There were solar panels made by KISS (Keep It Simple Systems). The Luna II was a set of flexible solar cells in a nylon case that you could be used for angling the solar energy properly. It will recharge or supplement battery power and works with all MP1x0 and 2000's. It does charge more slowly than a regular AC adapter though. A hard to find piece of Newton hardware.

I have heard there are other solar panels that may work, however you are risking your Newton's health by trying one. Be sure to ask on NewtonTalk first! Personally I would just get a solar charger that charges AA batteries, if you want to go solar and can't find a KISS solar unit. 100% safe that way. Of course you will need a battery cage for the MP2000 if you don't already have one.