The design and building of an ultrafast fluorescence measurement system with outstanding resolution has enabled us to collect data on the early events in molecules after light excitation (such as energy transfer and charge transfer). Temporal resolution is a key criterion for a time-resolved spectroscopy system. Thus, the equipment acquired in the project integrates the latest advancements in both the pulsed laser system and the streak camera detection module. These components have been combined into an optical setup designed to measure emission spectra and examine the photoelectrochemical energy conversion of the devices in the femtosecond time scale.
The information obtained with the new infrastructure has been applied to tune the design of photoactive molecules for Dye Sensitised Solar cells (DSSCs). In fact, we have designed and synthesised two organic photosensitisers, H6 and H7, featuring the bulky donor N-(2ʹ,4ʹ-bis(hexyloxy)-[1,1ʹ-biphenyl]-4-yl)-2ʹ,4ʹ-bis(hexyloxy)-N-methyl-[1,1ʹ-biphenyl]-4-amine (BPC6-DPA) and N-(2ʹ,4ʹ-bis(dodecyloxy)-[1,1ʹ-biphenyl]-4-yl)-2ʹ,4ʹ-bis(dodecyloxy)-N-methyl-[1,1ʹ-biphenyl]-4-amine (BPC12-DPA), respectively, along with bis-hexylthiophene as the π-linker and the electron acceptor 4-(benzo[c][1,2,5]thiadiazol-4-yl)benzoic acid (BTBA). Although the significantly longer alkyl chains do not alter the optical energy gap, for H7, we have been able to design molecular structures that exhibit longer excited-state lifetimes in both dye-grafted titania and alumina films compared to its H6 counterpart. The absorption kinetics suggest that the H7-grafted titania film forms a more compact packing pattern, effectively restricting the motion of copper (II) to the TiO2 surface and ensuring superb light harvesting. Both photosensitisers are utilized to create high photovoltage-output DSCs in conjunction with a copper (II/I) redox mediator. The DSC using the longer alkyl chain-based photosensitiser H7 achieves a high Voc of 1.22 V, comparable to the recently explored hybrid methyl ammonium lead-based perovskite semiconductors (PSK) in solar cells. The co-sensitised device combined with XY1b results in an efficient DSC with an impressive fill factor of 82.1% and an excellent power conversion efficiency (PCE) of 13.7% under simulated AM1.5 G conditions at 100 mW cm–2. Furthermore, the best device achieves an outstanding efficiency of up to 29.7% under dim light, surpassing PSK solar cells.
We have also tackled the durability of the DSC through the combination of co-sensitization and redox-active interfacial engineering. For that, a narrow-energy-gap sensitizer ((E)-3-(5-(6-(7-(4-(bis(2',4'-bis(hexyloxy)-[1,1'-biphenyl]-4-yl)amino)phenyl)benzo[c][1,2,5]thiadiazol-4-yl)-4-(2-ethylhexyl)-4H-dithieno[3,2-b:2',3'-d]pyrrol-2-yl)thiophen-2-yl)-2-cyanoacrylic acid, H4) is paired with a complementary blue-light-absorbing dye (4-(7-(4-(bis(2',4'-bis(hexyloxy)-[1,1'-biphenyl]-4-yl)amino)phenyl)-2-(2-ethylhexyl)-2H-benzo[d][1,2,3]triazol-4-yl)benzoic acid, H15), which possesses a strong absorption at ~410 nm and a prolonged excited-state lifetime, thereby compensating for the spectral response and reducing interfacial charge recombination in co-grafted titania films. Simultaneously, we introduce a hypervalent iodine (III) compound, 1-acetoxy-1,2-benziodoxol-3(1H)-one (IBA), into a cobalt-based electrolyte. The introduction of IBA facilitates fast oxidation of the triphenylamine electron-donor in H4, generating free radicals and enhancing intramolecular charge transfer. Furthermore, the redox byproduct 2-iodobenzoic acid (IA) plays a critical role in suppressing interfacial recombination by coordinating with lithium ion and forming halogen-bonded complexes with electrolyte additives. The synergistic effects of co-sensitization and the IBA additive in the electrolyte yield a co-sensitized DSC with a PCE of 12.84%, featuring excellent operational stability under indoor light soaking for 1000 h.
Moreover, we have also explored molecular photosensitizers based on the anthranil core were synthesized through a six-step linear synthesis featuring a Suzuki coupling at the C7 position and a controlled C3–H arylation. The introduction of donor and acceptor groups allowed the synthesis of unsymmetrical photosensitizers that were then investigated, revealing different spectroscopic and optoelectronic properties. Transient spectroscopy of excited species indicated that placing the acceptor at C7 and the donor group at C3 altered the energy levels around the anthranil core, making these dyes attractive for photovoltaic applications. Furthermore, we have investigated the use of the benzoselenadiazole core in the design and synthesis of a D-π-A photosensitizer for dye-sensitised solar cells that promotes the photocurrent generation when applied as a dye in DSSC.
