Abstract: Techniques for increasing conversion efficiency of thin film photovoltaic devices through back contact work function modification are provided. In one aspect, a photovoltaic device is provided having a substrate; a back contact on the substrate, wherein at least a portion of the back contact has a work function of greater than about 4.5 electron volts; an absorber layer on a side of the back contact opposite the substrate; a buffer layer on a side of the absorber layer opposite the back contact; and a top electrode on a side of the buffer layer opposite the absorber layer. The absorber layer preferably has thickness that is less than a depletion width+an accumulation width+a carrier diffusion length.

Abstract: Techniques for fabrication of kesterite Cu—Zn—Sn—(Se,S) films and improved photovoltaic devices based on these films are provided. In one aspect, a method of fabricating a kesterite film having a formula Cu2?xZn1+ySn(S1?zSez)4+q, wherein 0?x?1; 0?y?1; 0?z?1; and ?1?q?1 is provided. The method includes the following steps. A substrate is provided. A bulk precursor layer is formed on the substrate, the bulk precursor layer comprising Cu, Zn, Sn and at least one of S and Se. A capping layer is formed on the bulk precursor layer, the capping layer comprising at least one of Sn, S and Se. The bulk precursor layer and the capping layer are annealed under conditions sufficient to produce the kesterite film having values of x, y, z and q for any given part of the film that deviate from average values of x, y, z and q throughout the film by less than 20 percent.