  In this paper, first we introduce the notion of a post-Hopf algebra, which
gives rise to a post-Lie algebra on the space of primitive elements and there
is naturally a post-Hopf algebra structure on the universal enveloping algebra
of a post-Lie algebra. A novel property is that a cocommutative post-Hopf
algebra gives rise to a generalized Grossman-Larsson product, which leads to a
subadjacent Hopf algebra and can be used to construct solutions of the
Yang-Baxter equation. Then we introduce the notion of relative Rota-Baxter
operators on Hopf algebras. A cocommutative post-Hopf algebra gives rise to a
relative Rota-Baxter operator on its subadjacent Hopf algebra. Conversely, a
relative Rota-Baxter operator also induces a post-Hopf algebra. Then we show
that relative Rota-Baxter operators give rise to matched pairs of Hopf
algebras. Consequently, post-Hopf algebras and relative Rota-Baxter operators
give solutions of the Yang-Baxter equation in certain cocommutative Hopf
algebras. Finally we characterize relative Rota-Baxter operators on Hopf
algebras using relative Rota-Baxter operators on the Lie algebra of primitive
elements, graphs and module bialgebra structures.
