Multifunctional π-Conjugated Additives for Halide Perovskite

Crown-Assisted CsCu2I3 Growth and Trap Passivation for Perovskite Light-Emitting Diodes

Copper (Cu)-based perovskites are promising for lead-free perovskite light-emitting diodes (PeLEDs). However, it remains a significant challenge to achieve high performance devices due to the nonradiative loss caused by the disordered crystallization and lack of passivation. Crown ethers are known to form host-guest complexes by the interaction between C-O-C groups and certain cations, and 18-crown-6 (18C6) with an appropriate complementary size can interact with Cs+ and Cu+ cations. Herein, we studied the interaction between CsCu2I3 and two crowns with the same cyclic size, 18C6 and dibenzo-18-crown-6 (D18C6). Particularly, D18C6 can reduce the nonradiative recombination rate of CsCu2I3 film by passivating the defects and optimizing the film morphology effectively. The room mean square (RMS) decreased from 5.06 to 2.95 nm, and the PLQY was promoted from 4.71% to 19.9%. Besides, D18C6 can also decrease the barrier of hole injection. The PeLEDs based on D18C6-modified CsCu2I3 realized noticeable improvement with a maximum luminance and EQE of 583 cd/m2 and 0.662%, respectively.

New Nanophotonics Approaches for Enhancing the Efficiency and Stability of Perovskite Solar Cells

Over the past decade, the power conversion efficiency (PCE) of perovskite solar cells (PSCs) has experienced a remarkable ascent, soaring from 3.8% in 2009 to a remarkable record of 26.1% in 2023. Many recent approaches for improving PSC performance employ nanophotonic technologies, from light harvesting and thermal management to the manipulation of charge carrier dynamics. Plasmonic nanoparticles and arrayed dielectric nanostructures have been applied to tailor the light absorption, scattering, and conversion, as well as the heat dissipation within PSCs to improve their PCE and operational stability. In this review, it is begin with a concise introduction to define the realm of nanophotonics by focusing on the nanoscale interactions between light and surface plasmons or dielectric photonic structures. Prevailing strategies that utilize resonance-enhanced light-matter interactions for boosting the PCE and stability of PSCs from light trapping, carrier transportation, and thermal management perspectives are then elaborated, and the resultant practical applications, such as semitransparent photovoltaics, colored PSCs, and smart perovskite windows are discussed. Finally, the state-of-the-art nanophotonic paradigms in PSCs are reviewed, and the benefits of these approaches in improving the aesthetic effects and energy-saving character of PSC-integrated buildings are highlighted.

Passivating A-Site and X-Site Vacancies Simultaneously via N-Heterocyclic Amines for Efficient Cs2AgBiBr6 Solar Cells

To address the toxicity and stability issues of traditional lead halide perovskite solar cells (PSCs), the development of lead-free PSCs, such as Cs2AgBiBr6 solar cells, is of great significance. However, due to the low defect formation energy of Cs2AgBiBr6, a large number of vacancies, including A-site vacancies and X-site vacancies, form during the fabrication process of the Cs2AgBiBr6 film, which seriously damage the performance of the devices. The traditional phenylethylammonium (PEA) cation, mainly focusing on passivating A-site vacancies, is incapable of reducing X-site vacancies and so results in a limited performance improvement in Cs2AgBiBr6 solar cells. Herein, inspired by the capability of the Lewis base to coordinate with metal cations, a series of N-heterocyclic amines are introduced to serve as a dual-site passivator, reducing A-site and X-site vacancies at the same time. The highest power conversion efficiency of modified Cs2AgBiBr6 solar cells has been increased 36% from 1.10 to 1.50%. Further investigation reveals that the higher electron density of additives would lead to a stronger interaction with metal cations like Ag+ and Bi3+, thus reducing more X-site defects and improving carrier dynamics. Our work provides a strategy for passivating perovskite with various kinds of defects and reveals the connection between the coordination capability of additives and device performance enhancement, which could be instructive in improving the performance of lead-free PSCs.

Exploring the effect of C6H5-x/FxBr (x = 0-3) passivating agent on surface properties at different termination ends: first principles

To prevent further decomposition of organic-inorganic hybrid perovskite by defects, in this work density functional theory was applied to explore the electronic properties, carrier surface mobility and theoretical photoelectric conversion efficiency (PCE) of passivating molecules with different fluorine atom content at the symmetric site of the benzene ring at different termination ends of MAPbI3, which shed light on the control of perovskite surface passivation by different element atoms in the same molecule. We found that the same molecule acts as a different passivation agent at different termination faces. Passivating molecules on the surface termination end by MAI play a Lewis acid role, with molecules with stronger dipole moments narrowing the band gap from the original 1.77 to 1.73 eV. The exciton binding energy of molecules with stronger dipole moments (0.187-0.292 meV) is significantly lower than that of MAPbI3 (0.332 meV), so the effective separation of interface electrons and holes can be realized. Bromopenta-fluorobenzene has a lower adsorption energy of -0.17 eV, which can stably adsorb on the surface of perovskite and increase visible light absorption. Ultimately, the theoretical PCE increased from 15.8% to 16.16%. In addition, on the surface terminated by PbI2, BrB with a strong dipole moment can provide electrons for Pb2+ and act as a Lewis base. At the surface end, it can form an ionic bond with Pb2+, while the antibonding molecular orbital characteristic is dominant, which increases the band gap from 1.76 to 1.87 eV. After increasing to 4-F-BrB, the fluorine atom has strong electronegativity and can easily bond with Pb2+. The conjugate π cycle intensifies the promotion of electron transfer, reducing the work function from 5.262 to 4.703 eV, reducing the effective electron and hole mass (0.514, 0.204 m0), and improving the photovoltaic performance. Finally, increasing the number of passivation molecules resulted in a decrease in the PCE from 15.93% to 14.75%.

Nanotechnology for brain tumor imaging and therapy based on π-conjugated materials: state-of-the-art advances and prospects

In contemporary biomedical research, the development of nanotechnology has brought forth numerous possibilities for brain tumor imaging and therapy. Among these, π-conjugated materials have garnered significant attention as a special class of nanomaterials in brain tumor-related studies. With their excellent optical and electronic properties, π-conjugated materials can be tailored in structure and nature to facilitate applications in multimodal imaging, nano-drug delivery, photothermal therapy, and other related fields. This review focuses on presenting the cutting-edge advances and application prospects of π-conjugated materials in brain tumor imaging and therapeutic nanotechnology.

Suppressing Excess Lead Iodide Aggregation and Reducing N-Type Doping at Perovskite/HTL Interface for Efficient Perovskite Solar Cells

Excess lead iodide (PbI2 ) aggregation at the charge carrier transport interface leads to energy loss and acts as unstable origins in perovskite solar cells (PSCs). Here, a strategy is reported to modulate the interfacial excess PbI2 by introducing π-conjugated small-molecule semiconductors 4,4'-cyclohexylbis[N,N-bis(4-methylphenyl)aniline] (TAPC) into perovskite films through an antisolvent addition method. The coordination of TAPC to PbI units through the electron-donating triphenylamine groups and π-Pb2+ interactions allows for a compact perovskite film with reduced excess PbI2 aggregates. Besides, preferred energy level alignment is achieved due to the suppressed n-type doping effect at the hole transport layer (HTL) interfaces. As a result, the TAPC-modified PSC based on Cs0.05 (FA0.85 MA0.15 )0.95 Pb(I0.85 Br0.15 )3 triple-cation perovskite achieved an improved PCE from 18.37% to 20.68% and retained ≈90% of the initial efficiency after 30 days of aging under ambient conditions. Moreover, the TAPC-modified device based on FA0.95 MA0.05 PbI2.85 Br0.15 perovskite produced an improved efficiency of 23.15% compared to the control (21.19%). These results provide an effective strategy for improving the performance of PbI2 -rich PSCs.

Recent Progress of Layered Perovskite Solar Cells Incorporating Aromatic Spacers

Layered two dimensional (2D) or quasi-2D perovskites are emerging photovoltaic materials due to their superior environment and structure stability in comparison with their 3D counterparts. The typical 2D perovskites can be obtained by cutting 3D perovskites along  < 100 >  orientation by incorporation of bulky organic spacers, which play a key role in the performance of 2D perovskite solar cells (PSCs). Compared with aliphatic spacers, aromatic spacers with high dielectric constant have the potential to decrease the dielectric and quantum confinement effect of 2D perovskites, promote efficient charge transport and reduce the exciton binding energy, all of which are beneficial for the photovoltaic performance of 2D PSCs. In this review, we aim to provide useful guidelines for the design of aromatic spacers for 2D perovskites. We systematically reviewed the recent progress of aromatic spacers used in 2D PSCs. Finally, we propose the possible design strategies for aromatic spacers that may lead to more efficient and stable 2D PSCs.

Hydrazone dye passivator for high-performance and stable perovskite solar cells

There has been rapid development of organic-inorganic perovskite solar cells (PSCs) in recent years, but efficiency and stability challenges remain due to the massive defects in perovskite films. Here, the organic dye Th-azi-Pyr (ethyl 2-(2-(3-methyl-5-oxo-1-phenyl-1,5-dihydro-4H-pyrazol-4-ylidene) hydrazineyl)-4,5,6,7-tetrahydrobenzo[b]thiophene-3-carboxylate) with carbonyl, pyrazolone structure, and benzene ring was synthesized and used to prepare high-quality perovskite film as an additive. Th-azi-Pyr formed relatively stable intermediate ligands with uncoordinated Pb and I, slowing the crystal growth and reducing the grain boundary defects of perovskite. In addition, the benzene ring in the dye protected against moisture and increased the stability of the perovskite film. Therefore, the Th-azi-Pyr-modified PSC assembled in an air environment exhibited a promising power conversion efficiency (PCE) of 19.27%, which is superior to the 15.33% of the control PSC. Additionally, 88% of the original performance was maintained after 300 h at 25 ± 5 °C and 50 ± 10% relative humidity, implying that the modified PSCs exhibited greater stability than the untreated PSCs. This work indicates that simple and low-cost organic dyes are excellent defect passivators for high-performance PSCs.

Orotic Acid as a Bifunctional Additive for Regulating Crystallization and Passivating Defects toward High-Performance Formamidinium-Cesium Perovskite Solar Cells

Formamidinium-cesium (FA-Cs) perovskites are an attractive candidate for perovskite solar cells (PSCs) with high stability, but they tend to suffer from high intrinsic defect density, especially at grain boundaries. Herein, a common heterocyclic conjugated molecule, orotic acid (ORO), was employed as a novel bifunctional additive to simultaneously achieve crystallization regulation and defect passivation of an FA-Cs perovskite toward efficient and stable PSCs. ORO was introduced to an FA-Cs perovskite precursor solution as an effective coordination-induced crystallization regulator to improve the grain size and crystallinity. Furthermore, under the assistance of π electrons, its carboxyl group bonded with undercoordinated Pb²⁺ defects at grain boundaries, and it was also able to form hydrogen bonds with undercoordinated I^- defects, thus significantly reducing defect density. The average power conversion efficiency of the produced PSC devices with the ORO additive was promoted from 17.81% for the control PSCs to 19.32%, and a champion efficiency of 20.62% with negligible hysteresis was achieved. Additionally, the optimized devices exhibited high resistance to moisture incursion, leading to decent environmental stability. This work provides a convenient yet efficient approach to improve crystallization and passivate defects toward PSCs with enhanced efficiency and stability.

In-Depth Insight into the Effect of Hydrophilic-Hydrophobic Group Designing in Amidinium Salts for Perovskite Precursor Solution on Their Photovoltaic Performance

Amidinium salts have been utilized in perovskite precursor solutions as additives to improve the quality of perovskite films. The design of hydrophilic or hydrophobic groups in amidinium salts is of great importance to photovoltaic device performance and stability in particular. Here we report a contrast study of a guanidinium iodide (GUI) additive with a hydrophilic NH₂ group, and a N,1-diiodoformamidine (DIFA) additive with a hydrophobic C-I group, to investigate the group effect. The addition of GUI or DIFA was beneficial to achieve high quality perovskite film and superior photovoltaic device performance. Compared with GUI, the addition of the DIFA in a perovskite precursor solution enhanced the crystal quality, reduced the defect density, and protected the water penetration into perovskite film. The perovskite solar cell (PSC) devices showed the best power conversion efficiency (PCE) of 21.19% for those modified with DIFA, as compared to 18.85% for the control, and 20.85% for those modified with GUI. In benefit to the hydrophobic C-I group, the DIFA-modified perovskite films and PSC exhibited the best light stability, thermal stability, and humidity stability in comparison to the control films and GUI-modified films. Overall, the introduction of a hydrophobic group in the amidinium salts additive was demonstrated to be an efficient approach to achieve high quality and stable perovskite film and PSC devices.