A monolithic solar cell includes a first solar cell that is a sequential stack of an electrode, a silicon substrate, and an n-type emitter layer; a recombination layer disposed on the n-type emitter layer; an interfacial layer that is a double layer constituted of PEDOT:PSS and poly-TPD or PEDOT:PSS and PCDTBT, and that is disposed on the recombination layer; and a second solar cell that includes a p-type hole selective layer and a perovskite layer disposed on the p-type hole selective layer, the a p-type hole selective layer contacting and being integrated onto the interfacial layer of the first solar cell in a heat treatment during which the interfacial layer is partially decomposed, wherein the presence of the interfacial layer prevents a reduction in photoelectric conversion efficiency that occurs if the first solar cell and the second solar cell are combined without the presence of the interfacial layer.