If you just want a rough model of a half wave rectifier, you can use a Saturation block, from the Simulink/Discontinuities library. You could pass it an upper limit of Inf and a lower limit of 0.
If you want a more precise half wave rectifier that models the leakage on the other half wave, and you have a table of input and output voltages, you can use a 1-D Lookup Table from the Simulink/Lookup Tables library. Pass the input voltages in as the breakpoints, and the output voltages as the table data. The lookup table will interpolate between the provided values.
