Harvesting UV photons for solar energy conversion applications

Abstract

We report the synthesis and characterization of five new donor-p-spacer-acceptor dye molecules with a diphenylamine donor, fluorene-1,2,5-oxadiazole spacers and a range of acceptor/anchor groups (carboxylic acid 1, cyanoacrylic acid 2 and 3, alcohol 4 and cyano 5) to facilitate electron injection from the excited dye into the TiO 2 photoanode in dye-sensitized solar cells (DSSCs). Detailed photophysical studies have probed the dyes' excited state properties and revealed structure-property relationships within the series. Density functional theory (DFT) and time dependent DFT (TDDFT) calculations provide further insights into how the molecular geometry and electronic properties impact on the photovoltaic performance. A special feature of these dyes is that their absorption features are located predominantly in the UV region, which means the dye-sensitized TiO 2 is essentially colorless. Nevertheless, DSSCs assembled from 1 and 2 exhibit photovoltaic power conversion efficiencies of Z = 1.3 and 2.2%, respectively, which makes the dyes viable candidates for low-power solar cells that need to be transparent and colorless and for applications that require enhanced harvesting of UV photons. IntroductionInterfacial electron transfer between semiconductor nanoparticles and molecular adsorbates has been the subject of intense research activities in the past few decades. 1 In terms of solar energy conversion, the semiconductor-liquid junction solar cell is established as a very promising approach for solar energy conversion and different strategies have been proposed to generate electrical energy from sunlight. For instance, direct collection of light by the semiconductor is feasible when the photonic energy exceeds the energy gap between the valence band and the conduction band of the semiconducting material. In such a case, an electron is promoted from the valence band to the conduction band leaving a positively charged hole behind. Then, usually the hole migrates to the semiconductor/solution interface where it oxidizes a redox-active species in solution.In contrast, the electron moves away from the interface towards the electrode and an external circuit is set up. On the way to the counter electrode its free energy can be partially extracted, whereas after reaching the counter electrode it is captured by the oxidized redox-active molecule. Importantly, the net chemical reaction in such an arrangement is nil, since every oxidation process at the interface has its counterpart in an interfacial reduction reaction. 2 For semiconductor materials that can absorb significant portions of the solar spectrum (bandgaps: 1-2 eV) such a direct energy conversion strategy may be considered very efficient. Unfortunately, many of these materials with suitable bandgaps easily undergo destructive hole-based reactions or react with water or oxygen to generate electrically insulating barrier layers. 3,4 On the other hand, materials that are kinetically resistant to photocorrosion often exhibit relatively large bandgaps. One of the cond...

Microwave-Assisted Synthesis of an Alternant Poly(fluorene–oxadiazole). Synthesis, Properties, and White Light-Emitting Devices

“…For the structural unit, an almost complete separation of the HOMO and LUMO levels was noted, HOMO orbitals being mainly located on the fluorene moiety (E HOMO = −6.0927eV), while LUMO orbitals were located in particular on the oxadiazole segment (E LUMO = −2.0599eV) (Figure 6). Similar data were also reported for compounds containing these two building blocks [52].…”

Charge migration and charge transfer in molecular systems

“…Molecular modelling reveals that with the increase of the electron affinity of the anchor, the dipole moment varies and it increases the electronic decoupling of the HOMO and the LUMO, which in turn helps electron injection in TiO 2. 218 In contrast, the regeneration process is found independent of the dye structure modification at the interface. A series of fluorenes dyes with different electron donor and acceptor units was investigated too.…”

Facile synthesis of Bi nanoparticle modified TiO 2 with enhanced visible light photocatalytic activity

“…For the pure TiO 2 , the spectra from 400 to 750 nm should be ascribed to the formed oxygen vacancy in surface (SOV) during the H 2 treatment process [32]. Obviously, these SOV have nearly no effect on the visible light irradiation degradation, which is opposite to the former work because of the rapid recombination of electron-hole pairs [33].…”