Why can't solar panels produce 1 Kw per 1 square meter? This is the energy of the Sun's radiation per square meter on Earth but solar panels don't come close. Why can't we trap all that energy? Where is the rest of the energy going?
The Wikpedia article on solar cell efficiency gives a number of reasons that solar cells are less than 100% efficient. One of the large ones is the thermodynamic limit-a photon of less energy (longer wavelength) than the silicon band gap cannot produce an electron and one with higher energy can only produce as much voltage as the band gap. Even if you could choose the band gap, this limits efficiency to 34%. There are many more, smaller, factors that reduce the overall panel efficiency to around 20%.
No energy conversion process completes at 100% efficiency. Using the sun, gasoline or nuclear, none of these will generate power at 100% of total potential power available.
That figure of 1KW/m^2 is for total available light energy correct? Due to how solar panels work (I don't think I have enough knowledge to speak on this part directly) solar panels are tuned to specific wavelengths to maximize performance. They simply cannot absorb efficiently over the entire solar spectrum. The wavelengths outside of the targeted band are either reflected or absorbed as heat. Furthermore, some energy is lost due to internal resistance. Something that compounds this issue is that solar panels lose efficiency as they heat up, so it is very important to keep them relatively cool.
There are other factors such as how they behave under actual load and how things such as MPPT's come into play but I think I've covered the meat of the question.