Efficient Lead-Free Solar Cells Based on Hollow {en}MASnI3

Sep 27, 2017 - Tin-based perovskites have very comparable electronic properties to lead-based perovskites and are regarded as possible lower toxicity ...
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Cite This: J. Am. Chem. Soc. 2017, 139, 14800-14806

Efficient Lead-Free Solar Cells Based on Hollow {en}MASnI3 Perovskites Weijun Ke, Constantinos C. Stoumpos, Ioannis Spanopoulos, Lingling Mao, Michelle Chen, Michael R. Wasielewski, and Mercouri G. Kanatzidis* Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States S Supporting Information *

ABSTRACT: Tin-based perovskites have very comparable electronic properties to lead-based perovskites and are regarded as possible lower toxicity alternates for solar cell applications. However, the efficiency of tinbased perovskite solar cells is still low and they exhibit poor air stability. Here, we report lead-free tin-based solar cells with greatly enhanced performance and stability using so-called “hollow” ethylenediammonium and methylammonium tin iodide ({en}MASnI3) perovskite as absorbers. Our results show that en can improve the film morphology and most importantly can serve as a new cation to be incorporated into the 3D MASnI3 lattice. When the cation of en becomes part of the 3D structure, a high density of SnI2 vacancies is created resulting in larger band gap, larger unit cell volume, lower trap-state density, and much longer carrier lifetime compared to classical MASnI3. The best-performing {en}MASnI3 solar cell has achieved a high efficiency of 6.63% with an open circuit voltage of 428.67 mV, a short-circuit current density of 24.28 mA cm−2, and a fill factor of 63.72%. Moreover, the {en}MASnI3 device shows much better air stability than the neat MASnI3 device. Comparable performance is also achieved for cesium tin iodide solar cells with en loading, demonstrating the broad scope of this approach.

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PCE around 6%.23,24 Since 2014, many efforts have been devoted to improve the performance of tin-based perovskite solar cells.25−32 Seok et al. reported formamidinium tin iodide (FASnI3) perovskite solar cells with regular mesoporous TiO2 structure using pyrazine addition to achieve a PCE of 4.8%.28 Recently, some remarkable improvements have been achieved for the inverted planar tin-based perovskite solar cells. Yan and co-workers reported inverted planar FASnI3 solar cells using solvent-engineering method to achieve a high PCE of 6.22%.30 Later, Liao et al. also reported a high PCE of 5.94% for the inverted planar cells based on low-dimensional tin halide perovskites.33 More recently, Zhao et al. employed mixedorganic-cation tin iodide as absorber for the inverted planar cells and reported a PCE of 8.12%.34 However, the record efficiency of tin-based perovskite solar cells is still much lower than that of lead-based perovskite solar cells. In addition, all the tin-based perovskites would be fast degraded after exposed to the atmosphere. The typical tin-based perovskite materials such as methylammonium tin iodide (MASnI3), FASnI3, and cesium tin iodide (CsSnI3) have ideal band gaps (1.2−1.4 eV) with correspondingly high carrier mobility but suffer from lower stability and metallic-like conductivity due to the easy oxidation of Sn2+ to Sn4+.2 In this case, Sn4+ acts as a p-type dopant within the material via a process referred to as self-doping, which leads

INTRODUCTION Organic−inorganic halide perovskites are now viable alternative candidates for third generation photovoltaic technology because of their high absorption coefficients, tunable band gaps, long carrier diffusion lengths, high defect tolerance, and simple fabrication process.1−6 The power conversion efficiency (PCE) of lead-based perovskite solar cells has rapidly increased from 9.7% in 2012 to the currently certified 22.1% in a short time.7−15 However, for the further commercialization, in addition to high efficiency, long-term stability, and lower toxicity are being seriously considered. The toxicity of lead remains a concern for the large-scale implementation of this technology. Therefore, it is imperative to explore replacements or substitutes to mitigate some of these issues. Extensive efforts are under way to identify nontoxic or low-toxicity and air stable halide perovskites as solar cell absorbers, such as germanium, antimony, and bismuth-based compounds.16−21 Among the lead-free compounds, only tin-based perovskites have shown a promising performance so far due to their excellent optical and electrical properties, which are comparable with lead-based perovskites.2 Although the fundamental properties of the tin perovskites are very similar to those of the lead based ones and, in principle, they should be capable of comparable efficiencies, nevertheless, the facile p-type doping occurring in the tin perovskites raises the dark carrier density and harms the excited state properties.22 The first report on tin-based perovskite solar cells with regular mesoporous structure achieved the highest © 2017 American Chemical Society

Received: August 23, 2017 Published: September 27, 2017 14800

DOI: 10.1021/jacs.7b09018 J. Am. Chem. Soc. 2017, 139, 14800−14806

Article

Journal of the American Chemical Society

Figure 1. Schematic view of (a) a hypothetical unit cell of hollow perovskite with SnI2 vacancy and (b) solar cell device structure of {en}MASnI3.

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to a low Voc and even shorted devices. Theoretically, tin-based absorbers can achieve higher short-circuit current density (Jsc) than lead-based absorbers because of their narrower band gap and higher mobility.2,35 The lower performance of tin-based perovskite solar cells is caused mainly by the much lower opencircuit voltage (Voc) which is caused partly from the high dark carrier density in the materials. Therefore, it is highly desirable to explore methods to improve the performance and stability of tin-based perovskite solar cells by changing the intrinsic semiconductor properties of the tin-based perovskite absorbers. Recently, it has been reported that some new organic cations, such as butylammonium and phenylethylammonium, can adjust the properties of tin and lead-based perovskites, forming a low dimensional structure and therefore improving the stability of perovskites.33,36−40 In these cases, the cations change the 3D perovskite structure and yield 2D phases which exhibit higher stability than MAPbI3-based devices. In our recent work, we demonstrated that ethylenediammonium (en) can serve as a new A cation in the 3D FASnI3 perovskite structure to form a hybrid 3D perovskite, {en}FASnI3.41 The en can easily tune the band gap of FASnI3 films and significantly improve the performance of {en}FASnI3 solar cells, which achieved the highest PCE of 7.14%.41 Here, we report that the incorporation of en is a general effect and can be implemented in other perovskites. We show that MASnI3, when blended with a new cation of en, can also significantly improve the optoelectronic properties of films. Similar to {en}FASnI3, the en becomes a dication and enters the MASnI3 crystal structure without lowering the dimensionality to 2D. The resulting material is still a 3D perovskite but with different optoelectronic properties. The 3D structure is a new type of hollow perovskite with SnI2 vacancies. We show that solar cells using hollow {en}MASnI3 absorber can achieve high reproducible efficiencies of up to 6.63% with a Voc of 428.67 mV, a Jsc of 24.28 mA cm−2, and a fill factor (FF) of 63.72%. By comparison the respective device with MASnI3 itself shows much lower efficiency because the much lower Voc and FF. The trap-filled limit voltage and time-resolved photoluminescence (TRPL) measurements show that the {en}MASnI3 film with 15% en loading has ∼50% lower electron trap-state density and ∼9 times longer carrier lifetime than that the film without en loading. We also show that {en}CsSnI3 solar cell can achieve a high PCE of 3.79%.

EXPERIMENTAL DETAILS

Device Fabrication. For the substrate preparation, TiO2 compact layers and mesoporous TiO2 layers (1 μm thick) were deposited on FTO substrates as described previously.29 We prepared the dication enI2 as a white polycrystalline powder by slowly dropping hydriodic acid (2 mL) into en (485 μL) at 0 °C. After stirring for 30 min, the precipitate was filtered by suction filtration and dried in vacuum-oven and washed by diethyl ether for three times. The MASnI3 precursor solution was prepared by adding 447 mg of homemade SnI2,2 160 mg of homemade MAI, and 23.5 mg of SnF2 (Sigma, 99%) into 70.8 μL of dimethyl sulfoxide (DMSO) and 632.8 μL of N,N-dimethylformamide (DMF). After all the materials were dissolved, 18, 36, 54, and 72 mg of enI2 were added to the solution for the {en}MASnI3 precursors with 5%, 10%, 15%, and 20% en, respectively. The CsSnI3 precursor solution was prepared by adding 372.5 mg of homemade SnI2,2 181.9 mg of CsI (Sigma-Aldrich, 99.999%), and 23.5 mg of SnF2 (SigmaAldrich, 99%) into 70.8 μL of DMSO and 632.8 μL of DMF. The {en}CsSnI3 precursor solution with 10% en loading includes extra 36 mg of enI2. The MASnI3, {en}MASnI3, CsSnI3, and {en}CsSnI3 precursors were spin-coated on the mesoporous TiO2 layers with a spin rate of 1500 rpm for 60 s. Then, the films were annealed for 10 min at 70 °C on a hot plate. The solution of hole transporting material, consisting of 32 mg of poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) (Sigma-Aldrich, 99%) and 3.6 mg of 4-isopropyl-4′methyldiphenyliodonium tetrakis(pentafluorophenyl)borate (TCI America) in 1.6 mL of chlorobenzene, was spin-coated on the perovskite films at 1500 rpm for 30 s and then annealed at 70 °C for 5 min. To complete the device, an 80 nm thick Au electrode was thermally evaporated on top of hole transporting layer using a metal mask. The active area of the solar cells was 0.09 or 0.39 cm2. Film and Device Characterization. The morphology of the films, and devices was characterized by a high-resolution field emission scanning electron microscopy (SEM) (Hitachi SU8030). Proton nuclear magnetic resonance (1H NMR) spectra were recorded on Bruker Avance III 600 MHz system with BBI probe. X-ray diffraction (XRD) spectra of the perovskite films were obtained on a Rigaku Miniflex600 pXRD (Cu Kα graphite, λ = 1.5406 Å) operating at 40 kV/15 mA with a Kβ foil filter. PL and TRPL spectra were measured with a streak camera setup (Hamamatsu C4334 Streakscope). The instrument response function was approximately 1% of the sweep window. A commercial direct diode-pumped 100 kHz amplifier (Spirit 1040-4, Spectra Physics) produces a fundamental beam of 1040 nm (350 fs, 4.5 W). This light was used to pump a non-collinear optical parametric amplifier (Spirit-NOPA, Spectra-Physics) which delivers high repetition rate pulses. The samples were excited with 545 nm, 0.15 nJ pulses. Photocurrent density−voltage (J−V) curves were measured with a Keithley model 2400 instrument under AM1.5G simulated irradiation with a standard solar simulator (Abet Technologies). The light intensity of the solar simulator was calibrated 14801

DOI: 10.1021/jacs.7b09018 J. Am. Chem. Soc. 2017, 139, 14800−14806

Article

Journal of the American Chemical Society

Figure 2. Top view SEM images of the MASnI3 perovskite films (a) without and (b) with 15% en deposited on mesoporous TiO2. (c) UV−vis absorption, (d) XRD patterns, and (e) PL spectra of the MASnI3 perovskite films prepared without and with 15% en deposited on mesoporous TiO2. (f) 1H NMR spectra of the enI2, MASnI3, and {en}MASnI3 powders. by a NREL-certified monocrystalline silicon solar cell. External quantum efficiency (EQE) spectrum was characterized by an Oriel model QE-PV-SI instrument equipped with a NIST-certified Si diode. Ultraviolet−visible (UV−vis) absorption spectra of the films were performed on a Shimadzu UV-3600 UV−vis−NIR spectrometer operating in the 200−2000 nm region at room temperature.

perovskite. Figure 1a shows the structural model which is referred to here as {en}MASnI3. Figure 1b shows the device structure of the {en}MASnI3-based solar cells using a compact TiO2 film as an electron transporting/hole blocking layer. Mesoporous TiO2 filled with {en}MASnI3 was used as an electron transporting layer and absorber skeleton. The perovskite absorbers were fabricated by adding various amounts of enI2 in the regular MASnI3 perovskite precursor solutions containing 15% SnF2, which has been shown to reduce the background hole carrier density and improve the solar cell performance.42,43 PTAA and gold were used as the hole transporting layer and metal electrode, respectively. In general, tin-based perovskites undergo a much faster crystallization process than Pb-based perovskites due to their lower solubility.44 Because of this, it is more difficult to obtain smooth tin-based perovskite films, which is critical to fabricate high-performance devices. Recently, it reported that using

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RESULTS AND DISCUSSION Our pervious results indicate that a 3D hollow FASnI3 perovskite structure with massive numbers of Schottky vacancies was formed after en loading.41 In this work, we wanted to investigate whether the use of the MA cation in combination with en would also result in the same 3D structure as the MASnI3 perovskite but with the en cation incorporated so that the material contains both en and MA cations. Our XRD and NMR analyses indicate that is in fact the case and the 3D structure having SnI2 vacancies thus the term hollow MASnI3 14802

DOI: 10.1021/jacs.7b09018 J. Am. Chem. Soc. 2017, 139, 14800−14806

Article

Journal of the American Chemical Society

Figure 3. (a) Cross-sectional SEM image of a completed device. (b) J−V curves of the solar cells using the MASnI3 perovskite absorbers with and without 15% en in the dark and under AM1.5G illumination, measured under reverse voltage scan. (c) EQE and integrated Jsc measured from an MASnI3 solar cell with 15% en. (d) Histograms of PCEs for 72 MASnI3 solar cells with 15% en.

3D crystal structure, as confirmed by XRD. As shown in Figure 2d, the film with en loading exhibits a slight shift of the Bragg reflections to smaller 2θ angles, without any evidence of lower angle reflections that would indicate the formation of 2D perovskites, suggesting a small increase in the unit cell volume. These results have the same trend as our previous FASnI3 material with en loading,41 demonstrating the hollow structure with massive numbers of Schottky vacancies was also formed for the MASnI3 material after en loading. Figure 2e shows the PL spectra of the perovskite films without and with en addition deposited on mesoporous TiO2. The PL peak is slightly blueshifted after en loading, which is in good agreement with the blue shift observed in the UV−vis absorption spectrum. It should be noted that not only the PL peak is shifted but also the intensity is significantly enhanced, attributed to the better film quality. To confirm the successful incorporation of en and quantify the amount of en dication in the final perovskite films, we measured the 1H NMR spectra of the enI2 powder and the {en}MASnI3 perovskite powder obtained from scratching away the films. The enI2 and perovskite powders were then dissolved in DMSO-d6 and the en signals accurately quantified against the MA signals, as shown in Figure 2e. From the 1H NMR spectra, we can estimate the molar ratio of en and MA cations in the final film which is ∼0.16:1, very close to the nominal composition (0.15:1). The above analyses reveal that the en cation not only regulates the crystallization and morphology of perovskite absorbing layers, but also incorporates in the crystal structure and influences the band gap, which has a positive effect on device performance. Figure 3a shows a cross-sectional SEM

vapor-assisted and solvent-engineering methods can result in uniform perovskite films.29,30,44 Incorporation of certain additives, such as pyrazine and thiocyanate, have been shown to slow the crystallization process and improve the perovskite film morphology.28,45 en in trace amounts has been reported to improve the selenium film morphology previously.46 Here, we see that massive amounts of en can also improve the MASnI3 perovskite film morphology without using complex or multistep methods, but just a simple one-step method. As shown in Figure 2a, the film without en has poor film coverage. There are large MASnI3 grains covering the mesoporous TiO2 but more TiO2 area is still exposed and available to contact the PTAA HTL directly, which leads to serious electron−hole recombination and even device short circuit. In stark contrast, the film with en loading becomes much smoother and pin-hole-less, ensuring efficient electron−hole separation and transfer, as shown in Figure 2b. It is also noted that the film with en loading has smaller grain size and is more compact. The reason for this is that a new type of perovskite is formed which contains mixed cations of MA and en in the structure. To systematically investigate the effect of the added en dication on film morphology, we fabricated films with various amounts of en. We observe that these films become smoother as the en loading increases while keeping the good morphology above 15% loading (Figure S1). In addition to the film morphology, en can change the film’s optical properties. Figure 2c shows the UV−vis absorption spectra of the MASnI3 films with 0% and 15% en loading. Remarkably, the band gap of the film is significantly increased when using ∼15% en from ∼1.25 eV to ∼1.40 eV. Even though the band gap changes, the film maintains the same perovskite 14803

DOI: 10.1021/jacs.7b09018 J. Am. Chem. Soc. 2017, 139, 14800−14806

Article

Journal of the American Chemical Society

Figure 4. (a) Aging test on unencapsulated MASnI3 solar cells without and with 15% en under constant AM1.5G illumination in ambient air. (b) Dark I−V curves of the electron-only devices. (c) TRPL spectra of the MASnI3 perovskite films without and with 15% en deposited on mesoporous TiO2. (d) J−V curve of a solar cell using an MASnI3 perovskite absorber with 15% en prepared under reducing vapor atmosphere, measured under reverse voltage scan.

image of a typical {en}MASnI3 device, consisting of a 1 μm thick mesoporous TiO2 layer infiltrated with perovskite, a 500 nm thick perovskite capping layer, a thin PTAA layer and an 80 nm thick gold film. Figure 3b shows the J−V curves of two representative solar cells using the perovskite absorbers without and with 15% en loading. The devices were used without encapsulation to record the J−V curves in the dark and under the light. The neat MASnI3-based solar cell achieved a typically low PCE of 0.17% with a Voc of 42.20 mV, a Jsc of 23.76 mA cm−2, and an FF of 38.24%. The dark current curve shows that device with the neat MASnI3 absorber was nearly shortcircuited. This can be attributed to the poor film coverage which raises the conductivity of the films. The leakage current of this device is very large under the negative bias voltage. The solar cell performance, however, is significantly enhanced when the absorber is made with 15% en loading. As shown in Figure 3b, the {en}MASnI3 solar cell achieved a much higher PCE of 5.49% with a Voc of 372.93 mV, a Jsc of 24.03 mA cm−2, and an FF of 61.30%. The dark current curve shows that the device has a good diode behavior with a large onset voltage and a very low current under negative bias voltage. Therefore, the dark carrier density and conductivity of the {en}MASnI3/SnF2 films are much lower than that of the neat MASnI3/SnF2 film, resulting in much lower charge carrier recombination. Figure 3c shows the measured EQE spectrum of the solar cell using the absorber with 15% en, displaying a high average value in the 300−900 nm wavelength range. The Jsc integrated from the EQE curve is around 23.90 mA cm−2, which is in good agreement with the extracted current from the J−V curves.

To optimize the amount of en in the {en}MASnI3 film, we fabricated the devices using the perovskite absorbers with various amounts of en addition. Figure S2 shows the J−V curves of the solar cells with 5%, 10%, 15% and 20% en. The corresponding photovoltaic parameters are summarized in Table S1. The device efficiency first increases with en loading up to 15% and then decreases with increasing en content. The optimum performance was obtained from the device using the perovskite absorber with 15% en loading. To study the hysteresis of the {en}MASnI3 devices, we tested the J−V curves measured under different voltage scan directions. Figure S3 shows the device using the MASnI3 absorber with 15% en under the reverse voltage scan achieved a PCE of 5.34% with a Voc of 371.99 mV, a Jsc of 23.74 mA cm−2, and an FF of 60.50%. The device achieved a similar PCE of 5.06% with a Voc of 356.84 mV, a Jsc of 23.90 mA cm−2, and an FF of 59.33% when measured under the forward voltage scan, showing a small hysteresis. To further confirm the reproducibility of the devices, we fabricated 72 individual devices with 15% en loading, showing a high average PCE of 5.26 ± 0.47% with a Voc of 373.76 ± 19.94 mV, a Jsc of 23.03 ± 1.67 mA cm−2, an FF of 61.20 ± 3.52% (Figure 3d). We also fabricated a {en}MASnI3 solar cell with larger active area. Figure S4 shows the device with active area of 0.39 cm2 still achieved a high PCE of 5.09% with a Voc of 354.29 mV, a Jsc of 25.02 mA cm−2, and an FF of 57.40%, attributed to the higher uniformity and pin-hole-less character of the {en}MASnI3 film. To evaluate the stability of the {en}MASnI3 film compared with the classical MASnI3 film, we exposed the unencapsulated 14804

DOI: 10.1021/jacs.7b09018 J. Am. Chem. Soc. 2017, 139, 14800−14806

Article

Journal of the American Chemical Society

of the solar cells using the CsSnI3 absorbers with and without 10% enI2. The neat CsSnI3 solar cell achieved a low PCE of 0.45% with a Voc of 97.18 mV, a Jsc of 18.75 mA cm−2, and an FF of 24.81%. While the CsSnI3 solar cell with 10% enI2 obtains a much higher PCE of 3.79% with a Voc of 280.93 mV, a Jsc of 25.07 mA cm−2, and an FF of 53.82%. Similar to the {en}MASnI3 solar cells, the stability of the {en}CsSnI3 solar cells is also significantly enhanced after en loading, as shown in Figure S6. Even though the performance is not fully optimized, we can still conclude that the hollow perovskite with en loading can work well for all MA, FA, and Cs perovskites. We also anticipate that en loading is applicable to lead-based perovskites as well.

devices to ambient conditions. Figure 4a shows the device performance as a function of the time under constant AM1.5G illumination in air at room temperature. The plot shows that the unencapsulated solar cell using the neat MASnI3 absorber degraded rapidly. Just after 2 min, the efficiency decreased to 0% and the device was short circuit, due to the fast oxidation of the MASnI3 film. In contrast, the unencapsulated {en}MASnI3 device shows much better stability under the same conditions, retains ∼60% of its initial efficiency after 10 min. Thus, not only the efficiency but also the air stability can be significantly improved after en loading. To investigate the change of the intrinsic property of the perovskite films caused by the incorporation of the en cations in the structure, we carried out dark current−voltage (I−V) analysis for electron-only devices, which can yield insights on the electron trap-state density in the perovskite films.47,48 Figure 4b shows the I−V curves of the electron-only devices using the MASnI3 and {en}MASnI3 films measured in the dark. The electron-only device structure is shown in the inset of Figure 4b. The plot at low bias voltage shows a linear relation, indicating an ohmic-contact response, while the current increases sharply at higher bias voltage, indicating the trap-states are fully filled. The voltage at which all the traps are filled can be marked as trap-filled limit voltage (VTFL), determined by eq 1:49 VTFL = entL2 /2εε0

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CONCLUSIONS We have shown that en can be incorporated into the MASnI3 perovskite and cause significant changes in the optoelectronic properties without a change in structure type. The en addition can increase the band gap and PL intensity of {en}MASnI3 films because it enters the cages of the 3D perovskite structure, replacing MA cations. These new types of 3D perovskites which contain en dications in the cages are not fully dense and have significantly reduced electron trap-state density and increased carrier lifetime, thereby improving device efficiency and stability. The best-performing {en}MASnI3 solar cell based on mesoporous TiO2 structure achieved the highest PCE of 6.63%. Similar results have also been demonstrated for the Cs tin-based perovskite solar cells with en loading. Our results suggest that the incorporation of new cations can keep the 3D structure, and this is a new method to simultaneously improve the efficiency and stability of tin-based perovskite solar cells. Recent reports have described planar cells with promising results based on (FA)(MA)SnI3 with 8.12% efficiency.34 These are very promising results; however, our own attempts to prepare such planar cells have not resulted in efficiencies higher than 3%. Our future work will be focused on transitioning our hollow perovskite materials to a planar platform.

(1)

where nt is the trap-state density, L is the thickness of the perovskite film, e is the elementary charge of the electron, and ε and ε0 are the relative dielectric constant of perovskite and the vacuum permittivity, respectively. According to eq 1, a lower trap-state density of the materials can result in a lower VTFL. The device using the {en}MASnI3 film has a much lower VTFL than the device using the neat MASnI3 film. It can be estimated that the trap-state density nt of the MASnI3 and {en}MASnI3 films are 1.8 × 1014 and 0.9 × 1014 cm−3, respectively. The significantly reduced trap density can be mainly attributed to the improved film quality after en loading, resulting in the improved FF and Voc of the devices. To compare the electron extraction and transport properties, we measured TRPL lifetimes of the different films. The TRPL spectra in Figure 4c show that the neat MASnI3 film has a very short lifetime of 0.41 ± 0.04 ns, while the {en}MASnI3 film has a much longer lifetime of 3.5 ± 0.2 ns. This suggests that the {en}MASnI3 film has a much longer electron−hole diffusion length than that the film without en loading and this is consistent with the lower carrier density of the former. In our previous results, we mentioned that the glovebox atmosphere can seriously affect the property of tin-based perovskite films.29 Specifically, a reducing vapor atmosphere such as hydrazine can improve the performance and especially Voc of the tin-based perovskite solar cells.50 To further improve the device performance, we have combined the en addition with the reducing vapor atmosphere to achieve higher efficiency of the {en}MASnI3 solar cells. Figure 4d shows the J−V curve of the solar cell using the {en}MASnI3 absorber fabricated under mild hydrazine atmosphere. The device achieved a higher PCE of 6.63% with a significantly improved Voc of 428.67 mV, a Jsc of 24.28 mA cm−2, and an FF of 63.72% when measured under the reverse voltage scan. The en addition affects not only the MA perovskites but also the Cs perovskites. To better understand the trend across the other cations, we also fabricated the CsSnI3-based device with en loading in the same fashion. Figure S5 shows the J−V curves

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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/jacs.7b09018. SEM images and J−V curves of the MASnI3 perovskites with various amounts of en loading; J−V curves of the device with larger area and under different voltage scan directions; J−V curves and stability test of the CsSnI3 solar cells without and with en, including Figures S1−S6 and Table S1 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Weijun Ke: 0000-0003-2600-5419 Constantinos C. Stoumpos: 0000-0001-8396-9578 Lingling Mao: 0000-0003-3166-8559 Michael R. Wasielewski: 0000-0003-2920-5440 Mercouri G. Kanatzidis: 0000-0003-2037-4168 Notes

The authors declare no competing financial interest. 14805

DOI: 10.1021/jacs.7b09018 J. Am. Chem. Soc. 2017, 139, 14800−14806

Article

Journal of the American Chemical Society

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ACKNOWLEDGMENTS This work was supported in part by the ANSER Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award DE-SC0001059. This work made use of the EPIC facility (NUANCE Center-Northwestern University), which has received support from the MRSEC program (NSF DMR-1121262) at the Materials Research Center, and the Nanoscale Science and Engineering Center (EEC-0118025/ 003), both programs of the National Science Foundation; the State of Illinois; and Northwestern University.

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DOI: 10.1021/jacs.7b09018 J. Am. Chem. Soc. 2017, 139, 14800−14806