Tiny circuit for solar charging a supercap to 2.7 volts. The PCB measures just 1/4" x 1/2" in size. Plug your solar panel into the "IN" port and plug a supercap and the mote to be powered by the supercap into the other two ports. Uses just two parts: an LDO and a diode. Design places diode before LDO rather than afterward so that a full charge is achieved faster by avoiding the very long charging tail of the opposite configuration. Fortunately, the reverse current leakage of this particular part combination is only around 6 micro-amps. Both diode and LDO are currently stocked by Digikey.
You may also plug a battery or other power source into the "IN" port first if you want to pre-charge the supercap before connecting the mini solar panel.