Is there a way to charge a Li-Po battery using solar panels to increase the flight time of a quadcopter during its flight?
The short answer is no. This is not to say that someone won't extend the flight time of a copter using solar energy one day. But if you attach off the shelf solar panels to off the shelf LiPo batteries and use an off the shelf or quickly fabricated custom power conversion electronics, then the reduction in flight time from the extra mass will be similar to any energy gained from the solar panels.
You can review further discussion on this topic here: How do I create a portable solar panel lipo charger?