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Photovoltaic Performance of Vapor-Assisted Solution-Processed Layer Polymorph of Cs3Sb2I9 Anupriya Singh, Karunakara Moorthy Boopathi, Anisha Mohapatra, Yang-Fang Chen, Dr. Gang Li, and Chih Wei Chu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16349 • Publication Date (Web): 27 Dec 2017 Downloaded from http://pubs.acs.org on December 28, 2017

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ACS Applied Materials & Interfaces

Photovoltaic Performance of Vapor-Assisted Solution-Processed Layer Polymorph of Cs3Sb2I9 Anupriya Singh,†,‡,§ Karunakara Moorthy Boopathi,†Anisha Mohapatra† Yang Fang Chen,‡,§ Gang Liδ and Chih Wei Chu*†,§,∞, ξ †

Research Center for Applied Science, Academia Sinica, Taipei 115, Taiwan (R.O.C.)

‡

Department of Physics, National Taiwan University, Sec. 4, Roosevelt Road, Taipei 106, Taiwan (R.O.C.), §

Nano Science and Technology, Taiwan International Graduate Program, Academia Sinica and National Taiwan University, Taiwan (R.O.C.) ∞ College of Engineering, Chang Gung University, Taoyuan City, Taiwan (R.O.C.) ξ

Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan (R.O.C.) δ

Department of Electronic and Information Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China.

KEYWORDS: perovskite solar cells, lead-free, antimony based, all-inorganic, solution processed ABSTRACT: The presence of toxic lead (Pb) remains a major obstruction to the commercial application of perovskite solar cells. Although antimony (Sb)-based perovskite-like structures A3M2X9 can display potentially useful photovoltaic behavior, solution-processed Sb-based perovskite-like structures usually favor the dimer phase, which has poor photovoltaic properties. In this study, we prepared a layered polymorph of Cs3Sb2I9 through solution-processing and studied its photovoltaic properties. The exciton binding energy and exciton lifetime of the layerform Cs3Sb2I9 were approximately 100 meV and 6 ns, respectively. The photovoltaic properties

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of the layered polymorph were superior to those of the dimer polymorph. A solar cell incorporating the layer-form Cs3Sb2I9 exhibited an open-circuit voltage of 0.72 V and a power conversion efficiency of 1.5%—the highest reported for an all-inorganic Sb-based perovskite. INTRODUCTION: The needs of future generation depend vastly on the access to cheap and abundant sources of energy. An ideal energy conversion process is photovoltaic (PV) which has potential to meet this requirement. Dye/semiconductor sensitized solar cells,1–5 organic solar cells,6 and inorganic–organic hybrid solar cells7 show promising results among the novel photovoltaic devices. Amongst these, the discovery of lead halide perovskites opened the gates for low-cost solution-processable solar cells with efficiencies competitive with those of high-cost Si solar cells. Over the years, the photo-conversion efficiencies (PCEs) of lead halide perovskite solar cells have improved from 3.8% in 2009,8 to 22.1% in 2017.9 Even higher efficiencies of PV cells can be achieved with the help of tandem configuration due to the leveraging of multiple semi-conducting materials that absorb different sections of the solar irradiance spectrum.10,11 Still, the lead halide perovskites have problems of high toxicity (Pb) and low stability. Accordingly, alternatives to Pb are being sought to reduce the toxicity, and various ideas have been postulated to improve the stability. The presence of organic cations [e.g., CH3NH3, CH(NH2)2] in the hybrid perovskites appears to be the main reason for the device instability, because they have considerably lower thermal decomposition temperatures and high sensitivity to moisture.12,13 As a result, several different cations have been explored, and attempts have been made to replace the organic cations with inorganic cations (e.g., Cs, Rb).14,15 The Cs-based lead halide perovskite (CsPbI3) crystallizes in a photo-inactive, orthorhombic δphase (“yellow phase”) at room temperature; its photoactive perovskite phase (“black phase”) is stable only at temperatures above 300 °C.16 Although a black phase stable at room temperature


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has recently been developed,16 the material still contains toxic lead. First-principles calculations have revealed that the extraordinary performance of lead-based perovskites is related to the outermost s electrons of the metal cation hybridizing with the p orbitals of the halogen (anion) to give anti-bonding character to the valence band maximum, as well as a lower ionization energy.17 Thus, elements that have an electronic configuration similar to that of Pb2+, having outer s electrons, should act as good replacements for toxic lead.18 An obvious choice, in the same group as Pb, is Sn; the maximum efficiency was achieved using all-inorganic CsSnI3 (3.31%).19CsSnI3 undergoes a rapid phase transformation upon exposure to air from the black orthorhombic phase (B-γ-CsSnI3) to the yellow orthorhombic phase (γ-CsSnI3), followed by irreversible oxidation into Cs2SnI6 within several hours.20 Moving to the next group in the periodic table, Bi and Sb also have electronic configurations similar to Pb and can form A3B2X9 structures; because they exist only in the +3 state, their ability to form three-dimensional cornersharing octahedral structures is, however, constrained. As reported, Bi gives the best results with Cs cation, with the efficiency reaching 1.09%; nevertheless, its higher band gap and defect states limit its efficiency in solar cells.21 In addition, the binding energies of excitons are relatively high in Bi-based perovskites.21 Antimony is another promising element that has recently been reported as an alternative to Pb in solar cells.18, 22-25 Our group recently studied the use of methyl ammonium (MA) and Cs cations for 0D Sb-based perovskite-like structures. Although, the MA cations provided a higher maximum efficiency of 1.1% (without HI additive), the Cs cations was shown to have benefit of providing more stable devices because of its all-inorganic nature.26 So, to go for a lead- free stable material Cs3Sb2I9 which also occurs as a 2D polymorph is a good alternative that need to be explored.


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Two types of polymorphs occur for Cs3Sb2I9: one has two-dimensional corrugated layers of polyanions and the other has isolated bi-octahedral Sb2X39– anions. These two polymorphs can be obtained selectively, depending on the preparation conditions. The polymorph having corrugated layers of polyanions, abbreviated as “layer form (2D),” has been reported to form at 250 °C; the other, having bi-octahedral Sb2X39– anions, abbreviated as “dimer form (0D),” is formed at 150 °C.27 The dimer form of Cs3Sb2I9 is preferred over the layered form when prepared through solution processing;23 the PCE of the dimer form, as has been reported by our group, was 0.62%. A nearly direct band gap, higher electron and hole mobility, and better tolerance to defects (because of higher dielectric constants) make the layered form of Cs3Sb2I9 more suitable than the dimer form for PV applications.18, 28 In contrast to the finding that the dimer form is preferred through solution-processing, Pauling’s rule states that the layered form (corner-sharing octahedral) should be preferred over the dimer form (face-sharing octahedral) to maximize the distance between two adjacent Sb atoms.29 The main constraint to obtain layered Cs3Sb2I9 is the need for high-temperature annealing in SbI3 vapor, making the device difficult to reproduce.18 In addition, density functional theory (DFT) calculations have been reported to reveal that the material has deep level defects, which limit its open-circuit voltage (Voc) to less than 0.2 V and its short-circuit current density (Jsc) to less than 0.2 mA cm–2 .18 In this present study, the layered form of Cs3Sb2I9 has been prepared through solution-processing and the investigation for the photovoltaic (PV) applications of both the layered and dimer forms has been done. Here, SbI3 was sequentially introduced to get the stoichiometric layered polymorph of Cs3Sb2I9. The morphology of the film was controlled through pre-annealing, studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM), while verified the stoichiometry using energy dispersive X-ray spectroscopy (EDX). A comparative


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analysis of the properties of the dimer and layered polymorphs of Cs3Sb2I9 and photovoltaic performance of both forms has been studied. Devices prepared with a planar architecture exhibited champion efficiencies of 1.5% for the layered form and 0.89% for the dimer form. The layered form displayed superior PV performance because of its better material properties; it appears to have potential as a material for further studies for other optoelectronic applications. To deposit thin films of layered Cs3Sb2I9 through solution-processing, a single precursor solution

with

different

molar

ratios

of

CsI

and

SbI3

for

spin-coating

onto

poly(ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS)-coated substrates was used. For a better morphology, a mixture of DMF and DMSO as the solvent for dissolving SbI3 and CsI was used.30 The spin-coated substrates were pre-annealed at 70 °C for 15 min to remove the solvents, giving a smoother surface with a large number of small nuclei that could later grow into large crystallites. For the crystallization of layered Cs3Sb2I9, high-temperature annealing at 250 °C was required, but, because the vapor pressure of SbI3 at 250 °C is in the range 20–30 mm Hg, it evaporated from the films giving SbI3-deficient structures.31 Thus, sequential introduction of SbI3 after transferring the films on 250 °C was done. The films were placed in a bottle and then 30 µL of 30% SbI3 in DMF was placed inside the bottle to expose the film to SbI3 vapors, as displayed schematically in Figure 1. The detailed processing conditions for preparation of thin films is given under device fabrication part of Supporting Information (SI). Figure S1(SI) presents the schematics for thin film preparation corresponding to the dimer form. Figure S2(SI) provides real-time photographs of the vapor-assisted annealing of the layer form. EDX was used to measure the stoichiometries of the films; Table S1(SI) lists the EDX data of perovskite thin films prepared at various molar ratios on the PEDOT:PSS substrates. A CsI-to-SbI3 molar ratio


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of 1:0.25 provided the expected stoichiometric ratio, as measured using EDX (SI, Figure S3); the device performance was also the best at this molar ratio (SI, Table S2). Table S1 reveals that the amount of SbI3 in the precursor solution affected the final stoichiometry of the film. The absence of SbI3 in the precursor solution resulted in no change in color after annealing at 250 °C in the SbI3 vapors, suggesting that the small amounts of SbI3 in the film obtained after spin-coating acted as attractive centers for SbI3 vapors and gave the final stoichiometric Cs3Sb2I9 perovskite. A higher content of SbI3 in the precursor solution also resulted in a decrease in efficiency (SI, Table S2), because of the non-stoichiometric Cs3Sb2I9 as determined using EDX. Effects of higher temperatures (up to 300 °C) was also tested (SI, Table S2), but no significant changes were observed, other than the data being less reproducible because of a lack of control over the loss of SbI3. The thin films prepared under the optimized conditions were used as active layer for solar cell fabrication, using the device architecture displayed in Figure 1b. The device specifications are given in device fabrication part of SI. Figure 1c presents a cross-sectional view of the device, recorded using field emission scanning electron microscopy (FESEM), revealing a thickness of approximately 130 nm for the active layer. The absorption coefficient (α) at 450 nm for the layered material Cs3Sb2I9, formed using the procedure described above, was approximately 1.5 × 105 cm–1—higher than the value of 1 × 105 cm–1 reported for Bi-based perovskites but lower than the value of 2 × 105 cm–1 reported for Pbbased perovskite.21,32 The absorption coefficient of the material obtained herein was higher than that reported for the dimer form of Cs3Sb2I9.26 Figure 2a presents the normalized (to maximum) absorptions for thin films of the layer and dimer polymorphs; the layer Cs3Sb2I9 exhibited enhanced absorption. The band gap absorption peak for the layer Cs3Sb2I9 appeared at 2.05 eV,


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quite distinct from the absorption peak of the dimer form of Cs3Sb2I9 at 2.3 eV. The higher absorption coefficients in Cs3Sb2I9 can be attributed to the p-orbital character of both the valence and conduction bands leading to a high joint density of states similar as that for CH3NH3PbI3,33 but needs further work for confirming exact reason behind this increment. Thus, the higher value of α for the layered form relative to that for the dimer form implies a higher density of states in the layered form. Three absorption peaks appeared in the absorption spectra, with all three undergoing similar drifts in their peak positions for both dimer and layer form, suggesting that these peaks may have arisen from the same band. The layer form exhibited stronger absorption coefficients for all three peaks, presumably due to the higher density of states in it. The signal at 610 nm could be attributed to the exciton peak, as confirmed by the photoluminescence obtained for the layered form at 633 nm (Figure 2d). Determining the origin of the other two peaks will require further investigation. A Tauc plot revealed that the band gap of the layer form was 2.05 eV (Figure 2b) if the transition were considered as direct, while that of the dimer was 2.3 eV (Figure 2c). This band gap matches the value reported for the layer form of Cs3Sb2I9,23 but it differs from the exciton peak energies in the photoluminescence curve (Figure 2d). The Wannier–Mott exciton binding energy in the layer-form Cs3Sb2I9 was estimated from the optical band gap and the PL peak,34 to be approximately 100 meV. This value is significantly lower than that for Bi-based perovskites, which have exciton binding energies on the order of 300 meV,21 but higher than those of Pb-based perovskites, with exciton binding energies of 25–30 meV.35 No PL peak for the dimer form was observed, presumably because this material had an indirect band gap.23 Time-resolved photoluminescence was measured to provide a better understanding of the excitons produced in active layer which is shown in Figure 2e.


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The growth of the crystalline samples of both polymorphs was studied using X-Ray Diffraction (XRD), by recording XRD spectra of the as-deposited and pre-annealed samples, and after transferring to 250 °C for the layered sample and after annealing at 150 °C for the dimer (Figure 3a, b). The strongest peak in the simulated XRD spectrum of Cs3Sb2I9 appeared at a value of 2θ of 25.7°, corresponding to the (201) plane.23,27 The as-deposited film featured its strongest peak at 39.5°, corresponding to CsI, with a small peak at 25.7° revealing the presence of crystallites of Cs3Sb2I9. The ratios of the intensities of the peaks at 39.5 to 25.7° were lower in the pre-annealed films, implying an increase in the content of Cs3Sb2I9 crystallites. The narrower full width at half maximum (FWHM) of the peaks revealed an increase in the grain size of the Cs3Sb2I9 crystallites in the samples. After annealing at 250 °C, no sign of the peak corresponding to CsI at 39.5° was observed, suggesting the complete reaction of the precursors to give the desired stoichiometry. In addition to the signature peak of Cs3Sb2I9 at 25.7° [i.e., for the (201) plane], peaks also appeared at 8.67, 17.24, and 25.86°, corresponding to the (001), (002), and (003) planes, respectively, revealing the preferred C-axis orientation of the film. To further investigate the crystallinity of the perovskites, the sizes of the perovskite crystals from the FWHMs of the peaks at a value of 2θ of 25.7° for both the dimer and layer forms were calculated. The average crystallite sizes were estimated using the Scherrer equation (1): D = 0.89 λ/β cos θ

(1)

where D is the crystallite size, λ is the wavelength of the X-rays, β is the FWHM of the diffraction peak, θ is the diffraction angle, and the constant 0.89 is the shape factor.36 The crystallite sizes of the layer and dimer Cs3Sb2I9 were 218 and 50 nm, respectively. Thus, the


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crystallinity and crystallite size of the layered perovskite film were both significantly higher than those of the dimer form. SEM and AFM were used to study the morphologies of the samples (Figure 3c–f). The changes in morphology were studied with the help of AFM; the average roughness of the film decreased from 57 to 20 nm upon pre-annealing the film, as revealed in Figure 3(c´, d´). For better understanding, height variation in the samples by AFM imaging has been shown in Figure S4(SI). Figure 3(e, e´) present SEM and AFM images of the layer Cs3Sb2I9 film obtained after annealing at 250 °C, revealing grain sizes of approximately 150–500 nm. In contrast, the SEM and AFM images of the films of the dimer form revealed that they were very smooth. Indeed, AFM imaging indicated that the average roughness of the dimer form was only 6 nm. Urbach energy was calculated to gain insight regarding the structural disorder, impurities, deep traps, phonon effects, and excitons. The inverse of the slope of the exponential tail for the energies just below the band edge in the absorption spectra gives the urbach energy.37,38 The urbach energies for the Cs3Sb2I9 films synthesized through this solution-processing method were 134 meV for the layered form and 162 meV for the dimer form (SI, Figure S5a). The effect of pre-annealing in the case of the layered form was examined by calculating the urbach energy with and without pre-annealing. Without pre-annealing, the urbach energy was 220 meV (SI, Figure S5b), much higher than that of the films processed with pre-annealing at 70 °C. This reduction in urbach energy with improvement in morphology by pre-annealing indicates that further improvements in morphology should improve the PV properties of these Sb-based perovskite-like structures substantially. But, still these values are very high in comparison with those of conventional photovoltaic materials—including MA lead iodide, which exhibits an urbach energy of only approximately 15 meV39—indicative of the significant effects of the


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mentioned defects related to urbach energy in Cs3Sb2I9 and, hence, a major reason for the poorer PV properties. Ultraviolet photoelectron spectroscopy (UPS) was used to determine the valence band maximum (Figure 4a, b); the corresponding energy band alignment of the various layers in the device are presented in (Figure 4c). Inverted planar heterojunction (PHJ) solar cells were fabricated, incorporating both the dimer and layered Cs3Sb2I9 thin films as light absorbers to explore their photovoltaic performance. Initially, the devices were prepared using the architecture (ITO/PEDOT:PSS/Cs3Sb2I9/PC70BM/C60/BCP/Al) which uses double fullerene layers. Preannealing was introduced to improve the morphology in the case of the layered polymorph; its effect on the device performance is evident in Figure 5a. The use of double-fullerene layers (PCBM + C60) in Pb-based perovskites is required to fill the surface trap states;40 Cs3Sb2I9, however, has been reported to have deep level defects, rather than surface trap states,23 and, therefore, devices with only one fullerene layer (ITO/PEDOT:PSS/Cs3Sb2I9/PC70BM/Al) were prepared, as represented in the device architecture displayed in Figure 1b. The substitution of PCBM/C60/BCP to only PCBM led to a considerable increase in the value of Jsc while maintaining the same value of Voc (Figure 5b). This increase in Jsc can be due to the excitons which are produced by the photons getting reflected back from the top electrode. If C60/BCP is present near the top electrode, it absorbs some light and thus photons reaching and reflected back from Al electrode reduces and hence lesser Jsc. (Figure 5c) displays the photocurrent density– voltage (J–V) curves of the best devices obtained using the dimer and layered forms. Negligible hysteresis was found in devices for both the polymorphs as can be seen in Figure S6(SI). The EQE spectra of the devices in Figure 5d are consistent with the UV–Vis absorbance spectra of the perovskite films in Figure 2a. Thus, Cs3Sb2I9 acted as the primary contributor to the


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photocurrent and as a good light absorbing material. Table 1 summarizes the photovoltaic parameters. The champion device based on the Pb-free inorganic layered Cs3Sb2I9 perovskite exhibited a value of Jsc of 5.31 mA cm–2, a value of Voc of 0.72 V, and a fill factor (FF) of 38.97% under AM1.5G solar illumination, corresponding to a PCE of 1.49%; the champion device incorporating the dimer form exhibited a value of Jsc of 2.82 mA cm–2, a value of Voc of 0.77 V, and a FF of 40%, giving a PCE of 0.89%. The active layer thickness was varied by varying spin speed and the speed is optimized to 6000 rpm. A thinner or thicker film results in reduction of efficiency of the device (SI, Table S3) because of inappropriate carrier transfer to the respective electrode. The dimer form gave the higher value of Voc because of its larger band gap. According to detailed balance theory,41 the maximum Voc (Voc-max) of a semiconductor absorber is approximately equal to its band gap energy (Eg) subtracted by 0.25 eV. Thus, the values of Vocmax

for the layer and dimer forms were 1.80 and 2.05 V, respectively. Accordingly, the

experimental values were very low in comparison with the theoretical values. In general, the non-radiative recombination induced by, for example, defect states in the band gap, may seriously reduce the performance of the solar cell. Consequently, the PL response showed in Figure 2d combined with lower values of urbach energy for layer Cs3Sb2I9 suggests lower losses in non-radiative recombination, dimer form on the other hand showed no PL response and higher value of urbach energy. These factors may be related to the better photocurrent and voltage for the solar cell based on layer as compared to the dimer form and makes it an interesting material for other optoelectronic applications, such as light emitting diodes or lasers. To gain insight into the lower efficiency, the time-resolved PL spectra (Figure 2e) was studied for the PL peak of the layered form obtained at 633 nm. By fitting the data using a single exponential fit, the lifetime of


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the exciton was found to be only 6 ns, quite low in comparison with that of MAPbI3 perovskites, which have a lifetime of approximately 9.6 ns,42 presumably because the high content of deep level defects in Cs3Sb2I9 resulted in faster decay of excitons through trap states. The PL of the sample after excitation at 375 nm was measured before and after the TRPL measurements to check the stability of the sample during TRPL measurements; the intensity remained approximately the same. Careful defect passivation should be able to minimize defects (which is evident from the urbach energy calculations) and can increase both the PL intensity and the exciton lifetime,42 but will require further investigation. In addition, the devices had very poor FFs, presumably because the high exciton binding energy (ca. 100 meV) made it much easier for the charges to recombine.43 Furthermore, grain boundaries can also provide a high obstacle for charge transport, thereby giving low FFs.44 The stability of devices were examined; the layerform Cs3Sb2I9 was found to be more stable than the dimer-form Cs3Sb2I9. The operational stability of the devices was evaluated through continuous illumination for 18 h inside the glove box for device with layer form as active layer, observing a loss of only 8% (SI, Figure S7). Solution-processed layered Cs3Sb2I9 structures have been reported as non-reproducible,18 but, with this present technique, the efficiencies of 1.2–1.3% were found to be reproducible (SI, Figure S8). A comparison of toxicity and efficiency of Pb and Sb is given in Table S4(SI) which showed that Pb is indeed very toxic as compared to Sb but has an advantage of giving outstanding efficiency which has improved through years from 2009 to 2017. This preliminary study gives a way to further work and improve the efficiency for layer Cs3Sb2I9. Apart from this, an all inorganic Sb (Cs3Sb2I9) based device showed an better stability as compared to conventional

Pb

based

perovskite

device

with

structure

(ITO/PEDOT:PSS/MAPbI3/PC60BM/C60/BCP/Al) has been shown in Figure S9(SI). These


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results together with results from several characterization tools have suggested this material to be a potential active layer, it should be further investigated in various applications, including light emitting diodes, transistors, and photodetectors. CONCLUSION: In summary, the inorganic Sb-based perovskite Cs3Sb2I9 in both layer and dimer form have been prepared through solution-processing and obtained efficiencies of 1.5 and 0.89%, respectively, for corresponding champion solar cells having the inverted device architecture. PL spectra revealed an exciton peak at 633 nm, while TRPL gave an exciton lifetime of approximately 6 ns for the layer polymorph, with no such PL peak evident for the dimer form. The exciton binding energy for the layer polymorph was 100 meV. A decrease in the Urbach energy and an improvement in device performance occurred after introducing a preannealing step in the preparation of the layered Cs3Sb2I9. The Urbach energy of the layer-form Cs3Sb2I9 was less than that of the dimer-form Cs3Sb2I9. A double-fullerene layer was not necessary for this inorganic Sb-based device. The device stability of the layered polymorph was greater than that of the dimer polymorph. It can be concluded that the layer-form Cs3Sb2I9 perovskite has very promising properties that suggest its use in various optoelectronic devices; and solar cell performance of the dimer-form is also appreciable, and should be studied further. ASSOCIATED CONTENT: The Supporting Information is available free of charge on the ACS Publications website. Schematic representation of the preparation of dimer-form Cs3Sb2I9 with a real-time photograph of the film, Real-time photographs of the dropping of a SbI3 solution in DMF at 250 °C to provide extra SbI3 to give a final stoichiometric layered-form Cs3Sb2I9 at 250 °C. EDX spectra of


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the layer-form Cs3Sb2I9 films, Urbach energy curves, Stability study graphs and statistical study for 50 devices for layer form. AUTHOR INFORMATION

Corresponding Author Dr. Chih-Wei Chu E-mail: [email protected] Phone No.: +886-2-27873183 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Dr. Chu thanks the Ministry of Science and Technology (MOST) of Taiwan (104-2221-E-001014-MY3) and the Career Development Award of Academia Sinica, Taiwan (103-CDA-M01), for financial support. REFERENCES (1) Kamazani, M. M.; Zarghami, Z.; Niasari, M. S. Facile and Novel Chemical Synthesis, Characterization, and Formation Mechanism of Copper Sulfide (Cu2S, Cu2S/CuS, CuS) Nanostructures for Increasing the Efficiency of Solar Cells. J. Phys. Chem. C 2016, 120, 2096−2108 (2) Gholamrezaei, S.; Niasari, M. S. An Efficient Dye Sensitized Solar Cells Based on SrTiO3 Nanoparticles Prepared from a New Amine-Modified Sol-Gel Route, J Mol Liq. 2017, 243, 227– 235


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(19) Lyu, M.; Yun, J.-H.; Chen, P.; Hao, M.; Wang, L. Methylammonium Bismuth Iodide as a Lead-Free, Stable Hybrid Organic–Inorganic Solar Absorber. Adv. Energy Mater. 2017, 1602512. (20) Kontos, A. G.; Kaltzoglou, A.; Siranidi, E.; Palles, D.; Angeli, G. K.; Arfanis, M. K.; Psycharis, V.; Raptis, Y. S.; Kamitsos, E. I.; Trikalitis, P. N.; Stoumpos, C. C.; Kanatzidis, M. G.; Falaras, Structural Stability, Vibrational Properties, and Photoluminescence in CsSnI3 Perovskite upon the Addition of SnF2. P. Inorg Chem 2017, 56, 84-91. (21) Park, B.-W.; Philippe, B.; Zhang, X.; Rensmo, H.; Boschloo G.; Johansson, E. M. J. Bismuth Based Hybrid Perovskites A3Bi2I9 (A: Methylammonium or Cesium) for Solar Cell Application. Adv. Mater. 2015, 27, 6806-6813. (22) Hebig, J.-C.; Kühn, I.; Flohre, J.; Kirchartz, T. Optoelectronic Properties of (CH3NH3)3Sb2I9 Thin Films for Photovoltaic Applications. ACS Energy Letters 2016, 1, 309-314. (23) Saparov, B.; Hong, F.; Sun, J.-P.; Duan, H.-S.; Meng, W; Cameron, S.; Hill, I. G.; Yan Y.; Mitzi, D. B. Thin-Film Preparation and Characterization of Cs3Sb2I9: A Lead-Free Layered Perovskite Semiconductor. Chem. Mater. 2015, 27, 5622-5632. (24) Ganose, A. M.; Savory, C. N.; Scanlon, D. O. Beyond Methylammonium Lead Iodide: Prospects for the Emergent Field of ns2 Containing Solar Absorbers. Chem. Commun. 2017, 53, 20-44. (25) Zuo, C.; Ding, L. Lead-free Perovskite Materials (NH4)3Sb2IxBr9−x. Angew. Chem. Int. Ed. (English), 2017, 56, 6528-6532.


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(42) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science, 2013, 342, 341-344. (43) Ran, C.; Wu, Z.; Xi, J.; Yuan, F.; Dong, H.; Lei, T.; He, X.; Hou, X. Construction of Compact Methylammonium Bismuth Iodide Film Promoting Lead-Free Inverted Planar Heterojunction Organohalide Solar Cells with Open-Circuit Voltage over 0.8 V. J. Phys. Chem. Lett 2017, 8, 394-400. (44) Correa-Baena, J.-P.; Abate, A.; Saliba, M.; Tress, W.; Jesper Jacobsson, T.; Gratzel, M.; Hagfeldt, A. The Rapid Evolution of Highly Efficient Perovskite Solar Cells. Energy Environ Sci. 2017, 10, 710-727.


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Tables Table 1. Average (± standard deviation from the mean) characteristics of the optimized solar cells based on both dimer and layer form of Cs3Sb2I9 extracted from J–V measurements under reverse direction. Brackets have the best device performance for each condition. Electrode

Voc (V)

Jsc (mA cm–2)

FF (%)

PCE (%)

Cs3Sb2I9(D)*

C60/BCP/Al

0.58 ±0.04 (0.62)

1.98 ± 0.48 (2.34)

44.25 ± 2.15 (46.18)

0.62 ± 0.05 (0.67 )

Cs3Sb2I9(D)*

Al

0.73 ± 0.04 (0.77)

2.32 ± 0.51 (2.82)

38.12 ± 2.32 (40.98)

0.89 (0.83)

Cs3Sb2I9(L)*

C60/BCP/Al

0.58 ± 0.05 (0.64)

2.48 ± 0.42 (2.96)

42.25 ± 2.25 (44.34)

0.84 (0.80)

Pre-annealed Cs3Sb2I9(L)*

C60/BCP/Al

0.62 ± 0.06 (0.68)

3.12 ± 0.52 (3.69)

38.12 ± 1.51 (41.52)

0.92 ± 0.03 (0.95)

Pre-Annealed Cs3Sb2I9(L)#

Al

0.68 ± 0.04 (0.72)

4.62 ± 0.71 (5.31)

37.14 ± 2.75 (38.97)

1.26 ± 0.18 (1.49)

Material

*Average calculated from 10 devices; # average calculated from 50 devices


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Figures

Figure 1. (a) Schematic representation of the sequential introduction of precursor for the preparation of the stoichiometric layer-form Cs3Sb2I9; accompanied by an SEM image of the layered perovskite morphology. (b) Sketch of the solar cell architecture. (c) Cross-sectional SEM image of a solar cell device prepared using layer-form Cs3Sb2I9.


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Figure 2. a) Normalized UV–Vis absorption spectra of layer and dimer form. (b, c) Tauc plots for the b) dimer form and c) layer form, giving the band gap. d) Absorption spectra (squares) revealing the exciton peak and PL spectra (spheres) of the layer form revealing the difference in energy between the wavelengths of the exciton peak. e) Time-resolved photoluminescence of the PL peak at 633 nm for the layer form.


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Figure 3. a) XRD patterns revealing the crystallinity at different stages in the growth of the layer form. b) XRD patterns of both polymorphs; (c–f) SEM and (c´–f´) AFM images of (c, c´) the asspin-coated film, (d, d´) the film pre-annealed at 70 °C, (e, e´) the layer form, and (f, f´) the dimer form.


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Figure 4. a) Full ultraviolet photoelectron spectra of both the dimer and layer forms, revealing their near-perfect overlapping. b) Secondary-electron cut-off (left) and valence-band region (right), determined from the UPS spectra. c) Energy level diagram of solar cell devices incorporating the dimer and layer forms.


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Figure 5. (a, b) J–V characteristics of the layered form (a) with and without pre-annealing and (b) with different electrodes (C60/BCP/Al and Al). (c) J–V characteristics of the layer and dimer forms with only Al as the electrode (All results are shown for reverse scanning with scanning rate) (d) EQE spectra of the layered and dimer forms; the y-axis on the right-hand side reveals the integrated values of Jsc of the corresponding devices.


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