Probably a poly-crystalline solar panel directly connected to some supercapacitors with a boost circuit to get the voltage up high enough to be useful.
Batteries are bad due to temperature sensitivity and cycle limits. Supercaps are better in these regards.
I want as few components as possible. I'll increase the complexity of the smarts as this project progresses. This is the general progression I'll follow, I think:
- Solar, supercap, boost, LED,
- Transistor/capacitor to blink LED,
- Microcontroller to blink morse message,
- Microcontroller to read non-volatile storage,
- Time passes…
- FPGA reading from multiple redundant flash storage chips with error correction.