吴晓晓

The design principle for lead-free perovskites and the progress of typical lead-free perovskite photodetectors are reviewed and discussed. The outlook for future research and applications is then explored.

Abstract

State-of-the-art photodetectors which apply hybrid perovskite materials have emerged as powerful candidates for next-generation light sensing. Among them, lead-based ones are the most popular beyond doubt on account of their unique and superior optoelectronic properties. Nevertheless, trade-off toward commercialization exists between nontoxicity and high performance, with the poor stability of lead-based perovskites, indicating that it is indispensable to substitute lead with nontoxic element meanwhile bringing about a comparable figure of merit of photodetectors and relatively long-term stability. Herein, recent advances in lead-free perovskite photodetectors are reviewed, analyzing the principle while designing new materials and highlighting some remarkable progress, which are comparable, even superior, to lead-based photodetectors. Furthermore, their potential strategy in optical communication, image sensing, narrowband photodetection, etc., is examined and a perspective on developing new materials and photodetectors with superior properties for more practical applications is provided.

An electrical loss management strategy by using SMe-TATPyr molecule manipulating dopant-free Poly(3-hexylthiophene) (P3HT) has been developed and employed to fabricate efficient and thermally stable CsPbI₂Br solar cells. The P3HT/SMe-TATPyr presents optimized molecular orientation, favorable energy level alignment and effective defect passivation. Based on P3HT/SMe-TATPyr HTLs, the fabricated devices yield a record-high efficiency of 16.93 % for CsPbI₂Br solar cells with dopant-free HTLs.

Abstract

Inorganic cesium lead halide perovskites offer a pathway towards thermally stable photovoltaics. However, moisture-induced phase degradation restricts the application of hole transport layers (HTLs) with hygroscopic dopants. Dopant-free HTLs fail to realize efficient photovoltaics due to severe electrical loss. Herein, we developed an electrical loss management strategy by manipulating poly(3-hexylthiophene) with a small molecule, i.e., SMe-TATPyr. The developed P3HT/SMe-TATPyr HTL shows a three-time increase of carrier mobility owing to breaking the long-range ordering of “edge-on” P3HT and inducing the formation of “face-on” clusters, over 50 % decrease of the perovskite surface defect density, and a reduced voltage loss at the perovskite/HTL interface because of favorable energy level alignment. The CsPbI₂Br perovskite solar cell demonstrates a record-high efficiency of 16.93 % for dopant-free HTL, and superior moisture and thermal stability by maintaining 96 % efficiency at low-humidity condition (10–25 % R. H.) for 1500 hours and over 95 % efficiency after annealing at 85 °C for 1000 hours.

Nature Communications, Published online: 26 February 2021; doi:10.1038/s41467-021-21493-w

A multidentate ligand synthetic strategy based on a linear amphiphilic PMMA‐b‐PAA diblock copolymer prepared from RAFT polymerization enables the synthesis of PMMA‐tethered CsPbBr₃/TiO₂ core/shell nanosheets (NSs). These NSs have markedly enhanced stabilities (i.e., thermal, photostability, moisture, polar solvent, aliphatic amine, etc.), and thus potential applications in optoelectronic materials and devices.

Abstract

Approaches to achieve stable perovskite nanocrystals (PNCs) of interest, in particular those with large structural anisotropy, through protective coating of the inorganic shell at a single‐nanocrystal (NC) level are comparatively few and limited in scope. Reported here is a robust amphiphilic‐diblock‐copolymer‐enabled strategy for crafting highly‐stable anisotropic CsPbBr₃ nanosheets (NSs) by in situ formation of a uniform inorganic shell (1st shielding) that is intimately ligated with hydrophobic polymers (2nd shielding). The dual‐protected NSs display an array of remarkable stabilities (i.e., thermal, photostability, moisture, polar solvent, aliphatic amine, etc.) and find application in white‐light‐emitting diodes. In principle, by anchoring other multidentate amphiphilic polymer ligands on the surface of PNCs, followed by templated‐growth of shell materials of interest, a rich variety of dual‐shelled, multifunctional PNCs with markedly improved stabilities can be created for use in optics, optoelectronics, and sensory devices.

Vacancy‐ordered double perovskites, A ₂RuCl₆ and A ₂RuBr₆, were studied. The synthesis, crystal structures, and optical and magnetic properties of the materials were determined. Differences in the halide and the A cation size influence the charge‐transfer spectra. On the basis of temperature‐dependent magnetic moments, accurate values were estimated for the spin‐orbit coupling constants of the Ru^IV d ⁴ ion.

Abstract

Vacancy‐ordered double perovskites are attracting significant attention due to their chemical diversity and interesting optoelectronic properties. With a view to understanding both the optical and magnetic properties of these compounds, two series of Ru^IV halides are presented; A ₂RuCl₆ and A ₂RuBr₆, where A is K, NH₄, Rb or Cs. We show that the optical properties and spin‐orbit coupling (SOC) behavior can be tuned through changing the A cation and the halide. Within a series, the energy of the ligand‐to‐metal charge transfer increases as the unit cell expands with the larger A cation, and the band gaps are higher for the respective chlorides than for the bromides. The magnetic moments of the systems are temperature dependent due to a non‐magnetic ground state with J _eff=0 caused by SOC. Ru‐X covalency, and consequently, the delocalization of metal d‐electrons, result in systematic trends of the SOC constants due to variations in the A cation and the halide anion.

Carbon nanoparticles can serve as versatile auxiliary components for metal halide perovskites: they promote crystallization, passivate defects, tune charge transfer/transport characteristics, and improve the performance of perovskite‐based optoelectronic devices. This review focuses on the tunability of the electronic properties of carbon nanoparticles such as graphene quantum dots and carbon dots, analyzes their interactions with perovskite components, and introduces different strategies of their implementation in solar cells, light‐emitting diodes, luminescent solar concentrators, and photodetectors.

Abstract

Metal halide perovskite‐based optoelectronics has experienced an unprecedented development in the last decade, while further improvements of efficiency, stability, and economic gains of such devices require novel engineering concepts. The use of carbon nanoparticles as versatile auxiliary components of perovskite‐based optoelectronic devices is one strategy that offers several advantages in this respect. In this review, first, a brief introduction is offered on metal halide perovskites and on the major performance characteristics of related optoelectronic devices. Then, the versatility and merits of different kinds of carbon nanoparticles, such as graphene quantum dots and carbon dots, are discussed. The tunability of their electronic properties is focused upon, their interactions with perovskite components are analyzed, and different strategies of their implementation in optoelectronic devices are introduced, which include solar cells, light‐emitting diodes, luminescent solar concentrators, and photodetectors. It is shown how carbon nanoparticles influence charge carriers extraction and transport, promote perovskite crystallization, allow for efficient passivation, block ion migration, suppress hysteresis, enhance their environmental stability, and thus improve the performance of perovskite‐based optoelectronic devices.

Long-term thermal stability is critical for perovskite solar cells (PSCs) operation. Un-encapsulated co-evaporated MAPbI₃ PSCs, contrary to similar spin-coated PSCs, exhibit remarkable structural robustness and retain 80% of their initial efficiency after 3600 h aging at 85 °C. This excellent intrinsic stability is driven by the strain-stress-free MAPbI₃ grown by co-evaporation, where post-annealing is not required to fully form the perovskite.

Abstract

Thermal stability is a critical criterion for assessing the long-term stability of perovskite solar cells (PSCs). Here, it is shown that un-encapsulated co-evaporated MAPbI₃ (TE_MAPbI₃) PSCs demonstrate remarkable thermal stability even in an n-i-p structure that employs Spiro-OMeTAD as hole transport material (HTM). TE_MAPbI₃ PSCs maintain over ≈95% and ≈80% of their initial power conversion efficiency (PCE) after 1000 and 3600 h respectively under continuous thermal aging at 85 °C. TE_MAPbI₃ PSCs demonstrate remarkable structural robustness, absence of pinholes, or significant variation in grain sizes, and intact interfaces with the HTM, upon prolonged thermal aging. Here, the main factors driving TE_MAPbI₃ stability are assessed. It is demonstrated that the excellent TE_MAPbI₃ thermal stability is related to the perovskite growth process leading to a compact and almost strain-stress-free film. On the other hand, un-encapsulated PSCs with the same architecture, but incorporating solution-processed MAPbI₃ or Cs_0.05(MA_0.17FA_0.83)_0.95Pb(I_0.83Br_0.17)₃ as active layers, show a complete PCE degradation after 500 h under the same thermal aging condition. These results highlight that the control of the perovskite growth process can substantially enhance the PSCs thermal stability, besides the chemical composition. The TE_MAPbI₃ impressive long-term thermal stability features the potential for field-operating conditions.

Publication date: 17 March 2021

Source: Joule, Volume 5, Issue 3

Author(s): Chaoyang Kuang, Zhangjun Hu, Zhongcheng Yuan, Kaichuan Wen, Jian Qing, Libor Kobera, Sabina Abbrent, Jiri Brus, Chunyang Yin, Heyong Wang, Weidong Xu, Jianpu Wang, Sai Bai, Feng Gao

Publication date: Available online 19 February 2021

Source: Joule

Author(s): Kaitlyn T. VanSant, Adele C. Tamboli, Emily L. Warren

A 2D zinc‐based metal–organic frameworks (MOFs) with plenty of active sites are prepared through a molecular self‐assembly strategy and used to solve the incompatibility between MOFs and perovskite films. The tunable crystallographic orientation and functionalized framework of this material are considered for regulating the photovoltaic performance and stability of perovskite solar cells.

Abstract

Perovskite degradation induced by surface defects and imperfect grain boundaries of films seriously damages the performance of perovskite solar cells (PSCs). Meanwhile, conventional organic molecules cannot maintain the long‐time passivation effects under the stimulation of external environmental factors. Here, efficient and stable grain passivation in perovskite films is realized by preparing formic acid‐functionalized 2D metal–organic frameworks (MOFs) as the terminated agent. Through robust interactions between exposed active sites and PbI₂, the 2D MOFs tightly caps the surface of PbI₂‐terminated perovskite grains to stabilize the perovskite phases and aids the adhesion of adjacent grains. The MOFs mainly distributed at the grain boundaries of the perovskite film is directly observed at the microscopic scale. The modified perovskite films have regular morphology, lower defect density, and superior optoelectronic properties. Benefiting from the suppressed charge recombination and faster charge extraction, a power conversion efficiency of 21.28% is achieved for the best‐performing PSC device. The unencapsulated PSCs with the MOFs modification maintain 88% and 81% of their initial efficiency after 750 h heating at 85 °C under N₂ atmosphere and more than 1000 h storage in ambient environment (25 °C, RH ≈ 40%), respectively.

The strong interaction between F and Sn changes the electron cloud density around Sn atoms by introducing RbF into SnO₂ colloidal dispersion, contributing to the improved electron mobility of SnO₂. While spin‐coating RbF onto the SnO₂ surface, the Rb⁺ cations escape into the bulk perovskite, which inhibits ion migration and decreases the trap density.

Abstract

Regulating the electron transport layer (ETL) has been an effective way to promote the power conversion efficiency (PCE) of perovskite solar cells (PSCs) as well as suppress their hysteresis. Herein, the SnO₂ ETL using a cost‐effective modification material rubidium fluoride (RbF) is modified in two methods: 1) adding RbF into SnO₂ colloidal dispersion, F and Sn have a strong interaction, confirmed via X‐ray photoelectron spectra and density functional theory results, contributing to the improved electron mobility of SnO₂; 2) depositing RbF at the SnO₂/perovskite interface, Rb⁺ cations actively escape into the interstitial sites of the perovskite lattice to inhibit ions migration and reduce non‐radiative recombination, which dedicates to the improved open‐circuit voltage (V _oc) for the PSCs with suppressed hysteresis. In addition, double‐sided passivated PSCs, RbF on the SnO₂ surface, and p‐methoxyphenethylammonium iodide on the perovskite surface, produces an outstanding PCE of 23.38% with a V _oc of 1.213 V, corresponding to an extremely small V _oc deficit of 0.347 V.

Temperature‐dependent transient absorption and time‐resolved photoluminescence investigations provide insights into electronic processes in heterogeneous organic‐inorganic halide perovskites. By taking 3D and quasi‐2D perovskite model systems, evidence is provided that charge carrier transfer (a double charge transfer) rather than energy migration dominates in heterogeneous quasi‐2D perovskite films.

Abstract

Heterogeneous organic‐inorganic halide perovskites possess inherent non‐uniformities in bandgap that are sometimes engineered and exploited on purpose, like in quasi‐2D perovskites. In these systems, charge carrier and excitation energy migration to lower‐bandgap sites are key processes governing luminescence. The question, which of them dominates in particular materials and under specific experimental conditions, still remains unanswered, especially when charge carriers comprise excitons. In this study transient absorption (TA) and transient photoluminescence (PL) techniques are combined to address the excited state dynamics in quasi‐2D and other heterogeneous perovskite structures in broad temperature range, from room temperature down to 15 K. The data provide clear evidence that charge carrier transfer rather than energy migration dominates in heterogeneous quasi‐2D perovskite films.