I see that you are increasing the current from your (12volt?) solar cell by stepping up the duty cycle of the PWM (R3/C1), and measuring the voltage across the solar cell (R1/R2/A1) and 1ohm/5watt current sense resistor (R4/A0).
I assume U1's supply voltage is >=12volt (the source could be 5volt).
Circuit seems ok, except for the ripple on C1.
With ~500hz PWM, it is ~0.6volt t/t @ 50%PWM.
That could introduce a measuring fault of ~8%.
Connecting R3 to pin 5 or 6 doubles the PWM frequency, and halves the error.
Have you written any code?