If you have 6V you can use one diode to lower voltage by 0,7V, and give 5,3V. I think that phone will work even on 6V, because charging controller IC is in phone, and have tolerance at least 1V. I will try first with one diode for 6V to get 5,3V, and if you have 7,5V use multiple diodes in serie, each diode for lowering 0,7V.
Diode can be 1N4007. In SMD can be very small.
For usage of 7805 voltage regulators you need at least 3V Vi-Vo difference, to get 5V on output. You can check LDO they requast lower Vi-Vo difference.
One of reason for factory 5V needs for phones chargers, is to keep charging IC power dissipation in ok ranges.
I'm curious, that voltages what you mention, 6V, 7V, thise voltages are constant, or voltage varies ?
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Better search solar case for phone, that can be more practical solution.