High Electronic Coupling between Cu Complexes and Oxidized Dyes Confirmed by Measurements of Driving Force Dependent Regeneration Kinetics in Minimal Electrolyte System.

We confirm fast regeneration kinetics between copper complexes and oxidized organic dyes and the major contribution of electronic coupling (HDA). The highest efficiency of dye-sensitized TiO2 solar cells has been shown by employing Cu complex redox couples. Various groups have reported a fast regeneration rate of oxidized dyes by Cu complexes giving a low driving force attributed to low reorganization energy (λ), but the effect of HDA has not been evaluated. The values of HDA and λ can be derived from driving force dependent transient absorption (TA) measurements. However, analyzing TA decay using Cu complexes is not trivial because accelerated recombination by the presence of Cu2+ complexes and biphasic TA decay often complicates the analysis. Here we employ 16 Cu1+ and Co2+ complexes and two dyes. To simplify the system, i.e., making a minimal electrolyte system, Cu2+ and Co3+ complexes and a common additive of 4-tert-butylpyridine are not used. From the driving force dependent TA decays of oxidized dyes by both Cu1+ and Co2+ complexes, λ for the combination of the Cu complexes and dyes is found to be about 0.15 eV lower than that of Co complexes. Approximately 3 to 5 times higher HDA values of Cu complexes than those of Co complexes are obtained, which is the dominant factor for faster rates. The values vary with the structure of the molecules, showing the possibility of increasing the HDA values further. The higher HDA values of a Cu complex than that of a Co complex are also reproduced by quantum chemical calculations.

Catalytic Decomposition of an Organic Electrolyte to Methane by a Cu Complex-Derived In Situ CO2 Reduction Catalyst.

Metal complexes are often transformed to metal complex-derived catalysts during electrochemical CO2 reduction, enhancing the catalytic performance of CO2 reduction or changing product selectivity. To date, it has not been investigated whether metal-complex derived catalysts also enhance the decomposition of the solvent/electrolyte components as compared to an uncoated electrode. Here, we tested the electrochemical stability of five organic solvent-based electrolytes with and without a Cu complex-derived catalyst on carbon paper in an inert atmosphere. The amount of methane and hydrogen produced was monitored using gas chromatography. Importantly, the onset potential for methane production was reduced by 300 mV in the presence of a Cu complex-derived catalyst leading to a significant amount of methane (417.7 ppm) produced at -2.17 V vs Fc/Fc+ in acetonitrile. This suggests that the Cu complex-derived catalyst accelerated not only CO2 reduction but also the reduction of the electrolyte components. This means that Faradaic efficiency (FE) measurements under CO2 in acetonitrile may significantly overestimate the amount of CH4. Only 28.8 ppm of methane was produced in dimethylformamide under an inert atmosphere, much lower than that produced under CO2 (506 ppm under CO2) at the same potential, suggesting that dimethylformamide is a more suitable solvent. Measurements in propylene carbonate produced mostly hydrogen gas while in dimethyl sulfoxide and 3-methoxypropionitrile neither methane nor hydrogen was detected. A strong linear correlation between the measured current and the amount of methane produced with and without the Cu complex-derived catalyst confirmed that the origin of methane production is solvent/electrolyte decomposition and not the decomposition of the catalyst itself. The study highlights that in a nonaqueous system, highly active catalyst in situ deposited during electrochemical testing can significantly influence background measurements as compared to uncoated electrodes, therefore the choice of solvent is paramount for reliable testing.

Effect of the Cu2+/1+ Redox Potential of Non-Macrocyclic Cu Complexes on Electrochemical CO2 Reduction.

Cu2+/1+ complexes facilitate the reduction of CO2 to valuable chemicals. The catalytic conversion likely involves the binding of CO2 and/or reduction intermediates to Cu2+/1+, which in turn could be influenced by the electron density on the Cu2+/1+ ion. Herein we investigated whether modulating the redox potential of Cu2+/1+ complexes by changing their ligand structures influenced their CO2 reduction performance significantly. We synthesised new heteroleptic Cu2/1+ complexes, and for the first time, studied a (Cu-bis(8-quinolinolato) complex, covering a Cu2+/1+ redox potential range of 1.3 V. We have found that the redox potential influenced the Faradaic efficiency of CO2 reduction to CO. However, no correlation between the redox potential and the Faradaic efficiency for methane was found. The lack of correlation could be attributed to the presence of a Cu-complex-derived catalyst deposited on the electrodes leading to a heterogeneous catalytic mechanism, which is controlled by the structure of the in situ deposited catalyst and not the redox potential of the pre-cursor Cu2+/1+ complexes.

Solvent-Dependent Functional Aggregates of Unsymmetrical Squaraine Dyes on TiO2 Surface for Dye-Sensitized Solar Cells.

Alkyl group wrapped donor-acceptor-donor (D-A-D) based unsymmetrical squaraine dyes SQ1, SQ5, and SQS4 were used to evaluate the effect of sensitizing solvents on dye-sensitized solar cell (DSSC) efficiency. A drastic change in DSSC efficiency was observed when the photo-anodes were sensitized in acetonitrile (bad solvent when considering dye solubility) and chloroform (good solvent) with an Iodolyte (I-/I3-) electrolyte. The DSSC device sensitized with squaraine dyes in acetonitrile showed better photovoltaic performance with enhanced photocurrent generation and photovoltage compared to the device sensitized in chloroform. In a good sensitizing solvent, dyes with long hydrophobic alkyl chains are deleterious forming aggregates on the TiO2 surface, which results in an incident photon-to-current conversion efficiency (IPCE) response mostly from monomeric and dimeric structures. Meanwhile, a bad sensitizing solvent facilitates the formation of well-packed self-assembled structures on the TiO2 surface, which are responsible for a broad IPCE response and high device efficiencies. The photoanode sensitized in the bad sensitizing solvent showed enhanced VOC values of 642, 675, and 699 mV; JSC values of 6.38, 11.1, and 11.69 mA/cm2; and DSSC device efficiencies of 3.0, 5.63, and 6.13% for the SQ1, SQ5, and SQS4 dyes in the absence of a coadsorbent (chenodeoxycholic acid (CDCA)), respectively, which were further enhanced by CDCA addition. Meanwhile, the photoanode sensitized in the good sensitizing solvent showed relatively low photovoltaic VOC values of 640, 652, and 650 mV; JSC values of 5.78, 6.79, and 6.24 mA/cm2; and device efficiencies of 2.73, 3.35, and 3.20% for SQ1, SQ5, and SQS4 in the absence of CDCA, respectively, which were further varied with equivalents of CDCA. The best DSSC device efficiencies of 6.13 and 3.20% were obtained for SQS4 without CDCA, where the dye was sensitized in acetonitrile (bad) and chloroform (good) sensitizing solvents, respectively.

Insight into the Origin of Trapping in Polymer/Fullerene Blends with a Systematic Alteration of the Fullerene to Higher Adducts.

The bimolecular recombination characteristics of conjugated polymer poly[(4,4'-bis(2-ethylhexyl)dithieno[3,2-b:2',3'-d]silole)-2,6-diyl-alt-(2,5-bis 3-tetradecylthiophen-2-yl thiazolo 5,4-d thiazole)-2,5diyl] (PDTSiTTz) blended with the fullerene series PC60BM, ICMA, ICBA, and ICTA have been investigated using microsecond and femtosecond transient absorption spectroscopy, in conjunction with electroluminescence measurements and ambient photoemission spectroscopy. The non-Langevin polymer PDTSiTTz allows an inspection of intrinsic bimolecular recombination rates uninhibited by diffusion, while the low oscillator strengths of fullerenes allow polymer features to dominate, and we compare our results to those of the well-known polymer Si-PCPDTBT. Using µs-TAS, we have shown that the trap-limited decay dynamics of the PDTSiTTz polaron becomes progressively slower across the fullerene series, while those of Si-PCPDTBT are invariant. Electroluminescence measurements showed an unusual double peak in pristine PDTSiTTz, attributed to a low energy intragap charge transfer state, likely interchain in nature. Furthermore, while the pristine PDTSiTTz showed a broad, low-intensity density of states, the ICBA and ICTA blends presented a virtually identical DOS to Si-PCPDTBT and its blends. This has been attributed to a shift from a delocalized, interchain highest occupied molecular orbital (HOMO) in the pristine material to a dithienosilole-centered HOMO in the blends, likely a result of the bulky fullerenes increasing interchain separation. This HOMO localization had a side effect of progressively shifting the polymer HOMO to shallower energies, which was correlated with the observed decrease in bimolecular recombination rate and increased "trap" depth. However, since the density of tail states remained the same, this suggests that the traditional viewpoint of "trapping" being dominated by tail states may not encompass the full picture and that the breadth of the DOS may also have a strong influence on bimolecular recombination.

Enhanced interfacial electron transfer kinetics between Co2+/3+ complexes and organic dyes with free space near their backbone.

Fast electron transfer (ET) between surface-bound dye molecules and electron donor molecules dissolved in electrolytes with simultaneous reduction in recombination rates are crucial to improve the photon-to-electron conversion efficiency of photo-electrochemical technologies. Here, the electron transfer characteristics of a new dye molecule PX47 with only two alkyl chains placed in the anti configuration of the π-conjugated quarterthiophene backbone is studied. It is anticipated that the appropriate free space between the alkyl chains allowed the approach of the Co(c1-bpy)3 redox mediator to near the backbone of the dye anchored to a TiO2 electrode even at complete coverage of the TiO2 surface, thereby enhancing electronic coupling. The ET kinetics measured by transient absorption spectroscopy were enhanced by a factor of six between PX47 and Co(c1-bpy)3 as compared to the structurally similar MK2 dye with four alkyl chains. The ET rates between PX47 and the larger nonyl-substituted (Co(c9-bpy)3 or tert-butyl substituted Co bipyridine (Co(dtb-bpy)3) were reduced by half and one third as compared to Co(c1-bpy)3, respectively, indicating a blocking effect of longer or bulky substituents on the redox mediators. For MK2 with four alkyl chains near the backbone, the ET rate was very similar between Co(c1-bpy)3 and Co(dtb-bpy)3 and was enhanced for (Co(c9-bpy)3, the latter explained by trapping the mediator inside the dye layer due to alkyl-alkyl interactions. Unexpectedly, two distinctly different recombination rates were measured between the oxidized Co(c1-bpy)3 mediators and TiO2 electrons in the PX47-TiO2 samples, which is explained by two possible arrangements of the PX47 on the TiO2 surface. The dominant arrangement allowed the adsorption of Co(c1-bpy)3 on the TiO2 surface enhancing recombination. The findings suggest that by strategically placing alkyl chains around the electronic units of dye molecules, the redox mediator can be intercalated into the dye layer increasing proximity and better electronic coupling. Although this work presents an effective strategy to enhance ET rates by designing structurally complementary electron donor-acceptor pairs, additional design strategies to further reduce recombination in such open dye structures are needed.

Flexible Polymer X-ray Detectors with Non-fullerene Acceptors for Enhanced Stability: Toward Printable Tissue Equivalent Devices for Medical Applications.

There is growing interest in the development of novel materials and devices capable of ionizing radiation detection for medical applications. Organic semiconductors are promising candidates to meet the demands of modern detectors, such as low manufacturing costs, mechanical flexibility, and a response to radiation equivalent to human tissue. However, organic semiconductors have typically been employed in applications that convert low energy photons into high current densities, for example, solar cells and LEDs, and thus existing design rules must be re-explored for ionizing radiation detection where high energy photons are converted into typically much lower current densities. In this work, we report the optoelectronic and X-ray dosimetric response of a tissue equivalent organic photodetector fabricated with solution-based inks prepared from polymer donor poly(3-hexylthiophene) (P3HT) blended with either a non-fullerene acceptor (5Z,5'Z)-5,5'-((7,7'-(4,4,9,9-tetraoctyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene-2,7-diyl)bis(benzo[c][1,2,5]thiadiazole-7,4-diyl))bis(methanylylidene))bis(3-ethyl-2-thioxothiazolidin-4-one) (o-IDTBR) or a fullerene acceptor, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Indirect detection of X-rays was achieved via coupling of organic photodiodes with a plastic scintillator. Both detectors displayed an excellent response linearity with dose, with sensitivities to 6 MV photons of 263.4 ± 0.6 and 114.2 ± 0.7 pC/cGy recorded for P3HT:PCBM and P3HT:o-IDTBR detectors, respectively. Both detectors also exhibited a fast temporal response, able to resolve individual 3.6 µs pulses from the linear accelerator. Energy dependence measurements highlighted that the photodetectors were highly tissue equivalent, though an under-response in devices compared to water by up to a factor of 2.3 was found for photon energies of 30-200 keV due to the response of the plastic scintillator. The P3HT:o-IDTBR device exhibited a higher stability to radiation, showing just an 18.4% reduction in performance when exposed to radiation doses of up to 10 kGy. The reported devices provide a successful demonstration of stable, printable, flexible, and tissue-equivalent radiation detectors with energy dependence similar to other scintillator-based detectors used in radiotherapy.

Multisample Correlation Reveals the Origin of the Photocurrent of an Unstable Cu2O Photocathode during CO2 Reduction.

The reliable characterization of the photoelectrochemical (PEC) performance of unstable photoelectrodes, often the simplest devices used as a baseline, is a huge challenge. By performing a correlation analysis of more than 100 parameters of Cu2O photocathodes electrodeposited under the same conditions, we discovered a strong positive correlation (R = 0.866) between the photocurrent in argon and the deposition current peak magnitude during electrodeposition, while a strong negative correlation (R = -0.787) was found in CO2. In argon, a positive correlation between the photocurrent during PEC tests and the post-PEC dark current suggests the dominance of photodegradation. In CO2, the higher photocurrent in PEC tests correlates well with the lower post-PEC dark current, revealing the dominance of photocatalytic CO2 reduction during the rapid PEC tests. Correlation analysis provides statistically robust insights into the operation of unstable electrodes based on routinely measured parameters and thus constitutes a simple yet previously unexplored methodology for characterizing photoelectrodes within the first minutes of operation.

The impact of insufficient time resolution on dye regeneration lifetime determined using transient absorption spectroscopy.

Dye regeneration lifetimes of a combination of dyes and redox mediators were determined by two transient absorption (TA) spectrometers with 0.5 ns (sub-ns) and 6 ns (ns) time resolutions to elucidate the impact of insufficient time resolution on the measurements of dye regeneration kinetics in dye-sensitised semiconductor electrodes. Due to the disordered nature of the dye-sensitised electrodes, the dye regeneration lifetime is often characterised by half-decay time (τ1/2) of the initial signal magnitude. Alternatively, τ1/2,S is calculated from stretched-exponential lifetime (τww) and the distribution of lifetimes characterised by the stretch parameter (ß). Stretched-exponential functions were numerically modelled, showing that to keep the error in τ1/2 ≤ 10%, τww needs to be at least 20 times longer than the time resolution in case of non-dispersive transients (ß = 0.9) but at least 870 times longer when dispersive (ß = 0.5). To test the predictions, TA decays of a combination of organic and porhyrin dyes and three cobalt-complex mediators are analysed, spanning a range of τww and ß. These examples show that a 262% error in τ1/2 is possible if the time resolution of the TA setup is only 13 times faster than τww and smaller ß results in larger error when τww is similar. Determining τ1/2,S by stretched-exponential fitting generally reduces the error compared to that determined directly from the graph. However, if the stretched-exponential function does not correctly describe the early signal transient, even a larger error by stretched-exponetial fitting is introduced. The key requirement for accurate measurement is to have a fast-enough TA setup to resolve the initial plateau of the TA signal. To demonstrate the impact of the measured errors, the measured regeneration lifetimes are plotted versus the driving force of the reaction and modelled using Marcus theory. Erroneous regeneration rates lead to an underestimated electronic coupling term by 2.2 times in case of a series of porphyrin dyes matched with Co complex electrolytes, a significant impact when the interpretation of factors affecting electron transfer at dye-sensitised semiconductor/electrolyte interface is discussed.

Substrate-Dependent Electron-Transfer Rate of Mixed-Ligand Electrolytes: Tuning Electron-Transfer Rate without Changing Driving Force.

To meet various requirements for electron transfer (ET) at the substrate/electrolyte interface, mixed redox couples assigned to different functions have been applied. While in all studies the mixed redox species had different redox potentials, such redox systems inherently lose energy by ET between the species. We report interfacial ET kinetics employing mixed-ligand electrolytes based on Co2+/3+ complexes with mixtures of dimethyl- and dinonyl-substituted bipyridyl (bpy) ligands with the same redox potential. The ET rates of the mixed electrolytes decrease with the increasing ratio of the dinonyl-bpy ligand, with substrates adsorbed by molecules without alkyl chains due to a blocking effect. However, when the molecules on substrates have four alkyl chains, the ET rate between the molecules and the electrolytes with increasing ratio of the dinonyl-bpy ligand is enhanced. The substrate-dependent behavior is explained by selective intermolecular interactions. The results open design flexibility for mixed-redox electrolyte systems to control ET at multi-substrate interfaces and provide a novel means to tune ET rates simultaneously for various ET processes in a system without losing energy by the ET.