Investigations on Semiconductor Material Synthesis and on Morphology Manipulation Towards High Efficiency and Stable Solar Technologies

Abstract

Solar cells, which harvest renewable solar energy via the photovoltaic (PV) effect, have been considered as one of the cleanest and most promising technologies to address the press energy crisis and environmental issues. In order to make solar cells the feasible and best replacement of fossil fuels, the concept of the third-generation PV devices based on inorganic nanomaterials, dye molecules, conjugated polymers and perovskite materials was proposed to largely decrease the cost of the solar cell devices, at the same time maintaining comparable or even achieving higher power conversion efficiency (PCE) compared to that of the existing ones. Additionally, luminescent solar concentrators (LSCs), which convert a wide range of photons into concentrated light in a specific range, are another potential approach to achieve these goals by reducing the use of expensive PV materials. In principle, the development of both PV and LSC devices depends on the material chemistry and device engineering, which are the two main directions to optimise the performance of these solar technologies. In this thesis, the work performed focuses on the synthesis of near infrared (NIR) absorbing quantum dots (QDs), their characterizations, and solar-related applications, as well as morphology optimisation of the photoactive film of polymer solar cells (PSCs), leading to enhanced PCE and stabilities.

In the first part, we successfully synthesized high-quality NIR PbS QDs in very small sizes using PbCl₂ and elemental sulfur (S) as lead and sulfur precursor, respectively, by introducing tributylphosphine (TBP) into the reaction. This route is much “greener” and facile as compared to the glove-box-involved method using toxic bis(trimethylsilyl)sulfide as sulfur precursor. Afterwards, the synthesis mechanism of very small QDs and their optical properties, morphology, dispersity, surface properties and LSC application were systematically investigated. In detail, this work can be divided into three sections (I-III): two sections are related to the PbS QDs synthesis, and the last section is about LSC application based on these synthesized QDs.

In section Ⅰ, the S-oleylamine (OLA) solution was mixed with different contents of TBP prior to its injection into the pre-heated PbCl₂-OLA solution. For the TBP-free reaction, the shortest absorption wavelength of synthesized PbS QDs was limited to ~1056 nm. With increasing the TBP content to 40 μL, the first-excitonic absorption peak of obtained PbS QDs was gradually blue-shifted to ~705 nm, due to the decreased QD size. Further increasing the contents of TBP, no QDs could be collected. The key factor leading to the blue-shifted first-excitonic absorption peak and final disappearance of QDs with increasing TBP content is the formation of strong bond between S and TBP. The S-TBP could have participated into the QD growth process, which prevented the QDs further growing to larger size with TBP as a type of rigid ligands on the surface. However, the stable S-TBP itself was not able to start the nucleation reaction with the lead precursor, then failed to form the QD once the S-OLA concentration was below a critical point. In addition to decreasing the size of QDs, as extra ligands located at the S sites on the QD surface (rather than on the Pb site as OLA ligands), TBP could change the QD surface composition, morphology and dispersity, and benefit its optical properties.