Insert the 100 ohm resistor into the breadboard and connect each leg of it to one leg of the photodiode. Connect the probes of the digital multimeter to the legs of the resistor. Turn the meter to the lowest voltage setting (millivolts) and you should see a very low voltage reading.
A photodiode is a PN junction or a PIN structure. When a photon hits the diode with sufficient energy, it excites an electron, creating a free electron (and a positively charged electron hole). This mechanism is also referred to as an internal photoelectric effect.
If the absorption occurs in the depletion region of the junction or a diffusion length thereof away, these carriers are wiped away from the built-in field of the depletion region by the junction. In this way, holes in the direction of the anode and electrons move in the direction of the cathode, and a photocurrent is generated. This photocurrent is the sum of the dark current (without light) and the light current. Therefore, the dark current must be minimized to improve the sensitivity of the device.