Spectroscopic and Spectroelectrochemical Characterization of Fundamental Interfacial Charge Transfer Processes Relevant to Efficient Solar Energy Conversion

Abstract

Solar energy conversion is accomplished by multilayered devices consisting of various conducting and semiconducting materials. Because the layers are only 10s - 100s of nm thick, device behavior is governed primarily by interfacial molecular dynamics that often differ from the bulk behavior of these materials. The thermodynamics and kinetics of the interfacial interactions are particularly interesting, as interfacial electron transfer strongly influence the efficiency of photovoltaics and devices used in solar hydrogen production. This work focuses specifically on interfacial charge transfer processes occurring at three interfaces relevant to thin film organic/inorganic solar energy conversion devices. i) A potential-step polymer electrochemical deposition and doping procedure was developed and used to create poly(3-hexylthiophene) (e-P3HT) interlayer films for organic photovoltaics. Photoelectron spectroscopies suggest that an interface dipole forms spontaneously at the polymer donor/fullerene acceptor interface through partial interfacial charge transfer prior to photoexcitation; this doping-dependent interfacial dipole was correlated to the electrical properties of these critical heterojunctions. ii) Potential-modulated fluorescence spectroscopy (PMF) was developed and used examine the kinetics of the reversible oxidation of the (e-P3HT) films in attempt to elucidate the ITO/e-P3HT charge transfer rates. However, the optical switching increased linearly as the polymer film decreased, indicating that the molecular-level process probed by PMF was rate-limited by counter-ion movement into and out of the polymer film. iii) Potential-modulated attenuated total reflectance spectroscopy (PM-ATR) was used to examine the reversible reduction of CdSe semiconductor nanocrystals tethered to indium tin oxide electrodes as well as the surface-coverage dependent bleaching of these nanocrystals. A new equivalent circuit model describing the CdSe/ITO electrode is proposed, and a PM-ATR simulation program was used to quantify Faradiac resistances to interfacial charge transfer that trend with the magnitude of overpotential. The insights gained through these experiments add to a growing understanding of the fundamental, molecular-level competition between photoinduced charge generation and parasitic charge recombination at these critical interfaces.