A photodiode can't be used as a transmitter. And even as a receiver, you'll require lots more circuitry (unless you want it to operate in complete darkness).
The simplest circuit would use a torch as the transmitter and a photodiode (or perhaps a phototransistor or CdS cell) as the receiver. The signal would be amplified and used to drive a relay.
Pointing the torch at the sinsor would cause the relay to pull in.
The relay could then be wired to normally apply power so the motor runs forward with the car in darkness and backward when in sufficient light (e.g. when the torch is shining on it).