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Basic Electronic Structures and Charge Carrier Generation in Organic Optoelectronic Materials
Published in Sam-Shajing Sun, Larry R. Dalton, Introduction to Organic Electronic and Optoelectronic Materials and Devices, 2016
The photoinduced charge separation due to the presence of a donor (D)/acceptor (A1) pair (a weak D/A pair with weak coupling [WC]) is also called photodoping (or photoelectric) process as schematically illustrated in Figures 3.27a, 3.28a, 3.29, and 3.33a. Specifically, in the photodoping process, the A1-LUMO is close and slightly lower than the D-LUMO, and the free energy difference (ΔE1) for the D-LUMO → A1-LUMO electron transfer could be optimum (e.g., ΔE1 = λ1 as exhibited in Figures 3.28a and 3.29) so that such transition can be much faster than the donor exciton decay (i.e., electron transfer from D-LUMO to D-HOMO) [16]. Note if the D-HOMO/A-LUMO coupling is stronger than D-HOMO/D-LUMO (such as in the thermal or chemical doping cases shown in Figures 3.28b, c, 3.30 through 3.32, and 3.33c), then photo (or thermal)-driven electron transfer from D-HOMO to A-LUMO may proceed directly to form an intermolecular uncorrelated or weakly correlated charge pair. This is a strong donor/acceptor pair with WC case as exhibited in Figure 3.33c and is the predominant charge carrier generation mechanism in chemical or thermal doping processes.
Optimization of Organic Solar Cells in Both Space and Energy–Time Domains
Published in Sun Sam-Shajing, Sariciftci Niyazi Serdar, Organic Photovoltaics, 2017
Sam-Shajing Sun, Carl E. Bonner
For a donor–acceptor pair where both can harvest light, the exciton quenching parameter for the pair can be expressed as: Yeq(D+A)=YeqDYeqA using ∂Yeq(D+A)/∂δE=0, this gives δEeq(D+A)=[(ED−EA)/λsA−2]/(1/λsD+1/λSA)−EB=−0.55eV The exciton quenching parameters Yeq(A), Yeq(D), and Yeq(D+A) versus the LUMO offset are plotted in Figure 8.22. As shown in Figure 8.22, Yeq(D+A) represents an overlap area where both donor and acceptor would harvest light efficiently, and the optimum offset is around −0.55 eV. The actual RO-PPV/SF-PPV-I LUMO offset of −0.9 eV appears a little larger than this optimum. Further improvement of photoinduced charge separation can be achieved via either reducing the LUMO level of RO-PPV, or increasing the LUMO level of SF-PPV-I via molecular engineering.
Two-Dimensional Photocatalytic Heterojunction Hybrid Nanomaterials for Environmental Applications
Published in A. Pandikumar, K. Jothivenkatachalam, S. Moscow, Heterojunction Photocatalytic Materials, 2022
R. Baby Suneetha, Suguna Perumal, P. Karpagavinayagam, C. Vedhi
Generally, the configuration of heterojunction photocatalysts can be classified as type I, type II, Z-scheme system, and Schottky junction system [29, 30], each providing different charge transfer processes. In type I, both electrons and holes will accumulate in one of the semiconductors whose VB and CB are within the bandgap of the other semiconductor.In type II heterostructures containing both the components with staggered energy band alignment, during the photocatalytic process, the CB electrons of one semiconductor would transfer to the CB of the other semiconductor, while the VB holes would transfer in the opposite direction between them.In the Z-scheme system, the band alignment of components is similar to type II heterojunction, but the migration of electrons is different. During the photocatalytic process, the electrons in the CB of one semiconductor can transfer to the VB of the other semiconductor to combine with holes, leading to the effective photoinduced charge separation.The Schottky junction system is obtained by coupling cocatalyst to semiconductor. In this system, photogenerated electrons can flow from the semiconductor to the cocatalyst through the interface, which is driven by the potential difference of Fermi levels in the two materials. Thus, the semiconductor possesses positive charges, while the cocatalyst has excess negative charges near the interface. This fact would lead to the formation of a space charge layer at the interface that induces a Schottky junction between the semiconductor and the cocatalyst. Consequently, the formed Schottky junction can act as an electron trap to efficiently capture photoinduced electrons, leading to the enhanced photocatalytic activity.Recently, a new step-scheme (S-scheme) heterojunction concept has been proposed based on Z-scheme photocatalysts [41–46]. Here two n-type semiconductor photocatalysts are coupled together to form the S-scheme heterojunction, in which one acts as an oxidation photocatalyst and the other as a reduction photocatalyst. Owing to the internal electric field created in S-scheme heterojunction, the excited electrons in the CB of oxidation photocatalysts will easily recombine with holes in the VB of the reduction photocatalysts. Thus, the strongly oxidative holes in the VB of oxidation photocatalysts and the strongly reductive electrons in the CB of reduction photocatalysts will be spatially separated. This S-scheme can not only prevent the recombination of photogenerated charges but also enhance the reducing capacity and oxidizing ability of the heterojunction in the photocatalysis process.
Synthesis and spectroscopic properties of osmium based polypyridyl compound, cis-OsII(phen)2Cl2, and its one-electron oxidation product [cis-OsIII(phen)2Cl2](PF6)
Published in Journal of Coordination Chemistry, 2022
Transition metal complexes based on polypyridyl ligands are the paragon of photosensitizers, with numerous reports available on their most famous representative [RuII(bpy)3]2+ (bpy = 2,2′-bipyridine) [1, 2]. These complexes show favorable properties, such as well-behaved redox steps, long-lived triplet metal-to-ligand charge transfer (3MLCT) states, as well as electrochemiluminescence [3–5]. Some of the aforementioned properties make the application of this compound as a photoredox catalyst possible [6–8]. Osmium-based complexes have received attention due to good photostability, stable coordination structure and faster electron transfer [9]. Luminescent and redox-active dendrimers have been the center of much attention since they can exhibit a combination of light absorption, intercomponent energy and/or electron transfer [10, 11], and excited-state luminescence making them capable to behave as light harvesting antennae and as multicomponent (supra)molecular systems for photoinduced charge separation [12–14]. Among dendrimers made of metal centers, those based on Os(II) polypyridine building blocks show potential in this regard because of their outstanding photophysical and redox properties which arise mainly due to the metal-to-ligand charge transfer (MLCT) excited states as well as for favorable thermodynamic and kinetic stabilities at physiological pH [15].
Enhanced optical properties and photocatalytic activity of TiO2 nanotubes by using magnetic activated carbon: evaluating photocatalytic reduction of Cr(VI)
Published in Environmental Technology, 2021
Seyed Ghorban Hosseini, Javad Vahabzadeh Pasikhani
To investigate the photoinduced charge separation, electron migration, and e−–h+ pairs recombination, the photoluminescence (PL) analysis was performed which the results are illustrated in Figure 6. Actually, higher PL intensity implies the higher recombination rate of e−–h+ pairs. In contrast, lower PL intensity means the lower recombination rate, and more electrons can migrate and participate in the photocatalytic reduction process. Based on Figure 6, the major peak of PL emission spectrum at around 361 and 390 nm are attributed to the band to band transition. At longer wavelengths, PL emission spectrum at 445 and 481 nm are assigned to the surface state emission [22]. According to Figure 6, TNTs/AC and all TNTs/MAC samples indicated lower PL intensity as compared to the TNTs. This result reveals that the addition of AC and MAC can inhibit the recombination of photogenerated charge carriers due to trapping of the photo-excited electron by AC as well as the formation of a Schottky barrier at the interface of Fe and TNTs. Based on Figure 6, among all photocatalysts, TNTs/MAC (2%) exhibited the lowest PL intensity. Therefore, it is expected that TNTs/MAC (2%) photocatalysts can contribute to their higher photoreduction activity due to the fact that they have the lowest recombination rate of photoinduced charge carriers.