Low-Temperature Synthesized Nb-Doped TiO2 Electron Transport Layer Enabling High-Efficiency Perovskite Solar Cells by Band Alignment Tuning

Niobium-doped conductive TiO-TiO2 heterostructure supported bifunctional catalyst for efficient and stable zinc-air batteries

The development of highly active and durable nonprecious metal-based bifunctional electrocatalysts for oxygen reduction/evolution reaction (ORR/OER) is important for rechargeable zinc-air batteries. Herein, a three-dimensional conductive niobium-doped TiO-TiO2 heterostructure supported ZIF-67-derived Co-NC bifunctional catalyst was fabricated. In the Co-NC@Nb-TiOx catalyst, the Nb doping promoted the formation of TiO-TiO2 heterojunction support, enhanced its conductivity and stability and provided strong electron metal-support interaction between Co-NC and Nb-TiOx. Also, the supported Co-NC nanoparticles provided abundant active sites with excellent ORR/OER activity. Experimental analysis reveals that the high OER activity of Co-NC@Nb-TiOx can be attributed to the in-situ generated CoOOH species. It exhibits excellent ORR activity, as shown by its onset potential (0.95 V vs. RHE) and half-wave potential (0.86 V vs. RHE). Its OER overpotential at 10 mA cm-2 is 480 mV. The zinc-air battery realizes outstanding cycling stability over 225 h cycles tested at 10 mA cm-2. This work demonstrates the importance of designing highly stable metal oxide-supported catalysts in electrochemical energy conversion devices.

Tabrizi B, D’Elia V, Del Gobbo S, Jabbour GE. Microwave-Assisted Non-aqueous and Low-Temperature Synthesis of Titania and Niobium-Doped Titania Nanocrystals and Their Application in Halide Perovskite Solar Cells as Electron Transport Layers

Undoped and Nb-doped TiO₂ nanocrystals are prepared by a microwave-assisted non-aqueous sol-gel method based on a slow alkyl chloride elimination reaction between metal chlorides and benzyl alcohol. Sub-4 nm nanoparticles are grown under microwave irradiation at 80 °C in only 3 h with precise control of growth parameters and yield. The obtained nanocrystals could be conveniently used to cast compact TiO₂ or Nb-doped TiO₂ electron transport layers for application in formamidinium lead iodide-based photovoltaic devices. Niobium doping is found to improve the cell performance by increasing the conductivity and mobility of the electron transport layer. At the same time, a measurable decrease in parasitic light absorption in the low wavelength portion of the spectrum was observed.

Plasma Oxidized Ti3C2Tx MXene as Electron Transport Layer for Efficient Perovskite Solar Cells

Recently, the two-dimensional material Ti₃C₂T_x MXene has attracted interest from researchers in perovskite solar cells (PSCs) with its great advantages in terms of high transmittance, high conductivity, tunable work function, and solution processability. However, the MXene-based PSC performance has still been inferior to that of the traditional TiO₂- or SnO₂-based counterpart up until now. Some critical issues regarding to the MXene/perovskite interface still have not been well addressed. Herein, we used the Ti₃C₂T_x MXene as electron transport layer in PSCs via a room-temperature solution process followed by oxygen plasma treatment. Various characterization techniques were taken to establish the correlation between the surface properties and termination groups of MXene. We showed that oxygen plasma treatment could break parts of Ti-C bonds and generate abundant Ti-O bonds randomly distributed on MXene. The surface modification resulted in tunable work functions of MXene, as well as reduced trap states and improved electron transport close to the interface. In addition, the surface tension of MXene and corresponding perovskite morphology were thoroughly investigated by the contact angle and topography measurements. High-resolution XPS spectra indicated the Pb-O interactions between perovskite and MXene, which contributed to the device stability improvement.

Low-temperature processed tantalum/niobium co-doped TiO2electron transport layer for high-performance planar perovskite solar cells

A low-temperature preparation process is significantly important for scalable and flexible devices. However, the serious interface defects between the normally used titanium dioxide (TiO₂) electron transport layer (ETL) obtained via a low-temperature method and perovskite suppress the further improvement of perovskite solar cells (PSCs). Here, we develop a facile low-temperature chemical bath method to prepare a TiO₂ETL with tantalum (Ta) and niobium (Nb) co-doping. Systematic investigations indicate that Ta/Nb co-doping could increase the conduction band level of TiO₂and could decrease the trap-state density, boosting electron injection efficiency and reducing the charge recombination between the perovskite/ETL interface. A superior power conversion efficiency of 19.44% can be achieved by a planar PSC with a Ta/Nb co-doped TiO₂ETL, which is much higher than that of pristine TiO₂(17.60%). Our achievements in this work provide new insights on low-temperature fabrication of low-cost and highly efficient PSCs.

Visualization of halide perovskite crystal growth processes by in situ heating WAXS measurements

We observed the crystallization dynamics of halide perovskite crystals (CH3NH3PbI3) by in situ heating wide-angle X-ray scattering measurements. As a result, we revealed that crystal growth occurs during the conversion of complexes to perovskite crystals.

Control of Molecular Orientation of Spiro-OMeTAD on Substrates

2,2',7,7'-Tetrakis(N,N-di-p-methoxyphenylamino)-9,9'-spirobifluorene (spiro-OMeTAD) is utilized as a p-type semiconductor layer in perovskite solar cells and solid-state dye-sensitized solar cells. Spiro-OMeTAD has been known to have a spiro center, leading to a random orientation. Although the molecular orientation of organic semiconductor materials influences the conductivity, which is directly related to semiconductor device characteristics, the molecular orientation of spiro-OMeTAD has not been fully discussed. In this study, we prepared spiro-OMeTAD layers on various substrates and investigated their orientation by grazing-incidence wide-angle X-ray scattering (GIWAXS) and near-edge X-ray absorption fine structure (NEXAFS). Additionally, we demonstrated that the molecular orientation of spiro-OMeTAD could be controlled by changing their surface energies by changing the substrate materials. Consequently, we could improve the electrical conductivity by improving its molecular orientation. The results of this study provide a guideline for the preparation of organic semiconductor material layers using the wet-coating method.

Cesium Acetate-Induced Interfacial Compositional Change and Graded Band Level in MAPbI3 Perovskite Solar Cells

Compositional engineering and interfacial modifications have played pivotal roles in the accomplishment of high-efficiency perovskite solar cells (PSCs). Different interfaces in the PSCs influence the performance remarkably either by altering the crystallization of the active material or shifting the energy levels or improving the electrical contact. This work reports how a thin layer of cesium acetate on the TiO₂ electron transport layer (ETL) induces generation of a PbI₂-rich methylammonium lead iodide (MAPbI₃) composition at the ETL/MAPbI₃ interface, which downshifts the conduction band level of MAPbI₃ to create an energy level gradient favorable for carrier collection, resulting in higher photocurrent, fill factor, and overall power conversion efficiency.