Polarons in Heterogeneous Photo(electro)Catalysts

ABSTRACTHeterogeneous photo(electro)catalysis involves sequential steps of photon absorption, charge separation, polaron formation, trapping, bulk and surface recombination, charge extraction, and surface catalysis. Among these, the formation and dynamics of polarons, quasiparticles resulting from strong electron‐lattice interactions, play a pivotal yet often underappreciated role. With ultrafast lifetimes ranging from femtoseconds to picoseconds, polarons are challenging to control, but they crucially influence photon absorption, charge carrier mobility, recombination rates, and catalytic reactivity. Recent advances in time‐resolved spectroscopy, scanning probe microscopy, and theoretical modeling have enabled direct observation and mechanistic interpretation of polaronic states in various photoactive semiconductors. This minireview aims to provide a comprehensive and pedagogical overview of polaron phenomena in heterogeneous photo(electro)catalysts, with a focus on how they affect key material functionalities. Special emphasis is placed on correlating material performance with polaron behavior through state‐of‐the‐art experimental characterization and modeling techniques. By highlighting mechanistic insights and unifying design principles, this minireview aims to guide the rational engineering of semiconductors with tailored polaronic properties for enhanced photo(electro)catalytic performance.

Revealing fundamentals of charge extraction in photovoltaic devices through potentiostatic photoluminescence imaging

In recent years, heterogeneous photocatalysis has emerged as a very appealing approach, not only for the degradation of pollutants, but also for the synthesis of chemicals. Although the main use of heterogeneous photocatalysis so far has been the mineralization and complete degradation of organic compounds, interest in the application of heterogeneous (photo)catalysts in organic synthesis is growing due to their potential application in the fabrication of renewable fuels as well as in the preparation of compounds and intermediates especially valuable to the chemical industry, such as pharmaceuticals or polymers. The synthesis of organic molecules assisted by heterogeneous photocatalysts has been dominated by the use of inorganic metal oxide semiconductors, especially titanium(IV) oxide; the use of other semiconductor materials, such as inorganic chalcogenides, carbon-based semiconductors, or metal–organic frameworks has been less explored. In this chapter we show that, in spite of the potential and the large number of heterogeneous photocatalysts already studied, the state of the art of heterogeneous photocatalysis in organic synthesis is still unsatisfactory and much below expectation, particularly in reactions other than oxidation and reduction, such as cross couplings, oxidative decarboxylations, and cycloadditions.

The relationship between charge carrier lifetime and mobility in a bulk heterojunction based organic solar cell, utilizing diketopyrrolopyrole-naphthalene co-polymer and PC71BM in the photoactive blend layer, is investigated using the photoinduced charge extraction by linearly increasing voltage technique. Light intensity, delay time, and temperature dependent experiments are used to quantify the charge carrier mobility and density as well as the temperature dependence of both. From the saturation of photoinduced current at high laser intensities, it is shown that Langevin-type bimolecular recombination is present in the studied system. The charge carrier lifetime, especially in Langevin systems, is discussed to be an ambiguous and unreliable parameter to determine the performance of organic solar cells, because of the dependence of charge carrier lifetime on charge carrier density, mobility, and type of recombination. It is revealed that the relation between charge mobility (μ) and lifetime (τ) is inversely proportional, where the μτ product is independent of temperature. The results indicate that in photovoltaic systems with Langevin type bimolecular recombination, the strategies to increase the charge lifetime might not be beneficial because of an accompanying reduction in charge carrier mobility. Instead, the focus on non-Langevin mechanisms of recombination is crucial, because this allows an increase in the charge extraction rate by improving the carrier lifetime, density, and mobility simultaneously.

Selective oxidation reactions are crucial in producing various chemicals in the industry. However, most catalytic processes for these reactions use harsh reaction conditions. Recently, heterogeneous photo(electro)catalysis has been considered promising for selective oxidation reactions under mild conditions. Semiconductor materials are frequently used as heterogeneous photocatalysts because they can produce active species that contribute to the oxidation of organic compounds under light irradiation. Among various semiconductor photocatalysts, tungsten oxide (WO3) is one of the most promising candidates due to its advantages, including high stability, wide light adsorption range, proper band structures, and unique catalytic mechanism. This paper reviews the WO3‐based materials for photocatalytic (PC) and photoelectrocatalytic (PEC) selective oxidations. We summarize the strategies for designing WO3 structures, which include morphology control, crystal facet optimization, and defect engineering. Then, the finely designed WO3 structures for PC and PEC oxidation of several typical organic compounds have been discussed based on catalytic mechanism and performance. Finally, we propose the prospects and challenges of WO3 photocatalysts and photoanodes in selective oxidation reactions.

Charge transfer and charge extraction mechanisms are two prevalent issues in the growing field of organic solar cells. Due to their complexity in nature, new methods need to be involved in addressing the fundamental properties associated with organic polymer solar cells. This dissertation has focused on developing a new method to estimate the charge collection lengths and surface recombination lengths of organic polymer solar cells. Photocurrent spectra have been analyzed systematically to observe the dependence on thickness of the active material. A red shift of the peak of the normalized photocurrent with respect to the device thickness has been further analyzed for two major material systems used in organic polymer solar cells, namely MDMO-PPV: PCBM and P3HT: PCBM. A theoretical model that measures the charge extraction of bulk hetero junction solar cell structures has been used taking into account of three main parameters including charge carrier collection length, absorption variation and surface recombination. This model has led to estimate two important parameters associated with charge transfer, recombination and extraction of organic solar cells which will provide opportunities for improvements in the performance of organic electronic devices. Key results are summarized as follows. A complete analysis of photocurrent spectra has been done to see its variation with active material thickness of well-known two material systems of bulk heterojunction organic solar cells. Results of these preliminary measurements suggest that peak of the photocurrent for both systems red shift with increasing thickness. Charge extraction model is introduced to explain the initial red shift of the photocurrent. This model fits well with the experimental results. Further analysis of the model suggests that the charge collection lengths can be estimated for organic polymer structures. Theoretical model gives higher collections lengths for MDMO-PPV solar cells while a lower collection length for P3HT solar cells. This model also has the capability to estimate the surface recombination length of organic bulk heterojunction solar cells. Different interfacial layers have been used to fit to the model calculation. These results suggest that the least surface recombination lengths were achieved with solar cells of PEDOT-PSS. This method can be used to optimize the interfacial layers to improve the efficiency in organic solar cells. AC photocurrent measurements have been carried out to observe the frequency dependence of organic solar cells. Main results show that increasing response time from the light source increases the performance of the solar cells. Further analysis of these

The present work analyses the fundamental transport property of photogenerated carriers named low field mobility for Al0.5Ga0.5N/AlN/Sapphire-based MSM detector using five conventional mobility models. Using Atlas- Silvaco TCAD simulator, appropriate mobility models for differently doped structures were investigated for the least dark current density and improved photocurrent. For each doping case, low electric field mobility models have been compared for I-V characteristics, conduction current density, recombination rate, velocity and mobility of charge carriers. Since there are uncertainties for the most fundamental transport property like field-dependant mobility, therefore more study is required to model mobility for high performance of Wide Band Gap materials. In this work, the mobilities of carriers (mup and mun values) are not specified manually in the Silvaco simulation program however mobility model has been defined. The present work analyses various mobility models on dark current density, photocurrent, mobility and velocity of charge carriers in a photo-absorbent layer. Earlier experimental and theoretical studies have proved that variation in dark current density is greatly affected by doping concentration, structural dimension parameters and optical and electrical properties of detector’s materials. The main objective of the present work is selecting the suitable mobility model to get high Conduction current density in light to Conduction current density in dark ratio for each doping case. We also found that selection of models has a significant impact on spectral response. This simulation work could help align theoretical performance with experimental data and utilising AlGaN MSM detectors in modern UV detection and low noise applications.

Charge trapping and the formation of polarons is a pervasive phenomenon in transition-metal oxide compounds, in particular at the surface, affecting fundamental physical properties and functionalities of the hosting materials. Here we investigate via first-principles techniques the formation and dynamics of small polarons on the reduced surface of titanium dioxide, an archetypal system for polarons, for a wide range of oxygen vacancy concentrations. We report how the essential features of polarons can be adequately accounted for in terms of a few quantities: the local structural and chemical environment, the attractive interaction between negatively charged polarons and positively charged oxygen vacancies, and the spin-dependent polaron-polaron Coulomb repulsion. We combined molecular-dynamics simulations on realistic samples derived from experimental observations with simplified static models, aiming to disentangle the various variables at play. We find that depending on the specific trapping site, different types of polarons can be formed, with distinct orbital symmetries and a different degree of localization and structural distortion. The energetically most stable polaron site is the subsurface Ti atom below the undercoordinated surface Ti atom, due to a small energy cost to distort the lattice and a suitable electrostatic potential. Polaron-polaron repulsion and polaron-vacancy attraction determine the spatial distribution of polarons as well as the energy of the polaronic in-gap state. In the range of experimentally reachable oxygen vacancy concentrations, the calculated data are in excellent agreement with observations, thus validating the overall interpretation.

We present a study of the dynamics of the excitonic magnetic polarons in Cd1−xMnxTe epilayers. The transient photoluminescence measured by a streak camera under resonant excitation is analyzed with a special attention devoted to line shape of the emission signal. A nonradiative spectral region is observed in zero field and is strongly suppressed in a field of 6 T. This observation is understood to be directly related to the alignment of the Mn2+ spins and is therefore closely related to the formation of magnetic polarons. A few Mn spins are found to be aligned already within 10 ps, but the total Mn spin polarization in the sample is found to increase exponentially with a rise time of 37 ps in Cd0.73Mn0.27Te epilayers. Furthermore, the values of 19.2 and 12.4 meV were determined for the energies of the excitonic magnetic polarons in Cd0.73Mn0.27Te and Cd0.79Mn0.21Te epilayers, respectively.

The dynamics of polarons with interaction quadratic in the phonon coordinates is studied. An expression for the charge carrier mobility using the many‐body theory is derived. The temperature dependence of the charge carrier mobility exhibits similar behaviour to that for one‐phonon processes, i.e. the mobility decreases with the temperature. It is found that the higher‐order processes only decrease the value of the mobility but do not change the character of its temperature dependence.

The detection of hot electrons and understanding the correlation between hot electron generation and surface phenomena are challenging questions in the surface science and catalysis community. Hot electron flow generated on a gold thin film by photon absorption (or internal photoemission) appears to be correlated with localized surface plasmon resonance. It has been found that the hot electron flux generated under photon absorption and exothermic chemical reaction is the major mediator of energy conversion process [1-3]. In this talk, I introduce the research direction to attempt to detect the surface plasmon driven hot carrier at the nanometer scale by using scanning probe microscopy. To detect and utilize the hot electron flows at the macroscale level, the metal-semiconductor nanodiodes were constructed. At the nanometer scale, we utilized photoconductive atomic force microscopy to observe photoinduced hot electrons on a triangular Au nanoprism on n-type TiO2 under incident light. This is the direct proof of the intrinsic relation between hot electrons and localized surface plasmon resonance. We observed surface plasmon induced hot hole by using the system of Au nanoprism on p-type GaN [4]. I will discuss the impact of hot carriers in the photocatalytic activity under photoelectrochemical water splitting by using Au-based plasmonic nanostructures [5].

Small polaron formation in transition metal oxides is believed to be a reaction bottleneck in many solar energy conversion applications. While polaron formation has been previously confirmed, the microscopic interaction between a small polaron and its host material is largely unexplored. Here, using femtosecond XUV reflection spectroscopy, we report the evidence of electronic and structural interaction between the small polaron and its host material in CuFeO2, a photoelectrode material for CO2 reduction. Initial small polaron formation is observed as a spectral blue shift occurring within the first 100 fs. After polaron formation, we observe an increased coherent oscillation signal around the polaron sites, which is attributed to polaron-induced optical phonons. This observation suggests that the polaron-associated local lattice distortion can launch optical phonons in neighboring unit cells. In addition to structural coupling, the electronic states in the host materials can also be modified during polaron formation. As an example, we report an increase of Fe oxidation states after photoexcitation. The population of these highly oxidized Fe atoms strongly correlates with polaron dynamics, suggesting that polaron can alter its surrounding electronic states in host materials.

Charge extraction methods are popular for measuring the charge carrier density in thin film organic solar cells and to draw conclusions about the order and coefficient of nongeminate charge recombination. However, results from such studies may be falsified by inhomogeneous steady state carrier profiles or surface recombination. Here, we present a detailed drift-diffusion study of two charge extraction methods, bias-assisted charge extraction (BACE) and time-delayed collection field (TDCF). Simulations are performed over a wide range of the relevant parameters. Our simulations reveal that both charge extraction methods provide reliable information about the recombination order and coefficient if the measurements are performed under appropriate conditions. However, results from BACE measurements may be easily affected by surface recombination, in particular for small active layer thicknesses and low illumination densities. TDCF, on the other hand, is more robust against surface recombination due to its transient nature but also because it allows for a homogeneous high carrier density to be inserted into the active layer. Therefore, TDCF is capable to provide meaningful information on the order and coefficient of recombination even if the model conditions are not exactly fulfilled. We demonstrate this for an only 100 nm thick layer of a highly efficient nonfullerene acceptor (NFA) blend, comprising the donor polymer PM6 and the NFA Y6. TDCF measurements were performed as a function of delay time for different laser fluences and bias conditions. The full set of data could be consistently fitted by a strict second order recombination process, with a bias- and fluence-independent bimolecular recombination coefficient k2 = 1.7 × 10−17 m3 s−1. BACE measurements performed on the very same layer yielded the identical result, despite the very different excitation conditions. This proves that recombination in this blend is mostly through processes in the bulk and that surface recombination is of minor importance despite the small active layer thickness.

Modeling of bias-induced changes of organic field-effect transistor characteristics

¶ Recombination within the bulk and at the surfaces of crystalline silicon has been investigated in this thesis. Special attention has been paid to the surface passivation achievable with plasma enhanced chemical vapour deposited (PECVD) silicon nitride (SiN) films due to their potential for widespread use in silicon solar cells. The passivation obtained with thermally grown silicon oxide (SiO2) layers has also been extensively investigated for comparison. ¶ Injection-level dependent lifetime measurements have been used throughout this thesis to quantify the different recombination rates in silicon. New techniques for interpreting the effective lifetime in terms of device characteristics have been introduced, based on the physical concept of a net photogeneration rate. The converse relationships for determining the effective lifetime from measurements of the open-circuit voltage (Voc) under arbitrary illumination have also been introduced, thus establishing the equivalency of the photoconductance and voltage techniques, both quasi-static and transient, by allowing similar possibilities for all of them. ¶ ...