Organic solar cells (OSCs) have experienced great development in the past years. Organic materials have many advantages when compared to the inorganic materials such as chemical tunability, mechanical flexibility and compatibility with roll-to-roll printing flexibility. Whereas, the understanding of structure-property relationships and donor-acceptor interactions is still not enough. In this thesis, I will introduce my research focused on the molecular design of small molecular acceptor and polymer donor materials, aiming to provide comprehensions into highly efficient OSCs. This thesis is constituted of five chapters:

Chapter I is the general background of organic solar cells, including the related device structures, the working mechanisms, active layer materials and the motivation of...[ Read more ]

Organic solar cells (OSCs) have experienced great development in the past years. Organic materials have many advantages when compared to the inorganic materials such as chemical tunability, mechanical flexibility and compatibility with roll-to-roll printing flexibility. Whereas, the understanding of structure-property relationships and donor-acceptor interactions is still not enough. In this thesis, I will introduce my research focused on the molecular design of small molecular acceptor and polymer donor materials, aiming to provide comprehensions into highly efficient OSCs. This thesis is constituted of five chapters:

Chapter I is the general background of organic solar cells, including the related device structures, the working mechanisms, active layer materials and the motivation of this thesis.

In Chapter II, an A–D–A-type non-fullerene acceptor (named IDTS-4F) with an alkyl thiophenyl side chain and dimethoxy thiophene bridging unit is reported. Compared with conventional acceptor IT-4F, the IDTS-4F has a smaller optical bandgap and higher lowest unoccupied molecular orbital (LUMO) level, which are beneficial to increase the J_SC and V_OC of the devices. The ternary device by incorporating 0.2 of IDTS-4F and 0.8 of IT-4F (with PM6 as the donor polymer) can simultaneously achieve a higher V_OC and J_SC. In the attempts to further increase the performance of excellent PM6:IT-4F system, it’s challenging to maintain the optimal film morphology. However, with addition of crystalline IDTS-4F, morphology characterizations indicate increased phase purity and reduced domain size. The PCE above 14% for the ternary devices are among the top values reported before 2019.

Generally, for most state-of-the-art polymers, the device performance is highly sensitive to their molecular weight, which raised the difficulty of synthesis and purification. In Chapter III, a chlorinated donor polymer named D18-Cl is reported, which can achieve high performance with a wide range of polymer molecular weight. The devices based on D18-Cl show a higher V_OC due to the slightly deeper energy levels and an outstanding J_SC owing to the appropriate domain sizes of blend films. They can achieve high efficiencies (17.30–17.97%) when its number-averaged molecular weight (M_n) is ranged from 45 to 72 kDa. Such property and performance make D18-Cl a promising donor polymer for scale-up and low-cost production.

The morphology optimization of active layers is a major challenge for the ternary OSCs. We believed that The well compatible Y6 and its derivatives may prefer to form an alloyed state for efficient electron transport in ternary active layers. In Chapter IV, a series of ternary OSCs are fabricated with the polymer D18-Cl as donor, and Y6 and Y6-1O as acceptors. The energy loss can be minimized by incorporating Y6-1O, leading to the V_OC improvement of ternary OSCs. By finely adjusting the Y6-1O content, 17.91% efficiency is achieved in the OSCs with 30 wt% Y6-1O in acceptors. The J_SC and FF improvement should be ascribed to comprehensively optimal photon harvesting, exciton dissociation and charge transport in active layers. This work also introduces the magnetic-controlled photocurrent method into the ternary system for the first time, which proves that the more effective charge separation within the ternary blend, and provides more solid evidence for analyzing the mechanism of the ternary device.

Chapter V is the summary of the thesis, and future perspectives development of high-performance non-fullerene OSCs.