Chen Weijie

Many applications of TiO₂ nanoparticles, such as photocatalytic water splitting or water remediation, occur in aqueous environment. However, the impact of solvation on TiO₂ electronic structure remains unclear because only few experimental methods are currently available to probe nanoparticle–water interface. Soft X-ray absorption spectroscopy has been extensively used to characterize the electronic structure of TiO₂ materials, but so far only in vacuum conditions. Here, oxygen K edge and titanium L edge X-ray absorption spectroscopy characterization of TiO₂ nanoparticles measured directly in aqueous dispersion is presented. For this purpose, a new method to probe nanomaterials in liquid using a holey membrane-based flow cell is introduced. With this approach, the X-ray transmission of the membrane is increased, especially in the water window, compared to solid membranes.

TiO₂ nanoparticles are characterized in situ in aqueous dispersions by soft X-ray absorption spectroscopy at the oxygen K edge and titanium L edge. For this purpose, a new flow cell method using a holey membrane to increase X-ray transmission of the membrane is introduced. The influence of hydration on the electronic structure of TiO₂ nanoparticles is discussed.

In this work, a novel lamination method employing hydrogen-bond interaction to assemble a highly conductive free standing poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) film as a common electrode is demonstrated in a solution processed metal-free foldable integrated photocapacitor (IPC) composed of a monolithic organic solar cell (OSC) and a capacitor. The highlights of the work are: (1) micrometer free standing PEDOT:PSS electrode is successfully laminated onto a relatively large area (1 cm²) OSCs; (2) a free standing capacitor based on the PEDOT:PSS electrode is achieved; (3) the IPC demonstrates an overall efficiency of 2% and an energy storage efficiency of 58%, which is comparable with those of IPCs based on metallic common electrodes; (4) the novel lamination method for PEDOT:PSS electrode enables free standing PEDOT:PSS broad applications in solution processed flexible organic electronics, especially tandem or/and integrated organic electronic devices. Furthermore, the IPC is foldable with excellent cycling stability (no decay after 100 recycles at 1 mA cm⁻²). These results indicate that free standing PEDOT:PSS film is a promising candidate as common electrodes for IPCs to break the restrictions of metal electrodes. The demonstrated lamination method will greatly extend the applications of PEDOT:PSS electrodes to large area flexible organic electronic devices.

A novel lamination method for free standing poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) electrode is developed and applied successfully onto large area metal-free organic solar cells and solution processed foldable integrated photocapacitors assisted by hydrogen-bond interaction. This novel lamination method for PEDOT:PSS electrode will greatly extend applications of PEDOT:PSS electrode in other kinds of large area flexible organic electronics.

2,2′,7,7′-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene (spiro-MeOTAD) hole transport material (HTM) is a milestone in the history of perovskite solar cells (PSCs). Proper choice of HTMs is key factor for efficient charge extraction and stability in solar cells. Spiro-MeOTAD is proven to be the most suitable HTM for testing PSCs due to its facile implementation and high performance. Similarly, spiro-MeOTAD is receiving attention in other applications other than in solar cells due to its desirable properties. However, spiro-MeOTAD is under debate regarding the topics of cost-performance, long-term stability, degradation issues (induced by temperature, additives, film quality, and environmental conditions), coating technologies compatibility, reliance on additives, and hysteresis. In this review, the advent of spiro-MeOTAD, and related aforementioned issues about spiro-MeOTAD are discussed. In addition, spiro-MeOTAD properties, alternative and new additives, other applications, and new HTMs that is comparable or outperforms spiro-MeOTAD in PSCs are summarized. In the outlook, the future research directions based on reported results that warrant further investigations are outlined.

Spiro-MeOTAD is the most commonly used hole transport material for perovskite solar cells. In this article, various aspects of spiro-MeOTAD including its properties, additives, stability, alternatives, its applications in perovskite solar cells as well as other applications are reviewed.

2D layered van der Waals (vdW) atomic crystals are an emerging class of new materials that are receiving increasing attention owing to their unique properties. In particular, the dangling-bond-free surface of 2D materials enables integration of differently dimensioned materials into mixed-dimensional vdW heterostructures. Such mixed-dimensional heterostructures herald new opportunities for conducting fundamental nanoscience studies and developing nanoscale electronic/optoelectronic applications. This study presents a 1D ZnO nanowire (n-type)–2D WSe₂ nanosheet (p-type) vdW heterojunction diode for photodetection and imaging process. After amorphous fluoropolymer passivation, the ZnO–WSe₂ diode shows superior performance with a much-enhanced rectification (ON/OFF) ratio of over 10⁶ and an ideality factor of 3.4–3.6 due to the carbon–fluorine (CF) dipole effect. This heterojunction device exhibits spectral photoresponses from ultraviolet (400 nm) to near infrared (950 nm). Furthermore, a prototype visible imager is demonstrated using the ZnO–WSe₂ heterojunction diode as an imaging pixel. To the best of our knowledge, this is the first demonstration of an optoelectronic device based on a 1D–2D hybrid vdW heterojunction. This approach using a 1D ZnO–2D WSe₂ heterojunction paves the way for the further development of electronic/optoelectronic applications using mixed-dimensional vdW heterostructures.

A novel mixed-dimensional 1D ZnO–2D WSe₂ van der Waals heterojunction device for photodetection and imaging processes is demonstrated. The ZnO–WSe₂ heterojunction diode exhibits a spectral photodetection range from ultraviolet to near infrared. A prototype visible imager is demonstrated using the ZnO–WSe₂ heterojunction diode as an imaging pixel.

Monolayer graphene is used as an electrode to develop novel electronic device architectures that exploit the unique, atomically thin structure of the material with a low density of states at its charge neutrality point. For example, a single semiconductor layer stacked onto graphene can provide a semiconductor–electrode junction with a tunable injection barrier, which is the basis for a primitive transistor architecture known as the Schottky barrier field-effect transistor. This work demonstrates the next level of complexity in a vertical graphene–semiconductor architecture. Specifically, an organic vertical p-n junction (p-type pentacene/n-type N,N′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C₈)) on top of a graphene electrode constituting a novel gate-tunable photodiode device structure is fabricated. The model device confirms that controlling the Schottky barrier height at the pentacene–graphene junction can (i) suppress the dark current density and (ii) enhance the photocurrent of the device, both of which are critical to improve the performance of a photodiode.

Organic vertical p–n junction stacked on top of a graphene electrode is demonstrated for gate-tunable photodiodes. The model device confirms that controlling the Schottky barrier height at the pentacene–graphene junction can (i) suppress the dark current density and (ii) enhance the photocurrent of the device.

PtCoFe nanowires with different alloying compositions are chemically prepared and acted as counter electrodes (CEs) in dye-sensitized solar cells (DSSCs) with Ru(II)-based dyes. Due to their superior reduction activity, PtCoFe nanowires with rich (111) facets enhance the performance of DSSCs. Hence, N719 DSSCs with PtCoFe nanowires, respectively, produce better power conversion efficiency (PCE) of 8.10% for Pt₃₃Co₂₄Fe₄₃ nanowire, 8.33% for Pt₇₄Co₁₂Fe₁₄ nanowire, and 9.26% for Pt₄₉Co₂₃Fe₂₈ nanowire in comparison to the PCE of Pt CE (7.32%). Further, the PRT-22 DSSC with Pt₄₉Co₂₃Fe₂₈ nanowire exhibits a maximum PCE of 12.29% with a certificated value of 12.0%, which surpass the previous PCE record of the DSSCs with Ru(II)-based dyes. The photovoltaic and electrochemical results reveal the composition-dependent activity along with a volcano-shaped trend in the I⁻/ redox reaction. Theoretical work on the adsorption energies of I₂, the desorption energies of I⁻, and the corresponding absolute energy demonstrates that the reduction activity followed in the order of Pt₄₉Co₂₃Fe₂₈(111) plane > Pt₇₄Co₁₂Fe₁₄(111) plane > Pt₃₃Co₂₄Fe₄₃(111) plane, proving Pt₄₉Co₂₃Fe₂₈ nanowire to be a superior cathode material for DSSCs.

PtCoFe nanowires with rich (111) planes are chemically prepared and act as the counter electrodes of dye-sensitized solar cells (DSSCs). Evidently, Pt₄₉Co₂₃Fe₂₈ nanowires demonstrate the superior catalytic properties with respect to the I⁻/ redox reactions. Thus, PRT-22 DSSC based on Pt₄₉Co₂₃Fe₂₈ nanowire produces an impressive power conversion efficiency of 12.29%.

MoSe₂ as a typical transition metal dichalcogenide holds great potential for energy storage and catalysis but its performance is largely limited by its poor conductivity. Bi₂Se₃ nanosheets, a kind of topological insulators, possess gapless edges on boundary and show metallic character on surface. According to the principle of complementary, a novel integrated quasiplane structure of MoSe₂/Bi₂Se₃ hybrids is designed with artistic heteronanostructures via a hot injection in colloidal system. Interestingly, the heteronanostructures are typically constituted by single-layer Bi₂Se₃ hexagonal nanoplates evenly enclosed by small ultrathin hierarchical MoSe₂ nanosheets on the whole surfaces. X-ray photoelectron spectroscopy investigations suggest obvious electron transfer from Bi₂Se₃ to MoSe₂, which can help to enhance the conductivity of the hybrid electrode. Especially, schematic energy band diagrams derived from ultraviolet photoelectron spectroscopy studies indicate that Bi₂Se₃ has higher E_F and smaller Φ than MoSe₂, further confirming the electronic modulation between Bi₂Se₃ and MoSe₂, where Bi₂Se₃ serves as an excellent substrate to provide electrons and acts as channels for high-rate transition. The MoSe₂/Bi₂Se₃ hybrids demonstrating a low onset potential, small Tafel slope, high current density, and long-term stability suggest excellent hydrogen evolution reaction activity, whereas a high specific capacitance, satisfactory rate capability, and rapid ions diffusion indicate enhanced supercapacitor performance.

Integrated quasiplane heteronanostructures of MoSe₂/Bi₂Se₃ hexagonal nanosheets are fabricated via a hot injection process for the first time. Investigations suggest there is obvious electron transfer from Bi₂Se₃ to MoSe₂, which can help enhance the conductivity of the hybrid electrode. Owing to synergistic structural features, the MoSe₂/Bi₂Se₃ hybrids exhibit significantly enhanced electrocatalytic water splitting activity and supercapacitive performance.

A highly flexible perovskite film prepared by incorporating elastomer polyurethane (PU) is described by Licheng Tan, Yiwang Chen, and co-workers in article number 1703061. By cross-linking the grain boundaries between neighboring perovskite crystals, a PU network is formed to significantly improve the bendability of perovskite films. This strategy is expected to be of profound significance for the realization of perovskite solar cells as flexible and wearable devices.

The pervasive use of non-scalable fabrication methods in research greatly hinders the translation to industrial manufacturing of perovskite and organic solar cells. Inkjet printing is a scalable technique suitable for lab to industrial scale production. In article number 1703704, Dechan Angmo and co-workers comprehensively review inkjet printing application in the fabrication of different functional layers in organic and perovskite solar cells.

A simple, inexpensive, ecofriendly and scalable technique for fabricating highly efficient selective solar absorbers (SSAs) is reported by Nanfang Yu, Yuan Yang, and co-workers in article number 1702156. The technique yields SSAs with excellent wide-angle solar absorptance (≈96% at 0° and ≈79% at 70°), which is higher than, and low thermal emittance (<10%), which is on par with, those of existing designs.

An effective light trapping scheme in perovskite solar cells is achieved by incorporating bio-inspired moth-eye nanostructures into metal back electrode to improve the sunlight absorption. The enhanced light harvesting is ascribed by the collective effects of broadband polarization-insensitive light scattering and surface plasmonic excitation. This is reported by Yan-Qing Li, Chun-Sing Lee, Jian-Xin Tang, and co-workers in article number 1700492.

Organic-inorganic mixed perovskite solar cells demonstrate superiority for the application as low-cost printable solar energy, yet common processing of solvent-controlled crystallization routes contain highly toxic solvents that cause safety and/or environmental issues. In article number 1700576, Fuzhi Huang, Jie Zhong, and co-workers demonstrate a green solvent engineering incorporated interface optimization method to address this topic, and high performance devices are obtained.