Charging a Li-Ion battery with a photovoltaic cell during constant current phase
You can charge Li-Ion cells at very low currents, as long as you respect the maximum voltage, and the dwell time at maximum voltage. The cell will take longer to charge than at the recommended rate, but it will still charge as efficiently.

If you expect your setup to remain on charge for a long time, then you will need to reduce your maximum voltage, 4.2v is not a safe level for a Li-Ion cell to be left at indefinitely, regardless of current.