Conventional 3D organic-inorganic halide perovskites have recently undergone unprecedented rapid development. Yet, their inherent instabilities over moisture, light, and heat remain a crucial challenge prior to the realization of commercialization. By contrast, the emerging 2D Ruddlesden-Popper-type perovskites have recently attracted increasing attention owing to their great environmental stability. However, the research of 2D perovskites is just in their infancy. In comparison to 3D analogues, they are natural quantum wells with a much larger exciton binding energy. Moreover, their inner structural, dielectric, optical, and excitonic properties remain to be largely explored, limiting further applications. This review begins with an introduction to 2D perovskites, along with a detailed comparison to 3D counterparts. Then, a discussion of the organic spacer cation engineering of 2D perovskites is presented. Next, quasi-2D perovskites that fall between 3D and 2D perovskites are reviewed and compared. The unique excitonic properties, electron-phonon coupling, and polarons of 2D perovskites are then be revealed. A range of their (opto)electronic applications is highlighted in each section. Finally, a summary is given, and the strategies toward structural design, growth control, and photophysics studies of 2D perovskites for high-performance electronic devices are rationalized.

Organic/inorganic thermoelectric nanocomposites (TENCs) have seized great attention because they integrate the advantages of inorganic (i.e., high electrical conductivity) and organic (i.e., low thermal conductivity and mechanical flexibility) components. Major barriers that obstruct the development of this field are the lack of n‐type TE materials and their relatively low performance, leaving the construction of TE devices difficult to realize. This review article is therefore focused on recent advances on n‐type TENCs that primarily comprise carbon nanotube (CNT) and inorganic nanocrystal (NC)‐based hybrids. CNT‐based n‐type TENCs are fabricated mainly by transforming the p‐type CNT to n‐type with organic dopants or by blending CNTs with n‐type semiconducting polymers. NC‐based n‐type TENCs are typically obtained by blending semiconductor nanocrystals or metallic nanostructures with polymers. Additionally, the fabrication and thermoelectric performance of 2D layered superlattice structures are also reviewed. Finally, an outlook of n‐type TENCs is given with a perspective for their possible future improvements.

𝝅-Conjugated donor (D)−acceptor (A) copolymers have been extensivelystudied as organic photovoltaic (OPV) donors yet remain largely unexplored in organic thermoelectrics (OTEs) despite their outstanding mechanical bendability, solution processability and flexible molecular design. Importantly, they feature high Seebeck coefficient (S) that are desirable in room-temperature wearable application scenarios under small temperature gradients. In this work, the authors have systematically investigated a series of D−A semiconducting copolymers possessing various electron-deficient A-units (e.g., BDD, TT, DPP) towards efficient OTEs. Upon p-type ferric chloride (FeCl 3 ) doping, the relationship between the thermoelectric characteristics and the electron-withdrawing ability of A-unit is largely elucidated. It is revealed that a strong D−A nature tends to induce an energetic disorder along the 𝝅-backbone, leading to an enlarged separation of the transport and Fermi levels, and consequently an increase of S. Meanwhile, the highly electron-deficient A-unit would impair electron transfer from D-unit to p-type dopants, thus decreasing the doping efficiency and electrical conductivity (𝝈). Ultimately, the peak power factor (PF) at room-temperature is obtained as high as 105.

Emerging organic–inorganic thermoelectric nanocomposites (TENCs) are promising candidates for the realization of high‐performance flexible thermoelectric generators (TEGs), yet there is an absence of effective means to precisely regulate the film morphology of TENCs. Here, the use of a magnetic field to improve thermoelectric performance of solution fabricated n‐type metallic TENCs is reported. Of particular relevance is that the magnetic field gives rise to aligned Co nanowires (NWs) within a poly(vinylidene fluoride) (PVDF) matrix. Such oriented TENCs exhibit significantly increased electrical conductivity in comparison to identical nanocomposites that are randomly oriented. As a result, the best power factor of oriented Co NWs (80 wt%)/PVDF TENCs reaches 523 µW m−1 K−2 at 320 K, which is among the highest reported n‐type TENCs. By pairing these n‐type TENCs with benchmark p‐type poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonate) (PEDOT:PSS) thin films, the fabrication of flexible and planar TEGs that yield a maximum output voltage and power of 26.4 mV and 5.2 µW when ∆T = 50 K, respectively, is reported.

Thermoelectric (TE) materials can realize the direct transformation between heat and electricity, thereby facilitating the recycling of waste heat. Semiconducting π-conjugated polymers (π-CPs) have been largely explored as organic TE materials thanks to the facile molecular tunability of their electronic properties, their room-temperature solution-processability, their intrinsic low thermal conductivity, and their outstanding mechanical flexibility. In this Focus Review, we describe two key strategieschemical doping and structural tailoringin polymeric TEs for strengthening TE power factors of π-CPs. First, the doping mechanisms are unraveled by a sequential process of charge transfer and free carrier release, followed by the introduction of various doping methods for enhancing the chemical doping. Second, the design principles for polymeric structures including the π-backbone and side-chain engineering are presented. Third, supplementary strategies such as polymer chain alignment and construction of polymer blends are identified. Finally, the existing prime obstacles to future development are discussed and an outlook on feasible solutions to resolving them is provided.