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Two-dimensional (2D) hybrid perovskite solar cells have induced widespread research interest owing to their higher stability as compared to three-dimensional (3D) analogues. The photovoltaic performance, however, is still limited. Here, we report a perovskite film dominated by 3D components that are modulated by formamidinium thiocyanate in the 2D PEA(2)FA(3)Pb(4)I(13) perovskite composition. We demonstrate that a dimensionality transformation from a 2D/3D to predominantly 3D perovskite phase in the presence of SCN-based treatment enhances the optical absorption in visible and near-infrared regions. As a result, the power conversion efficiency improved 4.5 times with respect to the control to above 14%. The unencapsulated cell exhibited excellent moisture stability during the shelf lifetime measurement over 70 days under ambient conditions.

Despite considerable development in performance, both poor operational stability and high costs associated with hole conductors such as 2,2 ',7,7 '-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9 '-spirobifluorene (spiro-OMeTAD) and Poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine (PTAA) of perovskite solar cells (PSCs) need to be addressed by the research community. Here, two nonspiro hole transporting materials (HTMs), namely HTM-1 and HTM-2, are designed and straightforwardly synthesized exhibiting remarkable electrochemical properties and hole mobilities. In particular, the PSC based on the methoxy derivative (HTM-2) exhibits a remarkable efficiency of 21.2% (stabilized efficiency of 20.8%), which is superior to the benchmark HTM spiro-OMeTAD (stabilized efficiency of 20.4%). These results establish that the molecular design is effective in improving the performance of PSCs. Importantly, these two HTMs show admissible long-term stability under different harsh conditions such as thermal stress up to 85 degrees C, high humidity level of 60% +/- 10%, and continuous illumination over 1000 h. These insights allow correlating the impact of molecular design on optoelectronic properties of nonspiro-based hole conductors with the overall device performance.

Despite the outstanding role of mesoscopic structures on the efficiency and stability of perovskite solar cells (PSCs) in the regular (n–i–p) architecture, mesoscopic PSCs in inverted (p–i–n) architecture have rarely been reported. Herein, an efficient and stable mesoscopic NiOx (mp-NiOx) scaffold formed via a simple and low-cost triblock copolymer template-assisted strategy is employed, and this mp-NiOx film is utilized as a hole transport layer (HTL) in PSCs, for the first time. Promisingly, this approach allows the fabrication of homogenous, crack-free, and robust 150 nm thick mp-NiOx HTLs through a facile chemical approach. Such a high-quality templated mp-NiOx structure promotes the growth of the perovskite film yielding better surface coverage and enlarged grains. These desired structural and morphological features effectively translate into improved charge extraction, accelerated charge transportation, and suppressed trap-assisted recombination. Ultimately, a considerable efficiency of 20.2% is achieved with negligible hysteresis which is among the highest efficiencies for mp-NiOx based inverted PSCs so far. Moreover, mesoscopic devices indicate higher long-term stability under ambient conditions compared to planar devices. Overall, these results may set new benchmarks in terms of performance for mesoscopic inverted PSCs employing templated mp-NiOx films as highly efficient, stable, and easy fabricated HTLs.

Hole transport materials (HTM) are an important component in perovskite solar cells (PSC). Despite a multitude of HTMs developed in recent years, only few of them lead to solar cells with efficiencies over 20%. Therefore, it is still a challenge to develop high-performing HTMs, which have ideal energy levels of the frontier orbitals, are highly efficient in transporting charges, and stabilize the solar cell at the same time. In this work, the development of a structurally novel molecular HTM, CPDA 1, is described which is based on a common cyclopentadiene core and can be efficiently and inexpensively synthesized from readily available starting materials, which is important for future realization of low-cost photovoltaics on larger scale. Due to excellent optoelectronic, thermal, and transport properties, CPDA 1 not only meets the envisioned properties by reaching high efficiencies of 23.1%, which is among the highest reported to date, but also contributes to a respectable long-term stability of the PSCs.

Hybrid halide perovskite materials have become one of the leading candidates for various optoelectronic applications. They are based on organic-inorganic structures defined by the AMX(3) composition, were A is the central cation that can be either organic (e.g., methylammonium, fonnamidinium (FA)) or inorganic (e.g., Cs+), M is a divalent metal ion (e.g., Pb2+ or Sn2+), and X is a halide anion (I-, Br-,( )or Cl-). In particular, FAPbI(3) perovskites have shown remarkable optoelectronic properties and thermal stabilities. However, the photoactive alpha-FAPbI(3) (black) perovskite phase is not thermodynamically stable at ambient temperature and forms the delta-FAPbI(3) (yellow) phase that is not suitable for optoelectronic applications. This has stimulated intense research efforts to stabilize and realize the potential of the alpha-FAPbI(3) perovskite phase. In addition, hybrid perovskites were proven to be unstable against the external environmental conditions (air and moisture) and under device operating conditions (voltage and light), which is related to various degradation mechanisms. One of the strategies to overcome these instabilities has been based on low-dimensional hybrid perovskite materials, in particular layered two-dimensional (2D) perovskite phases composed of organic layers separating hybrid perovskite slabs, which were found to be more stable toward ambient conditions and ion migration. These materials are mostly based on S(x)A(n-1)Pb(n)X(3n+1) composition with various mono- (x = 1) or bifunctional (x = 2) organic spacer cations that template hybrid perovskite slabs and commonly form either Ruddlesden-Popper (RP) or Dion Jacobson (DJ) phases. These materials behave as natural quantum wells since charge carriers are confined to the inorganic slabs, featuring a gradual decrease in the band gap as the number of inorganic layers (n) increases from n = 1 (2D) to n = co (3D). While various layered 2D perovskites have been developed, their FA-based analogues

Despite rapid improvements in efficiency, long-term stability remains a challenge limiting the future up-scaling of perovskite solar cells (PSCs). Although several approaches have been developed to improve the stability of PSCs, applying ammonium passivation materials in bilayer configuration PSCs has drawn intensive research interest due to the potential of simultaneously improving long-term stability and boosting power conversion efficiency (PCE). This review focuses on the recent advances of improving n-i-p PSCs photovoltaic performance by employing ammonium halide-based molecular modulators. The first section briefly summarizes the challenges of perovskite materials by introducing the degradation mechanisms associated with the hygroscopic nature and ion migration issues. Then, recent reports regarding the roles of overlayers formed from ammonium-based passivation agents are discussed on the basis of ligand and halide effects. This includes both the formation of 2D perovskite films as well as purely organic passivating layers. Finally, the last section provides future perspectives on the use of organic ammonium halides within bilayer-architecture PSCs to improve the photovoltaic performances. Overall, this review provides a roadmap on current demands and future research directions of molecular modulators to address the critical limitations of PSCs, to mitigate the major barriers on the pathway toward future up-scaling applications.

It is well established that the lack of understanding the crystallization process in a two-step sequential deposition has a direct impact on efficiency, stability, and reproducibility of perovskite solar cells. Here, we try to understand the solid-solid phase transition occurring during the two-step sequential deposition of methylammonium lead iodide and formamidinium lead iodide. Using metadynamics, x-ray diffraction, and Raman spectroscopy, we reveal the microscopic details of this process. We find that the formation of perovskite proceeds through intermediate structures and report polymorphs found for methylammonium lead iodide and formamidinium lead iodide. From simulations, we discover a possible crystallization pathway for the highly efficient metastable alpha phase of formamidinium lead iodide. Guided by these simulations, we perform experiments that result in the low-temperature crystallization of phase-pure alpha-formamidinium lead iodide.