My earlier attempt using LM7805 voltage regulator was a success but it was not of a good design. I got another solar panel supplying voltage but the extra charge was all wasted because LM7805 dissipates heat when it drops voltage. Along with that, current is also lost.
The mobile phone power bank that I am using is rated at 20,000 mAh and takes two whole days to get fully charged from the mains. With direct sunlight available only for 6 hours, it takes 2 days of plugging in to the solar panels to get just one of the four LEDs to light up.
In this, I am using a DC-to-DC voltage converter module. This module drops the voltage while increasing the current. I have removed the LM7805 and zener diode from the charger circuit. Using the voltage converter module is a no-brainer. It has to inputs and two outputs. Inputs are connected two 2.0mm DC sockets to which the solar pannels are plugged in. The two output in the converter are connected to the USB ports.
For safe housing, I cut holes in a plastic box. Now only the USB port and the DC sockets are outside.