Cuprous oxide is commonly used as a pigment, a fungicide, and an antifouling agent for marine paints.  Rectifier diodes based on this material have been used industrially as early as 1924, long before silicon became the standard.  Copper(I) oxide is also responsible for the pink color in a positive Benedict's test.
In December 2021, Toshiba announced the creation of a transparent cuprous oxide (CuO) thin-film solar cell. The cell achieved an 8.4% energy conversion efficiency, the highest efficiency ever reported for any cell of this type as of 2021. The cells could be used for high-altitude platform station applications and electric vehicles.