Optical and electronic properties of dye-sensitized heterojunction solar cells

This research studies the optical and electronics properties of Solid-State Dye-Sensitized Solar cells (SSDSCs). The propagation of light inside dye anchored TiO2 mesoporous layer includes scattering and absorption of photons. The light absorption behavior of different particle sizes TiO2 (from 20-150 nm) and two different dyes(Z-907 and K-68) is explored. With a 4-flux optical model and experimental data, the photon absorbed fluxes of various thickness are characterized. The electronic properties of the dye-sesitized heterojunction were scrutinized with photocurrent and photovoltage transient method. This approach provides a way to analyze the charge recombination and transport processes and to compare devices made with different treatments or materials. The effects of particle size, and surface modification were carried out. The example of an ultra high open circuit voltage cell (more than 1 volt) was studied to understand the origin of the high open-circuit voltage (Voc). The interface between the organic and inorganic material is a critical issue for the performance of SSDSC. Since the contact is ohmic between the Spiro-MeOTAD (A hole transport material, HTM) and fluorine doped tin-oxide(FTO), a compact hole blocking underlayer is required to form a rectifying junction at this interface. One chapter was spent on the issue of the blocking layer by studying the flat junction (FTO/blocking layer/HTM) behavior. A new material of Nb2O5 was evaluated and showed similar capabilities as the TiO2 spray pyrolysis layer. New materials for charge collection to diversify the application of SSDSCs were conducted. Metal foils, TiO2 nanotube on FTO and indium doped tin-oxide(ITO) transparent counter electrode were successfully integrated into SSDSC devices and presented reasonable photovoltaic performances. With the result in the optical modeling, the internal quantum efficiency (IQE) was estimated from front and back side irradiation using different transparent window layer (FTO and ITO). The IQE was found to be around 80% across the 400-650 nm spectrum for the front side illumination. Photoinduced absorption spectroscopy showed that the infiltration of Spiro-MeOTAD into thick film (7 um) is not complete and leaves some dyes not being contact with HTM where regeneration will not occur effectively. This technique demonstrated a qualitative characterization for the pore-filling issue. In the end, we summarized some general conclusions and some outlooks for the future research and development of the dye-sensitized heterojunction solar cells.