Despite the various synthesis methods to obtain carbon dots (CDs), the bottom‐up methods are still the most widely administrated route to afford large‐scale and low‐cost synthesis. However, as CDs are developed with increasing reports involved in producing many CDs, the structure and property features have changed enormously compared with the first generation of CDs, raising classification concerns. To this end, a new classification of CDs, named carbonized polymer dots (CPDs), is summarized according to the analysis of structure and property features. Here, CPDs are revealed as an emerging class of CDs with distinctive polymer/carbon hybrid structures and properties. Furthermore, deep insights into the effects of synthesis on the structure/property features of CDs are provided. Herein, the synthesis methods of CDs are also summarized in detail, and the effects of synthesis conditions of the bottom‐up methods in terms of the structures and properties of CPDs are discussed and analyzed comprehensively. Insights into formation process and nucleation mechanism of CPDs are also offered. Finally, a perspective of the future development of CDs is proposed with critical insights into facilitating their potential in various application fields.

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The mixed halide perovskites have emerged as outstanding light absorbers for efficient solar cells. Unfortunately, it reveals inhomogeneity in these polycrystalline films due to composition separation, which leads to local lattice mismatches and emergent residual strains consequently. Thus far, the understanding of these residual strains and their effects on photovoltaic device performance is absent. Herein we study the evolution of residual strain over the films by depth-dependent grazing incident X-ray diffraction measurements. We identify the gradient distribution of in-plane strain component perpendicular to the substrate. Moreover, we reveal its impacts on the carrier dynamics over corresponding solar cells, which is stemmed from the strain induced energy bands bending of the perovskite absorber as indicated by first-principles calculations. Eventually, we modulate the status of residual strains in a controllable manner, which leads to enhanced PCEs up to 20.7% (certified) in devices via rational strain engineering.

CsPbX (X = Cl, Br, I) perovskite quantum dots (QDs) are potential emitting materials for illumination and display applications, but toxic Pb is not environment- and user-friendly. In this work, we demonstrate the partial replacement of Pb with Mn through phosphine-free hot-injection preparation of CsPbMnCl QDs in colloidal solution. The Mn substitution ratio is up to 46%, and the as-prepared QDs maintain the tetragonal crystalline structure of the CsPbCl host. Meaningfully, Mn substitution greatly enhances the photoluminescence quantum yields of CsPbCl from 5 to 54%. The enhanced emission is attributed to the energy transfer of photoinduced excitons from the CsPbCl host to the doped Mn, which facilitates exciton recombination via a radiative pathway. The intensity and position of this Mn-related emission are also tunable by altering the experimental parameters, such as reaction temperature and the Pb-to-Mn feed ratio. A light-emitting diode (LED) prototype is further fabricated by employing the as-prepared CsPbMnCl QDs as color conversion materials on a commercially available 365 nm GaN LED chip.

Carbon nanodots (C-dots) synthesized by electrochemical ablation and small molecule carbonization, as well as graphene quantum dots (GQDs) fabricated by solvothermally cutting graphene oxide, are three kinds of typical green fluorescence carbon nanomaterials. Insight into the photoluminescence origin in these fluorescent carbon nanomaterials is one of the important matters of current debates. Here, a common origin of green luminescence in these C-dots and GQDs is unraveled by ultrafast spectroscopy. According to the change of surface functional groups during surface chemical reduction experiments, which are also accompanied by obvious emission-type transform, these common green luminescence emission centers that emerge in these C-dots and GQDs synthesized by bottom-up and top-down methods are unambiguously assigned to special edge states consisting of several carbon atoms on the edge of carbon backbone and functional groups with C═O (carbonyl and carboxyl groups). Our findings further suggest that the competition among various emission centers (bright edge states) and traps dominates the optical properties of these fluorescent carbon nanomaterials.

We report on a significant power conversion efficiency improvement of perovskite solar cells from 8.81% to 10.15% due to insertion of an ultrathin graphene quantum dots (GQDs) layer between perovskite and TiO2. A strong quenching of perovskite photoluminescence was observed at ∼760 nm upon the addition of the GQDs, which is pronouncedly correlated with the increase of the IPCE and the APCE of the respective cells. From the transient absorption measurements, the improved cell efficiency can be attributed to the much faster electron extraction with the presence of GQDs (90-106 ps) than without their presence (260-307 ps). This work highlights that GQDs can act as a superfast electron tunnel for optoelectronic devices.

Highly crystalline SnO2 is demonstrated to serve as a stable and robust electron-transporting layer for high-performance perovskite solar cells. Benefiting from its high crystallinity, the relatively thick SnO2 electron-transporting layer (≈120 nm) provides a respectable electron-transporting property to yield a promising power conversion efficiency (PCE)(18.8%) Over 90% of the initial PCE can be retained after 30 d storage in ambient with ≈70% relative humidity.

enhanced device performance. [3b] Besides, 2D perovskite also has been deposited by cation exchange or spin-casting onto CH 3 NH 3 PbI 3 crystalline films as a capping layer to improve device stability. [4] To our knowledge, however, such as capping layer has always imposed a significant barrier to charge transfer due to the quantum-confined energy level structure of 2D perovskite. [3a,b] Herein, we developed a solution process based in situ growth route, which allowed us to form a 2D capping layer together with 3D perovskite, essentially a 3D-2D graded interface. Remarkably, the 3D-2D graded perovskite layer was made atomically sharp at the near surface by using such a simple in situ growth process. More importantly, that dimensionally graded layer advantageously modified the interface energy level, consequently reduced the charge recombination at a perovskite/phenyl-C 61 -butyric acid methyl ester (PCBM) interface, and at the same time facilitated the interfacial charge transfer. This innovation has led to a pronounced promotion of the performance of NiO based p-i-n solar cells, with V oc reaching up to 1.17 V, which is among the highest for triiodide perovskite on NiO, alongside J sc of 21.80 mA cm −2 and FF of 0.78, resulting in an overall PCE of 19.89%. More than expected, the large hydrophobic groups at the interface as well as the grain boundaries tremendously enhanced the device ambient stability; our design of the graded layer not only enhanced moisture stability by abating water penetration but also improved thermal stability by suppressing the cross-layer ion migration. Figure 1A shows schematically the deposition method of the (CH 3 NH 3 PbI 3 -PEA 2 Pb 2 I 4 ) 3D-2D graded perovskite film. The deposition process is based on the solvent engineering method but with a key modification: [5] Phenethylammonium iodide (PEAI)/toluene solution was used instead of pure toluene for the solvent dripping process prior to conversion to a crystallized perovskite film by annealing at 95 °C. To facilitate comparison, three kinds of perovskite films were deposited under different conditions as shown in Table 1. In brief, the 3D-2D Graded sample was fabricated by the method mentioned above, while the 3D sample was deposited by the conventional toluene dripping process, and the 2D sample was made simply 2D halide perovskite materials have shown great advantages in terms of stability when applied in a photovoltaic device. However, the impediment of charge transport within the layered structure drags down the device performance. Here for the first time, a 3D-2D (MAPbI 3 -PEA 2 Pb 2 I 4 ) graded perovskite interface is demonstrated with synergistic advantages. In addition to the significantly improved ambient stability, this graded combination modifies the interface energy level in such a way that reduces interface charge recombination, leading to an ultrahigh V oc at 1.17 V, a record for NiO-based p-i-n photovoltaic devices. Moreover, benefiting from the graded structure induced continuously upshifts energy level, th...