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:

1. Solar, supercap, boost, LED,
2. Transistor/capacitor to blink LED,
3. Microcontroller to blink morse message,
4. Microcontroller to read non-volatile storage,
5. Time passes…
6. FPGA reading from multiple redundant flash storage chips with error correction.