When did they stop making solid copper pennies?

Solid copper pennies were last minted in 1857 and were much larger than today's penny. Pennies were made mostly of copper until mid-1982, when the composition was changed to 97.5% zinc with a plating of 2.5% copper.

The only exception was in 1943 when wartime copper shortages forced the Mint to make pennies out of scrap steel. Bronze cents were resumed the next year, 1944.