Investigation on Functionalized Ruthenium-Based Sensitizers to Enhance Performance and Robustness of Dye-Sensitized Solar Cells

It has been increasingly aware to the world today that reserves of fossil fuels are limited and their use has serious environmental side effects. Encouraged by this realization was the evolution of the use of cleaner alternative energy, among which Dye-sensitized solar cells (DSCs) are potentially attractive candidates for the lower cost of producing devices which convert an abundant amount of energy from the sun into electricity. Dye-sensitizer in DSCs plays a crucial role as the chlorophyll in plants; to harvest solar light and transfer the energy via electron transfer to a suitable material (TiO2 in this case) to produce electricity. The topic of interest for this thesis is to further enhance the photovoltaic performance and the robustness of DSCs by tuning the optical properties of the dye-sensitizer (Ruthenium complex, in this case) using several strategies including an extension of the π-conjugation system, an introduction of antenna molecules and a modification of the Ru-complex structure. This work focuses on the DSC device fabrication and photovoltaic characterization in order to investigate more insight into structure-property-device performance relationship. New benchmarks for high performance DSCs with ruthenium complex sensitizers with π-extension in their ancillary ligands were presented. The overall conversion efficiency of 9.6% and 8.5% have been achieved with Ru-based sensitizer containing ethylenedioxythiophene, using low-volatile electrolyte and solvent-free electrolyte, respectively. The Rusensitizer functionalized with hexylthio-bithiophene unit exhibited a conversion efficiency of 9.4% with low-volatile electrolyte. All these devices showed good stability under prolonged light soaking at 60 °C. Extending π-conjugation of the anchoring ligand with thiophene units in monoleptic Ru-sensitizer also yields an impressive conversion efficiency of 6.1% using 3-µm-thin mesoporous TiO2 film in corporate with low-volatile electrolyte. DSC devices based on ruthenium sensitizers functionalized with thienothiophene- and EDOT-conjugated bridge, together with carbazole moiety on the ancillary ligands were found efficient with conversion efficiencies of 9.4% and 9.6%, respectively, in presence of a volatile electrolyte. The carbazole-functionalized ruthenium-based DCSs also performed excellently in the stability test using a low-volatile electrolyte. Furthermore, the Ru-complexes synthesized by click-chemistry in association with triazole-derivative moieties were successfully used as DCS sensitizers. DSC devices sensitized with these dyes provided the overall conversion efficiency close to 10% with volatile electrolyte. Further studies with solvent-free electrolyte showed notable device stability under extending full sunlight intensity at 60 °C. The results presented here provide a fertile base for further investigation, which will focus on improving the spectral response of ruthenium dye-sensitizer to full sunlight by searching for new strategies to modify the sensitizer with more efficient functional groups. The target is to reach higher conversion efficiency of DSC devices while retaining their stability under standard reporting conditions

A Thiophene-Based Anchoring Ligand and Its Heteroleptic Ru(II)-Complex for Efficient Thin-Film Dye-Sensitized Solar Cells

A novel heteroleptic Ru-II complex (BTC-2) employing 5,5'-(2,2'-bipyridine-4,4'-diyl)-bis(thiophene-2-carboxylic acid) (BTC) as the anchoring group and 4,4'-dinonyl-2,2'-bipiridyl and two thiocyanates as ligands is prepared. The photovoltaic performance and device stability achieved with this sensitizer are compared to those of the Z-907 dye, which lacks the thiophene moieties. For thin mesoporous TiO2 films, the devices with BTC-2 achieve higher power conversion efficiencies than those of Z-907 but with a double-layer thicker film the device performance is similar. Using a volatile electrolyte and a double layer 7 + 5 mu m mesoporous TiO2 film, BTC-2 achieves a solar-to-electricity conversion efficiency of 9.1% under standard global AM 1.5 sunlight. Using this sensitizer in combination with a low volatile electrolyte, a photovoltaic efficiency of 8.3% is obtained under standard global AM 1.5 sunlight. These devices show excellent stability when subjected to light soaking at 60 degrees C for 1000 h. Electrochemical impedance spectroscopy and transient photovoltage decay measurements are performed to help understand the changes in the photovoltaic parameters during the aging process. In solid state dye-sensitized solar cells (DSSCs) using an organic hole-transporting material (spiro-MeOTAD, 2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene), the BTC-2 sensitizer exhibits an overall power conversion efficiency of 3.6% under AM 1.5 solar (100 mW cm(-2)) irradiation

Ruthenium Sensitizer with Thienothiophene-Linked Carbazole Antennas in Conjunction with Liquid Electrolytes for Dye-Sensitized Solar Cells

A new heteroleptic ruthenium complex, coded CYC-B12, incorporating an antenna ligand composed of sequential connections of a thienothiophene conjugated bridge and carbazole hole-transport moiety was prepared. This new sensitizer exhibits a lower energy MLCT band centered at 555 nm with a high molar absorption coefficient of 2.24 x 10(4) M-1 cm(-1). The device sensitized by CYC-B12 in conjunction with a volatile electrolyte shows a high photovoltaic efficiency of 9.4% under an illumination of standard global AM 1.5G sunlight. With a low-volatile electrolyte, the cell based on this new sensitizer shows not only a good conversion efficiency of 8.2% but also excellent durability (>96%) under light soaking at 60 degrees C in a simulated sunlight for 1000 h. The difference in the electron recombination kinetics caused by various liquid electrolytes or aging process is also investigated by employing the transient photoelectrical measurements

การสังเคราะห์อนุภาคเงินระดับนาโนเมตรในหางน้ำยางธรรมชาติสำหรับเซลล์สุริยะชนิดสีย้อมไวแสงSynthesis of Silver Nanoparticles in Skim Natural Rubber for Dye-sensitized Solar Cell

งานวิจัยนี้มุ่งเน้นสังเคราะห์อนุภาคเงินระดับนาโนเมตรในหางน้ำยางธรรมชาติ และศึกษารูปแบบที่เหมาะสมในการเจืออนุภาคเงินระหว่างชั้นของวัสดุกึ่งตัวนำไทเทเนียมไดออกไซด์ เพื่อปรับปรุงประสิทธิภาพของเซลล์สุริยะชนิดสีย้อมไวแสง (Dye-sensitized Solar Cell; DSSC) โดยอาศัยหลักการเกิดพลาสมอนเรโซแนนซ์บนพื้นผิวของอนุภาคเงินระดับนาโนเมตรเพื่อเพิ่มกระแสไฟฟ้าในเซลล์ให้สูงขึ้น ผลการทดสอบประสิทธิภาพการเปลี่ยนรูปพลังงานแสงอาทิตย์เป็นพลังงานไฟฟ้า (Photoconversion Efficiency; PCE) พบว่าอัตราส่วนอนุภาคเงินต่อเอทานอลและรูปแบบการเจืออนุภาคเงินระหว่างชั้นไทเทเนียมไดออกไซด์มีผลต่อการเพิ่มขึ้นของค่าความหนาแน่นกระแสลัดวงจร (Short Circuit Current Density; Jsc) อย่างมีนัยสำคัญ โดยอัตราส่วนอนุภาคเงินต่อเอทานอลที่ 1 : 500 และการเจืออนุภาคเงินระหว่างชั้นของไทเทเนียมไดออกไซด์ 3 ชั้น ให้ประสิทธิภาพ PCE สูงสุด 4.3% เพิ่มขึ้นคิดเป็น 58.9% เปรียบเทียบกับประสิทธิภาพของเซลล์ทดสอบ DSSC ที่ไม่ได้เจือด้วยอนุภาคเงินThis research focuses on synthesis of silver nanoparticles (AgNPs) in the skim natural rubber latex and study the doping pattern of AgNPs onto mesoporous nanocrystalline layers of TiO2 to improve the efficiency of Dye-Sensitized Solar Cell (DSSC). By the surface plasmon resonance effect of silver nanoparticles, the photocurrent of DSSC test devices can be increased. The result shows that the ratio of AgNPs to ethanol and the doping pattern in TiO2 film significantly affect the photocurrent density. DSSC test devices employing the ratio of AgNPs to ethanol 1 : 500 and the AgNPs doped between three layers of TiO2 film yield the best photoconversion efficiency of 4.3%, increased by 58.9% in comparison with those without AgNPs doping

"Click-chemistry" approach in the design of 1,2,3-triazolyl-pyridine ligands and their Ru(II)-complexes for dye-sensitized solar cells

The synthesis of new 1,2,3-triazolyl-pyridine ligands via "click-chemistry" and their corresponding Ru(L)(2,2'-bipyridyl-4,4'-dicarboxylic acid)(NCS)(2) complexes (L = 1,2,3-triazolyl-pyridine) are presented. The complexes have been photophysically and electrochemically characterized and have been used as sensitizers in dye-sensitized solar cells (DSSC). In DSSCs with an acetonitrile-based electrolyte the cells comprising of Ru(2-(1-(4-hexylphenyl)-1H-1,2,3-triazol-4-yl)pyridine)(2,2'-bipyridyl- 4,4'-dicarboxylic acid)(NCS)(2) TBA salt 1 showed an overall power conversion efficiency of 7.8% under full sunlight intensity, and Ru(2-(4-(4-hexylphenyl)-1H-1,2,3-triazol-1-yl)pyridine)(2,2'-bipyridyl- 4,4'-dicarboxylic acid)(NCS)(2) TBA salt 2 an efficiency of 4.7%. Transient photovoltage and photocurrent decay measurements showed an enhanced performance for dye 1 due to faster electron transport into the TiO2 film and lower recombination rate in comparison to dye 2 sensitized devices. Additionally, solid-state devices were prepared with 2 mm thick TiO2 films using spiro-OMeTAD as a hole-transport material. The solid-state dye-sensitized solar cells showed power conversion efficiencies of 1.92% and 0.38% for sensitizer 1 and 2, respectively

New Organic Sensitizer for Stable Dye-Sensitized Solar Cells with Solvent-Free Ionic Liquid Electrolytes

We report a high molar extinction coefficient metal-free sensitizer composed of a triarylamine donor in combination with the 2-(2,2'-bithiophen-5-yl)acrylonitrile conjugation unit and cyanoacrylic acid as an acceptor. In conjugation with a volatile acetonitrile-based electrolyte or a solvent-free ionic liquid electrolyte, we have fabricated efficient dye-sensitized solar cells showing a corresponding 7.5% or 6.1% efficiency measured under the air mass 1.5 global sunlight. The ionic liquid cell exhibits excellent stability during a 1000 h accelerated test under the light-soaking and thermal dual stress. Intensity-modulated photocurrent and photovolatge spectroscopies were employed along with the transient photoelectrical decay measurements to detail the electron transport in the mesoporous titania films filled with these two electrolytes

New Efficiency Records for Stable Dye-Sensitized Solar Cells with Low-Volatility and Ionic Liquid Electrolytes

We report a high molar extinction coefficient heteroleptic polypyridyl ruthenium sensitizer, featuring an electron-rich 3,4-ethylenedioxythiophene unit in its ancillary ligand. A nanocrystalline titania film stained with this sensitizer shows an improved optical absorption, which is highly desirable for practical dye-sensitized solar cells with a thin photoactive layer, facilitating the efficient charge collection. In conjunction with low-volatility and solvent-free electrolytes, we achieved 9.6-10.0% and 8.5-9.1% efficiencies under the air-mass 1.5 global solar illumination. These dye-sensitized solar cells retain over 90% of the initial performance after 1000 h full sunlight soaking at 60 degrees C

Click-Functionalized Ru(II) Complexes for Dye-Sensitized Solar Cells

Three heteroleptic ruthenium complexes incorporating new ancillary ligands synthesized by sequential connection of different alkyl functionalities with triazole as a linker are prepared using click chemistry. These sensitizers exhibit low-energy metal-to-ligand charge transfer bands centered at 540 nm with molar extinction coefficients of up to 1.54 x 10(4) L mol-1 cm-1. The devices using these sensitizers in conjunction with a volatile electrolyte show high photovoltaic conversion efficiencies of 8.7 to 9.9% under standard AM 1.5G sunlight (100 mW cm-2) conditions. Using an ionic liquid electrolyte, the cells show not only a good power-conversion efficiency of up to 7.1%, but also promising long-term stability under full sunlight intensity at 60 degrees C. The difference in the photovoltaic parameters during the ageing process is investigated by employing transient photoelectrical measurements

Energy-Level and Molecular Engineering of Organic D-pi-A Sensitizers in Dye-Sensitized Solar Cells

A series of organic D-pi-A sensitizers composed of different triarylamine donors in conjugation with the thienothiophene unit and cyanoacrylic acid as an acceptor has been synthesized at a moderate yield. Through tuning the number of methoxy substituents on the triphenylamine donor, we have gradually red-shifted the absorption of sensitizers to enhance device efficiencies. Further molecular engineering by the substitution of two hexyloxy chains in place of the methoxy groups allows fabricating a solvent-free dye-sensitized solar cell with a power conversion efficiency of 7.05% measured under the air mass 1.5 global sunlight. Time- and frequency-domain photoelectrical techniques have been employed to scrutinize the aliphatic chain effects with a close inspection on effective electron lifetime, diffusion coefficient, and diffusion length