We present a detailed mechanistic investigation of early stage photoelectrochemical deposition of platinum nanoparticles (Pt-NPs) on (PEC) platforms for the hydrogen evolution reaction (HER) using electrochemical, spectroscopic, and microscopy characterization, combined with density functional theory (DFT) modeling. The photoactive junction—formed from a blend of conjugated donor (PTB7-Th) and acceptor (P(NDI2OD-T2)) polymers—exhibits HER efficiencies that increase exponentially with Pt-NP loading (ca. 8.0 x 101 to 2.5 x 104 ng/cm2) of highly faceted Pt-NPs. The blended heterojunction demonstrates vertical phase segregation, with acceptor-rich domains driving photoelectrochemical Pt-NP deposition heterogeneously via progressive nucleation, at sites which we posit are the termini of “fast transport pathways.” X-ray photoelectron spectroscopy (XPS) and DFT modeling suggest that Pt-S interactions involving the bi-thiophene groups in (n-doped) P(NDI2OD-T2) dominate at the earliest stages of Pt deposition, while larger 14- and 38-atom Pt clusters also demonstrate Pt–C and Pt–O=C interactions on naphthalene diimide cores. Collectively, our findings reveal that sub-micron environments in soft photoelectrode materials function as integrated architectures where charge transport and catalytic sites operate in concert. The hierarchical assembly of these nanoenvironments will provide a transformative framework for scalable energy conversion that extends beyond hydrogen evolution to more complex energy carriers, such as multielectron and proton redox reactions.

Hybrid (photo)cathodes consisting of conjugated polymer and hydrogen evolution reaction (HER) cocatalysts are an emerging platform for low-cost solar fuel generation. Poly{[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)}, known as P(NDI2OD-T2) or N2200, is a promising electron accepting material for bulk heterojunction photocathodes. Unlike inorganic (photo)electrodes, much less is known about the energetic alignment of conjugated polymer electrode/metal/electrolyte junctions. In this work, we investigate the electrical doping behavior in an N2200 cathode and its Fermi-level alignment with gold nanoparticles, which is used here as a model for the hydrogen evolution metal cocatalyst. Through UV/visible, Raman, and attenuated total-reflectance infrared spectroelectrochemistry, we observe the impact of electrical doping on the vibrational frequencies of neutral, anion, and dianion species in N2200, which suggests that electron density changes within the corresponding naphthalene-diimide (NDI) units. Upon one-electron reduction, the C═O stretching frequency of the NDI anion unit (polaron) shows a red shift by ∼ 68 cm-1. Additionally, the C═O stretching frequency of neutral units in the doped N2200 shows a minor red shift of ∼ 5 cm-1, suggesting charge transfer from neighboring polaron units. Surface-enhanced Raman spectroscopy measurements of a gold nanoparticle-functionalized N2200 electrode revealed that the Au Fermi level only shifts with that of N2200 upon polaron formation; thus, the formal potential of polymer polaron formation determines the behavior of the catalyst Fermi level, which we posit will modulate reaction capability. This mechanistic study provides a new approach for understanding the nanometer-scale energetics at the conjugated polymer/cocatalyst junction and provides critical insights for the future design of HER (photo)cathodes.

Herein, we demonstrate how Fermi-level pinning at buried contacts impacts solar fuel generation in all-polymer photocathodes by systematically comparing the effects of work function, hydroxyl coverage, and hydrogen evolution using chemically modified indium tin oxide (ITO) supports. Photovoltages and net photocathode performance are improved when the ITO is passivated using phosphonic acids, independent of work function, suggesting that the passivation reduces Fermi-level pinning at the buried interface arising from blended heterojunction interactions with surface metal hydroxyls. Transient photovoltage decay reveals differences in recombination mechanisms, supported by light intensity-dependent measurements. Briefly, nonpassivated, hydrophilic contacts exhibit trap-assisted recombination, while passivated, hydrophobic contacts follow bimolecular recombination. We then investigate changes in electroactivity of hole-transfer processes as a function of scan rate and repetitive cycling using a diffusion-controlled molecular redox probe, analogous to a hole-only device achieved via the electrolyte. The nonpassivated buried contacts exhibit higher overpotentials for oxidation, indicative of hole injection/extraction barriers. We observe irreversible electron transfer via the hole-transport level of the blended heterojunction and a strong cycle dependence, consistent with changes in the hole trap state density. Passivation results in more reversible redox behaviors, consistent with more Ohmic-like contacts. Collectively, these results provide context toward the realization of durable organic photoelectrodes with optimized photovoltages and net solar-to-hydrogen conversion efficiencies via fundamental understanding of the rates of carrier generation, recombination, and transport in high-dielectric aqueous environments and opportunities to characterize buried interfaces under device-relevant electric fields.

We combine fluence-dependent photoluminescence (PL) and electrochemical characterization to estimate defect-dependent trapping rates, energetic distributions, and densities of defects, in lead halide perovskite films Cs0.1FA0.9PbI3 and Cs0.17FA0.83Pb(I0.75Br0.25)3, before and after trap passivation with vapor-deposited (3-aminopropyl)trimethoxysilane (APTMS). For both compositions, the PL studies show that energetically shallow subgap near-valence defect densities lead to bimolecular nonradiative recombination which is reduced by 92% after APTMS treatment. Electrochemical characterization of these same active layers shows for the first time that APTMS passivation impacts mainly on near-valence (mobile anionic) defects that decrease in density by 4 orders of magnitude, from 1018 cm-3 to 1014 cm-3, as a result of silane modification. The combination of PL and electrochemical characterization promises a unique approach to speciation and quantification of a broader distribution of trap states in perovskites and provides straightforward assessments of the efficacy of defect mitigation strategies.

Metal halide perovskite films in the top cell of triple-junction tandems require bandgaps around 2.0 eV to achieve current matching, assuming that the middle absorbing layer is the commonly used FAPbI(3) composition and the bottom cell has a bandgap around 1.1 eV. Unfortunately, mixed organic/inorganic metal halide perovskites that have the necessary Br content to reach a bandgap of 2.0 eV segregate into iodine-rich and bromine-rich phases under illumination, limiting their obtainable voltage. Previous reports have shown improved photostability using either Cs-based inorganic compositions or Cl incorporation on the X-site. Here, we investigate the inorganic triple halide compositional space CsPb(I1-x-yBryClx)(3) where bandgaps near 2.0 eV are expected based on the knowledge that CsPbI2Br has a bandgap of 1.90 eV. Incorporation of Cl occurs readily for x <= 0.07-0.10 within perovskites with a Br content of 0.3 <= y <= 0.42. When x >0.1, X-ray diffraction and photoluminescence (PL) measurements indicate that multiple compositional phases form. We hypothesize that the variable sizes of the three halide ions are not supported within the rigid Cs lattice, resulting in the formation of multiple compositional phases. The photoluminescence quantum yield of the single-phase compositional space & horbar;CsPb(I1-x-yBryClx)(3) where x <= 0.07 & horbar;was typically 0.001-0.004%, most likely as a result of a high defect density, including mobile iodine species. PL light-soaking measurements of many perovskite compositions with bandgaps in the range of 1.89-2.05 eV demonstrate that phase segregation occurs when initial bandgaps are above 1.95 eV regardless of halide content: indicating further iodide oxidation and corresponding migration under illumination. The conclusion is that further compositional or additive engineering is necessary for the development of inorganic triple halide compositions that accomplish the elusive goal of fabricating high-quality and photostable 2.0 eV films for use in multijunction tandems.

Halide perovskite solar cells (PSCs) have emerged as one of the most promising next-generation photovoltaic technologies, owing to high conversion efficiencies, low fabrication costs, and broad material tunability without significant synthetic effort. Early progress directed material screening to boost efficiencies via compositional and morphological optimization. Outstanding challenges include refined control over parasitic loss pathways, such as surface recombination at charge harvesting electrodes, and long-term stability. In this review, we identify critical design criteria of interfaces to promote both performance and durability. Key points of consideration include near-surface chemistry-electronic property connections underpinning charge harvesting selectivity, how defect-mediated recombination at interfaces lowers device performance, and how interfacial instabilities underlie many failure mechanisms in perovskite solar cells. This review surveys current materials strategies for interfacial engineering, including transport layer design and surface passivation, as well as advanced characterization techniques that offer insights into materials surfaces and buried interfaces. By integrating theoretical understanding with empirical approaches, we aim to provide a framework for rational interface design to unlock scalable, stable, and efficient perovskite photovoltaics.

Control over charge injection and extraction processes across buried interfaces is fundamental for all (opto)electronic multilayer device platforms, necessitating detailed understanding of local structural and chemical differences that promote defect formation, distort energetic band-edge alignments, and alter charge transport processes. Herein, the implementation of a low-cost electroanalytical methodologies' tool suite is described to quantitatively characterize buried interfaces and redox reactions in printable, mixed electrical-ionic, and redox-active metal halide perovskites and a prototypical hole-transporting nickel oxide (NiOx) thin film. The objective is to demonstrate the power of electrochemical methodologies to improve the nanoscale understanding of complex interfaces within optoelectronic devices by providing case studies on how to: i) differentiate between electronic and chemical properties in NiOx contacts; ii) measure changes in reversibility of halide redox reactions via NiOx surface states; iii) assess energy alignment and charge transport across (modified) buried interfaces; and iv) quantify defects at buried interfaces that change with modifiers and differences in perovskite processing, including increasing defect concentrations when films are slot-die-coated versus spin-cast. The collective approach addresses major challenges in understanding the precise energy landscape and interface reactivity under relevant electric fields that mimic operando conditions (away from equilibrium) and across length scales in thin film device formats.

The buried interfaces between charge-selective contacts and metal halide perovskites are critical to photovoltaic device performance, and as such, a number of additives are proposed to control and modify energy level alignment. Interface engineering strategies ultimately require the successful detection of near-band energetics as well as the assessment of charge transport processes, including injection and/or extraction barriers under relevant electric fields. Herein, the study utilizes a low-cost, straightforward electroabsorption approach for operando characterization of buried interfaces of NiOx hole-selective layers/metal halide perovskite photoactive layers, complemented with full solar cell device performance. The spectroelectrochemical approach is used to quickly optimize the functionalization process of nickel oxide nanoparticles (NiOx NPs) with 1-ethyl-3-methyl-imidazolium iodide (EMIMI) using ITO/NiOx NPs/perovskite half-stack structures and correlating the transport phenomena with changes in surface chemistry and energetics. The spectroelectrochemical tool proves to be a valuable analytical methodology to study the quality and properties of buried interfaces in perovskite-based (opto)electronic devices, is agnostic with both hole- and electron-selective contacts, and conceivably extended to other semiconductor junctions, as it allows for quantification of surface effects and can predict device performance under operando conditions.

Immersing polymer solar cells in aqueous electrolyte for photoelectrochemical (PEC) hydrogen production is likely to cause photophysical changes that could present both challenges and opportunities for engineering functional and durable devices. Herein we study the bulk heterojunction blend poly(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b ']dithiophene-2,6-diyl)-alt-(2-(((2-ethylhexyl)oxy)carbonyl)-3-fluorothieno[3,4-b]thiophene-4,6-diyl):poly(N,N '-di(2-octyldodecyl)naphthalene-1,8:4,5-bis(dicarboximide)-2,6-diyl)-alt-(2,2-bithiophene-5,5 '-diyl) (PTB7-Th:N2200) excited-state dynamics in electrolyte from femtosecond to millisecond time scales using pump-probe microwave conductivity and absorption spectroscopy. While the blend swells very little, electrolyte exposure increases the microwave-frequency mobility and possibly the yield of photogenerated charges while also decreasing crystallinity. These results indicate an enhancement in key performance metrics, implying that any limitations on the performance of PEC test devices do not arise from active layer-electrolyte interactions. For the PTB7-Th:N2200 blend or similar photocathode systems, our results indicate that improving the interfacial kinetics and/or the carrier lifetime should be prioritized, not protecting the active layer from the electrolyte. Since this observation may not be universal to all polymer systems, future research should focus on identifying their limiting photophysical processes.

Immersing polymer solar cells in aqueous electrolyte for photoelectrochemical (PEC) hydrogen production is likely to cause photophysical changes that could present both challenges and opportunities for engineering functional and durable devices. We present a study of the excited-state dynamics of a bulk heterojunction blend of the polymers PTB7-Th and N2200 in electrolyte from femtosecond to millisecond timescales using microwave conductivity and transient absorption spectroscopy. While the blend swells very little, electrolyte exposure increases the microwave-frequency mobility and possibly the yield of photo-generated charges while also decreasing crystallinity. These results indicate an enhancement in key performance metrics, implying that any limitations on the performance of PEC test devices do not arise from active layer-electrolyte interactions. Future research must therefore focus on improving the interfacial kinetics and/or the carrier lifetime in polymer photocathodes, not protecting the active layer from the electrolyte. Additionally, adding an electron transport layer (ETL) at the interface of a hydrophilic polymer capable of mixed ionic-electronic conduction (MIEC) shelters and stabilizes photogenerated charge carriers, extending their lifetimes and providing a greater opportunity to carry out photoelectrochemical processes.

Semiconductor photoelectrochemistry is a dynamic and interdisciplinary field at the forefront of research in solar fuels, energy conversion, and catalysis. This Perspective captures the collective insights from the second Gerischer Electrochemistry Today Symposium, held at Colorado State University in Fort Collins, CO, in August 2024, which convened leading researchers, early-career scientists, and industry partners to define the critical next steps for the field. Through interactive sessions, technical talks, panel discussions, and training initiatives-including a Semiconductor Electrochemistry Bootcamp-the symposium emphasized three pillars of advancement: (i) facilitating the exchange of new ideas in semiconductor electrochemistry and charge separation; (ii) fostering the development of future researchers, research topics, and participation in the semiconductor workforce; and (iii) building community. This Energy Focus distills key themes from the meeting and identifies major knowledge gaps in the following areas: mechanisms of charge separation and recombination, role of defects and disorder, dynamic and operando characterization methods, interfacial chemistry and surface passivation, theoretical and modeling limitations, and standardization and benchmarking. The inclusive and collaborative structure of the symposium enabled the generation of this comprehensive report that will serve as a roadmap for fundamental and applied research in the rapidly evolving field of semiconductor electrochemistry over the next decade.

Herein, we quantify rates of O2-photoactivated corrosion and recovery processes within triple cation CsFAMAPb(IBr)3 perovskite active layers using dynamic near-ambient pressure X-ray photoemission spectroscopy (NAP-XPS). Activated corrosion is described as iodide oxidation and lead reduction, which occurs only in the presence of both O2 and light through photoinduced electron transfer. We observe electron density reorganization from the Pb-I bonds consistent with ligand exchange, evident from the nonstoichiometric redox change (i.e., <1 e-). Approximately half of the Pb centers are reduced to weakly coordinated Pb-higher oxidation number than metallic Pb-with a rate coefficient of ∼3 (±0.3) × 10-4 atomic percent/s. Hole capture by I- yields I3- and is accompanied by increased concentrations of near-surface bromides, hypothesized to be due to anion vacancies and/or oxidation of mobile iodide resulting from ion demixing. Activated corrosion is found to be quasi-reversible; initial perovskite stoichiometry slowly recovers when the O2/light catalyst is removed, postulated to be due to mobile halide species present within the film below XPS sampling depth. Small deviations in near-surface composition (<2%) of the perovskite are used to connect reaction rates to quantified, near-band edge donor and acceptor defect concentrations, demonstrating two energetically distinct sites are responsible for the redox process. Collectively, environmental flux and rate quantification are deemed critical for the future elucidation of chemical degradation processes in perovskites, where rate-dependent reaction pathways are expected to be very system dependent (environment and material).

Organic electrochemical transistors (OECTs) have attracted significant interest in cutting-edge sensing and bioelectronics. One fundamental approach to amplify signals and improve the performance of the OECT sensing is the use of redox reactions at gate electrodes and/or polymer channels. Faradaic processes increase the electric field drop at the semiconductor channel, which can promote an increased transconductance. However, many biological analytes, including peptides and proteins, are not redox-active and are not immediately amenable to Faradaic OECT biosensing. Herein, we establish a set of systematic redox interface modifications to floating gate (FG) OECTs to enable significantly greater transduction at lower operating voltages over capacitive gating in FG architectures. The proof-of-concept device platform consists of (1) a second FG that operates using mixed monolayers of redox-active species and a selective binding probe and (2) a solid-state OECT element with a redox mediator in an ion gel as the electrolyte to align the energy level for efficient doping and amplification. As a point of validation, a Faradaic electrochemical immunoassay for neuropeptide Y (NPY) is used; the NPY immunoassay is composed of ternary monolayers of redox labels and biorecognition elements, with a 439 pM detection of NPY in artificial sweat. Overall, we successfully demonstrate a power-saving Faradaic FG OECT device in the solid state, which is compatible with easy miniaturization and suitable for printable/wearable sensors and bioelectronics for various applications, from point-of-care diagnostics to healthcare.

Mixed ionic/electronic conducting polymers are versatile systems for, e.g., energy storage, heat management (exploiting electrochromism), and biosensing, all of which require electrochemical doping, i.e., the electrochemical oxidation or reduction of their macromolecular backbones. Electrochemical doping is achieved via electro-injection of charges (i.e., electronic carriers), stabilized via migration of counterions from a supporting electrolyte. Since the choice of the polymer side-chain functionalization influences electrolyte and/or ion sorption and desorption, it in turn affects redox properties, and, thus, electrochemically induced mixed conduction. However, our understanding of how side-chain versus backbone design can increase ion flow while retaining high electronic transport remains limited. Hence, heuristic design approaches have typically been followed. Herein, we consider the redox and swelling behavior of three poly(propylenedioxythiophene) derivatives, P(ProDOT)s, substituted with different side-chain motifs, and demonstrate that passive swelling is controlled by the surface polarity of P(ProDOT) films. In contrast, active swelling under operando conditions (i.e., under an applied bias) is dictated by the local side-chain free volume on the length scale of a monomer unit. Such insights deliver important design criteria toward durable soft electrochemical systems for diverse energy and biosensing platforms and new understanding into electrochemical conditioning ("break-in") in many conducting polymers.

Impedance-based biosensors for microbes, such as yeast and bacteria, have been widely used as an alternative to optical sensing techniques. Yet, impedance-based techniques require complex electronics and data processing that are difficult to implement. Ultrasensitive, drift-correcting electronic devices are needed for biological sensing in complex environments. Organic electrochemical transistors (OECTs) provide high signal amplification at low voltages, but most studies have focused extensively on semiconductor materials within the channels used as amplifiers. This study uses a number of interfacial electrochemical modifications to floating gate OECTs to create a universal platform for biological sensing focused on real-time monitoring of yeast concentrations as a proof-of-concept. Specifically, Y. lipolytica was selected as a model system, as it has been studied extensively for use within the oleochemical industry for production of biofuels and plastics. Real-time monitoring of the presence of these cells is needed for process control and storage in relatively complex environments. Using a combination of interfacial Faradaic mechanisms and reference and sample channel calibrations, a working range of 10(3)-10(6) CFU/mL is established, with a sensitivity of similar to 6% per decade. One key advantage is that the output response using the dual floating gate OECT platform in this device architecture is significantly simpler than the output from electrochemical impedance spectroscopy and can be used as a base platform for future real-time field biosensors.

Semiconductor material optimization requires quantification of performance and stability-dependent near-valence maximum and near-conduction minimum defects with sufficient energy resolution and sensitivity. Herein, we utilize a spectroscopy-electrochemistry approach to resolve the energy-distinct donor and acceptor defect concentrations in wide-gap (Cs(.05)FA(.79)MA(.16))Pb(I.87Br.13)(3) perovskites, benchmarked against photoluminescence and photovoltaic device performance. Monitoring charge transfer events to electron acceptor and donor molecules within solid electrolyte top contacts enables defect quantification below 10(15) cm(-3) at an energy resolution of 10 meV under device-relevant bias, well below levels reported by other methods. Further method sensitivity is demonstrated for defects arising from <2% formamidinium concentration modifications, mimicking compositional imperfections resulting from nonoptimized processing. This method provides the first complete perovskite energetic diagrams with small changes in composition, is nondestructive, compatible with in-line processing characterization, and will enable the semiconductor community to link molecular origins of defects with limitations in device performance across a wide array of optoelectronic platforms.

An azide-modified phosphonic acid can be used to covalently tether polymers to indium tin oxide, preventing delamination and dissolution.