Performance Enhancement of Lead-Free Tin-Based Perovskite Solar

Subscriber access provided by HACETTEPE UNIVERSITESI KUTUPHANESI

Letter

Performance Enhancement of Lead-Free Tin-based Perovskite Solar Cells with Reducing Atmosphere-Assisted Dispersible Additive Tze-Bin Song, Takamichi Yokoyama, Shinji Aramaki, and Mercouri G. Kanatzidis ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00171 • Publication Date (Web): 20 Mar 2017 Downloaded from http://pubs.acs.org on March 21, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Energy Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26

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

ACS Energy Letters

Performance Enhancement of Lead-Free Tin-based Perovskite Solar Cells with Reducing AtmosphereAssisted Dispersible Additive Tze-Bin Song1 †, Takamichi Yokoyama1, 2 †, Shinji Aramaki2 and Mercouri G. Kanatzidis1* 1

Department of Chemistry, Northwestern University, 2145, Sheridan Road, Evanston, Illinois

60208, United States 2

Mitsubishi Chemical Group Science & Technology Research Center, Inc., 1000 Kamoshida-

cho, Aoba-ku, Yokohama 227-8502, Japan Corresponding Author *Email: [email protected]

1 Environment ACS Paragon Plus

ACS Energy Letters

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

ABSTRACT Sn-based halide perovskite materials have attracted tremendous attention and have been employed successfully in solar cells. However, their high conductivities resulting from the unstable divalent Sn state in the structure cause poor device performance and poor reproducibility. Herein, we used excess tin iodide (SnI2) in Sn-based halide perovskite solar cells (ASnI3, A = Cs, methylammonium and formamidinium tin iodide as the representative light absorbers), combined with a reducing atmosphere to stabilize the Sn2+ state. Excess SnI2 can disperse uniformly into the perovskite films and functions as a compensator as well as a suppressor of Sn2+ vacancies thereby effectively reducing the p-type conductivity. This process significantly improved the solar-cell performances of all the ASnI3 materials on mesoporous TiO2. Optimized CsSnI3 devices achieved a maximum power conversion efficiency of 4.81%, which is the highest among all inorganic Pb-free perovskite solar cells to date.

TOC


Page 2 of 26

Page 3 of 26

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

ACS Energy Letters

Metal halide perovskite compounds in the form of ABX3 [A = cesium (Cs), methylammonium (MA), or formamidinium (FA); B = Pb, Sn, or Ge; X = I, Br, Cl, or F] have attracted significant attention as promising materials for highly efficient and low-cost optoelectronic devices such as lasers, light-emitting diodes (LEDs), photodetectors, and photovoltaic cells.1-4 Among these materials, Pb-based perovskites (APbX3), in particular, have demonstrated remarkable solar-cell performance with power conversion efficiencies (PCEs) higher than 20%.5,6 These high PCE values have been attributed to a number of remarkable properties, including long carrier diffusion lengths (>100 µm), low Urbach energy (15 meV), and high absorption coefficients (>105 cm−1).7,8 However, for achieving even higher performances approaching the Shockley–Queisser limit, Pb-based perovskites with bandgaps (Eg) ranging from 1.45 to 2.4 eV are not ideal considering the ideal Eg in this regard to be ~1.34 eV for single-junction solar-cell applications. Additionally, the use of Pb poses a serious concern for large-scale fabrication and commercialization owing to its toxicity and systematic examinations of the practical conditions using Pb are still under evaluation.9 Therefore, various Pb-free perovskite derivatives such as substitution of Pb with its group 14 congeners, Sn or Ge, 2D perovskite Cs3Sb2I9, molecular Cs3Bi2I9 iodobismuthate analogue, and ordered double perovskites Cs2AgBiX6 and Cs2AgInX6, have been recently explored to overcome those concerns.10-16 Among them, Sn-based halide perovskites (ASnX3), which are an analog to Pbbased perovskites, are particularly promising alternatives. This is because ASnX3 perovskites possess less toxicity with narrower Eg values compared with Pb-based counterparts’; particularly ASnI3 has Eg ranging from 1.2 to 1.4 eV which covers the ideal Eg value. If the encapsulation is not perfect, the perovskite layer decomposes to AI and SnX2 (or PbX2). SnX2 will eventually hydrolyze into SnO2 but PbX2 will convert to toxic PbO. SnO2 is harmless but PbX2 is toxic to


ACS Energy Letters

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

the environment. Further, ASnI3 has extremely high carrier mobility (~2000 cm2V−1s−1) and demonstrated the highest device performances of over 5% PCE among all the lead-free perovskite materials.17-20 A great challenge in the field of Sn-based perovskites is that despite being regarded as semiconductors, they often exhibit metallic behavior because of a strong “self-doping” effect, i.e., a high p-type doping concentration is spontaneously induced owing to the easy oxidation of Sn2+ to Sn4+ and the low formation energy of Sn vacancies.21-24 Hence, without appropriate modifications, ASnX3-based perovskite solar cells generally exhibit near short-circuit diode behavior.10,25-31 It has been reported that adding SnF2 to ASnX3 enables the fabrication of a working solar cell.20,28-32 Because SnF2 is one of the most chemically stable Sn2+ compounds, it effectively turns the films into Sn2+-rich materials that can suppress the formation of Sn vacancies.33 However, SnF2 forms a separate phase in perovskite films because of agglomeration unless it is chemically treated.20,28-31 Consequently, SnF2 additives can only be applied to certain Sn-based perovskites that display a relatively mild self-doping effect. Recently, we showed that instead of SnF2, other excess divalent Sn compounds can also be used as Sn vacancy suppressors to reduce the background hole carrier density (or p-doping level).34 Moreover, we developed a reducing vapor atmosphere process utilizing hydrazine that can suppress Sn4+ formation resulting it in improved carrier lifetime and reduced defects/traps-induced recombination in Sn-based perovskite films. By combining this atmosphere process with a conventional SnF2 additive, we successfully improved the device performances of various Sn-based perovskite solar cells.35 Despite this, there is still some Sn4+ alongside a deficit of Sn2+ present in the films; this results in Sn vacancies that need to be compensated or suppressed to achieve cells with higher efficiencies.27, 28 Thus, based on the knowledge gained from these studies, we hypothesized that


Page 4 of 26

Page 5 of 26

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

ACS Energy Letters

we should be able to further suppress Sn vacancy formation if we could effectively incorporate other Sn2+ compounds that have better dispersion than SnF2. In this study, to test this hypothesis, we introduced SnI2 as an excess Sn2+ compound for ASnI3 perovskites; further, we combined this with our reducing vapor atmosphere approach to stabilize their Sn2+ state. We find that excess SnI2 does not affect the perovskite phase formation and has good dispersibility in the system. Moreover, excess SnI2 could supply more Sn2+ to the system and compensate for Sn2+ lost in oxidization of Sn2+ to Sn4+ during the growth of the perovskite film; this is contrasting to the currently popular SnF2 additive which remains intact in the film owing to its chemical stability. Thus, the formation of Sn defects stemming from such oxidization might be hardly prevented with the SnF2 additive approach. Herein, we focus mainly on CsSnI3 because of its promising properties, as discussed in our previous report,35 and examined CsI/SnI2 in molar ratios ranging from 0.2 to 1. Surprisingly, the optimum CsI/SnI2 ratio for the best device performance was found to be ~0.4; such excess amount of Sn2+ hardly ever realized good perovskite films with SnF2 due to its problematic agglomeration. The optimized CsSnI3 devices achieved a PCE of 4.81%, which to our best knowledge is the highest efficiency achieved with an all-inorganic Pb-free system.36-38 To verify the universal applicability of this approach, MASnI3 and FASnI3 devices were also fabricated, and similar performance improvement for these devices are demonstrated in this study. CsSnI3 perovskite films were prepared on mesoporous TiO2 (meso-TiO2) substrates from solutions with various molar CsI/SnI2 ratios ranging from 1.0 (stoichiometric ratio) to 0.2 in a mixed solvent of dimethylformamide (DMF) and dimethyl sulfoxide (DMOS);28 these films were fabricated under a weak hydrazine atmosphere by dropping 50 µl of hydrazine solution in a spin-coater chamber in a glovebox, followed by purging of the glovebox for a few hours, as


ACS Energy Letters

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

described in our previous report.35 This weak hydrazine atmosphere does not cause the overreduction of Sn-perovskite films, and thus allowed us to systematically study the effect of excess SnI2.28 Note that these films were formed using the conventional so-called one-step method, which is more compatible with practical industrial processes, e.g., roll-to-roll printing, than the previously reported vapor-assisted solution process (VASP), the anti-solvent drop, and hot casting methods.10,20,28,35 The morphologies of the films were characterized by scanning electron microscopy (SEM), as shown in Figure 1a. Films with good coverage were observed with a wide range of CsI/SnI2 ratios. It should be emphasized that 0.4-CsI/SnI2 films (i.e., films with a molar ratio of CsI/SnI2= 0.4) contained 120-mol% excess SnI2. Nevertheless, no agglomeration was visible in the films; this demonstrates the advantage of SnI2’s dispersibility. On the other hand, needle-shaped SnI2 crystals were observed on the 0.2-CsI/SnI2 films (Figure 1a), which displayed a deteriorated surface coverage. This would be because the morphology of the film is dominated by crystallization of the excess SnI2, which impedes the growth of high surface coverage films. In the XRD pattern of Figure 1b the 0.2-CsI/SnI2 films exhibit weak CsSnI3 Bragg peaks with a strong SnI2 peak at 2θ=12.8°, which corresponds to the structure visible in the SEM image. Conversely, all other films had strong CsSnI3 Bragg peaks, indicating that SnI2 does not disturb CsSnI3 film formation. Furthermore, no additional Bragg peaks were observed from the films, suggesting that the SnI2 is amorphous-like and certainly not crystalized in these films. If SnI2 were to form a completely separate phase from CsSnI3 within the films, it is unlikely that SnI2 would remain amorphous-like after annealing. We did not observe a yellow color, which would be seen if SnI2 had segregated, either on the surface of the film or at the TiO2 interface (Figure S1). The elemental mapping over the 0.4-CsI/SnI2 film (top-view and cross section) with SEM/energy-dispersive X-ray spectroscopy (EDS) also indicated no observable


Page 6 of 26

Page 7 of 26

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

ACS Energy Letters

SnI2 agglomeration as shown in Figure S2. It is thus plausible to consider that SnI2 disperses uniformly within the films or forms a very thin nanolayer at the CsSnI3 grain boundaries. The absorption spectra and Tauc plots of the films are shown in Figures 1c and S3, respectively. While the absorbance of the films weakened as the CsI/SnI2 ratio decreased, their Eg was the same as the previously reported value (~1.3 eV).21,29 This is reasonable because Eg is determined by CsSnI3 even if it is diluted by SnI2. These results indicate that a dispersive SnI2 additive can help achieve good CsSnI3 films growth over a wide range of excess Sn2+ inclusion in the films.


ACS Energy Letters

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

Figure 1. (a) SEM images of the CsSnI3 perovskite films grown with various CsI/SnI2 molar ratios, together with a neat SnI2 film. Uniform, high-surface coverage CsSnI3 films were observed over a wide range of excess SnI2 used in this study. Needle-shaped SnI2 crystals were seen only in the 0.2-CsI/SnI2 films. (b) XRD patterns of the CsSnI3 and neat SnI2 films as well as the simulated CsSnI3 patterns. Additional peak was observed only in the 0.2-CsI/SnI2 films. (c) Absorption spectra of the CsSnI3 films. Scale bars are 5µm. Further, we completed the assembly of the solar cells by depositing poly[bis(4phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) doped with tetrakis(pentafluorophenyl)borate


Page 8 of 26

Page 9 of 26

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

ACS Energy Letters

(TPFB) and Au as the hole transporting layer (HTL) and top electrode, respectively.10 Representative current density–voltage (J–V) curves as well as the device parameters as a function of the molar ratio of CsI/SnI2 are shown in Figure 2. CsSnI3 devices prepared from solutions with 0.8- and 1-CsI/SnI2 molar ratios exhibited short-circuit behavior, whereas the device performances were improved when the ratio was ≤0.6, Figure 2a. The highest average PCE of 2.41% was achieved in the 0.4-CsI/SnI2 devices. Although, according to the XRD results, the 0.6-CsI/SnI2 devices were expected to contain a larger amount of CsSnI3 phase in the films for light absorption, the low shunt resistance (320 ohm·cm2 for 0.4-CsI/SnI2 and 175 ohm·cm2 for 0.6-CsI/SnI2), which was estimated from the slope at 0V, resulted in poor open circuit voltage (VOC). This low shunt resistance may have originated from the slightly higher carrier density in the 0.6-CsI/SnI2 film. The 0.2-CsI/SnI2 devices exhibited good diode behavior; however, because of the low amount of CsSnI3 phase present, the short-circuit photocurrent density (JSC) was limited, and coupled with the poor film coverage, resulting in a lower VOC than that in the 0.4CsI/SnI2 devices. The shunt resistance of the 0.2-CsI/SnI2 devices was still comparable to that of other CsI/SnI2 ratios, which contained smaller SnI2 excess. These results suggest that excess SnI2 did not significantly impede carrier transport and collection but suppressed the problematic high conductivity in the CsSnI3 perovskite films. The optimum ratio of CsI/SnI2 was thus found to be 0.4; at this ratio, JSC was similar to that of 0.6-CsI/SnI2 devices and had a fill factor (FF) comparable to that of the 0.2-CsI/SnI2 devices. We also applied the same approach to examine MASnI3 and FASnI3 perovskite materials; similar trends in the device performances were observed, as shown in Figure S4. The optimum ratios of MAI/SnI2 and FAI/SnI2 were 0.4–0.6 and 0.6–0.8, and the best PCE of each system was 2.01% and 2.33%, respectively. The top-view SEM images of the films with the stoichiometric


ACS Energy Letters

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

Page 10 of 26

and optimum ratios of MASnI3 and FASnI3 are shown in Figure S5; once again there is no significant deterioration in the morphologies owning to the excess SnI2. The obtained results suggest that it is difficult to achieve functional devices with stoichiometric solutions in various Sn-based perovskite solar cells; this is in agreement with the short-circuit diode behaviors observed in previous studies.10,25-27,29-31 In addition, the optimum ratio varied for different cation systems; this might reflect on the different formation energies of Sn2+ vacancies in their respective lattices; the higher the optimum AI/SnI2 ratio, the higher the formation energy of the Sn2+ vacancy. Among the three perovskite materials, FASnI3 devices have an optimum ratio that is closest to the stoichiometric ratio (0.6 to 0.8). This ratio we obtained is in agreement with that previously reported for optimized FASnI3 devices using SnF2 as an additive. It is interesting to mention that MASnI3 films prepared using LT-VASP are formed from ~0.67-MAI/Sn2+ compounds [SnI2, SnO and Sn(OH)2], which is coincidently very close to the optimum ratio (0.4 to 0.6) of MASnI3 devices obtained in the current study.34 The steady-state current measured at a maximum power point and stabilized power outputs of CsSnI3, MASnI3 and FASnI3 devices from their optimized AI/SnI2 ratios are shown in Figure S6. Interestingly, while the MASnI3-based devices showed constant output value, a distinct improvement was seen in CsSnI3-based devices. On the contrary, the FASnI3-based devices degraded with time, as has been observed in a previous report by Lee et al.20 Although this difference may reflect the long term durability of each device under inert atmosphere, underling mechanisms are still under investigation. In this study, we used the first J-V scan results to report the device performances. One more thing to note is that all the devices were measured after encapsulation; without encapsulation devices degraded severely as shown in Figure S7.


Page 11 of 26

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

ACS Energy Letters

We also prepared CsSnI3 devices in a pure N2 atmosphere for comparison. Unfortunately, those devices showed short-circuit behavior even at the optimum CsI/SnI2 ratio (Figure S8) that we achieved in a weak hydrazine vapor atmosphere. Among the three Sn-based systems we tested, we were only able to obtain devices with VOC >0.15 V for FASnI3, with an FAI/SnI2 ratio of 0.2 and 0.4, without utilizing a hydrazine vapor atmosphere (Figure S9). Hence, to suppress SnI2 from forming Sn4+ and ensure that it effectively compensates the Sn2+ vacancies, it is essential to utilize the hydrazine vapor atmosphere for obtaining the most efficient devices. However, one report in the literature highlighted that CsSnI3 device performances were enhanced by simply incorporating an excess of only 10% SnI2.39 The possible reasons for this discrepancy in comparison to our results may be caused by unknown impurities in the films (visible in the reported XRD patterns) or the interaction between CsSnI3 and CuI used as a HTL, which may reduce p-doping levels.


ACS Energy Letters

(a)

30

J (mA/cm )

CsI/SnI2

2

1.0 0.8 0.6 0.4 0.2

20 10 0 -0.1

0.0

0.1

0.2

0.3

0.4

Voltage (V) 0.3

3.0 2.5 2.0 1.5 1.0 0.5 0.0

VOC (V)

PCE (%)

(b)

0.2 0.1 0.0 0.2 0.4 0.6 0.8 1.0

0.2 0.4 0.6 0.8 1.0

CsI/SnI2 ratio

CsI/SnI2 ratio 25 20 15 10 5 0

FF (%)

JSC (mA/cm2)

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

Page 12 of 26

0.2 0.4 0.6 0.8 1.0

CsI/SnI2 ratio

50 40 30 20 10 0 0.2 0.4 0.6 0.8 1.0

CsI/SnI2 ratio

Figure 2 (a) J–V characteristics of the CsSnI3-based devices fabricated using various CsI/SnI2 molar ratios. The devices were functional when the CsI/SnI2 ratio was 0.2, 0.4, or 0.6. The highest PCE was obtained when the CsI/SnI2 ratio was 0.4. (b) Each device parameter is plotted as a function of the CsI/SnI2 molar ratio. The results for three devices in the same batch are shown. The fill factor was plotted as 0 when VOC was lower than 0.02V (short-circuit behaviour). The solid line indicates the average of the three devices.

The energy level of the CsSnI3 film was characterized via photoemission spectroscopy in air (PESA), as shown in Figure 3a, to further understand the obtained device performance. The


Page 13 of 26

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

ACS Energy Letters

valence band maximum (VBM) of the CsSnI3 films was estimated to be −4.58 eV relative to the vacuum level, and the conduction band minimum (CBM) was estimated to be −3.28 eV by adding Eg. Together with the literature values of MASnI3, FASnI3, TiO2 and PTAA,10,40,41 the energy band alignments of Sn-based perovskites are shown in Figure 3b. It is noted that type II band alignment is found in all three materials,42 with a significant conduction band mismatch of >0.3 eV. Thus, the severe VOC loss (from the valued expected from the position of the VBM of Sn-based perovskites and CBM of TiO2) is not surprising because type II band alignment increases the recombination rate, which is further enhanced in highly p-doped materials such as Sn-based perovskite.40 To evaluate how our approach affects the doping level, we performed Kelvin probe measurements to determine the Fermi level of the 1.0 and optimum ratio 0.4-CsI/SnI2 films, as shown in Figure 3c. Since our Kelvin probe was setup in air, Fermi level shifts were observed during the measurements, which is in line with the observed performance degradation of the devices in air (Figure S7). This phenomenon has been discussed in our previous report, in which we proposed that it occurs because of the self-doping effect of the MASnI3 system.10 Although this phenomenon may affect the measured values, it is clear that 0.4-CsI/SnI2 has a shallower Fermi level than 1.0-CsI/SnI2. This suggests that the p-doping level is decreased by the high amounts of SnI2 combined with the hydrazine vapor atmosphere resulting in the observed performance improvements. From the energy-band alignment and Kelvin probe measurements, we found that the Fermi level of the 0.4-CsI/SnI2 film was ca. −4.5 eV, which is closer to the VBM but far from the midgap energy of CsSnI3 such that it still allows room to further reduce the doping level in CsSnI3 devices.


ACS Energy Letters

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

Page 14 of 26

Figure 3. (a) PESA measurement results for the CsSnI3 film. The valence band was estimated to be −4.58 eV (with respect to the vacuum level) on the basis of the onset energy of the photoemission spectrum.(b) Energy-level diagram of CsSnI3. Reported values for MASnI3, FASnI3, TiO2, and PTAA are also depicted. (c) Fermi levels of 1.0- and 0.4-CsI/SnI2 films measured using a Kelvin probe in air. The 0.4-CsI/SnI2 sample shows a shallower Fermi level, indicating a lower p-doping level. On the basis of the above film characterization and device results, we then prepared CsSnI3 devices with a CsI/SnI2 ratio of 0.4 under more intense hydrazine atmosphere (~800 µl). Figure 4a shows the J–V curves of these devices measured via a reverse bias sweep for the best14 Environment ACS Paragon Plus

Page 15 of 26

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

ACS Energy Letters

performing CsSnI3 solar cells with a PCE of 4.81%, which is significantly improved from our previous results using the SnF2 even with the optimized reducing vapor atmosphere.28 The JSC value of ~25 mA/cm2 is similar to that ofthe devices prepared in a weak hydrazine atmosphere, and the rapid increases in the VOC and FF values contribute to the overall improvement of the PCE. The hydrazine atmosphere further suppresses Sn4+ formation (over and above the excess of SnI2) and reduces p-doping in Sn-based perovskites; this lead to fewer recombination sites and pathways in the CsSnI3 films. Significant hysteresis behavior was not observed in the forward (from JSC to VOC) and reverse (from VOC to JSC) scans of the J–V characteristics (Figure S10). In addition, the reproducibility of the device performance was examined by fabricating batches of devices and the statistics relating to their performances are shown in Figure S11. The representative external quantum efficiency (EQE) spectrum is shown in Figure 4b. The maximum EQE value reached 85% (Figure 4b), but the EQE value near Eg was lower presumably because a reduced amount of absorber was present in the 0.4-CsI/SnI2 films. Despite the slight loss of photocurrent near Eg, this EQE value could be further improved by fine tuning the CsI/SnI2 ratio and the film thickness.


ACS Energy Letters

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

Page 16 of 26

Figure 4. (a) J–V characteristic of the 0.4-CsI/SnI2-based devices that showed the best performance. (b) A representative EQE spectrum and integrated JSC. In conclusion, we successfully demonstrated that the AI/SnI2 ratio can be tuned over a wide range without deteriorating ASnI3 film quality by using a SnI2 additive to introduce more Sn2+ sources. Moreover, in a reducing vapor atmosphere, our current approach represents a universal process that can be used to prevent short-circuit behavior and achieve well-functioning solar cells of various Sn-based perovskite (namely, MASnI3-, FASnI3-, and CsSnI3-based solar cells). We systematically showed the device performances as a function of the AI/SnI2 ratio for these three materials; namely, FASnI3 was found to exhibit functional devices close to the stoichiometric ratio while the optimum ratios of MASnI3 and CsSnI3 were quite off-


Page 17 of 26

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

ACS Energy Letters

stoichiometric. We expect this to provide a general guide for future work on Sn-based perovskite solar cells. The viability of our approach was further demonstrated using optimized CsSnI3 devices with a PCE of 4.81%, which is currently the highest performance for an inorganic Pbfree perovskite solar cell. The results of this study pave a path for Sn-based perovskite solar cells to be more competitive with Pb-based devices.

Methods Materials: SnI2 synthesis and purification was performed following our previous work.10 CsI, hydrazine and poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) were purchased from Sigma Aldrich. Methylammonium iodide and formamidinium iodide were purchased from 1-material. 4-isopropyl-4'-methyldiphenyliodonium tetrakis(pentafluorophenyl)borate (TPFB) was purchased from TCI America. All these materials were used as received. Perovskite film fabrication: TiO2 compact layer and meso-TiO2 layer (350-nm-thick) were formed on the FTO substrate following our previous work.10 CsSnI3 solution was prepared by adding CsI and SnI2 into a mixed solvent of DMF and DMSO (4:1 volume ratio). The ratio of CsI and SnI2 was varied by changing the amount of CsI (MAI or FAI) from 0.2 M to 1.0 M with fixed amount of SnI2 (1.0 M). 50 µL hydrazine solution was dropped into the spincoater chamber, followed by purging the glovebox for a few hours to create weak hydrazine atmosphere. Subsequently, 45 µL of the CsSnI3 solution was dropped onto the meso-TiO2 substrate and the substrate was spun at 2500 rpm for 60 sec. Then, the films were annealed at 100°C for 20 min to complete the CsSnI3 perovskite films. The same procedure was used for MASnI3 and FASnI3 film fabrication. These films were used for characterization and device fabrication. For the optimized devices, the CsSnI3 films were made on 700-nm-thick meso-TiO2


ACS Energy Letters

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

Page 18 of 26

layer from DMSO solution of CsI/SnI2 (0.4 M/1.0 M) under intensive hydrazine atmosphere which was created by dropping 800 µL hydrazine into the spincoater chamber. All these procedures were performed in a N2-filled glovebox. For comparison, the perovskite films were also made using the same fabrication procedure in a different N2-filled glovebox where hydrazine has never been used. Perovskite film characterization: Transmittance of the perovskite films was obtained from 300 nm to 1200 nm at room temperature using a Shimadzu UV-3600 PC double-beam, double-monochromator spectrophotometer. UV-vis absorption spectra was calculated from the transmittance by using an equation of α = -log(T), where α and T are absorbance and transmittance, respectively. XRD patterns were collected using a Rigaku MiniFlex600 X-ray diffractometer (Cu Kα, 1.5406 Å) operating at 40 kV and 20 mA. Either Hitachi 4800 or 8030 SEM was used for surface morphology study. Valence band of CsSnI3 (stoichiometric ratio) films were measured using photoemission spectroscopy in air (PESA) (AC-2, Riken Keiki) where ultraviolet light is irradiated to the sample and electrons emitted from the surface via the photoelectric effect is measured with a photoelectron counter (open counter).43 The measurements were performed immediately after the films were taken out to the air to avoid oxidization of the films. Fermi level of the 1.0 and 0.4-CsI/SnI2 films were performed using a Kelvin probe (KP technology) in air. Solar cell characterization: The hole transport layer was prepared from 32 mg PTAA and 3.6 mg TPFB dissolved in chlorobenzene. An amount of 60 µl solution was dropped onto the perovskite films, followed by spinning the substrate at 1500 rpm for 30 sec to form a hole transporting layer. Subsequently, the films were annealed at 70°C for 5 min. Finally, 100-nmthick gold top electrodes were deposited in vacuum using a thermal evaporation to complete


Page 19 of 26

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

ACS Energy Letters

device fabrication. All devices were encapsulated using a 30-µm-thick hot-melting polymer and a microscope cover slip. These procedures were carried out in the pure N2 atmosphere. J-V characteristics of the devices were measured in air under 1-sun illumination (AM 1.5G, 100 mW/cm2) using a certified solar simulator (Abet Technologies) and Keithley 2400 source meter. The voltage was swept from positive bias to negative bias (namely, from VOC to 0 V) unless otherwise stated. The overlapping area between the un-etched FTO glass and Au electrode (namely, “nominal” active area of the devices) was between 0.12 and 0.15 cm2. In this study, the J-V characteristics without a shadow mask were shown because we observed that VOC decreases and FF increases in the J-V characteristics with the shadow mask and these changes occur and vary based on the ratio of the device active area and the aperture of the mask. The exact active area used for calculating JSC was estimated from the ratio of JSC values measured without and with a black shadow mask of 0.04 mm2 aperture. The JSC values were further confirmed with IPCE spectra.

ASSOCIATED CONTENT Supporting Information: Photographs of perovskite films; top and cross-section SEM/EDS mapping of perovskite films; Tauc plots of CsSnI3 perovskite films; J-V characteristics and device performances of MASnI3 and FASnI3 from different AI/SnI2 ratio; SEM images of MASnI3 and FASnI3 films; maximum power output and stabilized power outputs of CsSnI3, MASnI3 and FASnI3 devices; J-V characteristics of CsSnI3, MASnI3 and FASnI3 device stabilities; J-V characteristics of CsSnI3 devices from different environment; J-V characteristics of FASnI3 devices from different FI/SnI2 ratio; Hysteresis behavior of a 0.4-CsI/SnI2 device; Histogram of PCE of 0.4-CsI/SnI2 devices.


ACS Energy Letters

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

Page 20 of 26

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Author Contributions †

T. S. and T.Y. contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT This research made use of resources at the ANSER Center, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences (DE-SC0001059) and the Institute for Sustainable Energy at Northwestern University. T.-B.S. acknowledges financial support from Mitsubishi Chemical Group Science & Technology Research Center, Inc. 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 (EEC0118025/003), both programs of the National Science Foundation; the State of Illinois; and Northwestern University. The authors acknowledge Mr. Takuma Uryu (Hitachi High Technologies America, Inc.) and Mr. John Villalovos (RKI Instruments, Inc.) for performing PESA measurement and Dr. Yoshiyuki Nakajima (Riken Keiki Co. Ltd.) for fruitful discussion on the PESA results. T.-B.S and T.Y thank to Dr. Constantinos Stoumpos (Northwestern University) for synthesizing SnI2 used in this study.


Page 21 of 26

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

ACS Energy Letters

REFERENCES (1)

Zhu, H.; Fu, Y.; Meng, F.; Wu, X.; Gong, Z.; Ding, Q.; Gustafsson, M. V.; Trinh, M. T.;

Jin, S.; Zhu, X. Lead Halide Perovskite Nanowire Lasers with Low Lasing Thresholds and High Quality Factors. Nat. Mater. 2015, 14, 636-642. (2)

Dou, L.; Yang, Y. M.; You, J.; Hong, Z.; Chang, W.-H.; Li, G.; Yang, Y. Solution-

Processed Hybrid Perovskite Photodetectors with High Detectivity. Nat. Comm. 2014, 5, 5404. (3)

Stoumpos, C. C.; Kanatzidis, M. G. The Renaissance of Halide Perovskites and Their

Evolution as Emerging Semiconductors. Acc. Chem. Res. 2015, 48, 2791-2802. (4)

Stoumpos, C. C.; Kanatzidis, M. G. Halide Perovskites: Poor Man's High‐Performance

Semiconductors. Adv. Mater. 2016, 28, 5778-5793. (5)

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, 12, 1234-1237. (6)

Saliba, M.; Orlandi, S.; Matsui, T.; Aghazada, S.; Cavazzini, M.; Correa-Baena, J.-P.;

Gao, P.; Scopelliti, R.; Mosconi, E.; Dahmen, K.-H. A Molecularly Engineered HoleTransporting Material for Efficient Perovskite Solar Cells. Nat. Energy 2016, 1, 15017. (7)

Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J.; 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. (8)

De Wolf, S.; Holovsky, J.; Moon, S.-J.; Löper, P.; Niesen, B.; Ledinsky, M.; Haug, F.-J.;

Yum, J.-H.; Ballif, C. Organometallic Halide Perovskites: Sharp Optical Absorption Edge and its Relation to Photovoltaic Performance. J. Phys. Chem. Lett. 2014, 5, 1035-1039.


ACS Energy Letters

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

(9)

Page 22 of 26

Hailegnaw, B.; Kirmayer, S.; Edri, E.; Hodes, G.; Cahen, D. Rain on Methylammonium

Lead Iodide Based Perovskites: Possible Environmental Effects of Perovskite Solar Cells. J. Phys. Chem. Lett. 2015, 6, 1543-1547. (10)

Yokoyama, T.; Cao, D. H.; Stoumpos, C. C.; Song, T.-B.; Sato, Y.; Aramaki, S.;

Kanatzidis, M. G. Overcoming Short-Circuit in Lead-Free CH3NH3SnI3 Perovskite Solar Cells via Kinetically Controlled Gas-Solid Reaction Film Fabrication Process. J. Phys. Chem. Lett. 2016, 7, 776-782. (11)

Stoumpos, C. C.; Frazer, L.; Clark, D. J.; Kim, Y. S.; Rhim, S. H.; Freeman, A. J.;

Ketterson, J. B.; Jang, J. I.; Kanatzidis, M. G. Hybrid Germanium Iodide Perovskite Semiconductors: Active Lone Pairs, Structural Distortions, Direct and Indirect Energy Gaps, and Strong Nonlinear Optical Properties. J. Am. Chem. Soc. 2015, 137, 6804-6819. (12)

Krishnamoorthy, T.; Ding, H.; Yan, C.; Leong, W. L.; Baikie, T.; Zhang, Z.; Sherburne,

M.; Li, S.; Asta, M.; Mathews, N.; Mhaisalkar, S. G. Lead-Free Germanium Iodide Perovskite Materials for Photovoltaic Applications. J. Mater. Chem. A 2015, 3, 23829-23832. (13)

Savory, C. N.; Walsh, A.; Scanlon, D. O. Can Pb-Free Halide Double Perovskites

Support High-Efficiency Solar Cells? ACS Energy Lett. 2016, 1, 949-955. (14)

Volonakis, G.; Haghighirad, A. A.; Milot, R. L.; Sio, W. H.; Filip, M. R.; Wenger, B.;

Johnston, M. B.; Herz, L. M.; Snaith, H. J.; Giustino, F. Cs2InAgCl6: A New Lead-Free Halide Double Perovskite with Direct Band Gap. J. Phys. Chem. Lett. 2017, 8, 772-778. (15)

Saparov, B.; Hong, F.; Sun, J.; 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


Page 23 of 26

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

ACS Energy Letters

(16)

Giustino, F.; Snaith, H. J. Toward Lead-Free Perovskite Solar Cells. ACS Energy Lett.

2016, 1, 1233-1240. (17)

Hao, F.; Stoumpos, C. C.; Cao, D. H.; Chang, R. P.; Kanatzidis, M. G. Lead-Free Solid-

State Organic-Inorganic Halide Perovskite Solar Cells. Nat. Photonics 2014, 8, 489-494. (18)

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. (19)

Noel, N. K.; Stranks, S. D.; Abate, A.; Wehrenfennig, C.; Guarnera, S.; Haghighirad, A.-

A.; Sadhanala, A.; Eperon, G. E.; Pathak, S. K.; Johnston, M. B. Lead-Free Organic–Inorganic Tin Halide Perovskites for Photovoltaic Applications. Energy Environ. Sci. 2014, 7, 3061-3068. (20)

Liao, W.; Zhao, D.; Yu, Y.; Grice, C. R.; Wang, C.; Cimaroli, A. J.; Schulz, P.; Meng,

W.; Zhu, K.; Xiong, R. G. Lead‐Free Inverted Planar Formamidinium Tin Triiodide Perovskite Solar Cells Achieving Power Conversion Efficiencies up to 6.22%. Adv. Mater. 2016, 28, 93339340. (21)

Chung, I.; Song, J.-H.; Im, J.; Androulakis, J.; Malliakas, C. D.; Li, H.; Freeman, A. J.;

Kenney, J. T.; Kanatzidis, M. G. CsSnI3: Semiconductor or Metal? High Electrical Conductivity and Strong Near-Infrared Photoluminescence from a Single Material. High Hole Mobility and Phase-Transitions. J. Am. Chem. Soc. 2012, 134, 8579-8587. (22)

Takahashi, Y.; Hasegawa, H.; Takahashi, Y.; Inabe, T. Hall Mobility in Tin Iodide

Perovskite CH3NH3SnI3: Evidence for a Doped Semiconductor. J. Solid State Chem. 2013, 205, 39-43.


ACS Energy Letters

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

(23)

Page 24 of 26

Takahashi, Y.; Obara, R.; Lin, Z.-Z.; Takahashi, Y.; Naito, T.; Inabe, T.;

Ishibashi, S.; Terakura, K. Charge-Transport in Tin-Iodide Perovskite CH3NH3SnI3: Origin of High Conductivity. Dalton Trans. 2011, 40, 5563-5568. (24)

Iefanova, A.; Adhikari, N.; Dubey, A.; Khatiwada, D.; Qiao, Q. Lead Free

CH3NH3SnI3 Perovskite Thin-Film with P-Type Semiconducting Nature and Metal-Like Conductivity. AIP Adv. 2016, 6, 085312. (25)

Ogomi, Y.; Morita, A.; Tsukamoto, S.; Saitho, T.; Fujikawa, N.; Shen, Q.;

Toyoda, T.; Yoshino, K.; Pandey, S. S.; Ma, T. CH3NH3SnXPb(1–X)I3 Perovskite Solar Cells Covering up to 1060 nm. J. Phys. Chem. Lett. 2014, 5, 1004-1011. (26)

Zuo, F.; Williams, S. T.; Liang, P. W.; Chueh, C. C.; Liao, C. Y.; Jen, A. K. Y.

Binary‐Metal Perovskites Toward High‐Performance Planar‐Heterojunction Hybrid Solar Cells. Adv. Mater. 2014, 26, 6454-6460. (27)

Wang, F.; Ma, J.; Xie, F.; Li, L.; Chen, J.; Fan, J.; Zhao, N. Organic

Cation‐Dependent Degradation Mechanism of Organotin Halide Perovskites. Adv. Funct. Mater. 2016, 26, 3417-3423. (28)

Lee, S. J.; Shin, S. S.; Kim, Y. C.; Kim, D.; Ahn, T. K.; Noh, J. H.; Seo, J.; Seok,

S. I. Fabrication of Efficient Formamidinium Tin Iodide Perovskite Solar Cells through SnF2Pyrazine Complex. J. Am. Chem. Soc. 2016, 138, 3974-3977. (29)

Kumar, M. H.; Dharani, S.; Leong, W. L.; Boix, P. P.; Prabhakar, R. R.; Baikie,

T.; Shi, C.; Ding, H.; Ramesh, R.; Asta, M. Lead‐Free Halide Perovskite Solar Cells with High Photocurrents Realized through Vacancy Modulation. Adv. Mater. 2014, 26, 7122-7127. (30)

Sabba, D.; Mulmudi, H. K.; Prabhakar, R. R.; Krishnamoorthy, T.; Baikie, T.;

Boix, P. P.; Mhaisalkar, S. G.; Mathews, N. Impact of Anionic Br-Substitution on Open Circuit


Page 25 of 26

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

ACS Energy Letters

Voltage in Lead Free Perovskite (CsSnI3-xBrx) Solar Cells. J. Phys. Chem. C 2015, 119, 17631767. (31)

Koh, T. M.; Krishnamoorthy, T.; Yantara, N.; Shi, C.; Leong, W. L.; Boix, P. P.;

Grimsdale, A. C.; Mhaisalkar, S. G.; Mathews, N. Formamidinium Tin-based Perovskite with Low Eg For Photovoltaic Applications. J. Mater. Chem. A 2015, 3, 14996-15000. (32)

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. (33)

Xu, P.; Chen, S.; Xiang, H.-J.; Gong, X.-G.; Wei, S.-H. Influence of Defects and

Synthesis Conditions on the Photovoltaic Performance of Perovskite Semiconductor CsSnI3. Chem. Mater. 2014, 26, 6068-6072. (34)

Yokoyama, T.; Song, T.-B.; Cao, D. H.; Stoumpos, C. C.; Aramaki, S.;

Kanatzidis, M. G. The Origin of Lower Hole Carrier Concentration in Methylammonium Tin Halide Films Grown by Vapor-Assisted Solution Process. ACS Energy Lett. 2017, 2, 22-28. (35)

Song, T.-B.; Yokoyama, T.; Stoumpos, C. C.; Logsdon, J.; Cao, D. H.;

Wasielewski, M. R.; Aramaki, S.; Kanatzidis, M. G. The Role of Reducing and Basic Vapor Atmosphere in Sn-based Perovskite Solar Cells. J. Am. Chem. Soc. 2017, 139, 836-842. (36)

Wang, N.; Zhou, Y.; Ju, M. G.; Garces, H. F.; Ding, T.; Pang, S.; Zeng, X. C.;

Padture, N. P.; Sun, X. W. Heterojunction‐Depleted Lead‐Free Perovskite Solar Cells with Coarse‐Grained B‐γ‐CsSnI3 Thin Films. Adv. Energy Mater. 2016, 6, 1601130. (37)

Eperon, G. E.; Paternò, G. M.; Sutton, R. J.; Zampetti, A.; Haghighirad, A. A.;

Cacialli, F.; Snaith, H. J. Inorganic Cesium Lead Iodide Perovskite Solar Cells. J. Mater. Chem. A 2015, 3, 19688-19695.


ACS Energy Letters

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)

Page 26 of 26

Luo, P.; Xia, W.; Zhou, S.; Sun, L.; Cheng, J.; Xu, C.; Lu, Y. Solvent Engineering

for an Ambient-Air-Processed, Phase-Stable CsPbI3 in Perovskite Solar Cells. J. Phys. Chem. Lett. 2016, 7, 3603-3608. (39)

Marshall, K. P.; Walton, R. I.; Hatton, R. A. Tin Perovskite/Fullerene Planar

Layer Photovoltaics: Improving The Efficiency and Stability of Lead-Free Devices. J. Mater. Chem. A 2015, 3, 11631-11640. (40)

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. (41)

Zhang, W.; Smith, J.; Hamilton, R.; Heeney, M.; Kirkpatrick, J.; Song, K.;

Watkins, S. E.; Anthopoulos, T.; McCulloch, I. Systematic Improvement in Charge Carrier Mobility of Air Stable Triarylamine Copolymers. J. Am. Chem. Soc. 2009, 131, 10814-10815. (42)

Brandt, R. E.; Young, M.; Park, H. H.; Dameron, A.; Chua, D.; Lee, Y. S.; Teeter,

G.; Gordon, R. G.; Buonassisi, T. Band Offsets of N-Type Electron-Selective Contacts on Cuprous Oxide (Cu2O) for Photovoltaics. Appl. Phys. Lett. 2014, 105, 263901. (43)

Kirihata, H.; Uda, M. Externally Quenched Air Counter for Low‐Energy

Electron Emission Measurements. Rev. Sci. Instrum. 1981, 52, 68-70.


Performance Enhancement of Lead-Free Tin-Based Perovskite Solar

Recommend Documents