Electron Injection Dynamics Research Articles

Comparative Study of Quasi-Solid-State Dye-Sensitized Solar Cells Using Z907, N719, Photoactive Phenothiazine Dyes and PVDF-HFP Gel Polymer Electrolytes with Different Molecular Weights

The present study investigates the influence of photosensitizer selection and the polymer electrolyte composition on the performance of quasi-solid-state dye-sensitized solar cells (QsDSSCs). Two benchmark ruthenium dyes, N719 and Z907, alongside a novel photoactive phenothiazine dye were used. Each dye was incorporated into a QsDSSC architecture employing poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) as the gel electrolyte matrix, with varying molecular weights, to investigate their impacts on the overall device performance and long-term stability. Our results demonstrated that the N719 dye exhibited the highest power conversion efficiency (PCE), attributed to its strong absorption in the visible spectrum and efficient electron injection into the TiO2 photoanode. Z907, on the other hand, showed moderate PCE due to its broader absorption profile but slower electron injection kinetics. The phenothiazine dye revealed promising PCE, with tunable absorption properties and efficient charge transfer. Furthermore, the impact of PVDF-HFP polymer gel electrolytes with varying molecular weights on cell stability was explored. The QsDSSC incorporating the PVH80 polymer with the phenothiazine dye exhibited reduced dye desorption, due to the effective dye molecules’ immobilization by the gel matrix, and consequently enhanced long-term stability over 600 h. This comparative study sheds light on the interplay between dye selection, the polymer gel’s properties, and QsDSSCs’ performance. These insights are crucial in designing robust and efficient QsDSSCs for practical applications.

(Digital Presentation) Characterization of Ultrafast Dynamics of Refractory Metal Nanoparticles of Interest as Photocatalysts

Plasmonic noble metal nanoparticles are of interest as photocatalysts for several reasons, including a very large absorption cross section and the creation of a greatly enhanced evanescent electromagnetic field at the surface of the nanoparticles when excited at the plasmon resonance. Furthermore, the plasmon resonance is tunable over a wide wavelength range as a function of the size, shape, and composition of the nanoparticles. When these properties are combined with the known photocatalytic activity of noble metals for technologically important targets such as solar fuels, it is not surprising that these nanoparticle systems are widely studied. Noble metals do, however, possess a weakness in that they can be easily melted upon photoexcitation, due to the fact that noble metals dissipate absorbed photon energy efficiently through ultrafast electron-phonon coupling, in roughly one picosecond, to produce heat. As a result, the study of new classes of plasmonic materials, such as refractory metals, with much higher melting points than noble metals is growing rapidly. While there are still challenges in the size-selective synthesis of refractory metal nanoparticles, characterization efforts on these materials are of strong interest. We report here a study on the time-resolved spectroscopy of electron-phonon coupling in refractory metal nanoparticles. There are very significant variations in the efficiency and time-scales of electron-phonon coupling in refractory metal nanoparticles as compared to noble metal nanoparticles. Furthermore, due to the fact that many refractory metals are oxidized under ambient conditions to create a thin oxide layer on the surface of the nanoparticles, these materials also demonstrate dynamics related to electron injection from the metal to the oxide layer that noble metals do not show. Far from being a negative, the added electron injection dynamics offer new opportunities for photocatalysis that we discuss. Use of the Center for Nanoscale Materials, an Office of Science user facility, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

Modifying electron injection kinetics for selective photoreduction of nitroarenes into cyclic and asymmetric azo compounds

Modifying the reactivity of substrates by encapsulation is essential for microenvironment catalysts. Herein, we report an alternative strategy that modifies the entry behaviour of reactants into the microenvironment and substrate inclusion thermodynamics related to the capsule to control the electron injection kinetics and the selectivity of products from the nitroarenes photoreduction. The strategy includes the orchestration of capsule openings to control the electron injection kinetics of electron donors, and the capsule’s pocket to encapsulate more than one nitroarene molecules, facilitating a condensation reaction between the in situ formed azanol and nitroso species to produce azo product. The conceptual microenvironment catalyst endows selective conversion of asymmetric azo products from different nitroarenes, wherein, the estimated diameter and inclusion Gibbs free energy of substrates are used to control and predict the selectivity of products. Inhibition experiments confirm a typical enzymatic conversion, paving a new avenue for rational design of photocatalysts toward green chemistry.

Immersion solvent dependent photophysical and photovoltaic properties of Porphyrin/TiO2 interface

In this work, the role of immersion solvents on light harvesting efficiency, photoinduced electron injection and photovoltaic properties of porphyrin/TiO2 have been investigated. Time resolved fluorescence technique was used to characterize the kinetics of electron injection at the porphyrin/TiO2 interface. The results show that electron injection from photoexcited porphyrin to the conduction band of TiO2 is highly sensitive to the nature of the solvent used during self-assembly of the TiO2-porphyrin interface. Based on the findings, the t-BuOH:ACN solvent is suitable for achieving long lived electrons in a H2MP/TiO2 device.

Ultrafast electron injection kinetics and effect of plasmonic silver nanoparticle at organic dye-TiO2 interface

Ultrafast electron injection kinetics and effect of plasmonic silver nanoparticle at organic dye-TiO2 interface

The effects of laser polarization and wavelength on injection dynamics of a laser wakefield accelerator

Here, we investigate the effects of laser polarization and wavelength on electron injection dynamics in a laser wakefield accelerator. During the ionization process, electrons gain residual momentum and kinetic energy via above threshold ionization, which has a strong dependence on laser polarization. A circularly polarized laser pulse results in a much higher residual momentum and kinetic energy gain for the ionized electrons compared with the linearly polarized case. This residual momentum results in particle injection because of the sensitivity of particle trapping to the initial conditions and enhanced the total injected beam charge in both experiments and particle-in-cell simulations. Due to the strong correlation of above threshold ionization with laser wavelength, in this work we extended the investigation to long wavelength (up to 20 μm) drive pulses using particle-in-cell simulations. Owing to the gain in kinetic energy, it may be expected that the charge trapped would consistently increase for circular polarization with increasing laser wavelength, but this was not observed. Instead, there are oscillations with wavelength in the relative trapped charge between linear and circular polarization cases, which arise because of ionization and heating effects on the plasma. Our studies highlight the complex interplay between several different physical effects, including injection regimes—above threshold ionization assisted injection, wave-breaking injection by carrier-envelope-phase effects and ionization injection—ionization gradient induced laser pulse evolution, and thermal modifications to the wake structure that need considering when extrapolating laser wakefield acceleration to different wavelength regimes.

Complexity of Electron Injection Dynamics and Light Soaking Effects in Efficient Dyes for Modern DSSC

Electron transfer dynamics in dye sensitized solar cells (DSSCs) employing triphenylamine Y123 dye were investigated by means of femtosecond broadband transient absorption spectroscopy in the visible and mid-IR range of detection. The electron injection process to the titania conduction band was found to appear biphasically with the time constant of the first component within 350 fs and that of the second component between 80 and 95 ps. Subsequently, the effects of continuous irradiation on the ultrafast and fast electron transfer processes were studied in the systems comprising Y123 dye or carbazole MK2 dye in combination with cobalt- or copper-based redox mediators: [Co(bpy)3](B(CN)4)2/3 (bpy = 2,2′-bipyridine) or [Cu(tmby)2](TFSI)1/2 (tmby = 4,4′,6,6′ tetramethyl-2,2′-bipyridine, TFSI = bis(trifluoromethane)sulfonamide). We have found that the steady-state illumination led to acceleration of the electron injection process due to the lowering of titania conduction band edge energy. Moreover, we have observed that the back electron transfer to the oxidized dye was suppressed. These changes in the initial (up to 3 ns) charge separation efficiency were directly correlated with the photocurrent enhancement.

Multistep Electron Injection Dynamics and Optical Nonlinearity Investigations of π-Extended Thioalkyl-Substituted Tetrathiafulvalene Sensitizers

A comprehensive investigation is presented on the photophysical and third-order nonlinear optical (NLO) properties of two thioalkyl-substituted tetrathiafulvalene molecules (referred here as G1 and G3) to understand their utility as photosensitizers for dye-sensitized solar cell (DSSC) and optoelectronic applications. Both steady-state and time-resolved (in the fs–ns time regime) absorption and photoluminescence (PL) spectroscopy techniques were employed to comprehend the excited-state properties of the molecules in solution as well as in thin films deposited on both quartz and mesoporous TiO2 layers. The spectroscopy measurements in solution and thin films deposited on quartz provided the excited-state properties of dye molecules. Time-resolved PL measurements at the dye–TiO2 interface provided initial evidence of electron injection by fast PL quenching decay dynamics for both the molecules. Detailed target analysis of the femtosecond transient absorption spectroscopy (TAS) data of the dye–TiO2 sample revealed a multistep ultrafast electron injection for both molecules with the fastest injection component being 374 and 314 fs for G1 and G3 molecules, respectively. The ultrafast NLO properties of G1 and G3 were studied using the Z-scan technique with 800 nm, ∼70 fs laser pulses. The open aperture measurements showed three-photon absorption with magnitudes of coefficients 4.7 × 10–5 cm3/GW2 and 5.2 × 10–5 cm3/GW2, and the closed aperture measurements provided second hyperpolarizability (γ) values of 3.5 × 10–31 esu and 4.2 × 10–31 esu for G1 and G3, respectively. Additionally, the onset of optical limiting was estimated to be 5.8 × 10–3 J/cm2 and 5.7 × 10–3 J/cm2 for G1 and G3 molecules, respectively.

Role of Explicitly Included Solvents on Ultrafast Electron Injection and Recombination Dynamics at TiO2/Dye Interfaces.

Solvent effects are important for photovoltaic systems like dye-sensitized solar cells (DSCs) but remain largely unexplored, partly due to the complexity of explicitly including solvent molecules in atomistic simulations. To address these issues, we have systematically investigated the solvent effects in practical solar cells using ab initio excited state dynamics simulations. In this computational protocol and the extended model system, we considered giving a novel perspective on the excited state changes in response to solvation and finite temperature at the heterointerface of DSCs. By directly comparing the geometric stability of interface bonding, photoabsorption, interfacial electronic structure, and dynamics of vacuum and solvent systems, we obtain useful insights into how solvents influence the key factors that determine the efficiency of DSCs. Solvents significantly enhance the intensity of visible light absorption of chromophores (∼2 times) through two effects: (a) by inducing changes to the dye molecule structure due to intermolecular dye-solvent interactions, and (b) by the dielectric screening of the solvent. Furthermore, by adsorbing onto the TiO2 surface, solvent molecules adjust the interfacial band alignment to a favorable level and screen out the attraction force between injected electrons in the semiconductor substrate and holes left on the chromophore to a large extent, dramatically slowing down the recombination process (>8 times). Our findings provide a comprehensive picture of the explicit solvent effects on individual energy conversion steps in DSCs at the microscopic scale and lead to more accurate prediction of the performance of nanodevices in practical environments, contributing to the optimization of realistic renewable energy devices.

Molecular design of porphyrin dyes using different electron-withdrawing moieties for high performance dye-sensitized solar cells

In spite of having some favorable light harvesting properties, porphyrins are still inferior to ruthenium dyes in dye sensitized solar cells. Nevertheless, there are enormous scope for improving the efficacy of these dyes. To recognize the connection between the structure and light-harvesting properties, herein, we have made a systematic theoretical investigation on a series of Zn-porphyrin dyes. We have explored the effect of benzodithiazole based model acceptor groups on their photovoltaic properties, especially on the electron injection dynamics with TiO2 photoanode. The model dyes show enhance light absorption in the entire visible region of solar spectrum in comparison to the parent dye which has been synthesized recently. The interfacial electron transfer (IET) dynamics reveals that suitable heteroatom substitution in acceptor group offers faster (sub-picosecond) electron injection as compared to the parent dye.

Collaborative effect of plasmon-induced resonance energy and electron transfer on the interfacial electron injection dynamics of dye-sensitized solar cell.

It has been widely recognized that plasmonic metal nanoparticles (MNPs) can enhance the power convention efficiency (PCE) of dye-sensitized solar cells (DSSCs). This enhancement is ascribed to the combined effects of plasmon decay, scattering, near-field enhancement, and exciting charge carriers in semiconductors through plasmon-induced resonance energy transfer (PIRET) and hot electron injection (HEI). PIRET and HEI processes appeared between MNPs, and semiconductors have been intensively investigated; however, it is not clear how the collaborative effect of PIRET and photon-induced direct and indirect electron transfer (PICT) occurred between plasmonic metals and dyes, and the interference of different charge separation channels (CSCs) starting from PIRET and PICT affects the PCE of DSSCs. This work aims to address these issues. We apply a model Hamiltonian method, which obviously includes both PIRET and PICT processes from Au MNP to dye molecules and incorporates the dye's electron-phonon interaction, to investigate the carrier dynamics. It is found that PIRET deforms the wavepacket dynamics of the molecular excited state and results in ten-fold enhancement of dye absorption. MNPs augment light absorption and increase the electron density in empty molecular orbitals of the dye molecule. Consequently, this enhances the interfacial charge separation. Furthermore, we observed the interference behavior of two CSCs and gave a full-scale insight into the correlation between the constructive/destructive interference and the electronic-state properties as well as carrier-phonon interactions. This work provides a theoretical guidance to optimize DSSCs.

The effect of molecular structure and ultrafast electron injection dynamics on the efficiency of BODIPY sensitized solar cells

In an attempt to investigate the effect of charge transfer dynamics on the photovoltaic performance of the dye-sensitized solar cells (DSSCs), a new series of 4,4-difluoro-3a,4a-diaza-s-indacene (BODIPY) compounds were designed and sensitized. Charge transfer dynamics of the studied compounds were investigated by using femtosecond transient absorption (TA) spectroscopy technique. TA spectroscopy results revealed that excited state lifetime of the sensitized dyes can be controlled by altering the position of the anchoring group as well as the conjugation length of the studied compounds. Photovoltaic performances of the DSSCs produced with the sensitized dyes were evaluated by using incident photon to current efficiency (IPCE) spectra as well as IV measurements. It was found that the dye with longer conjugation length showed efficient electron injection to the conduction band of the semiconductor (TiO2) depending on the anchoring group position. In addition to that, the adsorption of the sensitizer on the TiO2 was affected by the position of OH moieties. Therefore, the results of IPCE and J-V measurements suggest that, in order to improve the photovoltaic performance, the anchoring group should be bound to the positions which do not prevent the molecular rotation. This work could be useful for developing new strategies towards molecular engineering for DSSC applications.

Strategies towards enhancing the efficiency of BODIPY dyes in dye sensitized solar cells

The goal of this study is to reveal the effect of electron donating and anchoring group of the dye on the electron injection dynamics and photovoltaic performance of BODIPY dye sensitized solar cells. The effect of electron donating properties was studied by choosing methoxyphenyl and triphenylamine (TPA) as electron donor moieties. Therefore, four new BODIPY derivatives were designed and sensitized to achieve these goals. In addition, attachment of the dye to TiO2 was studied by altering the surface morphology of TiO2 with fs laser ablation.The steady state absorption and emission spectra, femtosecond transient absorption spectra, incident photon-to-current conversion efficiency (IPCE) and I–V measurements of the studied dyes were investigated, systematically. It was found that methoxyphenyl moiety gives better performance than TPA moiety. BODIPY dye with methoxyphenyl moiety and one anchoring group demonstrated the best photovoltaic performance due to greater excited state lifetime in spite of the lower light harvesting ability, as compared to other studied dyes. Increasing the number of anchoring groups did not increase the photovoltaic performance of BODIPY dye with methoxyphenyl moiety. Anchoring efficiency, light-harvesting capability, and electron injection rate to TiO2 were enhanced by femtosecond laser ablation treatment of TiO2 surface. Experimental results proved that power conversion efficiency was increased 47% when photoanode was laser ablation treated.

Electron Dynamics in Anatase TiO2 Nanoparticles by Ultrafast Broadband Deep-Ultraviolet Spectroscopy

The optical bandgap of anatase TiO2 nanoparticles is dominated by bulk absorption bands in the deep-ultraviolet due to strongly bound excitons. These spectral features can be utilized as a sensitive probe of carrier and lattice dynamics inside the TiO2 nanoparticles. Here, we implement ultrafast broadband spectroscopy tuned to the exciton resonances in order to track the electron cooling in the conduction band of bare anatase nanoparticles and monitor the electron injection dynamics from an external dye in the case of sensitized anatase nanoparticles.

Coherent ultra-broadband laser-assisted injection radiation from a laser plasma accelerator

The injection of electrons into a laser wakefield accelerator (LWFA) is observed to generate an intense coherent ultra-broadband and ultrashort pulse radiation flash, consistent with the acceleration of electrons from rest to nearly the speed of light in a distance < ~ 1 $\mu$m. The flash is sufficiently bright to induce large nonlinear refractive index shifts in optical materials; we estimate a source brightness temperature of ~$10^{18}$ K. We present measurements of the flash spectra, coherence, pulse duration, polarization and angular distribution, providing a detailed picture of electron injection dynamics in LWFA. These are characteristic of laser-assisted injection of off-axis electrons, which preserves wake coherence.

Comparison of Interfacial Electron Transfer Efficiency in [Fe(ctpy)2]2+-TiO2 and [Fe(cCNC)2]2+-TiO2 Assemblies: Importance of Conformational Sampling.

Fe(II)-polypyridines have limited applications as chromophores in dye-sensitized solar cells due to the short lifetimes (∼100 fs) of their photoactive metal-to-ligand charge transfer (MLCT) states formed upon photoexcitation. Recently, a 100-fold increase in the MLCT lifetime was observed in a [Fe(CNC)2]2+ complex (CNC = 2,6-bis(3-methylimidazole-1-ylidine)pyridine) which has strong σ-donating N-heterocyclic carbene ligand in comparison to the weaker field parent [Fe(tpy)2]2+ complex (tpy = 2,2':6',2″-terpyridine). This study utilizes density functional theory (DFT), time-dependent DFT, and quantum dynamics simulations to investigate the interfacial electron transfer (IET) in [Fe(cCNC)2]2+ (cCNC = 4'-carboxy-2,6-bis(3-methylimidazole-1-ylidine)pyridine) and [Fe(ctpy)2]2+ (ctpy = 4'-carboxy-2,2':6',2″-terpyridine) sensitized TiO2. Our results suggest that the replacement of tpy by CNC ligand does not significantly speed up the IET kinetics in the [Fe(cCNC)2]2+-TiO2 assembly in comparison to the [Fe(ctpy)2]2+-TiO2 analogue. The high IET efficiency in the [Fe(cCNC)2]2+-TiO2 assemblies is therefore due to longer lifetime of [Fe(cCNC)2]2+ photoactive 3MLCT states rather than faster electron injection kinetics. It was also found that the inclusion of conformational sampling in the computational model is important for proper description of the IET processes in these systems, as the models relying on the use of only fully optimized structures may yield misleading results. The simulations presented in this work also illustrate various pitfalls of utilizing properties such as electronic coupling, number of available acceptor states, and driving force, as well as calculations based on Fermi's golden rule framework, to reach conclusions on the IET efficiency in dye-semiconductor systems.

Ultrafast Electron Injection from Photoexcited Perovskite CsPbI3 QDs into TiO2 Nanoparticles with Injection Efficiency near 99.

Photoexcited electron injection dynamics from CsPbI3 quantum dots (QDs) to wide gap metal oxides are studied by transient absorption spectroscopy. Experimental results show under a low excitation intensity that ∼99% of the photoexcited electrons in CsPbI3 QDs can be injected into TiO2 with a size-dependent rate ranging from 1.30 × 1010 to 2.10 × 1010 s-1, which is also ∼2.5 times faster than that in the case of ZnO. A demonstration QD-sensitized solar cell based on a CsPbI3/TiO2 electrode is fabricated that delivers a power conversion efficiency of 5%.

Linker Length-Dependent Electron-Injection Dynamics of Trimesitylporphyrins on SnO2 Films

Electron-injection dynamics of dye-sensitized photoelectrochemical cells depend on the length of the linker connecting the molecular photosensitizer to the metal oxide electron acceptor. However, systematic studies of the effect of chromophore–oxide distance are scarce. Here we present the synthesis, characterization, spectroscopy, and computational modeling of electron-injection dynamics from free-base trimesitylporphyrins with varying linker lengths into tin(IV) oxide (SnO2). In each system, the porphyrin core and metal oxide film remain the same while only the linker binding the porphyrin to the carboxylate anchor group is varied. A length range spanning 8.5–17.2 A is studied by employing phenylene, biphenylene, terphenylene, and benzanilide groups as the linker. We find a clear correlation between linker length and injection rates, providing insights that will be exploited in the optimization of dye-sensitized photoelectrochemical cells.

Ultrafast Electron Injection from the S₂ State of Carotenoids into TiO₂ Nanoparticles.

Dye molecules attached to TiO₂ nanoparticles have been of a great interest in the applications including dye-sensitized solar cells. In this paper, TiO₂ nanoparticle sensitized with a natural photosynthetic dye, 8′-apo-β-caroten-8′-oic acid (ACOA) was investigated by femtosecond transient absorption spectroscopy. Ultrafast excited state dynamics of ACOA and electron injection and recombination dynamics between dye molecules and TiO₂ nanoparticles were compared by changing the excitation wavelength between 403 and 480 nm. Ultrafast electron injection into TiO₂ nanoparticles with quantum yields of 0.33 was observed for both excitation wavelengths.

Enhanced light harvesting and charge recombination control with TiO2/PbCdS/CdS based quantum dot-sensitized solar cells

Charge recombination is considered as one of the most significant factors in hindering the photovoltaic performance of quantum dot-sensitized solar cells (QDSSCs). In addition, expand the light absorption range to the near-infrared region in QDSSCs increasing the generated photocurrent. A significant enhancement in the power conversion efficiency (PCE) of QDSSCs has been obtained by charge recombination control and enhanced light harvesting. In this respect, PbCdS layer is introduced between TiO2 and CdS QDs via a facile successive ionic layer absorption and reaction (SILAR) method in order to further improve the cell performance. For the first time a photoanode assembly composed of TiO2/PbCdS/CdS was prepared in QDSSCs. The photovoltaic parameters were significantly enhanced with the incorporation of a PbCdS intermediate layer between TiO2 and CdS QDs but varied appreciably with the SILAR cycles of PbCdS. When four SILAR cycle layer was applied, the PCE is as high as 3.35%, which is higher than the efficiency of 1.84% for the solar cell without PbCdS layer. The improved performance of the TiO2/PbCdS/CdS-based QDSSCs was attributed to the PbCdS layer can enhance the light harvesting to release more excitons. In addition, the PbCdS layer accelerates the electron injection kinetics and also functioning as a blocking layer to cover the TiO2 core from the outer QDs and redox couple, thereby reducing the recombination of electrons from the TiO2 with the electrolyte or with the QDs. Electrochemical impedance measurements has been measured and discussed in detail providing a detailed analysis of charge transport processes in QDSSCs.