  Energy dissipation and decoherence are at first glance harmful to acquiring
long exciton lifetime desired for efficient photovoltaics. In the presence of
both optically forbidden (namely, dark) and allowed (bright) excitons, however,
they can be instrumental as suggested in photosynthesis. By simulating quantum
dynamics of exciton relaxations, we show that the optimized decoherence that
imposes a quantum-to-classical crossover with the dissipation realizes a
dramatically longer lifetime. In an example of carbon nanotube, the exciton
lifetime increases by nearly two orders of magnitude when the crossover
triggers stable high population in the dark exciton.
