Correct solar panels to pick for drone-mod
TL/DR:

Even assuming an unrealistically large amount of collected energy , it is doubtful that solar panels on the drone will do you any good.



Lets rough out some numbers.

The energy stored in your battery is approximately 4.4 watt hours. That's 1.2 ampere hours times 3.7 volts.

To make the solar cells worth while, you'll need to collect a significant fraction of that energy during flight.

Solar power at the Earth's surface is around 1000 watts per square meter.  If you had a 100 percent efficient solar panel of one square meter surface area, you could get 1000 watts of power.

Your drone couldn't fly with that large surface hanging on it, and solar panels are far from 100 percent efficient.

I'll assume a (hopelessly) optimistic efficiency of 30 percent for your solar panels.

I'll also assume a solar panel about the size of your drone.  The drone is 27cm by 27cm.  I'll go with panels of 30cm by 30cm.  That's a surface area of 0.09 square meters.

1000 watts times 0.09 is 90 watts of incoming solar power.

At 30 percent efficiency, that's 27 watts of power from your solar panel.

Take a flight time of 15 minutes, and your solar panel will collect 6.75 watt hours of energy.

That sounds good - it is more than your battery, so it seems like you could get some benefit out of it.

Or not.  

Your drone now has to carry the weight of the solar panel and stabilize it in flight. 

Taking a WAG, I'd be surprised if your drone battery would last for half the normal flight time while carrying the solar panel (and associated electronics.)  That halves the time before you are dependent on the energy from the solar panel.  You'll have only gotten about 3.5 watts hours in solar energy by that point.

Your solar energy has to be used to charge the battery, so there will be losses.  You will lose energy in converting the voltage to be usable, and you will lose energy as you draw it back out of the battery.  

You might effectively end up with a couple of watt hours usable from the whole operating period.

If you lose half your flight time by carrying the solar panel, and then can't make it up with energy from the solar panel, then the whole thing is a loss.

Since I mostly ignored losses and assumed a really optimistic amount of energy from the solar panel, I think you will find that a solar panel in flight is a loss.  Even if you manage to make it extend your flight time a little, you still have to deal with your drone being ungainly and instable with the solar panels on it.



The better way to power your drone from solar power is to use a (big) solar charger to charge your drone battery on the ground.  Keep a spare battery at hand and recharging.  Swap as needed.  You won't extend the time of individual flights, but you can make more flights per day that way.