Charge transport and recombination in hybrid organic-inorganic solar cells

Abstract

Hybrid organic-inorganic solar cells have been cited as a promising alternative to presently commercialised photovoltaic technologies. However, despite over 20 years of research into hybrid organic-inorganic solar technologies, there are still many unknowns concerning the physical operation of these devices. The two most successful technologies comprising the discipline of hybrid organic-inorganic solar cells are the dye-sensitised solar cell (DSSC) and the perovskite-based solar cell (PSC). This thesis reports on charge transport and recombination mechanisms occurring within DSSC and PSC devices. The first phase of this work examined the role of electron traps on charge recombination in DSSCs, and whether reports of trap state passivation were justified. Electrochemical impedance spectroscopy (EIS) and intensity-modulated photovoltage spectroscopy (IMVS) were used to characterise nanoparticle TiO2 films treated with an atomic layer deposited (ALD) coating. The application of a theoretical model to the experimental data was used to characterise the relative contributions of conductive states and trap states to the total recombination rate. The interpretation of these results indicated that the ALD surface treatment reduced recombination from conductive and trap states evenly, and did not selectively passivate surface traps. The second phase of this thesis explored the physical characteristics of DSSCs formed on flexible plastic substrates, as well as electron transport rates within nanostructured TiO2 beads. EIS, IMVS, intensity-modulated photocurrent spectroscopy (IMPS) and transient measurements revealed that the benchmark fabrication techniques of cold isostatic pressing (CIP) and nanoglue treatments resulted in electron diffusion lengths 3 – 4 times shorter than those produced via a high-temperature sintering step. Characterisation of charge transport kinetics within mesoporous TiO2 beads identified two effective diffusion rates: one pertaining to intra-bead diffusion, and the other to inter-bead connections. The third phase of this work entailed the characterisation of planar PSCs primarily through EIS. Impedance spectroscopy measurements of planar PSC devices revealed high-frequency and low-frequency features that exhibited different dependencies on the charge carrier concentration. Based on additional photoluminescence (PL) and transient measurements, as well as the formative work of previous studies, an equivalent circuit model was proposed to describe the impedance spectroscopic response of planar PSC devices. The final phase of this thesis aimed to characterise the differences in transport and recombination dynamics between planar and mesoscopic architectures. Time-resolved microwave conductivity (TRMC) and time-resolved PL measurements revealed higher charge mobilities and lower trap-mediated recombination dynamics in planar perovskite films compared to mesoscopic perovskite films. The consolidation of the best aspects of each architecture into a new PSC assembly was found to increase the overall power conversion efficiency. In an extension of this work, a different perovskite precursor solution was used to produce a textured, mesoscopic perovskite layer without the use of a thick nanoparticle scaffold. This textured morphology was capable of enhancing further the optical and charge separation properties of the device, resulting in a maximum power conversion efficiency of 16.3 %. The work undertaken in the aforementioned studies highlights how a deeper understanding of hybrid organic-inorganic solar cell physics can be used to further optimise the solar cell performance. Awards: Winner of the Mollie Holman Doctoral Medal for Excellence, Faculty of Engineering, 2015.