Two-Step Perovskite Solar Cells with > 25% Efficiency: Unveiling the Hidden Bottom Surface of Perovskite Layer.

While significant efforts in surface engineering have been devoted to the conversion process of lead iodide (PbI2) into perovskite and top surface engineering of perovskite layer with remarkable progress, the exploration of residual PbI2 clusters and the hidden bottom surface on perovskite layer have been limited. In this work, a new strategy involving 1-butyl-3-methylimidazolium acetate (BMIMAc) ionic liquid (IL) additives is developed and it is found that both the cations and the anions in ILs can interact with the perovskite components, thereby regulating the crystallization process and diminishing the residue PbI2 clusters as well as filling vacancies. The introduction of BMIMAc ILs induces the formation of a uniform porous PbI2 film, facilitating better penetration of the second-step organic salt and fostering a more extensive interaction between PbI2 and the organic salt. Surprisingly, the oversized residual PbI2 clusters at the bottom surface of the perovskite layer completely diminish. In addition, advanced depth analysis techniques including depth-resolved grazing-incidence wide-angle X-ray scattering (GIWAXS) and bottom thinning technology are employed for a comprehensive understanding of the reduction in residual PbI2. Leveraging effective PbI2 management and regulation of the perovskite crystallization process, the champion devices achieve a power conversion efficiency (PCE) of 25.06% with long-term stability.

Hexylammonium Acetate-Regulated Buried Interface for Efficient and Stable Perovskite Solar Cells.

Due to current issues of energy-level mismatch and low transport efficiency in commonly used electron transport layers (ETLs), such as TiO2 and SnO2, finding a more effective method to passivate the ETL and perovskite interface has become an urgent matter. In this work, we integrated a new material, the ionic liquid (IL) hexylammonium acetate (HAAc), into the SnO2/perovskite interface to improve performance via the improvement of perovskite quality formed by the two-step method. The IL anions fill oxygen vacancy defects in SnO2, while the IL cations interact chemically with Pb2+ within the perovskite structure, reducing defects and optimizing the morphology of the perovskite film such that the energy levels of the ETL and perovskite become better matched. Consequently, the decrease in non-radiative recombination promotes enhanced electron transport efficiency. Utilizing HAAc, we successfully regulated the morphology and defect states of the perovskite layer, resulting in devices surpassing 24% efficiency. This research breakthrough not only introduces a novel material but also propels the utilization of ILs in enhancing the performance of perovskite photovoltaic systems using two-step synthesis.

Facile Tailoring of Metal-Organic Frameworks for Förster Resonance Energy Transfer-Driven Enhancement in Perovskite Photovoltaics.

Förster resonance energy transfer (FRET) has demonstrated its potential to enhance the light energy utilization ratio of perovskite solar cells by interacting with metal-organic frameworks (MOFs) and perovskite layers. However, comprehensive investigations into how MOF design and synthesis impact FRET in perovskite systems are scarce. In this work, nanoscale HIAM-type Zr-MOF (HIAM-4023, HIAM-4024, and HIAM-4025) is meticulously tailored to evaluate FRET's existence and its influence on the perovskite photoactive layer. Through precise adjustments of amino groups and acceptor units in the organic linker, HIAM-MOFs are synthesized with the same topology, but distinct photoluminescence (PL) emission properties. Significant FRET is observed between HIAM-4023/HIAM-4024 and the perovskite, confirmed by spectral overlap, fluorescence lifetime decay, and calculated distances between HIAM-4023/HIAM-4024 and the perovskite. Conversely, the spectral overlap between the PL emission of HIAM-4025 and the perovskite's absorption spectrum is relatively minimal, impeding the energy transfer from HIAM-4025 to the perovskite. Therefore, the HIAM-4023/HIAM-4024-assisted perovskite devices exhibit enhanced EQE via FRET processes, whereas the HIAM-4025 demonstrates comparable EQE to the pristine. Ultimately, the HIAM-4023-assisted perovskite device achieves an enhanced power conversion efficiency (PCE) of 24.22% compared with pristine devices (PCE of 22.06%) and remarkable long-term stability under ambient conditions and continuous light illumination.

Thiol-Functionalized Conjugated Metal-Organic Frameworks for Stable and Efficient Perovskite Photovoltaics.

Metal-organic frameworks (MOFs) have been investigated recently in perovskite photovoltaics owing to their potential to boost optoelectronic performance and device stability. However, the impact of variations in the MOF side chain on perovskite characteristics and the mechanism of MOF/perovskite film formation remains unclear. In this study, three nanoscale thiol-functionalized UiO-66-type Zr-based MOFs (UiO-66-(SH)2 , UiO-66-MSA, and UiO-66-DMSA) are systematically employed and examined in perovskite solar cells (PSCs). Among these MOFs, UiO-66-(SH)2 , with its rigid organic ligands, exhibited a strong interaction with perovskite materials with more efficient suppression of perovskite vacancy defects. More importantly, A detailed and in-depth discussion is provided on the formation mechanism of UiO-66-(SH)2 -assisted perovskite film upon in situ GIWAXS performed during the annealing process. The incorporation of UiO-66-(SH)2 additives substantially facilitates the conversion of PbI2 into the perovskite phase, prolongs the duration of stage I, and induces a delayed phase transformation pathway. Consequently, the UiO-66-(SH)2 -assisted device demonstrates reduced defect density and superior optoelectronic properties with optimized power conversion efficiency of 24.09% and enhanced long-term stability under ambient environment and continuous light illumination conditions. This study acts as a helpful design guide for desired MOF/perovskite structures, enabling further advancements in MOF/perovskite optoelectronic devices.

Revealing Size-Dependency of Ionic Liquid to Assist Perovskite Film Formation Mechanism for Efficient and Durable Perovskite Solar Cells.

Ionic liquids (ILs) are extensively utilized for the manipulation of crystallization kinetics of perovskite, morphology optimization, and defect passivation for the fabrication of highly efficient and stable devices. However, comparing ILs with different chemical structures and selecting the appropriate ILs from the many types available to enhance perovskite device performance remains a challenge. In this study, a range of ILs containing different sizes of anions are introduced as additives for assisting in film formation in perovskite photovoltaics. Specifically, ILs with various sizes significantly affects the strength of chemical interaction between ILs and perovskite composition, inducing varying degrees of conversion of lead iodide to perovskite as well as the formation of perovskite films with markedly disparate grain sizes and morphology. Theoretical calculations in conjunction with experimental measurements revealed that small-sized anion can more effectively reduce defect density by filling halide vacancies within perovskite bulk materials, resulting in suppression of charge-carrier recombination, an extended photoluminescence lifetime, and significantly improved device performance. Boosted by ILs with appropriate size, the champion power conversion efficiency of 24.09% for the ILs-treated device is obtained, and the unencapsulated devices retain 89.3% of its original efficiency under ambient conditions for 2000 h.

Tailoring Ionic Liquid Chemical Structure for Enhanced Interfacial Engineering in Two-Step Perovskite Photovoltaics.

Ionic liquids (ILs) have emerged as versatile tools for interfacial engineering in perovskite photovoltaics. Their multifaceted application targets defect mitigation at SnO2 -perovskite interfaces, finely tuning energy level alignment, and enhancing charge transport, meanwhile suppressing non-radiative recombination. However, the diverse chemical structures of ILs present challenges in selecting suitable candidates for effective interfacial modification. This study adopted a systematic approach, manipulating IL chemical structures. Three ILs with distinct anions are introduced to modify perovskite/SnO2 interfaces to elevate the photovoltaic capabilities of perovskite devices. Specifically, ILs with different anions exhibited varied chemical interactions, leading to notable passivation effects, as confirmed by Density Functional Theory (DFT) calculation. A detailed analysis is also conducted on the relationship between the ILs' structure and regulation of energy level arrangement, work function, perovskite crystallization, interface stress, charge transfer, and device performance. By optimizing IL chemical structures and exploiting their multifunctional interface modification properties, the champion device achieved a PCE of 24.52% with attentional long-term stability. The study establishes a holistic link between IL structures and device performance, thereby promoting wider application of ILs in perovskite-based technologies.

Dimethylammonium Cation-Induced 1D/3D Heterostructure for Efficient and Stable Perovskite Solar Cells.

Mixed-dimensional perovskite engineering has been demonstrated as a simple and useful approach to achieving highly efficient and more-durable perovskite solar cells (PSCs), which have attracted increasing research interests worldwide. In this work, 1D/3D mixed-dimensional perovskite has been successfully obtained by introducing DMAI via a two-step deposition method. The additive DMA+ can facilitate the crystalline growth and form 1D DMAPbI3 at grain boundaries of 3D perovskite, leading to improved morphology, longer charge carrier lifetime, and remarkably reduced bulk trap density for perovskite films. Meanwhile, the presence of low-dimension perovskite is able to prevent the intrusion of moisture, resulting in enhanced long-term stability. As a result, the PSCs incorporated with 1D DMAPbI3 exhibited a first-class power conversion efficiency (PCE) of 21.43% and maintained 85% of their initial efficiency after storage under ambient conditions with ~45% RH for 1000 h.

Understanding the Electrochemical Formation and Decomposition of Li2O2 and LiOH with Operando X-ray Diffraction.

The lithium air, or Li-O2, battery system is a promising electrochemical energy storage system because of its very high theoretical specific energy, as required by automotive applications. Fundamental research has resulted in much progress in mitigating detrimental (electro)chemical processes; however, the detailed structural evolution of the crystalline Li2O2 and LiOH discharge products, held at least partially responsible for the limited reversibility and poor rate performance, is hard to measure operando under realistic electrochemical conditions. This study uses Rietveld refinement of operando X-ray diffraction data during a complete discharge-charge cycle to reveal the detailed structural evolution of Li2O2 and LiOH crystallites in 1,2-dimethoxyethane (DME) and DME/LiI electrolytes, respectively. The anisotropic broadened reflections confirm and quantify the platelet crystallite shape of Li2O2 and LiOH and show how the average crystallite shape evolves during discharge and charge. Li2O2 is shown to form via a nucleation and growth mechanism, whereas the decomposition appears to start at the smallest Li2O2 crystallite sizes because of their larger exposed surface. In the presence of LiI, platelet LiOH crystallites are formed by a particle-by-particle nucleation and growth process, and at the end of discharge, H2O depletion is suggested to result in substoichiometric Li(OH)1-x , which appears to be preferentially decomposed during charging. Operando X-ray diffraction proves the cyclic formation and decomposition of the LiOH crystallites in the presence of LiI over multiple cycles, and the structural evolution provides key information for understanding and improving these highly relevant electrochemical systems.