Precision Structural Engineering of Low-Cost p-Type Organic Semiconductor Materials for Optimized Functional Properties

Abstract

Organic and perovskite solar cells have emerged as promising candidates for third-generation photovoltaic technologies. Organic solar cells (OSCs) have advantages, including lightweight, flexible device architectures, and low-cost, solution-based processing. Perovskite solar cells (PSCs), on the other hand, exhibit remarkable power conversion efficiencies and broad spectral absorption, and long carrier diffusion lengths. Structural modifications of organic materials used in solar cells, including active layer materials and interface layer materials, enable precise control over their optoelectronic properties as well as molecular packing and orientation. This tunability underscores the versatility of organic materials and their importance as key materials for advancing next-generation photovoltaic platforms. In this thesis, structurally tailored p-type organic materials are investigated, with their structures tuned differently for OSCs and PSCs, to provide insight into structure–property relationships for optimizing their functional roles.

In the first study, we present design considerations for p-type materials intended to act as donor materials within organic solar cell active layers. Conjugated donor polymers have been intensively studied in OSCs. Among them, polythiophene (PT) derivatives have long been valued for their synthetic simplicity and cost-effectiveness, with a simple conjugated backbone that can be readily tuned through molecular engineering. Building on these advantages, we investigate a terpolymer design strategy for PT donors, allowing fine structural modulation that refines their molecular properties and enhances their functionality in organic electronic devices. We synthesized new terpolymers by introducing a non-fluorinated thiophene-based third components into a backbone composed of thiazolothiazole (TTz)-based and 2F-thiophene monomers. The incorporation of the third components subtly increases backbone torsion, enabling fine control over aggregation behavior and solubility, while concurrently upshifting the frontier orbital energy levels. In addition, structural modulation governs molecular orientation in thin films. The terpolymers adopt predominantly edge-on orientation in neat film but undergo acceptor-induced reorientation toward face-on orientation with Y6 acceptor. Different third components generate diverse variations in polymer properties, which modulate film morphology and lead to varying degrees of reorientation. These combined variations ultimately influence device performance. By correlating these trends across multiple descriptors, we aim to identify governing factors and propose design guidelines for programmable molecular orientation.

In the second study, we present a novel p-type material for use in hole-transport layer (HTL) in PSCs. Self-assembled monolayers (SAMs) are widely used as HTL in inverted PSCs because they exhibit negligible parasitic absorption, introduce interfacial dipoles for favorable energy level alignment, and enable high efficiency via tunneling effect. Based on this framework, we designed two novel SAMs by introducing halogen group (-Cl, -Br) as an additional functional group to the parent SAM, aiming to increase dipole moment and thereby tuning the work function for more favorable hole extraction. While the halogenated SAMs improved VOC of the devices, their overall efficiencies were limited by reduced JSC and FF, which can be attributed to low coverage and the resulting increase in series resistance. Additionally, we adopted a co-SAM strategy by incorporating the halogenated SAM into the parent SAM. Owing to the structural similarity between the parent and halogenated SAM molecules, they can form well-mixed co-assembled monolayers. As a result, the co-SAM devices preserve the VOC benefit while improving the FF and overall efficiency.