Gas phase electronic structure of the DTDCTB small-molecule donor for vacuum-processed organic photovoltaics compared to its constituent building blocks

This study provides a comprehensive analysis of the electronic structure of the small-molecule (SM) donor DTDCTB in terms of its main molecular components, DPTA and BTD, with donor and acceptor characters, respectively. The occupied electronic states of gas phase DTDCTB and the two building block molecules are probed using photoelectron (PE) spectroscopy, while the unoccupied electronic states are probed using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Jointly, density functional theory (DFT) calculations of the electronic structure and X-ray absorption spectra are used to assign the experimental peaks. We find that the frontier characteristic peaks of the gas phase building blocks can be identified in the spectra of the DTDCTB molecule. In particular, the highest occupied molecular orbital (HOMO) of DTDCTB, corresponding to the first peak in the outer photoelectron valence spectrum, is attributed mainly to the DPTA moiety (∼70%), with only a small contribution (∼30%) from BTD and from the dicyanovinylene (CN) terminal group. In contrast, the lowest unoccupied molecular orbital (LUMO), identified as the first spectral feature of the C and N K-edge NEXAFS spectra, originates almost entirely from the moieties with acceptor character, BTD and CN (∼81%), with a marginal contribution (∼19%) from DPTA. This study aims to elucidate how the electronic structure of DTDCTB, crucial for its technological functionalities in small-molecule organic photovoltaics (SMOPVs), is a result of the electronic characteristics of its constituent building blocks. The DTDCTB molecule precisely combines the electronic properties of its constituent donor and acceptor building blocks: the inclusion of thiophene in the donor moiety facilitates π-electron delocalization from the donor side to the acceptor side of DTDCTB, thus leading to the formation of mesomeric structures. Therefore, the D-A-A molecular architecture is confirmed to be a strategic solution to guarantee the efficient charge transfer among the two electron-donor and electron-withdrawing counterparts.