Confused about photodiode current
According to this diagram the current at zero bias is down-&gt;reverse
current.How is that even possible?

First you must understand that this graph shows what happens when an external voltage is applied across the photodiode. By convention, when voltage is applied across a component any current flowing into it is considered positive, while current coming out of it (back into the voltage source) is negative.
In your graph P0 represents the photodiode with no light on it, so it is not generating any current and the curve looks like a standard diode (current passes through zero at zero applied voltage).
P1 and P2 show the results at two different light levels. In these cases the diode is generating a current, which appears as a negative value because it is coming out of the diode, not going into it. As forward bias voltage increases the 'regular' diode current increases exponentially, consuming the 'negative' photocurrent and (when bias voltage is high enough) making the external current become positive.



simulate this circuit &ndash; Schematic created using CircuitLab