Bismuth Incorporation Stabilized α‑CsPbI3 for Fully Inorganic Perovskite Solar Cells Yanqiang Hu,† Fan Bai,† Xinbang Liu,‡ Qingmin Ji,‡ Xiaoliang Miao,† Ting Qiu,† and Shufang Zhang*,† †
College of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China Herbert Gleiter Institute of Nanoscience, Nanjing University of Science and Technology, Nanjing 210094, China
‡
S Supporting Information *
ABSTRACT: All-inorganic CsPbI3 perovskite is emerging to be an alternative light-harvesting material in solar cells owing to the enhanced stability and comparable photovoltaic performance compared to organic−inorganic hybrid perovskites. However, the desirable black phase α-CsPbI3 is not stable at room temperature and degrades rapidly to a nonperovskite yellow phase δ-CsPbI3. Herein, we introduce a compositional engineering approach via incorporating Bi3+ in CsPbI3 to stabilize the α-phase at room temperature. Fully inorganic solar cells based on the Bi-incorporated αCsPb1−xBixI3 compounds demonstrate a high PCE of 13.21% at an optimal condition (incorporation of 4 mol % Bi3+) and maintain 68% of the initial PCE for 168 h under ambient conditions without encapsulation. This is the first attempt of partial substitution of the “B”-site of the perovskite to stabilize the α-CsPbI3, which paves the way for further developments of such perovskites and other optoelectronic devices. cells. However, the α-CsPbI3 is not stable and degrades rapidly to a nonperovskite yellow phase δ-CsPbI3 (Figure 1a) at room temperature.41−45 The undesirable phase transition is because the size of the cesium cation is too small to support the PbI6 polyhedral frame in the cubic perovskite structure.27,44,46 This has been confirmed by recent works with partial substitution of Cs+ by larger cations like MA+ and FA+,24,27,28 or partial substitution of the I− ion by a smaller Br− ion could stabilize the perovskite phase.37,43 On the other hand, Snaith et al. proved that α-CsPbI3 could be stabilized at room temperature under inert atmosphere by adding a small amount of hydroiodic acid (HI) in the precursor solution, and a preliminary PCE of 2.9% was obtained.44 Luo et al. further developed this method by a sequential isopropanol (IPA) treatment process.45 The αCsPbI3 could be further stabilized at room temperature under ambient air for several days, and the champion solar cell achieved a PCE of 4.13%. The HI-induced microstrain in the crystal lattice was believed to be the major reason for the stabilization of α-CsPbI3 at room temperature. Hence, we hypothesized that partially substituting Pb2+ (1.19 Å) with a smaller cation also could cause the lattice distortion to produce microstrain in the crystal lattice and stabilize the α-CsPbI3 at room temperature. Recently, other and our group’s parallel studies proved that the incorporation of Bi3+ (1.03 Å) into
O
rganic−inorganic halide perovskite materials have attracted tremendous research interest owing to their intriguing optical characteristics as well as promising application in next-generation optoelectronic devices.1−6 Among the various hybrid halide perovskites, CH3NH3PbI3 (MAPbI3) and HC(NH2)2PbI3 (FAPbI3) have been frequently studied and have achieved power conversion efficiencies (PCEs) exceeding 20% in solar cells.7−15 However, due to the hygroscopicity and thermally unstable nature of organic cation MA+, MAPbI3 is thermally unstable and vulnerable to moisture.16−20 Even for the more thermostable FAPbI3, the presence of hygroscopic FA+ also makes it suffer from the moisture stability issue.8,21−23 In order to improve the stability and photovoltaic performance of the devices, a series of Cs-incorporated systems have been developed,24−34 such as Cs x MA 1−x PbI 3 , 24 Cs x FA 1−x PbI 3 , 27,28,30 FA 0.83 Cs 0.17 Pb(I1−xBrx)3,31 and Csx(MA0.17FA0.83)1−xPb(I0.83Br0.17)3.29,32 However, these Cs-incorporated systems still face big challenges for the long-term stability due to the remaining organic components. Recently, all-inorganic cesium lead halide perovskites (CsPbX3) are emerging to be alternative light-harvesting materials in solar cells and have exhibited excellent ability to resist moisture and heat.35−39 Nevertheless, CsPbBr3 has a very large band gap of 2.3 eV, which is unable to absorb light with long-range wavelengths and usually results in low PCE of the solar cells.38−40 Compared to CsPbBr3, black phase α-CsPbI3 (Figure 1a) has a more suitable band gap of 1.73 eV for solar © 2017 American Chemical Society
Received: June 12, 2017 Accepted: August 31, 2017 Published: August 31, 2017 2219
DOI: 10.1021/acsenergylett.7b00508 ACS Energy Lett. 2017, 2, 2219−2227
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http://pubs.acs.org/journal/aelccp
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ACS Energy Letters
Figure 1. (a) Crystal structures and photographs of δ-CsPbI3 and α-CsPbI3, respectively. (b) Photograph of α-CsPbI3, δ-CsPbI3, and Biincorporated CsPb1−xBixI3 (0 ≤ x < 0.1) perovskite precursor solutions and corresponding films. (c) Top-view SEM images of the perovskite films; scale bar: 500 nm.
Figure 2. (a) XRD patterns of α-CsPbI3, δ-CsPbI3, and Bi-incorporated CsPb1−xBixI3 (0 ≤ x < 0.1) perovskite films. (b) EDS mapping of the CsPb0.96Bi0.04I3 film. Scale bar: 500 nm.
absorption and electrical conductivity are obtained. Fully inorganic CsPb1−xBixI3 solar cells in planar architecture with CuI as the hole transport material (HTM) demonstrate remarkable photovoltaic performances. At an optimal condition (incorporation of 4 mol % Bi3+), the solar cell achieves a high PCE of 13.21%, which far exceeds that of the devices based on the controlled α-CsPbI3 (8.07%). The optimal solar cell shows remarkable stability for 168 h under ambient conditions at 25 °C with relative humidity (RH) of 55%, maintaining 68% of the initial PCE without encapsulation.
MAPbBr3 and CsPbBr3 is capable of narrowing the band gap and improving the electrical conductivity.47,48 Therefore, it is anticipated that the incorporation of Bi3+ would further stabilize the α-CsPbI3 phase at room temperature and the solar cells based on the Bi-incorporated CsPbI3 film would exhibit improved PCE and stability. Herein, we attempt to stabilize the α-CsPbI3 at room temperature by partial substitution of the B-site of the perovskite with Bi3+ for the first time. By tuning the stoichiometric ratio of Bi3+, Bi-incorporated α-CsPb1−xBixI3 compounds with excellent stability as well as improved light 2220
DOI: 10.1021/acsenergylett.7b00508 ACS Energy Lett. 2017, 2, 2219−2227
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ACS Energy Letters Scheme 1. Possible Mechanisms for Stabilization of α-CsPbI3 by Adding HI or Bi3+ in the Precursor Solution
The Bi-incorporated CsPb1−xBixI3 (0 ≤ x < 0.1) compounds were prepared via the conventional one-step deposition method by employing precursor solutions combining PbI2 and BiI3 in certain molar ratios (see the Experimental Section for details). As a controlled sample, the pure α-CsPbI3 was prepared by a sequential solvent engineering method including the addition of HI in precursor solution and treatment of the film with IPA according to the literature.45 As shown in Figure 1b, the controlled α-CsPbI3 film is in black color, and the δ-CsPbI3 film prepared without adding HI and Bi3+ is shown in light yellow color, which is consistent with the typical black phase and yellow phase of CsPbI3 reported previously.44,45 The color of Bi-incorporated CsPb1−xBixI3 films changes gradually from light-brownish to dark-brownish with the concentration of Bi3+ increasing from 1 to 5 mol %, suggesting that the black phase perovskite gradually increased in the films. However, the color of the film changes to red rather than dark-brownish when adding 10 mol % Bi3+ in the perovskite precursor solution, implying collapse of the cubic perovskite induced by the excess of Bi3+. The elemental compositions of the target films are confirmed by elemental analyses through energy dispersive Xray spectroscopy (EDS). The results indicate that the compositions of these films are approximate to the precursor solutions (Figure S1). The ratios of metal (Pb/Bi) to iodine in the films are all close to 1:3. It should be noted that the iodine components in the Bi-incorporated films are slightly larger than those of the pure CsPbI3 films due to the higher valency of Bi3+ than Pb2+. For a simple expression, we still use CsPb1−xBixI3 (0 ≤ x < 0.1) to describe the Bi-incorporated films. The top-view scanning electron microscopy (SEM) images of the films are presented in Figure 1c. The pure α-CsPbI3 film shows a uniform surface with separated small grains, whereas the δCsPbI3 has a rough surface with very large grains. Inspiringly, the films surface flattens out gradually and become more and more uniform and compact with the addition of Bi3+. With the increase of Bi3+ concentration, the grain sizes in these Biincorporated CsPb1−xBixI3 films decrease gradually. Specially, the average grain size in the CsPb0.96Bi0.04I3 film is the smallest (less than 100 nm). According to previous works, the grain size is of critical importance for the stable α-CsPbI3 at low temperature. 49 Therefore, the small grains in the Biincorporated CsPb1−xBixI3 films are beneficial to stabilize the black phase.
X-ray diffraction (XRD) spectra were performed to further determine the phases of these perovskite films (Figure 2a). The crystalline phase of the controlled α-CsPbI3 film was in good agreement with the typical cubic perovskite lattice (Pm-3m). The yellow phase CsPbI3 prepared without adding HI or Bi3+ showed characteristic peaks of the orthorhombic structure with a space group of Pnma. With an increase of the Bi3+ component in the films, the peaks of (012), (112), and (122) planes of the orthorhombic phase gradually disappeared, and the (100), (110), (111), and (200) peaks of the cubic phase emerged, demonstrating clearly the phase transition from the δ-phase to α-phase. The δ-phase was completely transformed into the αphase at 4 mol % Bi3+ incorporation in the CsPbI3 lattice. Nevertheless, when the concentration of Bi3+ was increased to 10 mol %, the α-phase degrade into other impurity phases Cs3Bi2I9 (as shown in Figure S2). Therefore, it is very important to use a suitable amount of Bi3+ in the precursor solution for stabilization of the black phase perovskite. Under our experimental conditions, the concentration of Bi3+ should be controlled at 4−5 mol %. We note that although the αCsPbI3 can be stabilized by HI in the previous work, it is still a metastable phase and cannot be stable for a long time (99.5%), hydriodic acid (HI, 57 wt % in H2O), dimethyl sulfoxide (DMSO, anhydrous, 99.8%) and N,N-dimethylformamide (DMF, anhydrous, 99.8%) were purchased from Sigma-Aldrich. All salts and solvents were used as received without any further purification. All-Inorganic Perovskite Precursor Solutions Preparation. The αCsPbI3 precursor solution was prepared according to the procedure reported by Luo .45 HI (66 μL/mL) was added into the precursor solution with a mixture of DMSO and DMF (3:7, v:v) as the solvent. The Bi3+-incorporated CsPbI3 perovskite precursor solutions were prepared by fully dissolving a 0.01 mol CsI and 0.01 mol mixture of PbI2/BiI3 (molar ratios of BiI3 in the mixture ranged from 0 to 10 mol %) in 10 mL of solvent with mixing DMSO and DMF (3:7, v:v). Then, the precursor solutions were stirred at 70 °C for 12h. All-Inorganic Perovskite Solar Cell Fabrication. First, a fluorinedoped tin oxide (FTO) conducting glass (sheet resistance of 8 ohm/square) was ultrasonically cleaned by detergent, deionized water, and acetone for 20 min sequentially and then treated by a UV/O3 cleaner for 15 min. Then, a uniform dense TiO2 layer was deposited on the FTO glass substrate by spin coating 0.1 mol/L titanium diisopropoxide bis(acetylacetonate) in butanol at 3500 rpm and repeating the process twice and then sintering at 500 °C for 30 min. Subsequently, the perovskite film was deposited on the compact TiO2 by spin coating of the asprepared precursor solution at 800 rpm for 10 s and 3500 rpm for 35 s. The spin-coated film was annealed at 100 °C for 15 min to form the perovskite film. Then, 0.1 mol/L CuI solution (1:39 mixture of Pr2 S/chlorobenzene) was coated on the perovskite film by using a method similar to that previously reported by Jeffrey A. Christians and co-workers.55 The FTO substrate (2 cm × 2 cm) was first placed on a spin-coater, which was maintained at 80 °C; three drops of CuI solution were dropwise added onto the perovskite film. The glass bar was then moved back and forth rapidly (approximately 8 cm/s) over the surface of the perovskite film until a thin layer of solution remained on the surface, which was then spin at 6500 rpm for 30 s to form a overlayer in the perovskite layer. Finally, a 100 nm thick Au electrode was deposited on top of the device through a shadow mask by thermal evaporation under ∼5 × 10−5 Torr vacuum conditions. Measurement and Characterization. Powder XRD patterns were recorded by a Bruker D8 diffractometer with Cu Kα radiation (λ = 1.5406 Å). The surface morphology and elemental compositions were observed by field-emission scanning electron microscopy (FE-SEM; Quanta 250FEG) with EDS. X-ray photoelectron spectroscopy (XPS) measure-
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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/acsenergylett.7b00508. Supporting analysis of EDS results, XPS spectra and XRD data, photograph of solar cells, and J−V characteristic data (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Shufang Zhang: 0000-0003-0556-4078 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge financial support from the National Natural Science Foundation of China (21403112), the Natural Science Foundation of Jiangsu Province (BK20140778), the Fundamental Research Funds for the Central Universities (3091501333), and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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DOI: 10.1021/acsenergylett.7b00508 ACS Energy Lett. 2017, 2, 2219−2227
Letter
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DOI: 10.1021/acsenergylett.7b00508 ACS Energy Lett. 2017, 2, 2219−2227
