How can I invert a solar-powered oscillator's frequency?
I was able to simulate this circuit and play around with it a bit. The relationship between the voltage of the solar cell, the 330k resistor and the 10nF cap sets the operating frequency of the circuit. The difficulty in inverting this relationship is that the condition in which you want highest frequency (low light) also corresponds to the condition where VCC is lowest. Unless you can introduce an additional voltage supply independent of the solar cell, I don't see how you can achieve what you want. Sorry...