Many of the sunniest places on earth are also deserts, therefore the best place to put your solar cell is both very dusty and also very short of water to wash the dust off your panels, this can seriously reduce the efficiency of the system. Not only is this a problem on earth but the Mars rovers Spirit and Opportunity have been slowly loosing power and if it wasn't for the occasional martian mini whirlwind they would have conked out years ago.
Malay Mazumder, a from Boston University has been working on this problem for lunar landers and may have come up with a neat water free solution which will work on earth too. The idea is to cover the solar cell with small transparent electrodes and then occasionally apply high voltage to these electrodes. The voltages are arranged to form waves moving across the panel, which charge up and then push dust particles away from and then across they panel.
They can remove 90% of the dust on a panel in about 60 seconds, and the energy use is very small despite the high voltages involved as the currents are tiny. So future Mars missions may be less dependent on lucky dust devils, and on earth solar power could be cheaper in deserts.