AUTORIZACIÓN, CONSULTA Y REPORTE A CENTRALES DE INFORMACIÓN

Figure 3.4 draws together the ideal fundamental properties and the considerations for materials synthesis and device development which have been highlighted as important for good device performance in previous sections.1 _{Finding new materials with each property and optimising all}

the processing parameters is not simple. As a result, to date, the best performing bismuth-based device is ~30% of the efficiency of the best perovskite device.76,89,153 _{However, many advances}

in materials and device processing of bismuth-based materials have been made in recent years, providing an optimistic outlook for this group of compounds. The recent experimental advances of the most promising bismuth compounds, most of which have s contribution in their valence band, will be discussed in the following sections.

Figure 3.4 The fundamental properties, synthesis and device development considerations which

need to be optimum for an efficient photovoltaic device. Figure adapted from reference [1] (with permission under Creative Commons Attribution CC BY license).

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3.3.1 Methylammonium bismuth iodide and the double perovskites

The first compounds discussed are based around replication of the perovskite crystal structure. Initial approaches towards this involved direct substitution of Pb2+ _{with Bi}3+_{. However, the}

difference in valence of the metal ions resulted in the formation of the strongly excitonic, one- dimensional (CH3NH3)3Bi2I9, for which an exciton binding energy of 300 meV was

measured.95,154,155 _{These factors would likely limit carrier transport and lifetime properties.}

Regarding processing, compact morphologies were initially only achieved using two-step processes via a BiI3 intermediate, involving at least one vacuum processing step.52,156,157 This is

due to the poor solubility of bismuth salts in organic solvents causing fast crystallisation,1,158

making direct deposition of (CH3NH3)3Bi2I9 from solution challenging.159,160 Using a two-step

vacuum process the best efficiency to date for (CH3NH3)3Bi2I9 was achieved, at 1.64%,157 as a

result of compact films with grain hundreds of nanometres in size, whilst Brandt et al. have recorded an 8.1 ns minority carrier lifetime.4 _{Despite these efforts, device J}

SC and VOCs remain

severely limited,157 _{likely due to the large exciton binding energy.}1 _{However, it should be noted}

that Shin et al. have recently used a solvent engineering approach to overcome solubility problems in a one-step synthesis route, using additives to form BiI3-Lewis acid adducts in solution resulting

in slower crystallisation.158 _{This could be explored and adapted for other bismuth compounds.}1

The second family of bismuth compounds inspired by the perovskite structure is the double perovskites. Here Pb2+ _{is substituted with M}+ _{and M}3+_{, e.g. Ag}+ _{and Bi}3+_{, creating a three-}

dimensional network of alternating metal halide octahedra.1,161,162 _{This overcomes the problems}

associated with the one-dimensionality of (CH3NH3)3Bi2I9, resulting in good dispersion of the

conduction and valence bands and hole effective masses down to 0.14 (for Cs2AgBiBr6 cf. 0.18 –

0.4 for MAPbI3).1,162–164 Other examples synthesised include Cs2AgBiCl6, (CH3NH3)2AgBiBr6

and (CH3NH3)2KBiCl6.161,162,165 For a Cs2AgBiBr6 single crystal, Slavney et al. reported a

photoluminescence lifetime of 667 ns, which approaches the microsecond lifetimes measured for the lead halide perovskites.166,167 _{Hoye et al. have since proposed that the PL emission occurs via}

a defect state due to a red-shift in emission wavelength relative to the bandgap, and, using transient absorption spectroscopy, reported a lifetime > 1 μs.47 _{Despite these excellent carrier properties,}

the double perovskites mentioned all have band-gaps ≥ 2 eV,1 _{even wider than the optimum for}

tandem devices on silicon,32 _{whilst the metal reactants suffer from the same solubility problems}

as bismuth salts.168 _{As a result, few reports of a double perovskite thin films exist.}1,47,168 _{Of these,}

a champion device efficiency of 2.4% was reported for a (Cs2AgBiBr6) device fabricated by Greul

et al., using TiO2 and spiro-OMeTAD as the CTMs.168 Regarding the bandgap, thallium alloying

successfully reduced the band-gap of Cs2AgBiBr6 from ~ 2.2 eV to 1.4 eV, but the toxicity of

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Whilst the double perovskites have carrier lifetime properties comparable to the lead halide perovskites they are fundamentally limited by their band-gap energies (Table 3.1) and processing issues.1 _{Therefore, doping and solvent engineering routes need investigating. Aside}

from compounds inspired by the perovskite structure, advances have also been made in bismuth compounds of differing structure but, as discussed, with s orbital contribution to the valence band.

Table 3.1 The bandgap energy, best device efficiency and device architecture of the best devices

of methylammonium lead iodide, methylammonium bismuth iodide and a range of double perovskite compounds.

Material Bandgap / eV Best PCE / % Device architecture

MAPbI3 1.55[111] 21.6 [153] FTO|La-BaSnO3| MAPbI3|PTAA|Au (CH3NH3)3Bi2I9 2.9 [a,163] 1.64 [157] FTO|c-TiO2|m- TiO2|(CH3NH3)3Bi2I9| spiro- OMeTAD|Ag Cs2AgBiBr6 2.19 [162] 2.43 [168]

FTO|c-TiO2|m-TiO2|Cs2AgBiBr6|

spiro-OMeTAD|Au

Cs2AgBiCl6 2.77 [162] - -

(CH3NH3)2AgBiBr6 2.02 [165] - -

(CH3NH3)2KBiCl6 3.04 [161] - -

FTO = fluorine-doped tin oxide, PTAA = Poly[bis(4-phenyl)(2,4,6trimethylphenyl)amine, c-TiO2

= compact TiO2, m-TiO2 = mesoporous TiO2, spiro-OMeTAD = 2,2',7,7'-Tetrakis[N,N-di(4-

methoxy phenyl)amino]-9,9'-spirobifluorene. a _{Bandgap measured taking into account the}

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3.3.2 Other bismuth compounds based on replicating the defect tolerance of

methylammonium lead iodide

BiI3, an intermediate in (CH3NH3)3Bi2I9 synthesis, possesses the optimum bandgap for a tandem

device with a Si bottom cell at 1.7 eV. A photoluminescence lifetime of 7.3 ns has been measured for a film deposited by physical vapour transport at 77% of its melting point.51 _{However, bismuth}

vacancies are predicted to have deep defect transition level ~ 0.7 eV from the conduction band edge with a formation energy of ~ 0.75 eV under iodine rich conditions.4 _{Therefore, despite s}

orbital contribution to the valence band, resulting in antibonding character at the band-edge,51 _it

does not display the same tolerance to defects as the lead halide perovskites.1 _{Additionally, device}

performance has been limited due to poor film morphology which introduces shunt pathways into devices.1,51 _{As a result, the champion device (using TiO}

2 as ETM and V2O5 as HTM) reached only

1%,171 _{and the best open-circuit voltage values have reached only ~ 25% of the bandgap energy.}172

A family of materials with a more suitable bandgap for a single junction solar cell are the ternary alkali ternary alkali chalcogenides (MBiCh2 where M = Li, Na, K etc and Ch = S, Se).

Whilst there are few experimental reports on these materials, Rosales et al. recently reported the synthesis of rock-salt NaBiS2 and NaBiSe2 nanocrystals from solution.173 Band-gap energies of

1.01 – 1.24 eV and 1.1 eV were measured respectively, as well as α values of 104 _{– 10}6 _cm-1 _over

the solar spectrum. Further characterisation and processing into devices are needed to evaluate the potential of these materials for PV applications. However, substituting the alkali metal with group 11 Ag yields AgBiS2, which has achieved the highest efficiency for a solar cells using a

bismuth compound (champion device efficiency of 6.3%, Table 3.2).76 _{In this case, nanocrystals}

synthesised by the hot-injection method were spin cast and treated with tetramethylammonium iodide to replace insulating capping ligands. Whilst nanocrystals were only ~ 10 nm in size, they formed a compact layer.76 _{Devices also benefitted from the good spectral overlap of AgBiS}

2 (Eg

= 1.3 eV), yield JSC values ~18 mAcm-2. However open-circuit voltage values only reached 500

meV regardless of the HTM valence band energy, indicating the presence of deep trap states limiting the voltage due to high levels of SRH recombination.76 _{Defect formation energy}

calculations would be useful to confirm this, and determine processing temperatures at which deep trap states could be avoided.

The last group of materials which will be discussed are the bismuth chalcoiodides, BiChI (where Ch = S, Se, or O), of which BiOI will be investigated in this thesis and will be discussed in section 3.4. BiSI and BiSeI crystallise into 1-dimensional ribbon-like structures, proposed to be beneficial for carrier transport between electrodes in devices if the 1-dimensional back bone is situated perpendicular to the charge transport materials.174,175 _{However, BiSI forms needle-like}

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direction.1,176,177 _{For example, devices incorporating spray pyrolysed BiSI with CuSCN as the}

HTM were only 0.01% efficient, likely due to large pin-holes visible in the SEM. Devices also suffered from poor matching of the valence band of CuSCN (-5.3 eV)116 _{with the deep lying}

valence band of BiSI (-6.4 eV).174 _{It should be noted that this is a major cause of voltage loss with}

many of the bismuth-based absorbers, which tend to have valence band maxima at ~ -5.9 to -6.3 eV, whilst common HTMs have their valence band/highest occupied molecular orbital at -5.2 to -5.4 eV.1 _{An improvement in PCE to 0.25% was made using a liquid electrolyte (NaI + I}

2 in

acetonitrile), however this introduced stability issues.176 _{BiSeI is yet to be made into devices,}

however its bandgap has an improved overlap with the solar spectrum (<1.37 eV for BiSeI vs. 1.59 eV for BiSI).177 _{The third of the series, BiOI, has shown the best performance in photovoltaic}

devices,178 _{forming motivation for this thesis, and will be discussed in more depth in the following}

chapters.

Table 3.2 Bandgap energy, best device efficiency and the device architecture of the best

performing device for the bismuth containing compounds discussed in section 3.3.2.

Compound Bandgap / eV

Highest device

efficiency / % Device architecture Ref

BiI3 1.8 1.0 FTO|TiO2|BiI3|ZnO|Al [171]

NaBiS2 1.0 – 1.2 - - [173]

NaBiSe2 1.1 - - [173]

AgBiS2 0.9 6.3 ITO|ZnO|AgBiS2|PTB7|MoO3|Ag [76]

BiOI 1.9 1.0 FTO|TiO2|BiOI| I2/I3-|Pt [178]

BiSI 1.59 0.25 FTO|BiSI|NaI/I2 in acetonitrile|Pt [176]

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