Device engineering and material design for high-efficiency organic solar cell using a benzo-difuran-based donor polymer

Abstract

Organic solar cells (OSCs) are considered a highly promising photovoltaic technology owing to the inherent benefits of organic materials, such as flexibility, eco-friendliness, and suitability for large-scale roll-to-roll printing processes. Recent advancements in small molecular acceptors (SMAs) have propelled the OSC field into the non-fullerene era, achieving power conversion efficiencies exceeding 19% with Y-series acceptors. Given the significant impact of photoactive layer film morphology on OSC performance, device engineering, particularly additive strategy, play a crucial role in optimizing film morphology. This thesis will delve into the intricate process of device fabrication and material design tailored for achieving high-efficiency OSCs based on benzo-difuran (BDF) materials. By exploring the utilization of additive strategies, this research aims to provide valuable insights into the interplay between device performance metrics and film morphology characteristics. In Chapter II, a breakthrough in efficiency for BDF-based polymer OSC was achieved through device engineering. By combining the BDF-based polymer donor D18-Fu with L8-BO and optimizing the morphology using 2-CN as a solid additive, a record PCE of 18.4% was attained, marking a significant advancement for BDF polymers. The study delved into the morphology formation process using in-situ techniques and thoroughly investigated the mechanism of 2-CN in comparison to its isomer, 1-CN, through morphology and device analyses. It has been shown that 2-CN can improve the PCE of various material systems, suggesting its potential as a general solid additive. This research highlights the significance of careful material selection and morphology optimization in advancing the organic solar cell field and sheds light on the differences between isomeric CN additives in altering material morphology. In Chapter III, two polymer donors (D18-Fu and PM6), two small molecular acceptors (L8-BO and Y6), and two traditional solvent additives (1-CN and DIO) are investigated. 12 sets of binary OSCs were fabricated to systematically compare their performance. Physical features were analyzed using TAS, and morphology were studied through various measurements (in-situ UV-vis, AFM, FLAS, GISAXS and GIWAXS) on the active layer films. The results demonstrate that changing either the material or additives can influence film morphology and, consequently, device efficiencies. Ultimately, the ternary blend achieves an efficiency close to 19% through understanding-guided optimization.. This study serves as a complementary exploration in the rapid efficiency development of OSCs, offering insights into device optimization. In Chapter IV, a novel approach using a combination of volatile (2-CN) and non-volatile (MF) additives is proposed to enhance the PCE of D18-Fu:L8-BO binary. The evaporation of 2-CN creates vacancies in the polymer matrix, allowing for the introduction of the non-volatile MF additive, which increases the donor-acceptor interface area and facilitates the generation of more free charges. The addition of MF also reduces non-radiative losses. As a result, the binary organic solar cell efficiency using benzo-difuran core-based polymer donors achieves a PCE of over 19%, marking a significant advancement in the field.

Date of Award	2024
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	He YAN (Supervisor) & Sai Ho PUN (Supervisor)