Why LED lights are more efficient?

There is a big stone. You want it to possess some energy. What you can do? 

You can push it, push it really hard to make it moving. It gains some speed. But not very high. Or you can put it by a cliff. Give it a gentle push. The stone will fall, fall faster and faster. When it reaches the bottom of the cliff, it will release a big impact.

For regular lights, electric voltage pushes electrons, pushes them really hard. Electrons gain some speed. But most of them are not very high, not high enough to generate bright light. Most of the energy is released in feeble invisible infrared light. 

Or you can set up a cliff with LED (Light Emitting Diode). An LED contains a PN junction, which is really an energy cliff, or more accurately an energy dam. Electric voltage gently pushes electrons through the PN junction, or the dam. Electrons fall hard through the dam. When they land at the bottom, they release a big impact in the form of high energy photons.  That is why LED lights are more efficient than regular lights.

Solar cells work in a similar way. They are also diodes containing PN junctions.