Influence of Molecular Orientation on Charge-Transfer Processes at Phthalocyanine/Metal Oxide Interfaces and Relationship to Organic Photovoltaic Performance

Abstract

The effect of the molecular orientation distribution of the first monolayer of donor molecules at the hole-harvesting contact in an organic photovoltaic (OPV) on device efficiency was investigated. Two zinc phthalocyanine (ZnPc) phosphonic acids (PA) deposited on indium tin oxide (ITO) electrodes are compared: ZnPc­(PA)4 contains PA linkers in all four quadrants, and ZnPcPA contains a PA linker in one quadrant. ZnPcPA monolayers exhibited a broad distribution of molecular orientations whereas ZnPc­(PA)4 adsorption produced a monolayer with a narrower orientation distribution with the molecular plane more parallel to the ITO surface. We used potential-modulated attenuated total reflectance spectroelectrochemistry (PM-ATR) to characterize the charge-transfer kinetics of these films and show that the highest rate constants correspond to ZnPc subpopulations that are oriented more parallel to the ITO surface plane. For ZnPc­(PA)4, rate constants exceeded 104 s–1 and are among the highest ever reported for a surface-confined redox couple, which is attributable to both its orientation and the small ZnPc–electrode separation distance. The performance of OPVs with ITO hole-harvesting contacts modified with ZnPc­(PA)4 was comparable to that achieved with highly activated bare ITO contacts, whereas for ZnPcPA-modified contacts, the OPV performance was similar to that observed with (hole-blocking) alkyl-PA modifiers. These results demonstrate the synergism between molecular structure, energetics, and dynamics at interfaces in OPVs.

Characterization of Charge-Transfer Kinetics at Organic/Electrode Interfaces Using Potential-modulated Attenuated Total Reflectance (PM-ATR) Spectroscopy

“…24 Introducing a redox-active moiety into the surface modifier offers the possibility of enhancing the charge-collection efficiency when the redox potential is matched to the relevant charge-transport energy levels. 14,15,28 The following sections describe recent studies of redox-active PA modifiers on ITO, focusing on relationships between monolayer structure and charge-transfer kinetics.…”

“…2,4 Combining potential-modulation with the ATR geometry enables the measurement of charge-transfer kinetics of monolayer to submonolayer thin films on waveguide electrodes, with selectivity to molecular subpopulations in the film provided by appropriate choice of the applied potential and the wavelength and polarization of light, as described further below. This method has been applied to study a wide variety of electrode-supported materials, including small molecule and polymeric semiconductors, [13][14][15][16][17] proteins, 18 inorganic complexes, 19 and semiconductor nanocrystals.…”

Influence of Molecular Aggregation on Electron Transfer at the Perylene Diimide/Indium-Tin Oxide Interface

“…15,16,32 ATR spectra acquired as a function of potential applied to the ITO slide (the working electrode) were measured using TM polarized light following procedures described previously. 27 …”

“…61 However, if direct through-space tunneling occurs, r depends on θ and thus differences in θ could affect ET kinetics. 15,16,27 To assess this possibility, r values were estimated for θ values of 33° and 42° which are the respective mean tilt angles for alkyl-PDI-PA and (terphenyl) 2 -PDI-IP-PA films (Table 2). Standard ET rate constants ( !,! )…”

“…ZnPcPA-modified cells outperformed cells prepared using ITO modified with redox-inactive alkyl phosphonic acids. 15 Song et al 4 showed that diarylamine chlorosilane (Ar 2 N-(CH 2 ) n -SiCl 3 , Ar = 3,4-difluorophenyl) redox-active modifiers can serve as a hole-selective interfacial layer on an ITO electrode in bulk-heterojunction OPVs; the PCE of these devices was slightly higher than that of devices in which PEDOT:PSS was utilized as the interfacial layer. 4 These and other papers [15][16][17][18][19][20][21] have demonstrated that the molecular design of a redox-active modifier, such as the type, position and length of its bridging moiety and the presence of solubilizing groups or bulky substituents, may significantly affect the charge-collection kinetics and efficiency at modified TCO surfaces, because these structural parameters dictate properties such as molecular orientation, tunneling distance, degree of aggregation, and/or interface dipole magnitude and orientation.…”