Convert 3v solar cell to 1.5v
Use a voltage regulator.
You won't get exactly half the voltage and double the current. You can get close by using a switching voltage regulator. Texas Instruments has a nice selection that will step down 3V to 1.5V with fairly good efficiency.
For instance the TPS62671 can do this job for you with around 90% efficiency.
Resistor dividers are good for making reference voltages (negligible current draw) or scaling down analog inputs. They're are not good for voltage supplies with high or inconsistent current draw.
A linear regulator would work too, but will not be as efficient as a buck topology.
Note that the specs you gave are for open-circuit voltage and short-circuit current. You will not get both those values at the same time, ever. Using a regulator is the best option you have to provide a constant voltage while adding more panels in parallel on the other side of the regulator is the best way to make sure you can provide enough current.