  A prototypical organic photovoltaic material is a heterojunction composed of
the blend of regioregular poly(3-hexylthiophene) (P3HT) and
[6,6]-phenyl-C$_{61}$-butyric acid methyl ester (PCBM). Microscopic
understanding of the energy conversion mechanism in this system involves the
relationship between the electronic structure and the atomistic geometry of
P3HT:PCBM interfaces. In this work, the effect of the number of P3HT layers on
the electronic structure of the P3HT:PCBM interface is studied by means of
first-principles $GW$. We apply the substrate screening approach to accelerate
such calculations and to better understand the many-body dielectric screening
at the interface. The quasiparticle band gap of the entire interface is found
to decrease as the number of P3HT layers increases. The gaps of the individual
components of the interface are found to be smaller than their isolated
counterparts, with strong dependence on the number of P3HT layers. Importantly,
when comparing the system of P3HT:PCBM - where a single interface is present -
and the system of P3HT:PCBM:P3HT, where an interface is formed on either side
of PCBM, we find that the two systems exhibit very different quasiparticle
energy level alignments. We discuss possible implications of our findings in
related experiments. The observed trends in layer-dependent quasiparticle
electronic structure of P3HT:PCBM interfaces provide computational insight into
energy conversion pathways in these materials.
