NASA is making development of high-thrust solar electric propulsion (SEP) a priority for manned beyond-earth-orbit missions: Solar-electric Advancing For Deep-space Propulsion - Aviation Week.
The SEP wouldn't propel the crew vehicles but would move various support systems: A SEP system weighs a lot less than chemical propulsion. Given enough time it can move a lot of mass through space. That in-space advantage makes it particularly attractive for pre-positioning cargo—supplies, habitats and the like—to keep human explorers alive after they arrive on a faster vehicle to explore a distant location.
Some additional info:
/-- NASA Selects Companies To Study Solar Electric Propulsion Spacecraft - NASA - Nov.13.11
/-- NASA Seeks Game Changing Solar Array Systems Proposals - NASA - April.26.12
/-- Solar Electric Propulsion Could Slash the Price of Manned Missions - AmericaSpace - Aug.1.12