Plants are well known to be far less efficient than are solar panels, even given their millions of years of evolution which usually gives biology an edge. The explanation is as given in the GP: its take a lot more energy thermodynamically to collect carbon out of the air and turn it into fuel than the energy you get from the resulting fuel. If there is an application where we absolutely need fuel as an intermediary, than by all means, but anywhere else we are shooting ourselves in the foot by not using the energy directly.