RESUMEN
TiO2 has high chemical stability, strong catalytic activity and is an electron transport material in organic solar cells. However, the presence of trap states near the band edges of TiO2 arising from defects at grain boundaries significantly affects the efficiency of organic solar cells. To become an efficient electron transport material for organic photovoltaics and related devices, such as perovskite solar cells and photocatalytic devices, it is important to tailor its band edges via doping. Nitrogen p-type doping has attracted considerable attention in enhancing the photocatalytic efficiency of TiO2 under visible light irradiation while hydrogen n-type doping increases its electron conductivity. DFT calculations in TiO2 provide evidence that nitrogen and hydrogen can be incorporated in interstitial sites and possibly form NiHi, NiHO and NTiHi defects. The experimental results indicate that NiHi defects are most likely formed and these defects do not introduce deep level states. Furthermore, we show that the efficiency of P3HT:IC60BA-based organic photovoltaic devices is enhanced when using hydrogen-doping and nitrogen/hydrogen codoping of TiO2, both boosting the material n-type conductivity, with maximum power conversion efficiency reaching values of 6.51% and 6.58%, respectively, which are much higher than those of the cells with the as-deposited (4.87%) and nitrogen-doped TiO2 (4.46%).
RESUMEN
Transition-metal-oxide hybrids composed of high surface-to-volume ratio Ta2O5 matrices and a molecular analogue of transition metal oxides, tungsten polyoxometalates ([PW12O40](3-)), are introduced herein as a charge storage medium in molecular nonvolatile capacitive memory cells. The polyoxometalate molecules are electrostatically self-assembled on a low-dimensional Ta2O5 matrix, functionalized with an aminosilane molecule with primary amines as the anchoring moiety. The charge trapping sites are located onto the metal framework of the electron-accepting molecular entities as well as on the molecule/oxide interfaces which can immobilize negatively charged mobile oxygen vacancies. The memory characteristics of this novel nanocomposite were tested using no blocking oxide for extraction of structure-specific characteristics. The film was formed on top of the 3.1 nm-thick SiO2/n-Si(001) substrates and has been found to serve as both SiO2/Si interface states' reducer (i.e., quality enhancer) and electron storage medium. The device with the polyoxometalates sandwiched between two Ta2O5 films results in enhanced internal scattering of carriers. Thanks to this, it exhibits a significantly larger memory window than the one containing the plain hybrid and comparable retention time, resulting in a memory window of 4.0 V for the write state and a retention time around 10(4) s without blocking medium. Differential distance of molecular trapping centers from the cell's gate and electronic coupling to the space charge region of the underlying Si substrate were identified as critical parameters for enhanced electron trapping for the first time in such devices. Implementing a numerical electrostatic model incorporating structural and electronic characteristics of the molecular nodes derived from scanning probe and spectroscopic characterization, we are able to interpret the hybrid's electrical response and gain some insight into the electrostatics of the trapping medium.
RESUMEN
Molybdenum oxide is used as a low-resistance anode interfacial layer in applications such as organic light emitting diodes and organic photovoltaics. However, little is known about the correlation between its stoichiometry and electronic properties, such as work function and occupied gap states. In addition, despite the fact that the knowledge of the exact oxide stoichiometry is of paramount importance, few studies have appeared in the literature discussing how this stoichiometry can be controlled to permit the desirable modification of the oxide's electronic structure. This work aims to investigate the beneficial role of hydrogenation (the incorporation of hydrogen within the oxide lattice) versus oxygen vacancy formation in tuning the electronic structure of molybdenum oxides while maintaining their high work function. A large improvement in the operational characteristics of both polymer light emitting devices and bulk heterojunction solar cells incorporating hydrogenated Mo oxides as hole injection/extraction layers was achieved as a result of favorable energy level alignment at the metal oxide/organic interface and enhanced charge transport through the formation of a large density of gap states near the Fermi level.