ATL431: Exists a nanoamp shunt regulator to clamp voltage of indoor solar cell?

Part Number: ATL431

Is there a shunt regulator (or other part) that will clamp to 3.6V the voltage from an indoor solar cell  (say Amorton 1815 generating a few or tens of uA in dim light of say 200lux)?

In typical conditions, the solar cell V operating is low enough, but in stronger light the solar cell can generate a max voltage of say 5V (Voc open circuit of 8 cells@0.6V).  That is higher than the Vmax the load (an mcu) can tolerate.

I have looked at the TL431 but the "Imin Minimum cathode current for regulation" of 400uA leads me to believe it would not work.  (But I have no experience with shunt regulators.)

I have searched for a zener but can't find one that has an Ir low reverse leakage of less than a few uA and a sharp knee at Vz or Vr of 3.6V.  I have evaluated a TVS diode (for ESD protection) but it didn't seem to clamp the voltage.   Also, a TVS seems designed for a pulse, not for constant current (but I figured that the package could withstand the say 200uW from the solar cell.)

I have tested the BQ25504 (which will clamp the Vbat output voltage) but I am looking for a simpler solution.  (If I wanted to minimize the area of the solar cell, I would use the BQ25504.  But most of the available amorphous solar panels for indoor light are already larger area and higher voltage than I need, not requiring the boost or MPPT of the BQ25504.  Also, I am using a tantalum capacitor, not a super cap or battery which might require protection functions of the BQ25504.)

My application is nano power, duty-cycled radios.