Device characteristics under dark and
solar cells are important candidates for sustainable, low-cost energy
generation. Frequently, the active layers of these devices are comprised of two
organic semiconductor constituents that are chemically incompatible. The
tendency for one of the constituents to preferentially segregate to the surface
of thin films results in compositional heterogeneities along the film depth.
This work explores the impact of vertical compositional heterogeneities on
charge transport in polymer solar cells via modular construction of devices. By
investigating a series of solar cells with identical device architectures, but
with interfacial segregations that are reversed in the active layers, Kahn and
Loo et al.  found that the
device performance of bulk-heterojunction polymer
solar cells is, in fact, independent of interfacial segregation of active
layers, contrary to a widespread assumption.
Acknowledgment: Funding from ONR
(N00014-08-1-1175), SOLAR Initiative (DMR-1035217) and MRSEC program
(DMR-0819860) at the NSF. A.K. also acknowledges support from DOE
(DEFG02-04ER46165 and DE-S0001084).