Chlorine-incorporation-induced formation of the layered phase for

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Chlorine-incorporation-induced formation of the layered phase for antimony-based lead-free perovskite solar cells Fangyuan Jiang, Dongwen Yang, Youyu Jiang, Tiefeng Liu, Xin-Gang Zhao, Yue Ming, Bangwu Luo, Fei Qin, Jiacheng Fan, Hongwei Han, Lijun Zhang, and Yinhua Zhou J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b10739 • Publication Date (Web): 23 Dec 2017 Downloaded from http://pubs.acs.org on December 23, 2017

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Journal of the American Chemical Society

Chlorine-incorporation-induced formation of the layered phase for antimony-based lead-free perovskite solar cells Fangyuan Jiang,1,† Dongwen Yang,2,† Youyu Jiang,1,† Tiefeng Liu,1 Xingang Zhao,2 Yue Ming,1 Bangwu Luo,1 Fei Qin,1 Jiacheng Fan, Hongwei Han,1 Lijun Zhang,2,* and Yinhua Zhou1,* 1

Wuhan National Laboratory for Optoelectronics, School of Optical and Electronic Information,

Huazhong University of Science and Technology, Wuhan 430074, China 2

Key Laboratory of Automobile Materials of MOE, State Key Laboratory of Superhard Materials, and

College of Materials Science and Engineering, Jilin University, Changchun 130012, China †

These three authors contribute equally to this work.

*Corresponding author. E-mail: [email protected] or [email protected]

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ABSTRACT The environmental toxicity of Pb in organic-inorganic hybrid perovskite solar cells remains an issue, which has triggered intense research on seeking alternative Pb-free perovskites for solar applications. Halide perovskites based on group-VA cations of Bi3+ and Sb3+ with the same lone-pair ns2 state as Pb2+ are promising candidates. Herein, through a joint experimental and theoretical study, we demonstrate that Cl-incorporated methylammonium Sb halide perovskites (CH3NH3)3Sb2ClXI9-X show promise as efficient solar absorbers for Pb-free perovskite solar cells. Inclusion of methylammonium chloride into the precursor solutions suppresses the formation of the undesired zero-dimensional dimer phase and leads to the successful synthesis of high-quality perovskite films composed of the two-dimensional layered phase favored for photovoltaics. Solar cells based on the as-obtained (CH3NH3)3Sb2ClXI9-X films reach a record-high power conversion efficiency over 2%. This finding offers a new perspective for the development of non-toxic and low-cost Sb-based perovskite solar cells.

KEY WORDS: lead-free perovskite solar absorbers; antimony; layered perovskite phase; solution-based processing; solar cells

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INTRODUCTION Organic-inorganic hybrid lead (Pb) halide perovskites (APbX3 with A representing a small molecule and X being a halogen) have emerged as revolutionary solar absorbers with power conversion efficiencies (PCEs) dramatically increasing from the initial value of 3.8%

1

to above 22%.2-12 The

outstanding photovoltaic performance of this class of material is attributed to their unique optoelectronic properties, such as suitable band gap, high absorption coefficient,13-15 high and balanced carrier mobilites,16,17 defect-tolerant features,18-21 and low exciton binding energy22,23. These intrinsic material properties fundamentally originate from the chemistry of the Pb lone-pair 6 s2 state and the mixed ionic-covalent bonding of the perovskite lattice.13,24,25 However, the environmental toxicity of the key element Pb remains an issue, which has limited the large-scale commercialization of perovskite solar cells (PSCs). Thus, there is strong desire to seek new perovskite materials having similarly good photovoltaic performance that do not contain Pb.26-38 A straightforward strategy to eliminate toxic Pb in halide perovskites is replacing Pb2+ with other isovalent ions. The same-group ns2 cations Sn2+/Ge2+ were first considered. The Snaith and Kanatzidis groups independently reported entirely Pb-free CH3NH3SnI3-based PSCs with initial PCEs of approximately 6%.39,40 However, the devices suffered from rapid degradation when exposed to air, caused by the instability of Sn2+ cations (prone to oxidization into Sn4+). Though efforts have been devoted to improving the material synthesis and device engineering, a Sn-based PSC with enhanced efficiency and simultaneously satisfying stability has not yet been achieved.41-48 Ge-based halide perovskites have been synthesized and demonstrated to be non-ideal solar absorbers because of the similarly high oxidation tendencies of Ge2+ and emergence of deep defect states.49-51 Replacing Pb2+

3



with other divalent cations beyond the IVA group (such as Sr and Ba)52,53 have also been attempted, but these materials have shown oversized band gaps unsuitable for solar applications. The group-VA cations (M3+ = Bi3+ and Sb3+) have been explored as Pb2+ replacements with the aim of maintaining the chemistry of the lone-pair ns2 state beneficial for high photovoltaic performance. Because of their higher +3 oxidation state, the normal AMX3 perovskites composed of corner-sharing MX6 octahedra cannot form, and instead, A3M2X9 becomes the stable stoichiometry.54-59 In general, two main phases of A3M2X9 have been experimentally reported55,58,60: the hexagonal phase consisting of zero-dimensional (0D) bioctahedral face-sharing (M2I9)3− clusters (dimer phase, left panel of Figure 1a) and the phase composed of two-dimensional (2D) corrugated layers with partially corner-sharing MX6 octahedra (layered phase, right panel of Figure 1a). The 0D dimer phase can be easily synthesized via low-temperature solution processing.54,58,61 However, the 0D dimer phase based perovskites suffer from intrinsic problems that are unfavorable for photovoltaics,55,60 including a low-symmetry-induced indirect band gap, strong quantum-confinement effect caused oversized gap values and inferior hopping-like carrier transport. As a result, the dimer-phase-based A3M2X9 solar cells exhibit low PCEs, with the highest reported PCE of a solar cell based on A3Sb2X9 less than 0.5%.58 The 2D layered phase with less dimensional reduction can partially circumvent the above problems and is expected to show a direct band gap, smaller optical band gap value and good in-layer carrier transport.55,60 Unfortunately, the 2D layered phase turns out to be less thermodynamically favorable than the 0D dimer phase and was only synthesized for particular materials (e.g., K3Bi2I9 and Cs3Sb2I9) by a high-temperature solid-state reaction.55,57,60 Since no film sample with satisfactory quality has been made, a solar cell based on the 2D layered phase with a visible PCE has not been reported. Until quite recently, with the introduction of small-size Rb+ and NH4+ as the A-site cations, 4


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2D layered phases of Rb3Sb2I962 and (NH4)3Sb2I959 have been demonstrated using low-temperature solution processing. However, it is inexplicable that the fabricated solar cells show barely enhanced PCEs (0.66% for Rb3Sb2I962 and 0.51% for (NH4)3Sb2I959) in comparison with the 0D dimer-phase-based devices. Further efforts to synthesize high-quality films of the 2D layered phase are strongly needed to examine its actual photovoltaic performance. In this report, we demonstrate through a joint theoretical-experimental effort that incorporating a substantial amount of Cl into methylammonium (MA) antimony halide perovskites (MA3Sb2I9) is an efficient way to synthesize high-quality films of the 2D layered phase desired for solar cells. First-principles calculations indicate that the energetic advantage of the 0D dimer phase over the 2D layered phase evidently decreases with an increasing Cl content in mixed-halide MA3Sb2ClXI9-X. Guided by this insight, we perform comprehensive chemical composition engineering of the precursor solutions by incorporating MACl into the mixture of SbI3 and MAI and successfully realize a phase transformation from the 0D dimer phase to the 2D layered phase. The presence of the 2D layered phase is indicated by X-ray diffraction measurement, optical absorbance spectroscopy, and direct comparison of the experimental data with theoretical calculations. With high-quality films of the 2D layered MA3Sb2ClXI9-X perovskites as light absorbers, we fabricate solar cell devices reaching PCEs over 2%, which is a record-high value for Sb-based halide perovskites.

RESULTS AND DISCUSSION First-Principles Calculations. We began with energetic calculations of the dimer phase vs the layered phase for Cl-incorporated A3Sb2I9. Considering the computational cost of the Cl substitution calculations involving a large supercell with many atoms, we chose fully inorganic Cs instead of a 5



complex organic molecule as the A-site cation. For both phases of Cs3Sb2ClXI9-X, the decomposition enthalpy ∆‫ܪ‬ௗ௘௖ , defined as the free energy difference before and after the reaction of Csଷ ܾܵଶ ‫݈ܥ‬௑ ‫ܫ‬ଽି௑ → ܾܵ‫ܫ‬ଷ + ‫ ܫݏܥ‬+ ‫݈ܥݏܥ‬, was calculated. A more negative ∆‫ܪ‬ௗ௘௖ corresponds to a more energetically stable condition of A3Sb2ClXI9-X against decomposition. The results are shown in Figure 1b. For pure Cs3Sb2I9, the dimer phase is more stable than the layered phase, as indicated by the ∆‫ܪ‬ௗ௘௖ of -44 meV per formula, consistent with the fact that the dimer phase is commonly synthesized in experiments.58 With Cl incorporation, ∆‫ܪ‬ௗ௘௖ (for the lowest-energy configuration) between the dimer phase (black dash line) and the layered phase (red dash line) first slight decreases in Cs3Sb2ClI8 (-51 meV) and then increases to -31 meV in Cs3Sb2Cl2I7. In the case of complete Cl substitution (Cs3Sb2Cl9), ∆‫ܪ‬ௗ௘௖ changes to a large positive value of 152 meV, where the layered phase becomes energetically favored. These results indicate that Cl substitution for I is a promising approach to suppress the formation of the dimer phase and to stabilize the layered phase. We note that Figure 1b indicates the Cl-I order-disordering effect (for Cs3Sb2ClI8 and Cs3Sb2Cl2I7) has a considerable effect on the thermodynamic stability. The associated energetic change may be even comparable to the energy difference between the 2D layered and 0D dimer phases. However, the Cl-I order-disordering effect has a mild effect on the electronic band gap (Figure S1), showing a change of less than 0.2 eV. Alternative decomposition pathways involving other competing phases (e.g., SbCl3 or ASbCl6) were also considered (Figure S2), but these pathways were energetically less favored than the above pathway. In any case, such decomposition pathway evaluations do not affect our key finding, i.e., the 2D layered phase transforms into the 0D dimer phase with an increasing Cl content in Cs3Sb2ClXI9-X. Sb-based Perovskite Film Fabrication. Motivated by our theoretical results, we designed a solution-based procedure to synthesize A3Sb2I9 with Cl incorporation through chemical composition 6


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engineering of the precursor solutions. A standard one-step anti-solvent spin-coating approach was used to grow perovskite films (see Experimental Section). Mixed precursors containing SbI3, CsI, and CsCl were first attempted to synthesize the inorganic perovskites Cs3Sb2ClXI9-X. However, we find that inorganic CsI and CsCl have limited solubility in the solvents, which significantly affects the quality of the synthesized films. We thus turned to the precursors containing SbI3, MAI, and MACl with better solubility to synthesize the organic-inorganic hybrid perovskites MA3Sb2ClXI9-X. Films deposited from precursors containing SbI3, MAI and MACl with molar ratios of 1:1.5:0, 1:0:1.5, 1:0.5:1.5 and 1:1:1.2 are denoted as film 1-1.5-0, 1-0-1.5, 1-0.5-1.5 and 1-1-1.2, respectively. Film 1-1.5-0 acts as the control film for the pure-halide MA3Sb2I9 perovskite, and the other three films represent Cl-incorporated mixed-halide MA3Sb2ClXI9-X perovskites with different I/Cl ratios. Crystalline Structure and Optical Property Studies of Sb-Based Perovskites to Differentiate Dimer and Layered Phases. The crystalline structures of the four kinds of as-fabricated films were characterized by X-ray diffraction (XRD), as shown in Figure 2. The insets show the images of the four films, reflecting the light-absorbing properties that will be discussed below. It can be seen that the three Cl-incorporated films 1-0-1.5, 1-0.5-1.5 and 1-1-1.2 (MA3Sb2ClXI9-X, Figure 2b-2d) show similar XRD patterns, which differ from that of film 1-1.5-0 (pure MA3Sb2I9, Figure 2a). Particularly, in the 20o-30° region (denoted by a blue rectangular box), the pure-iodine MA3Sb2I9 film shows four diffraction peaks at 24.8°, 25.4°, 27.1°, and 29.4°, whereas the other three Cl-incorporated MA3Sb2ClXI9-X films only display two peaks at ~25° and ~29.5° (the other two diffraction peaks disappeared). We simulated the XRD patterns of the dimer and layered phases using the crystalline structures obtained from the theoretical structure optimizations. We find good agreements between the XRD pattern of the pure MA3Sb2I9 film and the simulated result of the dimer phase (as shown in 7



Figure 2a) and between the XRD patterns of the Cl-incorporated MA3Sb2ClXI9-X films and the simulated result of the layered phase (Figure 2b). This indicates that, while the pure MA3Sb2I9 film crystalizes in the 0D dimer phase, the Cl-incorporated MA3Sb2ClXI9-X films are of the 2D layered phase. We then measured the absorption spectra of the four types of films (shown in Figure 3a). As seen from the spectra from long to short wavelength, film 1-1.5-0 (MA3Sb2I9) shows a slow increase of absorbance from 600 to 500 nm, and the light absorbance sharply rises starting at approximately 500 nm. Distinctly, the Cl-incorporated films 1-0-1.5, 1-0.5-1.5 and 1-1-1.2 have abruptly increased absorbance at approximately 570 nm, with higher intensities. The different absorption spectral regions and intensities explain the yellow and orange-red colors of the pure-halide MA3Sb2I9 and Cl-incorporated mixed-halide MA3Sb2ClXI9-X films, respectively (insets of Figure 2). Figure 3b shows the Tauc plots of the absorption coefficients for the evaluating band gap values. For direct gap features, the fitting curves deliver band gap values of 2.44 eV for the MA3Sb2I9 film and a uniform value of 2.17 eV for the three Cl-incorporated MA3Sb2ClXI9-X films. Assuming an indirect band gap gives a lower value of 2.0 eV for the MA3Sb2I9 film (Supplementary Figure S3). Figure 3c shows the calculated electronic band structures for the dimer (upper panel) and layered (lower panel) phases of MA3Sb2I9. The dimer phase has a band gap with a clearly indirect nature of 2.17 eV (formed by the L → Γ valence-to-conduction band transition). The layered phase exhibits a slightly indirect band gap of 1.79 eV (determined by the valence band maximum at the position in proximity to Γ and the conduction band minimum at Γ), 31 meV lower than the direct band gap formed at Γ. Therefore, compared with the 0D dimer phase, a quasi-direct band gap with reduced magnitude occurs for the higher-dimensional 2D layered phase, consistent with the above experimental optical absorption 8


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measurements. This quasi-direct band gap feature, which resembles the case of MAPbI3,63,64 is expected to contribute to a slow radiative recombination process of photon-induced carriers beneficial for photovoltaic property. With Cl incorporation into the layered phase (Figure 3d), the band gap increases from 1.79 eV for MA3Sb2I9 to 2.01 eV for MA3Sb2Cl2I7, which is still lower than the calculated 2.17 eV for the dimer phase. We also considered the possibility of incorporating Br to realize the same purpose of suppressing the formation of the undesired 0D dimer phase of A3Sb2I9 and stabilizing the 2D layer phase favored for photovoltaics, but it turns out that the resulting effect was not as effective as that of Cl incorporation. This was indicated by the following: (i) first-principles calculations showed that, although Cs3Sb2Br9 is energetically favored in the 2D layered phase, the energy difference between the 2D layered and 0D dimer phases (4 meV/atom) is nearly one-third that of the case of Cs3Sb2Cl9 (11 meV/atom). This result indicates that the ability of Br incorporation to stabilize the 2D layered phase by suppressing the 0D dimer phase is weaker than that of Cl incorporation. (ii) We attempted to synthesize Br-incorporated MA3Sb2BrXI9-X by mixing SbI3 and MABr in the precursor solution. Optical characterization of the Br-incorporated product (with SbI3:MABr=1:1.5) indicated a blueshifted absorption threshold (i.e., a larger band gap) when compared with the Cl-incorporated sample of the same composition (SbI3:MACl=1:1.5), as shown in Figure S4. Considering the reduced visible-light absorption efficiency, we abandoned the route of Br incorporation and focused our study on Cl incorporation. Morphologies of the Sb-Based Perovskite Films. The chemical composition of the precursor solution was found to have a crucial effect on the morphology and quality of the synthesized perovskite film. Figure 4 shows the top-view morphologies of the four types of films from scanning 9



electron microscopy (SEM) measurements (left panels) and the statistical histogram of the individual perovskite grain sizes (right panels). Although all the four films show good coverage over the entire substrate, their grain sizes are different: films 1-1.5-0 and 1-0-1.5 with a MA:Sb:Cl/I ratio in the precursor solution of exactly 3:2:9 (Figure 4a and 4b) are generally composed of small-size crystals with diameters in the narrow range of 100-400 nm (Figure 4e and 4f). For films 1-0.5-1.5 and 1-1-1.2 with excess Cl/I, the grain sizes increase (Figure 4c and 4d). Substantially larger sized grains with diameters between 400 and 600 nm and diameters of 800 nm appear (Figure 4g and 4h). While the underlying mechanism needs further investigation, it seems that an excess amount of MAX (X denotes Cl or I) in the precursor solution plays an important role in controlling the crystal size and quality of the MA3Sb2ClXI9-X perovskites. Discussions on Cl in the Crystalline Lattice of Sb-Based Perovskites. Thus, we find that incorporating Cl into the precursor solution for Sb-based perovskites leads to the formation of the layered phase of MA3Sb2ClXI9-X. This is distinct from the MACl-assisted solution growth of the Pb-based perovskite MAPbI3, where Cl appears merely in the intermediate phases instead of the final MAPbI3 product.65 Note that the mixed-halide MAPbI3-XClX has been synthesized by deliberately incorporating Cl into MAPbI3.66-70 To further explore the Cl behavior: (i) additional four precursor solutions with different molar ratios of SbI3, MAI and MACl were used to synthesize MA3Sb2ClXI9-X perovskite films. The XRD patterns, together with the light absorption Tauc plots of all the eight fabricated films, are summarized in Figure S5. Figure S6 shows the derived band gaps as a function of the Cl/I ratio. From the XRD patterns, one clearly sees that, with an increasing Cl/I ratio in the precursor solutions, the 0D dimer phase is transformed to the 2D layered phase; the critical Cl/I ratio of the phase transformation lies between 0.22 to 0.25. While the 0D dimer phases have larger band 10


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gaps of approximately 2.45 eV, the 2D layered phases show uniformly smaller band gaps of approximately 2.15-2.18 eV. (ii) Energy-dispersive X-ray spectroscopy (EDX) measurements were performed to characterize the Cl contents in the final reaction products, as shown in Figure S7 and Tables S1-S4. For the three Cl-incorporated MA3Sb2ClXI9-X perovskite films (in Figure 2b-2d), the Cl atomic contents are 11.55%, 9.64% and 6.19%, which are much smaller than the corresponding percentages of Cl in the precursors. The results indicate only a small amount of Cl enters the final products, about 1-2 Cl atoms in each MA3Sb2ClXI9-X formula. Therefore, we can conclude from the above analysis that a certain amount of Cl in the precursor solution is an indispensable prerequisite for stabilizing the 2D layered mixed-halide perovskites MA3Sb2ClXI9-X. This is supported by the first-principles energetic calculations that indicate an increased tendency for the 2D layered phase to be stabilized with an increasing Cl content in MA3Sb2ClXI9-X (Figure 1b). The Cl atoms exist in the final products but in a small quantity. Considering the similar XRD patterns and the uniform optical gap of all the Cl-incorporated perovskite films, we speculate that there could exist an optimal Cl(X) content (1-2 Cl atoms in each MA3Sb2ClXI9-X formula). The difference in the Cl contents measured by EDX might be attributed to the existence of Cl at grain boundaries or surfaces of the films having different morphologies and grain sizes. We also attempted to synthesize the MA3Sb2ClXI9-X perovskites using a different precursor of SbCl3 as the Sb and Cl sources. We find that simply mixing transparent SbCl3 solid and white MAI powder gives rise to an immediate reaction with a red powder product. Further optimizing the chemical composition of the ܾܵ‫݈ܥ‬ଷ + ‫ ܫܣܯ‬precursor solution (with a molar ratio of 1:3) led us to successfully synthesize the layered phase of the MA3Sb2ClXI9-X perovskite, as indicated by the XRD pattern and 11



optical absorption measurement (Figures S8-S10). The different reaction pathway starting with the SbCl3 precursor with rather strong Sb-Cl chemical bonding supports the presence of Cl in the final product. The identical optical gap (2.17 eV) from the Tauc absorption edges for all the synthesized films (Figures 3b and S10) further suggests the possible existence of an optimal content of Cl (X) (1-2 Cl atoms in each MA3Sb2ClXI9-X formula) for MA3Sb2ClXI9-X perovskites. Photovoltaic Performances of the Sb-Based Perovskite Solar Cells. The as-prepared Sb-based perovskite films were then used as the light-absorbing layers to fabricate PSCs in a typical n-i-p mesoscopic structure, as shown in Figure 5a. We used TiO2 and spiro-OMeTAD as the electron and hole-transport layers, respectively. Figure 5b shows the current density-voltage (J-V) curves for the devices fabricated from four kinds of perovskite films. The corresponding device performance parameters are summarized in Table 1. The PSC based on film 1-1.5-0 (MA3Sb2I9 with dimer phase) shows a low short-circuit current density (JSC) of 0.35 mA/cm2, a moderate open-circuit voltage (VOC) of 0.48 V, and a fill factor (FF) of 0.53. This results in a low PCE of 0.09%. Turning to the Cl-incorporated films (the layered phase of mixed-halide MA3Sb2ClXI9-X), the PSC based on film 1-0-1.5 shows a much higher PCE of 1.74% resulting from the significantly enhanced JSC of 4.3 mA/cm2, VOC of 0.72 V, and FF of 0.56. For films 1-0.5-1.5 and 1-1-1.2, where excess Cl/I in the precursor solution leads to larger grain sizes and enhanced film quality as we speculated, the PCEs further increase to 1.96% and 2.19%, respectively. To verify the output efficiencies of the PSCs, we measured the steady-state photocurrents at the maximum power points of the J-V curves. Figure 5c (black curve) shows the steady-state photocurrent result for the device based on film 1-1-1.2 (with a maximum power point voltage Vmpp = 0.54 V, as labeled in Figure 5b). The actual power output calculated from the steady-state photocurrent is 2.17% (red curve), which is consistent with the PCE 12


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value obtained from the J-V measurement. The J-V characteristics were also measured by changing the scan directions. We do not observe photocurrent hysteresis during the forward and reverse scans (Figure 5d). Figure 5e shows the external quantum efficiency (EQE) of the highest-PCE cell based on film 1-1-1.2. We obtain an integrated JSC of 5.1 mA/cm2, in accordance with the JSC value from the J-V measurement (~5 mA/cm2). Figure 5f displays the PCE distribution based on 40 separate solar cell devices (fabricated from film 1-1-1.2). We obtained a statistical PCE value of 1.74 ± 0.31% (average value ± standard deviation). To evaluate the effect of existing defects in the fabricated films on the PSC efficiency, thermal admittance spectroscopy (TAS) measurements were used to probe the trap density of states (Figure S11). We find that the three 2D layered Cl-incorporated MA3Sb2ClXI9-X films 1-0-1.5, 1,0.5-1.5 and 1-1-1.2 clearly show reduced trap densities compared with the 0D dimer MA3Sb2I9 film 1-1.5-0. This explains the much larger VOC values (approximately 0.7 V) of the 2D layered MA3Sb2ClXI9-X films, compared with the small VOC (0.48 V) of the 0D dimer film (Figure 5b), as the VOC values of perovskite solar cells are influenced by non-radiative recombination caused by trap states.71,72 Apart from the VOC, the defect-induced trap states also have non-ignorable effects on JSC and FF. Among the three Cl-incorporated MA3Sb2ClXI9-X films, film 1-1-1.2 shows a minimal trap density and thus is accompanied with the best PSC device performance. Stability Tests of the Sb-Based Perovskite Films and Solar Cells. Finally, we examined the stabilities of the Cl-incorporated MA3Sb2ClXI9-X perovskite films as well as the fabricated PSCs. The stabilities of the MA3Sb2ClXI9-X films were measured by tracking the XRD patterns. As seen in Figure S12, no distinct differences were observed in the diffraction peaks of the pristine film 1-1-1.2 and the films stored for 11 and 30 days in air (relative humility: ~10%). The fabricated PSC (based on film 1-1-1.2) also exhibits good stability when stored in a N2-filled glovebox as well as in air for up to 18 13



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days (Figure S13). When compared with the intrinsically strong oxidation tendency of Sn2+/Ge2+-based lead-free perovskites, our results have proven that the MA3Sb2ClXI9-X perovskite films are chemically stable to air exposure. In contrast to the 2D Pb-based organic-inorganic hybrid perovskites, where the surficial layers of the existing long-chain organic molecule provide protection to the inner perovskite framework and leads to enhanced stability,73 the air stability of the 2D layered MA3Sb2ClXI9-X perovskites originates from the intrinsic thermodynamic stability. A possible mechanism for this stability is as follows: the reduced dimensional perovskite framework could allow for more strain relaxation

or

generally

better

stability

of

the

Bi/Sb

compounds

against

potential

disproportionations.55,60,62 The latter is supported by the observed unchanged +3 oxidation state of Sb from the X-ray photoelectron spectroscopy (XPS) measurements (Figure S14) when the MA3Sb2ClXI9-X 62,74 films are exposed to air.

CONCLUSION In summary, we report a joint theoretical-experimental study of Sb-based mixed-halide perovskites MA3Sb2ClXI9-X for lead-free perovskite solar cells. We find that Cl incorporation is an effective approach to suppress the formation of the 0D dimer phase and stabilize the 2D layered phase of Sb-based perovskites. In contrast to the 0D dimer MA3Sb2I9 with a large indirect band gap, the 2D layered MA3Sb2ClXI9-X perovskites exhibit quasi-direct band gaps with smaller magnitudes, thus increasing the visible-light capture efficiency. Chemical composition engineering of the precursor solutions allows us to grow high-quality MA3Sb2ClXI9-X films with large grain sizes. The solar cells fabricated from the 2D MA3Sb2ClXI9-X films show PCEs of over 2%, surpassing the state-of-the-art values in the literature for Sb-based perovskite solar cells (0.66%). Moreover, the MA3Sb2ClXI9-X films 14


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and solar cells exhibit good air stabilities. These results demonstrate the potential of Sb for the fabrication of non-toxic lead-free organic-inorganic halide perovskite solar cells.

EXPERIMENTAL SECTION MA3Sb2ClXI9-X Film Preparation. The SbI3-based perovskite precursor solutions were prepared by reacting SbI3 (Sigma Aldrich, 98%, Shanghai), MAI (Xi’an p-OLED Corp., ≥99.5%, Xi’an) and MACl (Xi’an p-OLED Corp., ≥99.5%, Xi’an) with different molar ratios in mixed solvents (dimethylsulfoxide (DMSO):γ-butyrolactone (GBL)=3:7, v/v), forming clear solutions with a weight ratio of 40%. Then, the perovskite solutions were spin-coated at 1000 rpm for 10 s and 4500 rpm for 30 s. Approximately 25 s after the initiation of the second spin-coating step, toluene (350 µL) was dropped onto the substrates. Then, the films were dried on a hot plate at 105 ºC for 45 min. The SbCl3-based perovskite precursor solution was prepared by dissolving SbCl3 (Sigma Aldrich, ≥99%) and MAI in a molar ratio of 1:3 in N,N-dimethylformamide (DMF) to form a solution with a weight ratio of 40%. It was spin-coated onto substrates at 4000 rpm for 20 s. Approximately 11 s after the initiation of the spin-coating, toluene (350 µL) was dropped onto the substrates. The resulting films were dried at 100 ºC for 20 min to achieve full crystallization. Solar Cell Fabrication. All solar cell devices were fabricated onto F-doped SnO2 glass (FTO, Nippon Sheet Glass, TCO-15, Wuhan), which had been sequentially sonicated in baths containing detergent in de-ionized water, de-ionized water, acetone and isopropanol. An approximately 40-nm-thick compact TiO2 (c-TiO2) layer was coated onto pre-heated FTO glass by aerosol spray pyrolysis, followed by sintering at 450 ºC for 30 min. Then, a 200-nm-thick mesoporous TiO2 (m-TiO2) layer was spin-coated from a butanol-dissolved TiO2 paste (Dyesol, ~18 nm) on top of c-TiO2, followed by sintering at 550 15



ºC for 35 min. The spiro-OMeTAD hole-transport layer was spin-coated onto the Sb-perovskite films at 4500 rpm for 30 s from a solution mixture consisting of chlorobenzene (1 mL), spiro-OMeTAD (80 mg, Nichem, Taiwan), 4-ter-butylpyridine (tBP, 28.8 µL, Sigma Aldrich, Shanghai) and a pre-dissolved lithium-bis(trifluoromethanesulfonyl)imide (Li-TFSI, 17.5 µL, Sigma Aldrich, Shanghai) solution (520 mg Li-TFSI in 1 mL acetonitrile). Films were then taken out of the glovebox to a dry basin for the oxidation of spiro-OMeTAD for at least 15 hours. Finally, a ~60-nm-thick gold film was evaporated on top as the back electrodes. First-Principles Calculations. All the calculations are carried out using plane-wave pseudopotential methods within density functional theory (DFT) as implemented in the Vienna Ab initio Simulation Package.75,76 We described the electron-ion interactions using the projected augmented wave pseudopotentials77 with the 5s25p66s for Cs, 5s25p3 for Sb, 5s25p5 for I, 3s23p5 for Cl, 2s22p2 for C, 2s22p3 for N, and 1s for H as valence electrons. The generalized gradient approximation formulated by Perdew, Burke, and Ernzerhof

78

was used as exchange correlation functional. We used a kinetic

energy cutoff of 400 eV for wave-function expansion and k-point meshes with spacings of 2π×0.02 Å-1 or less for electronic Brillouin zone integration. We optimized the equilibrium structural parameters (including both lattice parameters and internal coordinates) of the involved materials through total energy minimization with the residual forces on the atoms converged to below 0.01 eV/Å. To properly take into account the long-range van der Waals (vdW) interactions that play a non-ignorable role in the hybrid perovskites involving organic molecules, the vdW-optB86b functional79 was adopted. We used the Heyd-Scuseria-Ernzerhof80 hybrid functional approach (with standard 25% exact Fock exchange) to correct the band gap underestimation error of DFT calculations and obtain reliable band gaps.

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Film and Solar Cell Characterizations. For absorbance measurements, the Sb-perovskite films were deposited on substrates of glass/FTO/c-TiO2/m-TiO2, and the UV-Vis absorbance spectra were recorded with a UV-Vis-NIR spectrophotometer (UV-3600, Shimadzu Scientific Instruments). Films for XRD measurements were deposited on quartz to eliminate unnecessary diffraction peaks. The XRD measurements were conducted using a Philips diffractometer (X’pert PRO MRD) with a step of 0.013° and a step time of 13.77 s. Sb-perovskite films for SEM measurements were prepared on substrates of FTO/c-TiO2/m-TiO2. The top-view morphologies of the Sb-perovskite films were measured using an FEI Nova Nano-SEM 450 system. The grain size distribution diagrams and average grain values were obtained by randomly measuring the sizes of 100 grains of each Sb-perovskite sample using ImageJ software. The compositions of the Sb perovskites (powders) were obtained through EDX measurement. The current-voltage characteristics (J-V) of the solar cells were measured using a Keithley 2400 under simulated 100 mW cm-2 AM 1.5G irradiation. During the measurements, the solar cells were kept in the glovebox. The scan rate was 20 mV s-1. The EQE spectra were obtained using a 150 W xenon lamp (Newport, USA) with a monochromator as the light source. The valence state of Sb was characterized by XPS (AXIS-ULTRA DLD-600 W, Japan). The trap density was characterized by TAS with a device structure of FTO/c-TiO2/m-TiO2/perovskite/spiro-OMeTAD/Au. The measurement was performed in the dark and analyzed according to previous literature results.81,82

Author Contributions †

These authors contributed equally to this work.

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ACKNOWLEDGMENTS We acknowledge support from the National Natural Science Foundation of China (Grant No. 21474035, 51403071, 11404131 and 11674121), the National High-tech R&D Program of China (863 Program, No. 2015AA034601), the Recruitment Program of Global Youth Experts in China, the HUST Key Innovation Team for Interdisciplinary Promotion (Grant No. 2016JCTD111), the Science and Technology Program of Shenzhen (JCYJ20160429182443609), the Postdoctoral Science Foundation of China (2016M602289) and the Special Fund for Talent Exploitation in Jilin Province of China. We thank Mr. Chao Chen and Prof. Jiang Tang for help with the TAS measurements and fruitful discussions.

Supporting Information Available: Additional XRD patterns, UV-Vis spectra, Tauc plots, TAS plots, EDX patterns, XPS patterns and stability measurements of various products or devices are available free of charge on the ACS Publications website.

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Figure 1. (a) Schematic plot of the Cl-addition-induced transformation from the 0D dimer phase of A3Sb2I9 to the 2D layered phase of A3Sb2ClXI9-X. (b) Calculated decomposition enthalpies ∆‫ܪ‬ௗ௘௖ (see text) for the 0D dimer (black) and 2D layered (red) phases of Cs3Sb2ClXI9-X with varied Cl contents. For each phase, the lowest-energy configuration at each Cl content is connected with a dashed line.

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Figure 2. X-ray diffraction (XRD) patterns of the films deposited from precursors containing SbI3, MAI and MACl with molar ratios of 1:1.5:0, 1:0:1.5, 1:0.5:1.5 and 1:1:1.2. The simulated XRD patterns of the 0D dimer phase and the 2D layered phase of MA3Sb2I9 are shown in (a) and (b) for comparison, respectively. The crystalline structures obtained from theoretical structure optimizations were used for the simulations. The insets show the images of the four films. The square box from 20 to 30° shows the difference between the dimer and layered phases.

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(a)

(b) SbI3-MAI-MACl 1-1.5-0 1-0-1.5 1-0.5-1.5 1-1-1.2

2.5 2.0

SbI3-MAI-MACl 1-1.5-0 1-0-1.5 1-0.5-1.5 1-1-1.2

(ahν)2 ( a.u.)

Absorbance (a.u.)

3.0

1.5 1.0 0.5

~2.17 eV ~2.44 eV

0.0

450

500

550

600

650

2.0

2.2

Wavelength (nm)

(c)

2.4 hν (eV)

2.6

2.8

(d) 2.4

3 2

Dimer

1

Layer Dimer

2.2

0 -1 X 3

Γ

Y

L

Γ Z

Eg (eV)

Energy (eV)

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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2

1.8

Layer

1

2.0

0

1.6

-1 X

Γ

Y

L

Γ

MA3Sb2I9

Z

MA3Sb2ClI8

MA3Sb2Cl2I7

Figure 3. (a) Measured UV-Vis absorbance spectra of the four types of films; (b) Tauc plots of the absorption coefficients for evaluating the band gap values, assuming direct gap features, of the pure-iodine perovskites MA3Sb2I9 and Cl-containing mixed-halide perovskites MA3Sb2ClXI9-X; (c) calculated electronic band structures of the 0D dimer phase and 2D layered phase of MA3Sb2I9. The valence band maximum was set to energy zero. The minimum band gap is denoted by a solid arrow line, and the direct gap of the layered phase is denoted by a dashed arrow line; (d) calculated band gaps for the dimer phase of MA3Sb2I9 and for the layered phase of MA3Sb2ClXI9-X with various Cl contents. 24


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Figure 4. (a-d) Top-view morphologies of the four types of films from scanning electron microscopy (SEM) measurements; (e-h) statistical histogram of the individual perovskite grain sizes for the corresponding films. The grain size values are extracted from ImageJ software by randomly measuring 100 separate perovskite grains for each film. Au nanoparticles were deposited on top of the perovskite films to increase the sample conductivities for the convenience of the SEM measurement.

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Figure 5. (a) Schematic solar cell device structure of the as-fabricated PSC. c-TiO2 and m-TiO2 denote compact TiO2 and mesoporous TiO2, respectively; (b) current density-voltage (J-V) curves for the devices fabricated with the four kinds of perovskite films; (c) steady-state photocurrent output for the device based on film 1-1-1.2 at the maximum power point in (b) (denoted by the red circle). Maximum power point voltage Vmpp is equal to 0.54 V. (d) Measured J-V curves at different scan directions for the device based on film 1-1-1.2; (e) external quantum efficiency (EQE) of the highest-PCE cell based on film 1-1-1.2; (f) PCE distributions of 40 separate PSC devices based on film 1-1-1.2.

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Table 1. Photovoltaic parameters of the fabricated PSCs based on the four films. The results are measured by the reverse scan directions at a scan rate of 20 mVs-1 under standard 100 mWcm-2 AM illumination. The average values and standard deviations are calculated based on 40 separate devices.

1-1.5-0 1-0-1.5 1-0.5-1.5 1-1-1.2

Average Best Average Best Average Best Average Best

JSC (mA/cm2) 0.30 ± 0.04 0.35 4.05 ± 0.32 4.30 4.54 ± 0.35 4.74 4.63 ± 0.41 5.04

VOC (V) 0.46 ± 0.02 0.48 0.67 ± 0.03 0.72 0.66 ± 0.04 0.70 0.66 ± 0.04 0.69

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FF 0.51 ± 0.02 0.53 0.56 ± 0.02 0.56 0.55 ± 0.03 0.59 0.57 ± 0.03 0.63

PCE (%) 0.07 ± 0.02 0.09 1.51 ± 0.20 1.74 1.65 ± 0.28 1.96 1.74 ± 0.31 2.19


ToC Graphic

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Chlorine-incorporation-induced formation of the layered phase for

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