2D/3D heterostructured CsPbI₂Br solar cells: a choice for a monolithic all-perovskite tandem device

All-inorganic CsPbI2Br perovskite solar cells (PSCs) have attracted intensive attention owing to their suitable bandgaps and excellent photo- and thermal stability, making them promising ideal top cells in high-performance monolithic all-perovskite tandem solar cells (all-PTSCs). However, CsPbI2Br PSCs are still facing a challenge due to their relatively low power conversion efficiency (PCE) and moisture sensitivity. Herein, a simple and effective in situ growth method was adopted to construct a two-dimensional/three-dimensional (2D/3D) heterostructure by spinning n-butylammonium bromide (BABr) on the top of a 3D CsPbI2Br perovskite film. The such fabricated high-quality 2D/3D CsPbI2Br film exhibits excellent hydrophobicity against moisture invasion, and a well-matched energy level with the hole transport layer (HTL) facilitating hole extraction at the perovskite/HTL interface. As a result, the optimized 2D/3D CsPbI2Br solar cell achieves an excellent PCE of 16.57% with improved stability. Based on this, we developed an all-PTSC having 2D/3D CsPbI2Br and MAPbI3 as the optical absorption layer of top and bottom cells, respectively, along with a solution-processed PEDOT:PSS film with high conductivity and high transmittance as the recombination layer. The fabricated all-PTSC exhibits a PCE of 10.22% and an ultrahigh open-circuit voltage of 2.33 V, approaching the sum of the light voltages of the two sub-cells. Benefiting from the outstanding thermal stability and hydrophobicity of 2D/3D CsPbI2Br, the all-PTSC showed superior stability under heat and air. This work demonstrates that all-inorganic perovskites as the photo-response cores of top cells are appropriate candidates for the fabrication of efficient and stable monolithic all-PTSCs.