André Felipe Vale Fonseca

We investigate the microscopic origin of the enhanced photoconductivity and charge carrier transport in a mixed-cation, mixed-halide hybrid perovskite passivated with CsPbBr3 quantum dots using optical pump-terahertz probe spectroscopy. We directly access the complex transient photoconductivity in the terahertz range, whose analysis allows us to elucidate the effect of CsPbBr3 quantum dot passivation on charge carrier transport between crystalline domains. We find that, for the passivated perovskites, transport properties are significantly improved, with the Drude–Smith backscattering parameter changing from −0.60 to −0.42, corresponding to a 43% increase in carrier mobility, while the intragrain carrier scattering time (≈110 fs) remains unchanged. These results demonstrate that the enhanced photoconductivity originates from reduced carrier backscattering at grain boundaries, rather than changes in intragrain carrier scattering. Our results provide microscopic insight into the mechanism underlying the improved performance of solar cells based on mixed-cation, mixed-halide hybrid perovskites passivated with CsPbBr3 quantum dots.

Metal halide perovskites excel under extreme space radiation but remain vulnerable to terrestrial conditions, especially thermal cycling. Advancing devices requires collaboration, interface passivation, quantum-dot integration, and in situ studies.

CuInS₂ quantum dots (QDs) have gained significant attention owing to their remarkable broadband emission, making them desirable for various optoelectronic applications requiring efficient luminescent nanomaterials. However, maximizing radiative recombination in CuInS₂ QDs necessitates minimizing intragap trap states. A common approach involves the introduction of Zn during the synthesis, which typically promotes the formation of a ZnS shell that passivates the QD surface. Despite its importance, the characterization and quantification of Zn incorporation using conventional techniques, such as optical spectroscopy or electron microscopy, remains challenging. In this study, we utilized X-ray absorption spectroscopy, in both X-ray absorption near-edge structure and extended X-ray absorption fine structure spectral ranges, to investigate Zn incorporation into CuInS₂ QDs, probing at the Zn, S, and Cu K-edges. This approach allowed us to detect the formation of a ZnS surface shell, tentatively quantifying its thickness, and to distinguish between Zn as a substituent at the shell or as an interstitial defect. Additionally, we explored the dynamical properties of CuInS₂ QDs using time-resolved optical spectroscopies, particularly in the presence of electron and hole acceptors (benzoquinone and phenothiazine), observing that hole transfer is highly sensitive to shell thickness. These results provide deeper insights into the Zn-induced shell.

Interfacial defects at the buried junction between the electron transport layer (ETL) and perovskite absorber critically hinder the performance of perovskite solar cells (PSCs). We report herein that a CdS quantum dot (QD) interlayer, deposited onto SnO₂ via a successive ionic layer adsorption and reaction method, provides an effective strategy to improve charge transport across this interface. The CdS QD layer not only suppresses oxygen vacancies but also reacts with hydroxyl groups on the SnO₂ surface, thereby improving surface potential uniformity and enhancing the electron extraction rate. Impedance spectroscopy further confirms improved interface homogeneity and charge transport, correlating with higher fill factor and short-circuit current densities. As a result, CdS modification enables a ∼25% efficiency enhancement on PSCs, highlighting the potential of QD-based interfacial engineering towards high-performance PSCs.

CuInS₂ quantum dots have been studied in a broad range of applications, but despite this, the fine details of their charge carrier dynamics remain a subject of intense debate. Two of the most relevant points of discussion are the hole dynamics and the influence of Cu:In synthesis stoichiometry. It has been proposed that Cu-deficiency leads to the formation of Cu²⁺, affecting the localization of holes into Cu defects. Importantly, it is precisely these confined hole states that are used to explain the interesting photoluminescence properties of CuInS₂ quantum dots. We use static X-ray spectroscopy to show no evidence for a measurable amount of native Cu²⁺ states in Cu-deficient samples (above 20%). Instead, the improved properties of these samples are explained by an increase of crystallinity, reducing the concentration of mid-gap states. Furthermore, to understand the charge carrier dynamics, herein, we employ ultrafast optical transient absorption and fluorescence up-conversion spectroscopies in combination with ultrafast X-ray absorption spectroscopy using a hard X-ray free electron laser. We demonstrate that in nonpassivated samples, holes are transferred from Cu atoms on subpicosecond time scales. Finally, we observe that Cu-deficient samples are more robust against photothermal effects at higher laser fluences. This is not the case for the Cu-rich sample, where heating effects on the structure are directly observed.

Several studies have demonstrated that low-dimensional structures (e.g., two-dimensional (2D)) associated with three-dimensional (3D) perovskite films enhance the efficiency and stability of perovskite solar cells. Here, we aim to track the formation sites of the 2D phase on top of the 3D perovskite and to establish correlations between molecular stiffness and steric hindrance of the organic cations and their influence on the formation and crystallization of 2D/3D. Using cathodoluminescence combined with a scanning electron microscopy technique, we verified that the formation of the 2D phase occurs preferentially on the grain boundaries of the 3D perovskite. This helps explain some passivation mechanisms conferred by the 2D phase on 3D perovskite films. Furthermore, by employing in situ grazing-incidence wide-angle X-ray scattering, we monitored the formation and crystallization of the 2D/3D perovskite using three cations with varying molecular stiffness. In this series of molecules, the formation and crystallization of the 2D phase are found to be dependent on both steric hindrance around the ammonium group and molecular stiffness. Finally, we employed a 2D/3D perovskite heterointerface in a solar cell. The presence of the 2D phase, particularly those formed from flexible cations, resulted in a maximum power conversion efficiency of 21.5%. This study provides insight into critical aspects related to how bulky organic cations' stiffness and steric hindrance influence the formation, crystallization, and distribution of 2D perovskite phases.