Dye-sensitised solar cells (DSSC) mimic photosynthesis by utilising light absorbing pigements (in nature this is chlorophyll). In p-n junction PV cells the semiconductors provides photoelectrons, charge carrier separation and transportation, whilst in the DSSC PV cells the semiconductor is used for charge transport and the photoelectrons are created by the separate photosensitive dye. Charge separation is formed at the surface between dye, semiconductor and electrolyte (normally an organic hole transport material). The basic design of the DSSC involves semiconductor structures formed between a photo-sensitised anode and an electrolyte. The semiconductor nanocrystals antennae that harvest the sunlight (photons) and the dye molecule perform charge separation (photocurrent), mimicking natural photosynthesis [33]. DSSC was introduced by O’Regan and Grätzel in 1991 [33, 34]. These cells are low- cost materials and are simple to manufacture. They release electrons from metals covered by a light absorbing pigment. The exposure to UV light and the use of a liquid electrolyte makes the cells prone to degradation and freezing respectively over time. Broadband dyes and electrolytes have 12% efficiency [35], compared to 4% to 5% of DSSC commercial efficiencies due to the presence of few dyes that can absorb a broad
11 spectral range. The advantage of incorporating nanomaterials in DSSCs makes them able to accommodate thousands of organic dyes.
The mechanism of the primary steps to convert photons into current for dye sensitised solar cell (DSSC) is demonstrated in Figure 1.4.
Figure 1.4: Schematic energy diagram and operating principle of DSSC [36]
The primary processes that occur in a DSSC for the solar energy–to–current conversion are given below:
1. Photosensitises (such as Ru complexes) adsorbed on a suitable substrate (TiO2) surface absorb the incident photon flux.
2. The photosensitsers are excited from the ground state (S) to the excited state (S∗) owing to the MLCT transition. The excited electrons are injected into the conduction band of the TiO2 electrode, resulting in the oxidation of the photosensitiser.
S + hν → S∗ (1) S∗ → S+ + e− (TiO
2) (2)
3. Injected electrons in the conduction band of TiO2 are transported between TiO2 nanoparticles with diffusion toward the back contact (TCO) and consequently reach the counter electrode through the external load and wiring.
4. The oxidized photosensitiser (S+) accepts electrons from the I− ion redox mediator, regenerating the ground state (S), and I− is oxidized to the oxidized state, I
12 S+ + e− →S (3)
5. The oxidized redox mediator, I3−, diffuses toward the counter electrode and is re- reduced to I− ions.
I3− + 2e− → 3I− (4)
Overall, electric power is generated without permanent chemical transformation. The performance of a DSSC is predominantly based on four energy levels of the components: the excited state (approximately LUMO) and the ground state (HOMO) of the photosensitiser, the Fermi level of the TiO2 electrode, which is located near the conduction band level, and the redox potential of the mediator (I−/I
3−) in the electrolyte. The photocurrent obtained from a DSSC is determined by the energy difference between the HOMO and the LUMO of the photosensitiser, analogous to the band gap, Eg, for inorganic semiconductor materials. The smaller the HOMO– LUMO energy gap, the larger the photocurrent will be because of the utilization of the long wavelength region in the solar spectrum. The energy gap between the LUMO level and the conduction-band level of TiO2, ΔE1, is important, and the energy level of the LUMO must be sufficiently negative with respect to the conduction band of TiO2 to inject electrons effectively. In addition, substantial electronic coupling between the LUMO and the conduction band of TiO2 also leads to effective electron injection. The HOMO level of the complex must be sufficiently more positive than the redox potential of the I−/I
3− redox mediator to accept electrons effectively (ΔE2). The energy gaps, ΔE1 and ΔE2, must be larger than approximately 200mV as driving force for each of the electron transfer reactions to take place with optimal efficiency [37]. The voltage in the DSSC is developed by the energy gap between the Fermi level of a TiO2 electrode and the redox potential of the I−/I3− in the electrolyte. The conduction band level of the TiO2 electrode and the redox potential of I−/I3− were estimated to be −0.5V versus normal hydrogen electrode (NHE) and 0.4V versus NHE, respectively, as shown in Figure 1.4. [36], (or −0.7V versus saturated calomel electrode (SCE) and 0.2V versus SCE, respectively [37, 38]). Thus, in the case of a DSSC using a TiO2 electrode and I−/I3− redox mediator, the maximum voltage is expected to be approximately 0.9V, depending on the electrolyte component because the Fermi level of the TiO2 electrode depends on the electrolyte components and their concentrations.
13 In contrast to conventional p-n-type solar cells, the mechanism of a DSSC does not involve a charge-recombination process between electrons and holes because electrons are only injected from the photosensitiser into the semiconductor and a hole is not formed in the valence band of the semiconductor. In addition, charge transport takes place in the TiO2 film, which is separated from the photon absorption site (i.e. the photosensitiser). Thus, effective charge separation is expected. This photon-to- current conversion mechanism in a DSSC is similar to the mechanism for photosynthesis in nature, in which chlorophyll functions as the photosensitiser and charge transport occurs in the membrane.
A part of this investigation will focus on a new class of organic dyes synthesised from 1,2,3-triazole compounds, synthesised using “click chemistry”, in combination with various base metals to form classical organometallic complexes. These complexes will be tested as DSSC cells and thus demonstrate, as a proof of concept, their feasibility within the field.