Functionalized Polyurethane as a Hole Transport Layer for Quantum Dot-Sensitized Solar Cells with Higher Open-Circuit Voltage and Fill Factor

ABSTRACT: A functionalized polyurethane-based hole transport layer (HTL) for quantum dot-sensitized solar cells (QDSSCs) has been developed by tailoring the redox behaviour of segmented polyurethane. The successful incorporation of ionic moieties into the hard segment was confirmed by Fourier-transform infrared (FTIR) and Nuclear magnetic resonance (NMR) spectroscopy. Sulfonation significantly improved the polymer’s electrical conductivity, electrochemical activity, and optical properties, thereby enabling efficient hole transport through favorable work-function alignment and reduced interfacial energy barriers between the photoactive layer and the Ag counter electrode. Ultrasmall spherical CuInS₂ quantum dots with an average size of 3.15 nm were synthesized using a capping-assisted method and characterized by X-ray diffraction (XRD), UV-Vis spectroscopy, and Transmission electron microscopy (TEM). The HOMO-LUMO and valence/conduction band energy levels, determined by cyclic voltammetry and UV-vis spectroscopy, revealed favorable energy level alignment for efficient charge separation and hole extraction. QDSSCs were fabricated with the architectures FTO/TiO₂/CuInS₂/SPU-2/Ag and FTO/SnO₂/TiO₂/CuInS₂/SPU-2/Ag. The introduction of an SnO₂ interfacial electron transport layer enhanced electron extraction by improving the energy band alignment with TiO₂, resulting in an increase in photocurrent density from 0.74 to 2.12 mA·cm⁻². The corresponding devices exhibited high open-circuit voltages of 0.89 and 0.75 V, fill factors of 67% and 56%, and power conversion efficiencies of 0.45% and 0.89%, respectively. These results demonstrate that the functionalized polyurethane HTL, combined with a SnO₂/TiO₂ bilayer electron transport layer (ETL), provides an effective strategy to improve charge transport and enhance the photovoltaic performance of CuInS₂-based QDSSCs.