Graded Heterojunction Engineering for Hole-Conductor-Free Perovskite Solar Cells with High Hole Extraction Efficiency and Conductivity

Despite great progress in the photovoltaic conversion efficiency (PCE) of inorganic-organic hybrid perovskite solar cells (PSCs), the large-scale application of PSCs still faces serious challenges due to the poor-stability and high-cost of the spiro-OMeTAD hole transport layer (HTL). It is of great fundamental importance to rationally address the issues of hole extraction and transfer arising from HTL-free PSCs. Herein, a brand-new PSC architecture is designed by introducing multigraded-heterojunction (GHJ) inorganic perovskite CsPbBrxI3-x layers as an efficient HTL. The grade adjustment can be achieved by precisely tuning the halide proportion and distribution in the CsPbBrxI3-x film to reach an optimal energy alignment of the valance and conduction band between MAPbI(3) and CsPbBrxI3-x. The CsPbBrxI3-x GHJ as an efficient HTL can induce an electric field where a valance/conduction band edge is leveraged to bend at the heterojunction interface, boosting the interfacial electron-hole splitting and photoelectron extraction. The GHJ architecture enhances the hole extraction and conduction efficiency from the MAPbI(3) to the counter electrode, decreases the recombination loss during the hole transfer, and benefits in increasing the open-circuit voltage. The optimized HTL-free PCS based on the GHJ architecture demonstrates an outstanding thermal stability and a significantly improved PCE of 11.33%, nearly 40% increase compared with 8.16% for pure HTL-free devices.