Aggregation of a Dibenzo[<i>b</i>,<i>def</i>]chrysene Based Organic Photovoltaic Material in Solution
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Abstract
Detailed
electrochemical studies have been undertaken on molecular
aggregation of the organic semiconductor 7,14-bis((triisopropylsilyl)-ethynyl)dibenzo[<i>b</i>,<i>def</i>]chrysene (TIPS-DBC), which is used
as an electron donor material in organic solar cells. Intermolecular
association of neutral TIPS-DBC molecules was established by using <sup>1</sup>H NMR spectroscopy as well as by the pronounced dependence
of the color of TIPS-DBC solutions on concentration. Diffusion limited
current data provided by near steady-state voltammetry also reveal
aggregation. Furthermore, variation of concentration produces large
changes in shapes of transient DC and Fourier transformed AC (FTAC)
voltammograms for oxidation of TIPS-DBC in dichloromethane. Subtle
effects of molecular aggregation on the reduction of TIPS-DBC are
also revealed by the highly sensitive FTAC voltammetric method. Simulations
of FTAC voltammetric data provide estimates of the kinetic and thermodynamic
parameters associated with oxidation and reduction of TIPS-DBC. Significantly,
aggregation of TIPS-DBC facilitates both one-electron oxidation and
reduction by shifting the reversible potentials to less and more positive
values, respectively. EPR spectroscopy is used to establish the identity
of one-electron oxidized and reduced forms of TIPS-DBC. Implications
of molecular aggregation on the HOMO energy level in solution are
considered with respect to efficiency of organic photovoltaic devices
utilizing TIPS-DBC as an electron donor material