PacSurf2018 Session EH-WeM: Efficient Power Conversion/Cells

Currently, the most important applications for polythiophenes are in the area of solar cells, thin-film transistors, light-emitting diodes, sensors and nonlinear optics with low-cost and low-temperature processing [1]. On the other hand, polythiophenes show a great promise also in the area of nonlinear optics and photonics [2] with enhanced second and third order nonlinearities. Hence, dynamics of polythiophenes gained an increased interest, because it is providing detailed understanding of the complex processes occurring in π-conjugated polymers.

A novel copolymer (poly(thiophene-2,5-diyl-2,5-di-n-octyloxycarbonyl-1,4-phenylene)) denoted as P33 is introduced as a potential material for photovoltaics. P33 dissolved in chloroform was investigated by steady-state absorption, linear/non-linear fluorescence spectroscopies and time-resolved fluorescence spectroscopy.

Molar extinction coefficient of P33 was determined as 18315 cm^-1.M^-1. The P33 fluorescence quantum yield and P33 singlet fluorescence lifetime were determined as 0.4 and 810 ps, respectively. The P33 fluorescence fast decay component shows decay times of 1.2 ps, 2.0 ps, and 0.5 ps for increasing wavelengths of 480 nm, 500 nm, and 520 nm, respectively. The fast component has been previously attributed to transport of nearly instantaneously formed excitons to localized states known as downhill energy transfer. Multi-photon excited fluorescence has been observed for the P33 solutions in chloroform and for 800 nm and 1200 nm pumping. The P33 TPA cross-section was evaluated as 6.9 GM. This spectroscopic study provides basic fluorescence characteristics of the novel thiophene copolymer P33.

This work was supported by VEGA 1/0400/16 .

References

[1] I. Etxebarria, J. Ajuria, R. Pacios, Solution-processable polymeric solar cells: A review on materials, strategies and cell architectures to overcome 10%. Org. Electron. 19 (2015) 34–60.

In recent years, researches on transparent photovoltaics has been attracting immerse interests as a key component of multifunctional window applications. Until now, there were enormous researches on transparent photovoltaics were based on organic materials, such as dye sensitized solar cell (DSSC) or organic solar cell (OSC), because of wide bandgap of the organic materials. However, these organic-based transparent solar cells are still suffering from the stability problem, which is one of critical obstacles for the commercialization of organic-based solar cells. In order to overcome this problem, some researchers are nowadays interested in the inorganic-based transparent solar cell technologies, such as ultra-thin film solar cells, patterned aperture solar cells, and so on. However, in these cases, device fabrication process is complicated and the device performance is limited because of restricted physical dimensions.

The electrocatalytic reactions at the electrode/electrolyte interface play a critical role in the performance of electrochemical energy storage and conversion devices. Understanding the structure and reaction processes at solid/liquid interface is of great importance in surface science and electrochemistry. In view of the dynamic and complex nature of the interface, in situ research approaches can provide valuable information of interfacial phenomena. In situ scanning tunneling microscopy (STM) is a powerful technique used for the interfacial investigation of electrochemical energy devices.

In this presentation, we employed high resolution electrochemical STM to investigate the typical electrochemical catalytic reactions, such as oxygen reduction reactions, oxygen evolution reactions, using the model molecular catalysts. The self-assembled metal porphyrin and phthalocyanine compounds show notable electrocatalytic activity. The real-time STM imaging provides direct evidence to study the interfacial electrochemical reactions at molecular level.

1. Gu, JY; Cai, ZF; Wang, D; Wan, LJ. ACS Nano. 2016, 10, 8746-8750.

2. Cai, ZF; Wang, X; Wang, D; Wan, LJ. ChemElectroChem, 2016, 3, 2048-2051.

Pt catalysts are used as anode catalysts for polymer electrolyte fuel cell (PEFC). The fuel in PEFC contains a small amount of impurities such as CO and H₂S. These impurities adsorb on active sites of Pt surfaces and degrade the hydrogen oxidation reaction activity of the anode catalyst. This loss of catalytic activity caused by impurities is known as impurity poisoning. Therefore, the development of the anode catalyst with the high impurity tolerance is strongly required. Takeguchi et al. experimentally found that adding SnO₂ as support material improves the CO tolerance of the Pt-based catalyst[1]. Furthermore, it is known that adsorbed impurities on the Pt catalyst can be removed by oxidation reaction. Kakati et al. reported that oxidation reaction by O and OH can recover from the H₂S poisoning[2]. In order to develop the high impurity tolerant catalyst, it is necessary to reveal the effect of SnO₂ nano-particles on H₂S tolerance and the recovery mechanism from H₂S poisoning by oxidation reaction. In this study, we analyzed the adsorption states of H₂S on Pt/SnO₂(110) model and the recovery process from H₂S poisoning by oxidation with OH using first-principles calculation.

For the calculation model, we put a Pt₂₉ cluster on SnO₂(110). The Pt₂₉ cluster exposes Pt(111) on the top (See supplementary document Fig. 1). We calculated the adsorption energy of H₂S on Pt/SnO₂(110) and compared with the one on Pt(111) to reveal the effect of SnO₂ on H₂S poisoning process. The adsorption energies of H₂S were -18.38 and -24.73 kcal/mol on the Pt cluster of Pt/SnO₂(110) and Pt(111), respectively. Thus, it was found that the adsorption of H₂S on Pt is suppressed by addition of SnO₂. Next, we analyzed the recovery process from H₂S poisoning by OH generated from dissociation of H₂O. It is known that H₂S adsorption on Pt is dissociative and adsorbed sulfur atom is generated. The sulfur atom adsorbed on the surface decreases the activity of the Pt catalyst. In this study, we considered the reaction process (See supplementary document Fig. 2) based on the intermediate stable species during the H₂S oxidation cascade in the gas phase[3] and calculated the activation energies of each elementary process. We found that the water dissociation is the rate-determining step on both Pt/SnO₂(110) and Pt(111) and the activation energies were 18.78 and 23.70 kcal/mol, respectively. Therefore, we demonstrated that an addition of SnO₂ promotes an oxidation reaction of adsorbed sulfur on the Pt catalyst.

1) T. Takeguchi, et. al., Catal. Sci. Technol. 6, 3214 (2016).

2) B. K. Kakati, et al., J. Power Sources 252, 317 (2014).

3) F. Tureček, et al., J. Am. Chem. Soc. 118, 11321 (1996).

Polymer electrolyte fuel cell (PEFC) needs anode materials with high tolerance to poisoning by impurities such as CO, NH₃, and H₂S in the fuel, which degrades performance of the PEFC. Recently, a composite of Pt and WO₃ (Pt/WO₃) is used as a catalyst in the anode, and this catalyst is effective for CO removal by oxidation [1]. For theoretical design of anode materials with high tolerance to impurity poisoning, the mechanism of high tolerance of Pt/WO₃ to CO should be revealed. In this study, we investigated CO oxidation processes on Pt/WO₃(001) by first-principles calculation. At first, we calculated the adsorption energies of CO on Pt/WO₃(001) and an isolated Pt cluster, to discuss an effect of WO₃ on CO tolerance of Pt. For a Pt/WO₃(001) model, a Pt₂₀ cluster is put on a WO₃(001) surface. The adsorption energy of CO on the Pt₂₀ cluster is 36.40 kcal/mol, while that on an isolated Pt₂₀ cluster is 45.65 kcal/mol. These results indicate that the combination of the WO₃ surface and Pt cluster decreases the adsorption energy of CO on the Pt cluster. To investigate the reason why the adsorption energy of CO decreases by the WO₃ surface, we calculated d-band center [2] of the Pt atom on Pt/WO₃(001) and Pt cluster. In general, downward shift of the d-band center increases the adsorption energy of CO. Here, the d-band center values of the Pt atom on Pt/WO₃(001) and isolated Pt cluster are -2.28 and -2.15 eV, respectively. This means that WO₃ modifies the electronic states of the Pt cluster and leads to the downward shift of the d-band center, which decreases the adsorption energy of CO. Next, we discuss CO oxidation on Pt/WO₃(001). The CO oxidation by H₂O proceeds as follows; (i) H₂O → OH^- + H⁺ and (ii) CO + OH^-→ CO₂ + H⁺ + 2e^-. Here, we firstly investigated H₂O dissociation on Pt/WO₃(001). In this calculation, the H₂O molecule adsorbs on the interface between the Pt cluster and WO₃(001) surface, and dissociates to H⁺ on the Pt atom and OH^- at the interface. The activation energy for the H₂O dissociation is 19.87 kcal/mol, which is lower than that on a pure Pt(111) surface (23.70 kcal/mol). Thus, we suggest that WO₃(001) can decrease an adsorption energy of CO and activation energy for H₂O dissociation on Pt catalyst during CO oxidation process.

[1] P.-Y. Olu, et al., Electrochem. Commun., 71, 69 (2016).

[2] B. Hammer, et al., Catal. Lett., 46, 31 (1997).

Harvesting sunlight enables conversion of photon energy to electronic energy and thermal energy. Among different classes of materials, metals have unique properties in light harvesting. Since metals are highly conductive and do not have bandgaps, metals can generate hot carriers even with low energy photons to be injected into an adjacent semiconductor. The excited hot carriers eventually become heat and heat the metals themselves and their surroundings. These photoelectric and photothermal effects can be enhanced by the optical resonances i.e. surface plasmon resonances. Hence, number of researches have taken advantage of plasmon resonances in photoelectric and photothermal conversions. As gold and silver are known to be excellent plasmonic materials, nanostructures made of these noble metals have been widely used in the recent studies.

In contrast, we have been working with titanium nitride (TiN) nanostructure to show that it can also be used in photoelectric and photothermal conversions. Titanium nitride is chemically stable and much cost-effective than gold or silver, making it a practical choice of material. In addition, TiN is plasmonic in visible and near infrared and superior to gold and silver in absorbing broad spectrum. In the first part, we present that TiN nanostructures can generate photocurrent by the irradiation of visible light, and can enhance the visible photocatalytic activities of carbon nitride which is a UV-active metal-free photocatalyst. In the second part, we show that TiN nanoparticles are efficient sunlight absorbers to generate solar heat. Since each TiN nanoparticle act as a nanoscale heater, solar heated TiN nanoparticles offer efficient water distillation and chemical reactions such as oxidation of carbon monoxide. Our results demonstrate that TiN nanostructures have the potential to replace gold and silver nanostructures in sunlight harvesting applications with better efficiencies.

Organic photovoltaics (OPV) are poised to play a major role in the global energy portfolio driven by their capability to be printed at high speeds across large areas using roll-to-roll (R2R) processing techniques; creating the tantalising vision of coating every roof and other suitable building surface with photovoltaic materials at extremely low cost. Indeed, recent full economic modelling of the balance of materials (BOM) and balance of system (BOS) costs, have highlighted the long-term commercial viability of OPV-based technology in today’s energy marketplace.

However, the chlorinated solvents that are used in current OPV technology are under continual regulatory pressure due to their hazardous and toxic nature. Indeed, increasingly harsh technical requirements for using these solvents means that their implementation in high speed printing lines will be highly problematic if not economically impractical. In addition, tailoring device morphology across large areas is fraught with difficulty due to the challenge of controlling phase segregation of polymer mixtures using conventional printing. Water-based polymer nanoparticle dispersions (solar paint) offer the prospect of simultaneously controlling the nanoscale architecture of the active layer whilst eliminating the need for hazardous organic solvents during device fabrication. However, the behaviour of these nanoparticulate devices is complex and thus understanding their structure-function relationships requires characterisation techniques that can probe chemical structure on the nanoscale. In this paper we review our progress in understanding the structure-function relationships of organic electronic nanoparticulate thin films. In particular, I will discuss how scanning transmission X-ray microscopy is an invaluable tool for characterising these materials.

Finally, I will explore the future prospects and economics for large scale manufacture of solar cells based on printing. I will discuss our recent achievements in the development of a fully operating R2R printing line and the installation of several large scale (> 100 m²) demonstrations of printed solar modules.