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1.	Yang, Wenxing, et al. (author) Surface passivation extends single and biexciton lifetimes of InP quantum dots 2020 In: Chemical Science. - : ROYAL SOC CHEMISTRY. - 2041-6520 .- 2041-6539. ; 11:22, s. 5779-5789 Journal article (peer-reviewed)abstract Indium phosphide quantum dots (InP QDs) are nontoxic nanomaterials with potential applications in photocatalytic and optoelectronic fields. Post-synthetic treatments of InP QDs are known to be essential for improving their photoluminescence quantum efficiencies (PLQEs) and device performances, but the mechanisms remain poorly understood. Herein, by applying ultrafast transient absorption and photoluminescence spectroscopies, we systematically investigate the dynamics of photogenerated carriers in InP QDs and how they are affected by two common passivation methods: HF treatment and the growth of a heterostructure shell (ZnS in this study). The HF treatment is found to improve the PLQE up to 16-20% by removing an intrinsic fast hole trapping channel (tau(h,non)= 3.4 +/- 1 ns) in the untreated InP QDs while having little effect on the band-edge electron decay dynamics (tau(e)= 26-32 ns). The growth of the ZnS shell, on the other hand, is shown to improve the PLQE up to 35-40% by passivating both electron and hole traps in InP QDs, resulting in both a long-lived band-edge electron (tau(e)> 120 ns) and slower hole trapping lifetime (tau(h,non)> 45 ns). Furthermore, both the untreated and the HF-treated InP QDs have short biexciton lifetimes (tau(xx)similar to 1.2 +/- 0.2 ps). The growth of an ultra-thin ZnS shell (similar to 0.2 nm), on the other hand, can significantly extend the biexciton lifetime of InP QDs to 20 +/- 2 ps, making it a passivation scheme that can improve both the single and multiple exciton lifetimes. Based on these results, we discuss the possible trap-assisted Auger processes in InP QDs, highlighting the particular importance of trap passivation for reducing the Auger recombination loss in InP QDs.
2.	Freitag, Marina, et al. (author) Copper Phenanthroline as a Fast and High-Performance Redox Mediator for Dye-Sensitized Solar Cells 2016 In: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 120:18, s. 9595-9603 Journal article (peer-reviewed)abstract The most commonly used redox mediators in dye-sensitized solar cells (DSCs), iodide/triiodide and cobalt trisbipyridine ([Co(bpy)(3)](2+/3+)), were successfully replaced by bis (2,9-dimethy1-1,10-phenanthroline) copp er (I/H) ([Cu(dmp)(2)](1+/2+)). The use of the copper complex based electrolyte led to an exceptionally high photovoltaic performance of 8.3% for LEG4-sensitized TiO2 solar cells, with a remarkably high open-circuit potential of above 1.0 V at 1000 W m(-2) under AM1.5G conditions. The copper complex based redox electrolyte has higher diffusion coefficients and is considerably faster in dye regeneration than comparable cobalt trisbipyridine based electrolytes. A driving force for dye regeneration of only 0.2 eV is sufficient to obtain unit yield, pointing to new possibilities for improvement in DSC efficiencies. The interaction of the excited dye with components of the electrolyte was monitored using steady-state emission measurements and time-correlated single-photon counting (TC-SPC). Our results indicate bimolecular reductive quenching of the excited LEG4 dye by the [Cu(dmp)(2)](2+) complex through a dynamic mechanism. Excited-state dye molecules can readily undergo bimolecular electron transfer with a suitable donor molecule. In DSCs this process can occur when the excited dye is unable to inject electrons into the TiO2. With a high electrolyte concentration the excited dye can be intercepted with an electron from the electrolyte resulting in the reduced state of the dye. Quenching of the reduced dye by the electrolyte competes with electron injection and results in a lower photocurrent. Quenching of excited LEG4 by complexes of [Cu(dmp)(2)](+), [Co(bpy)(3)](2+), and [Co(bpy)(3)](3+) followed a static mechanism, due ground-state dye-quencher binding. Inhibition of unwanted quenching processes by structural modifications may open ways to further increase the overall efficiency.
3.	Freitag, Marina, et al. (author) Supramolecular Hemicage Cobalt Mediators for Dye-Sensitized Solar Cells 2016 In: ChemPhysChem. - : Wiley. - 1439-4235 .- 1439-7641. ; 17:23, s. 3845-3852 Journal article (peer-reviewed)abstract A new class of dye-sensitized solar cells (DSSCs) using the hemicage cobalt-based mediator [Co(ttb)](2+/3+) with the highly preorganized hexadentate ligand 5,5 '', 5 ''''-((2,4,6-triethyl benzene-1,3,5-triyl)tris(ethane-2,1-diyl))tri-2,2'-bipyridine (ttb) has been fully investigated. The performances of DSSCs sensitized with organic D-p-A dyes utilizing either [Co(ttb)](2+/3+) or the conventional [Co(bpy)(3)](2+/3+) (bpy = 2,2'-bipyridine) redox mediator are comparable under 1000 Wm(-2) AM 1.5 G illumination. However, the hemicage complexes exhibit exceptional stability under thermal and light stress. In particular, a 120-hour continuous light illumination stability test for DSSCs using [Co(ttb)](2+/3+) resulted in a 10% increase in the performance, whereas a 40% decrease in performance was found for [Co(bpy)(3)](2+/3+) electrolyte-based DSSCs under the same conditions. These results demonstrate the great promise of [Co(ttb)](2+/3+) complexes as redox mediators for efficient, cost-effective, large-scale DSSC devices.
4.	Gao, Jiajia, et al. (author) Cation-Dependent Photostability of Co(II/III)-Mediated Dye-Sensitized Solar Cells 2015 In: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 119:44, s. 24704-24713 Journal article (peer-reviewed)abstract The electrolyte composition has a significant effect on the performance and stability of cobalt-based, dye-sensitized solar cells (DSSCs). The stability of DSSCs incorporating Co(II/III) tris(bipyridine) redox mediator has been investigated over 1000 h under full solar irradiation (with UV cutoff) at a temperature of 60 °C, the main focus being on monitoring the photovoltaic performance of the device and analyzing the internal charge-transfer dynamics in the presence of different cation coadditives (preferably added as tetracyanoborate salts). A clear cation-dependence is shown, not only of the early light-induced performance but also of the long-term photostability of the photovoltage of the device. These light-induced changes, which are attributed to the promotion of electron injection and less electron recombination loss, by transient spectral and electrochemical studies at the TiO2/dye/electrolyte interface, indicate that the main cation effects involve the TiO2 surface electric field and energy-state distribution. By examining the stability of adsorbed and solvated dye during aging, it has been found that the dye photodegradation is probably responsible for the decline in the photovoltage and that this is extremely dependent on the nature of the cation coadditives in the electrolyte. It is therefore suggested that optimization of the electrolyte cation composition is essential for improving the stability of cobalt-based DSSCs.
5.	Gao, Jiajia, et al. (author) Light-induced electrolyte improvement in cobalt tris(bipyridine)-mediated dye-sensitized solar cells 2019 In: Journal of Materials Chemistry A. - : Royal Society of Chemistry. - 2050-7488 .- 2050-7496. ; 7:33, s. 19495-19505 Journal article (peer-reviewed)abstract Lithium-ion-free tris(2,2′-bipyridine) Co(ii/iii)-mediated electrolytes have previously been proposed for long-term stable dye-sensitized solar cells (DSSCs). Such redox systems also offer an impressive DSSC performance improvement under light soaking exposure, manifested by an increase in photocurrent and fill factor without the expense of decreasing photovoltage. Kinetic studies show that charge transfer and ion diffusion at the electrode/electrolyte interface are improved due to the light exposure. Control experiments reveal that the light effect is unambiguously associated with electrolyte components, [Co(bpy)3]3+ and the Lewis-base additive tert-butylpyridine (TBP). Electrochemical and spectroscopic investigation of the [Co(bpy)3]3+/TBP mixtures points out that the presence of TBP, which retards the electrolyte diffusion, however causes an irreversible redox reaction of [Co(bpy)3]3+ upon light exposure that improves the overall conductivity. This discovery not only provides a new strategy to mitigate the typical Jsc-Voc trade-off in Co(ii/iii)-mediated DSSCs but also highlights the importance of investigating the photochemistry of a photoelectrochemical system.
6.	Hao, Yan, et al. (author) A small electron donor in cobalt complex electrolyte significantly improves efficiency in dye-sensitized solar cells 2016 In: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 7 Journal article (peer-reviewed)abstract Photoelectrochemical approach to solar energy conversion demands a kinetic optimization of various light-induced electron transfer processes. Of great importance are the redox mediator systems accomplishing the electron transfer processes at the semiconductor/electrolyte interface, therefore affecting profoundly the performance of various photoelectrochemical cells. Here, we develop a strategy-by addition of a small organic electron donor, tris(4-methoxyphenyl)amine, into state-of-art cobalt tris(bipyridine) redox electrolyte-to significantly improve the efficiency of dye-sensitized solar cells. The developed solar cells exhibit efficiency of 11.7 and 10.5%, at 0.46 and one-sun illumination, respectively, corresponding to a 26% efficiency improvement compared with the standard electrolyte. Preliminary stability tests showed the solar cell retained 90% of its initial efficiency after 250 h continuous one-sun light soaking. Detailed mechanistic studies reveal the crucial role of the electron transfer cascade processes within the new redox system.
7.	Hao, Yan, et al. (author) Efficient Dye-Sensitized Solar Cells with Voltages Exceeding 1 V through Exploring Tris(4-alkoxyphenyl)amine Mediators in Combination with the Tris(bipyridine) Cobalt Redox System 2018 In: ACS Energy Letters. - : American Chemical Society (ACS). - 2380-8195. ; 3:8, s. 1929-1937 Journal article (peer-reviewed)abstract Tandem redox electrolytes, prepared by the addition of a tris(p-anisyl)amine mediator into classic tris(bipyridine)cobalt-based electrolytes, demonstrate favorable electron transfer and reduced energy loss in dye-sensitized solar cells. Here, we have successfully explored three tris(4-alkoxyphenyl)-amine mediators with bulky molecular structures and generated more effective tandem redox systems. This series of tandem redox electrolytes rendered solar cells with very high photovoltages exceeding 1 V, which approaches the theoretical voltage limit of tris(bipyridine)cobalt-based electrolytes. Solar cells with power conversion efficiencies of 9.7-11.0% under 1 sun illumination were manufactured. This corresponds to an efficiency improvement of up to 50% as compared to solar cells based on pure tris(bipyridine)cobalt-based electrolytes. The photovoltage increases with increasing steric effects of the tris(4-alkoxyphenyl)amine mediators, which is attributed to a retarded recombination kinetics. These results highlight the importance of structural design for optimized charge transfer at the sensitized semiconductor/electrolyte interface and provide insights for the future development of efficient dye-sensitized solar cells.
8.	Hao, Yan, et al. (author) Novel Blue Organic Dye for Dye-Sensitized Solar Cells Achieving High Efficiency in Cobalt-Based Electrolytes and by Co-Sensitization 2016 In: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 8:48, s. 32797-32804 Journal article (peer-reviewed)abstract Blue and green dyes as well as NIR-absorbing dyes have attracted great interest because of their excellent ability of absorbing the incident photons in the red and near-infrared range region. A novel blue D-pi-A dye (Dyenamo Blue), based on the diketopyrrolopyrrole (DPP)-core, has been designed and synthesized. Assembled with the cobalt bipyridine-based electrolytes, the device with Dyenamo Blue achieved a satisfying efficiency of 7.3% under one sun (AM1.5 G). The co-sensitization strategy was further applied on this blue organic dye together with a red D-pi-A dye (D35). The successful co-sensitization outperformed a panchromatic light absorption and improved the photocurrent density; this in addition to the open-circuit potential result in an efficiency of 8.7%. The extended absorption of the sensitization and the slower recombination reaction between the blue dye and TiO2 surface inhibited by the additional red sensitizer could be the two main reasons for the higher performance. In conclusion, from the results, the highly efficient cobalt-based DSSCs could be achieved with the co-sensitization between red and blue D-pi-A organic dyes with a proper design, which showed us the possibility of applying this strategy for future high-performance solar cells.
9.	Hao, Yan, et al. (author) Peripheral Hole Acceptor Moieties on an Organic Dye Improve Dye-Sensitized Solar Cell Performance 2015 In: Advanced Science. - : Wiley-Blackwell. - 2198-3844. ; 2:11 Journal article (peer-reviewed)abstract Investigation of charge transfer dynamics in dye-sensitized solar cells is of fundamental interest and the control of these dynamics is a key factor for developing more efficient solar cell devices. One possibility for attenuating losses through recombination between injected electrons and oxidized dye molecules is to move the positive charge further away from the metal oxide surface. For this purpose, a metal-free dye named E6 is developed, in which the chromophore core is tethered to two external triphenylamine (TPA) units. After photoinduced electron injection into TiO2, the remaining hole is rapidly transferred to a peripheral TPA unit. Electron-hole recombination is slowed down by 30% compared to a reference dye without peripheral TPA units. Furthermore, it is found that the added TPA moieties improve the electron blocking effect of the dye, retarding recombination of electrons from TiO2 to the cobalt-based electrolyte.
10.	Hao, Yan, et al. (author) Triphenylamine Groups Improve Blocking Behavior of Phenoxazine Dyes in Cobalt-Electrolyte-Based Dye-Sensitized Solar Cells 2014 In: ChemPhysChem. - : Wiley. - 1439-4235 .- 1439-7641. ; 15:16, s. 3476-3483 Journal article (peer-reviewed)abstract Novel phenoxazine dyes are successfully introduced as sensitizers into dye-sensitized solar cells (DSCs) with cobalt-based electrolyte. In sensitizers with triphenylamine (TPA) groups recombination from electrons in the TiO2 conduction band to the cobalt(III) species is suppressed. The effect of the steric properties of the phenoxazine sensitizers on the overall device performance and on recombination and regeneration processes is compared. Optimized DSCs sensitized with IB2 having two TPA groups in combination with tris(2,2'-bipyridyl) cobalt( II/III) yield efficiencies of 6.3 %, similar to that of IB3, which is equipped with mutiple alkoxy groups. TH310 with only one TPA group gives lower efficiency and open circuit voltage, while IB1 without TPA groups performs even worse. These results demonstrate that both TPA groups on the IB2 are needed for an efficient blocking effect. These results reveal a possible new role for TPA units in DSC sensitizer design.
11.	Leandri, Valentina, et al. (author) Rapid Microwave-Assisted Self-Assembly of a Carboxylic-Acid-Terminated Dye on a TiO2 Photoanode 2018 In: ACS Applied Energy Materials. - : AMER CHEMICAL SOC. - 2574-0962. ; 1:1, s. 202-210 Journal article (peer-reviewed)abstract Self-assembly of carboxylic-acid-functionalized dyes on mesoporous, anatase TiO2 is at the heart of dye-sensitized solar cells (DSSCs). However, the process often requires 6-20 h of electrode immersion at room temperature in the dye-bath solutions. Here, we introduce a new, rapid microwave-assisted sensitization technique (MINAS), which significantly accelerates the sensitization process and yields high-quality, self-assembled films of an organic dye within 5 min. Targeted experiments show that the effects of the microwave radiation cannot be explained purely on the basis of the thermal component. The interaction of the microwave radiation with the conductive fluorine-doped tin oxide (FTO) electrical contact is a key aspect to consider and a unique feature of MWAS that is the likely cause for producing rapid self-assembly of the dye on the surface.
12.	Pati, Palas Baran, et al. (author) New dyes for DSSC containing triphenylamine based extended donor : Synthesis, photophysical properties and device performance 2017 In: Spectrochimica Acta Part A - Molecular and Biomolecular Spectroscopy. - : Elsevier BV. - 1386-1425 .- 1873-3557. ; 178, s. 106-113 Journal article (peer-reviewed)abstract Three new triphenylamine based dyes with Donor-Donor-Spacer-Acceptor (D-D-n-A) arrangement were designed and synthesized by convenient synthetic pathway. Unsymmetrical extended donor part may help to reduce the aggregation of dyes on the semiconductor surface. Wide range of absorption in the visible spectrum, electrochemical studies and theoretical optimization suggest that these dyes can be good members for DSSC. Further to check the performance of these dyes in device the solar cells were developed using iodine free Co-based electrolyte. Electronic characterisation concludes that devices based on 136 have the highest power conversion efficiency (4.7%) mostly due to an improved electron lifetime, which therefore improves both the Voc and Jac of the devices.
13.	Saki, Zahra, et al. (author) The synergistic effect of dimethyl sulfoxide vapor treatment and C-60 electron transporting layer towards enhancing current collection in mixed-ion inverted perovskite solar cells 2018 In: Journal of Power Sources. - : ELSEVIER SCIENCE BV. - 0378-7753 .- 1873-2755. ; 405, s. 70-79 Journal article (peer-reviewed)abstract Inverted perovskite solar cells (PSCs) have been introduced as better candidate for roll-to-roll printing and scaleup than their conventional configuration counterparts, while their fabrication is technically more demanding. The common light absorbing layer in inverted PSCs is the single cation methylammonium lead iodide (MAPbI(3)) perovskite, whereas mixed-ion perovskites are chemically more stable. In mixed-ion perovskites, where FA (formamidinium) is the main replacement for MA, the electron affinity is larger than in MAPbI3 perovskites, leading to possible barriers against photoelectron collection by the electron transporting layer (ETL). In this paper we report on a mixed-ion (FAPbI(3))(0.83)(MAPbBr(3))(0.17) inverted PSC with improved photocurrent through using a dimethyl sulfoxide vapor treatment of perovskite layer and replacing the conventional [6,6]-phenyl-C-71 butyric acid methyl ester (PC70BM) with C-60/bathocuproine (BCP) as more effective ETL. The treatment of perovskite layer results in reduction of impurity phases of 8-FAPbI(3) and Pbl(2). Photoluminescence and open circuit voltage decay data demonstrate better charge carrier collection by the C-60/BCP compared to the PC70BM ETL, and an electron barrier for the back flow of electrons from ETL to perovskite. Our improvements in perovskite crystalization and electron transfer layer simultaneously lead to increasing the current density from 10 to 21 mA cm(-2).
14.	Sveinbjornsson, Kari, et al. (author) Preparation of mixed-ion and inorganic perovskite films using water and isopropanol as solvents for solar cell applications 2018 In: Sustainable Energy & Fuels. - : Royal Society of Chemistry. - 2398-4902. ; 2:3, s. 606-615 Journal article (peer-reviewed)abstract Presently, the most efficient lead halide perovskite solar cells are manufactured by using high-boiling point organic solvents to dissolve the perovskite precursor materials prior to the perovskite formation. Previously, efforts have been made to exchange the said solvents for water with some success. Herein, we build on that work to develop a procedure for synthesising perovskite absorbers using only water and isopropanol as solvents. Our technique can be utilised for fabricating many different perovskite compositions, organic and inorganic. The technique is based on the high solubility of metal nitrates, such as lead(ii) nitrate and caesium(i) nitrate, in water and, respectively, their poor solubilities in isopropanol. The inclusion of CsNO3 to Pb(NO3)(2) films does not result in a phase separation of the perovskite material as one would expect when using lead(ii) halide precursor films. Using the perovskite composition Cs(0.1)FA(0.9)Pb(I0.83Br0.17)(3) we were able to reach an average solar cell power conversion efficiency of 13.0%. Furthermore, the technique can be applied to many different perovskite compositions making it appealing for large-scale manufacturing of perovskite solar cells.
15.	Sveinbjörnsson, Kári, et al. (author) Preparation of mixed-ion and inorganic perovskite films using water and isopropanol as solvents for solar cell applications 2018 In: Sustainable Energy & Fuels. - : The Royal Society of Chemistry. - 2398-4902. ; 2:3, s. 606-615 Journal article (peer-reviewed)abstract Presently, the most efficient lead halide perovskite solar cells are manufactured by using high-boiling point organic solvents to dissolve the perovskite precursor materials prior to the perovskite formation. Previously, efforts have been made to exchange the said solvents for water with some success. Herein, we build on that work to develop a procedure for synthesising perovskite absorbers using only water and isopropanol as solvents. Our technique can be utilised for fabricating many different perovskite compositions, organic and inorganic. The technique is based on the high solubility of metal nitrates, such as lead(ii) nitrate and caesium(i) nitrate, in water and, respectively, their poor solubilities in isopropanol. The inclusion of CsNO3 to Pb(NO3)2 films does not result in a phase separation of the perovskite material as one would expect when using lead(ii) halide precursor films. Using the perovskite composition Cs0.1FA0.9Pb(I0.83Br0.17)3 we were able to reach an average solar cell power conversion efficiency of 13.0%. Furthermore, the technique can be applied to many different perovskite compositions making it appealing for large-scale manufacturing of perovskite solar cells.
16.	Tian, Lei, et al. (author) Ultrafast dye regeneration in a core-shell NiO-dye-TiO2 mesoporous film 2018 In: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 20:1, s. 36-40 Journal article (peer-reviewed)abstract In this study, a core-shell NiO-dye-TiO2 mesoporous film was fabricated for the first time, utilizing atomic layer deposition technique and a newly designed triphenylamine dye. The structure of the film was confirmed by SEM, TEM, and EDX. Excitation of the dye led to efficient and fast charge separation, by hole injection into NiO, followed by an unprecedentedly fast dye regeneration (t(1/2) <= 500 fs) by electron transfer to TiO2. The resulting charge separated state showed a pronounced transient absorption spectrum caused by the Stark effect, and no significant decay was found within 1.9 ns. This indicates that charge recombination between NiO and TiO2 is much slower than that between the NiO and the reduced dye in the absence of the TiO2 layer (t(1/2) approximate to 100 ps).
17.	Tian, Lei, et al. (author) Ultrafast dye regeneration in a core-shell NiO-dye-TiO2 mesoporous film 2018 In: Physical Chemistry, Chemical Physics - PCCP. - : The Royal Society of Chemistry. - 1463-9076 .- 1463-9084. ; 20:1, s. 36-40 Journal article (peer-reviewed)abstract In this study, a core-shell NiO-dye-TiO2 mesoporous film was fabricated for the first time, utilizing atomic layer deposition technique and a newly designed triphenylamine dye. The structure of the film was confirmed by SEM, TEM, and EDX. Excitation of the dye led to efficient and fast charge separation, by hole injection into NiO, followed by an unprecedentedly fast dye regeneration (t1/2 [less-than-or-equal] 500 fs) by electron transfer to TiO2. The resulting charge separated state showed a pronounced transient absorption spectrum caused by the Stark effect, and no significant decay was found within 1.9 ns. This indicates that charge recombination between NiO and TiO2 is much slower than that between the NiO and the reduced dye in the absence of the TiO2 layer (t1/2 [approximate] 100 ps).
18.	Yang, Wenxing, et al. (author) A key discovery at the TiO2/dye/electrolyte interface : slow local charge compensation and a reversible electric field 2015 In: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 17:26, s. 16744-16751 Journal article (peer-reviewed)abstract Dye-sensitized mesoporous TiO2 films have been widely applied in energy and environmental science related research fields. The interaction between accumulated electrons inside TiO2 and cations in the surrounding electrolyte at the TiO2/dye/electrolyte interface is, however, still poorly understood. This interaction is undoubtedly important for both device performance and fundamental understanding. In the present study, Stark effects of an organic dye, LEG4, adsorbed on TiO2 were well characterized and used as a probe to monitor the local electric field at the TiO2/dye/electrolyte interface. By using time-resolved photo- and potential-induced absorption techniques, we found evidence for a slow (t > 0.1 s) local charge compensation mechanism, which follows electron accumulation inside the mesoporous TiO2. This slow local compensation was attributed to the penetration of cations from the electrolyte into the adsorbed dye layer, leading to a more localized charge compensation of the electrons inside TiO2. Importantly, when the electrons inside TiO2 were extracted, a remarkable reversal of the surface electric field was observed for the first time, which is attributed to the penetrated and/or adsorbed cations now being charge compensated by anions in the bulk electrolyte. A cation electrosorption model is developed to account for the overall process. These findings give new insights into the mesoporous TiO2/dye/electrolyte interface and the electron-cation interaction mechanism. Electrosorbed cations are proposed to act as electrostatic trap states for electrons in the mesoporous TiO2 electrode.
19.	Yang, Wenxing, et al. (author) Carrier Dynamics of Dye Sensitized-TiO2 in Contact with Different Cobalt Complexes in the Presence of Tri(p-anisyl)amine Intermediates 2018 In: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 122:26, s. 14345-14354 Journal article (peer-reviewed)abstract Heterogeneous charge transfer processes at sensitized wide bandgap semiconductor surfaces are imperative for both fundamental knowledge and technical applications. Herein, we focus on the investigation of carrier dynamics of a triphenylamine-based dye, LEG4, sensitized TiO2 (LEG4/TiO2) in contact with two types of electrolyte systems: pure cobalt-based electrolytes and in combination with an organic donor, tri(p-anisyl)amine (TPAA). Four different cobalt redox systems with potentials spanning a 0.3 V range were studied, and the carrier recombination and regeneration kinetics were monitored both at low and at high TiO2 (e(-)) densities (1.3 X 10(18) and 1.3 X 10(19) cm(-3), respectively). The results reveal that the introduction of the TPAA intermediate more effectively suppress the recombination loss of TiO2 (e(-)) under high charge conditions, close to open-circuit, as compared to low charge conditions. As a result, the charge transfer from the cobalt complexes to the oxidized dyes is significantly improved by the addition of TPAA. Dye-sensitized solar cells fabricated with the TPAA-containing electrolytes demonstrate remarkable improvement in both V-OC and J(SC) and lead to more than 25% increase of the light-to-electricity conversion efficiency. Furthermore, an unprecedented detrimental impact of TPAA on the device performance was identified when the redox potential of the TPAA donor and the cobalt complexes are close. This is ascribed to the formation of TPAA(center dot+) which can act as an active recombination centers and thus lower the solar cell performance. These insights point at a strategy to enhance the lifetimes of electrons generated in sensitized semiconductor electrodes by overcoming the charge recombination between TiO2 and the oxidized dye under high carrier densities in the semiconductor substrate and offer practical guidance to the design of future efficient electrolyte systems for dye-sensitized solar cells.
20.	Yang, Wenxing, et al. (author) Efficient aqueous dye-sensitized solar cell electrolytes based on a TEMPO/TEMPO+ redox couple 2015 In: RSC Advances. - : Royal Society of Chemistry (RSC). - 2046-2069. ; 5:34, s. 26706-26709 Journal article (peer-reviewed)abstract Aqueous electrolyte-based dye-sensitized solar cells (DSSCs) have recently emerged and shown to be a promising eco-friendly photovoltaic device. In the present study, we, for the first time, have developed 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) and TEMPO+ tetrafluoroborate salt as a redox couple in an aqueous electrolyte for DSSCs. With the hydrophobic dye LEG4 as a light absorber, we have achieved a power conversion efficiency of 4.3% and a record open circuit voltage of 955 mV in the device. This is attributed to the high formal redox potential of TEMPO/TEMPO+ (0.71 V vs. NHE) in water. In addition, despite the wide use of surfactants in previous studies, we have clearly shown that the addition of surfactants to the electrolyte is detrimental to solar cell performance. Therefore, the use of surfactants in aqueous DSSC electrolytes should be avoided or used with caution.
21.	Yang, Wenxing, et al. (author) Efficient dye regeneration at low driving force achieved in triphenylamine dye LEG4 and TEMPO redox mediator based dye-sensitized solar cells 2015 In: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 17:24, s. 15868-15875 Journal article (peer-reviewed)abstract Minimizing the driving force required for the regeneration of oxidized dyes using redox mediators in an electrolyte is essential to further improve the open-circuit voltage and efficiency of dye-sensitized solar cells (DSSCs). Appropriate combinations of redox mediators and dye molecules should be explored to achieve this goal. Herein, we present a triphenylamine dye, LEG4, in combination with a TEMPO-based electrolyte in acetonitrile (E-0 = 0.89 V vs. NHE), reaching an efficiency of up to 5.4% under one sun illumination and 40% performance improvement compared to the previously and widely used indoline dye D149. The origin of this improvement was found to be the increased dye regeneration efficiency of LEG4 using the TEMPO redox mediator, which regenerated more than 80% of the oxidized dye with a driving force of only similar to 0.2 eV. Detailed mechanistic studies further revealed that in addition to electron recombination to oxidized dyes, recombination of electrons from the conducting substrate and the mesoporous TiO2 film to the TEMPO+ redox species in the electrolyte accounts for the reduced short circuit current, compared to the state-of-the-art cobalt tris(bipyridine) electrolyte system. The diffusion length of the TEMPO-electrolyte based DSSCs was determined to be similar to 0.5 mu m, which is smaller than the similar to 2.8 mu m found for cobalt-electrolyte based DSSCs. These results show the advantages of using LEG4 as a sensitizer, compared to previously record indoline dyes, in combination with a TEMPO-based electrolyte. The low driving force for efficient dye regeneration presented by these results shows the potential to further improve the power conversion efficiency (PCE) of DSSCs by utilizing redox couples and dyes with a minimal need of driving force for high regeneration yields.
22.	Yang, Wenxing, 1988- (author) Exploring Electronic Processes at the Mesoporous TiO2/Dye/Electrolyte Interface 2017 Doctoral thesis (other academic/artistic)abstract Dye sensitized solar cells (DSSCs) are an attractive way to convert light into electricity. Its development requires a detailed understanding and kinetic optimization of various electronic processes, especially those occurring at the mesoporous TiO2/dye/electrolyte interface. This dissertation work is focused on the exploration of the various electronic processes at the sensitized-electrode/electrolyte interface by using various electrochemical and photochemical methods.Firstly, an alternative redox couple—TEMPO/TEMPO·+ with a relatively high positive redox potential—is explored, aiming to reduce the energy loss during the dye regeneration process. Despite the fast dye regeneration, the charge recombination between the electrons in the conduction band of mesoporous TiO2 and the oxidized redox species is found to be the limiting factor of the device. Further, a more efficient tandem-electrolyte system is developed, leading to DSSCs with the power conversion efficiency of 10.5 % and 11.7 % at 1 sun and 0.5-sun illumination, respectively. An electron-transfer cascade process during dye regeneration by the redox mediators is discovered to be beneficial. Further stability studies on the device suggest the crucial role of TiO2/dye/electrolyte interfaces in the long-term stability of cobalt bipyridyl electrolyte-based DSSCs.On the fundamental level, the local electric field and Stark effects at the TiO2/dye/electrolyte interface are investigated in various aspects—including the charge compensation mechanism, the factors affecting the electric field strength, as well as its impact on charge transfer kinetics. These results give further insights about the TiO2/dye/electrolyte interface, and contribute to the further development and understanding of DSSCs.
23.	Yang, Wenxing, et al. (author) Harvesting Sub-Bandgap IR Photons by Photothermionic Hot Electron Transfer in a Plasmonic p-n Junction 2021 In: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 21:9, s. 4036-4043 Journal article (peer-reviewed)abstract Plasmonic semiconductors are an emerging class of low-cost plasmonic materials, and the presence of a bandgap and band-bending in these materials offer new opportunities to overcome some of the limitations of plasmonic metals. Here, we demonstrate that in a plasmonic p-n heterojunction (Cu2-xSe-CdSe) the near-IR excitation (1.1 eV) of the hole plasmon in the p-Cu2-xSe phase results in rapid hot electron transfer to n-CdSe, with an energy 2.2 eV above the Fermi level. This hot electron generation and energy upconversion process can be well-described by a photothermionic mechanism, where the presence of a bandgap in p-Cu2-xSe facilitates the generation of energetic photothermal electrons. The lifetime of the transferred electrons in Cu2-xSe-CdSe can reach similar to 130 ps, which is nearly 100x longer than that of its metal-semiconductor counterpart. This result demonstrates a novel approach for harvesting the sub-bandgap near IR photons using plasmonic p-n junctions and the potential advantages of plasmonic semiconductors for hot carrier-based devices.
24.	Yang, Wenxing, 1988-, et al. (author) Impact of Local Electric Fields on Charge-Transfer Processes at the TiO2/Dye/Electrolyte Interface 2017 In: ACS Energy Letter. - : American Chemical Society (ACS). - 2380-8195. ; 2:1, s. 161-167 Journal article (peer-reviewed)abstract Photoinduced electron -transfer processes at the TiO2/ dye/electrolyte interface are vital for various emerging technologies. Here, the impact of the local electric field at this interface on the charge -transfer processes was investigated in two aspects: (a) charge recombination between the electrons accumulated within TiO2 and the photoxidized dye and (b) regeneration of the dyes by the cobalt bipyridyl redox mediators. The amplitude of the local electric field was changed by use of different cations in the electrolytic environment, in the order E-Ca(2+) > E-Mg(2+)> E-Na(+) > E-u(+) characterized by the transient absorption spectroscopy. For the charge recombination process, the kinetic time constant showed a remarkable linear correlation with the relative electric field strength, while for the regeneration process, no evident dependence was observed. These results collectively suggest the spatial confinement of the effects of the local electric field on the interfacial electron -transfer processes.
25.	Yang, Wenxing, et al. (author) Photoinduced Fano Resonances between Quantum Confined Nanocrystals and Adsorbed Molecular Catalysts 2021 In: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 21:13, s. 5813-5818 Journal article (peer-reviewed)abstract Interaction of surface adsorbate vibration and intraband electron absorption in nanocrystals has been reported to affect the photophysical properties of both nanocrystals and surface adsorbates and may affect the performance of hybrid photocatalysts composed of semiconductor nanocrystals and molecular catalysts. Here, by combining ultrafast transient visible and IR spectroscopic measurements, we report the observation of Fano resonances between the intraband transition of the photogenerated electrons in CdS and CdSe nanocrystals and CO stretching vibrational modes of adsorbed molecular catalysts, [Fe-2(cbdt)(CO)(6)] (FeFe; cbdt = 1-carboxyl-benzene-2,3-dithiolate), a molecular mimic for the active site of FeFe-hydrogenase. The occurrence of Fano resonances is independent of nanocrystal types (rods vs dots) or charge transfer character between the nanocrystal and FeFe, and is likely a general feature of nanocrystal and molecular catalyst hybrid systems. These results provide new insights into the fundamental interactions in these hybrid assemblies for artificial photosynthesis.
26.	Yang, Wenxing, et al. (author) Studies on the Interfacial Electric Field and Stark Effect at the TiO2/Dye/Electrolyte Interface 2016 In: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 120:39, s. 22215-22224 Journal article (peer-reviewed)abstract Interfaces of dye-sensitized TiO2 nanoparticles with electrolytes or hole conductors have been widely applied in photoelectrochemical cells. However, the fundamental understanding of their properties and function is still poor. Herein, we demonstrate that the spectral changes that occur in the-visible spectrum of dye-sensitized TiO2 films upon (a) Li+ titration, (b) potentiostatic electron accumulation in mesoporous TiO2, and (c) photoinduced electron injection into TiO2 can be explained by the Stark effect, which can then be used to characterize the change in the local electric field at the TiO2/dye/electrolyte interface. A quantitative analysis of the Stark effect indicates that the compact (Helmholtz) layer capacitance at the TiO2/dye/electrolyte interface strongly affects the strength of the local electric field. Systematic studies show that the Helmholtz layer capacitance depends strongly on the Li+ concentration and surface dye coverage but is independent of the concentrations of other electrolytic species and the light intensity. These results illustrate the potential of Stark spectroscopy for the in situ study of the TiO2/dye/electrolyte interfaces and provide substantial new insights into these widely applied interfaces related to photoelectrochemistry and other research fields.
27.	Yang, Wenxing, et al. (author) Surface-Ligand "Liquid" to "Crystalline" Phase Transition Modulates the Solar H2 Production Quantum Efficiency of CdS Nanorod/Mediator/Hydrogenase Assemblies 2020 In: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 12:31, s. 35614-35625 Journal article (peer-reviewed)abstract This study reports how the length of capping ligands on a nanocrystal surface affects its interfacial electron transfer (ET) with surrounding molecular electron acceptors, and consequently, impact the H-2 production of a biotic-abiotic hybrid artificial photosynthetic system. Specifically, we study how the H-2 production efficiency of a hybrid system, combining CdS nanorods (NRs), [NiFe] hydrogenase, and redox mediators (propyl-bridged 2,2'-bipyridinium, PDQ(2+)), depends on the alkyl chain length of mercaptocarboxylate ligands on the NR surface. We observe a minor decrease of the quantum yield for H-2 production from 54 +/- 6 to 43 +/- 2% when varying the number of methylene units in the ligands from 2 to 7. In contrast, an abrupt decrease of the yield was observed from 43 +/- 2 to 4 +/- 1% when further increasing n from 7 to 11. ET studies reveal that the intrinsic ET rates from the NRs to the electron acceptor PDQ(2+) are all within 10(8) -10(9) s(-1) regardless of the length of the capping ligands. However, the number of adsorbed PDQ(2+) molecules on NR surfaces decreases dramatically when n >= 10, with the saturating number changing from 45 +/- 5 to 0.3 +/- 0.1 for n = 2 and 11, respectively. These results are not consistent with the commonly perceived exponential dependence of ET rates on the ligand length. Instead, they can be explained by the change of the accessibility of NR surfaces to electron acceptors from a disordered "liquid" phase at n < 7 to a more ordered "crystalline" phases at n > similar to 7. These results highlight that the order of capping ligands is an important design parameter for further constructing nanocrystal/molecular assemblies in broad nanocrystal-based applications.
28.	Yang, Wenxing, et al. (author) Thermal Stability Study of Dye-Sensitized Solar Cells with Cobalt Bipyridyl-based Electrolytes 2016 In: Electrochimica Acta. - : Elsevier BV. - 0013-4686 .- 1873-3859. ; 213, s. 879-886 Journal article (peer-reviewed)abstract Dye-sensitized solar cells (DSSCs) with cobalt bipyridyl-based electrolytes can display higher solar cell performance than their iodide/triiodide counterpart. There is, however, little knowledge on their long term stability, which is a crucial aspect for potential commercial application. Herein, we studied the thermal stability of DSSCs using Co(bpy)(3)(2+/3+) redox electrolyte at 70 degrees C in the dark for 50 days, combining 3 different additives, 4-tert-butylpyridine (TBP), 1-methylimidazole (MBI) and 2,2'-bipyridyl (BPY), in a nonvolatile solvent 3-methoxypropionitrile (MPN). Significant voltage decreases were found for all the studied solar cells, with a mechanism involving both a positive shift of the conduction band edge potential of TiO2 and a decreased electron lifetime, characterized by time resolved transient modulation techniques. Furthermore electrochemical impedance spectroscopy and differential pulse voltammetry studies indicate that the stability of Co(bpy)(3)(3+) is limited, causing an increased diffusion resistance in the electrolyte, but, surprisingly, no substantial change of the short-circuit current density (Jsc) in the devices. Overall, the DSSCs fabricated with the addition of both MBI and BPY in the electrolyte show the highest stability, maintaining 96% of its initial efficiency after 50 days, resulting from the overall compensation effects between the open circuit voltage decrease and the Jsc increase. These results provide insights about the degradation mechanism and emphasize the importance of the stability of TiO2/dye/electrolyte interface for the device stability under thermal stress.
29.	Yang, Wenxing, et al. (author) Ultrafast and Long-Lived Transient Heating of Surface Adsorbates on Plasmonic Semiconductor Nanocrystals 2021 In: Nano letters (Print). - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 21:1, s. 453-461 Journal article (peer-reviewed)abstract Plasmonic photocatalysts have demonstrated promising potential for enhancing the selectivity and efficiency of important chemical transformations. However, the relative contributions of nonphotothermal (i.e., hot carrier) and photothermal pathways remain a question of intense current debate, and the time scale and extent of surface adsorbate temperature change are still poorly understood. Using p-type Cu2-xSe nanocrystals as a semiconductor plasmonic platform and adsorbed Rhodamine B as a surface thermometer and hot carrier acceptor, we measure directly by transient absorption spectroscopy that the adsorbate temperature rises and decays with time constants of 1.4 +/- 0.4 and 471 +/- 126 ps, respectively, after the excitation of Cu2-xSe plasmon band at 800 nm. These time constants are similar to those for Cu2-xSe lattice temperature, suggesting that fast thermal equilibrium between the adsorbates and nanocrystal lattice is the main adsorbate heating pathway. This finding provides insights into the transient heating effect on surface adsorbates and their roles in plasmonic photocatalysis.

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