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  • Journal article
    Kosco J, Gonzalez Carrero S, Howells CT, Fei T, Dong Y, Sougrat R, Harrison GT, Firdaus Y, Sheelamanthula R, Purushothaman B, Moruzzi F, Xu W, Zhao L, Basu A, De Wolf S, Anthopoulos TD, Durrant JR, McCulloch Iet al., 2022,

    Generation of long-lived charges in organic semiconductor heterojunction nanoparticles for efficient photocatalytic hydrogen evolution

    , Nature Energy, Vol: 7, Pages: 340-351, ISSN: 2058-7546

    Organic semiconductor photocatalysts for the production of solar fuels are attractive as they can be synthetically tuned to absorb visible light while simultaneously retaining suitable energy levels to drive a range of processes. However, a greater understanding of the photophysics that determines the function of organic semiconductor heterojunction nanoparticles is needed to optimize performance. Here, we show that such materials can intrinsically generate remarkably long-lived reactive charges, enabling them to efficiently drive sacrificial hydrogen evolution. Our optimized hetereojunction photocatalysts comprise the conjugated polymer PM6 matched with Y6 or PCBM electron acceptors, and achieve external quantum efficiencies of 1.0% to 5.0% at 400 to 900 nm and 8.7% to 2.6% at 400 to 700 nm, respectively. Employing transient and operando spectroscopies, we find that the heterojunction structure in these nanoparticles greatly enhances the generation of long-lived charges (millisecond to second timescale) even in the absence of electron/hole scavengers or Pt. Such long-lived reactive charges open potential applications in water-splitting Z-schemes and in driving kinetically slow and technologically desirable oxidations.

  • Journal article
    Du T, Macdonald TJ, Yang RX, Li M, Jiang Z, Mohan L, Xu W, Su Z, Gao X, Whiteley R, Lin C-T, Min G, Haque SA, Durrant JR, Persson KA, McLachlan MA, Briscoe Jet al., 2022,

    Additive-free, low-temperature crystallization of stable alpha-FAPbI(3) perovskite

    , Advanced Materials, Vol: 34, Pages: 1-10, ISSN: 0935-9648

    Formamidinium lead triiodide (FAPbI3) is attractive for photovoltaic devices due to its optimal bandgap at around 1.45 eV and improved thermal stability compared with methylammonium-based perovskites. Crystallization of phase-pure α-FAPbI3 conventionally requires high-temperature thermal annealing at 150 °C whilst the obtained α-FAPbI3 is metastable at room temperature. Here, aerosol-assisted crystallization (AAC) is reported, which converts yellow δ-FAPbI3 into black α-FAPbI3 at only 100 °C using precursor solutions containing only lead iodide and formamidinium iodide with no chemical additives. The obtained α-FAPbI3 exhibits remarkably enhanced stability compared to the 150 °C annealed counterparts, in combination with improvements in film crystallinity and photoluminescence yield. Using X-ray diffraction, X-ray scattering, and density functional theory simulation, it is identified that relaxation of residual tensile strains, achieved through the lower annealing temperature and post-crystallization crystal growth during AAC, is the key factor that facilitates the formation of phase-stable α-FAPbI3. This overcomes the strain-induced lattice expansion that is known to cause the metastability of α-FAPbI3. Accordingly, pure FAPbI3 p–i–n solar cells are reported, facilitated by the low-temperature (≤100 °C) AAC processing, which demonstrates increases of both power conversion efficiency and operational stability compared to devices fabricated using 150 °C annealed films.

  • Journal article
    Lee TH, Rao RR, Pacalaj RA, Wilson AA, Durrant JRet al., 2022,

    A Dual Functional Polymer Interlayer Enables Near-Infrared Absorbing Organic Photoanodes for Solar Water Oxidation

    , ADVANCED ENERGY MATERIALS, ISSN: 1614-6832
  • Journal article
    Du T, Richheimer F, Frohna K, Gasparini N, Mohan L, Min G, Xu W, Macdonald TJ, Yuan H, Ratnasingham SR, Haque S, Castro FA, Durrant JR, Stranks SD, Wood S, McLachlan MA, Briscoe Jet al., 2022,

    Overcoming Nanoscale Inhomogeneities in Thin-Film Perovskites via Exceptional Post-annealing Grain Growth for Enhanced Photodetection

    , NANO LETTERS, Vol: 22, Pages: 979-988, ISSN: 1530-6984
  • Journal article
    Macdonald T, Clancy A, Xu W, Jiang Z, Lin C-T, Mohan L, Du T, Tune D, Lanzetta L, Min G, Webb T, Ashoka A, Pandya R, Tileli V, McLachlan M, Durrant J, Haque S, Howard Cet al., 2021,

    Phosphorene nanoribbon-augmented optoelectronics for enhanced hole extraction

    , Journal of the American Chemical Society, Vol: 143, Pages: 21549-21559, ISSN: 0002-7863

    Phosphorene nanoribbons (PNRs) have been widely predicted to exhibit a range of superlative functional properties, however since they have only recently been isolated, these properties are yet to be shown to translate to improved performance in any application. PNRs show particular promise for optoelectronics, given their predicted high exciton binding energies, tunable band gaps, and ultrahigh hole mobilities. Here, we verify the theorized enhanced hole mobility in both solar cells and space-charge-limited-current devices, demonstrating the potential for PNRs improving hole extraction in universal optoelectronic applications. Specifically, PNRs are demonstrated to act as an effective charge-selective interlayer by enhancing hole extraction from polycrystalline methylammonium lead iodide (MAPbI3) perovskite to the poly(triarylamine) semiconductor. Introducing PNRs at the hole-transport/ MAPbI3 interface achieves fill factors above 0.83 and efficiencies exceeding 21% for planar p-i-n (inverted) perovskite solar cells (PSCs). Such efficiencies are typically only reported in single-crystalline MAPbI3-based inverted PSCs. Methylammonium-free PSCs also benefit from a PNR interlayer, verifying applicability to architectures incorporating mixed perovskite absorber layers. Device photoluminescence and transient absorption spectroscopy are used to demonstrate that the presence of the PNRs drives more effective carrier extraction. Isolation of the PNRs in space-charge-limited-current hole-only devices improves both hole mobility and conductivity; demonstrating applicability beyond PSCs. This work provides primary experimental evidence that the predicted superlative functional properties of PNRs indeed translate to improved optoelectronic performance.

  • Journal article
    Li Y, Xu W, Mussakhanuly N, Cho Y, Bing J, Zheng J, Tang S, Liu Y, Shi G, Liu Z, Zhang Q, Durrant JR, Ma W, Ho-Baillie AWY, Huang Set al., 2021,

    Homologous Bromides Treatment for Improving the Open-Circuit Voltage of Perovskite Solar Cells

    , ADVANCED MATERIALS, Vol: 34, ISSN: 0935-9648
  • Journal article
    Corby S, Rao R, Steier L, Durrant Jet al., 2021,

    The kinetics of metal oxide photoanodesfrom charge generation to catalysis

    , Nature Reviews Materials, Vol: 6, Pages: 1136-1155, ISSN: 2058-8437

    Generating charge carriers with lifetimes long enough to drive catalysis is a critical aspect for both photoelectrochemical and photocatalytic systems and a key determinant of their efficiency. This review addresses the charge carrier dynamics underlying the performance of metal oxides as photoanodes and their ability to drive photoelectrochemical water oxidation, alongside wider comparison with metal oxide function in photocatalytic and electrocatalytic systems. We start by highlighting the disparity between the ps–ns lifetimes of electron and holes photoexcited in bulk metal oxides versus the ms –s timescale of water oxidation catalysis. We go onto review recent literature of the dominant kinetic processes determining photoanode performance, namely charge generation, polaron formation and charge trapping, bulk and surface recombination, charge separation and extraction, and finally the kinetics of water oxidation catalysis. With each topic, we review current understanding and note areas of remaining uncertainty or controversy. We discuss the potential for material selection and examine approaches such as doping, nanostructuring, junction formation and/or co-catalyst deposition to enhance performance. Critically, we examine how such performance enhancements can be understood from analyses of carrier dynamics and propose design guidelines for further material or device optimisation.

  • Journal article
    Bozal-Ginesta C, Rao RR, Mesa CA, Liu X, Hillman SAJ, Stephens IEL, Durrant JRet al., 2021,

    Redox-state kinetics in water-oxidation IrOx electrocatalysts measured by operando spectroelectrochemistry

    , ACS Catalysis, Vol: 11, Pages: 15013-15025, ISSN: 2155-5435

    Hydrous iridium oxides (IrOx) are the best oxygen evolution electrocatalysts available for operation in acidic environments. In this study, we employ time-resolved operando spectroelectrochemistry to investigate the redox-state kinetics of IrOx electrocatalyst films for both water and hydrogen peroxide oxidation. Three different redox species involving Ir3+, Ir3.x+, Ir4+, and Ir4.y+ are identified spectroscopically, and their concentrations are quantified as a function of applied potential. The generation of Ir4.y+ states is found to be the potential-determining step for catalytic water oxidation, while H2O2 oxidation is observed to be driven by the generation of Ir4+ states. The reaction kinetics for water oxidation, determined from the optical signal decays at open circuit, accelerates from ∼20 to <0.5 s with increasing applied potential above 1.3 V versus reversible hydrogen electrode [i.e., turnover frequencies (TOFs) per active Ir state increasing from 0.05 to 2 s–1]. In contrast, the reaction kinetics for H2O2 is found to be almost independent of the applied potential (increasing from 0.1 to 0.3 s–1 over a wider potential window), indicative of a first-order reaction mechanism. These spectroelectrochemical data quantify the increase of both the density of active Ir4.y+ states and the TOFs of these states with applied positive potential, resulting in the observed sharp turn on of catalytic water oxidation current. We reconcile these data with the broader literature while providing a unique kinetic insight into IrOx electrocatalytic reaction mechanisms, indicating a first-order reaction mechanism for H2O2 oxidation driven by Ir4+ states and a higher-order reaction mechanism involving the cooperative interaction of multiple Ir4.y+ states for water oxidation.

  • Journal article
    Kosco J, Gonzalez-Carrero S, Howells CT, Zhang W, Moser M, Sheelamanthula R, Zhao L, Willner B, Hidalgo TC, Faber H, Purushothaman B, Sachs M, Cha H, Sougrat R, Anthopoulos TD, Inal S, Durrant JR, McCulloch Iet al., 2021,

    Oligoethylene glycol side chains increase charge generation in organic semiconductor nanoparticles for enhanced photocatalytic hydrogen evolution

    , Advanced Materials, ISSN: 0935-9648

    Organic semiconductor nanoparticles (NPs) composed of an electron donor/acceptor (D/A) semiconductor blend have recently emerged as an efficient class of hydrogen-evolution photocatalysts. It is demonstrated that using conjugated polymers functionalized with (oligo)ethylene glycol side chains in NP photocatalysts can greatly enhance their H2-evolution efficiency compared to their nonglycolated analogues. The strategy is broadly applicable to a range of structurally diverse conjugated polymers. Transient spectroscopic studies show that glycolation facilitates charge generation even in the absence of a D/A heterojunction, and further suppresses both geminate and nongeminate charge recombination in D/A NPs. This results in a high yield of photogenerated charges with lifetimes long enough to efficiently drive ascorbic acid oxidation, which is correlated with greatly enhanced H2-evolution rates in the glycolated NPs. Glycolation increases the relative permittivity of the semiconductors and facilitates water uptake. Together, these effects may increase the high-frequency relative permittivity inside the NPs sufficiently, to cause the observed suppression of exciton and charge recombination responsible for the high photocatalytic activities of the glycolated NPs.

  • Journal article
    Ling X, Zhu H, Xu W, Liu C, Pan L, Ren D, Yuan J, Larson BW, Gratzel C, Kirmani AR, Ouellette O, Krishna A, Sun J, Zhang C, Li Y, Zakeeruddin SM, Gao J, Liu Y, Durrant JR, Luther JM, Ma W, Gratzel Met al., 2021,

    Combined Precursor Engineering and Grain Anchoring Leading to MA-Free, Phase-Pure, and Stable alpha-Formamidinium Lead Iodide Perovskites for Efficient Solar Cells

    , ANGEWANDTE CHEMIE-INTERNATIONAL EDITION, Vol: 60, Pages: 27299-27306, ISSN: 1433-7851
  • Journal article
    Godin R, Durrant J, 2021,

    Dynamics of photoconversion processes: The energetic cost of lifetime gain in photosynthetic and photovoltaic systems

    , Chemical Society Reviews, Vol: 50, Pages: 13372-13409, ISSN: 0306-0012

    The continued development of solar energy conversion technologies relies on improved understanding of their limitations. In this review, we focus on a comparison of charge carrier dynamics underlying the function of photovoltaic devices with those of both natural and artificial photosynthetic systems. The efficiency of solar energy conversion is the product of the rate of generation of high energy species (charges for solar cells, chemical fuels for photosynthesis) and the energy contained in these species. It is known that the underlying kinetics of the photophysical and charge transfer processes affects the yield of high energy species. Comparatively little attention has been paid to how these kinetics are linked to the energy contained in the high energy species or the energy lost in driving the forward reactions. Here we review the operational parameters of both photovoltaic and photosynthetic systems to highlight the energy cost of extending the lifetime of charge carriers to levels that enable function. We show a strong correlation between the energy lost within the device and the necessary lifetime gain, even when considering natural photosynthesis alongside artificial systems. From consideration of experimental data across all these systems, the emprical energetic cost of each 10 fold increase in lifetime gain is 87 meV. This energetic cost of lifetime gain is approx. 50% greater than the 59 meV predicted from a simple kinetic model. Broadly speaking, photovoltaic devices show smaller energy losses compared to photosynthetic devices due to smaller necessary lifetime gains needed. This is because of faster charge extraction processes in photovoltaic devices compared to the complex multi-electron, multi-proton reactions to produce fuels by photosynthetic devices. The result is that in photosynthetic systems, larger energetic costs are paid to overcome unfavorable kinetic competition between the excited state lifetime and the rate of interfacial reactions. We a

  • Journal article
    Wu J, Cha H, Du T, Dong Y, Xu W, Lin C-T, Durrant JRet al., 2021,

    A Comparison of Charge Carrier Dynamics in Organic and Perovskite Solar Cells

    , ADVANCED MATERIALS, Vol: 34, ISSN: 0935-9648
  • Journal article
    Lin C-T, Xu W, Macdonald T, Ngiam J, Kim J-H, Du T, Xu S, Tuladhar P, Kang H, Lee K, Durrant J, McLachlan Met al., 2021,

    Correlating active layer structure and composition with device performance and lifetime in amino acid modified perovskite solar cells

    , ACS Applied Materials and Interfaces, Vol: 13, Pages: 43505-43515, ISSN: 1944-8244

    Additive engineering is emerging as a powerful strategy to further enhance the performance of perovskite solarcells (PSCs), with the incorporation of bulky cations and amino acid (AA) derivatives being shown as a promisingstrategy for enhanced device stability. However, the incorporation of such additives typically results inphotocurrent losses owing to their saturated carbon backbones hindering charge transport and collection. Herewe investigate the use of amino acids with varying carbon chain lengths as zwitterionic additives that enhancePSC device stability, in air and nitrogen, under illumination. We discover thatstability is insensitive to chain lengthhowever, as anticipated photocurrent drops as chain length increases. Using glycine as an additive results in animprovement in open circuit voltage from 1.10 to 1.14 V and a resulting power conversion efficiency of 20.2%(20.1% stabilized). Using time-of-flight secondary ion mass spectrometry we confirm that the AAs reside at thesurfaces and interfaces of our perovskite films and propose the mechanisms by which stability is enhanced. Wehighlight this with glycine as an additive, whereby an 8-fold increase in device lifetime in ambient air at 1-sunillumination is recorded. Short circuit photoluminescence quenching of complete devices are reported and revealthat the loss in photocurrent density observed with longer carbon chain AAs results from inefficient chargeextraction from the perovskite absorber layer. These combined results demonstrate new fundamentalunderstandings in the photophysical processes of additive engineering using amino acids and provide asignificant step forward in improving the stability of high-performance PSCs.

  • Journal article
    Wang Y, Godin R, Durrant JR, Tang Jet al., 2021,

    Efficient Hole Trapping in Carbon Dot/Oxygen‐Modified Carbon Nitride Heterojunction Photocatalysts for Enhanced Methanol Production from CO <sub>2</sub> under Neutral Conditions

    , Angewandte Chemie, Vol: 133, Pages: 20979-20984, ISSN: 0044-8249
  • Journal article
    Du T, Ratnasingham SR, Kosasih FU, Macdonald T, Mohan L, Augurio A, Ahli H, Lin C-T, Xu S, Xu W, Binions R, Ducati C, Durrant J, Briscoe J, McLachlan Met al., 2021,

    Aerosol assisted solvent treatment: a universal method for performance and stability enhancements in perovskite solar cells

    , Advanced Energy Materials, Vol: 11, ISSN: 1614-6832

    Metal-halide perovskite solar cells (PSCs) have had a transformative impact on the renewable energy landscape since they were first demonstrated just over a decade ago. Outstanding improvements in performance have been demonstrated through structural, compositional, and morphological control of devices, with commercialization now being a reality. Here the authors present an aerosol assisted solvent treatment as a universal method to obtain performance and stability enhancements in PSCs, demonstrating their methodology as a convenient, scalable, and reproducible post-deposition treatment for PSCs. Their results identify improvements in crystallinity and grain size, accompanied by a narrowing in grain size distribution as the underlying physical changes that drive reductions of electronic and ionic defects. These changes lead to prolonged charge-carrier lifetimes and ultimately increased device efficiencies. The versatility of the process is demonstrated for PSCs with thick (>1 µm) active layers, large-areas (>1 cm2) and a variety of device architectures and active layer compositions. This simple post-deposition process is widely transferable across the field of perovskites, thereby improving the future design principles of these materials to develop large-area, stable, and efficient PSCs.

  • Journal article
    Wang Y, Godin R, Durrant JR, Tang Jet al., 2021,

    Efficient Hole Trapping in Carbon Dot/Oxygen-Modified Carbon Nitride Heterojunction Photocatalysts for Enhanced Methanol Production from CO2 under Neutral Conditions

    , ANGEWANDTE CHEMIE-INTERNATIONAL EDITION, Vol: 60, Pages: 20811-20816, ISSN: 1433-7851
  • Journal article
    Adler C, Selim S, Krivtsov I, Li C, Mitoraj D, Dietzek B, Durrant JR, Beranek Ret al., 2021,

    Photodoping and Fast Charge Extraction in Ionic Carbon Nitride Photoanodes

    , ADVANCED FUNCTIONAL MATERIALS, Vol: 31, ISSN: 1616-301X
  • Journal article
    Clarke AJ, Luke J, Meitzner R, Wu J, Wang Y, Lee HKH, Speller EM, Bristow H, Cha H, Newman MJ, Hooper K, Evans A, Gao F, Hoppe H, McCulloch I, Schubert US, Watson TM, Durrant JR, Tsoi WC, Kim J-S, Li Zet al., 2021,

    Non-fullerene acceptor photostability and its impact on organic solar cell lifetime

    , CELL REPORTS PHYSICAL SCIENCE, Vol: 2
  • Journal article
    Murali G, Modigunta JKR, Park S, Lee S, Lee H, Yeon J, Kim H, Park YH, Park SY, Durrant JR, Cha H, An TK, In Iet al., 2021,

    Enhancing Light Absorption and Prolonging Charge Separation in Carbon Quantum Dots via CI-Doping for Visible-Light-Driven Photocharge-Transfer Reactions

    , ACS APPLIED MATERIALS & INTERFACES, Vol: 13, Pages: 34648-34657, ISSN: 1944-8244
  • Journal article
    Chang Y-H, Carron R, Ochoa M, Tiwari AN, Durrant JR, Steier Let al., 2021,

    Impact of RbF and NaF Postdeposition Treatments on Charge Carrier Transport and Recombination in Ga-Graded Cu(In,Ga)Se-2 Solar Cells

    , ADVANCED FUNCTIONAL MATERIALS, Vol: 31, ISSN: 1616-301X
  • Journal article
    Adler C, Selim S, Krivtsov I, Li C, Mitoraj D, Dietzek B, Durrant JR, Beranek Ret al., 2021,

    Photodoping and Fast Charge Extraction in Ionic Carbon Nitride Photoanodes

    <jats:p>Ionic carbon nitrides based on poly(heptazine imides) (PHI) represent a vigorously studied class of materials with possible applications in photocatalysis and energy storage. Herein, we study, for the first time, the photogenerated charge dynamics in highly stable and binder-free PHI photoanodes using <jats:italic>in operando</jats:italic> transient photocurrents and spectroelectrochemical photoinduced absorption measurements. We discover that light-induced accumulation of long-lived trapped electrons within the PHI film leads to effective photodoping of the PHI film, resulting in a significant improvement of photocurrent response due to more efficient electron transport. While photodoping has been previously reported for various semiconductors, it has never been shown before for carbon nitride materials. Furthermore, we find that the extraction kinetics of untrapped electrons are remarkably fast in these PHI photoanodes, with electron extraction times (ms) comparable to those measured for commonly employed metal oxide semiconductors. These results shed light on the excellent performance of PHI photoanodes in alcohol photoreforming, including very negative photocurrent onset, outstanding fill factor, and the possibility to operate under zero-bias conditions. More generally, the here reported photodoping effect and fast electron extraction in PHI photoanodes establish a strong rationale for the use of PHI films in various applications, such as bias-free photoelectrochemistry or photobatteries.</jats:p><jats:p />

  • Journal article
    Dong Y, Cha H, Bristow H, Lee J, Kumar A, McCulloch I, Bakulin A, Durrant Jet al., 2021,

    Correlating charge-transfer state lifetimes with material ener-getics in polymer:non-fullerene acceptor organic solar cells

    , Journal of the American Chemical Society, Vol: 143, Pages: 7599-7603, ISSN: 0002-7863

    Minimizing the energy offset between the lowest exciton and charge-transfer (CT) states is a widely employed strategy to suppress the energy loss (Eg/q – VOC) in polymer:non-fullerene acceptor (NFA) organic solar cells (OSCs). In this work, transient absorption spectroscopy is employed to determine CT state lifetimes in a series of low energy loss polymer:NFA blends. The CT state lifetime is observed to show an inverse energy gap law dependence and decreases as the energy loss is reduced. This behavior is assigned to increased mixing/hybridization between these CT states and shorter-lived singlet excitons of the lower gap component as the energy offset ΔECT-S1 is reduced. This study highlights how achieving longer exciton and CT state lifetimes has the potential for further enhancement of OSC efficiencies.

  • Journal article
    Jones B, Davies KR, Allan MG, Anantharaj S, Mabbett I, Watson T, Durrant JR, Kuehnel MF, Pitchaimuthu Set al., 2021,

    Photoelectrochemical concurrent hydrogen generation and heavy metal recovery from polluted acidic mine water

    , SUSTAINABLE ENERGY & FUELS, Vol: 5, Pages: 3084-3091, ISSN: 2398-4902
  • Journal article
    Bucci A, Garcia-Tecedor M, Corby S, Rao RR, Martin-Diaconescu V, Oropeza FE, de la Pena O'Shea VA, Durrant JR, Gimenez S, Lloret-Fillol Jet al., 2021,

    Self-supported ultra-active NiO-based electrocatalysts for the oxygen evolution reaction by solution combustion

    , JOURNAL OF MATERIALS CHEMISTRY A, Vol: 9, Pages: 12700-12710, ISSN: 2050-7488
  • Journal article
    Kyeong M, Lee J, Daboczi M, Stewart K, Yao H, Cha H, Luke J, Lee K, Durrant JR, Kim J-S, Hong Set al., 2021,

    Organic cathode interfacial materials for non-fullerene organic solar cells

    , JOURNAL OF MATERIALS CHEMISTRY A, Vol: 9, Pages: 13506-13514, ISSN: 2050-7488
  • Journal article
    Francas L, Selim S, Corby S, Lee D, Mesa CA, Pastor E, Choi K-S, Durrant JRet al., 2021,

    Water oxidation kinetics of nanoporous BiVO4 photoanodes functionalised with nickel/iron oxyhydroxide electrocatalysts

    , CHEMICAL SCIENCE, Vol: 12, Pages: 7442-7452, ISSN: 2041-6520
  • Journal article
    Mohapatra AA, Dong Y, Boregowda P, Mohanty A, Sadhanala A, Jiao X, Narayan A, McNeill CR, Durrant JR, Patil Set al., 2021,

    Rational design of donor-acceptor based semiconducting copolymers with high dielectric constants

    , The Journal of Physical Chemistry C: Energy Conversion and Storage, Optical and Electronic Devices, Interfaces, Nanomaterials, and Hard Matter, Vol: 125, Pages: 6886-6896, ISSN: 1932-7447

    The low dielectric constant of organic semiconductors limits the efficiency of organic solar cells (OSCs). In an attempt to improve the dielectric constant of conjugated polymers, we report the synthesis of three semiconducting copolymers by combining the thiophene-substituted diketopyrrolopyrrole (TDPP) monomer with three different monomeric units with varying electron donating/accepting strengths: benzodithiophene (BBT-3TEG-TDPP), TDPP (TDPP-3TEG-TDPP), and naphthalene diimide (P(gNDI-TDPP)). Among the series, BBT-3TEG-TDPP and P(gNDI-TDPP) exhibited the highest dielectric constants (∼5) at 1 MHz frequency, signifying the contribution of dipolar polarization from TEG side-chains. Furthermore, transient absorption spectroscopic studies performed on these polymers indicated low exciton diffusion length as observed in common organic semiconducting polymers. Our findings suggest that utilizing the polar side-chains enhances the dielectric constant in a frequency regime of megahertz. However, it is not sufficient to reduce the Coulombic interaction between hole and electron in excitonic solar cells.

  • Journal article
    Moss B, Wang Q, Butler K, Grau-Crespo R, Selim S, Regoutz A, Hisatomi T, Godin R, Payne D, Kafizas A, Domen K, Steier L, Durrant Jet al., 2021,

    Linking in situ charge accumulation to electronic structure in doped SrTiO3 reveals design principles for hydrogen-evolving photocatalysts

    , Nature Materials, Vol: 20, Pages: 511-517, ISSN: 1476-1122

    Recently, high solar-to-hydrogen efficiencies were demonstrated using La and Rh co-doped SrTiO3 (La,Rh:SrTiO3) incorporated into a low-cost and scalable Z-scheme device, known as a photocatalyst sheet. However, the unique properties that enable La,Rh:SrTiO3 to support this impressive performance are not fully understood. Combining in situ spectroelectrochemical measurements with density functional theory and photoelectron spectroscopy produces a depletion model of Rh:SrTiO3 and La,Rh:SrTiO3 photocatalyst sheets. This reveals remarkable properties, such as deep flatband potentials (+2 V versus the reversible hydrogen electrode) and a Rh oxidation state dependent reorganization of the electronic structure, involving the loss of a vacant Rh 4d mid-gap state. This reorganization enables Rh:SrTiO3 to be reduced by co-doping without compromising the p-type character. In situ time-resolved spectroscopies show that the electronic structure reorganization induced by Rh reduction controls the electron lifetime in photocatalyst sheets. In Rh:SrTiO3, enhanced lifetimes can only be obtained at negative applied potentials, where the complete Z-scheme operates inefficiently. La co-doping fixes Rh in the 3+ state, which results in long-lived photogenerated electrons even at very positive potentials (+1 V versus the reversible hydrogen electrode), in which both components of the complete device operate effectively. This understanding of the role of co-dopants provides a new insight into the design principles for water-splitting devices based on bandgap-engineered metal oxides.

  • Journal article
    Chang Y-H, Carron R, Ochoa M, Bozal-Ginesta C, Tiwari AN, Durrant J, Steier Let al., 2021,

    Insights from transient absorption spectroscopy into electron dynamics along the Ga-gradient in Cu(In,Ga)Se2 solar cells

    , Advanced Energy Materials, Vol: 11, ISSN: 1614-6832

    Cu(In,Ga)Se2 solar cells have markedly increased their efficiency over the last decades currently reaching a record power conversion efficiency of 23.3%. Key aspects to this efficiency progress are the engineered bandgap gradient profile across the absorber depth, along with controlled incorporation of alkali atoms via post‐deposition treatments. Whereas the impact of these treatments on the carrier lifetime has been extensively studied in ungraded Cu(In,Ga)Se2 films, the role of the Ga‐gradient on carrier mobility has been less explored. Here, transient absorption spectroscopy (TAS) is utilized to investigate the impact of the Ga‐gradient profile on charge carrier dynamics. Minority carriers excited in large Cu(In,Ga)Se2 grains with a [Ga]/([Ga]+[In]) ratio between 0.2–0.5 are found to drift‐diffuse across ≈1/3 of the absorber layer to the engineered bandgap minimum within 2 ns, which corresponds to a mobility range of 8.7–58.9 cm2 V−1 s−1. In addition, the recombination times strongly depend on the Ga‐content, ranging from 19.1 ns in the energy minimum to 85 ps in the high Ga‐content region near the Mo‐back contact. An analytical model, as well as drift‐diffusion numerical simulations, fully decouple carrier transport and recombination behaviour in this complex composition‐graded absorber structure, demonstrating the potential of TAS.

  • Journal article
    Limbu S, Park K-B, Wu J, Cha H, Yun S, Lim S-J, Yan H, Luke J, Ryu G, Heo C-J, Kim S, Jin YW, Durrant JR, Kim J-Set al., 2021,

    Identifying the Molecular Origins of High-Performance in Organic Photodetectors Based on Highly Intermixed Bulk Heterojunction Blends

    , ACS NANO, Vol: 15, Pages: 1217-1228, ISSN: 1936-0851

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