School of Chemistry - Research Publications

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    Alkyl Chain Length-Dependent Amine-Induced Crystallization for Efficient Interface Passivation of Perovskite Solar Cells
    Zhao, P ; Subbiah, J ; Zhang, B ; Hutchison, JA ; Ahluwalia, G ; Mitchell, V ; Ghiggino, KP ; Jones, DJ (WILEY, 2023-07)
    Abstract Efficient surface passivation of perovskite solar cells (PSC) using treatment with ammonium salts is demonstrated as an efficient method to enhance the device performance, owing to the affinity between the amine group and [PbI6]4− octahedron. However, due to their high solubility in polar solvents (DMF/DMSO), ammonium salts are more difficult to use in passivation of the interface between the electron transport layer and perovskite thin film in n‐i‐p structured PSCs. In this report, this work successfully links the amine group with a fullerene through a series of increasing carbon chain length, from two to twelve methylene units (FC‐X, X = 2, 6, 12), and then introduce the synthesized molecules as interface passivation layers into SnO2‐based planar n‐i‐p PSCs. Results show that the interface passivation effect is highly dependent on the side‐chain length, and the longer chain length amine‐functionalized fullerene is more beneficial for the device performance. A power conversion efficiency as high as 21.2% is achieved by using FC‐12. The surface energy, perovskite crystallite size and electron transfer capacity correlate with the linker chain length. This work develops an amine‐induced anchored crystallization of perovskite to unravel the mechanism of this passivation effect. As expected, enhanced device stability is also observed in the FC‐12 passivated PSCs.
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    Liquid Crystallinity as a Self-Assembly Motif for High-Efficiency, Solution-Processed, Solid-State Singlet Fission Materials
    Masoomi-Godarzi, S ; Liu, M ; Tachibana, Y ; Mitchell, VD ; Goerigk, L ; Ghiggino, KP ; Smith, TA ; Jones, DJ (WILEY-V C H VERLAG GMBH, 2019-08)
    Abstract Solution and solution‐deposited thin films of the discotic liquid crystalline electron acceptor–donor–acceptor (A‐D‐A) p‐type organic semiconductor FHBC(TDPP)2, synthesized by coupling thienyl substituted diketopyrrolopyrrole (TDPP) onto a fluorenyl substituted hexa‐peri‐hexabenzocoronene (FHBC) core, are examined by ultrafast and nanosecond transient absorption spectroscopy, and time‐resolved photoluminescence studies to examine their ability to support singlet fission (SF). Grazing incidence wide‐angle X‐ray (GIWAX) studies indicate that as‐cast thin films of FHBC(TDPP)2 are “amorphous,” while hexagonal packed discotic liquid crystalline films evolve during thermal annealing. SF in as‐cast thin films is observed with an ≈150% triplet generation yield. Thermally annealing the thin films improves SF yields up to 170%. The as‐cast thin films show no long‐range order, indicating a new class of SF material where the requirement for local order and strong near neighbor coupling has been removed. Generation of long‐lived triplets (µs) suggests that these materials may also be suitable for inclusion in organic solar cells to enhance performance.
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    Solution-Processable, Solid State Donor-Acceptor Materials for Singlet Fission
    Masoomi-Godarzi, S ; Liu, M ; Tachibana, Y ; Goerigk, L ; Ghiggino, KP ; Smith, TA ; Jones, DJ (WILEY-V C H VERLAG GMBH, 2018-10-25)
    Abstract The exploitation of singlet fission (SF) materials in optoelectronic devices is restricted by the limited number of SF materials available and developing new organic materials that undergo singlet fission is a significant challenge. Using a new strategy based on conjugating strong donor and acceptor building blocks, the small molecule (BDT(DPP)2) and polymer (p‐BDT‐DPP) systems are designed and synthesized knowing that bisthiophene‐2,5‐dihydropyrrolo[3,4‐c]pyrrole‐1,4‐dione (DPP) has a low lying triplet energy level, which is further confirmed by time‐dependent density functional theory (TD‐DFT) calculations. TD‐DFT and natural transition orbital (NTO) analysis are conducted to gain insight into the photophysical properties and features of excited states in BDT(DPP)2, respectively. Femtosecond and nanosecond transient absorption spectroscopies are used to investigate the excited state kinetics in the synthesized compounds. Fast formation of triplet pairs in thin film of p‐BDT‐DPP and BDT(DPP)2 and the equilibrium formation of correlated triplet pairs and S1 from triplet–triplet annihilation in solution of BDT(DPP)2 are further evidence of SF in these compounds. The short triplet lifetime, as a result of fast biexcitonic recombination, provides additional support for triplet pair formation through singlet fission.
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    Reduced Recombination and Capacitor-like Charge Buildup in an Organic Heterojunction
    Schwarz, KN ; Geraghty, PB ; Mitchell, VD ; Khan, S-U-Z ; Sandberg, OJ ; Zarrabi, N ; Kudisch, B ; Subbiah, J ; Smith, TA ; Rand, BP ; Armin, A ; Scholes, GD ; Jones, DJ ; Ghiggino, KP (American Chemical Society, 2020-02-05)
    Organic photovoltaic (OPV) efficiencies continue to rise, raising their prospects for solar energy conversion. However, researchers have long considered how to suppress the loss of free carriers by recombination—poor diffusion and significant Coulombic attraction can cause electrons and holes to encounter each other at interfaces close to where they were photogenerated. Using femtosecond transient spectroscopies, we report the nanosecond grow-in of a large transient Stark effect, caused by nanoscale electric fields of ∼487 kV/cm between photogenerated free carriers in the device active layer. We find that particular morphologies of the active layer lead to an energetic cascade for charge carriers, suppressing pathways to recombination, which is ∼2000 times less than predicted by Langevin theory. This in turn leads to the buildup of electric charge in donor and acceptor domains—away from the interface—resistant to bimolecular recombination. Interestingly, this signal is only experimentally obvious in thick films due to the different scaling of electroabsorption and photoinduced absorption signals in transient absorption spectroscopy. Rather than inhibiting device performance, we show that devices up to 600 nm thick maintain efficiencies of >8% because domains can afford much higher carrier densities. These observations suggest that with particular nanoscale morphologies the bulk heterojunction can go beyond its established role in charge photogeneration and can act as a capacitor, where adjacent free charges are held away from the interface and can be protected from bimolecular recombination.
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    Energy Migration in Organic Solar Concentrators with a Molecularly Insulated Perylene Diimide
    Banal, JL ; Soleimaninejad, H ; Jradi, FM ; Liu, M ; White, JM ; Blakers, AW ; Cooper, MW ; Jones, DJ ; Ghiggino, KP ; Marder, SR ; Smith, TA ; Wong, WWH (AMER CHEMICAL SOC, 2016-06-23)
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    Emissive Molecular Aggregates and Energy Migration in Luminescent Solar Concentrators
    Banal, JL ; Zhang, B ; Jones, DJ ; Ghiggino, KP ; Wong, WWH (American Chemical Society, 2017-01-17)
    Luminescent solar concentrators (LSCs) are light harvesting devices that are ideally suited to light collection in the urban environment where direct sunlight is often not available. LSCs consist of highly luminescent compounds embedded or coated on a transparent substrate that absorb diffuse or direct solar radiation over a large area. The resulting luminescence is trapped in the waveguide by total internal reflection to the thin edges of the substrate where the concentrated light can be used to improve the performance of photovoltaic devices. The concept of LSCs has been around for several decades, and yet the efficiencies of current devices are still below expectations for commercial viability. There are two primary challenges when designing new chromophores for LSC applications. Reabsorption of dye emission by chromophores within the waveguide is a significant loss mechanism attenuating the light output of LSCs. Concentration quenching, particularly in organic dye systems, restricts the quantity of chromophores that can be incorporated in the waveguide thus limiting the light absorbed by the LSC. Frequently, a compromise between increased light harvesting of the incident light and decreasing emission quantum yield is required for most organic chromophore-based systems due to concentration quenching. The low Stokes shift of common organic dyes used in current LSCs also imposes another optimization problem. Increasing light absorption of LSCs based on organic dyes to achieve efficient light harvesting also enhances reabsorption. Ideally, a design strategy to simultaneously optimize light harvesting, concentration quenching, and reabsorption of LSC chromophores is clearly needed to address the significant losses in LSCs. Over the past few years, research in our group has targeted novel dye structures that address these primary challenges. There is a common perception that dye aggregates are to be avoided in LSCs. It became apparent in our studies that aggregates of chromophores exhibiting aggregation-induced emission (AIE) behavior are attractive candidates for LSC applications. Strategic application of AIE chromophores has led to the development of the first organic-based transparent solar concentrator that harvests UV light as well as the demonstration of reabsorption reduction by taking advantage of energy migration processes between chromophores. Further developments led us to the application of perylene diimides using an energy migration/energy transfer approach. To prevent concentration quenching, a molecularly insulated perylene diimide with bulky substituents attached to the imide positions was designed and synthesized. By combining the insulated perylene diimide with a commercial perylene dye as an energy donor–acceptor emitter pair, detrimental luminescence reabsorption was reduced while achieving a high chromophore concentration for efficient light absorption. This Account reviews and reinspects some of our recent work and the improvements in the field of LSCs.
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    Highly Fluorescent Molecularly Insulated Perylene Diimides: Effect of Concentration on Photophysical Properties
    Zhang, B ; Soleimaninejad, H ; Jones, DJ ; White, JM ; Ghiggino, KP ; Smith, TA ; Wong, WWH (AMER CHEMICAL SOC, 2017-10-10)
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    Aggregation-induced emission-mediated spectral downconversion in luminescent solar concentrators
    Zhang, B ; Banal, JL ; Jones, DJ ; Tang, BZ ; Ghiggino, KP ; Wong, WWH (Royal Society of Chemistry, 2018-03-01)
    The light-harvesting efficiency of luminescent solar concentrators (LSCs) is encumbered by reabsorption of emission and concentration quenching. Energy transfer from a high-concentration donor to a low-concentration energy trap can reduce reabsorption losses while maintaining efficient light collection. Emissive aggregates enable this approach by reducing the impact of concentration quenching, which is detrimental to the entire energy transfer process. Here we describe a LSC that utilizes emissive aggregates as energy-transfer pairs for downconversion. We characterize the photophysics of a benzothiadiazole-based emissive aggregate, PITBT-TPE, that complements a highly emissive donor, DPATPAN, and functions as a highly emissive energy-transfer acceptor even at high concentrations in excess of 180 mM in the PMMA matrix. Monte Carlo simulations of LSCs that leverage these emissive aggregates as energy-transfer pairs predicted notable optical efficiencies at large concentrator dimensions. We demonstrate for the first time a LSC that utilizes donor and acceptor AIE chromophores to reduce reabsorption.