photodiode block measurement error in simulink

I tried to measure the OC voltage and SC current of the photodiode block in simulink. however, the current read is correct but the voltage is always zero no matter how much solar density it is. all the parameters I use is default value. anybody know what is going on here? BTW, it is better to use a solar cell block rather than photodiode block when simulation solar tracker system? Thank you!

regards Jingbo