Charge hopping mechanism

Once the electrons and holes have been injected, they migrate into ETL and HTL to form excited states referred to as polarons by physicists or radical ions by chemists. These polarons or radical ions move, by means of a so-called charge-hopping mechanism, through the electron and hole transport materials (ETMs and HTMs), which typically possess good charge mobility properties, and eventually into the EML. 

Figure 41. Two charge-hopping mechanisms. The donor injects an electron (or hole) into the bridge which consists of discrete redox units, (a) The bridge units are nearly degenerate. Consequently, the injected electron (or hole) moves randomly and reversibly up and down the bridge, finally becoming irreversibly trapped by the acceptor, (b) The bridge units constitute an ordered redox cascade the electron or hole moves essentially irreversibly along the bridge towards the acceptor.

A typical example of the charge hopping mechanism is shown by the tris(2,2 -bipyridine) ruthenium(II) complex 3 (Ru(bpy)3 ) incorporated into a Nafion membrane on its oxidation from a 2+ to a 3+ complex. The visible absorption spectral change of the Ru(bpy)3 /Nafion after potential steps from 0 to 1.3 V vs. SCE (Fig. 14-5) shows a decrease of the 2+ complex (453 nm) with a concomitant increase of the absorption by the 3+ complex (around 420 nm), whose rate was second order with respect to the concentration [40]. The second-order rate constant for the charge hopping was 2 = 1 1T0 moT s" (see Experiment 14-1, Section 14.5). 

Fig. 2 Illustration for charge transport (CT) in a polymer film by a physical displacement and b charge hopping mechanisms (Reprinted with permission from American Chemical Society [33])...

CT in methylviologen (MV " )/Nafion was studied using a potential-step chronoamperospectrometry (PSCAS) technique, and the results were compared with those in a poly(styrene)-pendant system. The CT by reduction of to MV " was foimd to occur by physical displacement of the center molecule with kp = 1.4 x 10 s [37]. By contrast, CT took place by a charge hopping mechanism in the poly(styrene)-pendant system. The redox couple of and MV + is considered to diffuse in a hydrophilic ion channel of Nafion, but in a poly(styrene)-pendant system, the diffusion should be difficult in the film by the covalent attachment of the center to the polymer matrix, which would result in a charge hopping mechanism [37]. 

A [Ru(bpy)3] (bpy = 2,2 -bipyridine)/Nafion film can be prepared by two methods (1) the complex is adsorbed from its aqueous solution into a Nafion film precoated on an electrode (adsorption method) or (2) an alcoholic mixture solution of Nafion and the complex is cast and air-dried on an electrode (mixtine casting method). CT by oxidation of Ru to Ru in the film prepared by a different method was compared [28]. CT was found to take place by a charge hopping mechanism for the films prepared by both the ad-... 

Henderson P T, Jones D, Hampikian G, Kan Y Z and Schuster G B 1999 Long-distance charge transport in dupiex DNA the phonon-assisted poiaron-iike hopping mechanism Proc. Natl Acad. Sc/., USA 96 8353-8... 

Electron-hopping is the main charge-transport mechanism in ECHB materials. There is precedence in the photoconductivity Held for improved charge transport by incorporating a number of redox sites into the same molecule. A number of attempts to adapt this approach for ECHB materials have been documented. Many use the oxadiazole core as the electron-transport moiety and examples include radialene 40 and dendrimer 41. However, these newer systems do not offer significant improvements in electron injection over the parent PBD. 

The localization of the HOMO is also important for another reason. Since it describes the distribution of a hole in a radical cation, it relates to the hindrance that a positive charge will encounter as it propagates along the chain. There is indeed experimental evidence (9) that the hole states of the polysilane chain are localized and that they move by a hopping mechanism. 

Keywords Charge injection Charge transfer Hopping mechanism Mismatches... 

The activated hopping mechanism that also involves adenines as charge carriers is validated by experiments in which we directly injected the positive charge into an adenine [19]. We have shown that photolysis of injection system 18 oxidizes the adenine base, maybe by through-bond charge transfer (19—>20). Therefore, incorporation of 18 into DNA duplexes makes it possible to inject the hole directly into an adenine base in the middle of an... 

Fig. 14 Reaction profile diagram for the hole transfer from a guanine radical cation (G +) to a distant GGG sequence via the activated hopping mechanism, which also involves adenines (A) as charge carriers...

Tran E, Cohen A, Murray R, Rampi MA, Whitesides GM (2009) Redox site-mediated charge transport in a Hg-SAM//Ru(NH3)63+/2+//SAM-Hg junction with a dynamic interelectrode separation compatibility with redox cycling and electron hopping mechanisms. J Am Chem Soc 131 2141-2150... 

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