Heterostructure‐driven graded energy band structure in halide perovskite films

Abstract

Perovskite/perovskite heterostructures provide a promising platform for engineering optoelectronic properties through compositional and energetic control. However, their fabrication using conventional sequential solution process is often hindered by chemical intermixing and partial dissolution of the underlying perovskite layer. Here, we demonstrate a polydimethylsiloxane (PDMS)‐assisted film transfer approach for constructing compositionally graded formamidinium methylammonium lead halide (FA _{1− x} MA _x PbI _{3(1− x )} Br _{3 x} ) heterostructures with controlled interfacial characteristics. A soft‐annealed FAPbI ₃ film was first transferred onto a MAPbBr ₃ layer, resulting in an incompletely crystallized upper layer. Subsequent thermal annealing promoted further crystallization and interfacial ion diffusion, producing a vertically graded FA _{1− x} MA _x PbI _{3(1− x )} Br _{3 x} composition in which the MA and Br content increased with depth rather than becoming uniformly distributed throughout the film. This compositional gradient produces a graded band structure, mainly through the valence‐band maximum, which is expected to drive holes toward the top surface and reduce carrier accumulation and recombination at the buried interface. The residual in‐plane tensile strain found in the FAPbI ₃ control is relieved in the near‐surface region of the heterostructure. The graded heterostructure also shows enhanced phase stability, largely retaining the photoactive perovskite phase after 3 h at 80%–90% relative humidity, whereas the control converts to non‐photoactive δ‐FAPbI ₃ .