Traps in metal halide perovskites: characterization and passivation

Simultaneous Defect Passivation and Electric Level Regulation with Rubidium Fluoride for High-Efficiency CsPbI2Br Perovskite Solar Cells

Due to the good balance of efficiency and stability, CsPbI2Br perovskite solar cells (PSCs) recently have attracted widespread attention. However, the improvement in photovoltaic performance for CsPbI2Br PSCs was mainly limited by massive defects and unmatched energy levels. Surface modification is the most convenient and effective strategy to decrease defect densities of perovskite films. Herein, we deposited rubidium fluoride (RbF) onto the surface of CsPbI2Br perovskite films by spin-coating. The numerous defects could be significantly passivated by RbF, resulting in suppressed nonradiative recombination. Furthermore, the CsPbI2Br perovskite film after RbF treatment exhibits a deeper Fermi level, and an additional built-in electric field forms to promote charge transport. Consequently, the champion device achieves a high efficiency of 10.82% with an improved VOC of 1.14 V, and it also exhibits excellent stability after long-term storage. This work offers a simple and effective approach to enhance the photovoltaic performance and stability of PSCs for broader applications in the future.

Ambient-Stable 2D Dion-Jacobson Phase Tin Halide Perovskite Field-Effect Transistors with Mobility over 1.6 Cm2 V-1 s-1

Solution-processed metal halide perovskites hold immense potential for the advancement of next-generation field-effect transistors (FETs). However, the instability of perovskite-based transistors has impeded their progress and practical applications. Here, ambient-stable high-performance FETs based on 2D Dion-Jacobson phase tin halide perovskite BDASnI4 , which has high film quality and excellent electrical properties, are reported. The perovskite channels are established by engineering the film crystallization process via the employment of ammonium salt interlayers and the incorporation of NH4 SCN additives within the precursor solution. The refined FETs demonstrate field-effect hole mobilities up to 1.61 cm2 V-1 s-1 and an on/off ratio surpassing 106 . Moreover, the devices show impressive operational and environmental stability and retain their functional performance even after being exposed to ambient conditions with a temperature of 45 °C and humidity of 45% for over 150 h.

Ferroelectric Wide-Bandgap Metal Halide Perovskite Field-Effect Transistors: Toward Transparent Electronics

Transparent field-effect transistors (FETs) are attacking intensive interest for constructing fancy "invisible" electronic products. Presently, the main technology for realizing transparent FETs is based on metal oxide semiconductors, which have wide-bandgap but generally demand sputtering technique or high-temperature (>350 °C) solution process for fabrication. Herein, a general device fabrication strategy for metal halide perovskite (MHP) FETs is shown, by which transparent perovskite FETs are successfully obtained using low-temperature (<150 °C) solution process. This strategy involves the employment of ferroelectric copolymer poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) as the dielectric, which conquers the challenging issue of gate-electric-field screening effect in MHP FETs. Additionally, an ultra-thin SnO₂ is inserted between the source/drain electrodes and MHPs to facilitate electron injection. Consequently, n-type semi-transparent MAPbBr₃ FETs and fully transparent MAPbCl₃ FETs which can operate well at room temperature with mobility over 10^-3  cm²  V^-1  s^-1 and on/off ratio >10³ are achieved for the first time. The low-temperature solution processability of these FETs makes them particularly attractive for applications in low-cost, large-area transparent electronics.

Strategy of Enhancing Built-in Field to Promote the Application of C-TiO2 /SnO2 Bilayer Electron Transport Layer in High-Efficiency Perovskite Solar Cells (24.3%)

Combining two kinds of electron transport layer (ETL) which have complementary advantages into a bilayer structure to form a bilayer ETL is an effective way to transcend inherent limitations of single-layer ETL, which is very helpful in the development of perovskite solar cells (PSCs). In this work, a strategy is proposed to break constraints on the application of the staggered bilayer ETL in high-efficiency PSC, namely utilizing a built-in field to overcome the dilemma in E_CBM making it possible to improve V_OC and FF simultaneously by tuning the Fermi level of ETLs properly. According to the strategy, a bilayer ETL structure comprised of C-TiO₂ and SnO₂ layer and corresponding Li-doping process are developed, and the characterization results confirm the effectiveness of the strategy, making the potentials of the C-TiO₂ (Li)/SnO₂ bilayer ETL fully released for its application in high-efficiency PSCs: a V_OC of 1.201 V for an ordinary triple-cation-perovskite-based PSC and a photoelectric conversion efficiency of 24.3% for a low-bandgap-perovskite-based PSC with high haze FTO superstrate are successfully achieved, indicating that the C-TiO₂ (Li)/SnO₂ bilayer ETL is a successful application paradigm of the proposed strategy and very promising in the application of high-efficiency PSCs.

Universal Surface Passivation of Organic-Inorganic Halide Perovskite Films by Tetraoctylammonium Chloride for High-Performance and Stable Perovskite Solar Cells

The power conversion efficiency (PCE) of perovskite solar cells has been showing rapid improvement in the last decade. However, still, there is an unarguable performance deficit compared with the Schockley-Queisser (SQ) limit. One of the major causes for such performance discrepancy is surface and grain boundary defects. They are a source of nonradiative recombination in the devices that not only causes performance loss but also instability of the solar cells. In this study, we employed a direct postsurface passivation strategy at mild temperatures to modify perovskite layer defects using tetraoctylammonium chloride (TOAC). The passivated perovskite layers have demonstrated extraordinary improvement in photoluminescence and charge carrier lifetimes compared to their control counterparts in both Cs_0.05(FAPbI₃)_0.83(MAPbBr₃)_0.17 and MAPbI₃-type perovskite layers. The investigation on electron-only and hole-only devices after TOAC treatment revealed suppressed electron and hole trap density of states. The electrochemical study demonstrated that TOAC treatment improved the charge recombination resistance of the perovskite layers and reduced the charge accumulation on the surface of perovskite films. As a result, perovskite solar cells prepared by TOAC treatment showed a champion PCE of 21.24% for the Cs_0.05(FAPbI₃)_0.83(MAPbBr₃)_0.17-based device compared to 19.58% without passivation. Likewise, the PCE of MAPbI₃ improved from 18.09 to 19.27% with TOAC treatment. The long-term stability of TOAC-passivated perovskite Cs_0.05(FAPbI₃)_0.83(MAPbBr₃)_0.17 devices has retained over 97% of its initial performance after 720 h in air.

Suppressed Blinking and Polarization-Dependent Emission Enhancement of Single ZnCdSe/ZnS Dot Coupled with Au Nanorods

Fluorescent quantum dots (QDs) have attracted extensive attention because of their promising applications in many fields such as quantum optics, optoelectronics, solid-state lighting, and bioimaging. However, photo-blinking, low emission efficiency, and instability are the drawbacks of fluorescent QD-based devices, affecting their optical properties and practical applications. Here, we report suppressed blinking, enhanced radiative rate, and polarization-dependent emission properties of single ZnCdSe/ZnS QDs assembled on the surface of Au nanorods (NRs). We found that the local surface plasmon (LSP) of Au NRs significantly regulates the excitation and emission properties of the composite ZnCdSe/ZnS QD-Au NRs (QD-Au NRs). The average number of photons emitted per unit time from single QD-Au NRs has been significantly enhanced compared with that of single ZnCdSe/ZnS QDs on the coverslip, accompanied by a drastically shortened lifetime and suppressed blinking. According to the experimental and simulation analysis, the photogenerated LSP field of Au NRs remarkably increases the excitation transition and the radiative rates of QD-Au NRs. Although the emission efficiency is slightly increased, the synergetic enhancement of excitation and radiative rates sufficiently competes with the nonradiative process to compensate for the low emission efficiency of QDs and ultimately suppress the photo-blinking of QD-Au NRs. Moreover, the polarization-dependent emission enhancement has also been observed and theoretically analyzed, demonstrating good consistency and confirming the contribution of excitation enhancement. Our findings present a practical strategy to improve the optical properties and stability of single QD-Au NR composite and provide essential information for a deep understanding of the interaction between emitters and the LSP field of metal nanoparticles.

Light Emission Properties of Thermally Evaporated CH3NH3PbBr3 Perovskite from Nano- to Macro-Scale: Role of Free and Localized Excitons

Over the past decade, interest about metal halide perovskites has rapidly increased, as they can find wide application in optoelectronic devices. Nevertheless, although thermal evaporation is crucial for the development and engineering of such devices based on multilayer structures, the optical properties of thermally deposited perovskite layers (spontaneous and amplified spontaneous emission) have been poorly investigated. This paper is a study from a nano- to micro- and macro-scale about the role of light-emitting species (namely free carriers and excitons) and trap states in the spontaneous emission of thermally evaporated thin layers of CH₃NH₃PbBr₃ perovskite after wet air UV light trap passivation. The map of light emission from grains, carried out by SNOM at the nanoscale and by micro-PL techniques, clearly indicates that free and localized excitons (EXs) are the dominant light-emitting species, the localized excitons being the dominant ones in the presence of crystallites. These species also have a key role in the amplified spontaneous emission (ASE) process: for higher excitation densities, the relative contribution of localized EXs basically remains constant, while a clear competition between ASE and free EXs spontaneous emission is present, which suggests that ASE is due to stimulated emission from the free EXs.

Study on the long time aging behavior of MAPbI3: from experiment to first-principles simulation

Both defect passivation and preventing ion diffusion are necessary for reaching the stable perovskite film.

Mitigation of vacancy with ammonium salt-trapped ZIF-8 capsules for stable perovskite solar cells through simultaneous compensation and loss inhibition

Due to the easy loss of ions during synthesis or usage, vacancies in perovskite film are ubiquitous, accelerating the degradation of perovskite materials and seriously hampering the stability of perovskite solar cells (PSCs). Herein, to simultaneously compensate for vacancies and reduce ammonium cation loss, a sustained release strategy was proposed by introducing multi-functional capsules consisting of zeolitic imidazolate framework-8 (ZIF-8) encapsulation agent and ammonium iodide salts as interlayer between the perovskite and hole transport layer. In the capsule interlayer, not only are ammonium iodide salts in ZIF-8 pores released to the perovskite layer, compensating for the vacancies, but the ZIF-8 also prevents the organic component of perovskite from evaporating and isolates the perovskite from moisture. As a consequence, decreased trap density, improved device efficiency, and enhanced stability of PSCs are obtained owing to the successful passivation of defects by the introduced capsules. ZIF-8@FAI shows the highest efficiency of 19.13% and a stabilized PCE over 93% of the initial efficiency at maximum power point for 150 h. This work provides a new strategy to improve efficiency and stability of PSCs based on the large family of porous materials.