A few points about solar panels:
- They are unlike a battery in that they give out a highly variable voltage, dependent on how much current you try to take from them
- Open-circuit (ie no current, nothing connected except a voltmeter) they'll show a voltage much higher than the nominal 6 volts shown on them
- When under load, the voltage drops considerably
- The "6 volts" printed on the label will only be accurate when you are taking a specified current from them (and assumes they are flooded with light). You should be able to work out the most efficient current by dividing the power rating of the panel in Watts (should be printed on it somewhere) by 6 volts.
To use solar panels, you usually need to have something to store the power and act as a buffer - either a battery or a big capacitor - it's unusual and difficult to run a circuit straight from the panel. Rather than figuring how to convert the solar power straight into 5v, look at powering your circuit from a rechargeable battery or a big capacitor, then look into solar charger circuits to keep the battery / cap charged from the panel.
h