  Electrocaloric (EC) materials show promise in eco-friendly solid-state
refrigeration and integrable on-chip thermal management. While direct
measurement of EC thin-films still remains challenging, a generic theoretical
framework for quantifying the cooling properties of rich EC materials including
normal-, relaxor-, organic- and anti-ferroelectrics is imperative for
exploiting new flexible and room-temperature cooling alternatives. Here, we
present a versatile theory that combines Master equation with Maxwell relations
and analytically relates the macroscopic cooling responses in EC materials with
the intrinsic diffuseness of phase transitions and correlation characteristics.
Under increased electric fields, both EC entropy and adiabatic temperature
changes increase quadratically initially, followed by further linear growth and
eventual gradual saturation. The upper bound of entropy change (dS_max) is
limited by distinct correlation volumes (V_cr) and transition diffuseness. The
linearity between V_cr and the transition diffuseness is emphasized, while
dS_max=300 kJ/(K.m3) is obtained for Pb0.8Ba0.2ZrO3. The dS_max in
antiferroelectric Pb0.95Zr0.05TiO3, Pb0.8Ba0.2ZrO3 and polymeric ferroelectrics
scales proportionally with V_cr^(-2.2), owing to the one-dimensional structural
constraint on lattice-scale depolarization dynamics; whereas dS_max in relaxor
and normal ferroelectrics scales as dS_max ~ V_cr^(-0.37), which tallies with a
dipolar interaction exponent of 2/3 in EC materials and the well-proven
fractional dimensionality of 2.5 for ferroelectric domain walls.
