DSSCs made from crystalline TiO2 electrodes is one of the most promising candidates in recent quest for cheap, clean and green alternative to fossil fuels. In recent years a lot of research is focused on the development of highly efficient and stable dyes. It was revealed in the late 1960s that upon illumination, organic dyes can generate electricity at oxide electrodes in electrochemical cells.[14] Current best PV research-cell efficiencies are displayed in Figure 1-9. To understand and simulate the primary processes in photosynthesis, the phenomenon was studied with chlorophyll extracted from spinach (bio-mimetic approach).[15] In 1972, these experiments lead to demonstrate electric power generation from solar cell via the dye sensitization principle.[16]
Figure 1-9. Current best photovoltaic research-cell efficiencies.[17]
However, The instability of the DSSC was recognized as a main challenge.[18]
Nanocrystalline semiconductor films have been used in the direct conversion of solar energy into chemical or electrical energy.[19-20] The conventional PVs having crystalline or amorphous silicon, have exceptional solar energy to electricity conversion efficiency of approximately 20%.[21] However, the fabrication of these PVs is expensive. CuInSe and CdTe thin film PV cells reach efficiencies of around 15%.[22] The scarcity of indium, selenium and tellurium can be a drawback for large scale production of these cells; also the high toxicity of cadmium has to be taken into account.
In 1991, Michael Grätzel and Brian O’Regan ignited the solar cell research area with a spark of 7% overall power conversion efficiency using a ruthenium sensitizer and porous TiO2
layer as semiconducting material.[23] Recently DSSCs achieved certified conversion efficiencies of around 11.9% for laboratory-scale devices based on ruthenium sensitizers[24]
and 12.7% for porphyrin-based devices[25] and 8.5% with small submodules.[26] The facile assembly, large choice of colors, transparency and mechanical flexibility are some features for extensive attraction towards the DSSCs.
1.4.1 Device structure
A schematic diagram of a typical DSSC device is shown in Figure 1-10. Dye-sensitized solar cells separate the two functions provided by silicon in a traditional cell design. Silicon not
only acts as the source of photoelectrons, but also it provides the electric field to separate the charges and create a current. In DSSCs, the semiconductor is used solely for charge transport, the photoelectrons are provided from a separate photosensitive dye. Charge separation occurs at the surfaces between the dye, semiconductor and electrolyte.
Figure 1-10. Schematic representation of Dye-Sensitized Solar Cell.
The nanocrystalline semiconductor is normally TiO2 with typical sizes of 20-30 nm, film thickness of ~10 µm with a porosity of ~60%, although other wide band gap oxides like ZnO and SnO can be used.[27] A monolayer of the sensitizer is attached to the surface of the semiconductor. A redox mediator, commonly iodide/tri-iodide redox couple in organic solvent is used as electrolyte. The electrode with the mesoporous film (the photoanode) is sandwiched together with a second conducting glass substrate. The second electrode is coated with catalytically active platinum for efficient reduction of oxidized redox species.
1.4.2 Electron Injection, Transport and Recombination
Efficient photon to current conversion occurs in DSSCs because of a judiciously well-adjusted interplay of different kinetic processes as illustrated in Figure 1-11. In the dark, the fermi level of electrons in the TiO2 semiconductor is in equilibrium with the redox energy level of the electrolyte. When a photon is absorbed by the sensitizer (S), it is excited to the higher energy level (eq. 4). The excited state molecule (S*) injects an electron into the conduction band (Ec) of the semiconductor in a femto to picosecond timescale (eq. 5) before the dye can relax back to its ground state (eq. 8). The oxidized sensitizer (S+) is regenerated by iodide in the electrolyte within a few microseconds (eq. 6), which generally occurs more rapidly than reduction by photoinjected electrons in the TiO2 (eq. 9). The tri-iodide formed upon the dye regeneration is reduced at the platinized counter electrode (eq. 7). The additional charge in the TiO2 under illumination defines a quasi-Fermi level EFn.Electrons in
the TiO2 are affected by two competing processes: Recombination with tri-iodide in the electrolyte (eq. 10) and diffusion through the mesoporous TiO2 to the front electrode. The effective time constants for these processes strongly depend on the trapping and detrapping events.
S → S* Photoexcitation (eq. 4)
∗ → Charge injection (eq. 5) 2I → I and 2I2● → I3 + I Dye regeneration (eq. 6) I3 2e → 3I Electrolyte regeneration (eq. 7)
∗ → Dye relaxation (eq. 8)
→ Recombination via dye (eq. 9) 2 I3 → 3I Recombination via electrolyte (eq. 10)
Figure 1-11. Schematic representation of the electron flow in DSSC.
Recombination occurs in the millisecond to second range, and diffusion ideally occurs on a timescale one to two orders of magnitude smaller such that a large fraction of electrons is extracted at the front electrode. The differences in the electrochemical potentials (or Fermi energies) of the electrons at the opposite electrodes, i.e. EFn and Eredox, defines the photovoltage generated by the cell, The quasi-fermi level EFn of electrons in the TiO2
depends on the charge generation rate in the TiO2, the transport rate, and the recombination rate.
1.4.3 Incident Photon to Current Efficiency (IPCE)
The solar cell performance is measured with several parameters like incident photon to current efficiency (IPCE), short circuit current (Jsc), open circuit voltage (Voc), fill factor (FF) and the overall efficiency of the photovoltaic cell (η). IPCE measures how efficiently
the incident photons are converted to electrons. The wavelength dependent IPCE can be expressed as the product of the light harvesting efficiency (LHE), quantum yield of charge injection (Φinj), charge collection efficiency (ηcoll) at the back contact and quantum yield of regeneration (Φreg) (eq. 11).