Small-molecule based organic devices continue to demonstrate the highest mobilities of any organic materials, making them the evident choice for better performing organic devices. Here we study a range of binary charge-transfer crystals where a fraction of electronic charge is transferred from the donor molecule to the acceptor molecule. Due to significant electron-phonon coupling, the vibrational motion in these materials has a strong impact on the electronic characteristics. To quantify this polaronic effect, we have completed the measurements of resonance Raman and absorption spectra of the charge-transfer excitations and performed quantum-chemical calculations on a range of binary organic crystals with the same acceptor and a range of donor molecules. Comparison of the reorganization energies of intermolecular vs intramolecular phonons in these materials helps us understand the relative contribution of the two major electron-phonon coupling mechanisms: local vs non-local. We find that small variations in the donor molecule and thereby the resultant crystal structure can have a large impact on the predominant electron-phonon coupling mechanism. Understanding the reasons for these variations is important in selecting and designing materials with suitable characteristics for the next generation of electronic devices.
First discovered at the beginning of the 20th century but still only partially understood today, organic semiconductors combine the electrical and optical properties typical of inorganic semiconductors with properties such as flexibility, low cost, and structural tunability via chemical modification. They are of significant interest due to their potential for optoelectronic applications such as displays, photosensors and solar cells. Crystalline organic charge-transfer compounds, combinations of two or more organic molecules in which one species acts as a donor of electric charge and the other as an acceptor, could provide new properties or improved performance to increase the range of application of organic semiconductors. Because of the hierarchy of bonding in these molecular crystals, the subtle interplay of electronic and vibrational states has far more influence on their properties than on those of covalent inorganic crystals. The further development of many applications of such compounds is limited by the lack of understanding of exciton dissociation and charge recombination processes and how these processes depend on the electronic and electron-vibration interactions. The charge-transfer states formed at the donor-acceptor interface play a key role, and both experimental and theoretical analyses depend on the arrangement of the donor and acceptor molecules at the nanoscale. By combining optical and transport measurements such as resonant Raman scattering, transient absorption and photocurrent with quantumchemical calculations it is possible to advance our understanding of the physics of these complex materials, paving the way for their application in 21st-century opto-electronic devices.