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  • Journal article
    Park SY, Labanti C, Pacalaj RA, Lee TH, Dong Y, Chin Y-C, Luke J, Ryu G, Minami D, Yun S, Park J-I, Fang F, Park K-B, Durrant JR, Kim J-Set al., 2023,

    The state-of-the-art solution-processed single component organic photodetectors achieved by strong quenching of intermolecular emissive state and high quadrupole moment in non-fullerene acceptors

    , Advanced Materials, Vol: 35, ISSN: 0935-9648

    A bulk-heterojunction (BHJ) blend is commonly used as the photoactive layer in organic photodetectors (OPDs) to utilize the donor (D)/acceptor (A) interfacial energetic offset for exciton dissociation. However, this strategy often complicates optimization procedures, raising serious concerns over device processability, reproducibility, and stability. Herein, highly efficient OPDs fabricated with single-component organic semiconductors are demonstrated via solution-processing. The non-fullerene acceptors (NFAs) with strong intrinsic D/A character are used as the photoactive layer, where the emissive intermolecular charge transfer excitonic (CTE) states are formed within <1 ps, and efficient photocurrent generation is achieved via strong quenching of these CTE states by reverse bias. Y6 and IT-4F-based OPDs show excellent OPD performances, low dark current density (≈10-9 A cm-2 ), high responsivity (≥0.15 A W-1 ), high specific detectivity (>1012 Jones), and fast photo-response time (<10 µs), comparable to the state-of-the-art BHJ OPDs. Together with strong CTE state quenching by electric field, these excellent OPD performances are also attributed to the high quadrupole moments of NFA molecules, which can lead to large interfacial energetic offset for efficient CTE dissociation. This work opens a new way to realize efficient OPDs using single-component systems via solution-processing and provides important molecular design rules.

  • Journal article
    Henderson C, Luke J, Bicalho I, Correa L, Yang E, Rimmele M, Demetriou H, Heutz S, Gasparini N, Heeney M, Bagnis D, Kim JSet al., 2023,

    Charge transfer complex formation between organic interlayers drives light-soaking in large area perovskite solar cells

    , Energy and Environmental Science, Vol: 16, Pages: 5891-5903, ISSN: 1754-5692

    Light soaking (LS) is a well-known but poorly understood phenomenon in perovskite solar cells (PSCs) which significantly affects device efficiency and stability. LS is greatly reduced in large-area inverted PSCs when a PC61BM electron transport layer (ETL) is replaced with C60, where the ETL is commonly in contact with a thin bathocuproine (BCP) interlayer. Herein, we identify the key molecular origins of this LS effect using a combination of surface photovoltage, ambient photoemission spectroscopy, Raman spectroscopy, integrated with density functional theory simulations. We find that BCP forms a photoinduced charge-transfer (CT) complex with both C60 and PC61BM. The C60/BCP complex accelerates C60 dimer formation, leading to a favourable cascading energetic landscape for electron extraction and reduced recombination loss. In contrast, the PC61BM/BCP complex suppresses PC61BM dimer formation, meaning that PC61BM dimerisation is not the cause of LS. Instead, it is the slow light-induced formation of the PC61BM/BCP CT complex itself, and the new energetic transport levels associated with it, which cause the much slower and stronger LS effect of PC61BM based PSCs. These findings provide key understanding of photoinduced ETL/BCP interactions and their impact on the LS effect in PSCs.

  • Journal article
    Luke J, Yang EJ, Labanti C, Park SY, Kim J-Set al., 2023,

    Key molecular perspectives for high stability in organic photovoltaics

    , Nature Reviews Materials, ISSN: 2058-8437

    Organic photovoltaics (OPVs) have rapidly improved in efficiency, with single-junction cells now exceeding 18% efficiency. These improvements have been driven by the adoption of new non-fullerene acceptors and the fine tuning of their molecular structures. Although OPVs are highly efficient, they often show extremely poor operational stability, primarily owing to the complex interplay between the morphological instability of the blended bulk heterojunction photoactive layers and the intrinsically poor photostability of the organic semiconductor materials themselves. To realize commercialization, it is vital to understand the degradation mechanisms of these organic materials to improve their stability. Efficiency increases have, in part, been driven by the rational molecular design of materials. In this Perspective, we examine how a similar bottom-up molecular design can be applied to OPV stability. Specifically, we highlight key molecular design parameters and demonstrate how each parameter impacts different degradation pathways. Looking forward, we propose that fundamental understanding of the molecular origin of OPV stability is a key research theme for next-generation OPVs. Additionally, we discuss the tools required, across length scales, to implement these design rules, particularly the use of in situ Raman spectroscopy as a critical bridge linking the molecular scale to the nanoscale and beyond.

  • Journal article
    Wang Y, Daboczi M, Zhang M, Briscoe J, Kim J-S, Yan H, Dunn Set al., 2023,

    Origin of the switchable photocurrent direction in BiFeO<sub>3</sub> thin films

  • Journal article
    Yang M, Cui J, Daboczi M, Law RV, Luke J, Kim J-S, Hankin A, Eslava Set al., 2023,

    Interplay between Collective and Localized Effects of Point Defects on Photoelectrochemical Performance of TiO<sub>2</sub> Photoanodes for Oxygen Evolution

  • Journal article
    Stewart K, Pagano K, Tan E, Daboczi M, Rimmele M, Luke J, Eslava S, Kim J-Set al., 2023,

    Understanding Effects of Alkyl Side-Chain Density on Polaron Formation Via Electrochemical Doping in Thiophene Polymers

  • Journal article
    Motai K, Koishihara N, Narimatsu T, Ohtsu H, Kawano M, Wada Y, Akisawa K, Okuwaki K, Mori T, Kim J-S, Mochizuki Y, Hayamizu Yet al., 2023,

    Correction to "Bifurcated Hydrogen Bonds in a Peptide Crystal Unveiled by X-ray Diffraction and Polarized Raman Spectroscopy".

    , Cryst Growth Des, Vol: 23, ISSN: 1528-7483

    [This corrects the article DOI: 10.1021/acs.cgd.3c00302.].

  • Journal article
    Hart LJF, Gruene J, Liu W, Lau T-K, Luke J, Chin Y-C, Jiang X, Zhang H, Sowood DJC, Unson DML, Kim J-S, Lu X, Zou Y, Gao F, Sperlich A, Dyakonov V, Yuan J, Gillett AJJet al., 2023,

    Understanding the Role of Triplet-Triplet Annihilation in Non-Fullerene Acceptor Organic Solar Cells

    , ADVANCED ENERGY MATERIALS, Vol: 13, ISSN: 1614-6832
  • Journal article
    Yan H, Cong S, Daboczi M, Limbu S, Hamilton I, Kwon S, Rapley CL, Tahir SM, Kerherve G, Payne D, Heeney M, Kim J-Set al., 2023,

    Ionic Density Control of Conjugated Polyelectrolytes via Postpolymerization Modification to Enhance Hole-Blocking Property for Highly Efficient PLEDs with Fast Response Times

  • Journal article
    Jiang Z, Du T, Lin C, Macdonald TJ, Chen J, Chin Y, Xu W, Ding B, Kim J, Durrant JR, Heeney M, McLachlan MAet al., 2023,

    Deciphering the role of hole transport layer HOMO level on the open circuit voltage of perovskite Solar cells

    , Advanced Materials Interfaces, Vol: 10, ISSN: 2196-7350

    With the rapid development of perovskite solar cells, reducing losses in open-circuit voltage (Voc) is a key issue in efforts to further improve device performance. Here it is focused on investigating the correlation between the highest occupied molecular orbital (HOMO) of device hole transport layers (HTLs) and device Voc. To achieve this, structurally similar HTL materials with comparable optical band gaps and doping levels, but distinctly different HOMO levels are employed. Using light-intensity dependent Voc and photoluminescence measurements significant differences in the behavior of devices employing the two HTLs are highlighted. Light-induced increase of quasi-Fermi level splitting (ΔEF) in the perovskite layer results in interfacial quasi-Fermi level bending required to align with the HOMO level of the HTL, resulting in the Voc measured at the contacts being smaller than the ΔEF in the perovskite. It is concluded that minimizing the energetic offset between HTLs and the perovskite active layer is of great importance to reduce non-radiative recombination losses in perovskite solar cells with high Voc values that approach the radiative limit.

  • Journal article
    Jeong S, Rana A, Kim J-H, Qian D, Park K, Jang J-H, Luke J, Kwon S, Kim J, Tuladhar PS, Kim J-S, Lee K, Durrant JR, Kang Het al., 2023,

    New ternary blend strategy based on a vertically self-assembled passivation layer enabling efficient and photostable inverted organic solar cells

    , Advanced Science, Vol: 10, Pages: 1-9, ISSN: 2198-3844

    Herein, a new ternary strategy to fabricate efficient and photostable inverted organic photovoltaics (OPVs) is introduced by combining a bulk heterojunction (BHJ) blend and a fullerene self-assembled monolayer (C60 -SAM). Time-of-flight secondary-ion mass spectrometry - analysis reveals that the ternary blend is vertically phase separated with the C60 -SAM at the bottom and the BHJ on top. The average power conversion efficiency - of OPVs based on the ternary system is improved from 14.9% to 15.6% by C60 -SAM addition, mostly due to increased current density (Jsc ) and fill factor -. It is found that the C60 -SAM encourages the BHJ to make more face-on molecular orientation because grazing incidence wide-angle X-ray scattering - data show an increased face-on/edge-on orientation ratio in the ternary blend. Light-intensity dependent Jsc data and charge carrier lifetime analysis indicate suppressed bimolecular recombination and a longer charge carrier lifetime in the ternary system, resulting in the enhancement of OPV performance. Moreover, it is demonstrated that device photostability in the ternary blend is enhanced due to the vertically self-assembled C60 -SAM that successfully passivates the ZnO surface and protects BHJ layer from the UV-induced photocatalytic reactions of the ZnO. These results suggest a new perspective to improve both performance and photostability of OPVs using a facial ternary method.

  • Journal article
    Mohapatra AA, Pranav M, Yadav S, Gangadharappa C, Wu J, Labanti C, Wolansky J, Benduhn J, Kim J-S, Durrant J, Patil Set al., 2023,

    Interface engineering in perylene diimide-based organic photovoltaics with enhanced photovoltage

    , ACS Applied Materials and Interfaces, Vol: 15, Pages: 25224-25231, ISSN: 1944-8244

    The introduction of nonfullerene acceptors (NFA) facilitated the realization of high-efficiency organic solar cells (OSCs); however, OSCs suffer from relatively large losses in open-circuit voltage (VOC) as compared to inorganic or perovskite solar cells. Further enhancement in power conversion efficiency requires an increase in VOC. In this work, we take advantage of the high dipole moment of twisted perylene-diimide (TPDI) as a nonfullerene acceptor (NFA) to enhance the VOC of OSCs. In multiple bulk heterojunction solar cells incorporating TPDI with three polymer donors (PTB7-Th, PM6 and PBDB-T), we observed a VOC enhancement by modifying the cathode with a polyethylenimine (PEIE) interlayer. We show that the dipolar interaction between the TPDI NFA and PEIE─enhanced by the general tendency of TPDI to form J-aggregates─plays a crucial role in reducing nonradiative voltage losses under a constant radiative limit of VOC. This is aided by comparative studies with PM6:Y6 bulk heterojunction solar cells. We hypothesize that incorporating NFAs with significant dipole moments is a feasible approach to improving the VOC of OSCs.

  • Journal article
    Motai K, Koishihara N, Narimatsu T, Ohtsu H, Kawano M, Wada Y, Akisawa K, Okuwaki K, Mori T, Kim J-S, Mochizuki Y, Hayamizu Yet al., 2023,

    Bifurcated Hydrogen Bonds in a Peptide Crystal Unveiled by X-ray Diffraction and Polarized Raman Spectroscopy

    , CRYSTAL GROWTH & DESIGN, Vol: 23, Pages: 4556-4561, ISSN: 1528-7483
  • Journal article
    Bellchambers P, Henderson C, Abrahamczyk S, Choi S, Lee J, Hatton RAet al., 2023,

    High Performance Transparent Silver Grid Electrodes for Organic Photovoltaics Fabricated by Selective Metal Condensation

    , Advanced Materials, Vol: 35, ISSN: 0935-9648

    <jats:title>Abstract</jats:title><jats:p>Silver grid electrodes on glass and flexible plastic substrates with performance that exceeds that of commercial indium‐tin oxide (ITO) coated glass are reported and show their suitability as a drop‐in replacement for ITO glass in solution‐processed organic photovoltaics (OPVs). When supported on flexible plastic substrates these electrodes are stable toward repeated bending through a small radius of curvature over tens of thousands of cycles. The grid electrodes are fabricated by the unconventional approach of condensation coefficient modulation using a perfluorinated polymer shown to be far superior to the other compounds used for this purpose to date. The very narrow line width and small grid pitch that can be achieved also open the door to the possibility of using grid electrodes in OPVs without a conducting poly(3,4‐ethylenedioxythiophene‐poly(styrenesulfonate) (PEDOT: PSS) layer to span the gaps between grid lines.</jats:p>

  • Journal article
    Lee TH, Fu Y, Chin Y-C, Pacalaj R, Labanti C, Park SY, Dong Y, Cho HW, Kim JY, Minami D, Durrant JR, Kim J-Set al., 2023,

    Molecular orientation-dependent energetic shifts in solution-processed non-fullerene acceptors and their impact on organic photovoltaic performance

    , Nature Communications, Vol: 14, Pages: 1-12, ISSN: 2041-1723

    The non-fullerene acceptors (NFAs) employed in state-of-art organic photovoltaics (OPVs) often exhibit strong quadrupole moments which can strongly impact on material energetics. Herein, we show that changing the orientation of Y6, a prototypical NFA, from face-on to more edge-on by using different processing solvents causes a significant energetic shift of up to 210 meV. The impact of this energetic shift on OPV performance is investigated in both bilayer and bulk-heterojunction (BHJ) devices with PM6 polymer donor. The device electronic bandgap and the rate of non-geminate recombination are found to depend on the Y6 orientation in both bilayer and BHJ devices, attributed to the quadrupole moment-induced band bending. Analogous energetic shifts are also observed in other common polymer/NFA blends, which correlates well with NFA quadrupole moments. This work demonstrates the key impact of NFA quadruple moments and molecular orientation on material energetics and thereby on the efficiency of high-performance OPVs.

  • Journal article
    Wang Y, Luke J, Privitera A, Rolland N, Labanti C, Londi G, Lemaur V, Toolan DTW, Sneyd AJ, Jeong S, Qian D, Olivier Y, Sorace L, Kim J-S, Beljonne D, Li Z, Gillett AJet al., 2023,

    The critical role of the donor polymer in the stability of high-performance non-fullerene acceptor organic solar cells

    , Joule, Vol: 7, Pages: 810-829, ISSN: 2542-4351

    The poor operational stability of non-fullerene electron acceptor (NFA) organic solar cells (OSCs) currently limits their commercial application. While previous studies have primarily focused on the degradation of the NFA component, we also consider here the electron donor material. We examine the stability of three representative donor polymers, PM6, D18, and PTQ10, paired with the benchmark NFA, Y6. After light soaking PM6 and D18 in air, we find an enhanced conversion of singlet excitons into trapped interchain polaron pairs on sub-100 femtosecond timescales. This process outcompetes electron transfer to Y6, significantly reducing the charge generation yield. However, this pathway is absent in PTQ10. We identify twisting in the benzo[1,2-b:4,5-b′]dithiophene (BDT)-thiophene motif shared by PM6 and D18 as the cause. By contrast, PTQ10 does not contain this structural motif and has improved stability. Thus, we show that the donor polymer can be a weak link for OSC stability, which must be addressed collectively with the NFA.

  • Journal article
    Luke J, Jo Y-R, Lin C-T, Hong S, Balamurugan C, Kim J, Park B, Lee K, Durrant JR, Kwon S, Kim B-J, Kim J-Set al., 2022,

    The molecular origin of high performance in ternary organic photovoltaics identified using a combination of in situ structural probes

    , Journal of Materials Chemistry A, Vol: 11, Pages: 1281-1289, ISSN: 2050-7488

    A ternary blend, wherein a tertiary acceptor is incorporated into a donor:non-fullerene acceptor (NFA) binary blend has emerged as a promising strategy for improving power conversion efficiency and stability of organic bulk heterojunction photovoltaics (OPVs). However, the effects of the tertiary component remain elusive due to the complex variation of crystallinity and morphology of donor and acceptor phases during thermal annealing. Herein a combination of in situ transmission electron microscopy and X-ray diffraction spectroscopy utilized during annealing identifies that (1) the addition of the tertiary component (O-IDFBR) delays the glass transition temperature of edge-on-oriented polymer donor (P3HT), prohibits the glass transition of face-on-oriented polymer donor (P3HT), broadens the crystallization temperature of O-IDTBR, and enhances the overall crystallinity of the donor and acceptor phases (P3HT and O-IDTBR), and (2) the ternary component induces homogeneously distributed nanoscale domains rather than a microscale separation between the donor and acceptor as observed in the binary blend. The optimized nanoscale domain morphology, driven by slower crystallization and enhanced overall crystallinity leads to a more stable morphology, resulting in superior device performance and stability.

  • Journal article
    Lee TH, Dong Y, Pacalaj RA, Park SY, Xu W, Kim J-S, Durrant JRet al., 2022,

    Organic Planar Heterojunction Solar Cells and Photodetectors Tailored to the Exciton Diffusion Length Scale of a Non-Fullerene Acceptor

  • Journal article
    Cui J, Daboczi M, Regue M, Chin Y, Pagano K, Zhang J, Isaacs MA, Kerherve G, Mornto A, West J, Gimenez S, Kim J, Eslava Set al., 2022,

    2D bismuthene as a functional interlayer between BiVO4 and NiFeOOH for enhanced oxygen-evolution photoanodes

    , Advanced Functional Materials, Vol: 32, Pages: 1-12, ISSN: 1616-301X

    BiVO4 has attracted wide attention for oxygen-evolution photoanodes in water-splitting photoelectrochemical devices. However, its performance is hampered by electron-hole recombination at surface states. Herein, partially oxidized two-dimensional (2D) bismuthene is developed as an effective, stable, functional interlayer between BiVO4 and the archetypal NiFeOOH co-catalyst. Comprehensive (photo)electrochemical and surface photovoltage characterizations show that NiFeOOH can effectively increase the lifetime of photogenerated holes by passivating hole trap states of BiVO4; however, it is limited in influencing electron trap states related to oxygen vacancies (VO). Loading bismuthene on BiVO4 photoanodes increases the density of VO that are beneficial for the oxygen evolution reaction via the formation of oxy/hydroxyl-based water oxidation intermediates at the surface. Moreover, bismuthene increases interfacial band bending and fills the VO-related electron traps, leading to more efficient charge extraction. With the synergistic interaction of bismuthene and NiFeOOH on BiVO4, this composite photoanode achieves a 5.8-fold increase in photocurrent compared to bare BiVO4 reaching a stable 3.4 (±0.2) mA cm–2 at a low bias of +0.8 VRHE or 4.7(±0.2) mA cm–2 at +1.23 VRHE. The use of 2D bismuthene as functional interlayer provides a new strategy to enhance the performance of photoanodes.

  • Journal article
    Guder F, Alshabouna F, Gonzalez-Macia L, Ji-Seon K, Asfour T, Lee HS, Tan E, nunez-bajo E, Cotur Y, Coatsworth P, Barandun Get al., 2022,

    PEDOT:PSS-modified cotton conductive thread for mass manufacturing of textile-based electrical wearable sensors by computerized embroidery

    , Materials Today, Vol: 59, Pages: 56-67, ISSN: 1369-7021

    The textile industry has advanced processes that allow computerized manufacturing of garments at large volumes with precise visual patterns. The industry, however, is not able to mass fabricate clothes with seamlessly integrated wearable sensors, using its precise methods of fabrication (such as computerized embroidery). This is due to the lack of conductive threads compatible with standard manufacturing methods used in industry. In this work, we report a low-cost poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)-modified cotton conductive thread (PECOTEX) that is compatible with computerized embroidery. The PECOTEX was produced using a crosslinking reaction between PEDOT:PSS and cotton thread using divinyl sulfone as the crosslinker. We extensively characterized and optimized our formulations to create a mechanically robust conductive thread that can be produced in large quantities in a roll-to-roll fashion. Using PECOTEX and a domestic computerized embroidery machine, we produced a series of wearable electrical sensors including a facemask for monitoring breathing, a t-shirt for monitoring heart activity and textile-based gas sensors for monitoring ammonia as technology demonstrators. PECOTEX has the potential to enable mass manufacturing of new classes of low-cost wearable sensors integrated into everyday clothes.

  • Journal article
    Lee J, Luke J, Ahn H, Kim D, Jin C-H, Kim MH, Won YS, Yoon M, Kim J-Set al., 2022,

    Efficient Charge Transport Driven by Strong Intermolecular Interactions in Cyclopentadithiophene-Based Donor-Acceptor Type Conjugated Copolymers

  • Journal article
    Tan E, Kim J, Stewart K, Pitsalidis C, Kwon S, Siemons N, Kim J, Jiang Y, Frost JM, Pearce D, Tyrrell JE, Nelson J, Owens RM, Kim Y-H, Kim J-Set al., 2022,

    The role of long-alkyl-group spacers in glycolated copolymers for high performance organic electrochemical transistors

    , Advanced Materials, Vol: 34, ISSN: 0935-9648

    Semiconducting polymers with oligoethylene glycol sidechains have attracted strong research interest for organic electrochemical transistor (OECT) applications. However, key molecular design rules for high-performance OECTs via efficient mixed electronic/ionic charge transport are still unclear. Herein, we synthesize and characterize new glycolated copolymers (gDPP-TTT and gDPP-TTVTT) with diketopyrrolopyrrole (DPP) acceptor and thiophene-based (TTT or TTVTT) donor units for accumulation mode OECTs, where a long-alkyl-group (C12 ) attached to DPP unit acts as a spacer distancing the oligoethylene glycol from the polymer backbone. gDPP-TTVTT shows the highest OECT transconductance (61.9 S cm-1 ) and high operational stability, compared to gDPP-TTT and their alkylated counterparts. Surprisingly, gDPP-TTVTT also shows high electronic charge mobility in field-effect transistor, suggesting efficient ion injection/diffusion without hindering its efficient electronic charge transport. The elongated donor unit (TTVTT) facilitates the hole polaron formation more localized to the donor unit, leading to faster and easier polaron formation with less impact on polymer structure during OECT operation, as opposed to the TTT unit. This is supported by molecular dynamics (MD) simulation. We conclude that these simultaneously high electronic and ionic charge transport properties are achieved due to the long-alkyl-group spacer in amphipathic sidechains, providing an important molecular design rule for glycolated copolymers. This article is protected by copyright. All rights reserved.

  • Journal article
    Wang B, Nam S, Limbu S, Kim J-S, Riede M, Bradley DDCet al., 2022,

    Properties and Applications of Copper(I) Thiocyanate Hole-Transport Interlayers Processed from Different Solvents

  • Journal article
    Yan H, Wade J, Wan L, Kwon S, Fuchter MJ, Campbell AJ, Kim J-Set al., 2022,

    Enhancing hole carrier injection <i>via</i> low electrochemical doping on circularly polarized polymer light-emitting diodes

    , JOURNAL OF MATERIALS CHEMISTRY C, Vol: 10, Pages: 9512-9520, ISSN: 2050-7526
  • Journal article
    Labanti C, Wu J, Shin J, Limbu S, Yun S, Fang F, Park SY, Heo C-J, Lim Y, Choi T, Kim H-J, Hong H, Choi B, Park K-B, Durrant J, Kim J-Set al., 2022,

    Light-intensity dependent photoresponse time of organic photodetectors and its molecular origin

    , Nature Communications, Vol: 13, Pages: 1-10, ISSN: 2041-1723

    Organic photodetectors (OPDs) exhibit superior spectral responses but slower photoresponse times compared to inorganic counterparts. Herein, we study the light-intensity-dependent OPD photoresponse time with two small-molecule donors (planar MPTA or twisted NP-SA) co-evaporated with C 60 acceptors. MPTA:C60 exhibits the fastest response time at high-lightintensities (>0.5 mW/cm 2), attributed to its planar structure favoring strong intermolecular interactions. However, this blend exhibits the slowest response at low-light intensities, which is correlated with biphasic photocurrent transients indicative of the pr esence of a low density of deep trap states. Optical, structural and en ergetical analyses indicate that MPTA molecular packing is strongly disrupted by C 60, resulting in a larger (370 meV) HOMO level shift. This results in greater energetic inhomogeneity including possible MPTA-C 60 adduct formation, leading to deep trap states which limit the low-light photoresponse time. This work provides important insights into the small molecule design rules critical for low charge-trapping and high-speed OPD applications.

  • Journal article
    Luke J, Yang EJ, Chin Y-C, Che Y, Winkler L, Whatling D, Labanti C, Park SY, Kim J-Set al., 2022,

    Strong intermolecular interactions induced by high quadrupole moments enable excellent photostability of non-fullerene acceptors for organic photovoltaics

    , Advanced Energy Materials, Vol: 12, Pages: 1-11, ISSN: 1614-6832

    Understanding degradation mechanisms of organic photovoltaics (OPVs) is a critical prerequisite for improving device stability. Herein, the effect of molecular structure on the photostability of non-fullerene acceptors (NFAs) is studied by changing end-group substitution of ITIC derivatives: ITIC, ITIC-2F, and ITIC-DM. Using an assay of in situ spectroscopy techniques and molecular simulations, the photodegradation product of ITIC and the rate of product formation are identified, which correlates excellently to reported device stability, with ITIC-2F being the most stable and ITIC-DM the least. The choice of acceptor is found to affect both the donor polymer (PBDB-T) photostability and the morphological stability of the bulk heterojunction blend. Molecular simulations reveal that NFA end-group substitution strongly modulates the electron distribution within the molecule and thus its quadrupole moment. Compared to unsubstituted-ITIC, end-group fluorination results in a stronger, and demethylation a weaker, molecular quadrupole moment. This influences the intermolecular interactions between NFAs and between the NFA and the polymer, which in turn affects the photostability and morphological stability. This hypothesis is further tested on two other high quadrupole acceptors, Y6 and IEICO-4F, which both show impressive photostability. The strong correlation observed between NFA quadrupole moment and photostability opens a new synthetic direction for photostable organic photovoltaic materials.

  • Journal article
    Hamilton I, Suh M, Bailey J, Bradley DDC, Kim J-Set al., 2022,

    Optimizing interfacial energetics for conjugated polyelectrolyte electron injection layers in high efficiency and fast responding polymer light emitting diodes

    , ACS Applied Materials and Interfaces, Vol: 14, Pages: 24668-24680, ISSN: 1944-8244

    Modification of the π-conjugated backbone structure of conjugated polyelectrolytes (CPEs) for use as electron injection layers (EILs) in polymer light emitting diodes (PLEDs) has previously brought conflicted results in the literature in terms of device efficiency and turn-on response time. Herein, we determine the energetics at the CPE and the light emitting polymer (LEP) interface as a key factor for PLED device performance. By varying the conjugated backbone structure of both the LEP and CPE, we control the nature of the CPE/LEP interface in terms of optical energy gap offset, interfacial energy level offset, and location of the electron–hole recombination zone. We use a wide gap CPE with a shallow LUMO (F8im-Br) and one with a smaller gap and deeper LUMO (F8imBT-Br), in combination with three different LEPs. We find that the formation of a type II heterojunction at the CPE/LEP interfaces causes interfacial luminance quenching, which is responsible for poor efficiency in PLED devices. The effect is exacerbated with increased energy level offset from ionic rearrangement and hole accumulation occurring near the CPE/LEP interface. However, a deep CPE LUMO is found to be beneficial for fast current and luminance turn-on times of devices. This work provides important CPE molecular design rules for EIL use, offering progress toward a universal PLED-compatible CPE that can simultaneously deliver high efficiency and fast response times. In particular, engineering the LUMO position to be deep enough for fast device turn-on while avoiding the creation of a large energy level offset at the CPE/LEP interface is shown to be highly desirable.

  • Journal article
    Lee HKH, Stewart K, Hughes D, Barbe J, Pockett A, Kilbride RC, Heasman KC, Wei Z, Watson TM, Carnie MJ, Kim J-S, Tsoi WCet al., 2022,

    Proton radiation hardness of organic photovltaics: an in-depth study

    , Solar RRL, Vol: 6, Pages: 1-10, ISSN: 2367-198X

    Recent developments of solution-processed bulk-heterojunction organic photovoltaic (OPV) cells have demonstrated power conversion efficiencies (PCEs) as high as 18% for single-junction devices. Such a high PCE in addition to its desirable lightweight property and high mechanical flexibility can realize high specific power and small stowed volume, which are key considerations when choosing PV for space missions. To take one important step forward, their resilience to ionizing radiation should be well studied. Herein, the effect of proton irradiation at various fluences on the performance of benchmark OPV cells is explored under AM0 illumination. The remaining device performance is found to decrease with increasing proton fluence, which correlates to changes in electrical and chemical properties of the active layer. By redissolving the devices, the solubility of the active layer is found to decrease with increasing proton fluence, suggesting that the active materials are likely cross-linked. Additionally, Raman studies reveal conformational changes of the polymer leading to a higher degree of energetic disorder. Despite a drop in performance, the retaining percentage of the performance is indeed higher than the current market-dominating space PV technology—III–V semiconductor-based PV, demonstrating a high potential of the OPV cell as a candidate for space applications

  • Journal article
    Xu W, Du T, Sachs M, Macdonald TJ, Min G, Mohan L, Stewart K, Lin C-T, Wu J, Pacalaj R, Haque SA, McLachlan MA, Durrant JRet al., 2022,

    Asymmetric charge carrier transfer and transport in planar lead halide perovskite solar cells

    , Cell Reports Physical Science, Vol: 3, Pages: 1-17, ISSN: 2666-3864

    Understanding charge carrier extraction from the perovskite photoactive layer is critical to optimizing the design of perovskite solar cells. Herein, we demonstrate a simple time-resolved photoluminescence method to characterize the kinetics of charge transport across the bulk perovskite and charge transfer from the perovskite layer to the interlayers, elucidating the dependence of these dynamics on film thickness, grain boundaries (GBs), and interlayers. Using asymmetric laser excitation, we selectively probe charge transport by generating charges both close to and far from the heterojunction interface and correlate these results with device performance. We observe that both film thickness and GBs affect the asymmetry between electron and hole charge transport across the bulk perovskite and charge transfer from the bulk perovskite to the respective interlayers.

  • Journal article
    Yan H, Tseng T-W, Omagari S, Hamilton I, Nakamura T, Vacha M, Kim J-Set al., 2022,

    Dynamic molecular conformational change leading to energy transfer in F8-5% BSP copolymer revealed by single-molecule spectroscopy

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