You'll need more than one diode to detect in which direction your burning candle is perhaps if you arrange them in a pattern you could steer your robot towards the biggest light output from the diodes.
You could get away with a single light sensing diode if you spin it in a horizontal circle and use a computer to interpret the changing light output with reference to a simultaneously rotating anglular position sensor.
Maybe an infrared photodiode might be better to sense fire, there's a lot more heat than visible light output from a candle.
Have fun with that one 8)
Klaus