Thin film solar cells based on layered chalcogenides: fundamentals and perspectives of van der Waals epitaxy

The preparation of thin films of layered chalcogenide semiconductors as MX and MX/sub 2/ (X=S, Se) based on the concept of van der Waals epitaxy (VDWE) is presented for different substrate/overlayer combinations of GaSe, InSe, SnSe/sub 2/, WS/sub 2/ on WSe/sub 2/, GaSe, MoTe/sub 2/, graphite and mica. In all cases stoichiometric films are formed either as epitaxial layers or strongly textured films with the c-axis aligned in spite of strong lattice mismatch. The interfaces are nonreactive and atomically abrupt. The electronic properties of the interfaces are mostly ideal showing band offsets according to the electron affinity rule and no operative interface states. However, doping of the films is still a problem which limits the band bending and the attainable surface photovoltage. The perspectives and preconditions for the further development of layered semiconductor VDWE films for solar cells is discussed.