Chen Weijie

SnO₂-based anode materials for lithium ion batteries suffer from inevitable pulverization and electrical disconnection during repeated charge–discharge cycles. Stabilizing nanostructure of SnO₂ particles by graphene is one of the most extensively studied strategies to achieve high capacity and long-term cyclability. However, in the wet chemistry, the reaggregation of the solvent–dispersed graphene and the SnO₂ make it difficult to fabricate SnO₂/graphene composites with desirable nanostructure which can be maintained during lithiation. Herein, the ionic liquid-assisted method is applied to prepare SnO₂ nanoparticles grafted on ionic liquid reduced graphene oxide (SnO₂@IL-RGO) composite through a novel “bridging effect” generated from the interaction between these two constituents and ionic liquid which efficiently maintains the desirable nanostructure and offers more conductive pathway upon cycling. The composite as an anode material achieves an increasing capacity up to 1508 mAh g⁻¹ in the 427th cycle at high current of 1 A g⁻¹. The ionic liquid-assisted strategy may be a promising approach to promote the strong combination and uniform dispersion of other metal oxides on the carbonaceous materials, providing a new way to prepare metal oxide–carbon composites for wide applications.

SnO₂@ionic liquid reduced graphene oxide is prepared with SnO₂ particles strongly and uniformly bound on carbonaceous sheets. The composite exhibits the lamellar structure of the carbonaceous sheets uniformly decorated by spherical-shaped SnO₂ nanoparticles which can be efficiently maintained upon cycling, avoiding the inevitable pulverization of SnO₂-based electrode with electric contact loss which provides more reactive sites for Li storage.

In article number 1701405, Hae Jung Son and co-workers develop D₁-A-D₂-A-type random terpolymers. Organic photovoltaics (OPVs) introducing the resulting polymer achieve a high efficiency of 10.31%. Furthermore, due to outstanding solution processability of the random terpolymer, 1 cm² OPVs reproducibly shows a high efficiency of up to 9.42% using thick active layers in the range of 250–380 nm.

Despite the rapid progress in solar power conversion efficiency of archetype organic–inorganic hybrid perovskite CH₃NH₃PbI₃-based solar cells, the long-term stability and toxicity of Pb remain the main challenges for the industrial deployment, leading to more uncertainties for global commercialization. The poor stabilities of CH₃NH₃PbI₃-based solar cells may not only be attributed to the organic molecules but also the halides themself, most of which exhibit intrinsic instability under moisture and light. As an alternative, the possibility of oxide perovskites for photovoltaic applications is explored here. The class of lead-free stable oxide double perovskites A₂M(III)M(V)O₆ (A = Ca, Sr, Ba; M(III) = Sb³⁺ or Bi³⁺; M(V) = V⁵⁺, Nb⁵⁺, or Ta⁵⁺) is comprehensively explored with regard to their stability and their electronic and optical properties. Apart from the strong stability, this class of double perovskites exhibits direct bandgaps ranging from 0.3 to 3.8 eV. With proper B site alloying, the bandgap can be tuned within the range of 1.0–1.6 eV with optical absorptions as strong as CH₃NH₃PbI₃, making them suitable for efficient single-junction thin-film solar cell application.

The class of lead-free stable oxide double perovskites A₂M(III)M(V)O₆ (A = Ca, Sr, Ba; M(III) = Sb³⁺ or Bi³⁺; M(V) = V⁵⁺, Nb⁵⁺, or Ta⁵⁺) is comprehensively explored with regard to their stability and their electronic and optical properties. Apart from the strong stability, this class of double perovskites exhibits direct bandgaps ranging from 0.3 to 3.8 eV. With proper B site alloying, the bandgap can be tuned within the range of 1.0–1.6 eV with strong optical absorptions, making them suitable for efficient single-junction thin-film solar cell application.

Solution-based processing of materials for electrical doping of organic semiconductor interfaces is attractive for boosting the efficiency of organic electronic devices with multilayer structures. To simplify this process, self-doping perylene diimide (PDI)-based ionene polymers are synthesized, in which the semiconductor PDI components are embedded together with electrolyte dopants in the polymer backbone. Functionality contained within the PDI monomers suppresses their aggregation, affording self-doping interlayers with controllable thickness when processed from solution into organic photovoltaic devices (OPVs). Optimal results for interfacial self-doping lead to increased power conversion efficiencies (PCEs) of the fullerene-based OPVs, from 2.62% to 10.64%, and of the nonfullerene-based OPVs, from 3.34% to 10.59%. These PDI–ionene interlayers enable chemical and morphological control of interfacial doping and conductivity, demonstrating that the conductive channels are crucial for charge transport in doped organic semiconductor films. Using these novel interlayers with efficient doping and high conductivity, both fullerene- and nonfullerene-based OPVs are achieved with PCEs exceeding 9% over interlayer thicknesses ranging from ≈3 to 40 nm.

Self-doping of perylene-diimide-based ionenes enables chemical and morphological control of interfacial doping and conductivity in organic electronic devices. Using these materials provides a straightforward and controllable method to modulate the interface between electrodes and active layers, affording both fullerene- and nonfullerene-based solar cells with high efficiencies over a wide range of doped interlayer thicknesses.

Molecular weight is an important factor determining the morphology and performance of all-polymer solar cells. Through the application of direct arylation polycondention, a series of batches of a fluorinated naphthalene diimide-based acceptor polymer are prepared with molecular weight varying from M_n = 20 to 167 kDa. Used in conjunction with a common low bandgap donor polymer, the effect of acceptor molecular weight on solar cell performance, morphology, charge generation, and transport is explored. Increasing the molecular weight of the acceptor from M_n = 20 to 87 kDa is found to increase cell efficiency from 2.3% to 5.4% due to improved charge separation and transport. Further increasing the molecular weight to M_n = 167 kDa however is found to produce a drop in performance to 3% due to liquid–liquid phase separation which produces coarse domains, poor charge generation, and collection. In addition to device studies, a systematic investigation of the microstructure and photophysics of this system is presented using a combination of transmission electron microscopy, grazing-incidence wide-angle X-ray scattering, near-edge X-ray absorption fine-structure spectroscopy, photoluminescence quenching, and transient absorption spectroscopy to provide a comprehensive understanding of the interplay between morphology, photophysics, and photovoltaic performance.

Excessively high molecular weights are shown to be detrimental to the performance of all-polymer solar cells. Increasing the molecular weight of the acceptor polymer to M_n = 167 kDa is found to result in liquid–liquid phase separation negatively impacting charge generation and collection. Intermediate molecular weights instead provide an optimum morphology with good carrier mobilities and improved molecular order.

Two wide bandgap star-shaped small molecular acceptors, para-TrBRCN and meta-TrBRCN, are synthesized for efficient nonfullerene polymer solar cells (PSCs). The tiny structural variation by just changing the linkage positions affects largely the inherent properties of the resulting molecules. Both molecules have a nonplanar 3D structure, which can prevent the excessively aggregation to realize the optimized morphology and ideal domain size in their active blends. Compared to para-TrBRCN, meta-TrBRCN exhibits the smaller distortions between the truxene skeleton and the benzothiadiazole units, which would also lead to the enhanced π–π stacking and charge transfer. When blending with PTB7-Th, high power conversion efficiencies (PCEs) of 10.15% and 8.28% are obtained for meta-TrBRCN and para-TrBRCN devices, respectively. To make up the weak absorption of above binary active blend in the longer wavelength region and increase the whole device performance further, low bandgap 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis(4-hexylthienyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]-dithiophene (ITIC-Th) is added as the second acceptor material to fabricate ternary blend PSCs. After adding 20 wt% of ITIC-Th, the resulting devices exhibit the well-balanced optical absorption and fine-tuned morphology, giving rise to the significantly improved PCE of 11.40% with much higher J _sc of 18.25 mA cm⁻² and fill factor of 70.2%.

Tow star-shaped wide bandgap molecular acceptors with truxene core were synthesized for efficient non-fullerene polymer solar cells. Both acceptors show high absorptions in the short wavelength region, which can match well with those of low bandgap polymer donors, giving rise to high power conversion efficiencies of 10.15% from binary blend devices and 11.40% from ternary blend devices, respectively.

A semiconductor p–n junction typically has a doping-induced carrier depletion region, where the doping level positively correlates with the built-in potential and negatively correlates with the depletion layer width. In conventional bulk and atomically thin junctions, this correlation challenges the synergy of the internal field and its spatial extent in carrier generation/transport. Organic–inorganic hybrid perovskites, a class of crystalline ionic semiconductors, are promising alternatives because of their direct badgap, long diffusion length, and large dielectric constant. Here, strong depletion in a lateral p–n junction induced by local electronic doping at the surface of individual CH₃NH₃PbI₃ perovskite nanosheets is reported. Unlike conventional surface doping with a weak van der Waals adsorption, covalent bonding and hydrogen bonding between a MoO₃ dopant and the perovskite are theoretically predicted and experimentally verified. The strong hybridization-induced electronic coupling leads to an enhanced built-in electric field. The large electric permittivity arising from the ionic polarizability further contributes to the formation of an unusually broad depletion region up to 10 µm in the junction. Under visible optical excitation without electrical bias, the lateral diode demonstrates unprecedented photovoltaic conversion with an external quantum efficiency of 3.93% and a photodetection responsivity of 1.42 A W⁻¹.

Strong charge depletion in a lateral perovskite nanosheet p–n junction is reported with a large depletion width (>10 µm) and a high built-in potential (≈0.5 eV) through local electronic doping at the surface of individual CH₃NH₃PbI₃ perovskite nanosheets. 2D electrostatics imaging further reveals a thickness dependence of the depletion width in the perovskite p–n junctions.

The discoloration rate of chrome yellow (CY), a class of synthetic inorganic pigments (PbCr1-xSxO4) frequently used by Van Gogh and his contemporaries, strongly depends on its sulfate content and on its crystalline structure (either monoclinic or orthorhombic). Macroscopic X-ray powder diffraction imaging of selected areas on Van Gogh's Sunflowers (Van Gogh Museum, Amsterdam) revealed the presence of two subtypes of CY: the light-fast monoclinic PbCrO4 (LF-CY) and the light-sensitive monoclinic PbCr1-xSxO4 (x ≈ 0.5; LS-CY). The latter was encountered in large parts of the painting (e.g., in the pale-yellow background and the bright-yellow petals, but also in the green stems and flower hearts) indicating their higher risk for past or future darkening. Overall, it is present in more than 50% of the CY regions. Preferred orientation of LS-CY allows to observe a significant ordering of the elongated nanocrystallites along the direction of Van Gogh's brush strokes.

In article number 1701140, Pavel Troshin and co-workers, show that planar junction solar cells based on antimony (V) bromide complexes, demonstrate an external quantum efficiency of ≈80% and power conversion efficiency of ≈4%. The discovery of the first perovskite-like compound ABX₆ exhibiting good photovoltaic performance opens wide opportunities for rational design of novel hybrid semiconductor materials for advanced electronic and photovoltaic applications.

Hybrid lead halide perovskites are promising materials for future photovoltaics applications. Their spectral response can be readily tuned by controlling the halide composition, while their stability is strongly dependent on the film morphology and on the type of organic cation used. Mixed cation and mixed halide systems have led to the most efficient and stable perovskite solar cells reported, so far they are prepared exclusively by solution-processing. This might be due to the technical difficulties associated with the vacuum deposition from multiple thermal sources, requiring a high level of control over the deposition rate of each precursor during the film formation. In this report, thermal vacuum deposition with multiple sources (3 and 4) is used to prepare for the first time, multications/anions perovskite compounds. These thin-film absorbers are implemented into fully vacuum deposited solar cells using doped organic semiconductors. A maximum power conversion efficiency of 16% is obtained, with promising device stability. The importance of the control over the film morphology is highlighted, which differs substantially when these compounds are vacuum processed. Avenues to improve the morphology and hence the performance of fully vacuum processed multications/anions perovskite solar cells are proposed.

Multiple-source (up to 4) thermal vacuum deposition is used to prepare for the first time multications/anions perovskite compounds. These thin-film absorbers are implemented into fully vacuum deposited solar cells using doped organic semiconductors. A maximum power conversion efficiency of 16% is obtained, with promising device stability.

The carrier concentration of the electron-selective layer (ESL) and hole-selective layer can significantly affect the performance of organic–inorganic lead halide perovskite solar cells (PSCs). Herein, a facile yet effective two-step method, i.e., room-temperature colloidal synthesis and low-temperature removal of additive (thiourea), to control the carrier concentration of SnO₂ quantum dot (QD) ESLs to achieve high-performance PSCs is developed. By optimizing the electron density of SnO₂ QD ESLs, a champion stabilized power output of 20.32% for the planar PSCs using triple cation perovskite absorber and 19.73% for those using CH₃NH₃PbI₃ absorber is achieved. The superior uniformity of low-temperature processed SnO₂ QD ESLs also enables the fabrication of ≈19% efficiency PSCs with an aperture area of 1.0 cm² and 16.97% efficiency flexible device. The results demonstrate the promise of carrier-concentration-controlled SnO₂ QD ESLs for fabricating stable, efficient, reproducible, large-scale, and flexible planar PSCs.

SnO₂ quantum dots (QDs) are synthesized by a simple and reproducible two-step low-temperature method, in which the carrier concentration of colloidal SnO² QDs is controlled. Planar perovskite solar cells with the efficiencies of 20.8% in small size (0.09 cm²), ≈19% in large size (1 cm²), and 16.97% for flexible devices with low-temperature processed SnO₂ QD electron-selective layers are obtained.