Efficiency is defined as (useful output)/input and it is machine dependent factor and can’t be increased for any particular panel.
If you direct sunlight from some giant mirror, first of all, no mirror reflects 100% energy, so energy will be lost while reflection , so we would get much less energy than from direct sunlight.
Moreover, we may increase the output per day, but efficiency would be the same.
It won’t make the device work any better, but just work through out the day producing more electricity.
It won’t be worth the effort at all.The above is just a theory.There will be a lot of scattering, absorption and uneven transmitting of radiations to different parts of the earth.
Moreover, we can’t get such a gigantic mirror halted somewhere in the sky that could cover the entire dark side of the earth.
Plants do photosynthesis during day time and give out oxygen.At night they intake oxygen for their survival too.
In the above case, they’ll be producing oxygen all the time and may not survive for long.
Present technology is no so obsessed about solar power.So, I believe though, we might be able to make it possible, it would be a really lame idea just to use some much of resource to build a mirror which would do more bad than good.