Electron Injection Dynamics at the SILAR Deposited CdS Quantum Dot/TiO₂ Interface

Semiconductor quantum dots with their tunable band gap energies offer new opportunities for controlling photoresponse and photoconversion efficiency of solar cells. Exciton dissociation, charge injection, and charge recombination are key steps for efficient interfacial electron transfer. Here, electron injection and carrier relaxation dynamics at the CdS quantum dot/TiO₂ interface were investigated by using ultrafast transient absorption spectroscopy and global fitting analyses. The CdS/TiO₂ composites were prepared by depositing CdS in the TiO₂ nanocrystalline films by the successive ionic layer adsorption and reaction (SILAR) technique. Comparing the transient absorption spectra of CdS/TiO₂ composites formed by different numbers of CdS coating cycles and CdS/Al₂O₃ revealed size-dependent electron injection from excited CdS into TiO₂ nanoparticles, which occurred on a time scale of 1–9 ps. We also demonstrated that holes are trapped in a localized state with a time constant of 0.3 ps, which was faster than the electron trapping with a time constant of about 30 ps

Near-Infrared Absorbing Cu₁₂Sb₄S₁₃ and Cu₃SbS₄ Nanocrystals: Synthesis, Characterization, and Photoelectrochemistry

Herein, we present the novel synthesis of tetrahedrite copper antimony sulfide (CAS) nanocrystals (Cu₁₂Sb₄S₁₃), which display strong absorptions in the visible and NIR. Through ligand tuning, the size of the Cu₁₂Sb₄S₁₃ NCs may be increased from 6 to 18 nm. Phase purity is achieved through optimizing the ligand chemistry and maximizing the reactivity of the antimony precursor. We provide a detailed investigation of the optical and photoelectrical properties of both tetrahedrite (Cu₁₂Sb₄S₁₃) and famatinite (Cu₃SbS₄) NCs. These NCs were found to have very high absorption coefficients reaching 10⁵ cm^–1 and band gaps of 1.7 and 1 eV for tetrahedrite and famatinite NCs, respectively. Ultraviolet photoelectron spectroscopy was employed to determine the band positions. In each case, the Fermi energies reside close to the valence band, indicative of a p-type semiconductor. Annealing of tetrahedrite CAS NC films in sulfur vapor at 350 °C was found to result in pure famatinite NC films, opening the possibility to tune the crystal structure within thin films of these NCs. Photoelectrochemistry of hydrazine free unannealed films displays a strong p-type photoresponse, with up to 0.1 mA/cm² measured under mild illumination. Collectively these optical properties make CAS NCs an excellent new candidate for both thin film and hybrid solar cells and as strong NIR absorbers in general

Identifying an Optimum Perovskite Solar Cell Structure by Kinetic Analysis: Planar, Mesoporous Based, or Extremely Thin Absorber Structure

Perovskite solar cells have rapidly been developed over the past several years. Choice of the most suitable solar cell structure is crucial to improve the performance further. Here, we attempt to determine an optimum cell structure for methylammonium lead iodide (MAPbI₃) perovskite sandwiched by TiO₂ and spiro-OMeTAD layers, among planar heterojunction, mesoporous structure, and extremely thin absorber structure, by identifying and comparing charge carrier diffusion coefficients of the perovskite layer, interfacial charge transfer, and recombination rates using transient emission and absorption spectroscopies. An interfacial electron transfer from MAPbI₃ to compact TiO₂ occurs with a time constant of 160 ns, slower than the perovskite photoluminescence (PL) lifetime (34 ns). In contrast, fast non-exponential electron injection to mesoporous TiO₂ was observed with at least two different electron injection processes over different time scales; one (60–70%) occurs within an instrument response time of 1.2 ns and the other (30–40%) on nanosecond time scale, while most of hole injection (85%) completes in 1.2 ns. Analysis of the slow charge injection data revealed an electron diffusion coefficient of 0.016 ± 0.004 cm² s^–1 and a hole diffusion coefficient of 0.2 ± 0.02 cm² s^–1 inside MAPbI₃. To achieve an incident photon-to-current conversion efficiency of >80%, a minimum charge carrier diffusion coefficient of 0.08 cm² s^–1 was evaluated. An interfacial charge recombination lifetime was increased from 0.5 to 40 ms by increasing a perovskite layer thickness, suggesting that the perovskite layer suppresses charge recombination reactions. Assessments of charge injection and interfacial charge recombination processes indicate that the optimum solar cell structure for the MAPbI₃ perovskite is a mesoporous TiO₂ based structure. This comparison of kinetics has been applied to several different types of photoactive semiconductors such as perovskite, CdTe, and GaAs, and the most appropriate solar cell structure was identified

Fluorene–Thiophene Copolymer Wire on TiO₂: Mechanism Achieving Long Charge Separated State Lifetimes

Insertion of interfacial molecules in bulk heterojunction and dye sensitized solar cells is effective to retard charge recombination reactions and thus to improve solar cell performance. So far, to extend charge separated state lifetime, the molecule was designed to increase distance between an n-type and a p-type semiconductors to reduce their electronic coupling. Here we investigated a series of thiophene–fluorene molecular wires on the TiO₂ nanoporous surface and propose a model to explain a long-lived charge separated state. The polymer wire acts as a sensitizer aligned in parallel to the TiO₂ surface and injects an electron into the TiO₂ with electron injection efficiency of >80%. Time-resolved microwave conductivity measurements suggest that a generated hole can be mobile, and we found with DFT calculation that a hole appears to be localized at the thiophene units which are not directly attached to the TiO₂ surface. Charge recombination between the mobile electron in the TiO₂ and the hole at the thiophene units is retarded to >100 ms compared to the reaction at the monomer/TiO₂ interface with ∼5 ms. Monte Carlo simulation supports that this slow charge recombination occurs with the localization of the hole at the thiophene units

Dye-Anchoring Functional Groups on the Performance of Dye-Sensitized Solar Cells: Comparison between Alkoxysilyl and Carboxyl Groups

We have compared two dye-anchoring functional groups, alkoxysilyl and carboxyl groups, to investigate their influence on the performance of dye-sensitized solar cells. (Dimethylamino)­azobenzene was selected as a chromophore possessing a donor–acceptor transition for the light absorption. Electrochemical and optical measurements were performed for 4-(dimethylamino)­azobenzene-4′-carboxylic acid and 4-(dimethylamino)­azobenzene-4′-triethoxysilane attached TiO₂ films. Electrochemical measurements and DFT calculations indicated almost identical potential energy levels and electron density of HOMO and LUMO states between these two dyes. Solar cell APCE spectra and charge recombination kinetics at the dye/TiO₂ interface revealed almost identical charge-transfer rates/yields from and to the dye. The difference was observed on improvement of an open circuit photovoltage (<i>V</i>_oc) by 60 mV and on the lifetimes of an electron in the TiO₂ conduction band for the dye with the alkoxysilyl functional group compared to the carboxyl group, suggesting that an alkoxysilyl functional group is more attractive to retard the charge recombination reaction between an electron in the TiO₂ conduction band and an oxidized form of an electrolyte redox couple. The highest solar energy conversion efficiency of 2.6% was achieved for dye-sensitized solar cells based on an azobenzene dye sensitizer under AM1.5<i>G</i>, one sun condition

Dye Regeneration Kinetics in Dye-Sensitized Solar Cells

The ideal driving force for dye regeneration is an important parameter for the design of efficient dye-sensitized solar cells. Here, nanosecond laser transient absorption spectroscopy was used to measure the rates of regeneration of six organic carbazole-based dyes by nine ferrocene derivatives whose redox potentials vary by 0.85 V, resulting in 54 different driving-force conditions. It was found that the reaction follows the behavior expected for the Marcus normal region for driving forces below 29 kJ mol^–1 (Δ<i>E</i> = 0.30 V). Driving forces of 29–101 kJ mol^–1 (Δ<i>E</i> = 0.30–1.05 V) resulted in similar reaction rates, indicating that dye regeneration is diffusion controlled. Quantitative dye regeneration (theoretical regeneration yield 99.9%) can be achieved with a driving force of 20–25 kJ mol^–1 (Δ<i>E</i> ≈ 0.20–0.25 V)

Improved Photovoltages for p‑Type Dye-Sensitized Solar Cells Using CuCrO₂ Nanoparticles

CuCrO₂ has been investigated as an alternative to conventionally used NiO in p-type dye sensitized solar cells (p-DSCs) using I^–/I₃^–- and [Co­(en)₃]^2+/3+-based redox mediators. The favorable valence band edge position of CuCrO₂ affords open circuit voltages as high as 734 mV when [Co­(en)₃]^2+/3+ is deployed as redox mediator. This is the highest reported open circuit voltage for any p-DSC to date, introducing mesoporous CuCrO₂ electrodes as a promising alternative to traditional NiO

Wafer-Scale Synthesis of Semiconducting SnO Monolayers from Interfacial Oxide Layers of Metallic Liquid Tin

Atomically thin semiconductors are one of the fastest growing categories in materials science due to their promise to enable high-performance electronic and optical devices. Furthermore, a host of intriguing phenomena have been reported to occur when a semiconductor is confined within two dimensions. However, the synthesis of large area atomically thin materials remains as a significant technological challenge. Here we report a method that allows harvesting monolayer of semiconducting stannous oxide nanosheets (SnO) from the interfacial oxide layer of liquid tin. The method takes advantage of van der Waals forces occurring between the interfacial oxide layer and a suitable substrate that is brought into contact with the molten metal. Due to the liquid state of the metallic precursor, the surface oxide sheet can be delaminated with ease and on a large scale. The SnO monolayer is determined to feature p-type semiconducting behavior with a bandgap of ∼4.2 eV. Field effect transistors based on monolayer SnO are demonstrated. The synthetic technique is facile, scalable and holds promise for creating atomically thin semiconductors at wafer scale