198 results found
Motai K, Koishihara N, Narimatsu T, et 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.].
Park SY, Labanti C, Pacalaj RA, et 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., Adv Mater
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, we demonstrate highly efficient OPDs fabricated with single-component organic semiconductors 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. Our work opens a new way to realize efficient OPDs using single-component systems via solution-processing and provides important molecular design rules. This article is protected by copyright. All rights reserved.
Stewart K, Pagano K, Tan E, et al., 2023, Understanding Effects of Alkyl Side Chain Density on Polaron Formation via Electrochemical Doping in Thiophene Polymers., Adv Mater
Polarons exist when charges are injected into organic semiconductors due to their strong coupling with the lattice phonons, significantly affecting electronic charge transport properties. Understanding the formation and (de)localisation of polarons is therefore critical for further developing organic semiconductors as a future electronics platform. However, there have been very few studies reported in this area. In particular, there has been no direct in situ monitoring of polaron formation and identification of its dependence on molecular structure and impact on electrical properties, limiting further advancement in organic electronics. Herein we demonstrate how a minor modification of side chain density in thiophene-based conjugated polymers affects the polaron formation via electrochemical doping, changing the polymers' electrical response to the surrounding dielectric environment for gas sensing. We find that the reduction in side chain density results in a multistep polaron formation, leading to an initial formation of localised polarons in thiophene units without side chains. Reduced side chain density also allows the formation of a high density of polarons with less polymer structural changes. More numerous but more localised polarons generate a stronger analyte response but without the selectivity between polar and non-polar solvents, which is different from the more delocalised polarons that show clear selectivity. Our results provide important molecular understanding and design rules for the polaron formation and its impact on electrical properties. This article is protected by copyright. All rights reserved.
Jiang Z, Du T, Lin C, et 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.
Jeong S, Rana A, Kim J-H, et 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.
Motai K, Koishihara N, Narimatsu T, et al., 2023, Bifurcated Hydrogen Bonds in a Peptide Crystal Unveiled by X-ray Diffraction and Polarized Raman Spectroscopy, Crystal Growth and Design, Vol: 23, Pages: 4556-4561, ISSN: 1528-7483
The strength of hydrogen bonds in molecular crystals has been investigated through X-ray structural analysis and vibrational spectroscopy, and the interatomic distances and the stretching vibrational frequency involving hydrogen bonds have been known to exhibit a linear relationship. However, these studies are limited to relatively small molecular compounds, such as amino acids. In this study, we employed tetrapeptide as a model system to investigate the hydrogen bond network in the peptide crystal. Single crystal X-ray diffraction revealed that the peptide crystal structure had bifurcated hydrogen bonds. Raman spectroscopy exhibited that the vibrational frequency of amide bonds correlated linearly to their interatomic distance, but the rate of the change was significantly low compared with previous works. Fragment molecular orbital calculations also revealed that bifurcated hydrogen bonds affect the strength of the hydrogen bonds. This study provides a valuable model system for discussing the effects of bifurcated hydrogen bonds in various crystals of peptides.
Mohapatra AA, Pranav M, Yadav S, et al., 2023, Interface Engineering in Perylene Diimide-Based Organic Photovoltaics with Enhanced Photovoltage., ACS Appl Mater Interfaces, Vol: 15, Pages: 25224-25231
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.
Lee TH, Fu Y, Chin Y-C, et 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.
Wang Y, Luke J, Privitera A, et 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.
Yang M, Cui J, Daboczi M, et al., 2023, Interplay between Collective and Localized Effects of Point Defects on Photoelectrochemical Performance of TiO<inf>2</inf> Photoanodes for Oxygen Evolution, Advanced Materials Interfaces
Among the various photoanode materials investigated for photoelectrochemical water splitting cells, TiO2 stands out due to its abundance, stability, and favorable valence band edge for water oxidation. In this study, the importance of introducing and combining oxygen and titanium vacancy point defects in anatase TiO2 photoanodes to improve their performance is unveiled, achieving a photocurrent density of 0.73 (±0.015) mA cm−2 at +1.23 VRHE under 100 mW cm−2 of simulated sunlight or 26.4 mA cm−2 at +1.23 VRHE under 100 mW cm−2 of 365 nm light. The characterization by X-ray photoelectron spectroscopy, surface photovoltage, and electron paramagnetic resonance demonstrates that these oxygen and titanium vacancies can have both collective and localized positive effects on the material, leading to a narrowing of the bandgap, an increase in donor density, and an increase in hydroxyl groups on the surface of TiO2. These result in enhanced light absorption, conductivity, and photovoltage, as well as a more negative flat-band potential and increase in hole flux to the semiconductor–electrolyte interface. These findings provide valuable insights into the role of point defects in modulating the properties of TiO2 and have important implications for the development of high-performance TiO2-based devices.
Luke J, Jo Y-R, Lin C-T, et 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.
Cui J, Daboczi M, Regue M, et 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.
Lee TH, Dong Y, Pacalaj RA, et al., 2022, Organic Planar Heterojunction Solar Cells and Photodetectors Tailored to the Exciton Diffusion Length Scale of a Non-Fullerene Acceptor, ADVANCED FUNCTIONAL MATERIALS, Vol: 32, ISSN: 1616-301X
Guder F, Alshabouna F, Gonzalez-Macia L, et 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.
Tan E, Kim J, Stewart K, et 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.
Labanti C, Wu J, Shin J, et 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.
Luke J, Yang EJ, Chin Y-C, et 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.
Yan H, Wade J, Wan L, et al., 2022, Enhancing hole carrier injection via low electrochemical doping on circularly polarized polymer light-emitting diodes, JOURNAL OF MATERIALS CHEMISTRY C, Vol: 10, Pages: 9512-9520, ISSN: 2050-7526
Hamilton I, Suh M, Bailey J, et 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.
Lee HKH, Stewart K, Hughes D, et 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
Lee J, Luke J, Ahn H, et al., 2022, Efficient Charge Transport Driven by Strong Intermolecular Interactions in Cyclopentadithiophene-Based Donor-Acceptor Type Conjugated Copolymers, ADVANCED ELECTRONIC MATERIALS, Vol: 8, ISSN: 2199-160X
Wang B, Nam S, Limbu S, et al., 2022, Properties and Applications of Copper(I) Thiocyanate Hole-Transport Interlayers Processed from Different Solvents, ADVANCED ELECTRONIC MATERIALS, Vol: 8, ISSN: 2199-160X
Yan H, Tseng T-W, Omagari S, et al., 2022, Dynamic molecular conformational change leading to energy transfer in F8-5% BSP copolymer revealed by single-molecule spectroscopy, JOURNAL OF CHEMICAL PHYSICS, Vol: 156, ISSN: 0021-9606
Marin-Beloqui J, Zhang G, Guo J, et al., 2022, Insight into the origin of trapping in polymer/fullerene blends with a systematic alteration of the fullerene to higher adducts, The Journal of Physical Chemistry C: Energy Conversion and Storage, Optical and Electronic Devices, Interfaces, Nanomaterials, and Hard Matter, Vol: 126, Pages: 2708-2719, ISSN: 1932-7447
The bimolecular recombination characteristics of conjugated polymer poly[(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(2,5-bis 3-tetradecylthiophen-2-yl thiazolo 5,4-d thiazole)-2,5diyl] (PDTSiTTz) blended with the fullerene series PC60BM, ICMA, ICBA, and ICTA have been investigated using microsecond and femtosecond transient absorption spectroscopy, in conjunction with electroluminescence measurements and ambient photoemission spectroscopy. The non-Langevin polymer PDTSiTTz allows an inspection of intrinsic bimolecular recombination rates uninhibited by diffusion, while the low oscillator strengths of fullerenes allow polymer features to dominate, and we compare our results to those of the well-known polymer Si-PCPDTBT. Using μs-TAS, we have shown that the trap-limited decay dynamics of the PDTSiTTz polaron becomes progressively slower across the fullerene series, while those of Si-PCPDTBT are invariant. Electroluminescence measurements showed an unusual double peak in pristine PDTSiTTz, attributed to a low energy intragap charge transfer state, likely interchain in nature. Furthermore, while the pristine PDTSiTTz showed a broad, low-intensity density of states, the ICBA and ICTA blends presented a virtually identical DOS to Si-PCPDTBT and its blends. This has been attributed to a shift from a delocalized, interchain highest occupied molecular orbital (HOMO) in the pristine material to a dithienosilole-centered HOMO in the blends, likely a result of the bulky fullerenes increasing interchain separation. This HOMO localization had a side effect of progressively shifting the polymer HOMO to shallower energies, which was correlated with the observed decrease in bimolecular recombination rate and increased “trap” depth. However, since the density of tail states remained the same, this suggests that the traditional viewpoint of “trapping” being dominated by tail states may not encompass the full picture
Park SY, Labanti C, Luke J, et al., 2022, Organic bilayer photovoltaics for efficient indoor light harvesting, Advanced Energy Materials, Vol: 12, Pages: 1-10, ISSN: 1614-6832
Indoor organic photovoltaics (OPVs) are a potential niche application for organic semiconductors due to their strong and well-matched absorption with the emission of indoor lighting. However, due to extremely low photocurrent generation, the device parameters critical for efficient indoor OPVs differ from those under 1 Sun conditions. Herein, these critical device parameters—recombination loss and shunt resistance (Rsh)—are identified and it is demonstrated that bilayer OPVs are suitable for indoor PV applications. Compared to bulk-heterojunction (BHJ), the open-circuit voltage loss of bilayer devices under low light intensities is much smaller, consistent with a larger surface photovoltage response, indicating suppressed recombination losses. The bilayer devices show a higher fill factor at low light intensities, resulting from high Rsh afforded by the ideal interfacial contacts between the photoactive and the charge transport layers. The high Rsh enables bilayer devices to perform well without a light-soaking process. Finally, the charge carriers are extracted rapidly in bilayers, which are attributed to strongly suppressed trap states possibly induced by isolated domains and non-ideal interfacial contacts in BHJs. This study highlights the excellent suitability of bilayer OPVs for indoor applications and demonstrates the importance of device architecture and interfacial structures for efficient indoor OPVs.
Chin Y-C, Daboczi M, Henderson C, et al., 2022, Suppressing PEDOT:PSS doping-induced interfacial recombination loss in perovskite solar cells, ACS Energy Letters, Vol: 7, Pages: 560-568, ISSN: 2380-8195
PEDOT:PSS is widely used as a hole transport layer (HTL) in perovskite solar cells (PSCs) due to its facile processability, industrial scalability, and commercialization potential. However, PSCs utilizing PEDOT:PSS suffer from strong recombination losses compared to other organic HTLs. This results in lower open-circuit voltage (VOC) and power conversion efficiency (PCE). Most studies focus on doping PEDOT:PSS to improve charge extraction, but it has been suggested that a high doping level can cause strong recombination losses. Herein, we systematically dedope PEDOT:PSS with aqueous NaOH, raising its Fermi level by up to 500 meV, and optimize its layer thickness in p-i-n devices. A significant reduction of recombination losses at the dedoped PEDOT:PSS/perovskite interface is evidenced by a longer photoluminescence lifetime and higher magnitude of surface photovoltage, leading to an increased device VOC, fill factor, and PCE. These results provide insights into the relationship between doping level of HTLs and interfacial charge carrier recombination losses.
Daboczi M, Ratnasingham SR, Mohan L, et al., 2021, Optimal Interfacial Band Bending Achieved by Fine Energy Level Tuning in Mixed-Halide Perovskite Solar Cells, ACS ENERGY LETTERS, Vol: 6, Pages: 3970-3981, ISSN: 2380-8195
Mohan L, Ratnasingham SR, Panidi J, et al., 2021, Determining out-of-plane hole mobility in CuSCN via the time-of-flight technique to elucidate its function in perovskite solar cells, ACS Applied Materials and Interfaces, Vol: 13, Pages: 38499-38507, ISSN: 1944-8244
Copper(I) thiocyanate (CuSCN) is a stable, low-cost, solution-processable p-type inorganic semiconductor used in numerous optoelectronic applications. Here, for the first time, we employ the time-of-flight (ToF) technique to measure the out-of-plane hole mobility of CuSCN films, enabled by the deposition of 4 μm-thick films using aerosol-assisted chemical vapor deposition (AACVD). A hole mobility of ∼10–3 cm2/V s was measured with a weak electric field dependence of 0.005 cm/V1/2. Additionally, by measuring several 1.5 μm CuSCN films, we show that the mobility is independent of thickness. To further validate the suitability of our AACVD-prepared 1.5 μm-thick CuSCN film in device applications, we demonstrate its incorporation as a hole transport layer (HTL) in methylammonium lead iodide (MAPbI3) perovskite solar cells (PSCs). Our AACVD films result in devices with measured power conversion efficiencies of 10.4%, which compares favorably with devices prepared using spin-coated CuSCN HTLs (12.6%), despite the AACVD HTLs being an order of magnitude thicker than their spin-coated analogues. Improved reproducibility and decreased hysteresis were observed, owing to a combination of excellent film quality, high charge-carrier mobility, and favorable interface energetics. In addition to providing a fundamental insight into charge-carrier mobility in CuSCN, our work highlights the AACVD methodology as a scalable, versatile tool suitable for film deposition for use in optoelectronic devices.
Vasilopoulou M, Mohd Yusoff ARB, Daboczi M, et al., 2021, High efficiency blue organic light-emitting diodes with below-bandgap electroluminescence, Nature Communications, Vol: 12, ISSN: 2041-1723
Blue organic light-emitting diodes require high triplet interlayer materials, which induce large energetic barriers at the interfaces resulting in high device voltages and reduced efficiencies. Here, we alleviate this issue by designing a low triplet energy hole transporting interlayer with high mobility, combined with an interface exciplex that confines excitons at the emissive layer/electron transporting material interface. As a result, blue thermally activated delay fluorescent organic light emitting diodes with a below-bandgap turn-on voltage of 2.5 V and an external quantum efficiency of 41.2% were successfully fabricated. These devices also showed suppressed efficiency roll-off maintaining an EQE of 34.8% at 1000 cd m-2. Our approach paves the way for further progress through exploring alternative device engineering approaches instead of only focusing on the demanding synthesis of organic compounds with complex structures.
Clarke AJ, Luke J, Meitzner R, et al., 2021, Non-fullerene acceptor photostability and its impact on organic solar cell lifetime, CELL REPORTS PHYSICAL SCIENCE, Vol: 2
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