I have several portable chargers. The best of all is 'JustMobile Gum Pro', 4400mAh and 120 grams, charges my phone fully about two times.
I have JustMobile for three years and used it with two phones and now with my actual phone. No need to buy a spare battery for each phone
I also have a solar charger (with a build-in battery) but it isn't very usefully. Often it becomes to hot because of the sun and shuts down the charging of its own battery. Li-Ion batteries has to be charged below +45C, so the so the charging control shuts down charging when the battery becomes to hot.
This is the crux: The Solar panel needs sun and when it gets sun it becomes hot, but the battery needs to be cool for charging.
But I use a solar panel (without a build-in battery). Outdoors it charges the battery of the portable charger, solar panel on the backpack, charger in the backpack. So there aren't any temperature problems for the battery

. And later (at night?) the phone can be charged with the portable charger.
The solar panel also is able to charge the phone directly. But this needs ongoing sunshine and takes its time for to charge the phone fully.
So I think it's the better way first to charges the charger with the solar panel and the charges the phone with the portable charger
But this method is for outdoor trips.
Most of time the JustMobile charges my phone when the phone's battery goes low in the late evening. Not outdoors ... no, when I'm sitting on the sofa
EDIT
Definition of solar charger:
This is a charger with a solar panel
and a build-in battery.
Without this build-in battery, it is just the solar panel.
But some solar panels have a charging circuit with 5V output and a USB connector for charging the batteries of phones and portable chargers.
Harry