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    Topic review

    Antimony/Bismuth Chalcohalides

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    Submitted by: Yong Chan Choi

    Definition

    Despite their comparable performance to commercial solar systems, lead-based perovskite (Pb-perovskite) solar cells exhibit limitations including Pb toxicity and instability for industrial applications. To address these issues, two types of Pb-free materials have been proposed as alternatives to Pb-perovskite: perovskite-based and non-perovskite-based materials. In this entry, we briefly introduce the crystal, energy band structures and theoretical insights of Sb/Bi chalcohalides as solar abosrbers.

    1. Introduction

    Since the Snaith group reported the 10.9% milestone power conversion efficiency (PCE) required for industrial applications for lead-based perovskite (Pb-perovskite) solar cells in 2012 [1], many types of Pb-perovskite solar cells have been fabricated, with performance significantly improving over the past few years [2][3][4][5][6][7]. At present, the certified PCE exceeds 25% [8], approaching the theoretical maximum efficiency for multi-junction Pb-perovskite solar cells [9]. This efficiency is also comparable to those of commercial solar systems based on Si, CdTe, and Cu(In,Ga)Se2. Moreover, high-performance Pb-perovskite solar cells can be manufactured through solution processing at a low temperature of <150 °C, which can reduce costs. Therefore, these characteristics make them the most promising alternative to current photovoltaic systems. However, Pb-perovskite solar cells exhibit limitations for commercialization, with potential health problems and stability being the two main barriers [6][7][10][11][12][13][14][15]. In particular, Pb can be easily released from the Pb-perovskite because of its instability, which can cause major health problems [10][11][12][13][14][15]. Although techniques of material and interface engineering, surface passivation, and encapsulation can significantly improve the stability of Pb-perovskite [7][15][16][17], thereby minimizing the Pb loss, the persistent toxicity problem requires attention to enhance commercialization.

    To address these issues, many researchers have focused on finding Pb-free and stable materials with comparable optoelectronic properties. The Pb-free photovoltaic materials proposed as alternatives to date are presented in Table 1. Replacing Pb by tin (Sn) or germanium (Ge), with similar ionic radius and belonging to the same group of the periodic table, in Pb-perovskites is a simple method for fabricating Pb-free materials while maintaining the perovskite structure. These materials are known as Pb-free perovskites. In particular, Sn-based perovskites ASnX3 (A = Cs+, organic cations; X = Cl, I, and Br) exhibit properties comparable to those of Pb-perovskites such as optimal band gaps (Egs) of 1.1–1.4 eV, high carrier mobilities, long carrier lifetimes, and long diffusion lengths [10][11]. Thus, many researchers have devoted attention to developing Sn-based perovskite solar cells [10][11]. Consequently, a record PCE of 11.4% was achieved through the FASnI3 (FA = CH5N2+) solar cell by introducing a phenylhydrazine hydrochloride [18]. However, Sn-perovskites still involve the serious disadvantage of rapid decomposition because Sn is readily oxidized from the +2 to +4 state on exposure to air [10][11]. Another approach for fabricating Pb-free perovskites involves replacing two Pb2+ ions with ions of two metals with oxidation states of +1 and +3 to form double perovskites represented as A2MIMIIIX6 [12][19], with the Cs2AgBiBr6 as a typical example. Alternatively, the two Pb2+ ions are replaced by a tetravalent metal ion, such as Sn4+ or Ti4+, forming compounds with the general formula A2MIVX6 [12][20]. Such compounds are termed vacancy-ordered double perovskites, with the Cs2SnI6 as a prime example. In addition, two-dimensional (2D) perovskites  are produced by replacing the Pb2+ ions with trivalent metal ions such as Sb3+ or Bi3+ [15][21]. Although these Pb-free double and 2D perovskites display significant stability improvement over Pb- and Sn-perovskites, efficiency remains a limitation.

    Table 1. Types of Pb-free photovoltaic materials and their best photovoltaic performance data.

     

    Metal (M) Ions

    Chemical Compound

    Record Device Performance

    PCE

    Material

    Ref.

    Perovskites

    Sn2+, Ge2+

    Perovskite/AMX3

    11.4%

    FASnI3

    [18]

    Ag+, Bi3+

    Double perovskite/A2MIMIIIX6

    2.84%

    Cs2AgBiBr6

    [19]

    Sn4+

    Vacancy-ordered double perovskite/A2MIVX6

    3.28%

    Cs2TiBr6

    [20]

    Sb3+, Bi3+

    2D perovskite/

    3.34%

    MA3Sb2I9−xClx

    [21]

    Sb/Bi-based non-perovskites

    Sb3+

    Sb chalcogenides/M2Ch3, CuMCh2

    10.5%

    Sb2(S,Se)3

    [22]

    Sb3+, Bi3+

    Ternary chalcohalides/MChX, M13Ch18X2

    4.07%

    Sb0.67Bi0.33SI

    [23]

    Sn2+, Pb2+, Sb3+, Bi3+

    Quaternary chalcohalides/

    4.04%

    Sn2SbS2I3

    [24]

    PCE— power conversion efficiency.

    Apart from these Pb-free perovskites, antimony/bismuth (Sb/Bi)-based non-perovskites are another alternative to Pb-perovskites. Unlike perovskites, most of these non-perovskites crystallize in a layered structure, with the layers linked by weak van der Waals forces. This anisotropic crystal structure provides unique and interesting properties that can significantly affect photovoltaic performance [25][26][27]. To date, many Sb/Bi non-perovskites for solar cells have been reported, and these comprise two types, according to elemental composition. The first type is the Sb chalcogenides involving an orthorhombic structure, such as the Sb2Ch3 and CuSbCh2 (Ch = S, Se). In fact, studies on these as photovoltaic materials predates those of Pb-perovskites because of their promising properties, such as the tunable Eg values of 1.0–1.8 eV, high visible light absorption coefficient, stability, low toxicity, and earth-abundant constituents [25][26]. Although varied engineering methods and device architectures have been employed to achieve high-efficiency for Sb chalcogenide solar cells, the performances of these cells remained below the 10% milestone until 2018 [26][27][28][29][30][31][32][33][34][35]. However, recently, a PCE of 9.2% was obtained from the [001]-oriented Sb2Se3 nanorod solar cells [36], and finally, a PCE of 10.5% was reported by Chen’s group from the hydrothermally deposited Sb2(S,Se)3 thin film solar cells [22][37].

    Sb/Bi chalcohalides represent the other type of Sb/Bi non-perovskites, comprising Sb/Bi-based semiconductors containing halides and chalcogenides. Following the initial application of Sb sulfoiodide (SbSI) in solar cells by the Seok group in 2018 [38], multiple Sb/Bi chalcohalide solar cells have been proposed. Thus far, the materials investigated for use in solar cells include ternary (MChX and M13Ch18X2, where M = Sb, Bi) [23][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53] and quaternary chalcohalides (, where MII = Sn, Pb; MIII = Sb, Bi) [23][54]. These chalcohalides commonly exhibit advantageous properties that can be adjusted for use in solar cells. In particular, the electronic structure of the most studied MChX family is similar to that of Pb-perovskites, with beneficial properties for solar cells such as high dielectric constant, low effective mass, and tunable Eg [39][42][43][44][55]. Therefore, high-performance MChX solar cells comparable to Pb-perovskite cells are expected due to these properties. Recently, the Seok group reported a PCE of 4.07% for Sb0.67Bi0.33SI solar cells, highlighting the high-efficiency potential for the MChX family [23]. In addition, the MChX family is suitable for other applications including the fabrication of room-temperature radiation detectors and p-type transparent conductors [39]. This wide-ranging applicability facilitates designing multifunctional devices. In addition to the MChX family, PCEs of 0.85% and 4.04% have been reported for solar cells based on M13Ch18X2 and , respectively. However, the highest PCE achieved for Sb/Bi chalcohalide solar cells remains at around 4%, although the performance has significantly improved over the past few years.

    2. Crystal and Energy Band Structures of Sb/Bi Chalcohalides

    In this section, the crystal and energy band structures of Sb/Bi chalcohalides used to date for solar cells are briefly presented. Depending on the number of elements and composition, Sb/Bi chalcohalides with different structures can be created, as shown in Table 2. The ternary chalcohalides employed for solar cells are the MChX and M13Ch18X2 types. The MChX type, such as SbSI and BiSI, involves the orthorhombic structure with the Pnma space group, crystallizing into an [(MChX)2]n double-chained structure, with the adjacent chains joined by van der Waals forces [57][58]. Conversely, the M13Ch18X2 type such as the Bi3S18I2 possesses a hexagonal structure with a ribbon-shaped (M4Ch6) subunit. The M4Ch6 subunits form six spokes around the central hexagonal channel at the corners of the unit cell, with iodine in between [53][58]. For the quaternary chalcohalides (M2IIMIIICh2X3), such as Pb2SbS2I3 and Sn2SbS2I3, crystallization produces the orthorhombic structure with the Cmcm space group [24][54][59].

    Table 2. Summarized data for the structural properties of Sb/Bi chalcohalides used for solar cells.

     

    Chemical Formula

    Structure/Space Group

    Typical Materials

    Ref.

    Ternary chalcohalides

    MChX

    Orthorhombic/Pnma

    SbSI, BiSI

    [23][38][39][40][41][42][43][44][45][46][47][49][58]

    M13Ch18X2

    Hexagonal/P63

    Bi13S18I2

    [53][58]

    Quaternary chalcohalide

    M2IIMIIICh2X3

    Orthorhombic/Cmcm

    Pb2SbS2I3,

    Sn2SbS2I3

    [24][54][59]

    To employ Sb/Bi chalcohalides in solar cells, the energy band structure deserves priority because of its importance in light harvesting and conversion. Specifically, the Eg should be checked because it determines the maximum PCE achievable for each material according to the Shockley–Queisser limit [60][61]. Thus, materials with an Eg value between 1.10 and 1.55 eV are preferred for solar cells. Figure 1 displays the energy band diagram of typical Sb/Bi chalcohalides reported to date. The positions of the conduction band minimum and valence band maximum as well as the Eg value vary depending on the elemental composition and number of elements. Along with the chalcohalides shown in Figure 1, Sb/Bi chalcohalides exhibit Eg values varying from 0.75 eV for Bi13S18I2 [53] to 2.31 eV for SbSBr [42]. These results indicate that their band structures can be tuned via chemical substitution, and that the electron transporting layer (ETL) and hole transporting layer (HTL) applications necessitate selectivity for each solar cell depending on the chalcohalide used. In addition to the band structures, other factors such as the optical absorption strength, charge effective mass, dielectric constant, and defects require consideration [44][61]. However, research on these remains insufficient, and this highlights the need for further studies.

    Figure 1. Energy band diagram of typical Sb/Bi chalcohalides. The SbSI, Sb0.67Bi0.33SI, BiSI, Pb2SbS2I3, and Sn2SbS2I3 energy levels were obtained from [23][38][47][54] and [24], respectively. For comparison, the energy levels for typical conducting oxides (F-doped SnO2 (FTO) and In-doped SnO2 (ITO)), the electron transporting layer (ETL), and hole transporting layer (HTL) are included. P3HT, PCPDTBT, and F8 denote poly(3-hexylthiophene), poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b’]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)], and poly(9,9-di-n-octylfluorenyl-2,7-diyl), respectively.

    3. Theoretical Insights on Sb/Bi Chalcohalides as Solar Absorbers

    Theoretical calculations, such as first-principle methods, provide further insight into the potential of specific materials (e.g., as solar absorbers) and clues for designing device structures. However, the research on such theoretical investigations is very limited because Sb/Bi chalcohalide solar cells are still in their early stages of development compared to the Pb-perovskite cells. Thus, in this section, theoretical insights into only the most studied MChX family are briefly introduced.

    Based on the first-principle calculations, Brandt et al. identified the MChX family as promising solar absorbers due to its low effective masses, large dielectric constants, and strong absorption, as shown in Table 3 [62]. They further found that BiSI and BiSeI are most suitable for achieving high-performance solar cells because of their much stronger spin-orbit coupling. The suitability of these Bi compounds for solar cells was also confirmed by other groups [39][43][44][63]. Ganose et al. suggested that the conducting oxide and HTL should be selected for efficient charge transfers by considering the electron affinity (EA = 4.9–5.0 eV) and ionized potential (IP = 6.2–6.4 eV) of these Bi chalcohalides, respectively [43]. They also concluded from the defect analysis that these Bi compounds represent intrinsic semiconductors regardless of fabrication conditions, making them best suited for application in p-i-n device architecture [44].

    Table 3. Summary of effective masses of hole (mh*) and electron (me*), static dielectric constant, and absorption coefficient of MChX family, calculated by different methods.

    MChX Compounds

    mh*

    me*

    Static Dielectric Constant

    Absorption Coefficient 1

    References

    Pb-perovskite 2

    0.10

    0.16

    20.07

    >1 × 105 cm−1

    [61][62]

    BiSI

    0.61–4.79

    0.53–2.33

    14.26–71.32

    >1 × 105 cm−1

    [39][44][62][63]

    BiSeI

    0.81–5.89

    0.25–1.61

    14.78–62.82

    >1 × 105 cm−1

    [39][44][62][63]

    SbSI

    0.27–2.06

    0.21–1.25

    10.56–69.38

    -

    [42][55][62][63][64]

    SbSeI

    0.57–4.37

    0.35–1.83

    14.70–57.18

    -

    [42][55][62][63][64]

    SbSBr

    0.24–3.55

    0.51, 0.52

    13.81–105.15

    -

    [42][55][62][63][64]

    1 Absorption coefficient values at visible region are presented. 2 Data of (CH3NH3)PbI3 are shown as typical of Pb-perovskites for comparison.

    Butler et al. analyzed the band structures of SbChX (SbSI, SbSeI, and SbSBr) by different calculation methods [42][55]. The effective masses were calculated to be below 0.65, indicating that SbChX have high charge carrier mobilities suitable for solar cells. They also found that the SbSBr have deeper IP energy (5.8 eV) than that of I-containing SbChX (5.3 eV for SbSeI and 5.4 eV for SbSI). This different IP energy suggests that contacting layers such as ETL and HTL should be selected depending on the halide ion of SbChX for optimal device performance [42]. For example, the contacting layers used in Cu2ZnSnS4 (CZTS) can be applied to SbSBr solar cells due to their similar IP value with that of CZTS. In addition, a heterojunction structure composed of SbSI/SbSBr with SbSBr epitaxially grown on SbSI was proposed for efficient charge separation based on their closely matched lattice parameters and band offsets [55].

    The entry is from 10.3390/nano10112284

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