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Ion migration activated by illumination is a critical factor responsible for the performance decline and stability degradation of perovskite solar cells (PSCs). While ion migration has been widely believed to be much slower than charge transport, recent research suggests that, despite the lack of understanding of the mechanism, it may also be involved in a series of rapid photoelectric responses of PSCs. Here, we report an improved circuit-switched transient photoelectric technique with nanosecond temporal resolution, which enables quantitative characterization of ion migration dynamics in PSCs across a fairly broad time window. Specifically, ion migration occurring within microseconds after illumination (corresponding to a diffusion length of ∼10−7 cm) is unambiguously identified. In conjunction with the composition engineering protocol, we justify that it arises from the short-range migration of halide anions and organic cations around the contact/perovskite interface. The rapid ion migration kinetics revealed in this work strongly complement the well-established ion migration model, which offers new insights into the mechanism of ion-carrier interaction in PSC devices. [ABSTRACT FROM AUTHOR]

The utilization of solar energy by light conversion processes in both natural and artificial photosynthesis systems has become a pivotal means of sustaining a consistent power supply for our daily lives and production. In general, these reactions rely on the visible spectra region where the sun produces abundant energy according to the black body's emission. Further expanding the adoption of the near-infrared (NIR) spectral region through upconversion has become an intelligent strategy to improve the entire utilization rate of solar energy. In this work, the two-photon upconversion (TPUC) properties were discovered in CsPbBr3 nanoplates (NPs) emitted at 450 nm, and a sub-gap energy level was observed to serve as the center to facilitate the upconversion process. The excitation threshold for TPUC is drastically lower than that for the two-photon absorption, another nonlinear upconversion path that requires strengthened excitation power. Moreover, a significant negative correlation between the excitation threshold for TPUC in the CsPbBr3 NPs and the exciton binding energy was unveiled, which could be attributed to the enhanced quantum confinement effects. Our work paved the way for a feasible way to expand the utilization of the NIR spectral region in light conversion applications. [ABSTRACT FROM AUTHOR]

Colloidal lead halide perovskite quantum dots (PQDs) display great potential for flexible light-emitting applications; however, the practicability, especially for the mixed halide PQD counterparts, is restricted by color deterioration and intensity decline in photoluminescence (PL) caused by unintentional variation in stoichiometric ratios due to ion migration upon exposure to light. Herein, we present a collaborative strategy to stabilize the PL by confining ion motion in colloidal PQDs. Specifically, we modify the surface of CsPbBr1.5I1.5 QDs with a zwitterionic ligand, sulfobetaine-18, to retard ion migration and accelerate its entropy-driven reverse process (ion intermixing in the dark) simultaneously. Furthermore, we propose a high-repetition, pulse-width-modulation-enabled optical excitation approach, whereby the ion migration/intermixing kinetics can be finely controlled by adjusting the excitation duty cycle. Taken together, we attain unprecedentedly stable PL from the colloidal mixed halide PQD solution, displaying constant intensity and negligible wavelength shifts (less than 3 nm) during the 25-minute quasi-continuous photoexcitation. [ABSTRACT FROM AUTHOR]

Lead halide perovskite quantum dots (PQDs) have shown remarkable single‐photon upconversion photoluminescence (SPUC‐PL) bringing them a brilliant perspective as the active media for biological imaging, optiag, and upconversion photovoltaics. However, the mechanism of SPUC in PQDs is still in dispute, and the specific strategy for optimizing PQDs toward a higher SPUC performance has yet to be established, which impedes the application of PQDs‐based SPUC‐PL in practice. In this work it is revealed that the PL quantum yield is declined by surface trap states, while the SPUC efficiency is promoted by Urbach tail states. These two types of intragap states can be chemically manipulated by engineering the surface ligand and the growth kinetics of perovskite nanocrystals, bringing on the improvement of light emission efficiency and the promotion of subgap low‐energy photon absorption, respectively. As a proof of concept, utilizing the synergistic effect of the proposed chemical manipulation method, the SPUC‐PL efficiency of PQDs is boosted by more than 40%, which gives rise to a spectral window of optical refrigeration gain as broad as ≈130 meV. [ABSTRACT FROM AUTHOR]

Ion accumulation in perovskite solar cells can be highly suppressed by a mesoporous TiO2 layer. This is evidenced by the decrease of the ion-related electrostatic potential with increasing the thickness of the mesoporous layer, accounted for by the electron population in the shallow trap states of the TiO2 nanocrystals. [ABSTRACT FROM AUTHOR]