  Carbon materials are excellent candidates for photovoltaic solar cells: they
are Earth-abundant, possess high optical absorption, and superior thermal and
photostability. Here we report on solar cells with active layers made solely of
carbon nanomaterials that present the same advantages of conjugated
polymer-based solar cells - namely solution processable, potentially flexible,
and chemically tunable - but with significantly increased photostability and
the possibility to revert photodegradation. The device active layer composition
is optimized using ab-initio density functional theory calculations to predict
type-II band alignment and Schottky barrier formation. The best device
fabricated is composed of PC70BM fullerene, semiconducting single-walled carbon
nanotubes and reduced graphene oxide. It achieves a power conversion efficiency
of 1.3% - a record for solar cells based on carbon as the active material - and
shows significantly improved lifetime than a polymer-based device. We calculate
efficiency limits of up to 13% for the devices fabricated in this work,
comparable to those predicted for polymer solar cells. There is great promise
for improving carbon-based solar cells considering the novelty of this type of
device, the superior photostability, and the availability of a large number of
carbon materials with yet untapped potential for photovoltaics. Our results
indicate a new strategy for efficient carbon-based, solution-processable, thin
film, photostable solar cells.
