Chasing Charge Transfer in Organic Systems.

[摘要] Charge transfer processes are important in biology, chemistry, and physics. Redox reactions, photosynthesis, the Krebs cycle, and most solar energy harvesting systems depend on intermolecular charge transfer. Improved understanding of photoinduced charge transfer may help guide the development of promising technologies such as organic photovoltaics. This dissertation therefore focuses on understanding photoinduced charge transfer and charge separation in organic photovoltaics.Since charge transfer typically takes place on femtosecond to nanosecond timescales, it is difficult to study using electrical methods. However, a wide variety of ultrafast nonlinear optical spectroscopic methods have adequate time resolution. Recently, a generalized pump probe spectroscopy known as time resolved second harmonic generation (TRSHG) has enabled particularly direct measurements of charge transfer. I develop a new version of TRSHG spectroscopy using optical heterodyne detection to reduce sensitivity to stray light and read noise. Using the new TRSHG spectroscopy, I study charge transfer in two different organic photovoltaic systems. In the first study, I examine a boron subphthalocyanine chloride / C60 planar heterojunction, and compare the results to theoretical predictions of charge transfer using a theory based on Fermi’s golden rule (FGR). The comparison sheds light on the microscopic structure of the SubPc / C60 heterojunction. In the second study, I investigate how charge separation changes in a tetraphenyldibenzoperiflanthene / C70 bulk heterojunction as a function of concentration ratio between the two molecules. I show that larger concentrations of C70 help charges separate farther on a sub-nanosecond timescale, before the steady state mobility dominates.All nonlinear spectroscopic methods rely on short pulses of light to achieve the high time resolution necessary to observe ultrafast charge transfer. An ultrafast pulse measurement method is necessary to characterize and optimize the time resolution of nonlinear spectroscopies. I present two new methods for measuring ultrafast pulses: a phase cycled variant of frequency resolved optical gating (FROG) and a method named spectral phase of electric field by analytic reconstruction (SPEAR). I also perform an extensive comparison of pulse measurement methods using a pulse shaper, and identify a few top performers.

[发布机构] University of Michigan

[效力级别] charge transfer

[关键词] organic photovoltaics;charge transfer;charge separation;ultrafast spectroscopy;ultrafast pulse measurement;Physics;Science;Physics