How SnF2 Impacts the Material Properties of Lead-Free Tin Perovskites

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How SnF Impacts the Material Properties of Lead-Free Tin Perovskites Satyajit Gupta, David Cahen, and Gary Hodes J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01045 • Publication Date (Web): 08 Apr 2018 Downloaded from http://pubs.acs.org on April 8, 2018

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The Journal of Physical Chemistry

How SnF2 Impacts the Material Properties of Lead-Free Tin Perovskites Satyajit Gupta1, David Cahen* and Gary Hodes* 1

Dept. of Materials & Interfaces Weizmann Institute of Science, Rehovot, 76100, Israel.

* corresponding authors: [email protected] [email protected]

1. Present address: Department of Chemistry, Indian Institute of Technology Bhilai, GEC Campus, Sejbahar, Raipur, Chhattisgarh, 492015, India. email: [email protected]

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The Journal of Physical Chemistry 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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Abstract Lead-based halide perovskites (APbX3) are fascinating optoelectronic materials. Due to toxicity issues of Pb, Sn-based halide perovskites are studied, although less so, as an alternative. Adding SnF2 often improves the properties of Sn halide perovskite-based devices. This effect is usually ascribed to suppression of Sn2+Sn4+ oxidation and /or decreased Sn vacancy concentration. These effects will change the doping, sometimes in opposite directions. Here we review the effect of addition of SnF2 during the formation of ASnX3 layers as observed by different groups, both to the properties of the layers themselves and to photovoltaic cells made from these layers. SnF2 can affect many different properties of the ASnX3 perovskites, including film morphology, doping, control over formation of unwanted crystal phases, material stability to various factors and energy level positions. It also improves (in general) the performance of photovoltaic cells made with these layers. Besides focusing on all these issues, we also describe possible doping scenarios for the perovskites, including some that do not appear to have been considered before and conclude that the doping mechanism depends strongly on whether the oxidation of Sn2+ to Sn4+ occurs during the materials preparation or after the film is formed, and if oxygen is involved.

Keywords: Tin-based halide perovskites, impact of SnF2, photovoltaic cells


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1. Introduction Power conversion efficiencies (PCEs) of lead (Pb2+)-based halide perovskite (HaP) photovoltaic (PV) cells (with the general perovskite formula of APbX3, where A=methyl ammonium (MA), formamidinium (FA) and/or cesium (Cs+), and X =I-, Br-, Cl-) have risen rapidly in the past half decade.1-4 Some specific compositions of HaPs have shown efficiencies beyond 22%.4 However, the toxicity of Pb, together with the non-negligible solubility of PbX2, the common decomposition product from the perovskites in water, results in environmental concerns that may restrict commercialization of Pb-based HaP cells. Thus, there is considerable interest to try to replace lead by non- or less-toxic elements5 such as tin (Sn2+),6,7 germanium (Ge2+)8 or a combination of bismuth and silver (Bi3+ and Ag+)9-11 to form lead-free halide perovskites. Of these elements, Sn is the most commonly-studied as an alternative to Pb. While much less studied than the analogous Pb HaPs, there is still a considerable literature on these Sn-based materials and cells (references. 6 and 7 are two recent reviews on Sn and Sn-alloy cells). The bandgaps of the Sn perovskites are lower than those of the corresponding Pb-based HaPs ones (e.g., MASnI3 has a gap of ~1.3 eV compared to ~1.6 eV for MAPbI3), and thus often closer to the optimal range for single junction cells, based on the ShockleyQueisser model.12 However, with the exception of two recent reports at 8.1%13 and 9.0%14, the highest reported efficiencies for these cells are around 6-7%15-19 and most are considerably lower, i.e., all devices based on Sn HaPs are much lower than the best reported values for cells containing Pb. From a comparison of absorption spectra of Sn HaPs compared to Pb HaPs over many papers, it can be seen that the absorption coefficients of the Sn HaPs are substantially lower than those of the Pb equivalents. This means that a thicker layer of Sn HaP is needed and thus a longer minority carrier diffusion length (LD). LD is a critical semiconductor parameter for photovoltaic cells since it indicates the distance the minority carrier (electrons for the usually p-type Sn perovskites) can move on average before recombination. Although there are limited data on these data, there is a wide range of values from (for MASnI3) 30 nm15 to 550 nm.20 (Single crystals, or large grain films which behave more like single crystals, give higher values). The corresponding LD for CsSnI3 films was reported to be as low as 16 nm.21 3 ACS Paragon Plus Environment


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The value of LD is determined by both the charge lifetime, τ, and diffusion coefficient, D, or related mobility, µ, by the Einstein relation: LD = (Dτ)1/2 = (kTµτ/e)1/2 where k is the Boltzmann constant, T the temperature and e the electronic charge. Again, there is very limited data on τ or µ for Sn HaP thin films. Compared to the corresponding Pb HaPs, Sn HaPs in general have much shorter lifetimes but larger µ (D). Mobility values (from Hall measurements) for pressed pellets of MASnI3, CsSnI3 and FASnI3 were reported to be ~2320, ~536 and ~103 cm2/V.s.22 Even assuming considerably-reduced values for thin film material, except maybe for the FASnI3, these are still much higher than the, at most, few tens cm2/V.s typical for films of the Pb HaPs. More details on LD and τ data are given in the Review below. SnF2 is commonly added to the preparation solution for Sn HaPs, ostensibly to reduce the oxidation of Sn2+ to Sn4+. However the effect of SnF2 on Sn HaPs is not limited to its effect on this oxidation reaction. In this review, we focus on the impact of SnF2 on the chemical and optoelectronic properties of Sn-based HaPs. Due to the lack of detailed explanation of the mechanism of p-type doping in these HaPs, we also consider the related defect chemistry and potential doping reactions. There are efforts on mixed Sn-Pb perovskite cells,6,7,23-25 with interesting results, but with questionable relevance for overcoming the Pb problem. Here we will, with a few exceptions, consider only pure Sn perovskites. 2. Sn2+ oxidation/doping The main reason for this large difference between Sn- and Pb-based perovskite cells is generally accepted to be the much easier oxidation of Sn2+ to Sn4+ compared to the oxidation of Pb2+ to Pb4+.15,26 The standard redox potential, E0, of Sn2+/Sn4+ = +0.15 V, compared to the Pb2+/Pb4+ system with E0 = +1.67 V; thus thermodynamically, it is much more difficult to oxidize Pb2+ to Pb4+ and, while Pb perovskites can in general be made in ambient atmosphere, even if control of humidity can be important, it is essential to prepare the Sn perovskites in the absence of oxygen to prevent oxidation of Sn2+ to Sn4+. Even under carefully controlled conditions, it is difficult to completely prevent some ex-


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posure to oxygen, and probably mainly for this reason, as well as for the presumed partial oxidation of the Sn2+ starting materials, some degree of oxidation of the Sn perovskites is likely to occur during preparation. The reason that this oxidation is of central importance for the Sn-based perovskites is that the presence of Sn4+ in a Sn2+-based perovskite can influence the doping. Experimentally the Sn perovskites are found to be p-type doped.26,27 There are several ways that can be envisioned for this to occur: charge neutrality requires that each Sn4+ on the Sn lattice site, which in the ideal, defect-free ASnI3 lattice should bear a 2+ charge, is charge-balanced by extra negative charges (e.g., two halide interstitials, each with one negative charge; see below), or by the absence of two positive charges, such as a Sn2+ vacancy or two A+ vacancies, or combinations thereof. All these defects can act as dopants. Surprisingly, in spite of the importance of this doping, there are almost no papers that give anything other than a very cursory explanation of the mechanism of this doping. Takahashi et al.28 suggested that when Sn2+ is replaced by adding some Sn4+ in the preparation step, the formal composition becomes MA1-2xSn(II)1-xSn(IV)xI3 or MASn(II)1-xSn(IV)x/2I3 resulting in MA or Sn deficiency respectively. We thus consider the various possibilities leading to the p-doping of Sn perovskites, based on free hole formation in the various possible processes. We also distinguish between the effect of Sn4+ added during preparation of the perovskite and that formed due to oxidation after formation. In the following we use, and will now briefly recall, the ‘Kröger-Vink’ notation29 to understand the defect chemistry/imperfections.30 In this notation, a species/entity is denoted by normal size characters such as, V, Sn and Br for vacancy, tin, and bromide, respectively. Subscript characters are used to denote lattice sites, such as “ i “ for an interstitial site (i.e., a site that is not occupied in a perfect lattice and “ Sn “ for the tin lattice site. Thus, ” Bri “ represents an bromide interstitial and “ SnSn “ a properly placed Sn, as it sits on the Sn lattice site.

In addition, the neutral lattice is considered as reference for the charges. For a species ‘X’, superscript characters such as XX, X´ and X• denote, neutral, negative and positive charges, relative to the charge that the lattice “expects” to be there on that site, i.e., a +1 5 ACS Paragon Plus Environment


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charge for the A site, a -1 charge for the bromide site and a +2 charge for the Sn site. While for interstitial sites absolute and relative charges are the same, this is not so for normal lattice sites. Thus, if the material formed with Sn4+, the charge in this notation is 2+, SnSn••. Naturally, SnSn•• presence would have to be charge-balanced, e.g., by 2Bri´ and/or VSn˝. Then, some of the following defect reactions can occur: 1. SnSn•• SnSnx + 2 h+ (SnSn•• + 2e- SnSnx; Sn4+ acts as acceptor; p-doping). Even if together with SnSn••, a Sn vacancy (VSn˝, neutral in terms of absolute charges) would be incorporated during compound formation, in view of the reported net p-doping effect, the following process

2.

VSn˝ VSnx + 2 e-

is unlikely to completely charge-balance SnSn••, i.e. it will not off-set the extra two positive charges of the SnSn•• (=Sn4+), because if VSn˝, which is doubly negatively charged with respect to the lattice, becomes neutral with respect to lattice, this would lead to ndoping. Naturally, if a neutral Sn vacancy, VSnx, is present (implying no Sn4+ on the Sn site, i.e., SnSn••, or that SnSn•• is charge balanced by another process than 2, such as 3 or 5 below), then a defect process opposite to 2 (VSnx VSn˝ + 2 h+) can occur and lead to p-doping. If the material formed with extra bromide (as interstitial) then extra bromide as interstitial can oxidize according to:

3.

2Bri´ 2Brix + 2e-

incorporating bromine on interstitial sites, to balance the extra two positive charges of the SnSn••, because discharging Bri´ will donate an electron; n-doping).


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Also, oxidation of Sn2+ to Sn4+ after ASnBr3 formation, in an originally defect-free material should not dominate as oxidation of SnSnx (=Sn4+) will, according to 4. SnSnx SnSn•• + 2 e-

lead to n-doping.

In addition to Sn vacancies, halide vacancies, which are often suggested to be important for the Pb halide perovskites, should be considered. Their presence can lead to n- or pdoping, depending on whether they are incorporated as neutral species with respect to the lattice 5. 2VBrx 2VBr• + 2 e- charging of Br vacancies, w.r.t. lattice; n-doping or as positively charged w.r.t. the lattice, i.e., neutral in absolute terms, 6. 2VBr• 2VBrx + 2 h+ discharging of Br vacancies, w.r.t. the lattice; p-doping. In addition to these possible defect chemical processes after ASnBr3 formation, a chemical oxidation to Sn4+ via reaction with O2 (oygenation), likely mostly at/near grain surfaces, should be considered. The overall reaction (changing to normal chemical notation) can be given by:

7. 2CsSnBr3 + O2 Cs2SnBr6 + SnO2. This reaction has been used to prepare Cs2SnBr6 by mild heating of CsSnBr3 in air.31 To understand how reaction 7 can dope CsSnBr3, we consider that it occurs only to a small extent, i.e., leaving most of the CsSnBr3 structure intact. As a structural reaction, this can be visualized as breaking some of the connections between SnBr6 octahedra (CsSnBr3 is made up of completely interconnected SnBr6 octahedra in three dimensions while Cs2SnBr6 contains isolated octahedra). Also, the oxidation will occur initially at grain surfaces, although diffusion of O2 into the bulk may occur over time. The Sn4+ chemical oxidation process can be given as: 7 ACS Paragon Plus Environment


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8. Sn2+ + O2 SnO2 + 2h+ with oxygen acting as an e- acceptor. This process will supply holes, i.e., act as a p-type dopant. Although SnO2 has not been observed as a separate phase in X-ray diffraction (XRD) (discussed below), it likely will occur as an amorphous material or in such small ‘crystalline’ aggregates that it is not detected by XRD. Although we assume that the SnO2 and CsSnBr3 make up two different phases, it is possible that the two phases are so well intermixed that the holes from reaction 8 are donated to the CsSnBr3. As a first step Sn at the grain surfaces will react with O2. Such a process: 8a. -Sn2+(on CsSnBr3 surface) + ½O2 + 2e- -Sn-O is well known for chalcogenide semiconductors to p-dope polycrystalline materials with µm grain sizes.32 Each or both processes can explain the p-type doping that occurs experimentally when CsSnBr3 is oxidized by O2. Likely 8a dominates in the early stages of the reaction, and 8 dominates in later stages as the perovskite structure breaks up. Such Sn-O species will remain in more or less its original position, especially in the early stages of oxidation. This hypothesis may be experimentally verified by atom tomography of grains, assuming that, based on the lack of X-ray diffraction evidence for SnO or SnO2, any tin oxide is amorphous or in the form of nanocrystals (< a few nm). Note that this proposed process obviates the need for formation of Sn vacancies in the bulk, which requires Sn mobility. It also implies that the p-type doping need not be due only to VSn but can also be due (at least in part) to Sn oxygenation/reduction of O2. We note that recently oxygenation leading to SnO2 and SnI4 was proposed for Sn iodide perovskites at elevated temperatures.33 While possible, we use here Occam’s razor, restricting ourselves to the simpler options. There is direct experimental evidence for tin being in a Sn4+ environment from photoemission data. However, since the reported binding energies of Sn4+ in SnO2 and in SnX4 are essentially identical, the Sn4+ observed may well come largely from SnO2, at least for perovskites that were oxidized to a relatively small extent. 8 ACS Paragon Plus Environment

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To summarize, oxidation reactions (2, 3, 4 and 5) clearly cannot dominate since they should give n-type material, which is not observed. Process 1 is likely to be dominant for cases where Sn4+ is present during material formation (process 6 is also possible but considered less likely). For material that has been oxidized by O2, processes 1 and, more likely 8(a) can be responsible for the doping. Excessive doping is undesirable for PV absorber material where it results in a narrowing of the space charge region as well as poorer material quality in general because of charge scattering by the dopants, reflected in reduced charge mobility and lifetimes and, through both these parameters, shorter charge diffusion/drift lengths. If oxidation of Sn2+ to Sn4+ and its incorporation into the material as it forms, results in formation of VSn, as is generally accepted in the literature, then a logical way to minimize formation of these vacancies is to add excess Sn during the preparation of the perovskite. This is routinely done, with SnF2 being the most common excess Sn source, but SnI2 and SnCl2 are also used. If the doping occurs through process 1 (during preparation) or 8a (after preparation), then this logic is less evident. In that case, the role of the SnF2 (a reducing compound) is presumably the reduction of the degree of oxidation. This could be due to creating/maintaining a reducing environment (maintaining a more negative potential) or by scavenging. It should be noted that SnF2 itself has been shown to undergo surface oxidation to SnO2.34 The emphasis of this paper is the role of SnF2 (or related) additives and their effects on the resulting Sn-based perovskite material properties and on related PV cells. We now critically review the relevant literature, considering both the effects of SnF2 on the doping and related properties of the perovskites, and other effects of the SnF2 that are not directly related to doping.

3. Review of the literature Doping/Sn4+/VSn The first report of SnF2 added to a Sn perovskite was by Chung et al. who added it to CsSnI3.35 The CsSnI3 in this work was not used directly as a light absorber, but rather as a hole transport material (HTM) for a solid-state dye-sensitized cell. However, it seems that 9 ACS Paragon Plus Environment


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(optical) absorption by the perovskite contributed to the cell photocurrent. SnF2 was added both in in the high-temperature solid state preparation of the perovskite starting material as well as during solution deposition of the perovskite. The role of SnF2 in improving the cell performance (mainly JSC) was not discussed in this report; SnF2 was treated as a dopant, but based on present understanding, it can reduce p-doping in the perovskite by reducing either oxidation of Sn2+ by oxygen to Sn4+ (reactions 7 or 8 above) and/or because of the law of mass-action, formation of VSn˝, i.e., Sn vacancies (that are neutral in terms of absolute charge, i.e., reaction 2 above is prevented or minimized). It is possible that de-doping the (probably as-prepared heavily-doped) perovskite, allows it to contribute more to the dye cell’s performance. Kumar et al. added SnF2 (optimal concentration of 20%) to CsSnI3 to fabricate Sn perovskite cells with high values of JSC (almost 23 mA.cm-2), low fill factor and low VOC, with the last observable being typical for most Sn-based perovskite cells.36 Very importantly, they measured the carrier densities of the CsSnI3 (by Hall and capacitance measurements) and found that the 20% SnF2 reduced NA from ~1019 cm-3 to ~1017 cm-3. They suggested that this was due to increase in the Sn chemical potential and, therefore, an increase in the formation energy of VSn, resulting in a decreased concentration of these vacancies. A hole carrier concentration of 4.5x1017 cm-3 was derived for melt-grown CsSnI3 with 100s µm crystallite size (no SnF2 used).21 This is comparable to the SnF2 > SnBr2 > SnI2 (all 10% excess) > no excess Sn with a 70% decrease in absorbance after 30 min with no excess Sn and only 10% decrease with SnCl2.44 Addition of SnF2 to CsSnI3 was also shown to increase photostability in another study. 38 Gupta et al. showed that pure CsSnBr3 films on TiO2 substrates that were irradiated for 30 min by X-rays in an XPS spectrometer exhibited pronounced elemental Pb peaks; such peaks were completely absent in the CsSnBr3 + SnF2 films under the same conditions.42 This shows that the SnF2 – containing perovskite is much more stable to Pb reduction by X-rays than the pure perovskite, which, among other things, makes characterization involving X-rays more reliable. More recently, we have found that this reduction to Pb metal is apparently not an intrinsic property of the CsSnBr3 itself but is determined by the substrate on which the HaP is deposited. This can be seen from Figure 3, which shows the effects of four different substrates on the Pb reduction by X-rays. For both TiO2 and PEDOT:PSS substrates, there was very clear formation of Sn0 after 30 min., although the amount of Sn0 did not increase much with further irradiation after this. 17 ACS Paragon Plus Environment


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On Au, there was a very small but visible shoulder at the Sn0 position after 5 min. which did not increase more even after a few hours irradiation. Finally, on ZnO, no Sn0 was observed even after 3 h of irradiation. In short, both TiO2 and PEDOT:PSS substrates show similar behavior and lead to some Sn reduction by X-rays, Au also shows some sign of reduction but much less, and with ZnO substrates, there is no sign whatsoever of elemental Sn even after a few hours of irradiation. We do not understand the reason for these differences. It is tempting to attribute them to interfacial effects occurring at the CsSnBr3/substrate interface. However, since the Sn0 is, in the absence of artifacts, measured at the surface of the films, this implies Sn0 migration through the films, which is not expected. There may be morphological explanations, e.g., the Sn0 is formed at the substrate interface but is detected through pinholes/voids in the films.

Figure 3. XPS spectra showing, the Sn 3d doublet, of CsSnBr3 on substrates of (A) PEDOT:PSS; (B) planar TiO2; (C) gold; (D) dense ZnO (Gupta, Cahen and Hodes, unpublished results).


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Nishikuba et al. showed that change in the valence band positions of MASnI3 or FASnI3 upon air exposure was slower if SnF2 was present in the perovskite.50 The stability of (unencapsulated) cells made with CsSnI3 films was relatively good in air with 25% RH (~30% drop in efficiency after 10 h cell operation), but there was no difference between Sn excess and no excess in this respect.42 On the other hand, improvement in the short term (hours) cell stability in an inert atmosphere was found for SnF2containing CsSnBr3 cells.44 Somewhat different, but related, we note that addition of SnF2 to a FASnI3 precursor solution was found to increase the stability of the solution as observed by changes in the solution color with time.41

Effects on energy level positions The work function (WF) of a semiconductor is expected to change upon doping since the position of the Fermi level shifts with respect to the energy band extrema, which, to a first approximation, are not expected to change. However, for CsSnBr3, Gupta et al. reported that the band edges of the CsSnBr3 moved up (towards the vacuum level, i.e., decrease in ionization energy) by ~0.2 eV when SnF2 was added (EVB = -6.3 eV without SnF2; -6.1 eV with SnF2) with no difference in the EVB position for concentrations of SnF2 between 5 and 80%.42 This shift could be beneficial both by promoting electron injection from the perovskite to the TiO2 and by reducing the large offset between perovskite and HTM EVB maxima, as any such offset results in a VOC loss. In contrast, Nishikubo et al. found no change in the valence band positions of MASnI3 or FASnI3 with or without SnF2, although they did report that this position changed more slowly upon air exposure when SnF2 was added (i.e., increased perovskite stability).50 The WF of the SnF2-free CsSnBr3 was only 50 meV below the conduction band minimum (CBM) and 0.15 meV below the CBM with SnF2.42 While it appears strange that the perovskite behaves as strongly n-type, the underlying TiO2 substrate can often dictate the measured work function of low-doped perovskites. As for the band positions, the WF did not vary for the different SnF2 concentrations between 5 and 80% (the range that was measured).



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A pronounced shoulder in the UPS valence band spectrum of FASnI3 was strongly reduced (to a tail) if SnF2 was added.39 It was suggested that this shoulder might arise from interface states. Since the relevant interface was with either the ambient or the vacuum of the UPS spectrometer, this implies surface states. Similar results were found in another study of FASnI3 and also MASnI3, although in the former case, the SnF2 removed altogether the tail/shoulder in the photoemission spectrum.50 Other methods to minimize Sn2+ oxidation It should be noted that, apart from excess Sn2+ (and of course employing an inert atmosphere during the film deposition), other methods have been used to minimize formation of Sn4+. Hydrazine vapor was used as a reducing atmosphere during film deposition of three different Sn halide perovskites – MASnI3, CsSnI3 and CsSnBr3.51 Compared to films made without hydrazine, films made with hydrazine vapor showed a reduction in Sn4+ of nearly 50% for MASnI3 and >20% for the other two perovskites, as measured by XPS. It is also interesting to compare the three perovskites: MASnI3 gave only slight photoactivity in the absence of hydrazine, which increased greatly using hydrazine; CsSnI3 gave better (but still poor) photoactivity which improved markedly when using hydrazine; while CsSnBr3 gave fairly good activity which improved only a little with hydrazine. This should be compared with the study of Sabba et al. on CsSn(I,Br)3, where they noted that addition of Br to initially CsSnI3 improved the cells, even in the absence of any SnF2.37 It seems that the Sn bromide perovskites are relatively less prone to oxidation than the iodide ones. Mancini et al.52 used ‘hypophosphoric’ acid (possibly they meant hypophosphorous acid, H3PO2, which is a known strong reducing agent, generally added to stabilize HI) for MA(Pb,Sn)Br3 cells. However, they did not discuss the role of the acid or compare cells made with and without it.

Recent conceptual advances in pure Sn perovskites 2017 has seen introduction of two conceptually-different and very promising types of Snbased perovskites.


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The first involves partial substitution of the FA cation by phenylethylammonium (PEA).14,17 PEA is a large cation and, by itself, would form a 2D material. When mixed in small amounts with other monovalent cations, the XRD patterns of the perovskites are essentially identical to those of the PEA-free material (except for preferential alignment of the crystals in the films). It seems that the PEA cations leave blocks of the 3D perovskite separated by occasional layers of PEA, hence the terminology of mixed 2D/3D perovskites, introduced as Pb-based HaP PV cells by Smith et al. in 2014.53 Besides reasonable VOC and respectable cell efficiencies, the PEA-containing HaPs have also been shown to improve the stability of the materials and their cells considerably. The second type, which might appear at first sight to be similar to the cells with PEA described above, partially substitutes the FA or MA cation with the dication, ethylene diamine (en).18,19 However, in this case, the pure 3D perovskite is maintained, although the en cation should be too large to fit in the lattice. The authors explain this by loss of SnI2 units, thus forming hollow structures. Additionally, the change in the A cation composition results in a major change in bandgap (e.g. from ~1.3 eV for pure FASnI3 to ~1.66 eV for 1:1 en:FA substitution); normally changes in A cation have only a rather small effect on bandgap. As with the PEA-substituted 2D/3D cells, the stability of the HaPs and their cells is much better than same HaP without en, and the cell efficiencies are respectable, although the VOC values are less good. Summary of some important cell properties contrasting presence and absence of SnF2 (or SnX2) In Table 1, we have collected values of VOC (and efficiency) of reported pure Sn halide perovskite PV cells, separating them into the different perovskite materials. In addition, we mark those cells for which the perovskite was not made with added SnF2 (and separately, deliberate excess of Sn from another source). The reason for this emphasis on VOC and SnF2 in the table can be seen in the first three rows for MASnI3, the most extensivelystudied Sn HaP and the first entry for MASnBr3: All four entries show unusually high values of VOC that have not been equaled since these papers were published (2014) and in all cases, were fabricated without SnF2 (common at that time). These four reports originate from two different groups, which requires them to be taken more seriously than if 21 ACS Paragon Plus Environment


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they all were reported by the same group (although they have apparently not been reproduced, something not uncommon for the early HaP work).

Material

SnF2

VOC (V)

PCE (%)

Year/Ref. No.

CsSnI3 CsSnI3 CsSnI3 CsSnI3 CsSnI31 CsSnI3 MASnI3 MASnI3 MASnI3 MASnI3 MASnI3 MASnI3

No Yes Yes xs SnI2 xs SnCl2 SnF2 + N2H4 No No No No Yes No

0.42 0.24 0.20 0.43 0.42 0.17 0.72 0.68 0.88 0.03 0.32 0.27

0.88 2.0 1.7 2.8 3.2 1.83 5.4 5.2 6.4 0.04 3.2 1.9

2012 / 54 2014 / 36 2015 / 37 2015 / 43 2016 / 44 2017 / 51 2014 / 55 2014 / 56 2014 / 15 2014 / 57 2015 / 58 2016 / 45

MASnI3 MASnI3 MASnI3 MASnI3 MASnI3

No -evap Yes No Yes SnF2 + N2H4

0.49 0.23 0.15 0.23 0.37

1.7 0.3 0.26 2.3 3.89

2016 / 47 2016 / 20 2016 / 23 2017 / 59 2017 / 51

MASnI3 MASnI3 MA0.9Cs0.1SnI3 FASnI3 FASnI3 FASnI3 FASnI3 FASnI32 FA0.8Cs0.2SnI3 FA0.75MA0.25SnI3 FA0.8PEA0.2SnI3 3 FA.92PEA.08SnI33 10%enFASnI3 4 15%enMASnI3 4 CsSnBr3 CsSnBr3 CsSnBr3 CsSnBr3

No Yes No Yes Yes Yes Yes No No Yes Yes Yes Yes Yes Yes Yes Yes-evap SnF2 + N2H4

0.59(5) 0.45 0.20 0.24 0.32 0.38 0.46(5) 0.33 0.23 0.61 0.59 0.52(5) 0.48 0.43 0.41 0.41 0.45 0.36

3.2 2.1 ~0.33 2.1 4.8 5.1 6.1 4.0 ~1.3 8.1 5.9 9.0 7.0 6.6 0.95 2.1 0.5 3.04

2017 / 25 2017 / 60 2016 / 61 2015 / 39 2016 / 40 2016 / 62 2016 / 16 2017 / 63 2016 / 61 2017 / 13 2017 / 17 2017 / 14 2017 / 18 2017 / 19 2015 / 37 2016 / 42 2016 / 49 2017 / 51

MASnBr3 MASnBr3 MASnBr3

No No-evap No

0.88 0.50 0.49

4.27 1.1 0.51

2014 / 56 2016 / 48 2017 / 46


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Table 1. PV parameters of reported Sn halide perovskite-based solar cells. 1. All four Sn dihalides were used; SnCl2 was the best. 2. FAI/PEDOT:PSS composite used with 2-step deposition (2nd step-evaporation of SnI2). 3. PEA = phenylethylammonium. 4. en = ethylenediamine.

This ‘apparent correlation’ between high values of VOC and absence of SnF2 leads one to wonder whether SnF2, while mostly improving cells overall, also has some negative effect that prevents attaining high VOC values. We submit that this is an important topic for future Sn-perovskite studies. At the same time, while there are clearly exceptions, also in the more modern studies, there is a good reason why SnF2 (or at least excess Sn) is commonly used – it does overall lead to better cells; in very many cases (again, not all), very poor performance is obtained if it is not used. Moderately high values of VOC (although still substantially less than the early values) have been only recently reported since 2014: MASnI3 - nearly 0.6 V25 (and notably also did not use SnF2 – see below for more details). All the following examples in this paragraph did use SnF2. FA0.75MA0.25SnI3 – 0.61 V13, FA0.8PEA0.2SnI3 – 0.59 V17 (PEA = phenylethylammonium), and FA.92PEA.08SnI3 – 0.53 V.14 The last three also exhibit relatively high efficiencies (8.1%, 5.9% and 9.0% respectively). The two containing PEA (discussed earlier) are not strictly 3D materials (see below), so are not directly comparable. This leaves the mixed A cation FA0.75MA0.25SnI3 as the sole example of a pure, 3D Sn HaP that used SnF2 yet gave a reasonably high value (0.61 V) of VOC. The example noted above of MASnI3 that gives nearly 0.6 V and without using SnF225 merits some expansion. This paper deals mainly with MA(Pb,Sn)I3 solid solutions, but also describes MASnI3. While made by a solution method, the substrate was initially heated to a high temperature (240 °C) and the films consisted (in general – it is not clear if this is true of the pure Sn HaP) of µm-sized crystals. It may be that these relatively large crystal-size films behaved between smaller-grained films and bulk material. It is also notable that, in the same paper, the composition MASn0.8Pb0.2I3, with a bandgap of 1.2 V, gave a VOC of 658 mV, while MASn0.6Pb0.4I3 with a bandgap of ~1.25 eV gave a VOC of 767 mV meaning that relatively modest amounts of Pb improve the low VOC found 23 ACS Paragon Plus Environment


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with Sn-perovskite cells quite dramatically. Ref. 25 also showed that mixing Sn with Pb increased the Urbach energy - which is very low for pure MAPbI3 (16 meV) and typically double or more for high Sn content halide perovskites (41 meV for pure tin HaP) - and introduces light-absorbing states that do not contribute to the photocurrent generation. Such experiments might be very useful to provide information on the nature of at least shallow defect states in the Sn perovskites.

4. Overall Summary and suggestions for future research SnF2 is most commonly added to Sn-based perovskites to minimize Sn2+ oxidation to Sn4+ and the consequent Sn vacancies, which can form to maintain charge balance. However, the mechanism involved is rarely dealt with. Sn4+, if added during material formation (whether deliberately or unintentionally, e.g. by using partially oxidized precursors) may not behave in the same way as HaP which is oxidized after formation, usually by oxygen. We are not aware of experiments that consider the effect of SnF2 on doping of HaP with deliberately-added Sn4+. The present literature considers the effect of SnF2 on oxidation to occur during or after formation of the HaP. In this paper, besides reviewing the literature, we also consider different doping pathways and show how Sn4+ formation through oxidation by oxygen can form p-type material. Excess of other Sn halides also reduces oxidation (or at least, free hole concentration which is usually taken as a marker for the oxidation), if not always as well as SnF2, and this is usually taken as a measure of reduction of Sn vacancies; it is still not clear whether the excess Sn reduces Sn vacancies simply by making their formation less probable or if the primary effect is to reduce the rate of oxidation and because of this, also reduce Sn vacancy density. Besides the effect of SnF2 on the HaP doping, normally the main reason for use of SnF2, this review also draws attention to the importance of other, non-related (to doping) effects of the SnF2 on the HaP: film morphology, prevention/minimizing formation of unwanted crystal phases, effects on both material and cell stability and on energy levels of the HaPs. Based on the data from the literature, we hypothesize that SnF2, while clearly mostly improving Sn-based cells, may also be detrimental to the VOC in certain cases. 24 ACS Paragon Plus Environment

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For some time, it has been felt in the community that pure Sn perovskites are unlikely to lead to commercial cells, due both to their very poor stability and relatively poor performance. A look at Table 1 and the clear improvements in both performance and stability that have been made in 2017 compared to the previous few years of relative stagnation, should be cause for more optimism. In particular, the HaPs containing some fraction of a large organic A cation, whether as a pure 3D or mixed 2D/3D material, are very encouraging and we expect to see much more effort and progress on these materials.

Acknowledgements We thank Marcus Bär and Regan Wilks of the Helmholz Center, Berlin, for their critical reading of parts of this manuscript. This research work was supported by the Israeli Ministry of National Infrastructures, Energy and Water Resources as part of an ERA-net project.

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TOC

morphology

ASnX3 + SnF 2

crystal phase oxidation/doping stability energy band position

ASnX3


How SnF2 Impacts the Material Properties of Lead-Free Tin Perovskites

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