Mesoscopic nitrogen-doped TiO2 spheres for quantum dot-sensitized solar cells

Improvement of Power Conversion Efficiency of Quantum Dot-Sensitized Solar Cells by Doping of Manganese into a ZnS Passivation Layer and Cosensitization of Zinc-Porphyrin on a Modified Graphene Oxide/Nitrogen-Doped TiO2 Photoanode

It is vital to acquire power conversion efficiencies comparable to other emerging solar cell technologies by making quantum dot-sensitized solar cells (QDSSCs) competitive. In this study, the effect of graphene oxide (GO), nitrogen, manganese, and a porphyrin compound on the performance of QDSSCs based on a TiO₂/CdS/ZnS photoanode was investigated. First, adding GO and nitrogen into TiO₂ has a conspicuous impact on the cell efficacy. Both these materials reduce the recombination rate and expand the specific surface area of TiO₂ as well as dye loading, reinforcing cell efficiency value. The maximum power conversion efficiency of QDSSC with a GO N-doped photoelectrode was 2.52%. Second, by employing Mn²⁺ (5 and 10 wt %) doping of ZnS, we have succeeded in considerably improving cell performance (from 2.52 to 3.47%). The reason for this could be for the improvement of the passivation layer of ZnS by Mn²⁺ ions, bringing about to a smaller recombination of photoinjected electrons with either oxidized dye molecules or electrolyte at the surface of titanium dioxide. However, doping of 15 wt % Mn²⁺ had an opposite effect and somewhat declined the cell performance. Finally, a Zn-porphyrin dye was added to the CdS/ZnS by a cosensitization method, widening the light absorption range to the NIR (near-infrared region) (>700 nm), leading to the higher short-circuit current density (J _SC) and cell efficacy. Utilizing an environmentally safe porphyrin compound into the structure of QDSSC has dramatically enhanced the cell efficacy to 4.62%, which is 40% higher than that of the result obtained from the TiO₂/CdS/ZnS photoelectrode without porphyrin coating.

Co-axial electrospray: a versatile tool to fabricate hybrid electron transporting materials for high efficiency and stable perovskite photovoltaics

We report a cost-effective and simple co-axial electrospray technique to fabricate a hybrid electron transporting material (ETM) consisting of a nanocomposite of hierarchically structured TiO₂ nanobeads (NBs) blended with ZnO nanofibers (NFs), namely ZnO NFs + TiO₂ NBs, for the first time ever. Owing to its large surface area, highly porous nature and fast electron transport, the hybrid ETM is further used in methylammonium lead iodide (CH₃NH₃PbI₃)-based perovskite solar cells (PSCs). The optimized cells utilizing the hybrid ETM exhibit a maximum power conversion efficiency (PCE_max) of 20.27%, the highest efficiency reported thus far for hybrid ETMs. Moreover, negligible hysteresis and highly reproducible values of PCE are observed for such cells. The PCE of devices based on the ZnO NF + TiO₂ NB hybrid ETM is found to be far superior to that of only ZnO NF and hierarchically structured TiO₂ NB-based ETMs. Light-induced transient measurement shows that the significantly rapid electron diffusion and longer electron lifetime of the ZnO NF + TiO₂ NB hybrid ETM than of only ZnO NF and hierarchically structured TiO₂ NB-based ETMs contribute to the enhanced efficiency in PSCs.

Electrosprayed TiO2 nanoporous hemispheres for enhanced electron transport and device performance of formamidinium based perovskite solar cells

Titanium dioxide (TiO₂) nanoporous hemispheres (NHSs) with a radius of ∼200 nm are fabricated by electrospraying a hydrothermally synthesized TiO₂ nanoparticle (NP) suspension solution. The resulting TiO₂ NHSs are highly porous, which are beneficial to the infiltration of perovskites and provide a larger contact area, as building blocks to construct a mesoporous TiO₂ layer for FA_0.81MA_0.15Pb(I_0.836Br_0.15)₃ based perovskite solar cells (PSCs). By varying the TiO₂ NHS collecting period (15 s, 30 s, 60 s and 90 s) during the electrospraying process, the performance of PSCs changes with different TiO₂ NHS distribution densities. The optimized PSC employing TiO₂ NHSs (60 s) exhibits a photovoltaic conversion efficiency (PCE) as high as 19.3% with a J_sc of 23.8 mA cm^-2, a V_oc of 1.14 V and a FF of 0.71. Furthermore, the PSC possesses a reproducible PCE value with little hysteresis in its current density-voltage (J-V) curves. The small perturbation transient photovoltage (TPV) measurement reveals a longer free carrier lifetime within the TiO₂ NHS based PSC than that in the TiO₂ NP based PSC, and the time of flight (TOF) photoconductivity measurement shows that charge mobilities in this system are also enhanced. These characteristics make TiO₂ NHSs a promising electron transport material for efficient photovoltaic devices.

Assembly of CdS nanoparticles on boron and fluoride co-doped TiO2 nanofilm for solar energy conversion applications

Boron and fluoride co-doped TiO₂ nanomaterial is successfully synthetized using a facile process, followed by chemical bath deposition in an organic solution to ensure high wettability and superior penetration ability of the B/F co-doped TiO₂ films.

Engineered band structure for an enhanced performance on quantum dot-sensitized solar cells

A photon-to-current efficiency of 2.93% is received for the Mn-doped CdS (MCdS)-quantum dot sensitized solar cells (QDSSCs) using Mn:ZnO (MZnO) nanowire as photoanode. Hydrothermal synthesized MZnO are spin-coated on fluorine doped tin oxide (FTO) glass with P25 paste to serve as photoanode after calcinations. MCdS was deposited on the MZnO film by the successive ionic layer adsorption and reaction method. The long lived excitation energy state of Mn2+ is located inside the conduction band in the wide bandgap ZnO and under the conduction band of CdS, which increases the energetic overlap of donor and acceptor states, reducing the “loss-in-potential,” inhibiting charge recombination, and accelerating electron injection. The engineered band structure is well reflected by the electrochemical band detected using cyclic voltammetry. Cell performances are evidenced by current density-voltage (J-V) traces, diffuse reflectance spectra, transient PL spectroscopy, and incident photon to current conversion efficiency characterizations. Further coating of CdSe on MZnO/MCdS electrode expands the light absorption band of the sensitizer, an efficiency of 4.94% is received for QDSSCs.

An ordered and porous N-doped carbon dot-sensitized Bi2O3 inverse opal with enhanced photoelectrochemical performance and photocatalytic activity

A novel ordered porous Bi2O3 inverse opal structure (IOS) was prepared using a polystyrene (PS) photonic crystal as the template for the first time. Nitrogen-doped carbon dots (N-CDs) were chosen to sensitize the as-prepared Bi2O3 IOS for improving photoelectrochemical performance and photocatalytic activity. The photocurrent density of the fabricated N-CDs/Bi2O3 IOS with favorable visible light absorption properties can achieve 0.75 mA cm(-2), which significantly enhanced performance two-, seven-, and thirty-fold compared with that of the CDs/Bi2O3 IOS, Bi2O3 IOS, and Bi2O3 nanoparticles (NPs), respectively. The N-CDs/Bi2O3 IOS also has increased photocatalytic activity for the decolorization of Rhodamine B (RhB), 4 times higher than Bi2O3 NPs. The above performance enhancement of N-CDs/Bi2O3 IOS is caused by the synergistic effect of N-CDs sensitization and the highly ordered IOS, which make it a promising material to be used in clean energy, solar cells, potential applications in water purification and so on.

The synthesis and characterization of lead sulfide with cube-like structure as a counter electrode in the presence of urea using a hydrothermal method

PbS counter electrodes at different concentrations of urea.

Quantum confinement effect of CdSe induced by nanoscale solvothermal reaction

We report a novel method, nanoscale solvothermal reaction (NSR), to induce the quantum confinement effect of CdSe on nanostructured TiO(2) by solvothermal route. The time-dependent growth of CdSe is observed in solution at room temperature, which is found to be accomplished instantly by heat-treatment in the presence of solvent at 1 atm. However, no crystal growth occurs upon heat-treatment in the absence of solvent. The nanoscale solvothermal growth of CdSe quantum dot is realized on the nanocrystalline oxide surface, where Cd(NO(3))(2)·4H(2)O and Na(2)SeSO(3) solutions are sequentially spun on nanostructured TiO(2), followed by heat-treatment at temperatures ranging from 100 °C to 250 °C. Size of CdSe increases from 4.4 nm to 5.3 nm, 8.7 nm and 14.8 nm, which results in decrease in optical band gap from 2.19 eV to, 1.95 eV, 1.74 eV and 1.75 eV with increasing the NSR temperature from 100 °C to 150 °C, 200 °C and 250 °C, respectively, which is indicative of the quantum confinement effect. Thermodynamic studies reveal that increase in the size of CdSe is related to increase in enthalpy, for instance, from 3.77 J mg(-1) for 100 °C to 8.66 J mg(-1) for 200 °C. Quantum confinement effect is further confirmed from the CdSe-sensitized solar cell, where onset wavelength in external quantum efficiency spectra is progressively shifted from 600 nm to 800 nm as the NSR temperature increases, which leads to a significant improvement of power conversion efficiency by a factor of more than four. A high photocurrent density of 13.7 mA cm(-2) is obtained based on CdSe quantum dot grown by NSR at 200 °C.