Janus MoSSe monolayers have been recently synthesized by replacing S by Se on one side of MoS2 (or vice versa for MoSe2). Due to the different electronegativities of S and Se, these structures carry a finite out-of-plane dipole moment. As we show here by means of density functional theory calculations, this intrinsic dipole leads to the formation of built-in electric fields when the monolayers are stacked to form N-layer structures. For sufficiently thin structures (N < 4), the dipoles add up and shift the vacuum level on the two sides of the film by ∼N·0.7 eV. For thicker films, the vacuum level shift saturates at around 2.2 eV due to compensating surface charges, which in turn leads to the formation of atomically thin n- and p-doped electron gasses at the surfaces. The doping concentration can be tuned between 5 × 1012 and 2 × 1013 e/cm2 by varying the film thickness. On the basis of band structure calculations and the Mott-Wannier exciton model, we compute the energies of intra- and interlayer excitons as a function of film thickness, suggesting that the Janus multilayer films are ideally suited for achieving ultrafast charge separation over atomic length scales without chemical doping or applied electric fields. Finally, we explore a number of other potentially synthesizable two-dimensional Janus structures with different band gaps and internal dipole moments. Our results open new opportunities for ultrathin optoelectronic components, such as tunnel diodes, photodetectors, or solar cells.