Performance limits for colloidal quantum dot and emerging thin-film solar cells

Abstract

Emerging thin-film solar photovoltaic (PV) technologies can be made lightweight and flexible with simple manufacturing methods, allowing rapid scale-up and ubiquitous deployment of solar power. However, for most emerging PV technologies-including colloidal quantum dots (QDs), perovskites, and organics-power conversion eciency and stability remain major obstacles to commercial development. In this thesis, we evaluate the long-term potential of emerging thin-film PV technologies, focusing on performance limits for QD solar cells in the face of inefficient charge extraction and energetic disorder. First, we introduce material complexity as a framework for analyzing PV technologies and assess the performance and scalability of all leading technologies on equal footing. This analysis points to a unique advantage of emerging thin films-high power-to-weight ratios. As a proof of concept, we demonstrate a process for producing thin, lightweight, transparent, laminable, and flexible PV substrates based on in situ vapor-phase growth of parylene-C films. This approach enables ultra-thin molecular organic solar cells with efficiencies and yields comparable to glass-based cells and weight-specfic power exceeding 6 W/g. Next, we address inefficient charge extraction in QD solar cells by demonstrating an ordered bulk heterojunction device architecture based on solution-grown ZnO nanowire arrays and PbS QDs. The nanowires decouple light absorption from charge extraction, improving the short-circuit current density by 50% and the power conversion eciency by 35%. Finally, we attempt to answer the question "Are QD solar cells worth pursuing further?" We use photothermal deflection spectroscopy to characterize disorder-induced band tailing in PbS QD films across dierent QD sizes, ligands, and processing conditions. Based on these measurements, we calculate radiative eciency limits ranging from 26% to 32%, which suggests that disorder does not severely constrain the long-term potential of PbS QD solar cells.