Imperial College London

DrArtemBakulin

Faculty of Natural SciencesDepartment of Chemistry

Research Fellow (Royal Society URF)
 
 
 
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G22aMolecular Sciences Research HubWhite City Campus

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Summary

 

Publications

Publication Type
Year
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45 results found

Nikolis VC, Dong Y, Kublitski J, Benduhn J, Zheng X, Huang C, Yüzer AC, Ince M, Spoltore D, Durrant JR, Bakulin AA, Vandewal Ket al., 2020, Field Effect versus Driving Force: Charge Generation in Small‐Molecule Organic Solar Cells, Advanced Energy Materials, Pages: 2002124-2002124, ISSN: 1614-6832

Journal article

Bristow H, Jacoutot P, Scaccabarozzi AD, Babics M, Moser M, Wadsworth A, Anthopoulos TD, Bakulin A, McCulloch I, Gasparini Net al., 2020, Nonfullerene-Based Organic Photodetectors for Ultrahigh Sensitivity Visible Light Detection., ACS Appl Mater Interfaces, Vol: 12, Pages: 48836-48844

It is well established that for organic photodetectors (OPDs) to compete with their inorganic counterparts, low dark currents at reverse bias must be achieved. Here, two rhodanine-terminated nonfullerene acceptors O-FBR and O-IDTBR are shown to deliver low dark currents at -2 V of 0.17 and 0.84 nA cm-2, respectively, when combined with the synthetically scalable polymer PTQ10 in OPD. These low dark currents contribute to the excellent sensitivity to low light of the detectors, reaching values of 0.57 μW cm-2 for PTQ10:O-FBR-based OPD and 2.12 μW cm-2 for PTQ10:O-IDTBR-based OPD. In both cases, this sensitivity exceeds that of a commercially available silicon photodiode. The responsivity of the PTQ10:O-FBR-based OPD of 0.34 AW-1 under a reverse bias of -2 V also exceeds that of a silicon photodiode. Meanwhile, the responsivity of the PTQ10:O-IDTBR of 0.03 AW-1 is limited by the energetic offset of the blend. The OPDs deliver high specific detectivities of 9.6 × 1012 Jones and 3.3 × 1011 Jones for O-FBR- and O-IDTBR-based blends, respectively. Both active layers are blade-coated in air, making them suitable for high-throughput methods. Finally, all three of the materials can be synthesized at low cost and on a large scale, making these blends good candidates for commercial OPD applications.

Journal article

Dong Y, Nikolis VC, Talnack F, Chin Y-C, Benduhn J, Londi G, Kublitski J, Zheng X, Mannsfeld SCB, Spoltore D, Muccioli L, Li J, Blase X, Beljonne D, Kim J-S, Bakulin AA, D'Avino G, Durrant JR, Vandewal Ket 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.

Journal article

Hopper TR, Jeong A, Gorodetsky A, Krieg F, Bodnarchuk MI, Huang X, Lovrincic R, Kovalenko MV, Bakulin Aet 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.

Journal article

Cha H, Zheng Y, Dong Y, Lee HH, Wu J, Bristow H, Zhang J, Lee HKH, Tsoi WC, Bakulin AA, McCulloch I, Durrant JRet 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.

Journal article

Xin C, Zhang J, Zhou X, Ma L, Hou F, Shi B, Pan S-J, Chen B, Wang P, Zhang D, Chen X, Zhao Y, Bakulin A, Li Y, Zhang Xet al., 2020, Defects Healing in Two-Step Deposited Perovskite Solar Cells via Formamidinium Iodide Compensation, ACS APPLIED ENERGY MATERIALS, Vol: 3, Pages: 3318-3327, ISSN: 2574-0962

Journal article

Schraff S, Maity S, Schleeper L, Dong Y, Lucas S, Bakulin AA, von Hauff E, Pammer Fet al., 2020, All-conjugated donor-acceptor block copolymers featuring a pentafulvenyl-polyisocyanide-acceptor, Polymer Chemistry, Vol: 11, Pages: 1852-1859, ISSN: 1759-9954

We report a fulvenyl-functionalized polyisocyanide (PIC2) with a high electron mobility of μe = 10−2 cm2 V−1 s−1. PIC2 has been incorporated into block-copolymers with either regioregular poly(3-dodecylthiophene) (P3DT → P(3DT-b-IC2)) or regioregular polythiazole (PTzTHX → P(TzTHX-b-IC2)). Block copolymer batches with different block-sizes have been isolated and their properties have been studied. Fluorescence quenching in the solid state and transient absorption spectroscopy indicate energy transfer from the donor- to the acceptor block upon photo-excitation. Fabrication of proof-of-principle organic photovoltaic cells with P(3DT-b-IC2) gave cells with an open circuit voltage (VOC) of ca. 0.89 V. The aggregation behavior of P(3DT-b-IC2) from solution was also studied, which revealed self-assembly into discreet microspheres of 1–8 μm diameter, with a size distribution of 1.72 (±0.37) μm under optimized aggregation conditions.

Journal article

Hopper TR, Gorodetsky A, Jeong A, Krieg F, Bodnarchuk MI, Maimaris M, Chaplain M, Macdonald TJ, Huang X, Lovrincic R, Kovalenko MV, Bakulin AAet al., 2020, Hot carrier dynamics in perovskite nanocrystal solids: role of the cold carriers, nanoconfinement and the surface, Nano Letters, Vol: 20, Pages: 2271-2278, ISSN: 1530-6984

Carrier cooling is of widespread interest in the field of semiconductor science. It is linked to carrier-carrier and carrier-phonon coupling, and has profound implications for the photovoltaic performance of materials. Recent transient optical studies have shown that a high carrier density in lead-halide perovskites (LHPs) can reduce the cooling rate through a “phonon bottleneck”. However, the role of carrier-carrier interactions, and the material properties that control cooling in LHPs, are still disputed. To address these factors, we utilize ultrafast “pump-push-probe” spectroscopy on LHP nanocrystal (NC) films. We find that the addition of cold carriers to LHP NCs increases the cooling rate, competing with the phonon bottleneck. By comparing different NCs and bulk samples, we deduce that the cooling behavior is intrinsic to the LHP composition, and independent of the NC size or surface. This can be contrasted with other colloidal nanomaterials, where confinement and trapping considerably influence the cooling dynamics.

Journal article

Hamid Z, Wadsworth A, Rezasoltani E, Holliday S, Azzouzi M, Neophytou M, Guilbert AAY, Dong Y, Little MS, Mukherjee S, Herzing AA, Bristow H, Kline RJ, DeLongchamp DM, Bakulin AA, Durrant JR, Nelson J, McCulloch Iet al., 2020, Influence of Polymer Aggregation and Liquid Immiscibility on Morphology Tuning by Varying Composition in PffBT4T-2DT/Nonfullerene Organic Solar Cells, ADVANCED ENERGY MATERIALS, Vol: 10, ISSN: 1614-6832

Journal article

Hopper TR, Jeong A, Gorodetsky A, Krieg F, Bodnarchuk MI, Huang X, Lovrincic R, Kovalenko MV, Bakulin AAet al., 2019, Kinetic modelling of carrier cooling in lead halide perovskite materials, Publisher: arXiv

The relaxation of high-energy "hot" carriers in semiconductors is known toinvolve the redistribution of energy between (i) hot and cold carriers and (ii)hot carriers and phonons. Over the past few years, these two processes havebeen identified in lead-halide perovskites (LHPs) using ultrafast pump-probeexperiments, but the interplay between these processes is not fully understood.Here we present a comprehensive kinetic model to elucidate the individualeffects of the hot and cold carriers in bulk and nanocrystal $CsPbBr_{3}$ filmsobtained from "pump-push-probe" measurements. In accordance with our previouswork, we observe that the cooling dynamics in the materials decelerate as thenumber of hot carriers increases, which we explain through a "hot-phononbottleneck" mechanism. On the other hand, as the number of cold carriersincreases, we observe an acceleration of the cooling kinetics in the samples.We describe the interplay of these opposing effects using our model, and byusing series of natural approximations, reduce this model to a simple formcontaining terms for the carrier-carrier and carrier-phonon interactions. Themodel can be instrumental for evaluating the details of carrier cooling andelectron-phonon couplings in a broad range of LHP optoelectronic materials.

Working paper

Zhang J, Futscher M, Lami V, Kosasih F, Cho C, Gu Q, Sadhanala A, Pearson A, Kan B, Divitini G, Wan X, Credgington D, Greenham N, Chen Y, Ducati C, Ehrler B, Vaynzof Y, Friend R, Bakulin Aet al., 2019, Sequentially deposited versus conventional nonfullerene organic solar cells: interfacial trap states, vertical stratification, and exciton dissociation, Advanced Energy Materials, Vol: 9, ISSN: 1614-6832

Bulk-heterojunction (BHJ) non-fullerene organic solar cells prepared from sequentially deposited donor and acceptor layers (sq-BHJ) have recently been promising to be highly efficient, environmentally friendly, and compatible with large area and roll-to-roll fabrication. However, the related photophysics at donor-acceptor interface and the vertical heterogeneity of donor-acceptor distribution, critical for exciton dissociation and device performance, are largely unexplored. Herein, steady-state and time-resolved optical and electrical techniques are employed to characterize the interfacial trap states. Correlating with the luminescent efficiency of interfacial states and its non-radiative recombination, interfacial trap states are characterized to be about 50% more populated in the sq-BHJ devices than the as-cast BHJ (c-BHJ), which probably limits the device voltage output. Cross-sectional energy-dispersive X-ray spectroscopy and ultraviolet photoemission spectroscopy depth profiling directly visualize the donor-acceptor vertical stratification with a precision of 1-2 nm. From the proposed “needle” model, the high exciton dissociation efficiency is rationalized. Our study highlights the promise of sequential deposition to fabricate efficient solar cells, and points towards improving the voltage output and overall device performance via eliminating interfacial trap states.

Journal article

Selim S, Pastor E, García-Tecedor M, Morris MR, Francas L, Sachs M, Moss B, Corby S, Mesa CA, Gimenez S, Kafizas A, Bakulin AA, Durrant JRet al., 2019, Impact of oxygen vacancy occupancy on charge carrier dynamics in BiVO4 photoanodes, Journal of the American Chemical Society, Vol: 141, Pages: 18791-18798, ISSN: 0002-7863

Oxygen vacancies are ubiquitous in metal oxides and critical to performance, yet the impact of these states upon charge carrier dynamics important for photoelectrochemical and photocatalytic applications, remains contentious and poorly understood. A key challenge is the unambiguous identification of spectroscopic fingerprints which can be used to track their function. Herein, we employ five complementary techniques to modulate the electronic occupancy of states associated with oxygen vacancies in situ in BiVO4 photoanodes, allowing us to identify a spectral signature for the ionisation of these states. We obtain an activation energy of ̴ 0.2 eV for this ionisation process, with thermally activated electron de-trapping from these states determining the kinetics of electron extraction, consistent with improved photoelectrochemical performance at higher temperatures. Bulk, un-ionised states however, function as deep hole traps, with such trapped holes being energetically unable to drive water oxidation. These observations help address recent controversies in the literature over oxygen vacancy function, providing new insights into their impact upon photoelectrochemical performance.

Journal article

Weu A, Kumar R, Butscher JF, Lami V, Paulus F, Bakulin AA, Yaynzof Yet al., 2019, Energy transfer to a stable donor suppresses degradation in organic solar cells, Advanced Functional Materials, Vol: 30, Pages: 1-9, ISSN: 1616-301X

Despite many advances toward improving the stability of organic photovoltaic devices, environmental degradation under ambient conditions remains a challenging obstacle for future application. Particularly conventional systems employing fullerene derivatives are prone to oxidize under illumination, limiting their applicability. Here, the environmental stability of the small molecule donor DRCN5T together with the fullerene acceptor PC70BM is reported. It is found that this system exhibits exceptional device stability, mainly due to almost constant short‐circuit current. By employing ultrafast femtosecond transient absorption spectroscopy, this remarkable stability is attributed to two separate mechanisms: 1) DRCN5T exhibits high intrinsic resistance toward external factors, showing no signs of deterioration. 2) The highly sensitive PC70BM is stabilized against degradation by the presence of DRCN5T through ultrafast, long‐range energy transfer to the donor, rapidly quenching the fullerene excited states which are otherwise precursors for chemical oxidation. It is proposed that this photoprotective mechanism be utilized to improve the device stability of other systems, including nonfullerene acceptors and ternary blends.

Journal article

Corby S, Pastor E, Dong Y, Zheng X, Francàs L, Sachs M, Selim S, Kafizas A, Bakulin AA, Durrant JRet al., 2019, Charge separation, band-bending, and recombination in WO3 photoanodes, Journal of Physical Chemistry Letters, Vol: 10, Pages: 5395-5401, ISSN: 1948-7185

In metal oxide-based photoelectrochemical devices, the spatial separation of photogenerated electrons and holes is typically attributed to band-bending at the oxide/electrolyte interface. However, direct evidence of such band-bending impacting upon charge carrier lifetimes has been very limited to date. Herein we use ultrafast spectroscopy to track the rapid relaxation of holes in the space-charge layer and their recombination with trapped electrons in WO3 photoanodes. We observe that applied bias can significantly increase carrier lifetimes on all time scales from picoseconds to seconds and attribute this to enhanced band-bending correlated with changes in oxygen vacancy state occupancy. We show that analogous enhancements in carrier lifetimes can be obtained by changes in electrolyte composition, even in the absence of applied bias, highlighting routes to improve photoconversion yields/performance, through changes in band-bending. This study thus demonstrates the direct connection between carrier lifetime enhancement, increased band-bending, and oxygen vacancy defect state occupancy.

Journal article

Vezie MS, Azzouzi M, Telford AM, Hopper TR, Sieval AB, Hummelen JC, Fallon K, Bronstein H, Kirchartz T, Bakulin AA, Clarke TM, Nelson Jet al., 2019, Impact of marginal exciton–charge-transfer state offset on charge generation and recombination in polymer:fullerene solar cells, ACS Energy Letters, Vol: 4, Pages: 2096-2103, ISSN: 2380-8195

The energetic offset between the initial photoexcited state and charge-transfer (CT) state in organic heterojunction solar cells influences both charge generation and open-circuit voltage (Voc). Here, we use time-resolved spectroscopy and voltage loss measurements to analyze the effect of the exciton–CT state offset on charge transfer, separation, and recombination processes in blends of a low-band-gap polymer (INDT-S) with fullerene derivatives of different electron affinity (PCBM and KL). For the lower exciton–CT state offset blend (INDT-S:PCBM), both photocurrent generation and nonradiative voltage losses are lower. The INDT-S:PCBM blend shows different excited-state dynamics depending on whether the donor or acceptor is photoexcited. Surprisingly, the charge recombination dynamics in INDT-S:PCBM are distinctly faster than those in INDT-S:KL upon excitation of the donor. We reconcile these observations using a kinetic model and by considering hybridization between the lowest excitonic and CT states. The modeling results show that this hybridization can significantly reduce Voc losses while still allowing reasonable charge generation efficiency.

Journal article

Hopper T, Qian D, Yang L, Wang X, Zhou K, Kumar R, Ma W, He C, Hou J, Gao F, Bakulin Aet al., 2019, Control of donor-acceptor photophysics through structural modification of a “twisting” push-pull molecule, Chemistry of Materials, Vol: 31, Pages: 6860-6869, ISSN: 0897-4756

In contemporary organic solar cell (OSC) research, small A-D-A molecules comprising electron donor (D) and acceptor (A) units are increasingly used as a means to control the optoelectronic properties of photovoltaic blends. Slight structural variations to these A-D-A molecules can result in profound changes to the performance of the OSCs. Herein, we study two A-D-A molecules, BTCN-O and BTCN-M, which are identical in structure apart from a subtle difference in the position of alkyl chains, which force the molecules to adopt different equilibrium conformations. These steric effects cause the respective molecules to work better as an electron donor and acceptor when blended with benchmark acceptor and donor materials (PC71BM and PBDB-T). We study the photophysics of these “D:A” blends and devices using a combination of steady-state and time-resolved spectroscopic techniques. Time-resolved photoluminescence reveals the impact of the molecular conformation on the quenching of the A-D-A emission when BTCN-O and BTCN-M are blended with PBDB-T or PC71BM. Ultrafast broadband transient absorption spectroscopy demonstrates that the dynamics of charge separation are essentially identical when comparing BTCN-M and BTCN-O based blends, but the recombination dynamics are quite dissimilar. This suggests that the device performance is ultimately determined by the morphology of the blends imposed by the A-D-A conformation. This notion is supported by X-ray scattering data from the “D:A” films, and electroluminescence data and pump-push-photocurrent spectroscopy on the “D:A” devices. Our findings provide insight into the remarkable structure-function relationship in A-D-A molecules, and emphasize the need for careful morphological and energetic considerations when designing high-performance OSCs.

Journal article

Hopper T, Gorodetsky A, Krieg F, Bodnarchuk M, Huang X, Lovrincic R, Kovalenko MV, Bakulin AAet al., 2019, Hot-carrier cooling in lead-bromide perovskite materials, Physical Chemistry of Semiconductor Materials and Interfaces XVIII, Publisher: Society of Photo-optical Instrumentation Engineers, ISSN: 0277-786X

Lead-halide perovskites are currently the highest-performing solution-processable semiconductors for solar energy conversion, with record efficiencies rapidly approaching that of the Shockley-Queisser limit for single-junction solar cells. Further progress in the development of lead-halide perovskite solar cells must overcome this limit, which largely stems from the ultrafast relaxation of high-energy hot carriers above the bandedge. In this contribution, we use a highly-specialized pump-push-probe technique to unravel the key parameters which control hot carrier cooling in bulk and nanocrystal (NC) lead bromide perovskites with different material composition, NC diameter and surface treatment. All samples exhibit slower cooling for higher hot carrier densities, which we assign to a phonon bottleneck mechanism. By comparing this density-dependent cooling behavior in the different samples, we find that the weak quantum confinement of electronic states and the surface defects in the NCs play no observable role in the hot carrier relaxation. Meanwhile, in accordance with our previous observations for bulk perovskites, we show that the cation plays a critical role towards carrier cooling in the perovskite NCs, as evidenced by the faster overall cooling in the hybrid FAPbBr3 NCs with respect to the all-inorganic CsPbBr3 NCs. These observations highlight the crucial role of the cations toward the phononic properties of lead-halide perovskites, and further point towards the defect tolerance of these emerging solution-processed semiconductors.

Conference paper

Bakulin A, Pastor E, Park J-S, Steier L, Kim S, Grätzel M, Durrant J, Walsh Aet al., 2019, In situ observation of picosecond polaron self-localisation in α-Fe2O3 photoelectrochemical cells, Nature Communications, Vol: 10, ISSN: 2041-1723

Hematite (α-Fe2O3) is the most studied artificial oxygen-evolving photo-anode and yet its efficiency limitations and their origin remain unknown. A sub-picosecond reorganisation of the hematite structure has been proposed as the mechanism which dictates carrier lifetimes, energetics and the ultimate conversion yields. However, the importance of this reorganisation for actual device performance is unclear. Here we report an in situ observation of charge carrier self-localisation in a hematite device, and demonstrate that this process affects recombination losses in photoelectrochemical cells. We apply an ultrafast, device-based optical-control method to resolve the subpicosecond formation of small polarons and estimate their reorganisation energy to be ~0.5 eV. Coherent oscillations in the photocurrent signals indicate that polaron formation may be coupled to specific phonon modes (<100 cm-1). Our results bring together spectroscopic and device characterisation approaches to reveal new photophysics of broadly-studied hematite devices.

Journal article

Dong Y, Cha H, Zhang J, Pastor E, Tuladhar PS, McCulloch I, Durrant JR, Bakulin AAet al., 2019, The binding energy and dynamics of charge-transfer states in organic photovoltaics with low driving force for charge separation, Journal of Chemical Physics, Vol: 150, ISSN: 0021-9606

Recent progress in organic photovoltaics (OPVs) has been enabled by optimization of the energetic driving force for charge separation, and thus maximization of open-circuit voltage, using non-fullerene acceptor (NFA) materials. In spite of this, the carrier dynamics and relative energies of the key states controlling the photophysics of these systems are still under debate. Herein, we report an in-depth ultrafast spectroscopic study of a representative OPV system based on a polymer donor PffBT4T-2OD and a small-molecule NFA EH-IDTBR. Global analysis of the transient absorption data reveals efficient energy transfer between donor and acceptor molecules. The extracted kinetics suggest that slow (∼15 ps) generation of charge carriers is followed by significant geminate recombination. This contrasts with the "reference" PffBT4T-2OD:PC71BM system where bimolecular recombination dominates. Using temperature-dependent pump-push-photocurrent spectroscopy, we estimate the activation energy for the dissociation of bound charge-transfer states in PffBT4T-2OD:EH-IDTBR to be 100 ± 6 meV. We also observe an additional activation energy of 14 ± 7 meV, which we assign to the de-trapping of mobile carriers. This work provides a comprehensive picture of photophysics in a system representing new generation of OPV blends with a small driving force for charge separation.

Journal article

Becker-Koch D, Rivkin B, Paulus F, Xiang H, Dong Y, Chen Z, Bakulin AA, Vaynzof Yet al., 2019, Probing charge transfer states at organic and hybrid internal interfaces by photothermal deflection spectroscopy, Journal of Physics: Condensed Matter, Vol: 31, ISSN: 0953-8984

In organic and hybrid photovoltaic devices, the asymmetry required for charge separation necessitates the use of a donor and an acceptor material, resulting in the formation of internal interfaces in the device active layer. While the core objective of these interfaces is to facilitate charge separation, bound states between electrons and holes may form across them, resulting in a loss mechanism that diminishes the performance of the solar cells. These interfacial transitions appear in organic systems as charge transfer (CT) states and as bound charge pairs (BCP) in hybrid systems. Despite being similar, the latter are far less investigated. Herein, we employ photothermal deflection spectroscopy and pump-push-probe experiments in order to determine the characteristics and dynamics of interfacial states in two model systems: an organic P3HT:PCBM and hybrid P3HT:ZnO photovoltaic layer. By controlling the area of the internal interface, we identify CT states between 1.4 eV and 1.8 eV in the organic bulk-heterojunction (BHJ) and BCP between 1.1 eV and 1.4 eV in the hybrid BHJ. The energetic distribution of these states suggests that they not only contribute to losses in photocurrent, but also significantly limit the possible maximum open circuit voltage obtainable from these devices.

Journal article

Yu H, Wang H, Zhang J, Lu J, Yuan Z, Xu W, Hultman L, Bakulin AA, Friend RH, Wang J, Liu X-K, Gao Fet al., 2019, Efficient and tunable electroluminescence from in situ synthesized perovskite quantum dots, Small, Vol: 15, ISSN: 1613-6810

Semiconductor quantum dots (QDs) are among the most promising next-generation optoelectronic materials. QDs are generally obtained through either epitaxial or colloidal growth and carry the promise for solution-processed high-performance optoelectronic devices such as light-emitting diodes (LEDs), solar cells, etc. Herein, a straightforward approach to synthesize perovskite QDs and demonstrate their applications in efficient LEDs is reported. The perovskite QDs with controllable crystal sizes and properties are in situ synthesized through one-step spin-coating from perovskite precursor solutions followed by thermal annealing. These perovskite QDs feature size-dependent quantum confinement effect (with readily tunable emissions) and radiative monomolecular recombination. Despite the substantial structural inhomogeneity, the in situ generated perovskite QDs films emit narrow-bandwidth emission and high color stability due to efficient energy transfer between nanostructures that sweeps away the unfavorable disorder effects. Based on these materials, efficient LEDs with external quantum efficiencies up to 11.0% are realized. This makes the technologically appealing in situ approach promising for further development of state-of-the-art LED systems and other optoelectronic devices.

Journal article

Nübling F, Hopper TR, Kuei B, Komber H, Untilova V, Schmidt S, Brinkmann M, Gomez E, Bakulin AA, Sommer Met al., 2019, Block junction-functionalized all-conjugated donor-acceptor block copolymers., ACS Applied Materials and Interfaces, Vol: 11, Pages: 1143-1155, ISSN: 1944-8244

Junction-functionalized donor-acceptor (D-A) block copolymers (BCPs) enable spatial and electronic control over interfacial charge dynamics in excitonic devices such as solar cells. Here we present the design, synthesis, morphology and electronic characterization of block junction-functionalized, all-conjugated, all-crystalline D-A BCPs. Poly(3-hexylthiophene) (P3HT), a single thienylated diketopyrrolopyrrole (ThxDPPThx, x=1 or 2) unit and poly{[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (PNDIT2) is used as donor, interfacial unit and acceptor, respectively. Almost all C-C coupling steps are accomplished by virtue of C-H activation. Synthesis of the macroreagent H-P3HT-ThxDPPThx, with x determining its C-H reactivity, is key to the synthesis of various BCPs of type H-P3HT-ThxDPPThx-block-PNDIT2. Morphology is determined from a combination of calorimetry, transmission electron microscopy (TEM) and thin film scattering. Block copolymer crystallinity of P3HT and PNDIT2 is reduced indicating frustrated crystallization. A long period lp, is invisible from TEM, but shows up in resonant soft X-ray scattering experiments at a length scale of lp~ 60 nm. Photoluminescence of H-P3HT-ThxDPPThx indicates efficient transfer of the excitation energy to the DPP chain end, but is quenched in BCP films. Transient absorption and pump-push-photocurrent spectroscopies reveal geminate recombination (GR) as the main loss channel in as-prepared BCP films independent of junction functionalization. Melt-annealing increases GR as a result of the low degree of crystallinity and poorly defined interfaces, and additionally changes backbone orientation of PNDIT2 from face-on to edge-on. These morphological effects dominate solar cell performance and cause an insensitivity to the presence of the block junction.

Journal article

Gallop NP, Selig O, Giubertoni G, Bakker HJ, Rezus YLA, Frost JM, Jansen TLC, Lovrincic R, Bakulin AAet al., 2018, Rotational cation dynamics in metal halide perovskites: Effect on phonons and material properties, Journal of Physical Chemistry Letters, Vol: 9, Pages: 5987-5997, ISSN: 1948-7185

The dynamics of organic cations in metal halide hybrid perovskites (MHPs) have been investigated using numerous experimental and computational techniques because of their suspected effects on the properties of MHPs. In this Perspective, we summarize and reconcile key findings and present new data to synthesize a unified understanding of the dynamics of the cations. We conclude that theory and experiment collectively paint a relatively complete picture of rotational dynamics within MHPs. This picture is then used to discuss the consequences of structural dynamics for electron–phonon interactions and their effect on material properties by providing a brief account of key studies that correlate cation dynamics with the dynamics of the inorganic sublattice and overall device properties.

Journal article

Zhang J, Kan B, Pearson AJ, Parnell AJ, Cooper JFK, Liu X-K, Conaghan PJ, Hopper TR, Wu Y, Wan X, Gao F, Greenham NC, Bakulin AA, Chen Y, Friend RHet al., 2018, Efficient non-fullerene organic solar cells employing sequentially deposited donor-acceptor layers, Journal of Materials Chemistry A, Vol: 6, Pages: 18225-18233, ISSN: 2050-7496

Non-fullerene acceptors (NFAs) have recently outperformed their fullerene counterparts in binary bulk-heterojunction (BHJ) organic solar cells (OSCs). Further development of NFA OSCs may benefit other novel OSC device structures that alter or extend the standard BHJ concept. Here, we report such a new processing route that forms a BHJ-like morphology between sequentially processed polymer donor and NFA with high power conversion efficiencies in excess of 10%. Both devices show similar charge generation and recombination behaviours, supporting formation of similar BHJ active layers. We correlate the ∼30 meV smaller open-circuit voltage in sq-BHJ devices to more substantial non-radiative recombination by voltage loss analysis. We also determine the exciton diffusion length of benchmark polymer PBDB-T to be 10 ± 3 nm. Our results demonstrate high-efficiency OSC devices using sequential deposition method and provide new opportunities to further improve performance of state-of-the-art OSCs.

Journal article

Hopper T, Gorodetsky A, Frost JM, Müller C, Lovrincic R, Bakulin Aet al., 2018, Ultrafast Intraband Spectroscopy of Hot-Carrier Cooling in Lead-Halide Perovskites, ACS Energy Letters, Vol: 3, Pages: 2199-2205, ISSN: 2380-8195

The rapid relaxation of above-band-gap “hot” carriers (HCs) imposes the key efficiency limit in lead-halide perovskite (LHP) solar cells. Recent studies have indicated that HC cooling in these systems may be sensitive to materials composition, as well as the energy and density of excited states. However, the key parameters underpinning the cooling mechanism are currently under debate. Here we use a sequence of ultrafast optical pulses (visible pump–infrared push–infrared probe) to directly compare the intraband cooling dynamics in five common LHPs: FAPbI3, FAPbBr3, MAPbI3, MAPbBr3, and CsPbBr3. We observe ∼100–900 fs cooling times, with slower cooling at higher HC densities. This effect is strongest in the all-inorganic Cs-based system, compared to the hybrid analogues with organic cations. These observations, together with band structure calculations, allow us to quantify the origin of the “hot-phonon bottleneck” in LHPs and assert the thermodynamic contribution of a symmetry-breaking organic cation toward rapid HC cooling.

Journal article

Gorodetsky A, Hopper T, Bakulin A, 2018, Ultrafast Carrier Cooling in Led Halide Perovskite Solar Cells, International Conference on Laser Optics (ICLO), Publisher: IEEE, Pages: 132-132

Conference paper

Qian D, Zheng Z, Yao H, Tress W, Hopper T, Chen S, Li S, Liu J, Chen S, Zhang J, Liu X-K, Gao B, Ouyang L, Jin Y, Pozina G, Buyanova I, Chen W, Inganäs O, Coropceanu V, Bredas J-L, Yan H, Hou J, Zhang F, Bakulin A, Gao Fet al., 2018, Design rules for minimizing voltage losses in high-efficiency organic solar cells, Nature Materials, Vol: 17, Pages: 703-709, ISSN: 1476-1122

The open-circuit voltage of organic solar cells is usually lower than the values achieved in inorganic or perovskite photovoltaic devices with comparable bandgaps. Energy losses during charge separation at the donor–acceptor interface and non-radiative recombination are among the main causes of such voltage losses. Here we combine spectroscopic and quantum-chemistry approaches to identify key rules for minimizing voltage losses: (1) a low energy offset between donor and acceptor molecular states and (2) high photoluminescence yield of the low-gap material in the blend. Following these rules, we present a range of existing and new donor–acceptor systems that combine efficient photocurrent generation with electroluminescence yield up to 0.03%, leading to non-radiative voltage losses as small as 0.21 V. This study provides a rationale to explain and further improve the performance of recently demonstrated high-open-circuit-voltage organic solar cells.

Journal article

Weu A, Hopper T, Lami V, Kreß J, Bakulin A, Vaynzof Yet al., 2018, Field-assisted exciton dissociation in highly efficient PffBT4T-2OD:Fullerene organic solar cells, Chemistry of Materials, Vol: 30, Pages: 2660-2667, ISSN: 0897-4756

Understanding the photophysics of charge generation in organic semiconductors is a critical step toward the further optimization of organic solar cells. The separation of electron–hole pairs in systems with large energy offsets is relatively well-understood; however, the photophysics in blends with low driving energy remains unclear. Herein, we use the material system PffBT4T-2OD:PC71BM as an example to show that the built-in electric field plays a critical role toward long-range charge separation in high-performance devices. By using steady-state and time-resolved spectroscopic techniques, we show that in neat films an energetic barrier impedes polymer exciton dissociation, preventing charge transfer to the fullerene acceptor. In complete devices, this barrier is diminished due to the built-in electric field provided by the interlayers/contacts and accompanying space-charge distribution. The observed behavior could also be relevant to other systems with low driving energy and emphasizes the importance of using complete devices, rather than solely films, for photophysical studies.

Journal article

Zhang J, Gu Q, Do TT, Rundel K, Sonar P, Friend RH, McNeill CR, Bakulin AAet al., 2018, Control of Geminate Recombination by the Material Composition and Processing Conditions in Novel Polymer: Nonfullerene Acceptor Photovoltaic Devices., Journal of Physical Chemistry A, Vol: 122, Pages: 1253-1260, ISSN: 1089-5639

Herein, we report on the charge dynamics of photovoltaic devices based on two novel small-molecule nonfullerene acceptors featuring a central ketone unit. Using ultrafast near-infrared spectroscopy with optical and photocurrent detection methods, we identify one of the key loss channels in the devices as geminate recombination (GR) of interfacial charge transfer states (CTSs). We find that the magnitude of GR is highly sensitive to the choice of solvent and annealing conditions. Interestingly, regardless of these processing conditions, the same lifetime for GR (∼130 ps) is obtained by both detection methods upon decomposing the complex broadband transient optical spectra, suggesting this time scale is inherent and independent of morphology. These observations suggest that the CTSs in the studied material blends are mostly strongly bound, and that charge generation from these states is highly inefficient. We further rationalize our results by considering the impact of the processing on the morphology of the mixed donor and acceptor domains and discuss the potential consequences of the early charge dynamics on the performance of emerging nonfullerene photovoltaic devices. Our results demonstrate that careful choice of processing conditions enables enhanced exciton harvesting and suppression of GR by more than 3 orders of magnitude.

Journal article

Kan B, Zhang J, Liu F, Wan X, Li C, Ke X, Wang Y, Feng H, Zhang Y, Long G, Friend RH, Bakulin AA, Chen Yet al., 2017, Fine-Tuning the Energy Levels of a Nonfullerene Small-Molecule Acceptor to Achieve a High Short-Circuit Current and a Power Conversion Efficiency over 12% in Organic Solar Cells., Advanced Materials, Vol: 30, ISSN: 0935-9648

Organic solar cell optimization requires careful balancing of current-voltage output of the materials system. Here, such optimization using ultrafast spectroscopy as a tool to optimize the material bandgap without altering ultrafast photophysics is reported. A new acceptor-donor-acceptor (A-D-A)-type small-molecule acceptor NCBDT is designed by modification of the D and A units of NFBDT. Compared to NFBDT, NCBDT exhibits upshifted highest occupied molecular orbital (HOMO) energy level mainly due to the additional octyl on the D unit and downshifted lowest unoccupied molecular orbital (LUMO) energy level due to the fluorination of A units. NCBDT has a low optical bandgap of 1.45 eV which extends the absorption range toward near-IR region, down to ≈860 nm. However, the 60 meV lowered LUMO level of NCBDT hardly changes the Voc level, and the elevation of the NCBDT HOMO does not have a substantial influence on the photophysics of the materials. Thus, for both NCBDT- and NFBDT-based systems, an unusually slow (≈400 ps) but ultimately efficient charge generation mediated by interfacial charge-pair states is observed, followed by effective charge extraction. As a result, the PBDB-T:NCBDT devices demonstrate an impressive power conversion efficiency over 12%-among the best for solution-processed organic solar cells.

Journal article

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