Publications
70 results found
Gallop NP, Maslennikov DR, Mondal N, et al., 2024, Ultrafast vibrational control of organohalide perovskite optoelectronic devices using vibrationally promoted electronic resonance., Nat Mater, Vol: 23, Pages: 88-94
Vibrational control (VC) of photochemistry through the optical stimulation of structural dynamics is a nascent concept only recently demonstrated for model molecules in solution. Extending VC to state-of-the-art materials may lead to new applications and improved performance for optoelectronic devices. Metal halide perovskites are promising targets for VC due to their mechanical softness and the rich array of vibrational motions of both their inorganic and organic sublattices. Here, we demonstrate the ultrafast VC of FAPbBr3 perovskite solar cells via intramolecular vibrations of the formamidinium cation using spectroscopic techniques based on vibrationally promoted electronic resonance. The observed short (~300 fs) time window of VC highlights the fast dynamics of coupling between the cation and inorganic sublattice. First-principles modelling reveals that this coupling is mediated by hydrogen bonds that modulate both lead halide lattice and electronic states. Cation dynamics modulating this coupling may suppress non-radiative recombination in perovskites, leading to photovoltaics with reduced voltage losses.
Pan J, Chen Z, Zhang T, et al., 2023, Operando dynamics of trapped carriers in perovskite solar cells observed via infrared optical activation spectroscopy, Nature Communications, Vol: 14, ISSN: 2041-1723
Conventional spectroscopies are not sufficiently selective to comprehensively understand the behaviour of trapped carriers in perovskite solar cells, particularly under their working conditions. Here we use infrared optical activation spectroscopy (i.e., pump-push-photocurrent), to observe the properties and real-time dynamics of trapped carriers within operando perovskite solar cells. We compare behaviour differences of trapped holes in pristine and surface-passivated FA0.99Cs0.01PbI3 devices using a combination of quasi-steady-state and nanosecond time-resolved pump-push-photocurrent, as well as kinetic and drift-diffusion models. We find a two-step trap-filling process: the rapid filling (~10 ns) of low-density traps in the bulk of perovskite, followed by the slower filling (~100 ns) of high-density traps at the perovskite/hole transport material interface. Surface passivation by n-octylammonium iodide dramatically reduces the number of trap states (~50 times), improving the device performance substantially. Moreover, the activation energy (~280 meV) of the dominant hole traps remains similar with and without surface passivation.
Liu W, Zhang H, Liang S, et al., 2023, The Synthesis of a Multiple D-A Conjugated Macrocycle and Its Application in Organic Photovoltaic., Angew Chem Int Ed Engl, Vol: 62
As a novel class of materials, D-A conjugated macrocycles hold significant promise for chemical science. However, their potential in photovoltaic remains largely untapped due to the complexity of introducing multiple donor and acceptor moieties into the design and synthesis of cyclic π-conjugated molecules. Here, we report a multiple D-A ring-like conjugated molecule (RCM) via the coupling of dimer molecule DBTP-C3 as a template and thiophenes in high yields. RCM exhibits a narrow optical gap (1.33 eV) and excellent thermal stability, and shows a remarkable photoluminescence yield (ΦPL ) of 11.1 % in solution, much higher than non-cyclic analogues. Organic solar cell (OSC) constructed with RCM as electron acceptor shows efficient charge separation at donor-acceptor band offsets and achieves a power conversion efficiency (PCE) of 14.2 %-approximately fourfold higher than macrocycle-based OSCs reported so far. This is partly due to low non-radiative voltage loss down to 0.20 eV and a high electroluminescence yield (ΦEL ) of 4×10-4 . Our findings emphasize the potential of D-A cyclic conjugated molecules in advancing organic photovoltaic technology.
Mondal N, Carwithen BP, Bakulin AA, 2023, Alloying metal cations in perovskite nanocrystals is a new route to controlling hot carrier cooling, Light: Science & Applications, Vol: 12, ISSN: 2095-5545
Hot carrier cooling is slowed down upon alloying tin in lead-halide perovskite nanocrystals through the engineering of carrier-phonon and carrier-defect interactions.
Wang Y, Yu J, Zhang R, et al., 2023, Origins of the open-circuit voltage in ternary organic solar cells and design rules for minimized voltage losses, NATURE ENERGY, Vol: 8, Pages: 978-988, ISSN: 2058-7546
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Meng Z, Pastor E, Selim S, et al., 2023, Operando IR optical control of localized charge carriers in BiVO4 photoanodes, Journal of the American Chemical Society, Vol: 145, Pages: 17700-17709, ISSN: 0002-7863
In photoelectrochemical cells (PECs) the photon-to-current conversion efficiency is often governed by carrier transport. Most metal oxides used in PECs exhibit thermally activated transport due to charge localization via the formation of polarons or the interaction with defects. This impacts catalysis by restricting the charge accumulation and extraction. To overcome this transport bottleneck nanostructuring, selective doping and photothermal treatments have been employed. Here we demonstrate an alternative approach capable of directly activating localized carriers in bismuth vanadate (BiVO4). We show that IR photons can optically excite localized charges, modulate their kinetics, and enhance the PEC current. Moreover, we track carriers bound to oxygen vacancies and expose their ∼10 ns charge localization, followed by ∼60 μs transport-assisted trapping. Critically, we demonstrate that localization is strongly dependent on the electric field within the device. While optical modulation has still a limited impact on overall PEC performance, we argue it offers a path to control devices on demand and uncover defect-related photophysics.
Zhang J, Qin J, Cai W, et al., 2023, Transport Layer Engineering Toward Lower Threshold for Perovskite Lasers, ADVANCED MATERIALS, Vol: 35, ISSN: 0935-9648
Jacoutot P, Scaccabarozzi A, Nodari D, et al., 2023, Enhanced Sub-1 eV detection in organic photodetectors through tuning polymer energetics and microstructure, Science Advances, Vol: 9, Pages: 1-9, ISSN: 2375-2548
One of the key challenges facing organic photodiodes (OPD) is increasing the detection into the IR region. Organic semiconductor polymers provide a platform for tuning the bandgap and optoelectronic response to go beyond the traditional 1000 nm benchmark. In this work, we present a NIR polymer with absorption up to 1500 nm. The polymer-based OPD delivers a high specific detectivity D* of 1.03×1010 Jones (-2 V) at 1200 nm and a dark current Jd of just 2.3×10-6 A cm-2 at -2V. We demonstrate a strong improvement of all OPD metrics in the NIR region compared to previously reported NIR-OPD, due to the enhanced crystallinity and optimized energy alignment which leads to reduced charge recombination. The high D* value in the 1100-1300 nm region is particularly promising for biosensing applications. We demonstrate the OPD as a pulse oximeter under NIR illumination, delivering heart rate and blood oxygen saturation readings in real-time without signal amplification.
Carwithen BP, Hopper TR, Ge Z, et al., 2023, Confinement and exciton binding energy effects on hot carrier cooling in lead halide perovskite nanomaterials, ACS Nano, Vol: 17, Pages: 6638-6648, ISSN: 1936-0851
The relaxation of the above-gap (“hot”) carriers in lead halide perovskites (LHPs) is important for applications in photovoltaics and offers insights into carrier–carrier and carrier–phonon interactions. However, the role of quantum confinement in the hot carrier dynamics of nanosystems is still disputed. Here, we devise a single approach, ultrafast pump–push–probe spectroscopy, to study carrier cooling in six different size-controlled LHP nanomaterials. In cuboidal nanocrystals, we observe only a weak size effect on the cooling dynamics. In contrast, two-dimensional systems show suppression of the hot phonon bottleneck effect common in bulk perovskites. The proposed kinetic model describes the intrinsic and density-dependent cooling times accurately in all studied perovskite systems using only carrier–carrier, carrier–phonon, and excitonic coupling constants. This highlights the impact of exciton formation on carrier cooling and promotes dimensional confinement as a tool for engineering carrier–phonon and carrier–carrier interactions in LHP optoelectronic materials.
Wang T, Hopper T, Mondal N, et al., 2023, Hot carrier cooling and trapping in atomically thin WS₂ probed by three-pulse femtosecond spectroscopy, ACS Nano, Vol: 17, Pages: 6330-6340, ISSN: 1936-0851
Transition metal dichalcogenides (TMDs) have shown outstanding semiconducting properties which make them promising materials for next-generation optoelectronic and electronic devices. These properties are imparted by fundamental carrier–carrier and carrier–phonon interactions that are foundational to hot carrier cooling. Recent transient absorption studies have reported ultrafast time scales for carrier cooling in TMDs that can be slowed at high excitation densities via a hot-phonon bottleneck (HPB) and discussed these findings in the light of optoelectronic applications. However, quantitative descriptions of the HPB in TMDs, including details of the electron–lattice coupling and how cooling is affected by the redistribution of energy between carriers, are still lacking. Here, we use femtosecond pump–push–probe spectroscopy as a single approach to systematically characterize the scattering of hot carriers with optical phonons, cold carriers, and defects in a benchmark TMD monolayer of polycrystalline WS2. By controlling the interband pump and intraband push excitations, we observe, in real-time (i) an extremely rapid “intrinsic” cooling rate of ∼18 ± 2.7 eV/ps, which can be slowed with increasing hot carrier density, (ii) the deprecation of this HPB at elevated cold carrier densities, exposing a previously undisclosed role of the carrier–carrier interactions in mediating cooling, and (iii) the interception of high energy hot carriers on the subpicosecond time scale by lattice defects, which may account for the lower photoluminescence yield of TMDs when excited above band gap.
Albaladejo-Siguan M, Becker-Koch D, Baird EC, et al., 2022, Interdot Lead Halide Excess Management in PbS Quantum Dot Solar Cells, Advanced Energy Materials, Vol: 12, ISSN: 1614-6832
Light-harvesting devices made from lead sulfide quantum dot (QD) absorbers are one of the many promising technologies of third-generation photovoltaics. Their simple, solution-based fabrication, together with a highly tunable and broad light absorption makes their application in newly developed solar cells, particularly promising. In order to yield devices with reduced voltage and current losses, PbS QDs need to have strategically passivated surfaces, most commonly achieved through lead iodide and bromide passivation. The interdot spacing is then predominantly filled with residual amorphous lead halide species that remain from the ligand exchange, thus hindering efficient charge transport and reducing device stability. Herein, it is demonstrated that a post-treatment by iodide-based 2-phenylethlyammonium salts and intermediate 2D perovskite formation can be used to manage the lead halide excess in the PbS QD active layer. This treatment results in improved device performance and increased shelf-life stability, demonstrating the importance of interdot spacing management in PbS QD photovoltaics.
Gallop NP, Ye J, Greetham GM, et al., 2022, The effect of caesium alloying on the ultrafast structural dynamics of hybrid organic-inorganic halide perovskites, Journal of Materials Chemistry A, Vol: 10, Pages: 22408-22418, ISSN: 2050-7488
Hybrid inorganic–organic perovskites have attracted considerable attention over recent years as promising processable electronic materials. In particular, the rich structural dynamics of these ‘soft’ materials has become a subject of investigation and debate due to their direct influence on the perovskites' optoelectronic properties. Significant effort has focused on understanding the role and behaviour of the organic cations within the perovskite, as their rotational dynamics may be linked to material stability, heterogeneity and performance in (opto)electronic devices. To this end, we use two-dimensional IR spectroscopy (2DIR) to understand the effect of partial caesium alloying on the rotational dynamics of the methylammonium cation in the archetypal hybrid perovskite CH3NH3PbI3. We find that caesium incorporation primarily inhibits the slower ‘reorientational jump’ modes of the organic cation, whilst a smaller effect on the fast ‘wobbling time’ may be due to distortions and rigidisation of the inorganic cuboctahedral cage. 2DIR centre-line-slope analysis further reveals that while static disorder increases with caesium substitution, the dynamic disorder (reflected in the phase memory of the N–H stretching mode of methylammonium) is largely independent of caesium addition. Our results contribute to the development of a unified model of cation dynamics within organohalide perovskites.
Maimaris M, Pettipher AJ, Azzouzi M, et al., 2022, Sub-10-fs observation of bound exciton formation in organic optoelectronic devices, Nature Communications, Vol: 13, ISSN: 2041-1723
Fundamental mechanisms underlying exciton formation in organic semiconductors are complex and elusive as it occurs on ultrashort sub-100-fs timescales. Some fundamental aspects of this process, such as the evolution of exciton binding energy, have not been resolved in time experimentally. Here, we apply a combination of sub-10-fs Pump-Push-Photocurrent, Pump-Push-Photoluminescence, and Pump-Probe spectroscopies to polyfluorene devices to track the ultrafast formation of excitons. While Pump-Probe is sensitive to the total concentration of excited states, Pump-Push-Photocurrent and Pump-Push-Photoluminescence are sensitive to bound states only, providing access to exciton binding dynamics. We find that excitons created by near-absorption-edge photons are intrinsically bound states, or become such within 10 fs after excitation. Meanwhile, excitons with a modest >0.3 eV excess energy can dissociate spontaneously within 50 fs before acquiring bound character. These conclusions are supported by excited-state molecular dynamics simulations and a global kinetic model which quantitatively reproduce experimental data.
Pastor E, Sachs M, Selim S, et al., 2022, Electronic defects in metal oxide photocatalysts, NATURE REVIEWS MATERIALS, Vol: 7, Pages: 503-521, ISSN: 2058-8437
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- Citations: 68
Garratt D, Misiekis L, Wood D, et al., 2022, Direct observation of ultrafast exciton localization in an organic semiconductor with soft X-ray transient absorption spectroscopy, Nature Communications, Vol: 13, ISSN: 2041-1723
The localization dynamics of excitons in organic semiconductors influence the efficiency of charge transfer and separation in these materials. Here we apply time-resolved X-ray absorption spectroscopy to track photoinduced dynamics of a paradigmatic crystalline conjugated polymer: poly(3-hexylthiophene) (P3HT) commonly used in solar cell devices. The π→π* transition, the first step of solar energy conversion, is pumped with a 15 fs optical pulse and the dynamics are probed by an attosecond soft X-ray pulse at the carbon K-edge. We observe X-ray spectroscopic signatures of the initially hot excitonic state, indicating that it is delocalized over multiple polymer chains. This undergoes a rapid evolution on a sub 50 fs timescale which can be directly associated with cooling and localization to form either a localized exciton or polaron pair.
Jacoutot P, Scaccabarozzi AD, Zhang T, et al., 2022, Infrared organic photodetectors employing ultralow bandgap polymer and non-fullerene acceptors for biometric monitoring, Small, Vol: 18, Pages: 1-10, ISSN: 1613-6810
Recent efforts in the field of organic photodetectors (OPD) have been focused on extending broadband detection into the near-infrared (NIR) region. Here, two blends of an ultralow bandgap push–pull polymer TQ-T combined with state-of-the-art non-fullerene acceptors, IEICO-4F and Y6, are compared to obtain OPDs for sensing in the NIR beyond 1100 nm, which is the cut off for benchmark Si photodiodes. It is observed that the TQ-T:IEICO-4F device has a superior IR responsivity (0.03 AW-1 at 1200 nm and −2 V bias) and can detect infrared light up to 1800 nm, while the TQ-T:Y6 blend shows a lower responsivity of 0.01 AW-1. Device physics analyses are tied with spectroscopic and morphological studies to link the superior performance of TQ-T:IEICO-4F OPD to its faster charge separation as well as more favorable donor–acceptor domains mixing. In the polymer blend with Y6, the formation of large agglomerates that exceed the exciton diffusion length, which leads to high charge recombination, is observed. An application of these devices as biometric sensors for real-time heart rate monitoring via photoplethysmography, utilizing infrared light, is demonstrated.
Becker-Koch D, Albaladejo-Siguan M, Kress J, et al., 2022, Oxygen-induced degradation in AgBiS<sub>2</sub> nanocrystal solar cells, NANOSCALE, Vol: 14, Pages: 3020-3030, ISSN: 2040-3364
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- Citations: 6
Azzouzi M, Nelson J, Eisner F, et al., 2022, Reconciling models of interfacial state kinetics and device performance in organic solar cells: Impact of the energy offsets on the power conversion efficiency, Energy and Environmental Science, Vol: 15, Pages: 156-1270, ISSN: 1754-5692
Achieving the simultaneous increases in the open circuit voltage (Voc), short circuit current (Jsc) and fill factor (FF) necessary to further increase the power conversion efficiency (PCE) of organic photovoltaics (OPV) requires a unified understanding of how molecular and device parameters affect all three characteristics. In this contribution, we introduce a framework that for the first time combines different models that have been used separately to describe the different steps of the charge generation and collection processes in OPV devices: a semi-classical rate model for charge recombination processes in OPV devices, zero-dimensional kinetic models for the photogeneration process and exciton dissociation and one-dimensional semiconductor device models. Using this unified multi-scale model in conjunction with experimental techniques (time-resolved absorption spectroscopy, steady-state and transient optoelectronic measurements) that probe the various steps involved in charge generation we can shed light on how the energy offsets in a series of polymer: non-fullerene devices affect the charge carrier generation, collection, and recombination properties of the devices. We find that changing the energy levels of the donor significantly affects not only the transition rates between local-exciton (LE) and charge-transfer (CT) states, but also significantly changes the transition rates between CT and charge-separated (CS) states, challenging the commonly accepted picture of charge generation and recombination. These results show that in order to obtain an accurate picture of charge generation in OPV devices, a variety of different experimental techniques under different conditions in conjunction with a comprehensive model of processes occurring at different time-scales are required.
Maimaris M, Pettipher AJ, Azzouzi M, et al., 2022, Sub-10fs Photocurrent and Photoluminescence Action Spectroscopies of Organic Optoelectronic Devices Reveals Ultrafast Formation of Bound Excitonic States
We apply ultrafast pump-push-photocurrent and pump-push-photoluminescence spectroscopies to polyfluorene organic diode to track in time the bound exciton formation. ‘Cold’-excitons become bound within 10-fs while ‘hot’-excitons can dissociate spontaneously within 50-fs before acquiring bound character.
Zheng X, Hopper TR, Gorodetsky A, et al., 2021, Multi-pulse terahertz spectroscopy unveils hot polaron photoconductivity dynamics in metal-halide perovskites, Journal of Physical Chemistry Letters, Vol: 12, Pages: 8732-8739, ISSN: 1948-7185
The behavior of hot carriers in metal-halide perovskites (MHPs) present avaluable foundation for understanding the details of carrier-phonon coupling inthe materials as well as the prospective development of highly efficient hotcarrier and carrier multiplication solar cells. Whilst the carrier populationdynamics during cooling have been intensely studied, the evolution of the hotcarrier properties, namely the hot carrier mobility, remain largely unexplored.To address this, we introduce a novel ultrafast visible pump - infrared push -terahertz probe spectroscopy (PPP-THz) to monitor the real-time conductivitydynamics of cooling carriers in methylammonium lead iodide. We find a decreasein mobility upon optically depositing energy into the carriers, which istypical of band-transport. Surprisingly, the conductivity recovery dynamics areincommensurate with the intraband relaxation measured by an analogousexperiment with an infrared probe (PPP- IR), and exhibit a negligibledependence on the density of hot carriers. These results and the kineticmodelling reveal the importance of highly-localized lattice heating on themobility of the hot electronic states. This collective polaron-latticephenomenon may contribute to the unusual photophysics observed in MHPs andshould be accounted for in devices that utilize hot carriers.
Chen Z, Li Z, Hopper T, et al., 2021, Materials, photophysics and device engineering of perovskite light-emitting diodes, Reports on Progress in Physics, Vol: 84, ISSN: 0034-4885
Here we provide a comprehensive review of a newly developed lighting technology based on metal halide perovskites (i.e. perovskite light-emitting diodes) encompassing the research endeavours into materials, photophysics and device engineering. At the outset we survey the basic perovskite structures and their various dimensions (namely three-, two- and zero-dimensional perovskites), and demonstrate how the compositional engineering of these structures affects the perovskite light-emitting properties. Next, we turn to the physics underpinning photo- and electroluminescence in these materials through their connection to the fundamental excited states, energy/charge transport processes and radiative and non-radiative decay mechanisms. In the remainder of the review, we focus on the engineering of perovskite light-emitting diodes, including the history of their development as well as an extensive analysis of contemporary strategies for boosting device performance. Key concepts include balancing the electron/hole injection, suppression of parasitic carrier losses, improvement of the photoluminescence quantum yield and enhancement of the light extraction. Overall, this review reflects the current paradigm for perovskite lighting, and is intended to serve as a foundation to materials and device scientists newly working in this field.
Labanti 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: 1936-0851
Non-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.
Garratt D, Misiekis L, Wood D, et al., 2021, Experimental and numerical investigations on optimal phase change material melting temperature utilized either alone or with night ventilation, Conference on Lasers and Electro-Optics Europe / European Quantum Electronics Conference (CLEO/Europe-EQEC), Publisher: IEEE, Pages: 1647-1651, ISSN: 2522-2708
Garratt D, Misiekis L, Wood D, et al., 2021, Ultrafast Exciton Dynamics in Poly(3-hexylthiophene) Probed with Time Resolved X-ray Absorption Spectroscopy at the Carbon K- edge, Conference on Lasers and Electro-Optics Europe / European Quantum Electronics Conference (CLEO/Europe-EQEC), Publisher: IEEE
Han S, Deng R, Gu Q, et al., 2020, Lanthanide-doped inorganic nanoparticles turn molecular triplet excitons bright, Nature, Vol: 587, Pages: 594-599, ISSN: 0028-0836
The generation, control and transfer of triplet excitons in molecular and hybrid systems is of great interest owing to their long lifetime and diffusion length in both solid-state and solution phase systems, and to their applications in light emission1, optoelectronics2,3, photon frequency conversion4,5 and photocatalysis6,7. Molecular triplet excitons (bound electron-hole pairs) are 'dark states' because of the forbidden nature of the direct optical transition between the spin-zero ground state and the spin-one triplet levels8. Hence, triplet dynamics are conventionally controlled through heavy-metal-based spin-orbit coupling9-11 or tuning of the singlet-triplet energy splitting12,13 via molecular design. Both these methods place constraints on the range of properties that can be modified and the molecular structures that can be used. Here we demonstrate that it is possible to control triplet dynamics by coupling organic molecules to lanthanide-doped inorganic insulating nanoparticles. This allows the classically forbidden transitions from the ground-state singlet to excited-state triplets to gain oscillator strength, enabling triplets to be directly generated on molecules via photon absorption. Photogenerated singlet excitons can be converted to triplet excitons on sub-10-picosecond timescales with unity efficiency by intersystem crossing. Triplet exciton states of the molecules can undergo energy transfer to the lanthanide ions with unity efficiency, which allows us to achieve luminescent harvesting of the dark triplet excitons. Furthermore, we demonstrate that the triplet excitons generated in the lanthanide nanoparticle-molecule hybrid systems by near-infrared photoexcitation can undergo efficient upconversion via a lanthanide-triplet excitation fusion process: this process enables endothermic upconversion and allows efficient upconversion from near-infrared to visible frequencies in the solid state. These results provide a new way to control triplet excitons
Nikolis VC, Dong Y, Kublitski J, et al., 2020, Field Effect versus Driving Force: Charge Generation in Small-Molecule Organic Solar Cells, ADVANCED ENERGY MATERIALS, ISSN: 1614-6832
Bristow H, Jacoutot P, Scaccabarozzi AD, et al., 2020, Nonfullerene-Based Organic Photodetectors for Ultrahigh Sensitivity Visible Light Detection, ACS APPLIED MATERIALS & INTERFACES, Vol: 12, Pages: 48836-48844, ISSN: 1944-8244
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- Citations: 33
Dong 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: 2041-1723
Organic 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.
Hopper TR, Jeong A, Gorodetsky A, et al., 2020, Kinetic modelling of intraband carrier relaxation in bulk and nanocrystalline lead-halide perovskites, Physical Chemistry Chemical Physics, Vol: 22, Pages: 17605-17611, ISSN: 1463-9076
The relaxation of high-energy “hot” carriers in semiconductors is known to involve the redistribution of energy between hot and cold carriers, as well as the transfer of energy from hot carriers to phonons. Over the past few years, these two processes have been identified in lead-halide perovskites (LHPs) using ultrafast pump-probe experiments, but their interplay is not fully understood. Here we present a practical and intuitive kinetic model that accounts for the effects of both hot and cold carriers on carrier relaxation in LHPs. We apply this model to describe the dynamics of hot carriers in bulk and nanocrystal CsPbBr3 as observed by multi-pulse “pump-push-probe” spectroscopy. The model captures the slowing of relaxation dynamics in the materials as the number of hot carriers increases, which has previously been explained by a “hot-phonon bottleneck” mechanism. The model also correctly predicts an acceleration of the relaxation kinetics as the number of cold carriers in the samples is increased. Using a series of natural approximations, we reduce our model to a simple form containing terms for the carrier-carrier and carrier-phonon interactions. The model can be instrumental for evaluating the details of carrier relaxation and carrier-phonon couplings in LHPs and other soft optoelectronic materials.
Cha H, Zheng Y, Dong Y, et al., 2020, Exciton and charge carrier dynamics in highly crystalline PTQ10:IDIC organic solar cells, Advanced Energy Materials, Pages: 1-11, ISSN: 1614-6832
Herein the morphology and exciton/charge carrier dynamics in bulk heterojunctions (BHJs) of the donor polymer PTQ10 and molecular acceptor IDIC are investigated. PTQ10:IDIC BHJs are shown to be particularly promising for low cost organic solar cells (OSCs). It is found that both PTQ10 and IDIC show remarkably high crystallinity in optimized BHJs, with GIWAXS data indicating pi‐pi stacking coherence lengths of up to 8 nm. Exciton‐exciton annihilation studies indicate long exciton diffusion lengths for both neat materials (19 nm for PTQ10 and 9.5 nm for IDIC), enabling efficient exciton separation with half lives of 1 and 3 ps, despite the high degree of phase segregation in this blend. Transient absorption data indicate exciton separation leads to the formation of two spectrally distinct species, assigned to interfacial charge transfer (CT) states and separated charges. CT state decay is correlated with the appearance of additional separate charges, indicating relatively efficient CT state dissociation, attributed to the high crystallinity of this blend. The results emphasize the potential for high material crystallinity to enhance charge separation and collection in OSCs, but also that long exciton diffusion lengths are likely to be essential for efficient exciton separation in such high crystallinity devices.
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