Monolithic Photoanodes of PTQ10:L8-BO with Improved Stability for Solar Water Oxidation and Enhanced Unassisted Water Splitting

Monolithic photoelectrodes are a promising way to exploit the excellent optoelectronic properties of organic polymer:non-fullerene bulk-heterojunctions (BHJ) for photoelectrochemical applications. However, the high voltage losses, poor photochemical stability and high synthetic complexity of the most commonly used polymer:non-fullerene combinations have limited their full potential. Here, we address these limitations by introducing a BHJ comprising the low-synthetic-complexity polymer PTQ10 and the near-infrared absorbing acceptor L8-BO. By integrating this new BHJ with a graphite sheet functionalised with a NiFeOOH catalyst, we achieve a low onset potential of +0.64 VRHE, a photocurrent density of 21 mA cm−2 at +1.23 VRHE and a t80 operational stability of 22 h under full AM1.5G illumination in monolithic photoanodes for water oxidation. These values represent a 40 mV increase in photovoltage and a sevenfold improvement in operational stability (t80 extended from 3 h to 22 h) compared to the reference system based on the ternary D18:PM6:L8-BO photoactive blend. Spectroscopic analyses reveal that these improvements stem from the reduced non-radiative voltage losses (from 0.24 V to 0.19 V) and superior photochemical and morphological stability of the PTQ10:L8-BO blend compared to the reference blend. Building on these advances, we demonstrate monolithic tandem photoanodes based on PTQ10:IDIC and PTQ10:L8-BO organic blends, achieving a solar-to-hydrogen efficiency of 6.2%. This work offers critical insights for developing stable, high-efficiency organic photoelectrodes for unassisted solar hydrogen generation.