Understanding the Performance-Limiting Factors of Cs₂AgBiBr₆ Double-Perovskite Solar Cells

Abstract

Cs 2 AgBiBr 6 thin film preparation for characterization. The double-perovskite thin films studied in this work were all prepared through sequential vapour deposition. In a vacuumsealed chamber, AgBr (99% Fluka), BiBr 3 (≥98% Sigma Aldrich) and CsBr (99.9% Sigma Aldrich) were placed in separate crucibles and sequentially thermally evaporated onto the substrates. In particular, the standard procedure we optimized evaporated 90 nm of AgBr, 120 nm of BiBr 3 and 150 nm of CsBr to obtain 300 nm of Cs 2 AgBiBr 6 . This basic stack was repeated the necessary number of times to achieve the desired total film thickness. To achieve thicknesses that are not multiples of 300 nm (like the 750 nm reported in the text), we ran the last evaporation cycle depositing half of each precursor thickness, keeping always the same precursors ratio (1:1.3:1.6 AgBr:BiBr 3 :CsBr). After the deposition of the desired thickness, we annealed the samples on a hotplate in air at 250 ºC for 30 minutes. The post-deposition annealing temperature and time were optimised to deliver maximum solar cell performance.Solar cell preparation. FTO or ITO coated glasses were cleaned by sequential sonication in soap, water, acetone and isopropanol. After being dried with a N 2 gun, the substrates were further cleaned by O 2 plasma for 10 minutes. Titanium isopropoxide (140 µl in 1 ml of EtOH) was added to 1 ml of acidic EtOH (10 µl of HCl 2M in 1 ml EtOH), and deposited on the FTO substrates by spincoating at 2000 rpm for 45 sec with 2000 rpm/sec acceleration. Following this, the films were annealed at 150°C for 15 min and 500°C for 30 min. SnO 2 layers were prepared by spincoating at 3000 rpm (200 rpm/sec) for 30 sec of a solution of SnCl 4 ⋅5H 2 O in isopropanol (17.5 mg/ml) on top of the FTO or ITO coated glasses. The so-prepared films were annealed at 100°C for 10 min followed by an annealing at 180°C for 30 min. The SnO 2 and TiO 2 films were placed in the vacuum chamber, and the Cs 2 AgBiBr 6 film was deposited as previously presented. The hole transport material (Spiro-OMeTAD, Lumtec) was dissolved in chlorobenzene (85 mg/ml) and doped with 20 µl of LiTFSI (500 mg/ml in BuOH) and 30 µl of tert-butylpyridine. The solution was then deposited on the active layer by spincoating in air at 2000 rpm (2000 rpm/sec) for 45 sec. The devices were then left overnight in a desiccator in air atmosphere, and then completed by the evaporation of 100 nm silver contacts. All the

Highly Absorbing Lead-Free Semiconductor Cu2AgBiI6 for Photovoltaic Applications from the Quaternary CuI–AgI–BiI3 Phase Space

“…Therefore, we have stabilized a close-packed iodide framework with three metal species that together give high absorption, and both the band gap and exciton binding energies are lower than those in a comparable bromide (Cs 2 AgBiBr 6 , Tauc plot E g ≈ 2.2 eV, E B ≈ 220 meV). 40 , 51 Both the strong absorption properties and low exciton binding energy are very encouraging for the potential use of Cu 2 AgBiI 6 as a solar absorber, in comparison to the previously reported double perovskites. We note that, although the band gap of the continuum of states at 2.06 eV appears to be quite large for PV applications, there exists considerable absorption at lower energies due to the excitonic states.…”

“…This is compared to the reported absorption coefficients of MAPbI 3 (blue dotted line) and Cs 2 AgBiBr 6 (blue dashed line), reproduced from Davies et al and Longo et al, respectively. 40 − 42 Also shown is the photoluminescence (PL) spectrum of Cu 2 AgBiI 6 . (b) Elliott model fitting of the absorption coefficient spectrum, giving a band gap of 2.06(1) eV and an exciton binding energy of 25(2) meV.…”

Tuning the Structural and Optoelectronic Properties of Cs2AgBiBr6 Double‐Perovskite Single Crystals through Alkali‐Metal Substitution

“…The zero‐frequency, long‐range mobility for the thin film is calculated to be μ thin‐film ≈30 cm 2 V −1 s −1 , given the value for the averaged Drude‐Smith c ‐parameter of −0.70 (μ thin‐film = μ (1 + c )). In addition, we applied a complementary approach [ 34,35 ] to estimate the lower limit for the mobility. This results in μ thin‐film > 2.6 cm 2 V −1 s −1 , consistent with previous reports [ 34,36,37 ] and mobility values inferred from our fittings.…”

“…In addition, we applied a complementary approach [ 34,35 ] to estimate the lower limit for the mobility. This results in μ thin‐film > 2.6 cm 2 V −1 s −1 , consistent with previous reports [ 34,36,37 ] and mobility values inferred from our fittings. The materials’ photoconductivity, σ = N c eμ where N c is carrier density, is shown in Figure S6d, Supporting Information for all samples.…”

Lead‐Free Halide Double Perovskite Nanocrystals for Light‐Emitting Applications: Strategies for Boosting Efficiency and Stability

“…The colloidal HDP nanocrystals have made great progress in preparation technology and performance modulation, showing rich and tunable optoelectronic properties, robust stability, low exciton binding energies, and high absorption efficiency. Lead‐free HDP nanocrystals have been documented wide application prospects in solar cells, [ 53–57 ] LEDs, [ 58 ] photocatalysis, [ 59,60 ] photodetectors, [ 61 ] and many other optoelectronic fields. [ 62,63 ] Although Sn 4+ ‐based, [ 64–67 ] Zr 4+ ‐based, [ 68 ] Na + /Bi 3+ ‐based, [ 69,70 ] Na + /In 3+ ‐based, [ 71 ] K + /In 3+ ‐based, [ 71 ] Ag + /In 3+ ‐based, [ 72–77 ] and Mn 2+ /Bi 3+ ‐based, [ 78,79 ] lead‐free HDP bulk materials with highly efficient emission ( Table 1 ) have been designed and synthesized in recent years, the photoluminescent (PL) properties of colloidal HDP nanocrystals still remain enormous challenge for application in LED devices compared with lead halide perovskite nanocrystals.…”