Photoelectrochemical (PEC) water splitting provides a compelling route to sustainable hydrogen production, and organic semiconductors offer unique advantages for this technology through their low cost, tuneable optoelectronic properties and compatibility with scalable, large-area solution processing. However, their application in PEC systems remains limited by modest photocurrent densities, instability in aqueous environments, and poor intrinsic photostability of many bulk-heterojunction (BHJ) blends, which require to be investigated and addressed in order to achieve long-term device operation.

In this talk, I will first present our work using a high-performing PM6:D18:L8-BO BHJ photoanode protected by a multifunctional graphite sheet that is electrically conductive, functionalised by an earth-abundant NiFeOOH catalyst, and readily fabricated using scalable processing. This protective layer prevents water-induced degradation while enabling efficient charge transfer to the catalyst, allowing photocurrent densities exceeding 25 mA cm^–2 at 1.23 V_RHE for water oxidation. Furthermore by integrating two photoactive layers with complementary absorption into a monolithic tandem photoanode, we demonstrate bias-free hydrogen generation with a solar-to-hydrogen (STH) efficiency of 5% [1].

I will then discuss how monolithic organic photoelectrodes can more broadly exploit the excellent optoelectronic properties of polymer:non-fullerene BHJs, while also addressing the remaining limitations of high voltage losses, poor photostability, and the high synthetic complexity typical of many donor–acceptor combinations. To overcome these constraints, we introduce a new BHJ comprising the low-synthetic-complexity polymer PTQ10 and the near-infrared-absorbing acceptor L8-BO. When paired with the NiFeOOH-functionalised graphite sheet, the resulting monolithic photoanodes achieve an onset potential of +0.64 V_RHE, a photocurrent density of 21 mA cm^–2 at +1.23 V_RHE, and a t₈₀ operational stability of 22 h under full AM 1.5G illumination. Compared to our earlier PM6:D18:L8-BO system, this represents a 40 mV increase in photovoltage and a sevenfold improvement in operational stability (t₈₀ extended from 3 h to 22 h). I will show how the superior photochemical and morphological stability of the PTQ10:L8-BO blend underpins these improvements, addressing the key degradation pathways that limit long-term operation. Building on this improved system, I will demonstrate monolithic tandem photoanodes based on PTQ10:IDIC and PTQ10:L8-BO absorbers, achieving bias-free water splitting with a record STH efficiency of 6.2%.

Finally, I will outline key directions for further enhancing both the stability and efficiency of integrated tandem organic photoelectrodes for bias-free photoelectrochemical hydrogen production.