Smartphone input voltage range
Samsung S8+ uses Quick Charge 2.0 spec, as evidenced by page 12 of QC device list. 
Quick Charge is a proprietary protocol that doesn't immediately provide voltages above 5V to a client device. Instead, the client device starts charging at 5V and negotiates higher voltage/power by requesting it from a charger. Charger should only provide higher voltage to a client device after negotiation. It is possible to implement the Quick Charge negotiation by "bitbanging" the protocol, see https://hackaday.com/2016/12/13/bitbanging-qualcomm-charge-controllers/

You're at risk of damaging your phone if trying to directly charge it with 9V without implementing the QC protocol first, because the phone really expects the charger to be at 5V at the moment of connecting.