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Journal articleYan H, Wade J, Wan L, et 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- Author Web Link
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- Citations: 8
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Journal articleLabanti 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: 2041-1723Organic 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.
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Journal articleLuke 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: 1614-6832Understanding 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.
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Journal articleHamilton 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: 1944-8244Modification 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.
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Journal articleLee 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: 2367-198XRecent 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
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Journal articleXu W, Du T, Sachs M, et 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: 2666-3864Understanding 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.
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Journal articleYan 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- Author Web Link
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- Citations: 1
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Journal articleMarin-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: 1932-7447The 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
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Journal articlePark 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: 1614-6832Indoor 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.
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Journal articleChin 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: 2380-8195PEDOT: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.
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Journal articleDaboczi 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- Author Web Link
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- Citations: 15
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Journal articleMohan 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: 1944-8244Copper(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.
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Journal articleVasilopoulou 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: 2041-1723Blue 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.
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Journal articleClarke 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- Author Web Link
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- Citations: 28
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Journal articleKyeong M, Lee J, Daboczi M, et 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- Author Web Link
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- Citations: 20
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Journal articleMarin-Beloqui JM, Toolan DTW, Panjwani NA, et al., 2021,
Triplet-Charge Annihilation in a Small Molecule Donor: Acceptor Blend as a Major Loss Mechanism in Organic Photovoltaics
, ADVANCED ENERGY MATERIALS, Vol: 11, ISSN: 1614-6832- Author Web Link
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- Citations: 12
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Journal articleLimbu S, Stewart K, Nightingale J, et al., 2021,
Solid-State Ionic Liquid: Key to Efficient Detection and Discrimination in Organic Semiconductor Gas Sensors
, ACS APPLIED ELECTRONIC MATERIALS, Vol: 3, Pages: 2152-2163- Author Web Link
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- Citations: 3
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Journal articleYun D-J, Lee S, Kim SH, et al., 2021,
Bevel Structure Based XPS Analysis as a Non-Destructive Chemical Probe for Complex Interfacial Structures of Organic Semiconductors
, SMALL METHODS, Vol: 5, ISSN: 2366-9608- Author Web Link
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- Citations: 1
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Journal articleLabanti C, Sung MJ, Luke J, et al., 2021,
Selenium-substituted non-fullerene acceptors: a route to superior Operational stability for organic bulk heterojunction solar cells.
, ACS Nano, Vol: 15, Pages: 7700-7712, ISSN: 1936-0851Non-fullerene acceptors (NFAs) for organic solar cells (OSCs) have significantly developed over the past five years with continuous improvements in efficiency now over 18%. However, a key challenge still remains in order to fully realize their commercialization potential: the need to extend device lifetime and to control degradation mechanisms. Herein, we investigate the effect of two different molecular engineering routes on the widely utilized ITIC NFA, to tune its optoelectronic properties and interactions with the donor polymer in photoactive blends. Heavier selenium (Se) atoms substitute sulfur (S) atoms in the NFA core in either outer or inner positions, and methyl chains are attached to the end groups. By investigating the effects of these structural modifications on the long-term operational stability of bulk-heterojunction OSC devices, we identify outer selenation as a powerful strategy to significantly increase device lifetime compared to ITIC. Combining outer selenation and methylation results in an impressive 95% of the initial OSC efficiency being retained after 450 h under operating conditions, with an exceptionally long projected half-lifetime of 5600 h compared to 400 h for ITIC. We find that the heavier and larger Se atoms at outer-core positions rigidify the molecular structure to form highly crystalline films with low conformational energetic disorder. It further enhances charge delocalization over the molecule, promoting strong intermolecular interactions among acceptor molecules. Upon methylation, this strong intermolecular interaction stabilizes acceptor domains in blends to be resilient to light-induced morphological changes, thereby leading to superior device stability. Our results highlight the crucial role of NFA molecular structure for OSC operational stability and provide important NFA design rules via heteroatom position and end-group control.
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Journal articleHeeney M, Kafourou P, Park B, et al., 2021,
One-step six-fold cyanation of benzothiadiazole acceptor Units for air-stable high-performance n-type organic field-effect transistors
, Angewandte Chemie International Edition, Vol: 60, Pages: 5970-5977, ISSN: 1433-7851We report a new high electron affinity acceptor end group for organic semiconductors, 2,1,3-benzothiadiazole-4,5,6-tricarbonitrile (TCNBT). An n-type organic semiconductor with an indacenodithiophene (IDT) core and TCNBT end groups was synthesized by a six-fold nucleophilic substitution with cyanides on a fluorinated precursor, itself prepared by a direct arylation approach. This one-step chemical modification was found to significantly impact the molecular properties: the fluorinated precursor, TFBT IDT, a poor ambipolar semiconductor, was converted into TCNBT IDT, a good n-type semiconductor. The highly electron-deficient end group TCNBT dramatically decreased the energy of the highest occupied and lowest unoccupied molecular orbitals (HOMO/LUMO) compared to the fluorinated analogue and improved the molecular orientation when utilized in n-type organic field-effect transistors (OFETs). Solution-processed OFETs based on TCNBT IDT exhibited a charge carrier mobility of up to µ e ≈ 0.15 cm 2 V -1 s -1 with excellent ambient stability for 100 hours, highlighting the benefits of the cyanated end group and the synthetic approach.
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Journal articleLuke J, Correa L, Rodrigues J, et al., 2021,
A Commercial Benchmark: Light-Soaking Free, Fully Scalable, Large-Area Organic Solar Cells for Low-Light Applications
, ADVANCED ENERGY MATERIALS, Vol: 11, ISSN: 1614-6832- Author Web Link
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- Citations: 33
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Journal articleRatnasingham SR, Mohan L, Daboczi M, et al., 2021,
Novel scalable aerosol-assisted CVD route for perovskite solar cells
, Materials Advances, Vol: 2, Pages: 1606-1612Organo-metal halide perovskite research has progressed rapidly, with photovoltaic (PV) devices achieving over 25% power conversion efficiency (PCE). However, scalable production of these devices is an ongoing challenge. We demonstrate the growth of methylammonium lead triiodide (MAPI) films via a novel two-step aerosol-assisted chemical vapour deposition (AACVD) method leading to the first ever perovskite-based PV devices using active layers deposited by AACVD. This is a scalable deposition process, requiring less complex equipment than conventional CVD. Furthermore, our method utilises methanol (MeOH) as the only solvent, as opposed to harmful solvents typically used in perovskite processing. Structural and optical characterization confirms successful formation of MAPI with no secondary phases and an optical bandgap of ∼1.58 eV. The final film had large grains (order of μm), with thickness ranging from 500–1100 nm. These films were used to fabricate working PV devices resulting in a champion PCE of 5.4%. While films demonstrated high structural and compositonal quality, we identified large film roughness as a limiting factor in device PCE, and elucidate the origin of this via detailed study of the film growth, which reveals a unique multi-step film formation process.
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Journal articleYiwen W, Jinho L, Xueyan H, et al., 2021,
Recent progress and Challenges toward highly stable nonfullerene acceptor‐based organic solar cells
, Advanced Energy Materials, Vol: 11, ISSN: 1614-6832Organic solar cells (OSCs) based on nonfullerene acceptors (NFAs) have made significant breakthrough in their device performance, now achieving a power conversion efficiency of ≈18% for single junction devices, driven by the rapid development in their molecular design and device engineering in recent years. However, achieving long‐term stability remains a major challenge to overcome for their commercialization, due in large part to the current lack of understanding of their degradation mechanisms as well as the design rules for enhancing their stability. In this review, the recent progress in understanding the degradation mechanisms and enhancing the stability of high performance NFA‐based OSCs is a specific focus. First, an overview of the recent advances in the molecular design and device engineering of several classes of high performance NFA‐based OSCs for various targeted applications is provided, before presenting a critical review of the different degradation mechanisms identified through photochemical‐, photo‐, and morphological degradation pathways. Potential strategies to address these degradation mechanisms for further stability enhancement, from molecular design, interfacial engineering, and morphology control perspectives, are also discussed. Finally, an outlook is given highlighting the remaining key challenges toward achieving the long‐term stability of NFA‐OSCs.
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Journal articleLimbu S, Park K-B, Wu J, et 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- Author Web Link
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- Citations: 17
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Journal articleTseng T-W, Yan H, Nakamura T, et al., 2020,
Real-time monitoring of formation and dynamics of intra- and interchain phases in single molecules of polyfluorene
, ACS Nano, Vol: 14, Pages: 16096-16104, ISSN: 1936-0851Poly(9,9-dioctylfluorene) (PFO) is one of the most important conjugated polymer materials, exhibiting outstanding photophysical and electrical properties. PFO is also known for a diversity of morphological phases determined by conformational states of the main chain. Our goal in this work is to address some of the key questions on formation and dynamics of one such conformation, the β-phase, by following in real time the evolution of fluorescence spectra of single PFO chains. The PFO is dispersed in a thin polystyrene film, and the spectra are monitored during the process of solvent vapor annealing with toluene. We confirm unambiguously that the PFO β-phase segments are formed on a true single-chain level at room temperature in the solvent-softened polystyrene. We further find that the formation of the β-phase is a dynamic and reversible process occurring on the order of seconds, leading to repeated spontaneous transitions between the glassy and β-phase segments during the annealing. Comparison of PFO with two largely different molecular weights (Mw) shows that chains with lower Mw form the β-phase segments much faster. For the high Mw PFO chains, a detailed Franck-Condon analysis of the β-phase spectra shows a large distribution of the Huang-Rhys factor, S, and even dynamic changes of this factor occurring on a single chain. Such dynamics are likely a manifestation of changing coherence length of the exciton. Further, for the high Mw PFO chains we observe an additional conformational state, a crystalline γ-phase. The γ-phase formation is also a spontaneous reversible process in the solvent-softened matrix. The phase can form from both the β-phase and the glassy phase, and the formation requires high Mw to enable intersegment interactions in a self-folded chain.
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Journal articleStewart K, Limbu S, Nightingale J, et al., 2020,
Molecular understanding of a π-conjugated polymer/solid-state ionic liquid complex as a highly sensitive and selective gas sensor
, JOURNAL OF MATERIALS CHEMISTRY C, Vol: 8, Pages: 15268-15276, ISSN: 2050-7526- Author Web Link
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- Citations: 20
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Journal articleDong Y, Nikolis VC, Talnack F, et al., 2020,
Orientation dependent molecular electrostatics drives efficient charge generation in homojunction organic sol
, Nature Communications, Vol: 11, ISSN: 2041-1723Organic solar cells usually utilise a heterojunction between electron-donating (D) and electron-accepting (A) materials to split excitons into charges. However, the use of D-A blends intrinsically limits the photovoltage and introduces morphological instability. Here, we demonstrate that polycrystalline films of chemically identical molecules offer a promising alternative and show that photoexcitation of α-sexithiophene (α-6T) films results in efficient charge generation. This leads to α-6T based homojunction organic solar cells with an external quantum efficiency reaching up to 44% and an open-circuit voltage of 1.61 V. Morphological, photoemission, and modelling studies show that boundaries between α-6T crystalline domains with different orientations generate an electrostatic landscape with an interfacial energy offset of 0.4 eV, which promotes the formation of hybridised exciton/charge-transfer states at the interface, dissociating efficiently into free charges. Our findings open new avenues for organic solar cell design where material energetics are tuned through molecular electrostatic engineering and mesoscale structural control.
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Journal articleKwon S, Pak Y, Kim B, et al., 2020,
Molecular-level electrochemical doping for fine discrimination of volatile organic compounds in organic chemiresistors
, JOURNAL OF MATERIALS CHEMISTRY A, Vol: 8, Pages: 16884-16891, ISSN: 2050-7488- Author Web Link
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- Citations: 7
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Journal articleJayaram AK, Pitsalidis C, Tan E, et al., 2020,
3D Hybrid Scaffolds Based on PEDOT:PSS/MWCNT Composites (vol 7, 363, 2019)
, FRONTIERS IN CHEMISTRY, Vol: 8, ISSN: 2296-2646 -
Journal articleWu J, Lee J, Chin Y-C, et al., 2020,
Exceptionally low charge trapping enables highly efficient organic bulk heterojunction solar cells
, Energy & Environmental Science, Vol: 13, Pages: 2422-2430, ISSN: 1754-5692In this study, we investigate the underlying origin of the high performance of PM6:Y6 organic solar cells. Employing transient optoelectronic and photoemission spectroscopies, we find that this blend exhibits greatly suppressed charge trapping into electronic intra-bandgap tail states compared to other polymer/non-fullerene acceptor solar cells, attributed to lower energetic disorder. The presence of tail states is a key source of energetic loss in most organic solar cells, as charge carriers relax into these states, reducing the quasi-Fermi level splitting and therefore device VOC. DFT and Raman analyses indicate this suppression of tail state energetics disorder could be associated with a higher degree of conformational rigidity and uniformity for the Y6 acceptor. We attribute the origin of such conformational rigidity and uniformity of Y6 to the presence of the two alkyl side chains on the outer core that restricts end-group rotation by acting as a conformation locker. The resultant enhanced carrier dynamics and suppressed charge carrier trapping are proposed to be a key factor behind the high performance of this blend. Low energetic disorder is suggested to be a key factor enabling reasonably efficient charge generation in this low energy offset system. In the absence of either energetic disorder or a significant electronic energy offset, it is argued that charge separation in this system is primarily entropy driven. Nevertheless, photocurrent generation is still limited by slow hole transfer from Y6 to PM6, suggesting pathways for further efficiency improvement.
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