ABSTRACT
BiFeO3 thin films have been widely studied for photoelectrochemical water splitting applications because of its narrow bandgap and good ferroelectricity which can promote the separation of photo-generated charges. Bismuth is well known as a volatile element and excess bismuth is usually added into the precursor to compensate the loss of bismuth during heat treatment, but the amount of excess bismuth required and how excess bismuth will affect PEC performance have not been clearly studied. Herein, self-doped Bi1+x FeO3 thin films are prepared via simple chemical solution deposition method with excess bismuth from 0-30% in the precursor. The loss of bismuth after annealing is confirmed by EDX and XPS. Multiple factors were investigated and it was found that non-stoichiometric Bi resulted in changes of structure, morphology, defects, electronic properties and PEC performance. An enhanced photocurrent is observed in bismuth-rich BiFeO3 films, which can be ascribed to the larger grain size, decreased oxygen vacancies, lattice distortion and supported charge separation. Moreover, the photocathodic performance can be further enhance by ferroelectric poling. Our work indicates that deficient bismuth should be carefully avoided during heat treatment and moreover, a slight excess of Bi is beneficial for PEC performance. Therefore, we offer a simple way to enhance PEC performance of BiFeO3-based ferroelectric materials through careful control of their stoichiometry.
ABSTRACT
Reliable fabrication of large-area perovskite films with antisolvent-free printing techniques requires high-volatility solvents, such as 2-methoxyethanol (2ME), to formulate precursor inks. However, the fabrication of high-quality cesium-formamidinium (Cs-FA) perovskites has been hampered using volatile solvents due to their poor coordination with the perovskite precursors. Here, this issue is resolved by re-formulating a 2ME-based Cs0.05FA0.95PbI3 ink using pre-synthesized single crystals as the precursor instead of the conventional mixture of raw powders. The key to obtaining high-quality Cs-FA films lies in the removal of colloidal particles from the ink and hence the suppression of colloid-induced heterogeneous nucleation, which kinetically facilitates the growth of as-formed crystals toward larger grains and improved film crystallinity. Employing the precursor-engineered volatile ink in the vacuum-free, fully printing processing of solar cells (with carbon electrode), a power conversion efficiency (PCE) of 19.3%, a T80 (80% of initial PCE) of 1000 h in ISOS-L-2I (85 °C/1 Sun) aging test and a substantially reduced bill of materials are obtained. The reliable coating methodology ultimately enables the fabrication of carbon-electrode mini solar modules with a stabilized PCE of 16.2% (average 15.6%) representing the record value among the fully printed counterparts and a key milestone toward meeting the objectives for a scalable photovoltaic technology.
ABSTRACT
Antisolvent-assisted spin coating has been widely used for fabricating metal halide perovskite films with smooth and compact morphology. However, localized nanoscale inhomogeneities exist in these films owing to rapid crystallization, undermining their overall optoelectronic performance. Here, we show that by relaxing the requirement for film smoothness, outstanding film quality can be obtained simply through a post-annealing grain growth process without passivation agents. The morphological changes, driven by a vaporized methylammonium chloride (MACl)-dimethylformamide (DMF) solution, lead to comprehensive defect elimination. Our nanoscale characterization visualizes the local defective clusters in the as-deposited film and their elimination following treatment, which couples with the observation of emissive grain boundaries and excellent inter- and intragrain optoelectronic uniformity in the polycrystalline film. Overcoming these performance-limiting inhomogeneities results in the enhancement of the photoresponse to low-light (<0.1 mW cm-2) illumination by up to 40-fold, yielding high-performance photodiodes with superior low-light detection.