Charge Transport And Recombination In Solid State Dye-sensitized Solar Cells

Compared to conventional energy, solar energy is inexhaustible, effective, clean and cheap. It has become a hot research and application area. Dye-sensitized solar cell (DSC) is a third generation solar cell, which is more simple technology, low cost and high profit, as compared to the silicon or inorganic thin-film solar cell.Since liquid-state DSC has lots of problem in stability due to the application of organic solvent, solid-state hole transporting material (HTM) was introduced to replace the liquid electrolyte. Solid-state DSC developed quickly these years as it is stable and reasonably owning higher power conversion efficiency. Nevertheless, the pore filling of HTM molecules in the mesoporous TiO2 films is low, and the hole mobility of HTM also limits the hole transport to the counter electrode, that leads to a fast recombination and relatively low efficiency of the solar cells. There is another parameter might also! affect the recombination process, which is the coverage of HTM on the TiO2 surface, though it has always been ignored. On the basis of these, we have done the workbelow:(1) The transmission spectra and steady-state current had been introduced to measure the pore filling fraction and the surface coverage of HTMs (Spiro-OMeTAD and P3HT). It was found that the extreme pore filling fraction was ～65% and ～30% for Spiro-OMeTAD and P3HT, respectively. However, their surface coverage showed no significant difference. Both of them gave a highest coverage fraction more than 90%. That indicates there is a big difference between the pore filling and the surface coverage, in another word, the surface coverage is another key factor needing to be considered when designing HTMs. Besides, Increasing the concentration of HTM solution could not continuously enhance the pore filling or surface coverage, but reach a peak value and further to form a thicker overlayer of HTMs. Regarding to the photovoltaic performance of solid-state DSCs, higher surface coverage of HTMs is needed. Results showed that the short-circuit current and the power conversion efficiency increased along with the increasing surface coverage.(2) A simple new method was utilized to fabricate mesoporous TiO2 films with different porosities. Higher porosity of TiO2 films allowed larger quantity of HTM penetrated into the pores. Results showed that higher pore filling and surface coverage of HTMs could enhance the short-circuit current, open-circuit voltage, fill factor and efficiency of the devices. After fitting the impedance spectra, it revealed that increasing the pore filling fraction of HTM could improve the hole transport, reduce the electron-hole recombination, lengthen the electron lifetime and diffusion length of the solar cells.(3) Annealing was applied to increase the crystallization and the oxidation of Spiro-OMeTAD. Consquently, the Spiro-OMeTAD became more conductive and the regeneration of dye molecules was improved. The function of Li-TFSI was responsible for the oxidation, however, its transportation to the surface of TiO2 nanoparticles decreased the conduction band of TiO2 resulting to a lower open-circuit voltage of the solar cells. On the other hand, when annealed at 140℃, the tBP evaporated to further reduce the TiO2 conduction band. Meanwhile, the short-circuit current was enormously decreased. Hence, it is important to find some new additives to tolerate high temperature when applying the DSCs in real conditions.On the other side, the recombination at the FTO/HTM interface is another key factor affecting the solar cell performance. Normally, a thin dense TiO2 compact layer is introduced to block the back-reaction of electrons at this interface. Lots of research suggests the property of compact layer is significant. According to this, we used physical vapor deposition method to fabricate the compact layer, which is believed to be controllableand suitable for a scale production. We also investigated the critical parameters of TiO2 compact layer that is required in solid-state DSCs.(1) Physical vapor deposited titanium oxide compact layer inhibited the recombination at the FTO/HTM interface efficiently. Moreover, the electron lifetime and the electron transport through the FTO were enhanced by the compact layer. Compared to bare DSCs,85 nm compact layer corresponded solar cell showed a highest increase in energy conversion efficiency of 814%, which was much larger than the improvement of 27% in liquid-state DSCs. It was found that the electron lifetime was-15 times shorter in solid-state DSCs than in liquid-state DSCs.(2) Spray pyrolysis turned to be more effective than physical vapor method to produce TiO2 compact layer for solid-state DSCs. We utilized spray pyrolysis method to fabricate four different TiO2 compact layers. Results showed that compact layers with higher permittivity, trap density and larger injection energy from the mesoporous TiO2 improved the cell performance more in Spiro-OMeTAD based solid-state DSCs. This is also suitable in liquid-state DSCs with I2/I3- electrolyte.