Great questions. There are solar power banks that can charge an iPhone. However, they are a bit large for a window sill, and unless the window is open they won't work well, because the window glass will absorb much of the sun's energy. The "serious" ones are also fairly expensive and are used mainly by expeditions of various kinds (archaeological, geological, etc). If you seriously want to look at options look for ones that can supply at least 1 amp at 5 volts and that have a USB power output. There are also "accumulator" types that have their own battery. The solar panel charges the battery throughout the day, and the battery can then be used to charge your phone at night.
Regarding your safety questions, as they all provide DC a lot of the concerns over cheap AC plug in power sources go away; they solar panels (like battery packs that can recharge phones) don't need to be MFI certified. As you point out, the cable should be, however. There is no fire risk, because you can't overcharge an iPhone; the charging circuit is in the phone and will stop charging when the battery reaches full capacity. But don't put the phone in the sun; only the solar panel.
Regarding carbon footprint, that is an extraordinarily difficult question, and experts haven't agreed. If you are considering total carbon footprint you must take into consideration the carbon footprint of the manufacturing process for solar panels, not just the use. Making a solar panel requires fusing quartz. That requires very high temperatures. So you need to consider the source of the heat that produces the high temperatures. If it comes from directly burning coal or oil it will be a losing proposition overall. Fortunately, most manufacturing comes from electricity, but you still need to consider where the electricity comes from. If it's solar, wind, geothermal, tidal or hydro power those will all be very clean; but even then you are still producing waste heat, which contributes to greenhouse effect and raises the entropy of the world. The problem is that most solar panels are manufactured in China, where the main source of electricity is from burning soft coal.
So the next question is how much energy you would save. An iPhone has around a 2 ampere-hour battery (different models are different sizes, but I'll make this simplifying assumption). As the charge process is probably around 70% efficient (a guess) that means to fully charge an iPhone battery will require 2.9 ampere-hours or, at the 5 volts into the phone from the power source about 15 watt-hours (rounding). I live in an expensive area for electricity; I pay 17¢ per kilowatt hour (or 1,000 watt-hours). So 15/1000 x 0.17 = $0.003. Thus one full charge costs me about 3 tenths of a cent.
Thank you for the opportunity to practice my math. If anyone notices an error in my calculation I would be happy to hear from you.
