The higher shunt resistance means that higher the polycrystalline lattice irregularity and many grain boundaries. This means higher current resistance and thus less current output, so efficiency of solar cell decreases. However, in the circuit model of a solar cell, a shunt resistor is connected in parallel to the diode and they say that it must have infinite resistance for the highest current output cuz it is in parallel. However infinite grain boundary irregularity means that there is infinite resistance to the whole current in the solar cell so no current flowing is produced.
There is a conflict here.
Warning: This is a crosspost, although noone answered my question, here are the links: