Effect of dimensionality on the optical absorption properties of CsPbI3 perovskite nanocrystals

Structural effects on the luminescence properties of CsPbI3 nanocrystals

Metal halide perovskite nanocrystals (NCs) are promising for photovoltaic and light-emitting applications. Due to the softness of their crystal lattice, structural modifications have a critical impact on their optoelectronic properties. Here we investigate the size-dependent optoelectronic properties of CsPbI3 NCs ranging from 7 to 17 nm, employing temperature and pressure as thermodynamic variables to modulate the energetics of the system and selectively tune the interatomic distances. By temperature-dependent photoluminescence spectroscopy, we have found that luminescence quenching channels exhibit increased non-radiative losses and weaker exciton-phonon coupling in bigger particles, in turn affecting the luminescence efficiency. Through pressure-dependent measurements up to 2.5 GPa, supported by XRD characterization, we revealed a NC-size dependent solid-solid phase transition from the γ-phase to the δ-phase. Importantly, the optical response to these structural changes strongly depends on the size of the NC. Our findings provide an interesting guideline to correlate the size and structural and optoelectronic properties of CsPbI3 NCs, important for engineering the functionalities of this class of soft semiconductors.

Tiny spots to light the future: advances in synthesis, properties, and application of perovskite nanocrystals in solar cells

Perovskites are in the hotspot of material science and technology. Outstanding properties have been discovered, fundamental mechanisms of defect formation and degradation elucidated, and applications in a wide variety of optoelectronic devices demonstrated. Advances through adjusting the bulk-perovskite composition, as well as the integration of layered and nanostructured perovskites in the devices, allowed improvement in performance and stability. Recently, efforts have been devoted to investigating the effects of quantum confinement in perovskite nanocrystals (PNCs) aiming to fabricate optoelectronic devices based solely on these nanoparticles. In general, the applications are focused on light-emitting diodes, especially because of the high color purity and high fluorescence quantum yield obtained in PNCs. Likewise, they present important characteristics featured for photovoltaic applications, highlighting the possibility of stabilizing photoactive phases that are unstable in their bulk analog, the fine control of the bandgap through size change, low defect density, and compatibility with large-scale deposition techniques. Despite the progress made in the last years towards the improvement in the performance and stability of PNCs-based solar cells, their efficiency is still much lower than that obtained with bulk perovskite, and discussions about upscaling of this technology are scarce. In light of this, we address in this review recent routes towards efficiency improvement and the up-scaling of PNC solar cells, emphasizing synthesis management and strategies for solar cell fabrication.

Temperature-dependence of the band gap in the all-inorganic perovskite CsPbI3 from room to high temperatures

Understanding the micro-mechanism of the temperature dependence of the band gap in all-inorganic perovskites is of great significance for their optoelectronic and photovoltaic applications in various temperature environments. Herein, based on the recently developed electron-phonon renormalization method, the temperature-dependent band gaps of the optoelectronic perovskite CsPbI₃ are studied from 300 K to 750 K (including orthorhombic, tetragonal, and cubic phases). It is found that the temperature-induced structural fluctuation makes the structure of perovskites deviate from the 0 K one, and the corresponding renormalized band gap differs from that at 0 K, especially for the high-temperature cubic phase (e.g., ΔE_g is ∼177 meV at 600 K). However, within the temperature range of each CsPbI₃ phase, the band gap E_g is enlarged slightly with the increase of temperature (e.g., ΔE_g is ∼26 meV from 600 K to 750 K for the cubic phase), showing the insensitivity of the structural fluctuation effect to the temperature change. The reason is that the chemical characters of band edges are determined by PbI₃^-, and due to the strong correlation between Pb and I, the Pb-I bond lengths and Pb-I-Pb bond angles are almost unchanged as the temperature increases. Our work provides a fundamental understanding of the temperature-dependent band gaps in all-inorganic perovskites and shed light on the commercialization of perovskites.

Measuring Exciton Fine-Structure in Randomly Oriented Perovskite Nanocrystal Ensembles Using Nonlinear Optical Spectroscopy: Theory

Lead halide perovskite nanocrystals (PNCs) exhibit unique optoelectronic properties, many of which originate from a purported bright-triplet exciton fine-structure. A major impediment to measuring this fine-structure is inhomogeneous spectral broadening, which has limited most experimental studies to single-nanocrystal spectroscopies. It is shown here that the linearly polarized single-particle selection rules in PNCs are preserved in nonlinear spectroscopies of randomly oriented ensembles. Simulations incorporating rotational averaging demonstrate that techniques such as transient absorption and two-dimensional coherent spectroscopy are capable of resolving exciton fine-structure in PNCs, even in the presence of inhomogeneous broadening and orientation disorder.

Exciton linewidth broadening induced by exciton-phonon interactions in CsPbBr3 nanocrystals

Quantum dephasing of excitonic transitions in CsPbBr₃ nanocrystals has been studied using two-dimensional electronic spectroscopy at cryogenic temperatures. The exciton-phonon interactions for acoustic and optical modes exhibit different effects on the coherent dynamics of excitonic transitions. The homogeneous linewidth shows a proportional dependence on the temperature, suggesting the primary dephasing channel of the elastic scattering between exciton and acoustic modes. The exciton-optical mode interaction is manifested as the beatings of off-diagonal signals in the population time domain at the frequencies of 29 and 51 cm^-1, indicating phonon replicas of excitonic transitions arising from coherent exciton-phonon interaction. The insight information of exciton homogeneous broadening in perovskite nanocrystals is essential for the potential application of quantum light sources.

Toward Engineering Intrinsic Line Widths and Line Broadening in Perovskite Nanoplatelets

Perovskite nanoplatelets possess extremely narrow absorption and emission line widths, which are crucial characteristics for many optical applications. However, their underlying intrinsic and extrinsic line-broadening mechanisms are poorly understood. Here, we apply multidimensional coherent spectroscopy to determine the homogeneous line broadening of colloidal perovskite nanoplatelet ensembles. We demonstrate a dependence of not only their intrinsic line widths but also of various broadening mechanisms on platelet geometry. We find that decreasing nanoplatelet thickness by a single monolayer results in a 2-fold reduction of the inhomogeneous line width and a 3-fold reduction of the intrinsic homogeneous line width to the sub-millielectronvolts regime. In addition, our measurements suggest homogeneously broadened exciton resonances in two-layer (but not necessarily three-layer) nanoplatelets at room-temperature.

Disentangling Electron-Phonon Coupling and Thermal Expansion Effects in the Band Gap Renormalization of Perovskite Nanocrystals

The complex electron-phonon interaction occurring in bulk lead halide perovskites gives rise to anomalous temperature dependences, like the widening of the electronic band gap as temperature increases. However, possible confinement effects on the electron-phonon coupling in the nanocrystalline version of these materials remain unexplored. Herein, we study the temperature (ranging from 80 K to ambient) and hydrostatic pressure (from atmospheric to 0.6 GPa) dependence of the photoluminescence of ligand-free methylammonium lead triiodide nanocrystals with controlled sizes embedded in a porous silica matrix. This analysis allowed us to disentangle the effects of thermal expansion and electron-phonon interaction. As the crystallite size decreases, the electron-phonon contribution to the gap renormalization gains in importance. We provide a plausible explanation for this observation in terms of quantum confinement effects, showing that neither thermal expansion nor electron-phonon coupling effects may be disregarded when analyzing the temperature dependence of the optoelectronic properties of perovskite lead halide nanocrystals.

Multidimensional coherent spectroscopy reveals triplet state coherences in cesium lead-halide perovskite nanocrystals

Advances in optoelectronics require materials with novel and engineered characteristics. A class of materials that has garnered tremendous interest is metal-halide perovskites, stimulated by meteoric increases in photovoltaic efficiencies of perovskite solar cells. In addition, recent advances have applied perovskite nanocrystals (NCs) in light-emitting devices. It was found recently that, for cesium lead-halide perovskite NCs, their unusually efficient light emission may be due to a unique excitonic fine structure composed of three bright triplet states that minimally interact with a proximal dark singlet state. To study this fine structure without isolating single NCs, we use multidimensional coherent spectroscopy at cryogenic temperatures to reveal coherences involving triplet states of a CsPbI₃ NC ensemble. Picosecond time scale dephasing times are measured for both triplet and inter-triplet coherences, from which we infer a unique exciton fine structure level ordering composed of a dark state energetically positioned within the bright triplet manifold.

Chemically Stable Black Phase CsPbI3 Inorganic Perovskites for High-Efficiency Photovoltaics

Research on chemically stable inorganic perovskites has achieved rapid progress in terms of high efficiency exceeding 19% and high thermal stabilities, making it one of the most promising candidates for thermodynamically stable and high-efficiency perovskite solar cells. Among those inorganic perovskites, CsPbI₃ with good chemical components stability possesses the suitable bandgap (≈1.7 eV) for single-junction and tandem solar cells. Comparing to the anisotropic organic cations, the isotropic cesium cation without hydrogen bond and cation orientation renders CsPbI₃ exhibit unique optoelectronic properties. However, the unideal tolerance factor of CsPbI₃ induces the challenges of different crystal phase competition and room temperature phase stability. Herein, the latest important developments regarding understanding of the crystal structure and phase of CsPbI₃ perovskite are presented. The development of various solution chemistry approaches for depositing high-quality phase-pure CsPbI₃ perovskite is summarized. Furthermore, some important phase stabilization strategies for black phase CsPbI₃ are discussed. The latest experimental and theoretical studies on the fundamental physical properties of photoactive phase CsPbI₃ have deepened the understanding of inorganic perovskites. The future development and research directions toward achieving highly stable CsPbI₃ materials will further advance inorganic perovskite for highly stable and efficient photovoltaics.

High Phase Stability in CsPbI3 Enabled by Pb-I Octahedra Anchors for Efficient Inorganic Perovskite Photovoltaics

CsPbI₃ inorganic perovskite has exhibited some special properties particularly crystal structure distortion and quantum confinement effect, yet the poor phase stability of CsPbI₃ severely hinders its applications. Herein, the nature of the photoactive CsPbI₃ phase transition from the perspective of PbI₆ octahedra is revealed. A facile method is also developed to stabilize the photoactive phase and to reduce the defect density of CsPbI₃ . CsPbI₃ is decorated with multifunctional 4-aminobenzoic acid (ABA), and steric neostigmine bromide (NGBr) is subsequently used to further mediate the thin films' surface (NGBr-CsPbI₃ (ABA)). The ABA or NG cation adsorbed onto the grain boundaries/surface of CsPbI₃ anchors the PbI₆ octahedra via increasing the energy barriers of octahedral rotation, which maintains the continuous array of corner-sharing PbI₆ octahedra and kinetically stabilizes the photoactive phase CsPbI₃ . Moreover, the added ABA and NGBr not only interact with shallow- or deep-level defects in CsPbI₃ to significantly reduce defect density, but also lead to improved energy-level alignment at the interfaces between the CsPbI₃ and the charge transport layers. Finally, the champion NGBr-CsPbI₃ (ABA)-based inorganic perovskite solar cell delivers 18.27% efficiency with excellent stability. Overall, this work demonstrates a promising concept to achieve highly phase-stabilized inorganic perovskite with suppressed defect density for promoting its optoelectronic applications.