A key limitation of Organic photovoltaic devices (OPVs) is their low open circuit voltage (VOC), which is typically &gt; 1.0 eV below the energy of the light absorbed. Using an SBCT chromophore in an OPV minimizes the energy offset needed at the donor/acceptor interface to drive charge transfer and charge separation, such that he rate of charge separation markedly exceeds the rate of back electron transfer (from A- to D+ at the interface). Incorporating the SBCT process into an OPV pushes the VOC close to the theoretical limits. These materials are symmetric molecules that spontaneously form an intramolecular charge transfer complex, with nearly complete one electron transfer from one part of the molecule to another. We have found several dimeric compounds, based on highly absorbing dipyrrin and other dyes. Excitation of these dye dimers initially gives an excited state localized on a single dye, which transitions to an inter-dye charge separated state (the SBCT state) on the ps time scale, and SBCT state lives for 0.5-2 ns. The SBCT state is lower in energy than the dye localized excited (LE) state in polar media and higher than LE state in nonpolar media, allowing us to use solvent choice to tune the nature of the excited state for our photophysical studies. We have investigated both meso bridged dyes and dyes coupled through Zn2+. The kinetics of formation of the SBCT process have been characterized by ultrafast spectroscopic techniques, both in solution and in the solid state. I will focus on two studies in my talk. The first is a study where we used solvent polarity to “tune” the energy of the SBCT state so that the energy difference between the LE and SBCT states is ca. 0, achieving charge transfer with no driving force. The second is a study were we used polarization anisotropy transient absorption measurements to determine whether it is the hole or electron that leaves the LE state, i.e. is it D*-D → D+-D- or D*-D → D--D+. We see some molecules where both of the pathways are equally likely and others where a specific carrier moves in the formation of the SBCT state.

Symmetry breaking charge transfer in BODIPY, dipyridylmethene and dipyrrin dimers

ACS Spring Meeting 2021

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