Development of Stable and Efficient Photovoltaic Devices Using Lead-Free Colloidal Nanocrystals

Abstract

Colloidal semiconductor nanocrystals, particularly colloidal quantum dots, offer unique characteristics suitable for solar cell applications, including tunable bandgaps based on crystal size, broad absorption range encompassing infrared rays, and the ability to generate multiple excitons. With advancements in synthesis technology, precise control over nanocrystal size and bandgap has become achievable, enabling the customization of their light absorption capacity within the range of 0.7 to 2.1 eV. PbS quantum dots have been extensively studied in the field of quantum dot solar cells and are typically synthesized using an alkyl chain ligand, such as oleic acid, as an interfacial ligand, resulting in high monodispersity crystal sizes. However, to incorporate quantum dot thin films into solar cells, an exchange process is required to replace the alkyl chain ligands with short organic-inorganic ligands. The exploration of engineering techniques involving ligand-induced dipole control, utilizing short organic-inorganic ligands containing functional groups like thiols, halides, and amines, has demonstrated the potential to control the energy levels of quantum dot solids. Ligand exchange processes have been identified as key technologies for achieving improved charge collection efficiency and enhanced performance in PbS quantum dot solar cells. For instance, treating the PbS quantum dot thin film with a 1,2-ethanedithiol (EDT) ligand solution enables its use as a hole transport layer material that blocks electrons while facilitating hole extraction. Solar cells incorporating EDT-treated PbS quantum dots have reported a certified power conversion efficiency of 8.55%, representing a significant achievement at the time.

However, the solid-state ligand exchange process used in the fabrication of existing quantum dot thin films has drawbacks, including surface defects and cracks that can occur during the process, ultimately affecting solar cell performance. To address these challenges, a process technology involving solution-phase ligand exchange and quantum dot inks has been developed. This approach minimizes surface defects and bandtail states, establishing a favorable energy level alignment within the solar cell. Furthermore, improvements in device architecture technology and synthesis techniques, such as device structure optimization, have contributed to the significant increase in power conversion efficiency, exceeding 15% for PbS quantum dot-based solar cells. Despite these advancements, the use of toxic elements like Pb and Cd poses limitations due to compliance with hazardous substances (RoHS) regulations.

Consequently, the development of eco-friendly basic materials is urgently needed to simultaneously advance environmental preservation and energy conversion technologies. Motivated by the need to address the aforementioned challenges, this thesis aims to provide innovative solutions and advancements in the field. The research conducted in this study focuses on developing high-efficiency and stable solar cells utilizing Pb-free and eco-friendly colloidal nanocrystals, namely AgBiS2 and InAs. By leveraging the unique properties of these nanocrystals as light absorbers or charge transport layers, this work strives to contribute to the development of sustainable and eco friendly photovoltaics.

The thesis comprises five chapters, each dedicated to exploring specific aspects of the research topic. Chapter 1 serves as an introduction, presenting the background, trends, and types of solar cells employed in colloidal nanocrystal-based solar devices. It establishes the foundation by providing fundamental knowledge regarding colloidal nanocrystals and their crucial characteristics for the development of eco-friendly nanocrystal-based solar cells.

Chapter 2 delves into the fabrication process of AgBiS2 nanocrystal ink, achieved through a novel solution-processed ligand exchange process. This chapter highlights the development of eco-friendly nanocrystal-based solar cells with minimized defects, aiming to improve their overall performance. Notably, the fabricated solar cells using the developed nanocrystal ink demonstrated an open-circuit voltage of 0.55 V and a commendable power conversion efficiency of 4.08%, setting a new benchmark for AgBiS2 solar cells in terms of open-circuit voltage.

In Chapter 3, an extensive study is conducted to characterize the AgBiS2 nanocrystals after ligand exchange. The investigation encompasses three short organic ligands: EDT, 3-mercaptopropionic acid (MPA), and malonic acid (MA). By analyzing the chemical, morphological, and electrical properties affected by the presence or absence of thiol and carboxyl acid groups, the chapter elucidates the significance of ligand selection. It also explores the impact of ligand properties on surface passivation and oxidation. Based on these findings, a bilayer AgBiS2 nanocrystal-based photovoltaic device with optimized TMAI-treated and MPA-treated films achieves a notable power conversion efficiency of 6.39%.

Chapter 4 shifts the focus towards the utilization of InAs colloidal quantum dots with discrete electron states as electron transport layers in solution-processed thin-film solar cells. This section highlights the introduction of a high-performance organic solar cell configuration using PM6:Y6 as the active material, which attains an impressive power conversion efficiency of 15.1%. Moreover, the chapter emphasizes the significant enhancement in operational stability, with the device maintaining over 80% of its original efficiency even after 1000 minutes of continuous illumination.

In conclusion, this thesis underscores the importance of energy band engineering through ligand tuning and device architecture optimization in driving advancements in the photovoltaic performance of colloidal nanocrystal-based solar cells. The necessity for eco-friendly materials that comply with environmental regulations is also emphasized, emphasizing the urgency to advance both environmental preservation and energy conversion technologies simultaneously. By addressing the research problem and presenting novel solutions and advancements, this thesis aims to make a valuable contribution to the field of high-efficiency and eco-friendly solar cells using Pb-free colloidal nanocrystals.