Improved fill factor in inverted planar perovskite solar cells with zirconium acetate as the hole-and-ion-blocking layer

Abstract

Planar perovskite solar cells (PSCs) have gained great interest due to their low-temperature solution preparation and simple process. In inverted planar PSCs, an additional buffer layer is usually needed on the top of the PCBM electron-transport layer (ETL) to enhance the device performance. In this work, we used a new buffer layer, zirconium acetate (Zr(Ac)). The inclusion of the Zr(Ac) buffer layer leads to the increase of FF from ∼68% to ∼79% and PCE from ∼14% to ∼17% in the planar PSCs. The UPS measurement indicates that the Zr(Ac) layer has a low HOMO level of -8.2 eV, indicating that the buffer layer can act as a hole-blocking layer. Surface morphology and surface chemistry investigations reveal that the elements I, MA and Pb can diffuse across the PCBM ETL, damaging the device performance. The covering Zr(Ac) molecules fill in the pinholes of the PCBM layer and effectively block the ions/molecules of the perovskite from diffusion across the ETL. The resulting more robust PCBM/Zr(Ac) ETL leads to weaker ionic charge accumulation and lower diode leakage current. The double role of hole-and-ion blocking of the Zr(Ac) layer explains the improved FF and PCE in the PSCs.

Inverted perovskite solar cells with enhanced lifetime and thermal stability enabled by a metallic tantalum disulfide buffer layer

“…Other buffer layers, such as AZO, 85,86 TPBi, 87 metal(acac) x , 88,89 Zr(Ac) 4 (ref. 90 ) have been successfully implemented in inverted PSCs between the PCBM and Ag layers. However, most of these studies focused on the PCE enhancement, without deeply discussing their effect on the device lifetime stability following an ISOS protocol.…”

Effect of Crystal Grain Orientation on the Rate of Ionic Transport in Perovskite Polycrystalline Thin Films

“…A specific problem related to degradation is the migration of ions. , Whereas the remarkable defect tolerance of perovskite materials (most defects only form shallow traps) enables highly efficient devices, the low formation energies of MA- and I-related defects (vacancies and interstitials) − within the perovskite crystal lattice give rise to significant ionic movement and in turn current–voltage ( I – V ) hysteresis and device decay. ,− It has been proposed that ion migration can be suppressed by engineering the electric field distribution in the device by using undoped charge blocking layers . Experimentally, several methods have been utilized in order to minimize the effect of ion migration, such as the improvement of perovskite crystallinity, , the addition of codoping cations, , and the inclusion of ion blocking layers in the device stack. − However, identifying the most effective solution to this problem requires the ability to precisely monitor ionic motion within the perovskite layer. Some techniques that have been used to accomplish this include X-ray photoemission spectroscopy (XPS), , secondary ion mass spectroscopy, , conductive atomic force microscopy (AFM), and kelvin probe force microscopy − and especially photoluminescence (PL) spectroscopy. ,− …”