First-principles investigation of organic photovoltaic materials C[subscript 60], C[subscript 70], [C[subscript 60]]PCBM, and bis-[C[subscript 60]]PCBM using a many-body G[subscript 0]W[subscript 0]-Lanczos approach

Abstract

We present a first-principles investigation of the excited-state properties of electron acceptors in organic photovoltaics including C[subscript 60], C[subscript 70], [6,6]-phenyl-C[subscript 61]-butyric-acid-methyl-ester ([C[subscript 60]]PCBM), and bis-[C[subscript 60]]PCBM using many-body perturbation theory within the Hedin's G[subscript 0]W[subscript 0] approximation and an efficient Lanczos approach. Calculated vertical ionization potentials (VIP) and vertical electron affinities (VEA) of C[subscript 60] and C[subscript 70] agree very well with experimental values measured in the gas phase. The density of states of all three molecules is also compared to photoemission and inverse photoemission spectra measured on thin films, and they exhibit a close agreement—a rigid energy-gap renormalization owing to intermolecular interactions in the thin films. In addition, it is shown that the low-lying unoccupied states of [C[subscript 60]]PCBM are all derived from the highest-occupied molecular orbitals and the lowest-unoccupied molecular orbitals of fullerene C[subscript 60]. The functional side group in [C[subscript 60]]PCBM introduces a slight electron transfer to the fullerene cage, resulting in small decreases of both VIP and VEA. This small change of VEA provides a solid justification for the increase of open-circuit voltage when replacing fullerene C[subscript 60] with [C[subscript 60]]PCBM as the electron acceptor in bulk heterojunction polymer solar cells.