  Regio-regular poly(3-hexylthiophene-2,5-diyl) (rr-P3HT), the work-horse of
organic photovoltaics, has been recently exploited in bulk heterojunction (BHJ)
configuration with phenyl-C61-butyric acid methyl ester (PCBM) for
solution-processed hydrogen-evolving photocathodes, reaching cathodic
photocurrents at 0 V vs. RHE (J0V vs RHE) of up to 8 mA cm-2. The
photoelectrochemical performance of these photocathodes strongly depends on the
presence of the electron (ESL) and hole (HSL) selective layer. While TiO2 and
its sub-stoichiometric phases are consolidated ESL materials, the currently
used HSLs (e.g., MoO3, CuI, PEDOTT:PSS, WO3) suffer electrochemical degradation
under hydrogen evolution reaction (HER)-working conditions. In this work, we
use solution-processed graphene derivatives as HSL to boost efficiency and
durability of rr-P3HT:PCBM-based photocathodes, demonstrating record-high
performance. In fact, our devices show cathodic J0V vs RHE of 6.01 mA cm-2,
onset potential (Vo) of 0.6 V vs. RHE, ratiometric power-saved efficiency
({\phi}saved) of 1.11% and operational activity of 20 hours in 0.5 M H2SO4
solution. Moreover, the designed photocathodes are effectively working in
different pH environments ranging from acid to basic. This is pivotal for their
exploitation in tandem configurations, where photoanodes operate only in
restricted electrochemical conditions. Furthermore, we demonstrate the
scalability of our all-solution processed approach by fabricating a large-area
(~9 cm2) photocathode on flexible substrate, achieving remarkable cathodic J0V
vs RHE of 2.8 mA cm-2, Vo of 0.45 V vs. RHE and {\phi}saved of 0.31%. This is
the first demonstration of high-efficient rr-P3HT:PCBM flexible photocathodes
based on low-cost and solution-processed manufacturing techniques.
