Cesium power: low Cs⁺levels impart stability to perovskite solar cells

Abstract

Towards increasing the stability of perovskite solar cells, the addition of Cs is found to be a rational approach. Recently triple cation based perovskite solar cells were found to be more effective in terms of stability and efficiency. Heretofore they were unexplored, so we probed the Cs/MA/FA (cesium/methyl ammonium/formamidinium) cation based perovskites by X-ray photoelectron spectroscopy (XPS) and correlated their compositional features with their solar cell performances. The Cs content was found to be optimum at 5%, when incorporated in the (MAFA)Pb(IBr) lattice, because the corresponding device yielded the highest fill factor compared to the perovskite without Cs and with 10% Cs. XPS studies distinctly reveal how Cs aids in maintaining the expected stoichiometric ratios of I : Pb, I : N and Br : Pb in the perovskites, and how the valence band (VB) edge is dependent on the Cs proportion, which in turn governs the open circuit voltage. Even at a low content of 5%, Cs resides deep within the absorber layer, and ensures minimum distortion of the VB level (compared to 0% and 10% Cs perovskites) upon Ar sputtering, thus allowing the formation of a stable robust material that delivers excellent solar cell response. This study which brings out the role of Cs is anticipated to be of paramount significance to further engineer the composition and improve device performances.

Investigation of NiOx-hole transport layers in triple cation perovskite solar cells

“…Compared to PEDOT:PSS (valence band of − 5.2 eV), the valence band (VB) of NiO x (− 5.49 eV) is well aligned to the VB of the mixed cation perovskite (− 5.50 eV). NiO x exhibits a high conduction band (CB) of − 1.85 eV whereas the CB of the perovskite is only − 3.88 eV, which makes NiO x a good electron blocking layer [6,9,26,31]. PC 60 BM (CB: − 4.2 eV) acts as electron transport layer in this device configuration [14,32].…”

“…Up to now, Cs-containing mixed ion perovskite solar cells were mostly fabricated in n-i-p device structures on mesoporous TiO 2 [4,6]. In our study, this efficient perovskite absorber is investigated in the planar p-i-n structure adopted from organic solar cells, which already gave promising PCE values with MAPbI 3 absorber layers ranging from 10% to above 18% [8,17,19,25,27,30].…”

“…This A-site cation combination results in the suppression of the photovoltaic non-active "yellow phase" and enhances perovskite crystallinity. However, so far this route is mainly investigated in n-i-p based perovskite solar cells [4][5][6]. In these structures, TiO 2 is used as ETL in most cases in combination with HTLs such as spiro-OMeTAD or other organic HTLs [7].…”

Recent Progress on the Long‐Term Stability of Perovskite Solar Cells

“…When the FAPbI 3 and FA 0.9 Cs 0.1 PbI 3 were exposed to continuous illumination (100 mA cm −2 ), the degree of degradation was severer for FAPbI 3 film (85.9%) than for FA 0.9 Cs 0.1 PbI 3 film (65.0%) after 19 h 40. The stability of FA 0.9 Cs 0.1 PbI 3 film was much better than the pristine FAPbI 3 film under 85% relative humidity at 25 °C in dark condition, due to more stabilized FA ions.…”

Suppressing X‐Migrations and Enhancing the Phase Stability of Cubic FAPbX3 (X = Br, I)

“…The orbitals of the countercations do not significantly contribute to the frontier bands, but influence the perovskite lattice type, lattice constants, and structural stability. Other experimental approaches to short-term stabilization of α-FAPbI 3 (besides rapid thermal quenching) include doping of the material with MA, [22][23][24][25] Cs, [26,27] and both MA and Cs, [28][29][30] but not with organic cations that bond stronger to the inorganic skeleton than FA. The higher the degree of covalent conjugation, the lower is the bandgap, the larger are radii of the excitons, the lower are the exciton binding energies, and, finally, the more efficient is the charge carrier transport in the material.…”