IoT device Power using 2.7V Super Capacitors Solar Charger Dc-DC
There are some problematic elements to your idea, but I think that it could be made to work.

Solar cells have a property called the Maximum Power Point, very simply this means that just connecting them to a load will typically have extremely poor results. You need to use something called Maximum Power Point Tracking to get good efficiency.

Supercapacitors typically have very poor leakage characteristics. This literally how much charge continually leaks out of them via internal processes, visualise a bucket with a hole in the bottom.
You will need to choose both the topology and the supercapacitor very carefully to ensure you don't waste your captured energy in leakage and efficiency losses.

Supercapcitor charging ICs are readily available (e.g. LTC3625). The MPPT technique is shown in this App Note: 


https://www.analog.com/en/design-center/reference-designs/circuit-collections/ltc3625-lt1784-solar-powered-super-capacitor-charger-with-mppt.html#cc-overview

For your power source selection into the load I would look at a 'dual ideal diode' configuration. This will allow you to 'OR' your two supplies together without allowing one to drive the other. This is usually the highest-efficiency way to accomplish this type of operation. (e.g. LTC4415).
You would have the 'primary' supply regulating at a couple of hundred millivolts higher than the 'backup' supply (but still within the acceptable range for the load), this ensures the primary feeds the load whenever possible.

You will need to look very carefully at your power-budget. It will take a very long time to charge your supercapacitor, your circuit will be taking current to operate its various components, and the supercapacitor will be leaking charge. You will need to balance the charge time and total energy per cycle that the panel can support with the energy being constantly lost, and also the large burst of energy that will be consumed at transmit time for your IOT device.