(i) The characteristic curve is of zener diode and curve is of solar cell.
(ii) The point P in Fig. represents zener break down voltage.
(iii) In Fig., the point Q represents zero voltage and negative current. It means light falling on solar cell with atleast minimum threshold frequency gives the current in opposite direction to that due to a battery connected to solar cell. But for point Q, the battery is short circuited. Hence Q represents the short circuit current.
In Fig., the point P represents some positive voltage on solar cell with zero current through solar cell. It means, there is a battery connected to a solar cell which gives rise to the equal and opposite current to that in solar cell by virtue of light falling on it. As current is zero for point P, hence we say P represents open circuit voltage.
