Band structure engineering in organic semiconductors

A key breakthrough in modern electronics was the introduction of band structure engineering, the design of almost arbitrary electronic potential structures by alloying different semiconductors to continuously tune the band gap and band-edge energies. Implementation of this approach in organic semiconductors has been hindered by strong localization of the electronic states in these materials. We show that the influence of so far largely ignored long-range Coulomb interactions provides a workaround. Photoelectron spectroscopy confirms that the ionization energies of crystalline organic semiconductors can be continuously tuned over a wide range by blending them with their halogenated derivatives. Correspondingly, the photovoltaic gap and open-circuit voltage of organic solar cells can be continuously tuned by the blending ratio of these donors. Electrical engineers can finetune the energetics of rigid photovoltaics and transistors by blending different semiconducting materials. However, it's hard to apply this tuning protocol to the flexible class of carbon-based semiconductors. Schwarze et al. now show that continuous band energy tuning is indeed possible by varying the blend ratios of certain organic phthalocyanines and their fluorinated or chlorinated derivatives (see the Perspective by Ueno). They demonstrated the effect, which they attribute to quadrupolar interactions, in model solar cells