Sn Perovskites with Ideal- Bandgap Yield Solar Cells with Higher

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Ethylenediammonium Based “Hollow” Pb/Sn Perovskites with Ideal-Bandgap Yield Solar Cells with Higher Efficiency and Stability Weijun Ke, Ioannis Spanopoulos, Qing Tu, Ido Hadar, Xiaotong Li, Gajendra S. Shekhawat, Vinayak P. Dravid, and Mercouri G. Kanatzidis J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b03662 • Publication Date (Web): 07 May 2019 Downloaded from http://pubs.acs.org on May 7, 2019

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Journal of the American Chemical Society

Ethylenediammonium Based “Hollow” Pb/Sn Perovskites with IdealBandgap Yield Solar Cells with Higher Efficiency and Stability Weijun Ke,1 Ioannis Spanopoulos,1 Qing Tu,2 Ido Hadar,1 Xiaotong Li,1 Gajendra S. Shekhawat,2 Vinayak P. Dravid,2 and Mercouri G. Kanatzidis1* 1Department

of Chemistry, Northwestern University, Evanston, IL 60208, United States

2Department

of Materials Science & Engineering, Northwestern University, Evanston, IL 60208,

United States Abstract The power conversion efficiency (PCE) of halide perovskite solar cells is now comparable to commercial solar cells. These solar cells are generally based on multi-cation mixed-halide perovskite absorbers with non-ideal bandgaps of 1.5 to 1.6 eV. The PCE should be able to rise further if the solar cells could use narrower-bandgap absorbers (1.2-1.4 eV). Reducing the Pb content of the semiconductors without sacrificing performance is also a significant driver in the perovskite solar cell research. Here, we demonstrate that mixed Pb/Sn-based perovskites containing the oversized ethylenediammonium (en) dication, {en}FA0.5MA0.5Sn0.5Pb0.5I3 (FA=formamidinium, MA=methylammonium), can exhibit ideal bandgaps of 1.27 to 1.38 eV, suitable for the assembly of single-junction solar cells with higher efficiencies. The use of en dication creates a three-dimensional (3D) hollow inorganic perovskite structure, which was verified through crystal density measurements and single crystal X-ray diffraction structural analysis as well as nuclear magnetic resonance measurements. The {en}FA0.5MA0.5Sn0.5Pb0.5I3 structure has massive Pb/Sn vacancies and much higher chemical stability than the same structure without en and vacancies. This new property reduces the dark current and carrier trap density and increases the carrier lifetime of the Pb/Sn-based perovskite films. Therefore, solar cells using

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{en}FA0.5MA0.5Sn0.5Pb0.5I3 light absorbers have substantially enhanced air stability and around 20% improvement

in

efficiency.

After

overlaying

a

thin

MABr

top

layer,

the

{5%

en}FA0.5MA0.5Sn0.5Pb0.5I3 material gives an optimized PCE of 17.04%. The results highlight the strong promise of 3D hollow mixed Pb/Sn perovskites in achieving ideal-bandgap materials with higher chemical stability and lower Pb content for high-performance single-junction solar cells or multi-junction solar cells serving as bottom cells.


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Introduction Halide perovskites have extremely high optical absorption coefficients, remarkably long carrier lifetimes, and tunable bandgaps despite the presence of massive structural defects and impurites.1-9 The

typical

Pb-based

halide

perovskites

are

MAPbI3,

CsPbI3,

and

FAPbI3

(MA=methylammonium, FA=formamidinium), with bandgaps of around 1.58, 1.73, and 1.48 eV, respectively.10-12 Perovskite solar cells have achieved a record power conversion efficiency (PCE) of 24.2%,13 outperforming those of multicrystalline silicon (22.3%), Cu(In,Ga)(Se,S)2 (23.3%), and CdTe (22.1%) solar cells however falling behind that of GaAs (29.1%) solar cells. In singlejunction solar cells, the ideal bandgaps should be around 1.1-1.4 eV for achieving the theoretical record efficiency limit.7, 14 High-performance perovskite solar cells are now using multi-cation mixed-halide perovskite absorbers (i.e. (Cs,FA,MA)PbI3-xBrx), adopting non-ideal bandgaps of around 1.5-1.6 eV.15-24 Halide perovskites have tunable bandgaps and their solar cells would be able to achieve higher PCEs if could take advantage of lower-bandgap absorbers. However, the lowest band gap of Pb-based perovskites is around 1.48 eV (FAPbI3).11 Previously, we reported that mixed Pb/Sn-based perovskites show an anomalous bandgap trend as a function of Pb/Sn ratio.25 Instead of following the normal linear dependence (also known as Vegard’s law), where the gaps vary smoothly from the wider bandgap end member MAPbI3 (around 1.58 eV) to the narrower bandgap end member MASnI3 (around 1.3 eV), intermediate compositions of MAPb1xSnxI3

exhibit bandgaps narrower than 1.2 eV.26 Therefore, the mixed Pb/Sn-based perovskites

have great potential for high-performance single-junction solar cells or bottom solar cells in multijunction solar cells,14, 27-28 targeting to achieve PCEs of over 28%. For the past few years, more and more researchers have been working on narrow-bandgap mixed Pb/Sn-based perovskite materials and have achieved sufficient breakthroughs.27-33 For example, the Yan, Snaith, McGehee,



and Jen groups have demonstrated high-performance two-terminal and four-terminal allperovskite tandem solar cells using mixed Pb/Sn-based perovskites as bottom solar cell absorbers.27, 29-30, 34-35 The single-junction narrow-bandgap mixed Sn/Pb-based perovskite solar cells, all-perovskite two-terminal, and four-terminal tandem solar cells generated record efficiencies of 19.03%,36 21.0%,35 and 23.1%,34 respectively. These systems are also attractive because of their lower content of Pb which is perceived to be highly toxic. However, since the mixed Pb/Sn-based perovskite materials contain Sn2+, they have the same chemical problem as the pure Sn-based perovskites, which makes them air-sensitive because of the easy formation of Sn4+, leading to heavy p-type doping of the materials, high conductivity, and quick degradation of the devices or ultimate decomposition.37-38 Therefore, it is imperative to seek more stable versions of these narrow-bandgap perovskite materials. Recently, we reported that the stability of threedimensional (3D) perovskite materials can be significantly improved by forming so-called “hollow” perovskite structures with certain organic cations.39-40 Specifically, we have demonstrated that diammonium cations which are too large to fit in the A cage of the ABX3 perovskite structure, do in fact enter the cage in apparent violation of the so-called Goldschmidt radius ratio rule.39-41 We have shown that this occurs by the formation of massive B metal vacancies (and possibly halide X vacancies) in the 3D perovskite “BX3” framework creating a “hollow” framework. This leads to more space near the A cage to accommodate the larger en dication and, at the same time, dramatically improves the stability of pure 3D Sn-based perovskites and their solar cells.40, 42-43 Here, we build on the above two discoveries and investigate the 3D mixed Pb/Sn hollow perovskites containing oversized ethylenediammonium (en) (3.743 Å) to create materials with lower lead content and much better chemical stability. We synthesized perovskites with the en


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dication in FA0.5MA0.5Sn0.5Pb0.5I3, which form a 3D hollow structure as determined by single crystal X-ray crystallographic data, powder x-ray diffraction (PXRD), proton nuclear magnetic resonance (1H-NMR) measurements, photoluminescence (PL), and ultraviolet−visible (UV−vis) absorption spectra. In addition to tuning the gap using the Pb/Sn ratio, a second knob serving to do same is the degree of incorporation of en. To be consistent with our previous notation,37 we refer to these “hollow” perovskite materials here as {en}FA0.5MA0.5Sn0.5Pb0.5I3 to distinguish them from the standard full perovskite FA0.5MA0.5Sn0.5Pb0.5I3. We show that just 5% en addition can dramatically reduce the trap density and increase the dark resistance and the carrier lifetime of the {en}FA0.5MA0.5Sn0.5Pb0.5I3 films, as confirmed by time-resolved PL (TRPL) and conductive atomic-force

microscopy

(CAFM)

measurements.

Hence,

solar

cells

using

{5%

en}FA0.5MA0.5Sn0.5Pb0.5I3 show much lower dark current and recombination, resulting in higher open-circuit voltage (Voc), fill factor (FF), PCE, and, most importantly, far superior air-stability. We find that the device performance improved further with a very thin MABr film laid over the perovskite to increase Voc, FF, and yield a champion PCE of 17.04% with negligible hysteresis. Results and discussion First, we synthesized the corresponding pure single crystals from solution as described previouly.10, 40 Various amounts of en were added into the hot precursors, and crystals precipitated from the solution by cooling down to room temperature. Figure 1a shows the scanning electron microscopy (SEM) images of the FA0.5MA0.5Sn0.5Pb0.5I3 and {en}FA0.5MA0.5Sn0.5Pb0.5I3 crystals, indicating a rhombic dodecahedral and octahedral shapes, respectively. Figure 1b shows the crystal structure of {en}FA0.5MA0.5Sn0.5Pb0.5I3, which was determined by single crystal XRD analysis. Given that the en cation (3.743 Å) has a much larger effective radius than FA (2.53 Å) and MA (2.17 Å), en cannot occupy the A-site position of the 3D FA0.5MA0.5Sn0.5Pb0.5I3 structure without



reducing the dimensionality of the 3D structure. Therefore, in order for the structure to maintain its 3D network character while at the same time incorporate the en molecules, it must create metal halide vacancies, as we have demonstrated in our previous reports in the Sn and Pb perovskites and we do so again below for the mixed Pb/Sn materials.39-40 The resulting FA0.5MA0.5Sn0.5Pb0.5I3 and {en}FA0.5MA0.5Sn0.5Pb0.5I3 crystals crystallize in the cubic Pm3m space group (α phase at room temperature).10 Detailed crystal data and structure refinement for the crystals with 0-20% en are summarized in Tables S1-15. The effects of en inclusion in the 3D structure are clear by examining closely the single crystal structure refinement data. The first effect is the expansion of the unit cell dimensions, ranging from a = 6.2983(1) Å for the 0% en-based crystal to a = 6.3063(4) Å and a = 6.3115(1) Å for 5% and 20% en-based ones, respectively. This is accompanied by the elongation of the M-I bond with values ranging from 3.14915(10) Å to 3.1532(4) Å and 3.15575(10) Å for the crystals with 0%, 5% and 20% en, respectively. Another evidence for vacancies in the structure is the less occupancy refinement of the heavy atoms. In the case of the hollow materials containing 5% and 20% en, the occupancy refinement of Sn and Pb atoms (0.9566 for 5% en and 0.8009 for 20% en) shows the massive vacancies present which is consistent with the replacement of some metal cations by en cations (Table S2, 7, and 12). This is accompanied by a significant decrease in the crystal density, ranging from 3.948 to 3.748 and 3.679 g/cm3 for the 0%, 5%, and 20% en-based samples (Table S1, 6, and 11), respectively. To verify these density profiles based on the crystallographic analysis, we also used a commercially available pycnometer to evaluate the densities of bulk materials. Accordingly, the experimental values follow the same trend, decreasing from 3.90 to 3.77 and 3.71 g/cm3. All these observations are in agreement with our previous studies,39-40 verifying clearly the hollow nature of this new family of mixed metal ion materials.


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Consistent with the single crystal data, the PXRD patterns of the bulk materials slightly shift to smaller 2θ angles after incorporation of en because of the expansion of the unit cell (Figure 1c, Figure S1). There are no Bragg reflection peaks lower than 10 deg, implying 2D perovskite structures are not formed. The presence of en dication in the dried {en}FA0.5MA0.5Sn0.5Pb0.5I3 crystals can be also verified by 1H-NMR measurements after dissolving the crystals in DMSO-d6. The 1H-NMR spectrum of the neat FA0.5MA0.5Sn0.5Pb0.5I3 (non-hollow) crystals shows the FA and MA signals (Figure S2), where δ=2.4, 7.5, 7.9, 8.6, and 9.0 ppm are assigned to -CH3 protons of MA, -NH3 protons of MA, =CH- proton of FA, -NH2 protons of FA, and -NH2 protons of FA, respectively. The molar ratio of FA and MA cations in the final crystals was found to be 1:1, which is consistent with that of the cations added in the precursor. The 1H-NMR spectra of the {5% en}FA0.5MA0.5Sn0.5Pb0.5I3 and {20% en}FA0.5MA0.5Sn0.5Pb0.5I3 shown in Figure S3-4 clearly prove the presence of en dication in the crystals, where δ = 3.0 and 7.8 ppm are assigned to -CH3 protons and -NH3 protons of en, respectively. The exact molar ratios of FA and en in the final crystals are around 1:0.05 and 1:0.155 for the nominal {5% en}FA0.5MA0.5Sn0.5Pb0.5I3 and {20% en}FA0.5MA0.5Sn0.5Pb0.5I3, respectively. In the following discussion, we still use the nominal component percentage to discuss the results although not all en can be incorporated into the final crystals because of the different solubility of en, MA, and FA cations in the crystal solvents. These unique structral variations render en dication a “knob” with which to finetune the material’s optical properties. UV-vis absorption spectra comparing the FA0.5MA0.5Sn0.5Pb0.5I3 and {en}FA0.5MA0.5Sn0.5Pb0.5I3 crystals are shown in Figure 1d. The pure FA0.5MA0.5Sn0.5Pb0.5I3 without en shows an absorption onset at around 1170 nm. By contrast, the materials with 5% and 20% en have absorption onsets at around 1100 and 1060 nm, respectively, showing blue-shifted absorption. The optical bandgaps estimated from the UV-vis absorption spectra of the crystals with



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0%, 5%, and 20% en are around 1.14, 1.17, and 1.24 eV, respectively (Figure S5). The photoluminescence (PL) spectra also show the same blue-shifted trend. The samples without and with 5% and 20% en exhibit emission peaks at around 954, 940, and 906 nm, respectively (Figure 1e). The slightly wider bandgaps of the materials can be attributed to the narrower bandwidths arising from the 3D hollow structure with metal halide vacancies induced by the larger en cations.40 Having investigated the optical properties of the crystals with various amounts of en, we then studied the film properties. In all films, 5 mol% SnF28 was added and they were deposited by the solvent-engineering

method44

using

mixed

dimethyl

sulfoxide

(DMSO)

and

N,N-

dimethylformamide (DMF) as the precursor solvent and diethyl ether as the anti-solvent. Similar to the crystals, the films with en adopt a 3D hollow structure, exhibit good crystalline quality, and feature two main Bragg reflections at 14.3 and 28.5 degrees, which can be indexed to the (100) and (200) reflections (Figure 2a). In addition, the XRD patterns of the films shift to lower angles after incorporating the en dication (Figure 2b), suggesting a slight expansion in the unit cell volume. Compared to the crystals, the films generally have slightly wider bandgaps (well known for (MA,FA)PbI3 and (MA,FA)SnI3 systems)10, 45 and the films samples containing en have the same blue-shifted absorption as the crystals (Figure 2c-d). The bandgap of the mixed FA0.5MA0.5Sn0.5Pb0.5I3 perovskite film without en is around 1.27 eV (Figure 2d). Even after adding 5% and 10% en, the films still have narrow bandgaps of 1.33 and 1.36 eV, respectively, which are located at the ideal-bandgap range for single-junction solar cells. Benefiting from the anomalous bandgap behavior in mixed Pb/Sn-based perovskites,25 all these materials have narrower bandgaps than the pure parent compounds of FASnI3 (1.48 eV) and MAPbI3 (1.58 eV). The films with 5% and 10% en loading show PL peaks at around 922 and 913 nm, respectively, whereas the neat FA0.5MA0.5Sn0.5Pb0.5I3 film has an emission peak at around 959 nm (Figure 2e).


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This is the same trend as in the corresponding crystals. To assess the effect of the en addition on photoexcited carrier lifetime, we measured the time-resolved PL (TRPL) spectra of the films. All films were deposited on fluorine-doped tin oxide (FTO)/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) substrates, which are the same as the device substrates described below. The TRPL spectra were fitted with double exponential curves. Figure 2d shows that the neat film has the shortest average lifetime of 29.6 ns. By contrast, the film with 5% en has a longer average lifetime of 45.6 ns, whereas at 10% en, the lifetime is extended to 47.6 ns. All data are summarized in Table S16. The longer PL lifetimes for the film samples with en addition are attributed to the reduced number of dark carriers and higher resistivity of the films. This was verified by atomic-force microscopy (AFM) and CAFM measurements (Figure 3a-f). Sn-based perovskite films can be very conductive because of the heavy p-type doping after Sn2+ oxidized to Sn4+.37 Figure 3a shows the AFM image of the pristine FA0.5MA0.5Sn0.5Pb0.5I3 film without en, having an average roughness of around 21.9 nm. The corresponding CAFM image is shown in Figure 3b, indicating a bright current image. The brighter contrast implies a much higher current and higher conductivity in the film. The whole area of the film shows high dark currents especially at the grain boundaries, with an averaged current of 57.14 ± 13.36 pA. Such high conductivity will lead to serious carrier recombination of the devices. In contrast, in the {en}FA0.5MA0.5Sn0.5Pb0.5I3 films, the dark currents in the grains as well as grain boundaries are significantly reduced. For example, the film with 5% en has a much darker image (Figure 3d) and a much lower average current of 2.78 ± 1.58 pA, which is over 90% lower than the film without en. Adding more en cation, the average current of the film with 10% en drops even lower at 0.78 ± 0.50 pA (Figure 3f). These data suggest that incorporation of en in



the perovskite structure inhibits the degree of Sn4+ doping via a mechanism that is likely associated with the hollow nature and massive Pb/Sn vacancies of the inorganic structure. Apart from the optical and electrical properties of films, the en addition also affects the film morphology. The films with 5% and 10% en exhibit smaller grains, no pinholes, and smoother surfaces with much lower average roughness of around 9.9 and 9.0 nm (Figure 3c and e), respectively, compared to 21.9 nm for the film without en. Figure 4a-c show SEM images comparing FA0.5MA0.5Sn0.5Pb0.5I3 films prepared without and with 5% and 10% en. The FA0.5MA0.5Sn0.5Pb0.5I3 film without en has grains with a size of ~ 500 nm (Figure 4a). In comparison, the grain size of the films with 5% and 10% en is around 200 nm (Figure 4b and c). After the films of the hollow {en}FA0.5MA0.5Sn0.5Pb0.5I3 perovskites were optimized and characterized, we fabricated solar cells employing an inverted planar architecture and a 500 nmthick perovskite film sandwiched between a bottom PEDOT:PSS hole transporting layer (HTL) deposited on a FTO substrate and a top phenyl-C61-butyric acid methyl ester (PCBM) electron transporting layer (ETL). The latter was covered with a thin 2,9-dimethyl-4,7-diphenyl-1,10phenanthroline (BCP) interface modification layer and an 80 nm-thick Ag electrode. All these layers were uniformly well packed on the substrates, which is critical for planar perovskite solar cells. A typical cross-sectional SEM image of a completed solar cell using a {5% en}FA0.5MA0.5Sn0.5Pb0.5I3 absorber is shown in Figure 4d. Figure 5a shows the photocurrent density-voltage (J-V) curves of the representative solar cells with 0%, 5%, and 10% en. All devices without encapsulation were measured in ambient air with a 1030% relative humidity at room temperature. The pristine FA0.5MA0.5Sn0.5Pb0.5I3 solar cell without en yielded a PCE of 11.03% with a high Jsc of 27.83 mA cm-2, a Voc of 0.65 V, and an FF of 61.38%. This 0% en-based solar cell generated the highest Jsc because the narrowest bandgap (1.27


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eV). However, the solar cell has the lowest Voc, which is similar to the pure Sn-based perovskite solar cells and is attributed to the high dark currents arising from adventitious p-type doping after oxidization of Sn2+ to Sn4+.37, 46 This effect can be partly suppressed by adding the en dication. The solar cells using the {5% en}FA0.5MA0.5Sn0.5Pb0.5I3 absorber realized a higher PCE of 13.19%, benefiting from lower dark currents and an improved Voc of 0.73 V and keeping a high Jsc of 27.60 mA cm-2, and an FF of 65.32%. If more en is added, the {10% en}FA0.5MA0.5Sn0.5Pb0.5I3 solar cell achieved an inferior PCE of 12.42% and a much lower Jsc of 24.35 mA cm-2 are obtained despite the highest Voc of 0.74 V and FF of 69.14%. The lower Jsc of the solar cells with 10% en addition could arise from the wider bandgap (1.36 eV vs. 1.33 eV of the 5% en sample) and higher resistance of the film (Figure 3f). Therefore, the amount of en is critical to the solar cell performance, and in this work is optimal for devices with 5% en. Figure 5b shows the external quantum efficiency (EQE) spectra of the various solar cells. All the devices exhibit high average EQE values ranging from visible-light to near-infrared light. The EQE spectrum of the neat FA0.5MA0.5Sn0.5Pb0.5I3 solar cell shows an onset at around 1015 nm, but the solar cells containing 5% en and 10% en show slightly blue-shifted EQE onsets at around 980 and 963 nm, which are consistent with the UV-vis absorption spectra shown in Figure 2c. The current density (J) integrated from the EQE curves of the solar cells employing the FA0.5MA0.5Sn0.5Pb0.5I3, {5% en}FA0.5MA0.5Sn0.5Pb0.5I3, and {10% en}FA0.5MA0.5Sn0.5Pb0.5I3 absorbers are 26.77, 26.50, and 24.27 mA cm-2, respectively, in good agreement with the Jsc trend in J-V measurements. To understand the improvement of the solar cell efficiency and Voc when using the hollow {en}FA0.5MA0.5Sn0.5Pb0.5I3 perovskites, we then studied the dark currents of the devices in more details. Figure 5c shows the comparison of the dark J-V curves for the solar cells employing different absorbers. It is apparent that the neat FA0.5MA0.5Sn0.5Pb0.5I3 solar cell has high dark



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currents under both the positive and negative bias voltages. The dark currents of this device are much higher than in the other two devices with en addition, which is consistent with the CAFM studies described above. The lower dark currents lead to lower charge recombination and longer carrier lifetime.43 The dark current–voltage (I–V) curves measured on hole-only devices without and with en were measured, can yield insights on the trap-state density in the perovskite films.47-48 In this context, we fabricated hole-only devices, including different perovskite absorbers between bottom FTO/PEDOT:PSS HTLs and top PTAA HTLs with Ag electrodes. Figure 5d shows that the I–V curves of the hole-only devices have a linear region at low bias voltages and then a sharp increase region at higher bias voltages. The voltages at which the currents start to sharply increase can be assigned to the trap-filled limit voltage (VTFL), which can be determined by the equation (1):49 VTFL=entL2/2εε0.

(1)

where e is the elementary charge of the electron, ε is the relative dielectric constant of the perovskite (around 32),50 ε0 is the vacuum permittivity, nt is the trap-state density, L is the thickness of the perovskite film. According to this equation, the nt of the perovskite materials can be estimated from the VTFL. The VTFL of the devices using 500 nm-thick neat FA0.5MA0.5Sn0.5Pb0.5I3, {5% en}FA0.5MA0.5Sn0.5Pb0.5I3, and {10% en}FA0.5MA0.5Sn0.5Pb0.5I3 films are around 1.47, 0.43, and 0.36 V, respectively, Figure 5d. According to equation (1), the nt of the FA0.5MA0.5Sn0.5Pb0.5I3, {5% en}FA0.5MA0.5Sn0.5Pb0.5I3, and {10% en}FA0.5MA0.5Sn0.5Pb0.5I3 films can be estimated at around 2.3 × 1016 and 6.7 × 1015, 5.6× 1015 cm-3 respectively. The neat FA0.5MA0.5Sn0.5Pb0.5I3 film has the highest trap-state density and therefore, the corresponding device should have the highest recombination rate, resulting in the lowest Voc, FF, and PCE of solar cells.


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Apart from the efficiency, the most important effect of en inclusion in the perovskite structure is the substantial improvement in air stability. As we mentioned above, Sn-based perovskites are very air sensitive and so are their solar cell devices.37 Compared to pure Sn-based perovskite solar cells which can degrade in several minutes,40 mixed Pb/Sn-based perovskites have less Sn and their solar cells have much better air stability. However, the stability of mixed Pb/Sn-based perovskite solar cells is lower than that of pure Pb-based perovskite solar cells. As shown in Figure 5e, the efficiency of the unencapsulated device based on neat FA0.5MA0.5Sn0.5Pb0.5I3 decreased by >50% of its initial efficiency just after exposure to air for 2 h. Only 18% of its initial efficiency could be retained after 25 h exposure to air. In stark contrast, the unencapsulated solar cells with {5% en}FA0.5MA0.5Sn0.5Pb0.5I3 solar cell retained 92% of the initial efficiency after exposure to air for 25 h under the same conditions and 28% after 122 h. The {10% en}FA0.5MA0.5Sn0.5Pb0.5I3 solar cell does have the best stability, retaining 61% of its initial efficiency after exposure to air for 122 h. Therefore, increasing incorporation of en in the perovskite structure promotes chemical stability, which is also reflected in device stability. Taking a look at the device performance, the Jsc of the mixed Pb/Sn-based solar cells can approach 30 mA cm-2, which is sufficient for single-junction solar cells. However, the Voc values of the devices are much lower than that the absorber bandgaps (1.27-1.36 eV), mainly arising from the serious recombination in the perovskite films and the recombination at the interfaces. Even after adding en dication, the Voc of the solar cells is still lower than 0.8 V. Their pure Pb-based analogs, FA1-xMAxPbI3, can achieve a high average Voc of around 1.1-1.2 V.20, 51-52 The device Voc could be further improved by promoting charge transport between the transporting layers and the perovskite absorbers. We observed that the charge transport and device performance can be improved with the use of an MABr overlayer on top of the {en}FA0.5MA0.5Sn0.5Pb0.5I3 layers. The presence of



Br has been reported to reduce the nonradiative recombination and device Voc.36, 51 A thin MABr top layer was first deposited by spin-coating an MABr solution (5 mg/mL in isopropanol (IPA)) on the prepared {en}FA0.5MA0.5Sn0.5Pb0.5I3 absorber layer. After annealing, a mixed-halide {en}FA1-xMAxSn0.5Pb0.5I3-yBry film actually forms judging from blue shift in the film optical absorption and PL emission from 1.33 to 1.38 eV (Figure S6). Energy-dispersive X-ray spectroscopy (EDS) mapping measurements confirm that Br ions were uniformly distributed on the entire surface of the 500 nm-thick film (Figure S7). MABr does not cause any apparent changes in the film morphology and conductivity, as shown in Figure S8. However, The VBM and CBM of the {en}FA1-xMAxSn0.5Pb0.5I3-yBry film shifted from -5.33 and -4.00 eV to -5.29 and -3.91 eV (Figure 6a and S9), respectively, suggesting the surface iodide atoms are replaced by Br atoms. The VBM band shift lowers the charge barrier with the HTL which could improve the charge transport in the devices. The charge transport in the device with MABr should be more fluent than that in the MABr-free device and could further reduce the carrier recombination. This is verified by the device performance (Figure 6b), where the efficiency of the solar cell using the {5% en}FA0.5MA0.5Sn0.5Pb0.5I3 is dramatically improved after incorporating MABr (5 mg/mL) from 13.19% to 17.04%, obtaining a Jsc of 27.21 mA cm-2, a Voc of 0.82 V, and an FF of 76.12% when measured under a forward voltage scan (from 0V to Voc). By contrast, the Br-free device shows a much lower Voc of 0.73 V and FF of 65.32% (Figure 5a). Additionally, the Br-based solar cell has negligible hysteresis. The corresponding solar cell measured under a reverse voltage scan (from Voc to 0 V) realized a similar PCE of 16.91% with a Jsc of 27.28 mA cm-2, a Voc of 0.82 V, and an FF of 75.70% (Figure 6b). Figure 6c shows the EQE spectrum of the {5% en}FA0.5MA0.5Sn0.5Pb0.5I3 device with MABr. The EQE onset of this device is at around 960 nm and slightly blue-shifted compared to the sample


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without MABr, which is consistent with the UV-vis absorption and PL emission results (Figure S6). The device with MABr has higher EQE values from 300 to 960 nm (Figure 6c), giving an integrated J of around 26.61 mA cm-2, which is close to the Jsc obtained from the J-V measurements (Figure 6b). Note that during all measurements, our devices were unencapsulated and measured in air and the EQE measurements were carried out after the J-V measurements. The air stability of the {en}FA0.5MA0.5Sn0.5Pb0.5I3 device with MABr was also slightly improved with respect to the devices without MABr and of course dramatically improved over devices without MABr and 5% en. As shown in Figure S10, the un-encapsulated device retained around 40% of its initial efficiency after air exposure at room temperature for 122 h. To assess the reproducibility of the devices, we fabricated 45 solar cells using the {5% en}FA0.5MA0.5Sn0.5Pb0.5I3 absorbers with MABr (5mg/mL). Figure 5d shows the PCE statistics for these solar cells, which own a high average PCE of 15.16 ± 0.90% with a Jsc of 26.06 ± 0.81 mA cm-2, a Voc of 0.81 ± 18.80 V, and an FF of 72.26 ± 0.90%. To further understand the improvement of the device performance by MABr, we measured the TRPL spectrum of the film (Figure 6e). The {5% en} FA0.5MA0.5Sn0.5Pb0.5I3 sample with MABr has a lifetime of 59.2 ns, which is longer than that of the {5% en} FA0.5MA0.5Sn0.5Pb0.5I3 film without MABr (45.6 ns). The longer carrier lifetime of the film can result in lower carrier recombination rates in the devices. The lower carrier recombination rates in the devices with MABr are also supported by the electrochemical impedance spectroscopy (EIS) experiments done on different devices. Figure 6f shows the Nyquist plots of the {5% en}FA0.5MA0.5Sn0.5Pb0.5I3 solar cells without and with MABr. The main semicircle in the Nyquist plots shown at low frequency is attributed mainly to the capacitance and recombination resistance (Rrec).46, 53-54 The recombination rate is inversely proportional to the Rrec.46, 53-54 It is clear that the device with MABr overlayer has



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a much higher Rrec, implying a much lower recombination rate. This is mainly attributed to the longer carrier lifetime in the perovskite film and the more efficient charge transport at the perovskite/transporting layer interface. All of these lead to improved device Voc, FF, and PCE. Taking a closer look at device performance, the Voc deficit (Eg/q- Voc) of our solar cells using FA0.5MA0.5Sn0.5Pb0.5I3,

{5%

en}FA0.5MA0.5Sn0.5Pb0.5I3,

and

{5%

en,

MABr}FA0.5MA0.5Sn0.5Pb0.5I3 absorbers are 0.62, 0.6, and 0.56 V, respectively. Therefore, the Voc deficit has been reduced by the en incorporation in the lattice and MABr overlayer while their full Pb-based analogs can have a Voc deficit as low as 0.4 V.51 Solar cells using thicker perovskite films with ideal bandgaps of around 1.3 eV and longer carrier lifetime should be able to achieve a Jsc over 30 mA cm-2. By combining the reduced Voc deficit (0.4 V) with the improved Jsc and FF, the mixed Pb/Sn-based single-junction hollow perovskite solar cells with en dication should have a path to much higher PCEs along with improved stability. Conclusions The incorporation of en in the strucutre of mixed Pb/Sn-based perovskites gives hollow perovskites with significantly improved semiconductor transport properties and chemical stability than the normal Pb/Sn perovskites. The {en}FA0.5MA0.5Sn0.5Pb0.5I3 hollow structures have massive Pb and Sn vacancies that seem to impart extra chemical stability, while at the same time attaining ideal bandgaps of 1.33-1.36 eV for single-junction solar cells. Devices using the {5% en}FA0.5MA0.5Sn0.5Pb0.5I3 absorbers have reduced dark current, lower trap-state density, and improved carrier lifetime, all of which result in improved Voc and FF. The greater air stability of the mixed Pb/Sn-based perovskite material is also reflected in the markedly improved air stability of the solar cells themselves. Furthermore, the incorporation of Br ions on the surface of the {5% en}FA0.5MA0.5Sn0.5Pb0.5I3 films gives longer carrier lifetime and creates better energy match


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between the perovskite VBM and the HTL level that facilitates the charge transport and further reduces the recombination . These solar cells exhibit a reduced Voc deficit, higher Voc and improved FF, delivering a champion PCE of 17.04% with a Jsc of 27.21 mA cm-2, a Voc of 0.82 V, and an FF of 76.12%. Although these materials are not Pb-free, they are do contain only half the lead content of the high performance (Cs,FA,MA)PbI3-xBrx materials. Our results suggest that the 3D hollow mixed Pb/Sn-based perovskite class of materials are good candidates for implementation in low Pb, high-performance, single-junction solar cells or bottom solar cells in tandem devices.

ASSOCIATED CONTENT Supporting Information Details of crystal synthesis, device fabrication, and characterization; XRD patterns, 1H-NMR spectra, and bandgaps of crystals; table of carrier lifetimes of films; photoemission spectroscopy measurements, UV-vis absorption spectra, PL spectra, XRD patterns, SEM, EDS, AFM, and CAFM images of films; Stability test of the device with MABr (PDF). Crystal data for FA0.5MA0.5Sn0.5Pb0.5I3 (CIF) Crystal data for {5% en}FA0.5MA0.5Sn0.5Pb0.5I3 (CIF) Crystal data for {20% en}FA0.5MA0.5Sn0.5Pb0.5I3 (CIF) AUTHOR INFORMATION Corresponding Author *[email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS



This work was supported as part of the Center for Light Energy Activated Redox Processes (LEAP), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0001059 (solar cell fabrication and characterization). Work in sample synthesis, processing and structural characterization was supported by grant Department of Energy, Office of Science grant SC0012541. PYSA measurements were carried out with equipment acquired by ONR grant N00014-18-1-2102. This work made use of the EPIC and SPID facility (NUANCE Center-Northwestern University), which have received support from the MRSEC program (NSF DMR-1720139) 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.

REFERENCES 1. Yin, W. J.; Shi, T.; Yan, Y. Unique properties of halide perovskites as possible origins of the superior solar cell performance. Adv. Mater. 2014, 26, 4653-4658. 2. Mitzi, D. B. Synthesis, structure, and properties of organic‐inorganic perovskites and related materials. Prog. Inorg. Chem. 1999, 48, 1-121. 3. Stoumpos, C. C.; Kanatzidis, M. G. The renaissance of halide perovskites and their evolution as emerging semiconductors. Acc. Chem. Res. 2015, 48, 2791-802. 4. Snaith, H. J. Perovskites: The Emergence of a New Era for Low-Cost, High-Efficiency Solar Cells. J. Phys. Chem. Lett. 2013, 4, 3623-3630. 5. Ansari, M. I. H.; Qurashi, A.; Nazeeruddin, M. K. Frontiers, opportunities, and challenges in perovskite solar cells: A critical review. J. Photochem. Photobiol. C: Photochem. Rev. 2018, 35, 1-24.


Page 18 of 32

Page 19 of 32 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


6. Correa-Baena, J.-P.; Abate, A.; Saliba, M.; Tress, W.; Jesper Jacobsson, T.; Grätzel, M.; Hagfeldt, A. The rapid evolution of highly efficient perovskite solar cells. Energy Environ. Sci. 2017, 10, 710-727. 7. Eperon, G. E.; Hörantner, M. T.; Snaith, H. J. Metal halide perovskite tandem and multiplejunction photovoltaics. Nat. Rev. Chem. 2017, 1, 0095. 8. Chung, I.; Lee, B.; He, J.; Chang, R. P.; Kanatzidis, M. G. All-solid-state dye-sensitized solar cells with high efficiency. Nature 2012, 485, 486-489. 9. Mao, L.; Stoumpos, C. C.; Kanatzidis, M. G. Two-Dimensional Hybrid Halide Perovskites: Principles and Promises. J. Am. Chem. Soc. 2019, 141, 1171-1190. 10. Stoumpos, C. C.; Malliakas, C. D.; Kanatzidis, M. G. Semiconducting tin and lead iodide perovskites with organic cations: phase transitions, high mobilities, and near-infrared photoluminescent properties. Inorg. Chem. 2013, 52, 9019-9038. 11. Stoumpos, C. C.; Kanatzidis, M. G. Halide perovskites: poor man's high-performance semiconductors. Adv. Mater. 2016, 28, 5778-5793. 12. Ke, W.; Spanopoulos, I.; Stoumpos, C. C.; Kanatzidis, M. G. Myths and reality of HPbI3 in halide perovskite solar cells. Nat. Commun. 2018, 9, 4785. 13.

National

Renewable

Energy

Laboratory.

Best

Research-Cell

Efficiencies,

https://www.nrel.gov/pv/assets/pdfs/pv-efficiency-chart.20190411.pdf 14. Leijtens, T.; Bush, K. A.; Prasanna, R.; McGehee, M. D. Opportunities and challenges for tandem solar cells using metal halide perovskite semiconductors. Nat. Energy 2018, 3, 828-838. 15. Ke, W.; Fang, G.; Wan, J.; Tao, H.; Liu, Q.; Xiong, L.; Qin, P.; Wang, J.; Lei, H.; Yang, G.; Qin, M.; Zhao, X.; Yan, Y. Efficient hole-blocking layer-free planar halide perovskite thin-film solar cells. Nat. Commun. 2015, 6, 6700.



16. Kim, H. S.; Lee, C. R.; Im, J. H.; Lee, K. B.; Moehl, T.; Marchioro, A.; Moon, S. J.; HumphryBaker, R.; Yum, J. H.; Moser, J. E.; Gratzel, M.; Park, N. G. Lead iodide perovskite sensitized allsolid-state submicron thin film mesoscopic solar cell with efficiency exceeding 9%. Sci. Rep. 2012, 2, 591. 17. Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites. Science 2012, 338, 643-7. 18. Zhou, H.; Chen, Q.; Li, G.; Luo, S.; Song, T. B.; Duan, H. S.; Hong, Z.; You, J.; Liu, Y.; Yang, Y. Interface engineering of highly efficient perovskite solar cells. Science 2014, 345, 542-546. 19. Yang, W. S.; Noh, J. H.; Jeon, N. J.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. High-performance photovoltaic perovskite layers fabricated through intramolecular exchange. Science 2015, 348, 1234-1237. 20. Saliba, M.; Matsui, T.; Domanski, K.; Seo, J.-Y.; Ummadisingu, A.; Zakeeruddin, S. M.; Correa-Baena, J.-P.; Tress, W. R.; Abate, A.; Hagfeldt, A.; Grätzel, M. Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance. Science 2016, 354, 206209. 21. Ke, W.; Fang, G.; Liu, Q.; Xiong, L.; Qin, P.; Tao, H.; Wang, J.; Lei, H.; Li, B.; Wan, J.; Yang, G.; Yan, Y. Low-Temperature Solution Processed Tin Oxide as an Alternative Electron Transporting Layer for Efficient Perovskite Solar Cells. J. Am. Chem. Soc. 2015, 137, 6730-6733. 22. Said, A. A.; Wagalgave, S. M.; Xie, J.; Puyad, A. L.; Chen, W.; Wang, Z.; Bhosale, S. V.; Bhosale, S. V.; Zhang, Q. NDI-based small molecules as electron transporting layers in solutionprocessed planar perovskite solar cells. J. Solid State Chem. 2019, 270, 51-57. 23. Gu, P.-Y.; Wang, N.; Wang, C.; Zhou, Y.; Long, G.; Tian, M.; Chen, W.; Sun, X. W.; Kanatzidis, M. G.; Zhang, Q. Pushing up the efficiency of planar perovskite solar cells to 18.2%


Page 20 of 32

Page 21 of 32 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


with organic small molecules as the electron transport layer. J. Mater. Chem. A 2017, 5, 73397344. 24. Wang, N.; Zhao, K.; Ding, T.; Liu, W.; Ahmed, A. S.; Wang, Z.; Tian, M.; Sun, X. W.; Zhang, Q. Improving Interfacial Charge Recombination in Planar Heterojunction Perovskite Photovoltaics with Small Molecule as Electron Transport Layer. Adv. Energy Mater. 2017, 7, 1700522. 25. Hao, F.; Stoumpos, C. C.; Chang, R. P.; Kanatzidis, M. G. Anomalous band gap behavior in mixed Sn and Pb perovskites enables broadening of absorption spectrum in solar cells. J. Am. Chem. Soc. 2014, 136, 8094-8099. 26. Im, J.; Stoumpos, C. C.; Jin, H.; Freeman, A. J.; Kanatzidis, M. G. Antagonism between SpinOrbit Coupling and Steric Effects Causes Anomalous Band Gap Evolution in the Perovskite Photovoltaic Materials CH3NH3Sn1-xPbxI3. J. Phys. Chem. Lett. 2015, 6, 3503-3509. 27. Eperon, G. E.; Leijtens, T.; Bush, K. A.; Prasanna, R.; Green, T.; Wang, J. T.-W.; McMeekin, D. P.; Volonakis, G.; Milot, R. L.; May, R.; Palmstrom, A.; Slotcavage, D. J.; Belisle, R. A.; Patel, J. B.; Parrott, E. S.; Sutton, R. J.; Ma, W.; Moghadam, F.; Conings, B.; Babayigit, A.; Boyen, H.G.; Bent, S.; Giustino, F.; Herz, L. M.; Johnston, M. B.; McGehee, M. D.; Snaith, H. J. Perovskiteperovskite tandem photovoltaics with optimized band gaps. Science 2016, 354, 861-865. 28. Wang, C.; Song, Z.; Li, C.; Zhao, D.; Yan, Y. Low-Bandgap Mixed Tin-Lead Perovskites and Their Applications in All-Perovskite Tandem Solar Cells. Adv. Funct. Mater. 2019, 1808801. 29. Rajagopal, A.; Yang, Z.; Jo, S. B.; Braly, I. L.; Liang, P. W.; Hillhouse, H. W.; Jen, A. K. Highly Efficient Perovskite-Perovskite Tandem Solar Cells Reaching 80% of the Theoretical Limit in Photovoltage. Adv. Mater. 2017, 29, 1702140. 30. Zhao, D.; Yu, Y.; Wang, C.; Liao, W.; Shrestha, N.; Grice, C. R.; Cimaroli, A. J.; Guan, L.; Ellingson, R. J.; Zhu, K.; Zhao, X.; Xiong, R.-G.; Yan, Y. Low-bandgap mixed tin–lead iodide



perovskite absorbers with long carrier lifetimes for all-perovskite tandem solar cells. Nat. Energy 2017, 2, 17018. 31. Chen, Z.; Liu, M.; Li, Z.; Shi, T.; Yang, Y.; Yip, H.-L.; Cao, Y. Stable Sn/Pb-Based Perovskite Solar Cells with a Coherent 2D/3D Interface. iScience 2018, 9, 337-346. 32. Zong, Y.; Zhou, Z.; Chen, M.; Padture, N. P.; Zhou, Y. Lewis-Adduct Mediated GrainBoundary Functionalization for Efficient Ideal-Bandgap Perovskite Solar Cells with Superior Stability. Adv. Energy Mater. 2018, 8, 1800997. 33. Chi, D.; Huang, S.; Zhang, M.; Mu, S.; Zhao, Y.; Chen, Y.; You, J. Composition and Interface Engineering for Efficient and Thermally Stable Pb-Sn Mixed Low-Bandgap Perovskite Solar Cells. Adv. Funct. Mater. 2018, 28, 1804603. 34. Zhao, D.; Wang, C.; Song, Z.; Yu, Y.; Chen, C.; Zhao, X.; Zhu, K.; Yan, Y. Four-Terminal All-Perovskite Tandem Solar Cells Achieving Power Conversion Efficiencies Exceeding 23%. ACS Energy Lett. 2018, 3, 305-306. 35. Zhao, D.; Chen, C.; Wang, C.; Junda, M. M.; Song, Z.; Grice, C. R.; Yu, Y.; Li, C.; Subedi, B.; Podraza, N. J.; Zhao, X.; Fang, G.; Xiong, R.-G.; Zhu, K.; Yan, Y. Efficient two-terminal allperovskite tandem solar cells enabled by high-quality low-bandgap absorber layers. Nat. Energy 2018, 3, 1093-1100. 36. Li, C.; Song, Z.; Zhao, D.; Xiao, C.; Subedi, B.; Shrestha, N.; Junda, M. M.; Wang, C.; Jiang, C.-S.; Al-Jassim, M.; Ellingson, R. J.; Podraza, N. J.; Zhu, K.; Yan, Y. Reducing SaturationCurrent Density to Realize High-Efficiency Low-Bandgap Mixed Tin-Lead Halide Perovskite Solar Cells. Adv. Energy Mater. 2018, 1803135. 37. Ke, W.; Stoumpos, C. C.; Kanatzidis, M. G. "Unleaded" Perovskites: Status Quo and Future Prospects of Tin-Based Perovskite Solar Cells. Adv. Mater. 2018, 1803230.


Page 22 of 32

Page 23 of 32 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


38. Ke, W.; Kanatzidis, M. G. Prospects for low-toxicity lead-free perovskite solar cells. Nat. Commun. 2019, 10, 965. 39. Spanopoulos, I.; Ke, W.; Stoumpos, C. C.; Schueller, E. C.; Kontsevoi, O. Y.; Seshadri, R.; Kanatzidis, M. G. Unraveling the Chemical Nature of the 3D "Hollow'' Hybrid Halide Perovskites. J. Am. Chem. Soc. 2018, 140, 5728–5742. 40. Ke, W.; Stoumpos, C. C.; Zhu, M.; Mao, L.; Spanopoulos, I.; Liu, J.; Kontsevoi, O. Y.; Chen, M.; Sarma, D.; Zhang, Y.; Wasielewski, M. R.; Kanatzidis, M. G. Enhanced Photovoltaic Performance and Stability with a New Type of Hollow 3D Perovskite {en}FASnI3. Sci. Adv. 2017, 3, e1701293. 41. Li, X.; Hoffman, J.; Ke, W.; Chen, M.; Tsai, H.; Nie, W.; Mohite, A. D.; Kepenekian, M.; Katan, C.; Even, J.; Wasielewski, M. R.; Stoumpos, C. C.; Kanatzidis, M. G. Two-Dimensional Halide Perovskites Incorporating Straight Chain Symmetric Diammonium Ions, (NH3CmH2 mNH3)(CH3NH3)n-1PbnI3n+1

(m = 4-9; n = 1-4). J. Am. Chem. Soc. 2018, 140, 12226-12238.

42. Ke, W.; Stoumpos, C. C.; Spanopoulos, I.; Mao, L.; Chen, M.; Wasielewski, M. R.; Kanatzidis, M. G. Efficient Lead-Free Solar Cells Based on Hollow {en}MASnI3 Perovskites. J. Am. Chem. Soc. 2017, 139, 14800-14806. 43. Ke, W.; Stoumpos, C. C.; Spanopoulos, I.; Chen, M.; Wasielewski, M. R.; Kanatzidis, M. G. Diammonium Cations in the FASnI3 Perovskite Structure Lead to Lower Dark Currents and More Efficient Solar Cells. ACS Energy Lett. 2018, 3, 1470-1476. 44. Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S. I. Solvent engineering for high-performance inorganic–organic hybrid perovskite solar cells. Nat. Mater. 2014, 13, 897-903. 45. Hao, F.; Stoumpos, C. C.; Cao, D. H.; Chang, R. P. H.; Kanatzidis, M. G. Lead-free solid-state organic–inorganic halide perovskite solar cells. Nat. Photon. 2014, 8, 489-494.



46. Ke, W.; Stoumpos, C. C.; Logsdon, J. L.; Wasielewski, M. R.; Yan, Y.; Fang, G.; Kanatzidis, M. G. TiO2-ZnS cascade electron transport layer for efficient formamidinium tin iodide perovskite solar cells. J. Am. Chem. Soc. 2016, 138 14998-15003 47. Dong, Q.; Fang, Y.; Shao, Y.; Mulligan, P.; Qiu, J.; Cao, L.; Huang, J. Electron-hole diffusion lengths> 175 μm in solution-grown CH3NH3PbI3 single crystals. Science 2015, 347, 967-970. 48. Yang, D.; Zhou, X.; Yang, R.; Yang, Z.; Yu, W.; Wang, X.; Li, C.; Liu, S.; Chang, R. P. H. Surface optimization to eliminate hysteresis for record efficiency planar perovskite solar cells. Energy Environ. Sci. 2016, 9, 3071-3078. 49. Sussman, A. Space‐Charge‐Limited Currents in Copper Phthalocyanine Thin Films. J. Appl. Phys. 1967, 38, 2738-2748. 50. Dong, Q.; Fang, Y.; Shao, Y.; Mulligan, P.; Qiu, J.; Cao, L.; Huang, J. Electron-hole diffusion lengths >175 μm in solution-grown CH3NH3PbI3 single crystals. Science 2015, 347, 967-970. 51. Luo, D.; Yang, W.; Wang, Z.; Sadhanala, A.; Hu, Q.; Su, R.; Shivanna, R.; Trindade, G. F.; Watts, J. F.; Xu, Z.; Liu, T.; Chen, K.; Ye, F.; Wu, P.; Zhao, L.; Wu, J.; Tu, Y.; Zhang, Y.; Yang, X.; Zhang, W.; Friend, R. H.; Gong, Q.; Snaith, H. J.; Zhu, R. Enhanced photovoltage for inverted planar heterojunction perovskite solar cells. Science 2018, 360, 1442-1446. 52. Shao, Y.; Yuan, Y.; Huang, J. Correlation of energy disorder and open-circuit voltage in hybrid perovskite solar cells. Nat. Energy 2016, 1, 15001. 53. Chandiran, A. K.; Yella, A.; Mayer, M. T.; Gao, P.; Nazeeruddin, M. K.; Gratzel, M. Subnanometer conformal TiO2 blocking layer for high efficiency solid-state perovskite absorber solar cells. Adv. Mater. 2014, 26, 4309-4312.


Page 24 of 32

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54. Gonzalez-Pedro, V.; Juarez-Perez, E. J.; Arsyad, W. S.; Barea, E. M.; Fabregat-Santiago, F.; Mora-Sero, I.; Bisquert, J. General working principles of CH3NH3PbX3 perovskite solar cells. Nano Lett. 2014, 14, 888-893.



(a)

FA0.5MA0.5Sn0.5Pb0.5I3

{5% en}FA0.5MA0.5Sn0.5Pb0.5I3

(b)

{20% en}FA0.5MA0.5Sn0.5Pb0.5I3

Intensity (a.u.)

(c) 20% en 5% en FA0.5MA0.5Pb0.5Sn0.5I3 10

(d)

(e)

1

PL Intensity (a.u.)

Absorbance (a.u.)

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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FA0.5MA0.5Pb0.5Sn0.5I3

0 300

5% en 20% en

600 900 Wavelength (nm)

1200

20 30 40 2θ (Degree)

50

60

FA0.5MA0.5Pb0.5Sn0.5I3 5% en 20% en

600

800 1000 Wavelength (nm)

1200

Figure 1 (a) SEM images of typical FA0.5MA0.5Pb0.5Sn0.5I3 and {en}FA0.5MA0.5Pb0.5Sn0.5I3 crystals. (b) Crystal structure of 3D hollow {en}FA0.5MA0.5Pb0.5Sn0.5I3. (c) PXRD patterns, (d) absorption, and (e) PL spectra of FA0.5MA0.5Pb0.5Sn0.5I3, {5% en}FA0.5MA0.5Pb0.5Sn0.5I3, and {20% en}FA0.5MA0.5Pb0.5Sn0.5I3 crystals.


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Figure 2 (a, b) XRD patterns, (c) UV-vis absorption spectra, (d) bandgaps, (e) PL spectra, and (f) TRPL spectra of a FA0.5MA0.5Pb0.5Sn0.5I3, a {5% en}FA0.5MA0.5Pb0.5Sn0.5I3, and a {10% en}FA0.5MA0.5Pb0.5Sn0.5I3 films, which were deposited on FTO/PEDOT:PSS substrates.



(a)

(b)

(c)

(d)

(e)

(f)

Figure 3. AFM and CAFM images of (a, b) a FA0.5MA0.5Pb0.5Sn0.5I3, (c, d) a {5% en}FA0.5MA0.5Pb0.5Sn0.5I3, and (e, f) a {10% en}FA0.5MA0.5Pb0.5Sn0.5I3 films deposited on ITO substrates. Scan sizes and applied bias are kept at 20 µm and 5V, respectively, for all samples.


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Figure 4. SEM images of (a) a FA0.5MA0.5Pb0.5Sn0.5I3, (b) a {5% en}FA0.5MA0.5Pb0.5Sn0.5I3, and (c) a {10% en}FA0.5MA0.5Pb0.5Sn0.5I3 films deposited on FTO/PEDOT:PSS substrates. (d) Crosssectional SEM image of a completed {5% en}FA0.5MA0.5Pb0.5Sn0.5I3 solar cell.



(b)100 75

20 10

FA0.5MA0.5Pb0.5Sn0.5I3 5% en 10% en

0 0.0

50

0 0.4 0.6 0.8 Voltage (V) -2 FA0.5MA0.5Pb0.5Sn0.5I3 (d)10 5% en 10% en 10-4

0.2

4


25

5% en 10% en

400 600 800 1000 Wavelength (nm)

Current (A)

J (mA cm-2)

(c) 10

EQE (%)

J (mA cm-2)

(a)30

100

FA0.5MA0.5Pb0.5Sn0.5I3 5% en 10% en

10-6

10-4

0.0

0.5 Voltage (v)

1.0

(e)1.0 Normalized PCE

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

0.1 1 Voltage (V)


0.5

0.0

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5% en 10% en 0

50

100 Time (hour)

Figure 5. (a) J-V, (b) EQE, and (c) dark J-V curves of the representative solar cells using a FA0.5MA0.5Pb0.5Sn0.5I3, a {5% en}FA0.5MA0.5Pb0.5Sn0.5I3, and a {10% en}FA0.5MA0.5Pb0.5Sn0.5I3 absorbers. (d) Dark I-V curves of the hole-only devices using various absorbers. (e) Comparison of the stability of representative un-encapsulated solar cells using various absorbers measured in ambient air with a 10-30% relative humidity at room temperature. The cells were stored in ambient air and in the dark.


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Figure 6. (a) Comparative diagram of the bandgap and band edge alignment structure for different materials. (b) J-V curves and (c) EQE spectrum of the representative solar cell using a {5% en}FA0.5MA0.5Pb0.5Sn0.5I3 absorber with MABr. (d) PCE statistics of 45 devices using the {5% en}FA0.5MA0.5Pb0.5Sn0.5I3 absorbers with MABr (e) Comparison of the TRPL spectra of the {5% en}FA0.5MA0.5Pb0.5Sn0.5I3 films without and with MABr deposited on FTO/PEDOT:PSS substrates. (f) Comparison of the Nyquist plots of solar cells using the {5% en}FA0.5MA0.5Pb0.5Sn0.5I3 absorbers without and with MABr. 31 ACS Paragon Plus Environment


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Sn Perovskites with Ideal- Bandgap Yield Solar Cells with Higher

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