Highly Efficient Carbon Counter Electrode with a MO3–x(M = Mo, W)/Poly(3-hexylthiophene) Hybrid Layer Facilitating an Efficient Hole Transport Pathway for Perovskite Solar Photovoltaic Cells

Inorganic oxides are widely used as hole transport materials (HTM) in perovskite solar cells (PSCs), due to their low cost and intrinsic chemical stability. However, these materials face challenges such as poor film formation on the perovskite layer, low power conversion efficiency (PCE), and limited stability in metal oxide-based modular n–i–p PSCs, which hinder their commercialization. In this work, we report the development of MoO_3–x and WO_3–x-based HTM from the VIB group, along with mixed halides APbI_2.7Br_0.3 (A = MA⁺, FA⁺, Cs⁺, MAFA⁺, CsMA⁺, CsFA⁺, and CsMAFA⁺) perovskite photoanode powders. These materials were successfully prepared via sustainable, energy-efficient one-pot microwave-assisted hydrothermal and solvothermal methods at 120–180 °C within 10 min without requiring any inert gas atmosphere. Subsequently, an inorganic–organic hybrid HTM layer was fabricated by impregnating 0.50 and 0.75% wt of nanorod-like WO_3–x and MoO_3–x oxides into a semiconducting poly(3-hexylthiophene) (P3HT) hybrid layer through a simple solution-processing technique. This approach was designed to address the aforementioned challenges. The obtained results show that PSCs with a carbon-coated ITO counter electrode and pristine P3HT/CsMAFA⁺ perovskite photoanode achieve a PCE of 12.8%. However, when WO_3–x/P3HT/CsMAFA⁺ and MoO_3–x/P3HT/CsMAFA⁺ are incorporated, the PCEs improve to 13.8 and 14.9%, respectively. These devices also exhibit enhanced optical and chemical stability during continuous maximum power point (MPP) tracking for 300 s, even under high-humidity conditions (RH ∼ 70 ± 5%, 30 °C). The observed PCE enhancement can be attributed to the improved molecular orientation of the polymer chains resulting from the higher-valent MoO_3–x and WO_3–x oxides. These materials, with their mixed rod-like morphologies, facilitate electron redistribution between P3HT and the oxides, increasing the M⁶⁺/M⁵⁺ ratio. This, in turn, enhances hole mobility and improves the uniformity of the HTM layer, allowing for better hole transport through the P3HT framework and significantly reducing energy losses during charge collection. This work presents a novel, simple method for integrating organic–inorganic hybrid P3HT/MoO_3–x(M = Mo, W) HTMs in PSCs, offering a promising pathway for achieving enhanced performance and stability in modular n–i–p PSCs, even under high-humidity conditions.