  As the race towards higher efficiency for inorganic/organic hybrid perovskite
solar cells (PSCs) is becoming highly competitive, a design scheme to maximize
carrier transport towards higher power efficiency has been urgently demanded.
Here, we unravel a hidden role of A-site cation of PSCs in carrier transport
which has been largely neglected, i.e., tuning the Frohlich electron-phonon
(e-ph) coupling of longitudinal optical (LO) phonon by A-site cations. The key
for steering Frohlich polaron is to control the interaction strength and the
number of proton (or lithium) coordination to halide ion. The coordination to I
alleviates electron-phonon scattering by either decreasing the Born effective
charge or absorbing the LO motion of I. This novel principle discloses lower
electron-phonon coupling by several promising organic cations including
hydroxyl-ammonium cation (NH$_3$OH$^+$) and possibly Li$^+$ solvating
methylamine (Li$^+$NH$_2$CH$_3$) than methyl-ammonium cation. A new perspective
on the role of A-site cation could help in improving power efficiency and
accelerating the application of PSCs.
