Improved Conductivity of 2D Perovskite Capping Layer for Realizing High-Performance 3D/2D Heterostructured Hole Transport Layer-Free Perovskite Photovoltaics

Perovskite solar cells (PSCs) have emerged as low-cost photovoltaic representatives. Constructing three-dimensional (3D)/two-dimensional (2D) perovskite heterostructures has been shown to effectively enhance the efficiency and stability of PSCs. However, further enhancement of device performance is still largely limited by inferior conductivity of the 2D perovskite capping layer and its mismatched energy level with the 3D perovskite layer. Here, we developed an effective surface modification strategy via synergically incorporating inorganic high valence-state niobium ion (Nb⁵⁺) metal dopants and organic ammonium halide salts to in situ construct a high-quality 2D perovskite capping layer on top of the underlying 3D perovskite layer. As a result, the conductivity of the 2D perovskite capping layer was effectively enhanced by 43%, the energy barrier between 3D and 2D perovskite layers was favorably reduced, and the built-in electric field of the 3D/2D heterostructured perovskite stacks has been enlarged. In addition, the 2D perovskite capping layer also effectively reduced the defect densities by up to 29%, as verified by the space-charge-limited-current (SCLC) tests. Benefiting from the facilitated charge extraction and suppressed non-radiative recombination, the blade-coated hole transport layer-free PSCs based on this optimized 3D/2D heterostructured perovskite film achieved an efficiency of 23.2%, ∼19% higher than that of the control device (19.5%), which represented one of the best-performing PSCs with simplified device architecture fabricated via a scalable fabrication technique. The modified 3D/2D perovskite-based device also exhibited improved operational stability.