Technical University of Denmark
Order your own solar cell for free at plasticphotovoltaics (dot) org
This is a see-through postcard that is a solar cell. If you contact the energy company listed above they will send you a fee postcard with your own cell. This one is postmarked in 2015 from Denmark.
Below is the side of the postcard where they attached the address and postage.
To learn more about PhotoVoltaics Solar Cells, click this link.