Long‐Lived Charges in Y6:PM6 Bulk‐Heterojunction Photoanodes with a Polymer Overlayer Improve Photoelectrocatalytic Performance

Photogenerating charges with long lifetimes to drive catalysis is challenging in organic semiconductors. Here, the role of a PM6 polymer overlayer on the photoexcited carrier dynamics is investigated in a Y6:PM6 bulk‐heterojunction (BHJ) photoanode undergoing ascorbic acid oxidation. With the additional polymer layer, the hole lifetime is increased in the solid state BHJ film. When the photoanode is electrically coupled to a hydrogen‐evolving platinum cathode, remarkably long‐lived hole polaron states are observed on the timescale of seconds under operational conditions. It is demonstrated that these long‐lived holes enable the organic photoanode with the polymer overlayer to show enhanced ascorbic acid oxidation performance, reaching ≈7 mA cm−2 at 1.23 VRHE without a co‐catalyst. An external quantum efficiency of 18% is observed using 850 nm excitation. It is proposed that the use of an organic overlayer can be an effective design strategy for generating longer charge carrier lifetimes in organic photoanodes for efficient oxidation catalysis.


Introduction
Photovoltaic (PV) technologies can achieve efficient, cost effective electricity generation from solar irradiation; however an increasing challenge is how to control and store this intermittently DOI: 10.1002/aenm.202300400 generated electricity. Solar-driven fuel synthesis is a key opportunity to make this green electricity a viable and sustainable energy source. [1] Photoelectrochemical (PEC) water splitting, which integrates solar-to-electricity and electricityto-fuel conversion in one semiconductor/electrolyte system, is receiving extensive attention as a possible low cost, scalable path to the solar-driven synthesis of green hydrogen. [2] PEC devices based on organic semiconductors have been attracting extensive attention, [1][2][3] particularly motivated by recent advances power conversion efficiency of organic PV (OPV) devices to 19%. [4] Highly efficient OPV devices have been developed based on bulk heterojunctions (BHJs) of a polymer donor and a small molecular acceptor. These are a promising alternative to the inorganic metal oxide semiconductors typically used in PEC devices, due in particular to their wide absorption range up to the near infrared (NIR), [5] whilst most metal oxides PEC can utilize only nearultraviolet or blue light. [6] Recent efficiency advances in OPVs have been driven by the development of non-fullerene small molecular acceptors, with high performances attributed in particular to their longer exciton lifetimes; [7] however, the nanosecondmicrosecond timescale of charges photogenerated in the BHJ is still much shorter than the millisecond-second timescales of photocatalysis. As such there is a significant kinetic challenge to drive charge transfer to the electrolyte because of short lifetimes of photogenerated charges in organic semiconductors. [8] Significant progress has been made recently in the performance of organic photoelectrodes based on conjugated polymers with the use of co-catalysts or metal overlayers to aid charge separation and drive desirable reduction/oxidation reactions. [1,3,9,10] However, whilst there have been extensive studies of charge carrier dynamics in organic photovoltaics, and more recently studies of such dynamics in polymeric photocatalysts in suspension, [11,12] studies of these dynamics in organic photoelectrodes have been very limited to date.
Here, we investigate how a polymer overlayer can extend the lifetime of the inherently short-lived charges generated by photoexcitation of the organic BHJ to drive an interfacial oxidation reaction. We have previously used a polymer overlayer in an organic photoanode to improve operational stability and suppress accumulated hole recombination. [5] In that study, we optimized organic BHJ-based photoanodes by sandwiching Y6:PM6 BHJ with ZnO nanoparticles and a PM6 overlayer, improving operational stability underwater. [5] After depositing Au/NiFe catalyst on the organic photoanode, a photocurrent density of 4.0 mA cm −2 at 1.23 V versus the reversible hydrogen electrode (1.23 V RHE ) was achieved with 100% Faradaic efficiency. Herein, we demonstrate that the polymer overlayer is able to drive organic oxidation reactions without any additional metal/catalyst overlayers, with the remaining photogenerated electrons being extracted to drive proton reduction. We focus herein on the polaron kinetics in the organic photoanode with and without polymer overlayer, and how this correlate with photoelectrocatalytic performance. Femtosecond transient absorption (TA) measurements indicate that the polymer overlayer increases the yield of long-lived polarons. Furthermore, by operando photoinduced absorption (PIA) spectroscopic analyses, the polymer overlayer is observed to extend the photogenerated charge lifetime to the seconds timescale, correlated with bias dependent photocurrent generation. This longlived charge generation is suggested to be critical to the remarkable organic oxidation photocurrent of 6.6 mA cm −2 coupled to H 2 generation demonstrated from our organic photoanode without metal/metal oxide electrocatalysts.

Results and Discussion
Molecular structures and absorption spectra of PM6 and Y6 are shown in Figure 1a,b. The full names of PM6 and Y6 are given in Supporting Information. PM6 was chosen as a polymer donor and Y6 as a non-fullerene acceptor (NFA), because their blended BHJ is widely used in high-performing photovoltaic devices which have wide absorption coverage from visible to near-infrared (NIR). [12,13] By comparison of the Y6:PM6 photoanode with the PM6 overlayer (denoted as Y6:PM6/PM6) versus a Y6:PM6 only photoanode (Figure 1c), we investigate the function of the PM6 overlayer, focusing on the generation of longlived hole polarons at the semiconductor/liquid junctions (with no added metals or metal oxides) from picoseconds to seconds timescale by using ultrafast TA and slow PIA measurements.
First, the photogeneration of charges in dry solid films were investigated by femtosecond TA spectroscopy measurements using a pump wavelength of 365 nm to preferentially excite PM6. The TA spectra of neat PM6 film exhibits a broad PM6 singlet exciton absorption in the NIR region with maximum at 1150 nm ( Figure  2a). In the TA spectra of Y6:PM6/PM6 and the Y6:PM6 films ( Figure 2c and Figure S1, Supporting Information), the PM6 exciton absorption at 1150 nm is reduced in amplitude and decays more rapidly compared to neat PM6 (Figure 2e), assigned to ultrafast electron/energy transfer from PM6 to Y6, consistent with previous literature on Y6:PM6 BHJ films and nanoparticles. [12,14] Also consistent with this conclusion, steady state photoluminescence (PL) measurements (excited at 532 nm) demonstrate that strong quenching of both PM6 and Y6 excitons in the BHJ ( Figure S2, Supporting Information). It is notable that the presence of the PM6 overlayer results in a similar decay with the Y6:PM6 film (see the decay kinetic of the Y6:PM6/PM6 film in Figure 2e), indicating that the PM6 overlayer does not affect to this ultrafast electron/energy transfer from PM6 to Y6 in the BHJ.
Previous research on similar BHJs have highlighted that energy transfer from PM6 to Y6 (and also to other NFAs), and hole transfer from Y6 to PM6 are the dominant charge generation pathways in such films. [14,15] As such hole transfer kinetics from Y6 to PM6 were also investigated by femtosecond TA spectroscopy using the pump wavelength of 750 nm, which excites Y6 selectively in the heterojunction film (see the absorption of PM6 and Y6 in Figure 1b). The neat PM6 film has negligible ground state bleaching (GSB) signals at the excitation of 750 nm ( Figure  S3a, Supporting Information), while neat Y6 film shows a GSB peak at around 700 nm ( Figure S3b, Supporting Information). The TA spectra of Y6:PM6/PM6 and Y6:PM6 films demonstrate GSB signals from the PM6 absorption region (550-650 nm), indicating ultrafast hole transfer from Y6 to PM6 by Y6 photoexcitation ( Figure S3c,d, Supporting Information). The corresponding carrier dynamics, probed at 580-590 nm, are displayed in Figure  S3e, Supporting Information. Both Y6:PM6/PM6 and Y6:PM6 exhibit hole transfer kinetics that are typical of NFA:polymer blends, [16] with rise half-times for the PM6 GSB signal of ≈2 ps.
When we probe the NIR region with an excitation of 750 nm, the neat Y6 film exhibits a Y6 singlet exciton absorption with maximum at 930 nm ( Figure 2b). In the Y6:PM6/PM6 and the Y6:PM6 film ( Figure 2d and Figure S4, Supporting Information), this absorption feature is broader with shoulders at ≈1000 nm. Corresponding pump fluence-dependent kinetics probed at 980-1000 nm ( Figure S5, Supporting Information) demonstrate fluence dependent kinetics in both heterojunction films, which can be assigned to bimolecular recombination of the photogenerated Scheme 1. Schematic representation of energy transfer (EnT) and electron transfer from PM6 to Y6 in the BHJ (left), hole transfer from Y6 to PM6 and bimolecular recombination in the BHJ (middle), and spatial separation of long-lived charges by the PM6 overlayer on the BHJ (right) polarons in these films. This polaron feature shows a long-lived decay (>6 ns) under low excitation fluence which clearly contrasts with the photoinduced absorption decay kinetics of neat Y6, which decay completely within 100 ps (Figure 2f). It is striking that the Y6:PM6/PM6 exhibits approximately threefold slower bimolecular recombination kinetics and a higher yield of longlived polarons than the Y6:PM6 film without the PM6 overlayer ( Figure 2f and Figure S5, Supporting Information). This indicates that the PM6 overlayer aids the spatial separation of charges even on the picosecond timescale, retarding bimolecular charge recombination compared to the Y6:PM6 BHJ alone, which could be a great advantage to drive photocatalytic reactions.
Overall, charge photogeneration in the organic heterojunction films can be summarized as illustrated in Scheme 1. Efficient PM6 exciton quenching is demonstrated in the Y6:PM6 by ultrafast electron transfer and energy transfer from PM6 to Y6 in <1 ps. Hole transfer from Y6 to PM6 then occurs in ≈2 ps, indicative of fast Y6 exciton separation. The right panel shows that introducing the PM6 overlayer on the organic BHJ photoanode causes an approximately threefold increase in polaron lifetime. A slightly red-shifted ground state absorption in the neat PM6 film compared to the Y6:PM6/PM6 film ( Figure S6, Supporting Information) demonstrates stronger molecular aggregation in the neat PM6 film compared to the blend film, possibly indicating energetic differences between pristine PM6 and PM6 in the blends. [17] The smaller bandgap of neat PM6 could facilitate hole extraction from the Y6:PM6 BHJ to the PM6 overlayer. Energy level alignment of polymer overlayers between BHJs and desired reduction/oxidation reactions would be a key point for the future PEC cell development. This spatial charge separation of holes by a polymer overlayer, which retards bimolecular recombination compared to the Y6:PM6 BHJ, can be expected to aid PEC function.
To understand the charge kinetics in the presence of electrolytes on longer timescales, our organic photoanodes were investigated by using photoinduced absorption (PIA) spectroscopy under operando PEC conditions using 5 s duration pulsed LED excitation. [18] PIA kinetics on the second timescales were collected in the presence/absence of ascorbic acid (AA) and applied bias (1.23 V RHE ). A three-electrode configuration was used: an Ag/AgCl reference electrode, a Pt mesh counter electrode, and the organic photoanode as the working electrode were prepared in 1 m borate buffer (pH 8.1) without or with 0.2 m AA (pH 5.5). The absorbance differences were measured at different probe wavelengths after the electrochemical cell was irradiated with quasi-continuous light-emitting diode (LED) pulses (40 mW cm −2 at 356 nm, 5 s on/off) through the glass/indium tin oxide.
In the case where no external bias is applied to the PEC cells ( Figures S7-S9, Supporting Information), negligible PIA signals (<0.25 mOD) are recorded from the Y6:PM6/PM6 irrespective of the presence of AA or Na 2 SO 3 . This is consistent with charge recombination occurring on the nanoseconds timescale in these materials, as discussed in the preceding paragraph ( Figure 2 and Scheme 1). Even under 1 sun illumination, which is more relevant than the ultrafast TA results from the high laser pulses, fast (microsecond) bimolecular recombination has been demonstrated in Y6:PM6 solar cells. [19] A small, negative PIA signal was observed in Y6:PM6 electrodes ( Figure S7b, Supporting Information), how this did not recover on the timescales measured, suggesting it derives most likely from the partial dissolution of the organic thin film into the electrolyte. This signal was negligible for the Y6:PM6/PM6 photoelectrode, indicating the PM6 overlayer enhances electrode stability, consistent with our previous study. [5] Under a bias of 1.23 V RHE (i.e., driving electron extraction from the organic photoanode), PIA spectra of our photoanodes at 2 s time delay after LED off are shown in Figure 3a,b. Corresponding spectra at different time delays are displayed in Figure S10a,b, Supporting Information. Both Y6:PM6/PM6 and Y6:PM6 photoanodes demonstrated strikingly long-lived, second-timescale PIA signals (probed at 950 nm) in the absence a sacrificial hole scavenger, AA (Figure 3c). These signals under the applied bias can be assigned as the long-lived PM6 + polaron features. [12] The PM6 + polaron absorption in the Y6:PM6/PM6 exhibited a longer lifetime and a higher PIA amplitude than in the Y6:PM6, indicating that the PM6 overlayer increases the accumulation of longlived photogenerated charges. This is in good agreement with the ultrafast TA data shown in Figure 2. PIA kinetics of the photoanodes under bias without AA probed from 600 to 1000 nm are exhibited in Figure S10c,d, Supporting Information.
Our observation of the photogeneration of PM6 + polarons with seconds timescale decay dynamics is remarkable, and orders of magnitude longer than charge carrier lifetimes in equivalent solar cell devices. There have been no reports of such long lived charge accumulation in organic photoelectrodes previously. It is analogous to similarly remarkable long lived polarons formed in PM6/PCBM nanoparticle photocatalysts. [12] Such long lived charge accumulation has previously been reported to be critical to the performance of metal oxide photoanodes. In metal oxides it is enabled by space charge layer formation at the semiconductor/electrolyte interface. The mechanistic origin of such long carrier lifetimes in our organic photoelectrodes is not yet established, although it is clearly enhanced by the PM6 overlayer, emphasizing the functional importance of this layer.
After incorporating 0.2 m AA in 1 m borate buffer, negligible PIA signals are recorded from both Y6:PM6/PM6 and Y6:PM6 (Figure 3d and Figure S11, Supporting Information), indicating long-lived PM6 + polarons on organic photoanodes are efficiently quenched by the sacrificial hole scavenger AA in the electrolyte. Analogous PIA signals with lower amplitude are also observed when using 0.2 m Na 2 SO 3 (an alternative hole scavenger) in 1 m borate buffer ( Figure S12, Supporting Information).
Photoelectrochemical performance of the Y6:PM6/PM6 and the Y6:PM6 photoanodes were tested with the same threeelectrode configuration as used in the PIA measurements with the electrolyte of 0.2 m AA in 1 m borate buffer (Figure 4a). Linear sweep voltammetry (LSV) scans of the Y6:PM6/PM6 and the Y6:PM6 under chopped 1 sun illumination are shown in Figure 4b. At 1.23 V RHE , the Y6:PM6/PM6 photoanode achieved a photocurrent density (J ph ) of 6.6 mA cm −2 , while the Y6:PM6 photoanode showed a J ph of 5.0 mA cm −2 . The remarkable J ph value of 6.6 mA cm −2 is a striking demonstration that thin-film photoanodes based on organic light absorbers can efficiently catalyze reactions (facile AA oxidation) without metal or metal oxide catalysts. It clearly demonstrates that long-lived polarons by the PM6 overlayer improves the photoanode performance and coupled hydrogen evolution (see the digital photograph of the PEC cell operation in Figure S13a, Supporting Information). The Faradaic efficiency for coupled H 2 evolution of the Y6:PM6/PM6 photoanode at 1.23 V RHE in 0.2 m AA in 1 m borate buffer under continuous 1 sun illumination is around 75% (Figure S13b, Supporting Information), indicating that a quarter of the photocurrent was used for undesired reactions in this PEC device. These photocurrents are higher (even after adjusting for lower the Faradaic efficiency) than that achieved by the Y6:PM6/PM6 photoanode from our previous study (4 mA cm −2 ), in which an Au/NiFe electrocatalyst top layer was used to catalyze water oxidation. [5] LSVs were also scanned with the electrolyte of 0.2 m Na 2 SO 3 in 1 m borate buffer (pH 8.1) and 0.5 m H 2 O 2 in 0.5 m NaOH (pH 13) for the Y6:PM6/PM6 and the Y6:PM6 photoanodes ( Figure S14, Supporting Information). Analogous to the result with 0.2 m AA (Figure 4b), the Y6:PM6/PM6 photoanode showed a higher J ph than the Y6:PM6 photoanode, indicating that the elongated carrier lifetime by the PM6 overlayer improves the oxidation performance of the organic photoanode with various hole sacrificial agents. The main reason of the higher photocurrent in AA oxidation originates from the higher oxidation potential of approximately −4.7 eV, [12] compared to the SO3 2− oxidation potential of approximately −5.2 eV, [3] indicating that the PM6 highest occupied molecular orbital (approximately −5.4 eV) has a suitable potential to oxidize AA.
We further compared the dependence of the Y6:PM6/PM6 photoanode performance on the thickness of the PM6 overlayer by controlling the concentration of PM6 solution (the thickness of 15, 30, and 45 nm by the concentration of 10, 15, and 20 mg mL −1 , respectively; see LSV in Figure S15, Supporting Information). By increasing the thickness from 15 to 30 nm, the photocurrent at 1.23 V RHE was almost identical; however, the photocurrent was drastically decreased to ≈1.5 mA cm −2 when the thickness increases to 45 nm, indicative of limited conductivity in the PM6 film. Overall, the highest J ph of 6.6 mA cm −2 was achieved with the PM6 thickness of 30 nm. Interestingly, the onset potential was reduced from 0.4 to 0.2 V RHE by decreasing the PM6 thickness from 45 to 15 nm, which is in line with the reduction of the onset potential by introducing the PM6 overlayer on the Y6:PM6 photoanode in Figure 4b. This indicates that to transfer photogenerated charges in the BHJ to the surface of the PM6 overlayer, a higher applied bias is required with thicker PM6 overlayer. Thus, the thickness of the polymer overlayer should be optimized in order to do not over the carrier diffusion limits of such organic semiconductors.
The external quantum efficiency (EQE) spectrum of the Y6:PM6/PM6 photoanode was measured in 0.2 m AA in 1 m borate buffer (pH 5.5) at 1.23 V RHE (Figure 4c). The EQE spectrum demonstrates photo-responses up to 900 nm, matching the Y6 ground state absorption (Figure 1b). This confirms that the measured J ph originates from light absorption by the organic photoactive layer across the entire visible spectrum. The maximum EQE of 17.8% was measured under 850 nm illumination, indicating that Y6 molecules were able to play a key role for photon-to-current conversion in this anode architecture. Due to the PM6 overlayer on the Y6:PM6 film, the maximum absorption is at around 600-650 nm where PM6 absorbs. However, the EQE at the PM6 absorption region was lower than the EQE at the Y6 absorption region (700-900 nm). This indicates that the primary function of the PM6 overlayer is as an interlayer in between the Y6:PM6 and the electrolyte, improving carrier lifetimes, rather than as a light sensitizer. The maximum EQE of Y6:PM6 nanoparticles (with 10% Pt) from our previous study was 5.0% in the 0.2 m AA solution, [12] suggesting that under applied bias the photoelectrode configuration with the polymer overlayer is able to achieve higher efficiencies than the photocatalytic nanoparticles.
To test the operational stability, chronoamperometry was measured in 0.2 m AA in 1 m borate buffer at 1.23 V RHE with 1 sun illumination ( Figure S16, Supporting Information). In the presence of the PM6 overlayer, the photocurrent stabilized at 40% of the initial photocurrent after 2500 s without any encapsulation layer (direct contacts between the organic semiconductor and the electrolyte). However, in the absence of the PM6 overlayer the photocurrent decays to <20% of the initial photocurrent within 1000 s. Here we show again that the PM6 overlayer functions to improve the underwater stability by its hydrophobic nature, consistent with our previous observations, but in this case without any further overlayer. [5] In our Y6:PM6/PM6 photoanodes, long-lived polaron signals can be observed only under applied bias to the Pt counter electrode. Hydrophobic, organic thin films are limited in their contact with water (electrolyte), so organic/electrolyte contact area is only given at the top film surface area. The bias-dependent photocurrent generation in Figure 4b indicates that an electrical driving force is required to enable polaron accumulation on the electrode surface where the photocatalytic reaction occurs in this thin film-based electrode system. This contrasts with our previous study on Y6:PM6 nanoparticles, where long-lived hole polarons were observed in the absence of applied bias. [12] It is possible that Y6:PM6 nanoparticle suspensions can allow more water penetration into the organic semiconductor, facilitating the generation of long-lived charges through greater dielectric screening, although further work is required to quantitatively compare the impact of electrolyte exposure on the charge carrier dynamics and performance of organic nanoparticles and photoelectrodes, also with controlling hydrophilicity of the organic surfaces.
Buried organic BHJ photoanodes, which are a combination of OPV and water oxidation electrocatalysis, have been reported, while require encapsulation to protect the OPV from the water (electrolyte) and a metal/metal oxide interlayer to transfer photogenerated charges to drive electrocatalysis. [1,5,9] Such systems may have only limited cost advantage over separated photovoltaic-electrolyser cells in terms of hydrogen generation cost. Organic-or carbon nitride-based photoanodes without cocatalysts have also been reported, but the photocurrent was around 0.1 mA cm −2 .
[20] In our Y6:PM6/PM6 photoanode, owing to the long-lived charges generated in the presence of the PM6 overlayer, the high photocurrent of 6.6 mA cm −2 and the EQE of 17.8% with concurrent hydrogen evolution is achieved by a direct organic/electrolyte junction. However, it is driven by electric bias and a facile oxidation of AA. Key challenges remain to control the hydrophilicity and energetics of the polymer surface to reduce onset potential and achieve direct water oxidation for efficient water splitting.

Conclusion
In summary, we demonstrate an NIR-absorbing Y6:PM6/PM6 organic BHJ photoanode which can efficiently carry out organic oxidation reactions. By introducing a PM6 overlayer between the BHJ and the electrolytes, the photocurrent of the photoanode increased from 5.0 to 6.6 mA cm −2 at 1.23 V RHE when oxidizing ascorbic acid without any additional metal or metal oxide catalyst layers. The broadband absorption of the Y6:PM6 BHJ is critical to device performance, with the highest EQE ≈18% measured under 850 nm illumination.
Femtosecond TA measurements showed that the lifetime of photoexcited PM6 polarons in the solid organic BHJ film increased from ≈2 ns to >6 ns when the PM6 overlayer was added. PIA kinetics of the corresponding PM6 polarons exhibited remarkably long lifetimes on the timescale of seconds under PEC reaction conditions, with a longer lifetime and greater accumulation being observed in the presence of the PM6 overlayer. These data suggest that the density of long-lived polarons is crucial for efficient photoanode performance, and that the PM6 overlayer increases the lifetime of PM6 polarons by increasing the spatial separation compared to the Y6:PM6 BHJ. To demonstrate the tandem PEC configuration with photocathodes for efficient green hydrogen production, design strategies of the efficient photoanodes should be required for concurrent oxidation reaction. This work demonstrates that future organic photoelectrode design should consider using a polymer overlayer technique to elon-gate the lifetime of photogenerated charges and enhance photoelectrode stability. Considering that these organic semiconductors have been designed to optimize photovoltaic performance, we believe the photocatalytic performance of solution-processed organic semiconductors can be improved further by using new molecular designs which are more fitted to the photoelectrochemical reactions.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.