Why can't smart phone screens also be solar panels?

Calculators take an order of magnitude less power. They are also not running all day.

Some back of envelope math tells me that a solar panel the size of my phone would roughly take 4-5 days to fully charge the phone.

If you are interested, you can buy solar charger phone cases - some even with fold out panels. But even then, it's hard to imagine any real utility from them unless you are actively backbacking.

One, they're not very big. Solar panels are on average only about 20% efficient (per square meter of full 1000W sunlight). An iPhone is only about 0.01 square meter, meaning 1000W/m^2 * 0.01m^2 * 20% = approximately 2W. That's 2W if the entire phone was a solar module, operating at peak power under full sun. A phone battery is about 10-12Wh, so it would take ~6 hours to fully charge under optimal conditions.

Two, you never get optimal conditions, especially if you're out hiking. Because of the way solar cells inside a module are connected, partial shade (your finger, a tree, a passing cloud) will plummet the output. If you shade just 20% of the solar module, you don't get 20% less power, you get something like 80% less... the tiniest bit of shade has huge effects on power output.

Three, it's terribly inconvenient. If you want to optimize charging, you can't be actively using your phone (because your fingers would shade the module), can't strap it to your backpack (the straps would shade it), can't hike through wooded areas (the trees would shade it). All of this would happen during the day, when you're active and wanting to use the phone. And if it gets too sunny and hot, you'll overheat your phone and either cause it to thermally shut down or else just damage it.

Generally it's just much more convenient to get an external solar panel, maybe one with a built-in battery pack. For backpacking, they can both be in the same portable unit. For car camping, you can have a huge (foldable) solar panel plus a big external battery. Here are a few of both kinds: https://www.outdoorgearlab.com/topics/camping-and-hiking/bes...

That way, you can just leave the solar charger at camp, in the sunniest spot (or dangle it from your backpack), where it'll slowly charge its own battery pack. At night (or whenever), you can plug in your phone to the battery and quickly charge your phone from the battery (which acts as a buffer/cache between your device and the sun) -- all without really changing your phone usage habits.

You don't really want your phone sitting in sunlight. The battery shouldn't be charged as it starts to heat up.

the solar panels in screen size nearly cannot charge the battery, and it heat up the phone probably cause damage of the battery. an external solar pack is more practical.

btw, thanks your question hints me another solution, if solar pack can be more slim and foldable, it will be great to be a "solar-phone-case".

Contradicting purposes, behaviours, limitations, and materials.

Like...

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The alternative to solar powered calculators is coin cell powered calculators. If you could run a smart phone on a coin cell, or anything close, this idea might have legs.

Solar panels absorb light so the screen wouldn’t look so great