Read voltage from solar cell

Hello, i have a problem using analog inputs of arduino uno. I would measure voltage of solar cell, so i connect to a divider it and send to buffer, the output of buffer to A0 input. What happen is that the read value (from 0 to 1024) seems to be right, but the voltage value (2*analogRead(A0)*5/1024) no. What is the matter? Thank you

Supply the whole program.

What are the R values?

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