Zumaira
Siddique
abc,
Julia L.
Payne
*c,
Muhammad Tariq
Sajjad
de,
Natalie
Mica
d,
David B.
Cordes
c,
Alexandra M. Z.
Slawin
c,
Ifor D. W.
Samuel
d,
Azhar
Iqbal
*a and
John T. S.
Irvine
*c
aDepartment of Chemistry, Quaid-i-Azam University, Islamabad, 45320, Pakistan. E-mail: aiqbal@qau.edu.pk
bDepartment of Chemistry, Government College University, Jhang Road, Faisalabad, 38000, Pakistan
cEaStCHEM School of Chemistry, University of St Andrews, North Haugh, St Andrews, KY16 9ST, UK. E-mail: jlp8@st-andrews.ac.uk; jtsi@st-andrews.ac.uk
dOrganic Semiconductor Centre, SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, KY16 9SS, UK
eLondon Centre for Energy Engineering, School of Engineering, London South Bank University, 103 Borough Road, London, SE1 0AA, UK
First published on 9th December 2022
Toxicity and regulatory concerns over the use of (CH3NH3)PbI3 in photovoltaic devices have resulted in significant interest in lead-free, organic–inorganic metal halides with excellent light absorbing properties and stability. Here we report the synthesis of three new lead-free bismuth halides which accommodate the symmetric conjugated p-phenylenediammonium cation (PPD = (H3NC6H4NH3)2+) in the structures. These are (PPD)BiI5, (PPD)[BiBr4]2·2H2O and (PPD)2BiCl7·H2O. We also synthesized β-(PPD)2Bi2I10. The band gap of the iodide, β-(PPD)2Bi2I10 (1.83 eV) is lower than that of the bromide, (PPD)[BiBr4]2·2H2O (2.64 eV) and chloride, (PPD)2BiCl7·H2O (2.93 eV). Photophysical studies show that β-(PPD)2Bi2I10 has the longest average charge carrier lifetime (>1 μs) of the materials studied here, of the same order of magnitude as that of (CH3NH3)PbI3 and has a low band gap. This suggests that β-(PPD)2Bi2I10 may be a promising candidate for use in lead-free photovoltaic devices.
Other metal cations such as tin(II), germanium(II) and bismuth(III) have been explored as alternatives to lead in halide perovskites.9–11 Bismuth(III) based hybrid perovskites have been widely used as a result of their lower toxicity, high absorption coefficients, iso-electronicity to Pb2+ (6s2, 6p0) and low oxidation in solution.12 Solution processable hybrid bismuth(III) based halides can form a range of different structure types, where bismuth halide octahedra may exist in the form of isolated zero dimensional octahedra,13,14 one dimensional interconnected chains of octahedra,15 two dimensional layers of octahedra or chains that extend in two directions,16–19 and three dimensional networks of octahedra.20 The connectivity of these octahedra could be through face, edge or corner sharing.21 Due to the greater choice and flexibility in organic cations, unique structural and optoelectronic properties can be linked with 1D, 2D and quasi-2D layered materials.22 So, by choosing versatile fabrication methodologies, exploring new materials and testing different device architectures, these materials should be capable of being fabricated at the industrial scale with the improved stability and performance that are required for commercialization.23
There are several examples of phenylenediammonium metal halides reported in the literature.24–27 In addition, organic cations that are closely related to phenylenediammonium have also been used in the preparation of organic–inorganic metal halides.28 However, many of these studies focus on the crystallography rather than the properties of these materials. The structure of p-phenylenediammonium (PPD) bismuth iodides (and a corresponding hydrate) and a p-phenylenediammonium bismuth chloride have previously been reported.24–27 PPDBi2I8·I2 consists of chains of edge sharing BiI6 octahedra and also contains molecules of I2, which form a pseudo-3-dimensional structure due to the short intermolecular I–I distances between iodine in the I2 molecules and the BiI6 octahedra.27 PPDBi2I8·I2 is stable up to temperatures of 100 °C and has a low band gap of 1.45 eV, although further properties of the compound are currently unexplored.27 PPD2Bi2I10 exists in two polymorphs, α and β, with the α–β phase transition occurring at −26 to −28 °C.24 Both α and β-PPD2Bi2I10 consist of isolated, edge sharing Bi2I10 dimers. In addition, the reported structure of the hydrate, PPD2Bi2I10·4H2O is very similar to the published β-PPD2Bi2I10 and again consists of Bi2I10 dimers.24,25 There is also one report of a p-phenylenediammonium bismuth chloride, PPD2BiCl7·H2O, in the literature which consists of isolated BiCl6 octahedra.26 A peak in differential scanning calorimetry data along with a change in the dielectric constant of PPD2BiCl7·H2O, both at 90 °C, provide evidence for a phase transition.26 It is thought that this phase transition relates to an order–disorder transition, probably driven by the reorganization of water or PPD molecules.26 Despite the improvement in optoelectronic properties by lowering non-radiative recombination in low dimensional materials, and enhanced moisture stability in organic metal halides containing aromatic amines, few studies have explored the use of symmetric conjugated amines, which are more compact and form rigid structural configurations with metal halides.29 Therefore, the aim of this work was to probe the synthesis, structural characterization and optoelectronic properties of p-phenylenediammonium bismuth halide materials. Compounds of this type may also have possible applications in ferroelectrics, lasers, light emitting diodes (LEDs), solar cells or super lattice heterojunction devices.30–32
The p-phenylenediamine dihydriodide salt (PPD·2HI) was prepared by the stoichiometric reaction of p-phenylenediamine with hydriodic acid. The p-phenylenediamine was dissolved in in 20 mL ethanol and HI was added dropwise to the solution in a round bottom flask at 0 °C to adjust the pH to around 6–7. The mixture was stirred for two hours, then the solvent was removed using a rotary evaporator at 80 °C. The crude yellowish-white powder was recrystallised three times from ethanol and diethyl ether. Afterwards the white product was filtered and dried at 70 °C in vacuum oven overnight. The equivalent procedure was followed for the synthesis of p-phenylenediamine dihydrobromide (PPD·2HBr) and p-phenylenediamine dihydrochloride salts (PPD·2HCl) by using HBr and HCl precursors instead of HI.
To prepare phase pure crystalline β-(PPD)2Bi2I10, stirring was discontinued after 20 minutes and the solution was cooled down to room temperature over 48 hours, which allowed the crystals to grow. After that, the crystals were filtered and dried at 70 °C in a vacuum oven for 24 hours. The atomic% (found/calculated) of Bi (17.76/16.67) and I (82.24/83.33) from EDX analysis of crystals confirmed the Bi:
I ratio in this sample. This compound matches the previously described compound β-PPD2Bi2I10.24
Images of the samples of β-(PPD)2Bi2I10, (PPD)[BiBr4]2·2H2O and (PPD)2BiCl7·H2O are shown in Fig. S1 (ESI†).
(PPD)BiI5 | (PPD)[BiBr4]2·2H2O | (PPD)2BiCl7·H2O | |
---|---|---|---|
CCDC code | 2180531 | 2180532 | 2180533 |
Formula weight | 953.64 | 601.72 | 695.46 |
Crystal description | Red chip | Yellow prism | Colourless prism |
Crystal size (mm3) | 0.05 × 0.03 × 0.02 | 0.09 × 0.06 × 0.03 | 0.15 × 0.13 × 0.03 |
Crystal system | Monoclinic | Monoclinic | Triclinic |
Space group | P21 | P21/c |
P![]() |
a (Å) | 10.0586(4) | 7.2340(17) | 9.505(2) |
b (Å) | 8.4358(3) | 12.090(3) | 10.376(2) |
c (Å) | 10.8613(5) | 12.753(3) | 13.322(3) |
α (°) | 99.260(4) | ||
β (°) | 107.582(4) | 98.210(6) | 110.582(4) |
γ (°) | 105.1820(10) | ||
Volume (Å3) | 878.55(6) | 1103.9(5) | 1139.6(4) |
Z | 2 | 4 | 2 |
ρ (calc, g cm−3) | 3.605 | 3.620 | 2.027 |
μ (mm−1) | 18.796 | 30.397 | 8.566 |
F(000) | 816 | 1052 | 664 |
Reflections collected | 11![]() |
13![]() |
4063 |
Independent reflections (Rint) | 3734 (0.0285) | 2018 (0.0770) | 4063 (0.046) |
Parameters, restraints | 145, 10 | 106, 28 | 269, 82 |
Goodness-of-fit on F2 | 1.015 | 0.968 | 1.180 |
R 1 | 0.0235 | 0.0394 | 0.0413 |
R 1 [I > 2sigma(I)] | 0.0204 | 0.0320 | 0.0402 |
wR2 | 0.0363 | 0.0757 | 0.1307 |
wR2 [I > 2sigma(I)] | 0.0357 | 0.0738 | 0.1300 |
Absolute structure parameter | −0.026(3) | ||
Largest diff. peak and hole (e Å−3) | 0.672 and −0.883 | 1.680 and −1.815 | 2.968 and −1.945 |
SEM images and EDS (Fig. S7, ESI†) were taken on a Jeol 6700 F microscope. FTIR spectra were obtained using an IR Infinity spectrometer in the range of 400–4000 cm−1.
![]() | ||
Fig. 1 Structure of (PPD)BiI5 obtained from single crystal XRD showing the view in the bc plane, indicating corner-sharing BiI6 octahedra which run along the b-axis. |
(PPD)BiI5 | (PPD)[BiBr4]2·2H2O | (PPD)2BiCl7·H2O | β-(PPD)2Bi2I10 | |
---|---|---|---|---|
Bi–I (Å) | Bi–Br (Å) | Bi–Cl (Å) | Bi–I (Å) | |
Bi–X (Å) | 2.9055(6) | 2.6619(11) | 2.567(3) | 2.923(3) |
Bi–X (Å) | 2.9658(6) | 2.6667(10) | 2.569(3) | 2.951(3) |
Bi–X (Å) | 2.9691(5) | 2.8474(10) | 2.662(3) | 3.064(2) |
Bi–X (Å) | 3.1554(5) | 2.8832(10) | 2.747(3) | 3.1013(19) |
Bi–X (Å) | 3.2627(6) | 3.1228(10) | 2.852(3) | 3.246(3) |
Bi–X (Å) | 3.3504(6) | 3.1251(11) | 2.941(3) | 3.271(3) |
Δd (×10−4) | 28.62 | 42.67 | 26.19 | 18.31 |
σ 2 | 16.72 | 12.21 | 19.52 | 1.65 |
The PPD bismuth iodide system is particularly interesting as a number of different structures have been reported. A summary of structural data from the Cambridge Structural Database40 are included in Table S1 (ESI†).24,25,27 Shestimerova et al. prepared (PPD)(BiI4)2·I2, which contains zig-zag, one-dimensional chains of edge sharing BiI6 octahedra running along the a-axis.27 The shortest inter-chain I–I distance is 3.837(1) Å along the c-axis and 3.862(1) Å along the b-axis. As suggested in the formula (PPD)(BiI4)2·I2, the material contains discrete iodine molecules, with an I–I distance of 2.7286(9) Å, which is typical of that found in molecular iodine (2.722(3) Å) at similar temperatures.27,41 Hrizi et al. have studied α- and β-(PPD)2Bi2I10 and a corresponding hydrate.24,25 These structures both contain isolated dimers of edge sharing bismuth iodide octahedra, forming Bi2I10 units. Our new material, (PPD)BiI5 is therefore the first PPD bismuth iodide to contain corner-sharing BiI6 octahedra. The I–Bi–I angles in (PPD)BiI5 vary further from 90° than other reported PPD bismuth iodides (see Table S1, ESI†). It is also interesting to note that in (PPD)BiI5, the Bi–Bi distances of 6.3718(4) Å are much longer than Bi–Bi distances in the other PPD bismuth iodides, which range from 4.5122(14) Å to 4.770(5) Å.24,25,27 Interestingly, the Bi–Bi distance in PPDBiI5 is comparable to the Pb–Pb distances in (CH3NH3)PbI3, which has a Pb–Pb distance of 6.306 Å in the cubic Pmm phase at 350 K.42
The preliminary PXRD pattern of the sample of (PPD)BiI5 which the single-crystal studied had come from indicated that three phases were present (Fig. S3, ESI†). Optimization of the synthetic procedure resulted in isolation of the second phase present as a pure phase: the PXRD pattern is shown in Fig. 2. The PXRD data can be indexed to a monoclinic cell of a = 11.488(3) Å, b = 12.9039(13) Å, c = 15.026(6) Å, β = 112.520(9)°, space group P21/n. This is in agreement with the published structural model of β-(PPD)2Bi2I10. The resulting Pawley fit to the PXRD data is shown in Fig. 2. The structure of β-(PPD)2Bi2I10 is shown in Fig. S4 (ESI†) and consists of isolated dimers of edge-sharing BiI6 octahedra, forming Bi2I10 units and therefore can be described as a zero dimensional perovskite.24 This is structurally very different to (PPD)BiI5, which consists of chains of corner-sharing octahedra forming one-dimensional chains. Attempts to prepare (PPD)BiI5, in a pure form were unsuccessful, so further studies focused solely on β-(PPD)2Bi2I10.
It was previously shown that the different compounds β-(PPD)2Bi2I10 and its hydrate (PPD)2Bi2I10·4H2O are structurally very similar, showing a small difference in unit cell parameters, which makes it difficult to distinguish between the two forms using powder X-ray diffraction.24,25 In order to investigate the possible incorporation of water into β-(PPD)2Bi2I10 sample, a combined TGA-DTA experiment was carried out (Fig. 3). A mass loss of 0.4% occurred between 50 and 60 °C and is accompanied by an exothermic peak in the DTA. This mass loss is smaller than the 3.6% expected for complete loss of all four water molecules in (PPD)2Bi2I10·4H2O, and is equivalent to the presence of 0.4 H2O per (PPD)2Bi2I10 formula unit. This provides preliminary evidence for the accommodation of a variable water content (i.e. a region of solid solution) in (PPD)2Bi2I10·xH2O, where based on literature and our current work, x can range from 0 to 4.
![]() | ||
Fig. 3 Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) of β-(PPD)2Bi2I10 carried out under an air atmosphere, black line represents TGA and red line represents the DTA. |
The structure of (PPD)[BiBr4]2·2H2O was determined from single crystal XRD data and is shown in Fig. 4. It crystallises in the space group P21/c and consists of one-dimensional chains of edge sharing BiBr6 octahedra, which run along the a-axis in a zig-zag motif. The PPD cation is positioned in the spaces between chains of BiBr6 octahedra. The asymmetric unit comprises of one bismuth atom, four crystallographically independent Br atoms, half a PPD cation and a molecule of water. The Bi–Br bond lengths range between 2.6619(11) Å and 3.1251(11) Å (Table 2), with Br–Bi–Br bond angles ranging from 83.86(3)° to 95.82(4)°. Bismuth atoms are linked alternately through Br2 and Br3 atoms; each pair of bringing Br atoms binding asymmetrically, showing one longer and one shorter bond [2.8474(10) and 3.1251(11), and 2.8832(10) and 3.1228(10) for Br2 and Br3, respectively], with Bi–Br–Bi angles of 92.85(3)° and 92.70(3)°, for Br2 and Br3, indicating some distortion of the bismuth octahedra. The inter-chain Br–Br distance is 3.8173(15) Å. Each potential hydrogen bond donor, both ammonium and water, forms a single hydrogen bond, two N–H⋯Br, one N–H⋯O and two O–H⋯Br. These show N–H⋯Br distances of 2.582(17) and 2.69(5) Å [N⋯Br separations of 3.518(8) and 3.564(8) Å], an N–H⋯O distance of 1.73(19) Å [N⋯O separation of 2.713(10) Å], and O–H⋯Br distances of 2.49(2) and 2.61(5) Å [O⋯Br separations of 3.451(8) to 3.508(8) Å]. The hydrogen bonds link the one-dimensional chains into a three-dimensional array (Fig. S5, ESI†).
![]() | ||
Fig. 4 Structure of (PPD)[BiBr4]2·2H2O obtained from single crystal XRD (a) ac plane (showing only chains of BiBr6 octahedra without the PPD cation for clarity) and (b) bc plane. |
The powder X-ray diffraction pattern of (PPD)[BiBr4]2·2H2O is shown in Fig. S6 (ESI†) and could be indexed to the same monoclinic cell as the single crystal XRD data. The resulting Pawley fit is shown in Fig. S6 (ESI†). A very small percentage of a secondary phase, attributed to the bromide analogue of β-(PPD)2Bi2I10, was present in the powder X-ray diffraction pattern. This will be discussed in a future publication.
The structure of (PPD)2BiCl7·H2O was determined from single crystal XRD data and is shown in Fig. 5. It crystallizes in the space group P and contains isolated BiCl6 octahedra. This is in contrast to the other PPD bismuth halides synthesized in this study, which showed either dimers or chains of octahedra. The closest inter-octahedra Cl–Cl distance is 4.836(4) Å. The asymmetric unit comprises one bismuth atom, seven crystallographically independent Cl atoms, (one not coordinated), one whole and two half crystallographically independent PPD cations and a molecule of water. The Bi–Cl bond lengths range from 2.567(3) to 2.941(3) Å (Table 2), with Cl–Bi–Cl bond angles ranging from 84.35(8) to 94.29(9)°. The Bi–Cl distances cluster with three shorter and three longer. Each potential ammonium hydrogen-bond donor, forms at least one hydrogen bond; all bar one to chloride, while the water forms three hydrogen bonds to chlorides. These show N–H⋯Cl distances ranging from 2.20(2) to 2.84(9) Å [N⋯Cl separations of 3.133(10) to 3.317(9) Å], an N–H⋯O distance of 1.84(4) Å [N⋯O separation of 2.786(16) Å], and O–H⋯Cl distances of 2.55(8) to 2.81(10) Å [O⋯Cl separations of 3.278(10) to 3.428(10) Å]. The hydrogen bonds link the BiCl6 octahedra into a three-dimensional array.
Fig. 6 shows a comparison of the Bi–X octahedra in the three materials and the corresponding bond lengths are summarized in Table 2.
It is interesting to note that a polymorph of (PPD)2BiCl7·H2O has been reported previously by Hrizi et al. although the synthetic conditions are different to those used here and the resulting structure is different to the (PPD)2BiCl7·H2O that we report here, with an approximate doubling of the b-axis.26 Attempts to fit our PXRD data with the model and unit cell parameters previously reported by Hrizi et al.26 were unsuccessful, using both Rietveld and Pawley refinement. However, our PXRD data could be fitted with the unit cell parameters that we obtained from single crystal X-ray diffraction (see Fig. S7, ESI†), indicating the difference in the unit cell parameters and structures of the two materials. PXRD data were collected after 8 months, to judge the long-term stability of the samples and are shown in Fig. S8 (ESI†). The SEM images of the samples synthesized are shown in Fig. S9 (ESI†). A detailed description of crystallographic parameters of the p-phenylenediammonium bismuth halides (PPD)BiI5, (PPD)[BiBr4]2·2H2O and (PPD)2BiCl7·H2O are given in Table 2 and Table S2 (ESI†). The FTIR spectra are shown in Fig. S10 and Table S3 (ESI†) summarizes the characteristic vibrational frequencies of these materials.
Compound | Band gap (eV) | Synthetic technique utilised | Ref. |
---|---|---|---|
Cs2AgBiBr6 | 2.02 | Top-seeded solution growth (TSSG) method | 49 |
Cs2AgBiBr6 | 2.19 | Solid state grinding and solution precipitation | 50 |
Cs2AgBiCl6 | 2.77 | Solid state grinding and solution precipitation | 50 |
(CH3NH3)2AgBiBr6 | 2.02 | Hydrothermal method | 51 |
(CH3NH3)2KBiCl6 | 3.04 | Hydrothermal method | 52 |
(CH3NH3)2TlBiBr6 | 2.16 | Hydrothermal method | 20 |
(TMP)[BiI5] | 2.02 | Solvothermal method | 53 |
(TMP)[BiBr5] | 2.67 | Solvothermal method | 53 |
(TMP)[BiCl5] | 3.21 | Solvothermal method | 53 |
(TMP)1.5[Bi2I7Cl2] | 2.10 | Solvothermal method | 53 |
(C6H13N)2BiI5 | 2.02 | Slow evaporation method | 54 |
(CH3NH3)3Bi2I9 | 2.06 | Solution grown powder method | 55 |
(CH3NH3)3Bi2I9 | 2.19 | Thin film in mixed solvents | 14 |
(CH3NH3)3Bi2I9 | 2.10 | Thin film formation in mixed solvents | 56 |
Cs3Bi2I9 | 2.20 | Thin film formation in mixed solvents | 56 |
AgBi2I7 | 1.87 | Solution processed thin film method | 57 |
[H3N(CH2)6NH3]BiI5 | 1.89 | Slow cooling solution method | 58 |
Cs2NaBiI6 | 2.43 | First principle study | 59 |
(CH3NH3)3Bi2Br9 | 2.5 | Collaborative solvent ligand-assisted re-precipitation method | 60 |
(C3H5N2S)BiI4 | 1.78 | Solution growth using solvent layering approach | 61 |
KBiI4·H2O | 1.76 | Slow evaporation method | 62 |
1,1-(1,n-Alkanediyl)bis(4-methylpyridinium)BiI5/H3C(C5H4N)(CH2)5(C5H4N)CH3 | 1.73 | Slow evaporation from solution | 63 |
(C17H18N4)BiI5 | 1.59 | Solution growth | 64 |
(AmV)BiI5, (AmV2+ = 4,4′-amino-bipyridinium) | 1.54 | Slow evaporation | 65 |
[(Me2DABCO)2(Bi2I10)2], [DABCO = 1,4-diazabicyclo[2.2.2]octane] | 1.44 | Solvothermal with slow cooling | 66 |
The room temperature steady state photoluminescence (PL) spectra of thin films of β-(PPD)2Bi2I10, (PPD)[BiBr4]2·2H2O and (PPD)2BiCl7·2H2O are shown in Fig. 7(a–c). The PL of β-(PPD)2Bi2I10 shows a broad emission peak at 703 nm with a full width half maximum (FWHM) of 119 nm and a separation between the longest wavelength absorption peak and the emission maximum of 164 nm. Compound (PPD)[BiBr4]2·2H2O shows a broad PL emission around 477 nm with FWHM of 146 nm and a shift of 51 nm between absorption and emission. The PL of (PPD)2BiCl7·H2O has a peak around 443 nm with a FWHM of 130 nm and has a peak around 443 nm. The shift between absorption and emission maxima is 91 nm. Table 4 represents the characteristic parameters of β-(PPD)2Bi2I10, (PPD)[BiBr4]2·2H2O and (PPD)2BiCl7·2H2O deduced from UV-vis and SSPL. According to Karunadasa et al. the broad emission is attributed to emission from defect sites which lead to the self-trapping of excitons and the distortion of the crystal structure.67 This distortion in the crystal structure has a strong correlation with the broadband light emission.68,69 There are three aspects of the crystal structure which have been attributed to the broad emission spectra. First, the chloride ions in these materials cause more octahedral distortion due to their small size compared to the other halide anions. Second, the rigid PPD cation also enhances octahedral distortion and contributes to the width of the PL peaks. Third, more hydrogen bonds form between the octahedra and diammonium cations than for monoammonium cations in perovskite structures enables the stronger interaction, which is also responsible for self-trapping of excitons for broad emission in perovskite materials.70
Compound | Absorption cutoff (nm) | Absorption peak (nm) | Band gap (eV) | PL peak (nm) | FWHM (nm) | Stokes shift (nm) | Stokes shift (eV) |
---|---|---|---|---|---|---|---|
β-(PPD)2Bi2I10 | 680 | 539 | 1.83 | 703 | 119 | 164 | 0.54 |
(PPD)[BiBr4]2·2H2O | 480 | 426 | 2.64 | 477 | 146 | 51 | 0.31 |
(PPD)2BiCl7·H2O | 395 | 352 | 2.93 | 443 | 130 | 91 | 0.72 |
A long exciton lifetime is desirable for efficient charge transfer and extraction. It is dependent on a number of parameters, but in general single crystals or films with a large grain size exhibit the highest charge carrier lifetimes. Materials which have optimum band gaps and long charge carrier lifetimes can result in photovoltaic devices with higher efficiency. However, the presence of defects or impurities quenches the light emission and is a major non-radiative loss pathway in perovskites. These defects are mostly due to surface halide vacancies.71 It has been shown that oxygen passivation can suppress these trap states efficiently.72,73 We have investigated oxygen passivation by spin-coating the films in the presence and absence of oxygen, i.e. outside and inside a nitrogen-filled glovebox. The time-resolved PL decays of spin-coated films of β-(PPD)2Bi2I10 with and without oxygen passivation are shown in Fig. 8(a). We found that the charge carrier lifetime of the film spin-coated in air is significantly enhanced compared to the film spin-coated inside the glovebox. Oxygen molecules are believed to bind strongly to surface halide vacancies and form superoxide in the presence of photo-excited electrons, which then leads to efficient passivation of these vacancy sites.74
![]() | ||
Fig. 8 Time-resolved PL decays of bismuth containing perovskite thin films, following excitation at 379 nm by a picosecond pulsed laser. (a) Effect of oxygen passivation on β-(PPD)2Bi2I10. The red curve (“with passivation”) is a decay of a film sample spin-coated in air. The black curve (“without passivation”) is a decay of a film a sample spin-coated in a nitrogen glove box. (b) PL decays of oxygen-passivated films of (PPD)2BiCl7·H2O (blue), (PPD)[BiBr4]2·2H2O (olive) and β-(PPD)2Bi2I10 (red) Black lines are three-exponential fit to the experimental data using eqn (1). (c) PL decay of β-(PPD)2Bi2I10 measured up to 3 μs. Black line is three-exponential fit. |
PL decays of passivated films of the other materials are shown in Fig. 8b. For comparison, the PL decay of passivated film of β-(PPD)2Bi2I10 is also plotted in Fig. 8b. The PL of compounds (PPD)[BiBr4]2·2H2O and (PPD)2BiCl7·H2O decayed completely within 200 ns, whereas β-(PPD)2Bi2I10 showed a much slower PL decay after fast initial decay. In order to get information about different processes, the PL decays of compounds (PPD)[BiBr4]2·2H2O and (PPD)2BiCl7·H2O were fitted with eqn (1) and the fitting parameters are given in Table 5.
R(t) = A1e(−t/τ1) + A2e(−t/τ2) + A3e(−t/τ3) | (1) |
![]() | (2) |
Compound | τ 1 | τ 2 | τ 3 |
---|---|---|---|
β-(PPD)2Bi2I10 | 8.2 ns (11.3%) | 123.6 ns (10.0%) | 1260.4 ns (78.7%) |
(PPD)[BiBr4]2·2H2O | 0.7 ns (65.7%) | 4.8 ns (21.2%) | 130.3 ns (13.1%) |
(PPD)2BiCl7·H2O | 0.9 ns (57.1%) | 8.8 ns (32.5%) | 46.5 ns (10.4%) |
To facilitate an approximate comparison with prior work we calculated the average PL lifetime using the intensity weighted average of the fitted exponentials by using eqn (3) and the calculated average lifetimes are given in Table 5.
![]() | (3) |
Compound | τ average | Techniques | Ref. |
---|---|---|---|
(CH3NH3)PbI3 | 1500 ns | TR microwave conductance | 78 |
(CH3NH3)PbI3 | 6.6 ns | TRPL-FTO/TiO2/perovskite | 79 |
(CH3NH3)PbI3 | 390 ns | Transient photovoltage method | 80 |
(CH3NH3)Pb(I0.71Br0.29)3 | 560 ns | Transient photovoltage method | 80 |
(CH3NH3)Pb(I0.46Br0.54)3 | 1070 ns | Transient photovoltage method | 80 |
(CH3NH3)PbI3 | 208 ns | TRPL-vapor-equilibrated method | 81 |
(CH3NH3)PbClxI3−x | 113.9 ns | TRPL-vapor-equilibrated method | 81 |
CsPbBr3 | 5.6 ns | TRPL-thiocyanate treatment | 82 |
CsPbBr3 | 16.8 ns | TRPL-SiO2 core shell | 83 |
CsPbBr3 | 3.9 ns | TRPL-pristine | 84 |
CsPbBr3 | 6.5 ns | TRPL-in situ passivation | 84 |
(CH5N2)PbBr3 | 180.3 ns | TRPL-rubidium doping | 85 |
(CH3NH3)PbBr3 | 10.1 ns | TRPL-with benzoquinone | 86 |
(CH3NH3)PbBr3 | 7.07 ns | TRPL | 87 |
Cs4PbBr6 | 1.9 ps | Femtosecond TA | 88 |
(PEA)2PbI4[(CH3NH3)PbI3]n−1 (1, 2, 3) | ∼150 ps | TRPL | 89 |
Cs2InCl5·H2O | 4.2 μs | TRPL | 90 |
(BPMA)2PbBr4 | 4.73 ms | Transient fluorescence | 91 |
(C4N2H14)PbBr4 | 201 ns | Pressure-induced PL | 92 |
(C4H14N2)2In2Br10 | 3.2 μs | TRPL | 93 |
(BA)2(CH3NH3)4Pb5I16−10xBr10x | 3.47 ns | TRPL | 94 |
(p-FC6H4C2H4NH3)2[PbI4] | 63 ns | TRPL | 95 |
(DMEN)PbBr4 | 0.75 μs | TRPL | 96 |
β-(PPD)2Bi2I10 | 1245 ns | TRPL | This work |
(PPD)[BiBr4]2·2H2O | 120 ns | TRPL | This work |
(PPD)2BiCl7·H2O | 31 ns | TRPL | This work |
Footnote |
† Electronic supplementary information (ESI) available. CCDC 2180531–2180533. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2tc02601e |
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