Photocell upper limit

Hi, I have an issue about my photocells. I hooked up with 10K resistor as suggested, but no matter how bright it shined on the photocell, the analog value won't go above 680. I even try without resistor which supposed to give me the max value 1024, but the value was still a constant 680.

Do you know why?

Can you tell more ?

Did you use the internal voltage reference of 1.1V ? What kind of photo cell ? Do you use a single solar cell of 0.7V ?

My bad, I wrongly hooked up the photocell on 3.3v output, which is why the value is almost half of 1024.

Thank you!