Modern solar panel technology is pretty damned awesome. I say this from personal experience, since my roof is pretty much maxed out. I may even move the inverter into the lounge to replace the television as my visual entertainment of choice.
Most people don't view solar panels as a source of entertainment, though. They want power, and the big thing that everyone talks about when it comes to power is the panels' efficiency: how many photons that hit them liberate electrons. The usual answer is... not many.
There is a fundamental limitation, called detailed balance, that helps limit the efficiency. Essentially, absorption of a photon and emission of a photon are the same thing (you just reverse the direction of time). So, if something is good at absorbing photons, it's also good at emitting photons. When your solar panel absorbs a lot of photons, there are lots of excited electrons around, and many of them will lose their energy by emitting other photons. In the end, where these two processes balance out helps set the maximum possible efficiency of a standard solar cell.