Before inventing the first solar powered wristwatch, Roger Riehl, had worked developing prototype electronic watches. In his monograph, The Invention of the Digital Watch, he wrote: “…What I believe to be the first digital watch ever constructed was put together (by me) in March 1968. The time keeping reference was the piezoelectric tuning fork at 1024 hertz made by Bramco of Piqua, Ohio. It was ¼” square by 1” long. The voltage supply was 5-6 volts. Thus it is a curious fact that the first digital electronic watch was neither LCD, LED, or Quartz."
Uranus produced a solar digital watch prototype in 1971. The solar cell supplemented the battery, rather than completely powering it.
The prototype Uranus watch was complex and difficult to make because it was made up a several chip with hundreds of soldering point connecting them, rather than a large scale integrated (LSD) chip.