Characterisation and applications of $Bbbk$-split bimodules

We describe the structure of bimodules (over finite dimensional algebras) which have the property that the functor of tensoring with such a bimodule sends any module to a projective module. The main result is that all such bimodules are $Bbbk$-split in the sense that they factor (inside the tensor category of bimodules) over $Bbbk$-vector spaces. As one application, we show that any simple $2$-category has a faithful $2$-representation inside the $2$-category of $Bbbk$-split bimodules. As another application, we classify simple transitive $2$-representations of the $2$-category of projective bimodules over the algebra $Bbbk[x,y]/(x^2,y^2,xy)$.