Probing Electronic Structures of Organic Semiconductors at Buried Interfaces by Electronic Sum Frequency Generation Spectroscopy

We use Electronic Sum Frequency Generation Spectroscopy (ESFG) to study the electronic structures at a buried solid/solid interface for the first time. The system is an organic thin film, poly(3-hexylthiophene-2,5-diyl) (P3HT), supported on a silicon surface. The ESFG measurement is only in resonance with electronic (or vibronic) excitations, thus capable of yielding rich information on the band gap and electronic structures of the P3HT film at interfaces. We find the bandgap of P3HT in contact with silicon is 2.2 eV, with a narrowed bandwidth and Lorentzian line shape. This is significantly distinct from the UV–vis spectra of bulk P3HT, which contains multiple broad Gaussian peaks. Our measurement demonstrates at interfaces regioregular P3HT has a uniform electronic structure, which could improve the short circuit currents. The unique capability of ESFG to probe electronic structures at buried interface under atmosphere will be useful for investigating many buried interfaces.