Photosensitive Diode or photo resistor?
There are two modes of operation of a PD. A PD that conducts a current when reverse-biased and illuminated will produce a voltage across the terminals when illuminated.
If you buy parts with proper datasheets (not random stuff without proper manufacturers and part numbers listed) they should characterize at least one mode of operation.
For example, this (probably similar) part

The photovoltaic mode shows an OC voltage of 400mV and a SC current of 35uA with the stated illumination wavelength and power density.
With the same illumination, and a -5V bias, the PD conducts 35uA, and when dark the current is typically 5nA.
From the fact that the parameters are only guaranteed for reverse bias operation, one might conclude that is the expected application.