Material design for high-performance organic and nanocrystal photovoltaics

Abstract

Recently, the third-generation photovoltaic technologies are captivating researchers’ attention owing to their lightweight, low-cost production, easy-tunable optical properties and solution-processable properties. This thesis mainly focuses on two emerging types of photovoltaic technologies: organic solar cells (OSCs) and Nanocrystal solar cells (NCSCs), which involves chemical modification of key activelayer materials of these two types of solar cells, with a focus on characterizing their performance and establishing the structure-performance relationships to guide future research of photovoltaic technologies. In Chapter II, a highly efficient polymer acceptor (PYFO-V) with enlarged bandgap is synthesized through the synergistic effects of side-chain and linkage modulation. Morphological analyses and transient photo-physics characterization reveal high crystallinity and effective charge transfer in the blend film of PM6:PYFO-V. As a result, the PM6:PYFO-V based all-polymer solar cells (all-PSCs) exhibit an impressive efficiency of 27.1% under indoor illumination with great photostability. Additionally, the PM6:PYFO-V based-all-PSCs processed by non-halogenated solvent and blade coating in 1 cm² devices also attain a decent indoor efficiency over 23%, demonstrating the huge potential of PYFO-V for commercialization. This study not only inspires future work on chemically constructing efficient polymer acceptors with enlarged bandgaps, but also sheds light on developing high-performance indoor all-PSCs for Internet of Things. Chapter III emphasizes the strategy of methoxylation on ending group units for efficient small molecule acceptors with enlarged bandgaps. Two main works of my research on ternary OSCs are summarized, including: 1) a novel fluoro-methoxylated ending group and asymmetric substitution strategy enabled acceptor with enlarged bandgaps for high-performance ternary OSCs; 2) dipole moment modulation of ending groups enabled asymmetric acceptors for high-performance ternary OSC. A systematic investigation underscores the effectiveness of designing ending groups with small dipole moments for enlarging the bandgap of small molecule acceptors. Chapter IV proposes an in-situ sodium passivating strategy for optimizing the size of AgBiS2 nanocrystals (NCs). Through TEM and XRD analysis, we demonstrates that sodium does not dope into the crystal lattice but instead caps the surface of the NCs, resulting in larger-sized NCs. The sodium passivation also helps to prevent the traps formation during the thin-film deposition, reducing the trap density and improving the film’s carrier mobilities. We further implements Na-passivated films to p-i-n NCSCs. The champion device basing on Na-passivated NCs achieves a JSC of 24.3 mA cm^-2 and an efficiency over 4.5%. This study suggests that sodium is particularly effective in improving the solar cell performance by forming a protective shell on the surface of AgBiS₂ NCs, which passivates traps and inhibits recombination pathways.

Date of Award	2025
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	He YAN (Supervisor) & Jonathan Eugene HALPERT (Supervisor)