lengths, low exciton binding energy, and ease of solution processability, which promise the perovskite-based photovoltaic devices both high efficiency and low manufacture cost. The intense effort to improve power conversion efficiencies (PCEs) has resulted in PCEs of perovskite solar cells (PSCs) increasing from 3.8% in 2009 [5] to over 23% in 9 years. [6,7] Changes in material composition (e.g., using mixed cations and halides together into a 3D lead−halide perovskites, such as [FA/MA]Pb[I/Br] 3 , [8] [FA/Cs]Pb[I/Br] 3 , [9,10] or [Cs/FA/MA]Pb[I/Br] 3[11] ) have played essential role to improve the performances and structural stability. However, the intrinsic instability of the 3D lead−halide perovskites with regard to moisture, heat, light, and oxygen remains to be entirely circumvented. [12,13] Two main measures have been taken to increase the stability of PSCs: providing sufficient protection to the perovskite material [14] and increasing the intrinsic stability of the perovskite material. [15] More importantly, techniques that can improve device stability without sacrificing the efficiency are critical for the future of PSCs. In this regard, strategies called multidimensional perovskite (MDP) using Ruddlesden-Popper type perovskite (Class I) or low dimensional polymorphs passivated 3D perovskites (Class II) were brought into being increasing the intrinsic stability of the perovskite material. [16] Ruddlesden-Popper phase layered perovskites ((RNH 3 ) 2 (MA) n−1 Pb n X 3n+1 , n = 1, 2, 3, 4, 5, …) [17][18][19][20][21] which was introduced by Smith et al. showed albeit low PCE the superior ambient stability, [18] where RNH 3 are large aliphatic or aromatic ammonium cations represented by n-butylammonium (BA) and 2-phenylethylammonium (PEA). (BA) 2 (MA) 2 Pb 3 I 10 as a light absorber retained its performance after exposure to a highhumidity environment for 60 d. [20] So far the highest reported PCE of Ruddlesden-Popper type perovskite-based solar cell is 15.42%. [22] Very recently, Zhang et al. utilized synchrotron source based in situ measurement to disclose the phase transition kinetics during the crystallization of Ruddlesden-Popper type perovskite. This study provided insight into the relationship between phase purity, quantum well orientation, and photovoltaic performance. [23] On the other hand, the approaches that involve low dimensional polymorphs passivated 3D perovskites, or 3D-2D perovskite stacked structures attracted extensive interest in recent years. This approach features not only the Supported by the density functional theory (DFT) calculations, for the first time, a fluorinated aromatic cation, 2-(4-fluorophenyl)ethyl ammonium iodide (FPEAI), is introduced to grow in situ a low dimensional perovskite layer atop 3D perovskite film with excess PbI 2 . The resulted (p-FC 6 H 4 C 2 H 4 NH 3 ) 2 [PbI 4 ] perovskite functions as a protective capping layer to protect the 3D perovskite from moisture. In the meantime, the thin layer facilitates charge transfer at the interfaces, thereby reducing the...

Chemically modified graphite is an economical material with promising applications in its own right or as an intermediate in the synthesis of graphene. However, because of its extreme chemical inertness, to date only two methods—oxidation and fluorination—have been found which can modify graphite with high yield and large throughput. Herein, we describe a third chemical approach for the synthesis of large quantities of highly modified graphite which uses a microwave-sparks-assisted halogenation reaction. The resulting graphite halide can easily be exfoliated into monolayer graphene in organic solvents. The structure and electronic properties of the original graphene can be recovered after thermal annealing of the graphene halide. Furthermore, the graphene halide can be further modified by a variety of organic functional groups. Solution-processed field-effect transistors based on the graphene halides resulted in device performances were comparable to, or even better than, that of graphene oxide.

Oxide perovskite materials have a long history of being investigated for thermoelectric applications. Compared to the state-of-the-art tin and lead chalcogenides, these perovskite compounds have advantages of low toxicity, eco-friendliness, and high elemental abundance. However, because of low electrical conductivity and high thermal conductivity, the total thermoelectric performance of oxide perovskites is relatively poor. Variety of methods were used to enhance the TE properties of oxide perovskite materials, such as doping, inducing oxygen vacancy, embedding crystal imperfection, and so on. Recently, hybrid perovskite materials started to draw attention for thermoelectric application. Due to the low thermal conductivity and high Seebeck coefficient feature of hybrid perovskites materials, they can be promising thermoelectric materials and hold the potential for the application of wearable energy generators and cooling devices. This mini-review will build a bridge between oxide perovskites and burgeoning hybrid halide perovskites in the research of thermoelectric properties with an aim to further enhance the relevant performance of perovskite-type materials.