Ionic Liquid Assisted Imprint for Efficient and Stable Quasi-2D Perovskite Solar Cells with Controlled Phase Distribution

Suppressing the Bottom Small n Phases of Quasi-2D Perovskites for High-Performance Photovoltaic Applications

The bottom small n phases in quasi-two-dimensional (Q-2D) perovskite films significantly hinder their photovoltaic performance development due to their severely low conductivity and nonideal band alignment in the corresponding solar cells. In this study, we successfully suppressed the growth of small n phases in Q-2D Ruddlesden-Popper (RP) perovskite (BA2MA4Pb5I16, ⟨n⟩ = 5) films by introducing 2,7-bis(diphenylphosphoryl)-9,9'-spirobifluorene (SPPO13) as an additive into the perovskite precursor solution. It is interesting to find that the hole transport layer poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) in our p-i-n device can attract the SPPO13 due to the π-π stacking effect. As a result, the SPPO13 concentrates at the bottom, and the coordination between SPPO13 and PbI2 leads to more [PbI6]4- octahedra gathering at the downside of the Q-2D perovskite film. Thereby, more large n phases remain at the bottom, and the unwanted small n phases are suppressed. The optimized device achieves a remarkable power conversion efficiency of 18.41%, which, according to our knowledge, is the highest value for the BA-MA-based perovskite. Moreover, our device also demonstrates outstanding stability, maintaining 99.5% and 95.3% of the initial efficiency after being stored for over 3500 h and under maximum power point tracking operation for over 400 h, respectively. Unlike conventional methods that primarily address bulk or interface properties, this approach uniquely combines π-π stacking effects and defect passivation through phosphine oxide groups, leading to enhanced crystallinity, vertical orientation, and suppressed nonradiative recombination. This work provides a new approach to regulate n-phase growth and promote the photovoltaic behavior of Q-2D perovskite solar cells.

Molecular Structure Tailoring of Organic Spacers for High-Performance Ruddlesden-Popper Perovskite Solar Cells

Layer-structured Ruddlesden-Popper (RP) perovskites (RPPs) with decent stability have captured the imagination of the photovoltaic research community and bring hope for boosting the development of perovskite solar cell (PSC) technology. However, two-dimensional (2D) or quasi-2D RP PSCs are encountered with some challenges of the large exciton binding energy, blocked charge transport and poor film quality, which restrict their photovoltaic performance. Fortunately, these issues can be readily resolved by rationally designing spacer cations of RPPs. This review mainly focuses on how to design the molecular structures of organic spacers and aims to endow RPPs with outstanding photovoltaic applications. We firstly elucidated the important roles of organic spacers in impacting crystallization kinetics, charge transporting ability and stability of RPPs. Then we brought three aspects to attention for designing organic spacers. Finally, we presented the specific molecular structure design strategies for organic spacers of RPPs aiming to improve photovoltaic performance of RP PSCs. These proposed strategies in this review will provide new avenues to develop novel organic spacers for RPPs and advance the development of RPP photovoltaic technology for future applications.

Enhanced amplified spontaneous emission performance through effective regulation of phase distribution in Ruddlesden-Popper perovskite films

Ruddlesden-Popper (RP) perovskites promise next-generation gain media for laser devices. However, most RP perovskite lasers are still suffering from inferior performance characteristics, such as inadequate energy transfer, unstable emission, and short lifetime. To address the above problems, high crystalline quality, compact, and smooth PEA2FA2Pb3Br10 films with uniform phase distribution were successfully prepared by ionic liquid (IL) methylammonium acetate (MAAc) in an air environment. Compared with the PEA2FA2Pb3Br10 film prepared by the traditional solvent dimethyl sulfoxide (DMSO), an enhanced amplified spontaneous emission (ASE) with a lower threshold of 58 µJ·cm-2 from the MAAc-treated film was obtained under nanosecond laser excitation. The transient absorption (TA) spectroscopy revealed that a uniform phase distribution and more efficient energy transfer processes were achieved in the PEA2FA2Pb3Br10-MAAc film, leading to an enhanced band-to-band spontaneous emission process. Furthermore, the films exhibited better stability, showing no signs of degradation under the 120 min pulsed laser pumping in air and stability of ASE spectra at even 95% humidity conditions. This study provides an important foundation for achieving high-performance optically pumped lasers based on the unique RP perovskites.

Unveiling Local Current Behavior and Manipulating Grain Homogenization of Perovskite Films for Efficient Solar Cells

Achieving high power conversion efficiency in perovskite solar cells (PSCs) heavily relies on fabricating homogeneous perovskite films. However, understanding microscopic-scale properties such as current generation and open-circuit voltage within perovskite crystals has been challenging due to difficulties in quantifying intragrain behavior. In this study, the local current intensity within state-of-the-art perovskite films mapped by conductive atomic force microscopy reveals a distinct heterogeneity, which exhibits a strong anticorrelation to the external biases. Particularly under different external bias polarities, specific regions in the current mapping show contrasting conductivity. Moreover, grains oriented differently exhibit varied surface potentials and currents, leading us to associate this local current heterogeneity with the grain orientation. It was found that the films treated with isopropanol exhibit ordered grain orientation, demonstrating minimized lattice heterogeneity, fewer microstructure defects, and reduced electronic disorder. Importantly, devices exhibiting an ordered orientation showcase elevated macroscopic optoelectronic properties and boosted device performance. These observations underscore the critical importance of fine-tuning the grain homogenization of perovskite films, offering a promising avenue for further enhancing the efficiency of PSCs.

Large-n quasi-phase-pure two-dimensional halide perovskite: A toolbox from materials to devices

Despite their excellent environmental stability, low defect density, and high carrier mobility, large-n quasi-two-dimensional halide perovskites (quasi-2DHPs) feature a limited application scope because of the formation of self-assembled multiple quantum wells (QWs) due to the similar thermal stabilities of large-n phases. However, large-n quasi-phase-pure 2DHPs (quasi-PP-2DHPs) can solve this problem perfectly. This review discusses the structures, formation mechanisms, and photoelectronic and physical properties of quasi-PP-2DHPs, summarises the corresponding single crystals, thin films, and heterojunction preparation methods, and presents the related advances. Moreover, we focus on applications of large-n quasi-PP-2DHPs in solar cells, photodetectors, lasers, light-emitting diodes, and field-effect transistors, discuss the challenges and prospects of these emerging photoelectronic materials, and review the potential technological developments in this area.