A different idea is to add a small solar panel/cell with some changes:

• Adding a small solar panel which mounts on top of the microcontroller and other circuitry on the top side. Some options are the following:
  • A polycrystalline small cell measuring 23mm * 20mm * 3.2mm like these
    • http://www.aliexpress.com/item/20pcs-0-5V-80MA-polycrystalline-silicon-solar-cell-panel-DIY-technology-Small-production-material/32586814132.html
    • http://www.aliexpress.com/item/20pcs-0-5V-80MA-polycrystalline-silicon-solar-cell-panel-DIY-technology-Small-production-material/32631751716.html
  • A high efficiency monocrystalline solar cell available on digikey measuring 22mm * 7mm * 1.8mm
    • http://ixapps.ixys.com/DataSheet/KXOB22-12X1L-DATA-SHEET-20130902-.pdf
    • http://www.digikey.com/product-search/en/sensors-transducers/solar-cells/1966326?k=solar panel
• A rechargeable lithium ion battery needs to be used such as the following:
  • http://www.aliexpress.com/item/high-quality-SD351530-95mah-3-7v-rechargeable-lithium-ion-battery/32531538474.html
• The solar panel provides 0.6V (max) at 50mA. A boost converter is needed to get this voltage up to charge the battery
• All the circuitry needs to be on the top side. No double sided load
• The solar panel (especially the 22mm * 7mm one) needs to be somehow mounted on the top-side circuitry