Effect of Extended π-Conjugation of the Donor Structure of Organic D–A−π–A Dyes on the Photovoltaic Performance of Dye-Sensitized Solar Cells

Two new D−A−π-spacer−A organic dyes, KM-10 and KM-11, containing a benzothiadiazole unit in a π-spacer and a cyanoacrylic acid as an acceptor have been synthesized and tested as sensitizers in dye-sensitized solar cells. Structural variations of the donor moiety, i.e., π-extension of the diphenylamine electron-donating groups, gave rise to different photovoltaic efficiencies –7.1% for KM-10 and 8% for KM-11– despite having comparable absorption properties. A detailed investigation, including transient photo- current and photovoltage decay measurement, transient absorption spectroscopy, and quantum chemical methods, provided important conclusions about the nature of the substitution on the photovoltaic properties of dyes.

Interfacial engineering for competitive Dye-Sensitized Solar Cells production

Natalie Flores Diaz

Renewable energy deployment and implementation of a circular economy will play a crucial role in decreasing CO2 emissions to meet the target from the Paris agreement. Dye-sensitized solar cells (DSSCs) can be designed in various colors and transparency for building-integrated photovoltaics (BIPV). This emerging technology has gained significant attention for its remarkably high power conversion efficiency (PCE) under indoor lighting up to 34%. To improve the performance of DSSCs, various approaches to study and reduce the interfacial electron recombination from the conduction band of the semiconductor to the electrolyte were implemented in this thesis. One of the approaches was the design of a tandem electrolyte containing two redox species, the radical AZADO (AZ+/0) and the [Co(bpy)3]2+/3+ complex. DSSCs with tandem electrolytes showed increased open-circuit voltage (VOC) up to 1009 mV and enhanced PCE of 9.15%. The VOC enhancement was determined to result from the higher redox potential of (AZ+/0) and the slower electron recombination rate. These findings demonstrate the advantages of employing tandem electrolytes to improve the VOC and overall performance of DSSCs. To decrease the solvent's volatility and the interfacial electron recombination in cobalt-based electrolytes, hyperbranched networks (HB) were designed through a click thiol-ene reaction between a thiol-siloxane and an acrylate monomer. The formed polymers were denoted HB1 (thiol-siloxane and PEGMA) and HB2 (thiol-siloxane and PEGMA). The addition of 20% of HB polymers to a cobalt-based electrolyte retained up to 98% of their initial weight until 150°C. The devices employing 10%(v/v) of HB1 attained the highest PCE under AM1.5 of 8.52%, comparable to 9.36% for the cobalt reference electrolyte. All the HB-electrolytes presented higher VOC values than the cobalt reference due to an upward shift in the CB of TiO2 and longer electron lifetimes. It was determined that the HB-polymers reduce the electron recombination and enhance the photovoltaic performance under low light intensity leading to a PCE of 23.19% under 1000 lux compared to 20.37% for the Co-reference. We then developed a new dye, R7, with a strong electron-donating PAH core and BPT2 as the central building block and a bulkier HF donor to decrease electron recombination at the interface TiO2/electrolyte. We found that in the presence of a copper electrolyte, the dye R7 outperformed devices with R6. EIS revealed that the bulky HF-donor introduction effectively suppressed the electron recombination between the TiO2 surface and the redox shuttle. Additionally, we employed a co-sensitized system of R7+Y123 to reach the outstanding parameters of JSC of 16.15 mAcm-2, VOC of 1035 mV, FF of 0.76 to achieve a top PCE of 12.7%. To the best of our knowledge, this is the highest for copper-based DSSCs using a blue photosensitizer. Lastly, the interfacial phenomena in DSSCs with carbon counter electrodes (CCEs) and copper-based electrolytes were studied to determine the feasibility of the production of monolithic DSSCs with copper-based hole-transporting materials. It was found that the electron transport in devices with CCEs is in the same order of magnitude as those with PEDOT. However, the devices with CCEs presented a decrease of 2 orders of magnitude in the recombination resistance, resulting in decreased VOC and JSC, leading to a PCE of 17% compared to 24.5% for PEDOT devices under 1000 lux illumination.

EPFL2021

Carbazol-phenyl-phenothiazine-based sensitizers for dye-sensitized solar cells

Shaik Mohammed Zakeeruddin, Felix Thomas Eickemeyer, Lukas Pfeifer, Zhongjin Shen, Dan Ren, Wei Huang, Yunfei Jiao, Yi Ding

Heteroaromatic units are commonly used as pi-spacers for sensitizers in dye-sensitized solar cells (DSSCs). Here, we investigated the influence of the pi-spacer on the photophysical properties, electrochemical properties and photovoltaic performance of carbazol-phenyl functionalized phenothiazine donor based organic dyes, i.e., CPPC-No (without a pi-spacer), CPPC-Th (thiophene as the pi-spacer), CPPC-Fu (furane as the pi-spacer), CPPC-Ph (phenyl as the pi-spacer) and CPPC-Py (pyridine as the pi-spacer). The solar to electric power conversion efficiency (PCE) achieved with these dyes strongly depends on the type of pi-spacer, increasing in the order of CPPC-Ph < No < Py < Th < Fu. The width of the absorption band and the photocurrent density increased with the electron richness of the pi-spacer. Theoretical calculations indicated that the planar molecular configuration of CPPC-Fu promotes the intramolecular charge transfer process. As a result, the CPPC-Fu based device showed a PCE of 7.4% under one sunlight illumination. The dyes using thiophene and pyridine as pi-spacers showed slightly lower performance, however, still significantly higher than the PCE achieved with the control compound lacking a pi-spacer. By contrast, introducing a phenyl group as the pi-spacer led to a decrease in device efficiency. These results give deeper insight into the effect of different pi-spacers on DSSC performance, showing the importance of molecular engineering in developing high efficiency organic dyes.

ROYAL SOC CHEMISTRY2021

Photoinduced Charge Transfer Dynamics in Blue-Coloured Diketopyrrolopyrrole (DPP) Dye-Sensitized Solar Cells

Heewon Bahng

The inception of push-pull organic dyes utilizing the donor-pi conjugated bridge-acceptor (D-pi-A) motif has created a paradigm shift in the design of efficient nanocrystalline dye-sensitized solar cells (DSSCs), offering control over aesthetic and performance properties. Especially, sensitizer embedding diketopyrrolopyrrole (DPP) core for solar cell application has attracted great attention because of its promising performance and aesthetic blue coloration. The core of this thesis is devoted to the understanding of the exact mechanism and dynamics of the charge separation in DPP-sensitized solar cells, where a number of photoinduced processes coexist and are kinetically entangled. To understand this complex combination of processes, fundamental backgrounds are introduced in the first chapter. Next, detailed descriptions of the instruments employed in this study are provided in the second chapter. In Chapter 3, detailed intermolecular interactions of D-pi-A DPP-based dye molecules on the redox-inactive Al2O3 is investigated at the exciton level. The results evidence that charge transfer (CT) excitons formed between neighboring dyes are involved in the lateral interaction. And this arises from the ordered molecular configuration on the surface due to the interaction between electron-rich donor and relatively electron-poor DPP core in the neighboring dye. On the other hand, when the donor is deprived of DPP dye, excimer-like excited states are formed upon photoexcita-tion and involved in the interaction due to pi-pi interaction. This suggests that the donor moiety plays a critical role in intermolecular interactions. In Chapter 4, the investigation is performed on how the lateral interaction studied in Chapter 3 affects the electron injection process of DPP-sensitized TiO2 films, by comparing the dynamics of donor-installed and donor-deprived DPP dye sensitizers. It is found that lateral intermolecular interactions exist even on TiO2, and takes place during several picoseconds. It is also found that this causes the charge injection process to become inefficient, especially in donor-deprived dye, because the delayed injection rate is slower than that of the lateral interaction. From these observations, it is suggested that the intermolecular interaction is another factor that can have a decisive influence on interfacial CT processes in DSSC. In Chapter 5, based on previous observations, we explore the reason why asymmetric DPP dye sensitizer offers better photovoltaic performances compared to that of the symmetric DPP one. An enhanced J-aggregates formation of symmetric thienyl-DPP (ThDPP) dye on TiO2 is observed com-pared to the case of asymmetric phenyl-DPP-thienyl (PhDPPTh) dye sensitizer. We find that these J-aggregates also inject electron upon excitation, and their oxidized form interacts with surrounding charge species. Moreover, their injection becomes inefficient as light intensity enhances. This result is in line with the nonlinear photocurrent dynamics of symmetric DPP dye sensitizer, which is be-lieved to the cause of device inefficiency. Through the studies carried out within this thesis using DPP dye sensitizers, a step-by-step understanding of the charge transfer mechanisms and in particular their dependence upon lateral in-termolecular electronic interactions is constructed. We hope that they will open new paths for further optimizations of the efficiency of DSSCs.