Really high impedances won't be able to drive the ADC input without significant error. For instance the Arduino analog inputs want 10k or less input impedance for best performance (both for accuracy and to avoid cross-talk on reading multiple channels - though the double-call-to-analogRead technique will help).
I'm just currious. Is it not possible (for more precission and faster readings) to use small solar cells (from low cost calculators) and connect these to your analog input?
Switching Time ?Light - dark: 0.0008s (0.8ms) at room temperature, 0.0003s (0.3ms) at 131 ? /55 ?