Smartphone internal charging circuit type and efficiency
It just depends on the charge current. Newer, higher powered smart phones use DC-DC conversion. But older phones that make do with 500mA or less USB current typically used linear charge control. If you do the power dissipation calculations you will see why. Once the charge current is higher than a few hundred mW, the dissipation for a linear regulator becomes difficult to manage. This is the driving force behind the choice. Thermal management (not efficiency, per-se).

Look at TI single cell charger IC's. I am not trying to endorse a specific brand. They just have a large variety of single-cell charger IC's. Efficiency will be 85% or higher.