Compositional engineering of a mixed cation/mixed halide perovskite in the form of (FAPbI)(MAPbBr) is one of the most effective strategies to obtain record-efficiency perovskite solar cells. However, the perovskite self-organization upon crystallization and the final elemental distribution, which are paramount for device optimization, are still poorly understood. Here we map the nanoscale charge carrier and elemental distribution of mixed perovskite films yielding 20% efficient devices. Combining a novel in-house-developed high-resolution helium ion microscope coupled with a secondary ion mass spectrometer (HIM-SIMS) with Kelvin probe force microscopy (KPFM), we demonstrate that part of the mixed perovskite film intrinsically segregates into iodide-rich perovskite nanodomains on a length scale of up to a few hundred nanometers. Thus, the homogeneity of the film is disrupted, leading to a variation in the optical properties at the micrometer scale. Our results provide unprecedented understanding of the nanoscale perovskite composition.
Interest
in delafossite (CuFeO2) as a candidate p-type
photocathode for photoelectrochemical (PEC) solar fuel production
has recently been increasing, mainly due to its excellent stability
in aqueous environments and favorable light absorption properties.
However, its PEC performance has remained poor for reasons that have
not yet been specifically determined. Herein, we report a detailed
investigation on sol–gel-processed CuFeO2 with a
range of spectroscopic, PEC, and microscopy techniques aimed at unraveling
the material properties governing photogenerated charge carrier harvesting
in this v. An analysis of the bulk transport properties using microwave
conductivity measurements reveals a good charge carrier mobility (0.2
cm2 V–1 s–1) and a
relatively long lifetime (200 ns) for photogenerated charge carriers.
Conversely, systematic PEC measurements with varied redox systems
reveal the existence of a high density of surface states (1014 cm–2) positioned 0.35 eV above the conduction
band, inducing Fermi level pinning at the semiconductor–liquid
junction. X-ray photoelectron spectroscopy suggests the presence of
a thin layer of metal hydroxide at the surface of the material. These
surface states were found to behave as electron traps, correlated
with an inversion of polarity at the surface of the semiconductor,
and thereby promoting charge recombination and limiting the photovoltage
developed at the junction. These findings suggest that if the detrimental
effects of the surface states can be eliminated, CuFeO2 would provide a sufficiently high photovoltage to be combined with
other solution-processed and stable photoanodes into an easily scalable
tandem PEC cell.
The search for ideal semiconductors for photoelectrochemical solar fuel conversion has recently recognized the spinel ferrites as promising candidates due to their optoelectronic tunability together with superb chemical stability.
The development of solution‐processable routes to prepare efficient photoelectrodes for water splitting is highly desirable to reduce manufacturing costs. Recently, sulfide chalcopyrites (Cu(In,Ga)S2) have attracted attention as photocathodes for hydrogen evolution owing to their outstanding optoelectronic properties and their band gap—wider than their selenide counterparts—which can potentially increase the attainable photovoltage. A straightforward and all‐solution‐processable approach for the fabrication of highly efficient photocathodes based on Cu(In,Ga)S2 is reported for the first time. It is demonstrated that semiconductor nanocrystals can be successfully employed as building blocks to prepare phase‐pure microcrystalline thin films by incorporating different additives (Sb, Bi, Mg) that promote the coalescence of the nanocrystals during annealing. Importantly, the grain size is directly correlated to improved charge transport for Sb and Bi additives, but it is shown that secondary effects can be detrimental to performance even with large grains (for Mg). For optimized electrodes, the sequential deposition of thin layers of n‐type CdS and TiO2 by solution‐based methods, and platinum as an electrocatalyst, leads to stable photocurrents saturating at 8.0 mA cm–2 and onsetting at ≈0.6 V versus RHE under AM 1.5G illumination for CuInS2 films. Electrodes prepared by our method rival the state‐of‐the‐art performance for these materials.
We propose a new mechanism by which the common electrolyte additive guanidinium thiocyanate (GdmSCN) improves efficiency in dye-sensitized solar cells (DSSCs). We demonstrate that binding of Gdm(+) to TiO2 is weak and does not passivate recombination sites on the TiO2 surface as has been previously claimed. Instead, we show that Gdm(+) binds strongly to the N719 and D131 dyes and probably to many similar compounds. The binding of Gdm(+) competes with iodine binding to the same molecule, reducing the surface concentration of dye-I2 complexes. This in turn reduces the electron/iodine recombination rate constant, which increases the collection efficiency and thus the photocurrent. We further observe that GdmNO3 can increase efficiency more than the current Gdm(+) source, GdmSCN, at least in some DSSCs. Overall, the results point to an improved paradigm for DSSC operation and development. The TiO2/electrolyte surface has long been held to be the key interface in DSSCs. We now assert that the dye layer/electrolyte interaction is at least, and probably more, important.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.