How to improve the current when the voltage is high in solar cell?
You want to operate your solar cell at the maximum power point. As you note, the power at open circuit is low, as is the power into a short; the optimum operating condition is somewhere inbetween.

You can determine this manually by putting an adjustable resistance across the cell and finding the point at which the I*V product is greatest. Your power converter needs to be designed with an input impedance that's close to this maximum power point. Manufacturers such as TI have charge controllers that can do this for you.

Note, though, that if you're only seeing 10 microamps closed-circuit current, your panel is not generating much energy: you can't expect more than about V_oc * I_s * 0.7, or about 4 microwatts in your case!