Publications
661 results found
Adler C, Selim S, Krivtsov I, et al., 2021, Photodoping and Fast Charge Extraction in Ionic Carbon Nitride Photoanodes
<jats:p>Ionic carbon nitrides based on poly(heptazine imides) (PHI) represent one of the most vigorously studied class of low-cost, tunable and stable polymeric materials with possible applications in photocatalysis and energy storage. However, the fundamental photophysical properties and processes that govern the performance of these materials are still poorly understood and have been studied mostly in form of particle suspensions. We study, for the first time, the photogenerated charge dynamics in highly stable and binder-free PHI photoanodes with excellent performance in photoelectrocatalytic alcohol conversions using <jats:italic>in operando</jats:italic> transient photocurrents (TPC) and spectroelectrochemical photoinduced absorption (PIA) measurements. Interestingly, we discover that light-induced accumulation of long-lived trapped electrons within the PHI film leads to effective photodoping of the PHI film, resulting in a significant improvement of photocurrent response due to more efficient electron transport. While photodoping has been previously reported for various inorganic and organic semiconductors, the here reported beneficial photodoping effect has never been shown before for carbon nitride materials. Furthermore, we find that the extraction kinetics of untrapped electrons are remarkably fast in these PHI photoanodes, with electron extraction times (ms) comparable to those measured for commonly employed metal oxide semiconductors (<jats:italic>e.g.</jats:italic>, TiO<jats:sub>2</jats:sub>, WO<jats:sub>3</jats:sub>, BiVO<jats:sub>4</jats:sub>). These results shed light on the excellent performance of PHI photoanodes in alcohol photoreforming, including very negative photocurrent onset, outstanding fill factor, and the possibility to carry out photoreforming under zero-bias conditions. More generally, the here reported photodoping effect and fast electron extraction in PHI photoanodes re
Mohapatra AA, Dong Y, Boregowda P, et al., 2021, Rational design of donor-acceptor based semiconducting copolymers with high dielectric constants, The Journal of Physical Chemistry C: Energy Conversion and Storage, Optical and Electronic Devices, Interfaces, Nanomaterials, and Hard Matter, Vol: 125, Pages: 6886-6896, ISSN: 1932-7447
The low dielectric constant of organic semiconductors limits the efficiency of organic solar cells (OSCs). In an attempt to improve the dielectric constant of conjugated polymers, we report the synthesis of three semiconducting copolymers by combining the thiophene-substituted diketopyrrolopyrrole (TDPP) monomer with three different monomeric units with varying electron donating/accepting strengths: benzodithiophene (BBT-3TEG-TDPP), TDPP (TDPP-3TEG-TDPP), and naphthalene diimide (P(gNDI-TDPP)). Among the series, BBT-3TEG-TDPP and P(gNDI-TDPP) exhibited the highest dielectric constants (∼5) at 1 MHz frequency, signifying the contribution of dipolar polarization from TEG side-chains. Furthermore, transient absorption spectroscopic studies performed on these polymers indicated low exciton diffusion length as observed in common organic semiconducting polymers. Our findings suggest that utilizing the polar side-chains enhances the dielectric constant in a frequency regime of megahertz. However, it is not sufficient to reduce the Coulombic interaction between hole and electron in excitonic solar cells.
Moss B, Wang Q, Butler K, et al., 2021, Linking in situ charge accumulation to electronic structure in doped SrTiO3 reveals design principles for hydrogen-evolving photocatalysts, Nature Materials, Vol: 20, Pages: 511-517, ISSN: 1476-1122
Recently, high solar-to-hydrogen efficiencies were demonstrated using La and Rh co-doped SrTiO3 (La,Rh:SrTiO3) incorporated into a low-cost and scalable Z-scheme device, known as a photocatalyst sheet. However, the unique properties that enable La,Rh:SrTiO3 to support this impressive performance are not fully understood. Combining in situ spectroelectrochemical measurements with density functional theory and photoelectron spectroscopy produces a depletion model of Rh:SrTiO3 and La,Rh:SrTiO3 photocatalyst sheets. This reveals remarkable properties, such as deep flatband potentials (+2 V versus the reversible hydrogen electrode) and a Rh oxidation state dependent reorganization of the electronic structure, involving the loss of a vacant Rh 4d mid-gap state. This reorganization enables Rh:SrTiO3 to be reduced by co-doping without compromising the p-type character. In situ time-resolved spectroscopies show that the electronic structure reorganization induced by Rh reduction controls the electron lifetime in photocatalyst sheets. In Rh:SrTiO3, enhanced lifetimes can only be obtained at negative applied potentials, where the complete Z-scheme operates inefficiently. La co-doping fixes Rh in the 3+ state, which results in long-lived photogenerated electrons even at very positive potentials (+1 V versus the reversible hydrogen electrode), in which both components of the complete device operate effectively. This understanding of the role of co-dopants provides a new insight into the design principles for water-splitting devices based on bandgap-engineered metal oxides.
Chang Y-H, Carron R, Ochoa M, et al., 2021, Insights from transient absorption spectroscopy into electron dynamics along the Ga-gradient in Cu(In,Ga)Se2 solar cells, Advanced Energy Materials, Vol: 11, ISSN: 1614-6832
Cu(In,Ga)Se2 solar cells have markedly increased their efficiency over the last decades currently reaching a record power conversion efficiency of 23.3%. Key aspects to this efficiency progress are the engineered bandgap gradient profile across the absorber depth, along with controlled incorporation of alkali atoms via post‐deposition treatments. Whereas the impact of these treatments on the carrier lifetime has been extensively studied in ungraded Cu(In,Ga)Se2 films, the role of the Ga‐gradient on carrier mobility has been less explored. Here, transient absorption spectroscopy (TAS) is utilized to investigate the impact of the Ga‐gradient profile on charge carrier dynamics. Minority carriers excited in large Cu(In,Ga)Se2 grains with a [Ga]/([Ga]+[In]) ratio between 0.2–0.5 are found to drift‐diffuse across ≈1/3 of the absorber layer to the engineered bandgap minimum within 2 ns, which corresponds to a mobility range of 8.7–58.9 cm2 V−1 s−1. In addition, the recombination times strongly depend on the Ga‐content, ranging from 19.1 ns in the energy minimum to 85 ps in the high Ga‐content region near the Mo‐back contact. An analytical model, as well as drift‐diffusion numerical simulations, fully decouple carrier transport and recombination behaviour in this complex composition‐graded absorber structure, demonstrating the potential of TAS.
Limbu 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
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- Citations: 17
Bozal-Ginesta C, Mesa CA, Eisenschmidt A, et al., 2021, Charge accumulation kinetics in multi-redox molecular catalysts immobilised on TiO<sub>2</sub>, CHEMICAL SCIENCE, Vol: 12, Pages: 946-959, ISSN: 2041-6520
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- Citations: 10
Rao RR, Stephens IEL, Durrant JR, 2021, Understanding What Controls the Rate of Electrochemical Oxygen Evolution, JOULE, Vol: 5, Pages: 16-18, ISSN: 2542-4351
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- Citations: 14
Wilson AA, Corby S, Francas L, et al., 2021, The effect of nanoparticulate PdO co-catalysts on the faradaic and light conversion efficiency of WO3 photoanodes for water oxidation, Physical Chemistry Chemical Physics, Vol: 23, Pages: 1285-1291, ISSN: 1463-9076
WO3 photoanodes offer rare stability in acidic media, but are limited by their selectivity for oxygen evolution over parasitic side reactions, when employed in photoelectrochemical (PEC) water splitting. Herein, this is remedied via the modification of nanostructured WO3 photoanodes with surface decorated PdO as an oxygen evolution co-catalyst (OEC). The photoanodes and co-catalyst particles are grown using an up-scalable aerosol assisted chemical vapour deposition (AA-CVD) route, and their physical properties characterised by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM) and UV-vis absorption spectroscopy. Subsequent PEC and transient photocurrent (TPC) measurements showed that the use of a PdO co-catalyst dramatically increases the faradaic efficiency (FE) of water oxidation from 52% to 92%, whilst simultaneously enhancing the photocurrent generation and charge extraction rate. The Pd oxidation state was found to be critical in achieving these notable improvements to the photoanode performance, which is primarily attributed to the higher selectivity towards oxygen evolution when PdO is used as an OEC and the formation of a favourable junction between WO3 and PdO, that drives band bending and charge separation.
Sachs M, Cha H, Kosco J, et al., 2021, Tracking Charge Transfer to Residual Metal Clusters in Conjugated Polymers for Photocatalytic Hydrogen Evolution (vol 142, pg 14574, 2020), JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, Vol: 143, Pages: 524-524, ISSN: 0002-7863
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- Citations: 1
Mesa CA, Rao RR, Francas L, et al., 2020, Reply to: Questioning the rate law in the analysis of water oxidation catalysis on haematite photoanodes, NATURE CHEMISTRY, Vol: 12, Pages: 1099-+, ISSN: 1755-4330
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- Citations: 8
Francàs L, Selim S, Corby S, et al., 2020, Water Oxidation Kinetics of Nanoporous BiVO4 Photoanodes Functionalised with Nickel/iron Oxyhydroxide Electrocatalysts
<jats:p>In this work, spectroelectrochemical techniques are employed to analyse the catalytic water oxidation performance of a series of three nickel/iron oxyhydroxide electrocatalysts deposited on FTO and BiVO<jats:sub>4</jats:sub>, at neutral pH. Similar electrochemical water oxidation performance is observed for each of the FeOOH, Ni(Fe)OOH and FeOOHNiOOH electrocatalysts studied, which is found to result from a balance between degree of charge accumulation and rate of water oxidation. Once added onto BiVO4 photoanodes, a large enhancement in the water oxidation photoelectrochemical performance is observed in comparison to the un-modified BiVO<jats:sub>4</jats:sub>. To understand the origin of this enhancement, the films were evaluated through time-resolved optical spectroscopic techniques, allowing comparisons between electrochemical and photoelectrochemical water oxidation. For all three catalysts, fast hole transfer from BiVO<jats:sub>4</jats:sub> to the catalyst is observed in the transient absorption data. Using operando photoinduced absorption measurements, we find that water oxidation is driven by oxidised states within the catalyst layer, following hole transfer from BiVO<jats:sub>4</jats:sub>. This charge transfer is correlated with a suppression of recombination losses which result in remarkably enhanced water oxidation performance relative to un-modified BiVO<jats:sub>4</jats:sub>. Moreover, despite similar electrocatalytic performance of all three electrocatalysts, we show that variations in water oxidation performance observed among the BiVO<jats:sub>4</jats:sub>/MOOH photoanodes stem from differences in photoelectrochemical and electrochemical charge accumulation in the catalyst layers. Under illumination, the amount of accumulated charge in the catalyst is driven by the injection of photogenerated holes from BiVO<jats:sub>4</jats:sub>, which is further affecte
Francàs L, Selim S, Corby S, et al., 2020, Water Oxidation Kinetics of Nanoporous BiVO4 Photoanodes Functionalised with Nickel/iron Oxyhydroxide Electrocatalysts
<jats:p>In this work, spectroelectrochemical techniques are employed to analyse the catalytic water oxidation performance of a series of three nickel/iron oxyhydroxide electrocatalysts deposited on FTO and BiVO<sub>4</sub>, at neutral pH. Similar electrochemical water oxidation performance is observed for each of the FeOOH, Ni(Fe)OOH and FeOOHNiOOH electrocatalysts studied, which is found to result from a balance between degree of charge accumulation and rate of water oxidation. Once added onto BiVO4 photoanodes, a large enhancement in the water oxidation photoelectrochemical performance is observed in comparison to the un-modified BiVO<sub>4</sub>. To understand the origin of this enhancement, the films were evaluated through time-resolved optical spectroscopic techniques, allowing comparisons between electrochemical and photoelectrochemical water oxidation. For all three catalysts, fast hole transfer from BiVO<sub>4</sub> to the catalyst is observed in the transient absorption data. Using operando photoinduced absorption measurements, we find that water oxidation is driven by oxidised states within the catalyst layer, following hole transfer from BiVO<sub>4</sub>. This charge transfer is correlated with a suppression of recombination losses which result in remarkably enhanced water oxidation performance relative to un-modified BiVO<sub>4</sub>. Moreover, despite similar electrocatalytic performance of all three electrocatalysts, we show that variations in water oxidation performance observed among the BiVO<sub>4</sub>/MOOH photoanodes stem from differences in photoelectrochemical and electrochemical charge accumulation in the catalyst layers. Under illumination, the amount of accumulated charge in the catalyst is driven by the injection of photogenerated holes from BiVO<sub>4&
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
Achilleos DS, Yang W, Kasap H, et al., 2020, Solar Reforming of Biomass with Homogeneous Carbon Dots, ANGEWANDTE CHEMIE-INTERNATIONAL EDITION, Vol: 59, Pages: 18184-18188, ISSN: 1433-7851
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- Citations: 52
Corby S, Tecedor M-G, Tengeler S, et al., 2020, Separating bulk and surface processes in NiO<sub><i>x</i></sub>electrocatalysts for water oxidation, SUSTAINABLE ENERGY & FUELS, Vol: 4, Pages: 5024-5030, ISSN: 2398-4902
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- Citations: 23
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.
Aitchison CM, Sachs M, Little MA, et al., 2020, Structure-activity relationships in well-defined conjugated oligomer photocatalysts for hydrogen production from water, CHEMICAL SCIENCE, Vol: 11, Pages: 8744-8756, ISSN: 2041-6520
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- Citations: 35
Moss B, Le H, Corby S, et al., 2020, Anisotropic electron transport limits performance of Bi2WO6 photoanodes, The Journal of Physical Chemistry C, Vol: 124, Pages: 18859-18867, ISSN: 1932-7447
Bi2WO6 is one of the simplest members of the versatile Aurivillius oxide family of materials. As an intriguing model system for Aurivillius oxides, BiVO4 exhibits low water oxidation onset potentials (∼0.5–0.6 VRHE) for driven solar water oxidation. Despite this, Bi2WO6 also produces low photocurrents in comparison to other metal oxides. Due to a lack of in situ studies, the reasons for such poor performance are not understood. In this study, Bi2WO6 photoanodes are synthesized by aerosol-assisted chemical vapor deposition. The charge carrier dynamics of Bi2WO6 are studied in situ under water oxidation conditions, and the rate of both bulk recombination and water oxidation is found to be comparable to other metal oxide photoanodes. However, the rate of electron extraction is at least 10 times slower than the slowest kinetics previously reported in an oxide photoanode. First-principles analysis indicates that the slow electron extraction kinetics are linked to a strong anisotropy in the conduction band. Preferred or epitaxial growth along the conductive axes is a strategy to overcome slow electron transport and low photocurrent densities in layered materials such as Bi2WO6.
Mesa CA, Steier L, Moss B, et al., 2020, Impact of synthesis route on the water oxidation kinetics of hematite photoanodes, The Journal of Physical Chemistry Letters, Vol: 11, Pages: 7285-7290, ISSN: 1948-7185
Operando spectroelectrochemical analysis is used to determine the water oxidation reaction kinetics for hematite photoanodes prepared using four different synthetic procedures. Whilst these photoanodes exhibit very different current / voltage performance, their underlying water oxidation kinetics are found to be almost invariant. Higher temperature thermal annealing was found to correlate with a shift in the photocurrent onset potential towards less positive potentials, assigned to a suppression of both back electron-hole recombination and of charge accumulation in intraband-gap states, indicating these intraband-gap states do not contribute directly to water oxidation.
Hou B, Kim B-S, Lee HKH, et al., 2020, Multiphoton Absorption Stimulated Metal Chalcogenide Quantum Dot Solar Cells under Ambient and Concentrated Irradiance, ADVANCED FUNCTIONAL MATERIALS, Vol: 30, ISSN: 1616-301X
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- Citations: 30
Windle CD, Wieczorek A, Xiong L, et al., 2020, Covalent grafting of molecular catalysts on C<sub>3</sub>N<sub><i>x</i></sub>H<sub><i>y</i></sub>as robust, efficient and well-defined photocatalysts for solar fuel synthesis, CHEMICAL SCIENCE, Vol: 11, Pages: 8425-8432, ISSN: 2041-6520
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- Citations: 14
Du T, Xu W, Xu S, et al., 2020, Light-intensity and thickness dependent efficiency of planar perovskite solar cells: charge recombination versus extraction, Journal of Materials Chemistry C, Vol: 8, Pages: 12648-12655, ISSN: 2050-7526
Photoactive layer thickness is a key parameter for optimization of photovoltaic power conversion efficiency (PCE), yet its impact on charge extraction and recombination hasn’t been fully understood in perovskite solar cells (PSCs). Herein we find that in planar PSCs the perovskite thickness yielding maximal PCE is strongly light-intensity dependent. Whilst under 1 sun irradiation the PCE is relatively invariant for perovskite thicknesses between 250 to 750 nm, at lower light intensities (0.1–0.5 sun) the thickest devices yield strongly enhanced PCE, but at higher light intensities (>1 sun) the thinnest devices give optimal PCE. Our results unravel that increased perovskite thickness leads to enhanced light absorption, reduced interfacial recombination at open circuit but greater bimolecular recombination losses at short circuit thus is suitable for devices working under weak illumination, typical of many real-world applications. Reducing perovskite thickness, however, shows the contrast trend and is suitable for PSCs working under concentrated illumination.
Bozal-Ginesta C, Mesa CA, Eisenschmidt A, et al., 2020, Charge Accumulation Kinetics in Multi-redox Molecular Catalysts Immobilised on TiO2
<jats:p>Multi-redox catalysis requires the transfer of more than one charge carrier and is crucial for solar energy conversion into fuels and valuable chemicals. In photo(electro)chemical systems, however, the necessary accumulation of multiple, long-lived charges is challenged by recombination with their counterparts. Herein, we investigate charge accumulation in two model multi-redox molecular catalysts for proton and CO<jats:sub>2</jats:sub> reduction attached onto mesoporous TiO<jats:sub>2</jats:sub> electrodes. Transient absorption spectroscopy and spectroelectrochemical techniques have been employed to study the kinetics of photoinduced electron transfer from the TiO<jats:sub>2</jats:sub> to the molecular catalysts in acetonitrile, with triethanolamine as the hole scavenger. At high light intensities, we detect charge accumulation in the millisecond timescale in the form of multi-reduced species. The redox potentials of the catalysts and the capacity of TiO<jats:sub>2</jats:sub> to accumulate electrons play an essential role in the charge accumulation process at the molecular catalyst. Recombination of reduced species with valence band holes in TiO<jats:sub>2</jats:sub> is observed to be faster than microseconds, while electron transfer from multi-reduced species to the conduction band or the electrolyte occurs in the millisecond timescale. Finally, under light irradiation, we show how charge accumulation on the catalyst is regulated as a function of the applied bias and the excitation light intensity.</jats:p>
Wang Y, Vogel A, Sachs M, et al., 2020, Current understanding and challenges of solar-driven hydrogen generation using polymeric photocatalysts (vol 4, pg 746, 2019), NATURE ENERGY, Vol: 5, Pages: 633-633, ISSN: 2058-7546
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- Citations: 6
Sachs M, Cha H, Kosco J, et al., 2020, Tracking charge transfer to residual metal clusters in conjugated polymers for photocatalytic hydrogen evolution, Journal of the American Chemical Society, Vol: 142, Pages: 14574-14587, ISSN: 0002-7863
Semiconducting polymers are versatile materials for solar energy conversion and have gained popularity as photocatalysts for sunlight-driven hydrogen production. Organic polymers often contain residual metal impurities such as palladium (Pd) clusters that are formed during the polymerization reaction, and there is increasing evidence for a catalytic role of such metal clusters in polymer photocatalysts. Using transient and operando optical spectroscopies on nanoparticles of F8BT, P3HT, and the dibenzo[b,d]thiophene sulfone homopolymer, P10, we demonstrate how differences in the timescale of electron transfer to Pd clusters translate into hydrogen evolution activity optima at extremely different residual Pd concentrations. For F8BT nanoparticles with common Pd concentrations of >1000 ppm (>0.1 wt. %), we find that residual Pd clusters quench photogenerated excitons via energy and electron transfer on the fs – ns timescale, thus outcompeting reductive quenching. We spectroscopically identify reduced Pd clusters in our F8BT nanoparticles from the µs timescale onwards and show that the predominant location of long-lived electrons gradually shifts to the F8BT polymer when the Pd content is lowered. While a low yield of long-lived electrons limits the hydrogen evolution activity of F8BT, P10 exhibits a substantially higher hydrogen evolution activity which we demonstrate results from higher yields of long-lived electrons due to more efficient reductive quenching. Surprisingly, and despite the higher performance of P10, long-lived electrons reside on the P10 polymer rather than on the Pd clusters in P10 particles, even at very high Pd concentrations of 27,000 ppm (2.7 wt. %). In contrast, long-lived electrons in F8BT already reside on Pd clusters before the typical timescale of hydrogen evolution. This comparison shows that P10 exhibits efficient reductive quenching but slow electron transfer to residual Pd clusters whereas the opposite is the case for F8
Lee J, Cha H, Yao H, et al., 2020, Toward Visibly Transparent Organic Photovoltaic Cells Based on a Near-Infrared Harvesting Bulk Heterojunction Blend, ACS APPLIED MATERIALS & INTERFACES, Vol: 12, Pages: 32764-32770, ISSN: 1944-8244
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- Citations: 38
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.
Wu 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: 1754-5692
In 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.
Lin Y-H, Sakai N, Da P, et al., 2020, A piperidinium salt stabilizes efficient metal-halide perovskite solar cells, Science, Vol: 369, Pages: 1-7, ISSN: 0036-8075
Longevity has been a long-standing concern for hybrid perovskite photovoltaics. We demonstrate high-resilience positive-intrinsic-negative perovskite solar cells by incorporating a piperidinium-based ionic compound into the formamidinium-cesium lead-trihalide perovskite absorber. With the bandgap tuned to be well suited for perovskite-on-silicon tandem cells, this piperidinium additive enhances the open-circuit voltage and cell efficiency. This additive also retards compositional segregation into impurity phases and pinhole formation in the perovskite absorber layer during aggressive aging. Under full-spectrum simulated sunlight in ambient atmosphere, our unencapsulated and encapsulated cells retain 80 and 95% of their peak and post-burn-in efficiencies for 1010 and 1200 hours at 60° and 85°C, respectively. Our analysis reveals detailed degradation routes that contribute to the failure of aged cells.
Mohapatra AA, Dong Y, Boregowda P, et al., 2020, Rational Design of Donor Acceptor Based Semiconducting Copolymers with High Dielectric Constant
<jats:p>An efficient photogeneration of free charge carriers has long been recognized as the paramount challenge in organic photovoltaic (OPV) devices. The low dielectric constant organic semiconductors fall short to reduce strong Coulombic interaction of tightly bound exciton and hence lead to a loss mechanism in OPVs due to charge-carrier recombination. To circumvent this problem, we adopt a strategy to enhance the dielectric constant of organic semiconductors by incorporating tetraethyleneglycol (TEG) side-chains. We report synthesis of three new semiconducting copolymers by combining thiophene substituted diketopyrrolopyrrole (TDPP) monomer with three other monomeric units with varying electron donating strength: benzodithiophene (BBT-3TEG-TDPP), TDPP (TDPP-3TEG-TDPP) and naphthalene diimide (PNDITEG-TDPP). BBT-3TEG-TDPP and PNDITEG-TDPP showed highest dielectric constants (~ 5) at 1MHz frequency suggesting efficient contribution of dipolar polarization from TEG side-chains. To understand the electronic contribution of the polymer backbone and the polarity of TEG side-chains, and the resulting enhancement of the dielectric constant, we further performed first-principles density functional theory calculations. Single-component organic solar cells (OSC) fabricated utilizing these polymers resulted in poor performance which is attributed to the absence of free charge generation. Furthermore, transient absorption spectroscopy studies show low exciton diffusion length as observed in donor-acceptor type conjugated polymers. Our results suggest that, the strategy of enhancing dielectric constant with polar side-chains is not sufficient to reduce Coulombic interaction between hole and electron in OSCs.</jats:p>
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