Charge Transfer Processes across Organic/Electrode Interfaces in Organic Photovoltaics

Abstract

Charge transfer efficiency at organic/electrode interfaces affects the performance organic photovoltaics (OPVs). It is hypothesized that the charge collection efficiency at organic/electrode interfaces is influenced by the structure of the first molecular layer adjacent to the electrode surface. Two differently oriented monolayers composed of two phosphonic acid (PA) functionalized zinc phthalocyanine (ZnPc) molecules, one with four PAs attached peripherally (ZnPcPA₄), one at each quadrant, and another that is functionalized with one PA (ZnPcPA), were tethered to indium tin oxide (ITO) surfaces as models for the donor/transparent conducting oxide (TCO) interface in OPVs to address the relationship between molecular orientation and electron transfer kinetics across the organic/ITO interface. The electron transfer rate constants across the monolayer/ITO interface were measured using potential modulated attenuated total reflectance (PM-ATR) spectroscopy in TE and TM polarizations, which allows rate constants to be determined for subpopulations of molecules that are oriented predominately in-plane and out-of-plane, respectively. The templating effect of the tethered monolayer on sequential layers of donor material for ZnPcPA and ZnPcPA₄ monolayers was assessed by depositing a few layers of CuPc on ZnPcPA- and ZnPcPA₄-modified ITO. The performance of OPV devices fabricated on ITO modified with these two monolayers was compared and correlated to their orientation and energy alignments with other layers in the devices. The dependence of kinetics of electron transfer on overpotential was studied using monolayer-tethered ZnPcPA on ITO. Measurements of apparent heterogeneous electron transfer rate constants (kapp) as a function of applied potential across the ZnPcPA/ITO interface were acquired using PM-ATR at various dc biases (Edc). With varying Edc imposed on the electrode, the fraction of adsorbed molecules in reduced or oxidized states is changed, and both the energetics and kinetics of the electrode reaction are affected. The results were discussed and compared to theoretical predictions. The photoinduced charge harvesting and photocurrent generation were assessed using a clicked ZnPc-perylene diimide (PDI) donor-acceptor assembly tethered to ITO via a PA anchoring group. This ZnPc-PDI donor-acceptor assembly was used to investigate the mechanism of photoinduced charge transfer and hole-capture by ITO in the presence of a solution phase redox mediator.