Lawrence Tien Lin Lee

Few-Layer MoSe2 Possessing High Catalytic Activity towards Iodide/Tri-iodide Redox Shuttles

Due to the two-dimensional confinement of electrons, single- and few-layer MoSe2 nanostructures exhibit unusual optical and electrical properties and have found wide applications in catalytic hydrogen evolution reaction, field effect transistor, electrochemical intercalation, and so on. Here we present a new application in dye-sensitized solar cell as catalyst for the reduction of I3− to I− at the counter electrode. The few-layer MoSe2 is fabricated by surface selenization of Mo-coated soda-lime glass. Our results show that the few-layer MoSe2 displays high catalytic efficiency for the regeneration of I− species, which in turn yields a photovoltaic energy conversion efficiency of 9.00%, while the identical photoanode coupling with “champion” electrode based on Pt nanoparticles on FTO glass generates efficiency only 8.68%. Thus, a Pt- and FTO-free counter electrode outperforming the best conventional combination is obtained. In this electrode, Mo film is found to significantly decrease the sheet resistance of the counter electrode, contributing to the excellent device performance. Since all of the elements in the electrode are of high abundance ratios, this type of electrode is promising for the fabrication of large area devices at low materials cost.

Fluorene-bridged organic dyes with di-anchoring groups for efficient co-adsorbent-free dye-sensitized solar cells

A series of new fluorene-bridged organic dyes with di-anchoring groups have been synthesized and well characterized. Such a molecular design strategy using two organic anchors inhibits the undesirable charge recombination and prolongs the electron lifetime which results in significant enhancement of the power conversion efficiency (η). These findings were supported by the results from electrochemical impedance spectroscopy (EIS) and open-circuit voltage decay (OCVD). Under standard AM 1.5 irradiation (100 mW cm−2), the best dye-sensitized solar cell (DSSC) exhibits a high η of 6.11% without the need for co-adsorbent addition. An open-circuit photovoltage (Voc) of 0.753 V, a short-circuit photocurrent density (Jsc) of 11.20 mA cm−2 and a fill factor (ff) of 0.725 were measured in such co-adsorbent-free cells. The high Voc value is mainly attributed to the improved electron lifetime (τn) and high resistance to the recombination of electrons (Rrec) of 422.38 Ω.

Printable highly catalytic Pt- and TCO-free counter electrode for dye-sensitized solar cells.

Here we show that a counter electrode based on carbon network supported Cu2ZnSnS4 nanodots on Mo-coated soda-lime glass for dye-sensitized solar cells can outperform the conventional best electrode with Pt nanoparticles on the fluorine-doped SnO2 conducting glass. In the as-developed electrode, all of the elements are of high abundance ratios with low materials cost. The fabrication is scalable because it is conducted by a screen-printing based approach. Therefore, this research lays a solid ground for the large area fabrication of high-performance dye-sensitized solar cell at reduced material cost.

Co-sensitization of 3D bulky phenothiazine-cored photosensitizers with planar squaraine dyes for efficient dye-sensitized solar cells

A series of new phenothiazine-cored 3D bulky organic sensitizers TP1–TP4 have been prepared and employed in dye-sensitized solar cells (DSSCs). The 3D bulky configuration of these molecules can effectively retard the charge recombination at the TiO2/electrolyte interface. Amongst the four dyes, the co-adsorbent-free DSSC based on the dye TP3 exhibited the best conversion efficiency (η) of 8.00%. Subsequently, the photosensitizer TP3 with strong UV-visible absorption and excellent performance in adsorbent-free DSSCs was co-sensitized with a near-infrared (NIR) absorbing squaraine dye YR6 to realize a UV-visible-NIR light-harvesting capability, which can effectively suppress the dye aggregation of YR6 with a planar structure and retard the charge recombination in the as prepared DSSC. Upon optimization, the co-sensitized DSSCs exhibited remarkable overall efficiency enhancements of 33% and 356% as compared with the devices based on TP3 and YR6 alone, respectively, and a high efficiency up to 9.84% was achieved at the TP3/YR6 molar ratio of 25 : 1.

Panchromatic light harvesting by N719 with a porphyrin molecule for high-performance dye-sensitized solar cells

We report efficient panchromatic light harvesting by co-adsorption of a porphyrin molecule and N719 in dye-sensitized solar cells. The co-sensitized device shows a considerably enhanced power conversion efficiency of 8.89%, while those individually sensitized by porphyrin and N719 display efficiencies of 7.21% and 8.02%, respectively. The porphyrin-sensitized device shows strong photocurrent generation in the Soret and Q bands, while N719 shows efficient spectral response in the 500–600 nm range; the combination of these two kinds of dye molecules displays a strong spectral response in the full-colour region. Mechanistic investigations are carried out by various spectral and electrochemical characterizations.

Synthesis and Characterization of Phenothiazine-Based Platinum(II)-Acetylide Photosensitizers for Efficient Dye-Sensitized Solar Cells.

Three new unsymmetrical phenothiazine-based platinum(II) bis(acetylide) complexes PT1-PT3 with different electron-donating arylacetylide ligands were synthesized and characterized. Their photophysical, electrochemical, and photovoltaic properties have been fully investigated and the density functional theory (DFT) calculations have been carried out. Under AM 1.5 irradiation (100 mW cm(-2)), the PT1-based dye-sensitized solar cell (DSSC) device exhibited an attractive power conversion efficiency (η) up to 5.78 %, with a short-circuit photocurrent density (J(sc)) of 10.98 mA cm(-2), an open-circuit photovoltage (V(oc)) of 0.738 V, and a fill factor (ff) of 0.713. These findings provide strong evidence that platinum-acetylide complexes have great potential as promising photosensitizers in DSSC applications.

Improving pore filling of gel electrolyte and charge transport in photoanode for high-efficiency quasi-solid-state dye-sensitized solar cells.

We demonstrate the enhancement of pore-filling and wettability of gel electrolyte in quasi-solid-state dye-sensitized solar cells (DSSCs) by developing a kinetically driven electrolyte infiltration approach, in which the air purging provides the driving force. This method renders fast electrolyte diffusion throughout the three-dimensional TiO2 nanoparticle network, promising for large-area device fabrication. In addition, for the first time we incorporate multiwalled carbon nanotubes into the anode of quasi-solid-state DSSCs to improve the charge transfer efficiency and fill factor. These advancements finally generate an efficiency exceeding 7.0%, much higher than the device efficiency of 5.5% fabricated by the conventional method.

Molecular engineering of starburst triarylamine donor with selenophene containing π-linker for dye-sensitized solar cells

A series of new D–π–A organic photosensitizers 7a–7d featuring a novel starburst electron donor unit and uncommon selenophene containing π-linker were synthesized, characterized, and applied for fabrication of dye-sensitized solar cells (DSSCs). Dyes 11d–13d with thiophene or phenyl ring as the π-linker also were synthesized for comparison. The best power conversion efficiency (PCE) of 6.67% was attained for 11d with a relatively high open-circuit voltage (Voc) of 0.825 V using conventional I−/I3− redox electrolyte in DSSCs, and this value reaches about 84% of the device based on standard dye N719 (7.91%) under the same device fabrication conditions. Electrochemical impedance spectroscopy (EIS) and open-circuit voltage decay (OCVD) were applied to verify the findings. All the results suggest that starburst electron donor design strategy can be used to minimize dye aggregation on TiO2 and to slow down the charge recombination kinetics in DSSCs to improve the photovoltaic performance. Effects of using selenophene as the π-linker building block on the photovoltaic parameters also were explored and evaluated.

Bis(phenothiazyl-ethynylene)-based organic dyes containing Di-anchoring groups with efficiency comparable to N719 for dye-sensitized solar cells

A new series of acetylene-bridged phenothiazine-based di-anchoring dyes have been synthesized, fully characterized, and used as the photoactive layer for the fabrication of conventional dye-sensitized solar cells (DSSCs). Tuning of their photophysical and electrochemical properties using different π-conjugated aromatic rings as the central bridges has been demonstrated. This molecular design strategy successfully inhibits the undesirable charge recombination and prolongs the electron lifetime significantly to improve the power conversion efficiency (η), which was proven by the detailed studies of electrochemical impedance spectroscopy (EIS) and open-circuit voltage decay (OCVD). Under a standard air mass (AM) 1.5 irradiation (100 mW cm-2 ), the DSSC based on the dye with phenyl bridging unit exhibits the highest η of 7.44 % with open-circuit photovoltage (Voc ) of 0.796 V, short-circuit photocurrent density (Jsc ) of 12.49 mA cm-2 and fill factor (ff) of 0.748. This η value is comparable to that of the benchmark N719 under the same conditions.

Plasmon-Enhanced Excitonic Solar Cells

The exciton dissociation in excitonic solar cells (XSCs) is controlled by interface, where the generated eletrons on one side and holes produced on the other side. In this device configuration, the interfacial characteristics are crucial for the operation, while the bulk property is less critical. Due to the unique characteristics of materials and operation mechanism, the fabrication of XSCs allows low-cost, large-scale solution processing, and the utilization of various printing techniques, instead of high-cost vacuum deposition and purification process applied in the fabrication of conventional p–n junction solar cells. To date, power conversion efficiencies exceeding 10 % have been achieved in some XSCs at lab scale. For the practical applications, the energy conversion efficiencies are required to be further improved, especially at module scale. In this chapter, we pay special attention to the recent development in the application of metal (e.g., Au and Ag) nanoparticles to increase the light utilization in XSCs for high photovoltaic conversion efficiency. This type of light trapping, on the ground of localized surface plasmon resonance and propagating surface plasmon polariton, provides an alternative approach to the development of new light absorbing materials to span the strong spectral response over broader ranges. The methodologies and enhancement mechanisms regarding a series of typical device architectures will be discussed.