Thin Films and Solar Cells Based on Semiconducting Two

Apr 5, 2017 - Department of Chemistry and Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, 2145 Sheridan Road, Evan...
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Thin Films and Solar Cells Based on Semiconducting Two-Dimensional Ruddlesden−Popper (CH3(CH2)3NH3)2(CH3NH3)n−1SnnI3n+1 Perovskites Duyen H. Cao,† Constantinos C. Stoumpos,† Takamichi Yokoyama,†,‡ Jenna L. Logsdon,† Tze-Bin Song,† Omar K. Farha,†,§ Michael R. Wasielewski,† Joseph T. Hupp,† and Mercouri G. Kanatzidis*,† †

Department of Chemistry and Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States ‡ Mitsubishi Chemical Group Science & Technology Research Center, Inc., 1000 Kamoshida-cho, Aoba-ku, Yokohama 227-8502, Japan § Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 22254, Saudi Arabia S Supporting Information *

ABSTRACT: Low electrical resistivity (high dark carrier concentration) of CH3NH3SnI3 often leads to short-circuiting in solar cells, and appropriate thin-film modifications are required to ensure functional devices. The longterm durability of organic−inorganic perovskite solar cells necessitates the protection of perovskite thin films from moisture to prevent material decomposition. Herein, we report that the electrical resistivity and the moisture stability of two-dimensional (2D) Ruddlesden−Popper (CH3(CH2)3NH3)2(CH3NH3)n−1SnnI3n+1 perovskites are considerably improved compared to those of the three-dimensional (3D) CH3NH3SnI3 perovskite and subsequently show the solar cell fabrication using a simple one-step spin-coating method. These 2D perovskites are semiconductors with optical band gaps progressively decreasing from 1.83 eV (n = 1) to 1.20 eV (n = ∞). The n = 3 and n = 4 members with optimal band gaps of 1.50 and 1.42 eV for solar cells, respectively, were thus chosen for in-depth studies. We demonstrate that thin films of 2D perovskites orient the {(CH3NH3)n−1SnnI3n+1}2− slabs parallel to the substrate when dimethyl sulfoxide solvent is used for deposition, and this orientation can be flipped to perpendicular when N,Ndimethylformamide solvent is used. We find that high-purity, single-phase films can be grown only by using precursor solutions of “pre-synthesized” single-phase bulk perovskite materials. We introduce for the first time the use of triethylphosphine as an effective antioxidant, which suppresses the doping level of the 2D films and improves film morphology. The resulting semiconducting 2D Sn-based iodide perovskite films were incorporated in solar cells yielding a power conversion efficiency of 2.5% from the Sn4I13 member. From the temporal stability standpoint, the 2D Sn perovskite solar cells outperform their 3D analogs.

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incompatibility of the perovskite layer with other parts of the device.7,8 There has been a growing effort to replace lead perovskites with environment-friendly lead-free compounds by substituting the divalent Pb with Sn or Ge9−12 or looking at materials with trivalent Sb or Bi.13,14Among the mentioned candidates, ASnI3

rganic−inorganic halide perovskite semiconductors have generated enormous excitement in the photovoltaic research community because devices can achieve over 20% efficiency. 1−5 Although progress in optimizing the power conversion efficiency (PCE) has been accelerating, the long-term prospects of perovskite photovoltaic technology still faces major challenges. One issue is the toxic lead present in the champion hybrid perovskite materials APbX3 (A = CH3NH3, HC(NH2)2, Cs and X = Cl, Br, I).6 The second issue is the long-term stability, which can be detrimentally affected by moisture, temperature, and the © 2017 American Chemical Society

Received: March 6, 2017 Accepted: April 5, 2017 Published: April 5, 2017 982

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http://pubs.acs.org/journal/aelccp

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ACS Energy Letters

ligand to improve the SnF2-perovskite film morphology. Overall, high-quality semiconducting 2D Sn-based iodide perovskite films and solar cells have been fabricated, demonstrating encouraging PCE of 1.94% and 2.53% from the n = 3 and n = 4 members, respectively, and promising device durability. Synthesis and Characterization of Bulk Materials. We initially focused our effort on the large-scale synthesis of the homologous 2D (CH3(CH2)3NH3)2(CH3NH3)n−1SnnI3n+1 perovskite series (n = 1−5). The series was synthesized from a stoichiometric reaction of SnCl2·2H2O, CH3NH3I (MAI), and C4H9NH3I (BAI) in an excess of aqueous HI/H3PO2 solutions. H3PO2 was utilized to suppress the oxidation of I− to I3− and/ or I2. To prevent any starting material loss caused by the exothermic reaction between HI acid and amine base, we used the MAI and BAI salts instead of their liquid amines. It is important to accurately weigh the starting materials because if the 2:(n−1):n ratio of BAI:MAI:SnCl2 is not met, a mixture of different multilayered products will form. Interestingly, we observed that the concentration of BAI necessary to obtain pure compounds from this reaction is stoichiometric, as opposed to the half-stoichiometric concentrations used in the 2D Pb-based perovskite series.33 In the Pb-based case, half of the starting BAI material is used to prevent the fast precipitation of the kinetic/thermodynamic product, namely, (BA)2(MA)Pb2I7. Because of the higher solubility of Sn-based materials in HI/H3PO2 solution, fast precipitation of Sn2I7 is prevented, allowing for the formation of the desired products. The use of stoichiometric BAI is advantageous because it raises the final yield of the reaction. Also of note is that the synthesis does not necessarily have to be carried out in a N2 atmosphere. The oxidation of Sn2+ to Sn4+ is prevented by the addition of reducing H3PO2. The subsequent nucleation and growth of the perovskite crystals is also protected by HI/H3PO2 acidic medium in which the lighter H3PO2 acid (50% w/w, d = 1.21 g/cm3) is believed to form a layer above the heavier HI acid (57% w/w, d = 1.70 g/cm3), preventing the entrance of unwanted species (O2, H2O, etc.). Additionally, we noticed that there always exists a compact solid layer of SnI4 at the liquid− air interface of the acidic mixture, physically isolating the perovskite crystals at the bottom of the vials from contact with the atmosphere (Figure S1, left picture). The solutions can be maintained for several months without oxidation, thus allowing for the slow growth of the perovskite crystals. Upon filtration, the secondary SnI4 phase persists, but this minor impurity can be physically removed by sublimation under vacuum (see the Supporting Information). The layered nature of these compounds is visually apparent in scanning electron microscopy (SEM) images (Figure S1, right picture). Typical crystal size is from 100 to 300 μm, though the crystal size should be tunable by varying the crystallization approach. The purity of the bulk crystalline materials was confirmed by single-crystal and powder X-ray crystallography. Figure S2a,b shows the experimental diffraction pattern of the bulk material versus the calculated pattern obtained from single-crystal diffraction measurement of the two representative compounds, namely (BA)2(MA)2Sn3I10 and (BA)2(MA)3Sn4I13. Clean material containing one single phase was achieved for both compounds. The successful synthesis of single-phase compounds containing thicknesses of 1 up to 6 perovskite layers has also been accomplished, and their structural details will be published elsewhere. Herein we focus only on the n = 3 and n = 4 members.

materials appear to be the most efficient light-absorbing material for lead-free perovskite solar cells. ASnI3 perovskites are known to possess high background carrier concentration due to Sn2+ vancancies,15 which are promoted by Sn’s susceptibility to oxidation (Sn2+ to Sn4+) in ambient conditions. Partial oxidation (p-doping) leads to degenerately doped ASnI3 perovskite which shorts the devices,16 and full oxidation leads to decomposition from three-dimensional (3D) (ASnI3) to zero-dimensional (0D) (A2SnI6).17 Recently, we have made progress in understanding these processes and developed solutions to suppress the doping level where functioning solar cells based on ASnI3 can be prepared.12,18−21 However, these cells are still temporally unstable. The challenge thus remains to further increase the stability of Sn-based solar cells. Moisture promotes the oxidation of the Sn2+ site of the perovskite by hydrolyzing the Sn−I bond and hydrogen bonding to the organic CH3NH3+ sites, a fact that has been shown in the lead perovskite family;22,23 thus, suppressing moisture attack will be highly beneficial. Two-dimensional (2D) lead-based perovskite thin films have been shown to be more moisture resistant than those of the 3D perovskites, and as a result 2D solar cells last longer. The first examples of 2D lead-based perovskite materials utilized in solar cells were (C6H5(CH2)2NH3)2(CH3NH3)2Pb3I10 compound and homologous (CH3(CH2)3NH3)2(CH3NH3)n−1PbnI3n+1 series, producing devices that are stable in medium humidity for several months with moderate power conversion efficiency.24,25 By optimizing the growth orientation of 2D films to achieve favorable charge transport direction, Tsai et al. have successfully increased the device efficiency of (CH3(CH2)3NH3)2(CH3NH3)3Pb4I13 to above 10%.26 The improved moisture stability observed from the aforementioned 2D perovskite compounds comes from the hydrophobic nature of C6H5(CH2)2NH3+ and CH3(CH2)3NH3+ cations. Recent examples of incorporating hydrophobic cations in 2D perovskites are the use of polyethylenimine cation27 and cyclopropylammonium cation.28 It is reasonable to predict that organic−inorganic 2D perovskites will be also beneficial for improving the stability of Sn-based perovskite solar cells. Previously, Mitzi et al. reported that layered 2D perovskites have metallic properties, albeit with lower conductivity than their 3D CH3NH3SnI3 analog.29,30 Later studies showed that 3D CsSnI3 and CH3NH3SnI3 materials are in fact semiconductors and that the apparent metallic property of some samples was due to doping effects.31,32 With these considerations in mind, we set out to study 2D organic−inorganic Snbased iodide perovskites to first examine if these materials are truly metallic or semiconducting and then carry out an initial assessment for solar cell applications. The materials of interest are the Ruddlesden−Popper (CH3(CH2)3NH3)2(CH3NH3)n−1SnnI3n+1 (n = 1, 2, 3, 4, and 5) perovskite series. In this work, we show that these 2D Snbased iodide perovskite compounds are actually semiconductors and can act as light-absorbing materials for solar cells. We briefly discuss the large-scale synthesis and properties of the bulk crystals before moving on to thin-film fabrication and thinfilm properties of two representative compounds, Sn3I10 and Sn4I13. We lay out the pathway to obtain useful 2D perovskite films for solar cell applications and highlight the importance of solvent choice, bulk perovskite material quality, and spincoating deposition conditions. Additionally, we introduce here for the first time the use of triethylphosphine (TEP) in SnF2perovskite precursor solution as an intermediate coordinating 983

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ACS Energy Letters Ruddlesden−Popper compounds, of which the n = 3 and n = 4 Sn-based perovskites discussed here are members, are regarded as natural multiple-quantum-well (MQW) structures in which the semiconducting inorganic layers are the “wells” and the insulating organic layers are the “barriers”. They were of great interest to chemists and physicists in the early 1990s because of their intriguing physical properties such as enhanced exciton confinement and room-temperature photoluminescence.34,35 The band gap (Eg) of the series gradually increases from 1.20 eV (MASnI3, n = ∞) to 1.83 eV ((BA)2SnI4, n = 1) because of the dimensional reduction effect (Figure 1). This Eg

Figure 2. Different thin-film growth orientations of the 2D (BA)2(MA)2Sn3I10 material: (a) (0k0) parallel oriented, (b) (111) almost perpendicular oriented, and (c) (202) perfectly perpendicular oriented. The distance between (020) planes in parallel growth and between (111) planes and (202) planes are shown. The perpendicular growth favors charge transporting along the I−Sn−I to the adjacent substrates.

orientation to a remarkable extent. Figure 3 shows that under otherwise identical film fabrication conditions, the Sn3I10 film

Figure 1. Band gaps of bulk polycrystalline material of the (BA)2(MA)n−1SnnI3n+1 series (n = 1, 2, 3, 4, 5, and ∞).

trend versus dimensionality is universal and similar to the 2D Pb-based perovskites.33 While excitonic peaks are clearly seen in the entirety of the 2D Pb-based iodide perovskite series at room temperature, in the tin compounds, they are observed only in the single-layered (BA)2SnI4 compound. This suggests that 2D Sn-based iodide perovskites have exciton binding energies that are much lower than those of their Pb-based analogs because of their higher dielectric constants.36 We also determined the absorption coefficients (α) of (BA)2(MA)2Sn3I10 via thin-film samples. Figure S3 plots the absorption coefficient as a function of energy, showing an α of ∼1 × 105 cm−1 at 550 nm (2.25 eV) and a steep onset that implies low energetic disorder for the compound. With the ideal Eg of 1.50 and 1.42 eV for (BA)2(MA)2Sn3I10 and (BA)2(MA)3Sn4I13, respectively, these compounds are chosen for further studies and solar cell device applications. We will abbreviate the two compounds as Sn3I10 and Sn4I13 for convenience throughout the remainder of this Letter. Path to Desired 2D Tin Iodide Perovskite Thin Films for Solar Cells. The thin-film characteristics of 2D layered Pb-based perovskites have previously been studied, but very little is known regarding the Sn-based series. We aimed to grow clean, single-phase 2D Sn iodide perovskite thin films that are compact, have high-quality grains, and preferentially orient the perovskite slabs to selective substrates to facilitate carrier transport. There are three possibilities for 2D perovskite thinfilm growth on a substrate: (1) perpendicular, (2) parallel, and (3) nonoriented. By “perpendicular” we mean that the [SnnI3n+1](1+n)− perovskite slabs are oriented normal to the substrate (illustrated in Figure 2b,c); “parallel” means that the extended planes of the [SnnI3n+1](1+n)− slabs are parallel to the substrate with the remaining layers stacked over them and separated by the BA+ cations (Figure 2a); “non-oriented” means the film does not have any preferentially oriented growth. The orientation of the 2D films is critical to solar cell performance. The perovskite precursor solution plays a major role in the final film quality. It is striking to see that the solvent used to dissolve the bulk perovskites controls the film growth and

Figure 3. Control of the n = 3 (BA)2(MA)2Sn3I10 perovskite thinfilm growth orientation using different solvents, with 100% DMF solvent giving perpendicular growth and 4DMSO:1DMF v/v mixed solvent giving parallel growth, as evidenced by thin-film XRD measurement.

grows parallel to the planar substrate if dimethyl sulfoxide (DMSO) is utilized and perpendicular if N,N-dimethylformamide (DMF) is utilized. The parallel orientation reveals all (0k0) planes (k is an even integer), and the perpendicular orientation shows mostly (111) and (202) planes. Note that if mesoporous oxide substrates are employed, the preferential orientations might not be fully retained inside the mesopores, as shown in the X-ray diffraction (XRD) pattern of mesoporous TiO2−Sn3I10 film (Figure S4) where 3 Bragg peaks with very low intensities from the parallel orientation are observed in the 2θ region from 2°−13°. The difference is attributed to (1) the better surface wetting of DMSO solvent on planar TiO2 leading to more solution being spread out and (2) the intermediate complexes that DMSO is known to form with Sn2+ (SnI2· 3DMSO).37−39 This solvent effect is expected also to be applicable to other 2D multilayered perovskites (n > 1). The ability to manipulate the 2D perovskite film orientation by solvent choice is important because it allows for fundamental studies (such as understanding carrier mobilities along or across the perovskite slabs) and different optoelectronic applications (such as solar cells or transistors). The type of perovskite bulk material is also crucial. For the 3D perovskites, the perovskite precursor solution can be 984

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Figure 4. Thin-film XRD pattern of n = 4 Sn4I13 perovskite compound prepared by mixing method, showing the formation of a mixture of 2and 3-layered compounds.

°C substrates is a successful approach for preparing singlephase, layered 2D Sn-based iodide perovskite films. The pristine Sn3I10 perovskite film shows highly uniform and compact morphology (Figure 5a). The electrical resistance of

prepared by mixing stoichiometric amounts of metal halide and ammonium halide salts without synthesizing the 3D bulk material itself. This “mixing method” is not applicable for preparing clean 2D perovskite films. We attempted to grow the 2D Sn iodide films (n = 3, 4, 5, and 6) using the mixing method and observed using XRD that the final films contained only two products: Sn2I7 (n = 2) and Sn3I10 (n = 3). Figure 4 depicts an example of attempted n = 4 film growth by the mixing method. The low-angle window (2θ = 2 to 12°) clearly shows the formation of a mixture of n = 2 and n = 3 compounds; no n = 4 Bragg peaks are present. As mentioned above, the use of pure bulk materials as film precursors ensures clean film formation of the desired perovskite. Thus, it is reasonable to presume that the dissolution of the “pre-synthesized perovskite” in DMF still leaves molecular “perovskite seeds” in solution, and these may homogeneously nucleate the film growth into the desired products.40 On the other hand, mixing the starting materials SnI2, MAI, and BAI in DMF forms only solvated ions which, upon spin-coating on substrates, crash out kinetically (low solubility), namely, n = 2, an event which then shifts the equilibrium to produce a mixture of phases. Inspired by a recently developed film deposition method known as “hot-casting”, which resulted in 2D Pb-based iodide perovskite solar cells with a promising PCE of 12%,26 we looked at film deposition at two different substrate temperatures: 25 and 120 °C. While in the 2D Pb-based case, the perpendicular growth is already obtained by room-temperature casting,25 the crystallinity of the films is much higher with “hotcasting”. The 2D Sn-based perovskites behave quite differently. For Sn3I10, room-temperature casting shows almost a nonoriented film in which XRD in both the parallel (080) and almost-perpendicular (111) Bragg reflections are present, with (080) having higher intensity (see Figure S5a, red XRD pattern). Casting on a 120 °C substrate markedly intensifies the perpendicular growth as highlighted by the green rectangle (Figure S5b). Unlike the Sn3I10 film, the Sn4I13 film already grows preferentially perpendicular to the substrate at roomtemperature casting, as shown in Figure S5c (red pattern). Hotcasting raises the intensity of the perfectly perpendicular (202) peak and lowers that of (111). In terms of crystallinity and polycrystalline island size, room-temperature versus hot-casting does not make a significant difference; the fwhm values of the (202) peak of the two films in Figure S5b,d are similar. Nevertheless, the hot-casting film fabrication method yields a thick perovskite layer without requiring a high concentration of the perovskite precursor solution. Accordingly, we conclude that the combination of using presynthesized perovskite bulk materials as precursors, DMF solvent, and hot-casting on 120

Figure 5. Top surface SEM images of (a) pristine 2D Sn3I10 film, (b) (Sn 3 I 10 + SnF 2 ) film, and (d) (Sn 3 I 10 + SnF 2 + triethylphosphine) film. Cross-sectional SEM images of (c) (Sn3I10 + SnF2) film and (e) (Sn3I10 + SnF2 + triethylphosphine) film. The poor morphology caused by SnF2 in panels b and c is overcome by the use of triethylphosphine coordinating ligand, as shown in panels d and e.

pristine Sn3I10 and Sn4I13 films (with no SnF2 added) was measured by four-point probe resistivity measurement to be about 106 Ω, which is equivalent to a resistivity of about 103 Ω· cm (Figure S7), verifying their semiconducting nature. This resistivity was however not high enough, leading to a poor device performance of 0.35% for the n = 3 Sn3I10 member (Figure S6). Therefore, for solar cell fabrication, 0.2 M SnF2 salt was added into a 0.2 M solution of Sn3I10 or Sn4I13; SnF2 salt acts as an excess Sn2+ source to suppress adventitious doping with Sn4+ as well as increases the resistivity of 2D perovskite thin films.10,41−43 Unfortunately, adding SnF2 to the precursor solution results in formation of perovskite aggregates as demonstrated in Figure 5b (surface view) and Figure 5c (cross-sectional view). Recently, Seok and co-workers reported the use of DMSO solvent and a pyrazine−SnF2 complex to overcome the poor SnF2−FASnI3 film morphology.44 We took another approach by simply using triethylphosphine (TEP) as a soft Lewis base and weak coordinating ligand for Sn2+ species to slow the precipitation and mitigate the morphology problem. In addition, because of the softer Lewis acid nature of Sn2+ relative to Sn4+, its coordination serves to inhibit any incidental conversion to Sn4+. In particular, a small amount of TEP (0.034 M) was added into 0.2 M perovskite precursor solution prior to the film spin-coating step. TEP weakly coordinates with Sn2+ species, forming intermediate complexes, slowing the perovskite crystallization process, and consequently improving the SnF2−Sn3I10 film morphology. Also, with a low boiling point of 985

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ACS Energy Letters 127 °C, TEP is believed to be completely removed from the final film during the annealing step after spin-coating as well as during the Au thermal evaporation step. We confirmed that the perpendicular film growth of our 2D perovskite films is not altered by TEP addition according to thin-film XRD (Figure S8). Solar Cells Based on 2D n = 3 Sn3I10 and n = 4 Sn4I13 Perovskites. Steady-state and time-resolved photoluminescence (TrPL) measurements were performed on both Sn3I10 and Sn4I13 films to study their carrier lifetimes. Each sample was excited with a 20 nJ, 600 nm pulse, and the TrPL data was fit to a monoexponential decay convoluted with the instrument response. The monomolecular recombination lifetimes (for both electrons and holes), τ, are 0.39 and 0.61 ns for Sn3I10 and Sn4I13, respectively (Figure 6). These carrier lifetimes are

average, Sn3I10 devices with SnF2 additive produce a Jsc of 7.73 mA/cm2, a Voc of 355 mV, and a FF of 42.0% for an average PCE of 1.16%. As discussed above, SnF2 promotes the growth of isolated perovskite islands, which results in a nonuniform film and thus a poor FF. The introduction of TEP into the SnF2-perovskite precursor solution leads to denser and more uniform film growth. As a result, devices with TEP show consistent increases in average FF (42.0 to 53.7%), Jsc (7.7 to 8.7 mA/cm2), and Voc (355 to 376 mV), which together raises the average PEC from 1.15% to 1.75%, with the champion device reaching ∼2% efficiency (see Figure 7a and Table 1).

Figure 7. J−V curves of (a) n = 3 Sn3I10 perovskite solar cells with and without triethylphosphine and (c) n = 3 Sn3I10 and n = 4 Sn4I13 perovskite solar cells with triethylphosphine; EQE and integrated Jsc of triethylphosphine-containing (b) Sn3I10 device and (d) Sn4I13 device.

With the fabrication parameters determined for Sn3I10, we used the same procedure for Sn4I13. The Sn4I13 devices produce impressively high Jsc (∼24 mA/cm2 in average) owing to the lower Eg of 1.42 eV and significantly higher effective external quantum efficiency (EQE) of ∼73% at 550 nm wavelength. The EQE onset of the Sn3I10 device (∼750 nm) (Figure 7b) matches well with the Eg of the Sn3I10−SnF2−TEP film (Figure S10c). However, we noticed that the EQE onset of the Sn4I13 device (Figure 7d) slightly deviates from the Eg of Sn4I13− SnF2−TEP film (Figure S10b) by ∼0.12 eV. A drop in Voc and a slight drop in FF were observed in Sn4I13 devices compared to the Sn3I10 devices. First, this can be due to a higher p-type doping in Sn4I13 film. Because Sn4I13 is closer to the 3D homologue, it is thus expected to have more Sn2+ vacancies than Sn3I10. This higher vacancy concentration will lead to a higher perovskite conductivity, resulting in poorer FF. Additionally, it also facilitates higher charge recombination within the perovskite layer itself and between the TiO2 and perovskite layers. Second, the quality of the Sn4I13 film is inferior to that of the Sn3I10 film, which is clearly seen in Figure 8 SEM images of the n = 3 and n = 4 films. While the Sn3I10 film is very shiny to the naked eye, the Sn4I13 film is rough and has grain boundaries of several micrometers, potentially resulting in more traps and more charge recombination, both of which can lower FF and Voc. Despite these issues, the superior Jsc in Sn4I13 devices leads to higher PCEs (2.53%) than those of Sn3I10 devices (∼2%). With further optimization in Sn4I13 thin-film morphologies, higher PCE is anticipated. Sn4I13 or another multilayered 2D tin iodide member that lies between n = 4 and ∞ might perhaps be

Figure 6. (a) Steady-state and (b, c) time-resolved photoluminescence of n = 3 Sn3I10 and n = 4 Sn4I13 perovskite films on glass substrate. Note that these films were deposited with the use of SnF2-triethylphosphine as additives.

relatively short compared to their 3D analogs.45 More importantly, the steady-state PL data of both Sn3I10 and Sn4I13 films show one single emission at 860 and 922 nm, respectively, which further supports the materials’ single-phase purity. Because of the relatively short carrier lifetimes in these films, we utilized mesoporous TiO2 as electron-accepting layer to facilitate the collection of photogenerated charge carriers.46 Poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) doped with tetrakis(pentafluorophenyl) borate (TPFB) was used as hole-transporting layer. A typical Sn3I10 device architecture consists of a 600 nm mesoporous TiO2 layer infiltrated with perovskite, a 500 nm perovskite capping layer, and a 170 nm PTAA layer (aside from the basic FTO, TiO2 blocking layer, and Au top electrode; see Figure S9). On 986

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ACS Energy Letters Table 1. Summary of Photovoltaic Performance of 2D Sn3I10 and Sn4I13 Perovskite Solar Cells with and without TEP Coordinating Liganda

a

perovskite

Jsc (mA/cm2)

Voc (mV)

FF (%)

efficiency (%)

Sn3I10 + SnF2 Sn3I10 + SnF2 + TEP Sn4I13 + SnF2 + TEP

8.9 (7.73 ± 0.55) 8.9 (8.64 ± 0.52) 24.1 (23.7 ± 0.64)

380 (355 ± 33) 382 (376 ± 27) 229 (230 ± 4)

43.9 (42.0 ± 4.7) 57.1 (53.7 ± 4.74) 45.7 (44.6 ± 1.1)

1.49 (1.16 ± 0.23) 1.94 (1.74 ± 0.19) 2.53 (2.43 ± 0.11)

Average value and standard deviation of 10 devices are shown in parentheses.

process proceeding; thus, the Fermi level values likely represent slightly oxidatively doped films. The KP data also indicate that the Sn3I10 film is more air stable than the Sn4I13 film based on the stability of the Fermi level versus air-exposure time (Figure S12). Although a Voc of almost 400 mV was achieved with the Sn3I10 perovskite compound, this output is still far from the compound’s optical bandgap Eg of 1.5 eV. The Voc output is expected to be limited by the thin-film background carrier concentration and the use of n-type TiO2 semiconductor electron-accepting layer which does not offer optimal band alignment. Improving the thin-film quality to suppress energy losses and replacing TiO2 with materials with a higher conduction band minimum will be required to further increase the Voc. The durability of 2D tin iodide perovskite solar cells was also examined. Figure 10 compares the air stability of unencapsulated MASnI3 and Sn4I13 devices fabricated in the same manner. The PCE of the MASnI3 device deteriorated to almost 0% after 3 min of exposure to air while the Sn4I13 devices stayed intact after the first minute and only gradually degraded over ∼30 min (Figure 10c). We speculate that the 2D organic−inorganic perovskite films are protected with the hydrophobic long butylammonium chains upon film formation, and this slows the reaction of perovskite with moisture. Clearly, the fabrication of air-stable Sn-based perovskite solar cells is a challenge. However, the better air-stability of the 2D tin iodide perovskite devices in this work compared to the 3D counterparts demonstrates that 2D structure and functionalization of perovskites with moisture-tolerant molecules are promising strategies toward stable and efficient Sn-based perovskite solar cells. Finally, when the 2D Sn iodide perovskite solar cells are encapsulated, they exhibit far better stability: a one-month old device retained more than 90% of its initial efficiency (Figure 10d,e and Table 2). The device encapsulation in this work was done by thermally sealing a glass slide on top of the solar cell device at 115 °C using a 30 μm thick polymer. It is anticipated that with better encapsulation methods, devices will last much longer. In conclusion, the 2D Ruddlesden−Popper (CH3(CH2)3NH3)2(CH3NH3)n−1SnnI3n+1 perovskites are semiconductors with optical band gaps gradually decreasing from 1.80 eV (n = 1) to 1.20 eV (n = ∞). The n = 3 and n = 4 members have optimal band gaps of 1.50 and 1.42 eV, respectively, were chosen for thin-film deposition studies and solar cell applications. To obtain useful 2D tin iodide perovskite films for photovoltaic applications, the solvent choice, quality of bulk perovskite material, and spin-coating deposition conditions are crucial. We also introduced for the first time triethylphosphine in SnF2-perovskite precursor solution as an intermediate coordinating ligand, which improves the SnF2perovskite film morphology and the device performance. Overall, high-quality semiconducting 2D tin iodide perovskite films have been obtained and incorporated in solar cells demonstrating an encouraging PCE of 2.5% (from the Sn4I13

Figure 8. Top surface SEM images of 2D (BA)2(MA)n−1SnnI3n+1 (n = 3 and 4) films. The scale bar is 10 μm.

the “sweet spot” that maximizes the light-harvesting efficiency of low Eg Sn-based perovskites while also taking advantage of the lower background carrier concentration that 2D materials offer compared to 3D materials. We further investigated the energy band alignment (Figure 9) of 2D tin iodide perovskites using ultraviolet photoemission

Figure 9. Band alignment of TiO2, 3D MASnI3, 2D Sn4I13, 2D Sn3I10, and PTAA.

spectroscopy (UPS) and Kelvin probe (KP) measurements. Thin films (as opposed to bulk perovskite) were chosen for these measurements because the band energy values are directly relevant to the charge dynamics in the final solar cell. The valence band maximum (VBM) of Sn3I10 is similar in energy to that of the 3D MASnI3 based on the UPS measurements (Figure S11). The KP measurements (Figure S12) show that the Fermi level gradually moves up in energy when going from 3D (∼ −4.7 eV) to 2D (∼ −4.4 eV). This implies that the concentration of p-type carriers (conductivity) is lower in the 2D films compared to 3D MASnI3 films. Note that the KP measurements were carried out in air, with the film oxidation 987

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Figure 10. J−V curves of (a) MASnI3 and (b) n = 4 Sn4I13 perovskite devices measured in air; (c) PCE retention of MASnI3 and Sn4I13 devices versus air exposure time; (d) J−V curves and (e) PCE retention of encapsulated Sn4I13 device after several months.

Notes

Table 2. Summary of Photovoltaic Performance of 2D Sn4I13 Perovskite Solar Cells over Time device age

Jsc (mA/cm2)

Voc (mV)

FF (%)

efficiency (%)

efficiency retention (%)

fresh 1 month 4 month

23.2 22.4 20.3

225 230 232

45.2 42.8 32.7

2.37 2.21 1.55

100 93.2 42.2

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.H.C. acknowledges support from the Link Foundation through the Link Foundation Energy Fellowship Program. T.B.S. acknowledges financial support from Mitsubishi Chemical Group Science & Technology Research Center, Inc. This work was supported as part of the ANSER Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award Number DE-SC0001059. This work made use of the EPIC and Keck-II facilities of Northwestern University’s NUANCE Center, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF NNCI-1542205); the MRSEC program (NSF DMR1121262) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN. D.H.C and J.L.L thank Dr. Lin Ma for her assistance with the photoluminescence measurement.

member). More importantly, 2D Sn perovskite solar cells outperform their 3D analogs in temporal stability. Encapsulated Sn4I13 devices retained more than 90% of their initial performance after 1 month and dropped only to ∼50% after 4 months. Better device encapsulation technology is expected to extend the durability of 2D tin iodide perovskite solar cells. Finally, the open-circuit voltage of 2D tin iodide perovskite solar cells is still far from its band gap value. Work in progress is directed toward improving the Voc by replacing the TiO2 electron acceptor, optimizing the thin-film qualities, and redesigning the device architecture.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00202. Experimental details, photographs, SEM images, XRD patterns, absorption coefficient, additional J−V curves, electrical resistance, ultraviolet photoemission spectroscopy data, Kelvin probe spectra (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Constantinos C. Stoumpos: 0000-0001-8396-9578 Takamichi Yokoyama: 0000-0002-0766-4550 Omar K. Farha: 0000-0002-9904-9845 Michael R. Wasielewski: 0000-0003-2920-5440 Joseph T. Hupp: 0000-0003-3982-9812 Mercouri G. Kanatzidis: 0000-0003-2037-4168 988

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