Novel solution processable n-type materials for organic photovoltaics

While PCBM is the reigning champion for soluble n-types in bulk-heterojunction solar cells, it is possible that we have approached the upper efficiency limit of the P3HT/PCBM blend after years of research on the same system. Alternatives to PCBM could yield improved device performance or stability as soluble n-type acceptors must be explored in order to move forward with OPV research. Previously, we have shown that soluble small molecules such as triisopropylsilylethynyl (TIPS) pentacene can be synthetically altered to perform as n-type materials by adding electron-withdrawing nitrile functional groups to improve electron-accepting properties and lower its HOMO-LUMO levels. Here, we report further investigations into electron-deficient pentacenes for photovoltaic applications, along with a significant increase in previously reported efficiencies for bulk-heterojunction OPVs fabricated from blends of P3HT:cyano-pentacenes from 0.3% to over 1%. We have also observed that cyano-pentacenes that stack in a 1D "sandwich herringbone" arrangement systematically outperform similar molecules that stack in 2D with otherwise analogous energy levels. We have also begun to apply our crystal engineering methods to pentacene-fullerene adducts, showing that by strategic functionalization of the acene we can both improve the open-circuit voltage of polymer:acenofullerene solar cells, as well as tune crystal packing to maximize device current. Furthermore, a series of novel functionalized pentacene-C60 adducts have been synthesized with preliminary device data showing modest photovoltaic efficiencies.