We report the observation of self-doping in perovskite. CH 3 NH 3 PbI 3 was found to be either n-or p-doped by changing the ratio of methylammonium halide (MAI) and lead iodine (PbI 2 ) which are the two precursors for perovskite formation. MAI-rich and PbI 2 -rich perovskite films are p and n self-doped, respectively. Thermal annealing can convert the p-type perovskite to n-type by removing MAI. The carrier concentration varied as much as six orders of magnitude. A clear correlation between doping level and device performance was also observed. V

Interfacial electronic properties of the CH3NH3PbI3 (MAPbI3)/MoOx interface are investigated using ultraviolet photoemission spectroscopy and X-ray photoemission spectroscopy. It is found that the pristine MAPbI3 film coated onto the substrate of poly (3,4-ethylenedioxythiophene) poly(styrenesulfonate)/indium tin oxide by two-step method behaves as an n-type semiconductor, with a band gap of ∼1.7 eV and a valence band edge of 1.40 eV below the Fermi energy (EF). With the MoOx deposition of 64 Å upon MAPbI3, the energy levels of MAPbI3 shift toward higher binding energy by 0.25 eV due to electron transfer from MAPbI3 to MoOx. Its conduction band edge is observed to almost pin to the EF, indicating a significant enhancement of conductivity. Meanwhile, the energy levels of MoOx shift toward lower binding energy by ∼0.30 eV, and an interface dipole of 2.13 eV is observed at the interface of MAPbI3/MoOx. Most importantly, the chemical reaction taking place at this interface results in unfavorable interface energy level alignment for hole extraction. A potential barrier of ∼1.36 eV observed for hole transport will impede the hole extraction from MAPbI3 to MoOx. On the other hand, a potential barrier of ∼0.14 eV for electron extraction is too small to efficiently suppress electrons extracted from MAPbI3 to MoOx. Therefore, such an interface is not an ideal choice for hole extraction in organic photovoltaic devices.

A homologous Ni-Co based nanowire catalyst pair, composed of Ni(x)Co(3-x)O4 nanowires and NiCo/NiCoO(x) nanohybrid, is developed for efficient overall water splitting. Ni(x)Co(3-x)O4 nanowires are found as a highly active oxygen evolution reaction (OER) catalyst, and they are converted into a highly active hydrogen evolution reaction (HER) catalyst through hydrogenation treatment as NiCo/NiCoO(x) heteronanostructures. An OER current density of 10 mA cm(-2) is obtained with the Ni(x)Co(3-x)O4 nanowires under an overpotential of 337 mV in 1.0 M KOH, and an HER current density of 10 mA cm(-2) is obtained with the NiCo/NiCoO(x) heteronanostructures at an overpotential of 155 mV. When integrated in an electrolyzer, these catalysts demonstrate a stable performance in water splitting.

The electronic structure and surface composition of CH3NH3PbI3 (MAPbI3) films fabricated by one-step method with different precursor ratios of PbI2 to CH3NH3I (PbI2/MAI) have been investigated with ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). It is found that the core levels of all components in the MAPbI3 film shift toward lower binding energy with decreasing the precursor ratio of PbI2/MAI, indicating that the electronic structures of the MAPbI3 film can be adjusted by the precursor ratio of PbI2/MAI. The elemental compositions of the MAPbI3 film also depend on the precursor ratio and annealing process, and the compositions are strongly correlated to the electronic properties of the films. The electronic properties remain unchanged with an annealing at 110 °C. However, a core level shift of 0.5 eV toward higher binding energy is observed with an annealing at 150 °C, together with noticeable composition change from the XPS core level analysis. The distribution of all chemical components in the MAPbI3 film is further investigated with angle-resolved XPS (AR-XPS). It is observed that annealing at 150 °C leads to relatively shallow distribution variations of I and Pb in the MAPbI3 film, accompanied by infiltration of metallic Pb into the bulk.

The electronic properties of interfaces formed between Au and organometal triiodide perovskite (CH3NH3PbI3) are investigated using ultraviolet photoemission spectroscopy (UPS), inverse photoemission spectroscopy (IPES) and X-ray photoemission spectroscopy (XPS). It is found that the CH3NH3PbI3 film coated onto the substrate of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS)/indium tin oxide (ITO) by a two-step method presents n-type semiconductor behavior, with a band gap of 1.7 eV and a valence band (VB) edge of 1.0 eV below the Fermi energy (EF). An interface dipole of 0.1 eV is observed at the CH3NH3PbI3/Au interface. The energy levels of CH3NH3PbI3 shift upward by ca. 0.4 eV with an Au coverage of 64 Å upon it, resulting in band bending, hence a built-in field in CH3NH3PbI3 that encourages hole transport to the interface. Hole accumulation occurs in the vicinity of the interface, facilitating the hole transfer from CH3NH3PbI3 to Au. Furthermore, the shift of the VB maximum of CH3NH3PbI3 toward the EF indicates a decrease of energy loss as holes transfer from CH3NH3PbI3 to Au.