Solar energy conversion through photoelectrochemical (PEC) cells represents a promising route for sustainable fuel production, but achieving efficient and stable devices still requires significant advances in materials and interfacial engineering. While inorganic semiconductors such as metal oxides have been widely investigated due to their low cost, abundance, and stability, their performance is often limited by charge recombination, low carrier mobility, and restricted visible-light absorption. To overcome these limitations, our group explores several strategies, including surface modification, cocatalyst incorporation, and, more recently, the integration of organic semiconductors into hybrid photoelectrodes.

Among the different organic materials, conjugated porous polymers (CPPs) have emerged as particularly attractive candidates because they combine light-harvesting capability, charge transport properties, high surface area, and enhanced chemical stability. However, their application in photoelectrochemical systems has been hindered by the difficulty of processing these materials into high-quality thin films, as they are commonly synthesized as micron-sized particles. To address this challenge, we have developed different approaches for integrating CPPs into photoelectrodes, with special emphasis on nanostructuring and electropolymerization techniques.

In this work, two carbazole-based CPPs, p-BCzB and p-TCzB, were synthesized as ultrathin films through electropolymerization and incorporated into Cu₂O photocathodes as multifunctional interfacial layers. Hybrid architectures were prepared by positioning the polymer either beneath or on top of the Cu₂O layer, allowing the influence of interface design on charge-transfer processes to be investigated. All hybrid electrodes exhibited improved photoelectrochemical performance compared with bare Cu₂O, with photocurrent densities increasing by up to a factor of three. The best results were obtained when the polymer was deposited as an ultrathin overlayer, highlighting the importance of nanoscale interfacial engineering.

Electrochemical impedance spectroscopy and transient absorption spectroscopy revealed the multifunctional role of the CPPs within the hybrid structures. When located beneath Cu₂O, the polymers form heterojunctions that promote directional charge separation and facilitate carrier transport. In contrast, when deposited on top of the semiconductor, they act primarily as passivation layers, suppressing surface recombination and extending the lifetime of photogenerated charges. Among the studied materials, p-TCzB exhibited superior performance due to its more extended π-conjugated structure, which enhances charge delocalization and interfacial charge transfer.

These findings demonstrate that CPPs can simultaneously function as light-harvesting, charge-transport, and passivation layers, leading to improved photocurrents, enhanced charge-transfer kinetics, and longer carrier lifetimes. Beyond photoelectrochemical applications, this strategy opens new opportunities for incorporating conjugated porous polymers into a wide range of optoelectronic devices where efficient interfacial charge management and high-quality thin-film fabrication are essential.