Photocathodes for High Brightness Photo Injectors

The development of the photo injector has become a key technology for accelerator-based light sources and for the electron collider. There are a lot of opportunities to improve the injector quality, as well as photocathode function. A better photocathode can reduce the specification requirements of the drive laser system. The identification of better photocathodes is always a principal technical challenge, especially for the high brightness injectors. In this contribution, we will briefly review the status of photocathode research and the developing trends that are foreseen for the future.