Co-Solvent Assisted Optimization of the CuSCN Hole Transport Layer for Enhancing the Efficiency of Ambient Processable Perovskite Solar Cells with Carbon Counter Electrodes

In recent perovskite solar cell (PSC) research, copper(I) thiocyanate (CuSCN) is an emerging inorganic hole transport layer (HTL) due to its suitable band gap, matched band edge positions with the perovskite and high stability under ambient conditions. However, being a coordination polymer typically requires sulfide-based solvents that strongly interact with Cu(I) for dissolution. Dipropyl sulfide (DPS) is generally used where it is very sparingly soluble of about 10–12 mg mL⁻¹, which leads to low surface coverage with pin-holes on the surface responsible for the generation of defects at the perovskite–HTL interface. In this study, addition of the optimized amount 100 μL of co-solvent Acetonitrile (ACN) increased the CuSCN dissolution from 10 to 35 mg mL⁻¹. ACN can act as a Lewis-base making it capable of donating electrons to a Lewis-acid like Cu⁺ from CuSCN. ACN is a polar aprotic solvent due to its highly polar CN bond and by adding CuSCN the dipole–dipole interactions can stabilize the CuSCN molecules in solution. The device with architecture (FTO/c-TiO₂/mp-TiO₂/MAPbI₃/CuSCN/carbon) showed the higher power conversion efficiency (PCE) of ≈11% with V_oc of 1.01 V and I_sc 24.65 mA cm⁻² showing excellent stability stored under ambient atmosphere which retains 80% of its initial efficiency after 10 days.