The hard part is not to make higher efficiency solar cells (within limits), the hard part seems to be to make them cheap enough so they're commercially viable.
For instance, these 35+ % efficient cells have been available for two years now:
But you wouldn't be putting them on your roof unless you wanted to achieve a specific number of Watts on a small surface.
The figure to monitor is $/Watt, and that's where any gains will come from. Any decrease in the price per Watt is real, immediate gain. Increases in efficiency are only good if they lead to a lower price per Watt.
In a typical installation, the cells aren't the major cost anymore. Because of that, efficiency does decrease $/Watt, even if the $/Watt price of the cells isn't lower.
For instance, these 35+ % efficient cells have been available for two years now:
http://gizmodo.com/5388795/sharp-triple-layer-solar-cell-set...
But you wouldn't be putting them on your roof unless you wanted to achieve a specific number of Watts on a small surface.
The figure to monitor is $/Watt, and that's where any gains will come from. Any decrease in the price per Watt is real, immediate gain. Increases in efficiency are only good if they lead to a lower price per Watt.