We use Electronic Sum Frequency Generation Spectroscopy (ESFG) to study electronic structures of organic semiconductor at a buried solid/solid interface for the first time. We demonstrate that ESFG can be a robust method to study band structures at the buried interface at atmosphere, which is complementary to the existing photoemission techniques, which can determine the interfacial electronic structure in ultra-high vacuum conditions.<br/>We focus on studying the spin-coated poly (3-hexylthiophene-2,5-diyl) (regioregular-P3HT) and silicon interfaces. The resonance ESFG signal is generated at the P3HT/Si surface only when the sum frequency of the two input pulses equals to the bandgap transition of P3HT at the interfaces. Therefore, we are able to get rich information of band gap of the P3HT at interface. We find there is a single ESFG peak at 2.2 eV, with a narrowed Lorentzian bandwidth, which indicates that the P3HT at interfaces for a uniform conformation and has bandgap close to 2.2 eV. This is in sharp contrast to the bulk P3HT thin film, which based on its UV-Vis spectrum, has multiple conformations with various electronic transitions and broad inhomogeneous distributions. The uniform P3HT conformation at the interface can delocalizes excitons and therefore critically facilitates faster charge separations. Our investigation indicates that at the interfaces both electronic and molecular structure of the organic semiconductors can be drastically different from the bulk, even when thin films are used. Therefore, true interface sensitive technique is needed to investigate the electronic structure of buried interfaces at solid/solid interfaces.

Probing band gaps of organic semiconductors at buried interfaces by electronic sum frequency generation spectroscopy

251st National Meeting (2016)

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