condition.

change in XRD pattern of CsPbI3 film containing 10 mol % Eu even after 30 days of storage in relatively dry (10−20% RH) ambient atmosphere (displayed peaks of only the α-CsPbI3, no peaks from δ-CsPbI3 were observed). Formation of α-CsPbI3 (black) was also evident from UV− vis and photoluminescence (PL) spectra. The UV−vis spectra (Figure S2a) of the films containing no Eu (yellow CsPbI3) C

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leads to formation of a distorted cubic structure19 at low temperature. Hence, it is believed that Eu inclusion into CsPbI3 crystal stabilizes the black phase via reduced grain size and increased microstrain in the crystals. However, SEM micrographs of films with higher (8 and 10%) Eu concentration showed some spike structures (Figures 4d and e), which we suspect to be excess of EuCl3 segregated/expelled out of crystal lattice. Although XRD patterns do not show any peaks from EuCl3, which can be due to insufficient crystallinity, it is most likely that EuCl3 is being separated out. However, compositional analysis exactly on these structures is required to confirm if these are excess of EuCl3 or any other secondary phase is being formed and separated out of the black phase. Regardless, with higher content of Eu3+ in the solution, the black phase had longer stability, which was consistent with smaller grains. Further, these spike structures, which are suspected to be excess of EuCl3, probably prevented grain growth during aging and retained the black phase for a longer time. It is worth mentioning here that we also found that Eu2+ (1.17 Å), which is almost same size of Pb2+, also stabilizes the black phase at low temperature (XRD of black phase stabilized by 5 and 8 mol % of EuI2 is given in Figure S9). However, the stability of Eu3+ stabilized α-CsPbI3 films were reasonably longer than those with Eu2+. Indeed, although we used EuCl3 as the starting precursor for Eu3+-doping, the XPS analysis (Figure S6) showed existence of small amount of Eu2+. These results, in fact, indicate that size of Eu (Eu3+ or Eu2+) and site of location in the crystals might not play a major role in stabilization of α-CsPbI3. Instead, reduction in grain size/ crystal size, which must be linked to its role in the process of crystallization, is the main reason for black phase stabilization at room temperature. This was indeed apparent from observed grain growth following degradation (turning yellow from brown after long time exposure to ambient conditions) of Eucontaining CsPbI3 films (Figure S10). To know if Cl plays any role in stabilizing the black phase, films were made from precursor solutions with 5 mol % of PbCl2 or CsCl replacing PbI2 and CsI respectively, and even

Figure 3. Shift of (100) (a) and (200) (b) XRD peaks of CsPbI3:xEu perovskite containing 4, 6, 8, and 10 mol % of EuCl3. Peak shift (2θ) vs Eu concentration graphs corresponding to the above two XRD peaks are given in Figure S7. Figures S7c and d show high resolution (012) and (112) XRD peaks (no splitting) of CsPbI3 (without Eu), respectively.

Scanning electron micrographs (Figure 4) of the films shows drastic reduction in grain size when Eu was included in the solution. Besides, for higher concentration of Eu (8 and 10 mol %), splitting 0f (110) and (200) XRD peaks (Figure 3 and Figure S7) implies microstrain in the crystal, which probably

Figure 4. SEM micrographs of CsPbI3 films with (a) 0, (b) 4, (c) 6, (d) 8, and (e) 10 mol % of EuCl3 (another set of micrographs at lower magnification is provided in Figure S8). D

DOI: 10.1021/acs.chemmater.8b01808 Chem. Mater. XXXX, XXX, XXX−XXX

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Figure 5. (a) PCE (determined from backward scan) of CsPbI3:xEu perovskite (Eu-stabilized black phase; x = 4, 5, 6, 7, 8, and 10 mol % EuCl3) solar cells (the box chart has been prepared from 10 cells of each condition). (b) Stabilized power output and (c) J−V curves of a set of CsPbI3:xEu cells with 4, 5, and 6 mol % EuCl3. Both forward and backward scan J−V curves of all the cases (0−10 mol % Eu) are given in Figure S15.

with little amount of HCl into the CsPbI3. All of three conditions resulted in yellow phase of CsPbI3 (no α-CsPbI3) (Figure S11), confirming that Cl does not help in formation of the black phase. This conversely indicated that Eu (either Eu2+ or Eu3+) is solely responsible for stabilization of the black phase. However, it is clear that only size (ionic radii) factor does not play role in stabilizing the black phase. We believe that interaction of Eu3+ or Eu2+ with other components in the solution affects the crystallization process so as to form the black phase. A thorough investigation of ion−ion interactions in the solution (colloids or cluster composition) can provide a deeper understanding about the mechanism of stabilization for both the cases of EuCl3 or EuI2. Nonetheless, the CsPbI3 films with 4, 5, 6, 7, 8, and 10% of Eu were stable enough to be employed in device to check their photovoltaic performances. Although the fabrication method has not been thoroughly optimized, for a set of cells made with freshly coated spiro-OMeTAD (not aged overnight for oxidation), the PCE was found to remain around 4−5% (backward scan) for 4, 5, 6, and 7 mol % of Eu while it was 2− 3% for 8% Eu and around 1% for 10% of Eu (Figure S12a). The yellow δ-CsPbI3 (without Eu), as expected, showed no photovoltaic property at all (Figure S12 b). In this case, we measured the cell performance without aging the spiroOMeTAD for oxidation because we found that LiTFSI in the spiro-OMeTAD caused degradation of the black phase. The perovskite films alone (without spiro-OMeTAD) or with spiro-OMeTAD not containing LiTFSI were stable for longer period while the ones with spiro-OMeTAD containing LiTFSI degraded faster (partially degraded in 24 h) (Figure S13).

Moreover, in some preliminary experiments, we found that P3HT (without dopants) does not cause such fast degradation to the perovskite (CsPbI3:xEu) (Figure S13d). It seems that, as noted by Yanqiang et al.,19 organic HTMs may not be suitable for these cells. A suitable inorganic HTM should be used for longer stability of CsPbI3:Eu cells. However, for all the cells, the fill factor (FF) and open circuit voltage (Voc) improved remarkably when the cells were measured the next day despite the black phase of CsPbI3 had already started turning yellow. The efficiency of the cells with 4, 5, and 6% Eu increased to above 6%, as measured on backward scan (Figure 5a), reaching 6.8% in the best cell (Table S2). Photovoltaic parameters (Jsc, Voc, FF, and PCE) of the best-performing cells are listed in Table S2. This result indicated that an HTM, which requires no oxidation process, could have evaluated the actual potential of the black phase formed by Eu inclusion. Both light and dark J−V curves of all the cases (0, 2, 4, 5, 6, 7, 8, and 10 mol % Eu) are given in Figure S15. Increasingly suppressed dark current for higher concentration of Eu, which was reflected as increment in Voc and improved stability, indicates reduction in recombination with increase in Eu content. Stabilized power output and J−V curves of a representative set of CsPbI3:xEu cells with 4, 5, and 6 mol % Eu are given in Figure 5b. Although PCE measured from backward scan was above 6%, the stabilized power output of the cells varied between 3.5 and 4.5%. Huge hysteresis observed in the J−V curves strongly indicates that the device structure and/or the interfaces lack in desired characteristics and need further structural optimization, which is one of the E

DOI: 10.1021/acs.chemmater.8b01808 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials our immediate future goals. Improvement in performance as well as stability is expected to be accomplished by further optimization. For higher concentration (7, 8, and 10%) of Eu, Jsc decreased significantly, while Voc remained higher for 7 and 8% Eu and decreased dramatically for 10% Eu (Figure S14). Although it seems that the optimum concentration of Eu is around 5−6 mol %, further investigation is needed to determine the optimum concentration more precisely because there are several other characteristics such as thickness and morphology which also vary with Eu content. Because solubility of EuCl3 is low in the solvents (DMF and DMSO), it was required to dilute the solutions accordingly with increasing concentration of Eu in the CsPbI3 precursor. Hence, thickness of the films decreased slightly for higher Eu concentration (Figure S16). However, it looked unconvincing that such significant drop (8 and 10 mol %) in Jsc (Figure S14a) was due to such small change in thickness only. Instead, it is suspected that segregation of excess of EuCl3, most likely at the grain boundaries, or separating out on the surface as some rod-like structures (Figures 4d and e) is responsible for photocurrent reduction. However, chemical analysis of selective sites on the films is required to verify the above possibility. Hence, the results obtained so far are not sufficient to conclude that inclusion of 5−6% of Eu is optimum for the device. It is also required to understand the mechanism of loss in the cells with higher concentration of Eu (7−10%) because PL intensity does not display a direct correlation; there was no obvious decrease in the PL intensity showing increased recombination with Eu content. Although the performances of the cells are substantially low in comparison to what has been reported recently for Bi3+ or EDA stabilized α-CsPbI3, the above results and several important observations in the present study indicate that the performance as well as the stability of CsPbI3:xEu cells can be further improved by thorough optimization of the device fabrication process. The observations such as degradation of the black phase being assisted by spiro-OMeTAD or the additives in it, significant change in film morphology by substrate preheating conditions (Figure S17), and dependence of solution aging time on morphology are being considered seriously for future development. We strongly believe that selection of a more suitable HTM, optimization of FTO-TiO2 substrate preheating conditions, use of solvents that can dissolve EuCl3 better (to make thicker films with higher Eu content), and protection of CsPbI3 film surface from ambient conditions (moisture and O2) by some chemical modification can improve the performance and longterm stability of Eu-stabilized CsPbI3 perovskite solar cells.

though the cells made in the present study were not made by a thoroughly optimized process, disclosed that the cells with CsPbI3 films containing 5 and 6% Eu can work at a power conversion efficiency of above 6%. However, degradation of the formed black phase caused by dopants/additives in spiroOMeTAD, changing morphology with the annealing conditions, and sensitivity of the black phase to environment (O2 and H2O) and light suggest that careful optimization of the method addressing the above issues can result in further improvement of both efficiency and stability of these Eustabilized α-CsPbI3 solar cells. Based on the results being reported here and many other minor observations during the experiments, we believe that (i) use of inorganic HTM, (ii) optimization of the annealing temperature and time, and (iii) chemical modification of the CsPbI3:xEu surface can enhance efficiency and stability of CsPbI3:xEu perovskite cells further.

CONCLUSIONS In summary, it was found that incorporation of Eu3+ (EuCl3) into CsPbI3 stabilizes the black phase (α-CsPbI3) at room temperature and in ambient conditions (not under N2 or argon environment in glovebox). The higher the content of Eu in CsPbI3, the longer the stability of the black phase. Although XRD analysis and SEM micrographs suggest crystal structure distortion and reduced grain size in the CsPbI3:xEu films to be origin of the black phase stabilization, a thorough compositional analysis and phase evolution of CsPbI3:xEu is required to draw a solid conclusion on the underlying mechanism of black phase stabilization. Nevertheless, evaluation of photovoltaic performance of this Eu-stabilized α-CsPbI3, even

ACKNOWLEDGMENTS The present research has been supported by Japan Science and Technology Agency Advanced Low Carbon Technology R&D program (ALCA). The authors acknowledge Professor H. Segawa for giving access to research facilities at Research Center for Advanced Science and Technology (RCAST), University of Tokyo. T.M. thanks the support of New Energy and Industrial Technology Development Organization (NEDO) as well as Japanese Society for Promotion of Science (JSPS) Grant-in-Aid for Scientific Research B. A.K.J. thanks Dr. Wang (Segawa lab) for his help in performing the UV−vis absorption measurements; Dr. Masatoshi Yanagida, Dr. Ayumi Narita, and Ms. Namrata Pant from NIMS, Japan for their help

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b01808. UV−visible absorption and photoluminescence spectra of CsPbI3 films with different amounts of Eu (4, 5, 6, 7, 8, and 10 mol %); DTA and TGA curves of CsPbI3 with 5 and 10% Eu; EDX and XPS analysis of CsPbI3:xEu films; photographs and XRD patterns of CsPbI3 8 mol % of Eu heated at different temperatures; XRD patterns of CsPbI3 with 0, 5, and 8 mol % of EuI2; photographs of CsPbI3:10Eu films with and without spiro-OMeTAD layer (aged for 30 days); Jsc and Voc of CsPbI3:xEu perovskite (Eu-stabilized black phase; x = 4, 5, 6, 7, 8, and 10 mol % EuCl3) solar cells; photographs of CsPbI3:4Eu films made on FTO-TiO2 substrate not preheated and preheated at 100 °C (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Ajay Kumar Jena: 0000-0002-9279-5079 Ashish Kulkarni: 0000-0002-7945-208X Yoshitaka Sanehira: 0000-0003-2030-2690 Tsutomu Miyasaka: 0000-0001-8535-7911 Notes

The authors declare no competing financial interest.

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DOI: 10.1021/acs.chemmater.8b01808 Chem. Mater. XXXX, XXX, XXX−XXX

Article

Chemistry of Materials

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in carrying out XPS measurements; and Youhei Numata and Gyumin Kim for sharing their thoughts relevant to the study.

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DOI: 10.1021/acs.chemmater.8b01808 Chem. Mater. XXXX, XXX, XXX−XXX