Huiping Gao

Influence of transition metal doping on the structural, optical, and magnetic properties of TiO2 films deposited on Si substrates by a sol–gel process

Transition metal (TM)-doped TiO2 films (TM = Co, Ni, and Fe) were deposited on Si(100) substrates by a sol–gel method. With the same dopant content, Co dopants catalyze the anatase-to-rutile transformation (ART) more obviously than Ni and Fe doping. This is attributed to the different strain energy induced by the different dopants. The optical properties of TM-doped TiO2 films were studied with spectroscopic ellipsometry data. With increasing dopant content, the optical band gap (EOBG) shifts to lower energy. With the same dopant content, the EOBG of Co-doped TiO2 film is the smallest and that of Fe-doped TiO2 film is the largest. The results are related to electric disorder due to the ART. Ferromagnetic behaviors were clearly observed for TM-doped TiO2 films except the undoped TiO2 film which is weakly magnetic. Additionally, it is found that the magnetizations of the TM-doped TiO2 films decrease with increasing dopant content.

Growth of Zr/N-codoped TiO2 nanorod arrays for enhanced photovoltaic performance of perovskite solar cells

In this paper, Zr and N co-doped TiO2 (Zr/N–TiO2) nanorod arrays were synthesized using a hydrothermal method and perovskite solar cells were fabricated using them as an electron transfer layer. The solar cells based on Zr/N–TiO2 presented an enhanced performance compared with those based on un-doped TiO2. The solar cell performance was optimized by changing the Zr doping content. The efficiency of solar cells based on Zr/N–TiO2 with a Zr doping content of 1% (Zr/Ti, atomic ratio) has been achieved at 12.6%, which was 31.6% higher than that of solar cells based on un-doped TiO2. To get an insight into the enhancement, some investigations were carried out. The results indicate that the larger open voltage (Voc) could be due to the larger conduction band offset resulting from the smaller energy band gap for Zr/N–TiO2, and the enlarged short current (Isc) could be attributed to the faster electron transfer and reduced recombination rate for Zr/N–TiO2 NRs. These induce the enhancement of solar cell efficiency.

Understanding the effect of Mn2+ on Yb3+/Er3+ upconversion and obtaining a maximum upconversion fluorescence enhancement in inert-core/active-shell/inert-shell structures

In this study, the energy transfer (ET) mechanism among Yb3+/Er3+/Mn2+ has been revealed by download conversion (DC) and upconversion (UC) transients and steady-state fluorescence spectra. Moreover, the novel structure, inert-core/active-shell/inert-shell cubic NaYF4@NaYF4:Er3+/Yb3+/Mn2+@NaYF4, has been synthesized via a convenient hydrothermal method. With the excitation of a 980 nm laser diode, bright single-red upconversion (UC) fluorescence is observed, which shows the longest UC lifetime and highest efficiency compared with the other structures of NaYF4:Er3+/Yb3+/Mn2+, NaYF4:Er3+/Yb3+/Mn2+@NaYF4, and NaYF4@NaYF4:Er3+/Yb3+/Mn2+. The inert-core/active-shell/inert-shell nanocubes are homogeneous with an ultra-small (sub-20 nm) size, which is suitable for distribution and elimination in biological tissue. Our results indicate that the Mn2+ doping of δ-doped structures is an effective method to significantly improve UC single-red light, which shows great potential application in vivo bioimaging.

Simultaneous size and luminescence control of KZnF3:Yb3+/Er3+ nanoparticles by incorporation of Mn2+

A strategy is demonstrated for simultaneous size and color tuning of KZnF3:Yb3+/Er3+ nanocrystals through transition metal Mn2+ doping. The experimental results indicate that the introduction of Mn2+ into the KZnF3:Yb3+/Er3+ reaction system facilitates the decrease of the crystal size. Moreover, in the case of a high concentration of Mn2+ doping, the downshifting (DS) and upconversion (UC) luminescence all show a single red band emission. In this work, we mainly investigate the DS and UC energy transfer process among Mn2+/Yb3+/Er3+ in KZnF3 nanocrystals by the detection and analysis of the absorption, excitation, emission, and transient fluorescence spectra. The results show that both Mn2+ and Yb3+ ions can decrease the green emission while improving the red luminescence of Er3+. This DS and UC single red color tuning is significant for the application of nanoparticles in light conversion, biological labeling, or plant photosynthesis.

The optimum titanium precursor of fabricating TiO2 compact layer for perovskite solar cells

Perovskite solar cells (PSCs) have attracted tremendous attentions due to its high performance and rapid efficiency promotion. Compact layer plays a crucial role in transferring electrons and blocking charge recombination between the perovskite layer and fluorine-doped tin oxide (FTO) in PSCs. In this study, compact TiO2 layers were synthesized by spin-coating method with three different titanium precursors, titanium diisopropoxide bis (acetylacetonate) (c-TTDB), titanium isopropoxide (c-TTIP), and tetrabutyl titanate (c-TBOT), respectively. Compared with the PSCs based on the widely used c-TTDB and c-TTIP, the device based on c-TBOT has significantly enhanced performance, including open-circuit voltage, short-circuit current density, fill factor, and hysteresis. The significant enhancement is ascribed to its excellent morphology, high conductivity and optical properties, fast charge transfer, and large recombination resistance. Thus, a power conversion efficiency (PCE) of 17.03% has been achieved for the solar cells based on c-TBOT.

Enhanced Power Conversion Efficiency of Perovskite Solar Cells with an Up-Conversion Material of Er3+-Yb3+-Li+ Tri-doped TiO2

In this paper, Er3+-Yb3+-Li+ tri-doped TiO2 (UC-TiO2) was prepared by an addition of Li+ to Er3+-Yb3+ co-doped TiO2. The UC-TiO2 presented an enhanced up-conversion emission compared with Er3+-Yb3+ co-doped TiO2. The UC-TiO2 was applied to the perovskite solar cells. The power conversion efficiency (PCE) of the solar cells without UC-TiO2 was 14.0%, while the PCE of the solar cells with UC-TiO2 was increased to 16.5%, which presented an increase of 19%. The results suggested that UC-TiO2 is an effective up-conversion material. And this study provided a route to expand the spectral absorption of perovskite solar cells from visible light to near-infrared using up-conversion materials.

A New Up-conversion Material of Ho 3+ -Yb 3+ -Mg 2+ Tri-doped TiO 2 and Its Applications to Perovskite Solar Cells

A new up-conversion nanomaterial of Ho3+-Yb3+-Mg2+ tri-doped TiO2 (UC-Mg-TiO2) was designed and synthesized with a sol-gel method. The UC-Mg-TiO2 presented enhanced up-conversion fluorescence by an addition of Mg2+. The UC-Mg-TiO2 was utilized to fabricate perovskite solar cells by forming a thin layer on the electron transfer layer. The results display that the power conversion efficiency of the solar cells based on the electron transfer layer with UC-Mg-TiO2 is improved to 16.3 from 15.2% for those without UC-Mg-TiO2. It is demonstrated that the synthesized UC-Mg-TiO2 can convert the near-infrared light to visible light that perovskite film can absorb to improve the power conversion efficiency of the devices.

Enhanced Performance of Perovskite Solar Cells by Using Ultrathin BaTiO3 Interface Modification

Efficiency promotion has been severely constrained by charge recombination in perovskite solar cells (PSCs). Interface modification has been proved to be an effective way to reduce the interfacial charge recombination. In this work, a mesoporous TiO2 (mp-TiO2) layer was modified by an ultrathin BaTiO3 layer to suppress charge recombination in PSCs. The ultrathin BaTiO3 modification layer was prepared by the spin coating method using a barium salt solution. The concentration of the barium salt solution was optimized, and the effect of the BaTiO3 modification layer on the performance of the cells was also investigated. The modification layer can not only successfully retard charge recombination but also effectively boost the rate of electron extraction at the interface, resulting in enhanced open-circuit voltage ( Voc), short circuit current density ( Jsc), and fill factor. Furthermore, the hysteresis of the PSCs was also significantly reduced after the modification. By optimizing and employing the BaTiO3 modification layer, the power conversion efficiency of the cells was increased from 16.13 to 17.87%.