Organic photovoltaics are being explored as the next generation material for semiconducting and optoelectronic devices but are limited by their stability and efficiency. One new candidate is crystallized Anthradithiophene (ADT), which is a promising photo-voltaic material due to its high quantum yield and adjustable side groups. The side derivatives TES-F, TDMS-F, and TSBS-F, change the distribution of excited state electrons, affecting charge transfer in the crystal system without changing of the molecule's optoelectronic properties. Previous experiments have observed the absorption, reflection, and transmission spectra of these derivatives in solution with respect to the polarization of incident light but the electronic transitions at specific emission bands have not been explored. Additionally, we discover in ADT variants different H-aggregate and J-aggregate dipole orientations.
An analysis of these side groups is done by measuring a photoluminescence excitation spectrum of ADT and the spectra is compared against the absorption as a function of wavelength. By varying the wavelength of incident light on the sample using a monochromator system, an EMMCD camera images the sample as it is being excited. The response is recorded as a function of the wavelength of incident light and single domain regions are analyzed. By analyzing multiple emission peaks and isolating these bands, we compare the excitation spectra across different bands to the absorption spectra. This confirms non-unique transition behavior across each derivative and determine that each PLE spectra fits to the absorption spectrum through a scalar constant.
From the polarization of light on the sample, we find a tunable response that determines an H-like and J-like response from the system, isolating a specific angle that maximizes the coupling from the H or J-aggregate dipole orientation. TES-F appears maximally J-like at 45 degrees and maximally H-like at 117 degrees. TDMS-F has a maximum J-like response at 90 degrees and a maximal H-like response at 45 degrees. TSBS-F exhibits J like behavior across all polarization measurements, with 90 degrees being the strongest.