Organic‐free perovskite solar cells (PSCs) have been of rising interest due to their remarkable resistance toward long‐term thermal stress. Nevertheless, the inorganic perovskite films usually suffer from poor crystallization and high‐density defects in bulk and near/at the interfaces, which leads to significant charge recombination loss and hence inferior device performance. Herein, it is demonstrated that high‐quality CsPbI2Br perovskite films could be prepared by using 2D non‐layered materials as additives, such as In2S3 nanoflakes (Nano‐In2S3) with well‐matched lattices and unsaturated dangling bonds on the surface. In addition, it is found that the introduction of Nano‐In2S3 results in not only defect passivation but also remarkable quasi‐Fermi level splitting across the perovskite film due to its gradient doping behavior, thereby enhancing the built‐in electric field in the inverted PSCs. As a result, the optimal devices based on Nano‐In2S3:CsPbI2Br absorber and all‐inorganic interfacial layers deliver a champion power conversion efficiency of 15.17% along with excellent ambient and thermal stabilities, superior to those of the pristine devices and comparable to the best organic‐free PSCs. A novel strategy for highly efficient and stable organic‐free photovoltaics by using 2D non‐layered materials as multifunctional additives is demonstrated.

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Inorganic perovskite CsPbI2Br has advantages of excellent thermal stability and reasonable bandgap, which make it suitable for top layer of tandem solar cells. Nevertheless, solution‐processed all‐inorganic perovskites generally suffer from high‐density defects as well as significant tensile strain near underlayer/perovskite interface, both leading to compromised device efficiency and stability. In this work, the defect density as well as interfacial tensile strain in inverted CsPbI2Br perovskite solar cells (PeSCs) is remarkably reduced by using a bilayer underlayer composed of dopant‐free 2,2′,7,7′‐tetrakis(N,N‐dip‐methoxyphenylamine)‐9,9′‐spirobifluorene (Spiro‐OMeTAD) and copper phthalocyanine 3,4′,4″,4′″‐tetrasulfonated acid tetrasodium salt (TS‐CuPc) nanoparticles. As compared to control devices with pristine Spiro‐OMeTAD, devices based on Spiro‐OMeTAD/TS‐CuPc exhibit remarkably improved photovoltaic performance and enhanced thermal/humidity stability due to the better perovskite crystallization, improved interfacial passivation, and hole‐collection as well as efficient interfacial strain release. As a result, a champion efficiency of 14.85% can be achieved, which is approaching to the best reported for dopant‐free and inverted all‐inorganic PeSCs. The work thus provides an efficient strategy to simultaneously regulate the defects density and strain issue related to inorganic perovskites.

In situ prepared 2D/3D hybrid heterojunction perovskites have attracted increasing interest in the field of photovoltaics due to their combined merits of 2D and 3D perovskites as well as approved applications in regular structured (n–i–p) perovskite solar cells (PSCs). Nevertheless, in situ building a 2D phase on 3D inorganic perovskites still remains challenging probably due to the resistance of the Cs+ ion toward the exchange reaction with most organic cations. In this work, we report the facile building of the 2D phase with n = 1 using highly conjugated benzimidazolium iodine (BIZI) instead of imidazolium iodine (IZI) for the posttreatment on a 3D CsPbI2Br absorber. The combined spectroscopic results reveal that the posttreatment and the as-formed 2D (BIZ)2PbI3Br result in not only moisture proof for the 3D CsPbI2Br absorber but also remarkable defect suppression/passivation and efficient hole blocking at the CsPbI2Br/PC61BM interface. As a result, the inverted (p–i–n) PSCs with an optimized 2D/3D structure deliver remarkably improved photovoltaic performance along with enhanced ambient stability compared to control devices. Thus, this study provides crucial information on how to in situ build a 2D/3D structure via ligand chemistry as well as how the 2D/3D structure influences the performance of inverted PSCs.

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