Organic solar cells (OSCs) are an attractive technology due to the advantages of low production cost, mechanical flexibility and compatibility with roll-to-roll printing. Recent advances in small molecular acceptors (SMAs) have brought a “paradigm shift” from fullerene devices to non-fullerene devices. Whereas, the structure-property relationships and donor-acceptor interactions regarding photophysical and morphological aspects remain unexplored. In this thesis, I will introduce my research efforts devoted to the molecular design of donor and acceptor materials aiming to provide insights into high-performing non-fullerene OSCs. This thesis is constituted of five chapters:

Chapter I is the general background of organic solar cells, including the working mechanisms, characterizat...[ Read more ]

Organic solar cells (OSCs) are an attractive technology due to the advantages of low production cost, mechanical flexibility and compatibility with roll-to-roll printing. Recent advances in small molecular acceptors (SMAs) have brought a “paradigm shift” from fullerene devices to non-fullerene devices. Whereas, the structure-property relationships and donor-acceptor interactions regarding photophysical and morphological aspects remain unexplored. In this thesis, I will introduce my research efforts devoted to the molecular design of donor and acceptor materials aiming to provide insights into high-performing non-fullerene OSCs. This thesis is constituted of five chapters:

Chapter I is the general background of organic solar cells, including the working mechanisms, characterization methods, development history of photo-active organic materials, and the motivation of this thesis.

Chapter II focus on the exploration of the structure-property-morphology-performance relationship established for ring-fused perylene-diimide (PDI) SMAs. Systematic comparisons among a series of non-fused/ring-fused PDI molecules are made, providing insights into the changes in the molecular properties and device performances including: (1) the formation of the double-decker molecular geometry featuring intramolecular stacking that facilitate charge transport; (2) the devices based on the ring-fused PDI molecules achieved simultaneously low driving force and non-radiative recombination losses, lowering the total voltage losses and enhancing the open-circuit voltages; (3) upon ring-fusion the PDI molecules tend to form stronger phase separation with donor polymers, leading to higher domain purity and electron mobilities. As a result, record efficiencies of up to 11% are achieved despite low voltage losses of ~0.5 V for ring-fused PDI-based devices.

Chapter III highlights the strategy of enhancing intramolecular charge transfer (ICT) to promote performances of ladder-type SMAs via backbones and end group engineering. One case is the simple chlorination of the thiophene end groups rendered the resulting SMA with the strengthened ICT effect and a red-shifted absorption by 50 nm relative to the non-chlorinated counterpart. In addition, the non-covalent interactions induced by the chlorine atoms can promote intermolecular packing leading to enhanced charge mobility and higher domain purity. Consequently, the chlorinated molecule achieved simultaneously improved J_SC, FF and PCE of 12.7% compared with the non-chlorinated one. The other case is the introduction of strong electron-donating moieties, i.e., cyclopentadithiophene (CPDT) and dithienopyran (DTPR), into the backbone to construct near-infrared absorbing SMAs. Compared with the benchmark SMAs based on thienothiophene, the electron-rich nature of CPDT and DTPR induced progressively larger dipole moments between the backbone and end groups, which results in dramatically enhanced ICT effects and thus extended absorption onset of ~1000 nm.

Chapter IV concentrates on modifications of donor polymers with fine-tuned temperature-aggregation (TDA) properties. Three cases of polymer design will be introduced involving: (1) energy level tuning and morphology control by terthiophene-based polymers incorporating different ratios of difluorobenzothiadiazole (ffBT) and difluorobenzoxiadiazole (ffBX); (2) the synthesis of an asymmetric difluorotriazole (FTAZ) moiety that alleviates the excessive aggregation of a high crystalline TDA polymer for better morphology compatibility with SMAs; (3) a D-A1-D-A2 terpolymer strategy employed to dramatically enhanced the TDA properties of a reported donor polymer. All the three projects demonstrated the importance of suitable TDA properties in controlling the blend film morphology by carefully designed copolymerized building blocks. The formation of high domain purity and small domain sizes enable various material combinations of non-fullerene OSCs to obtain greatly enhanced J_SC and FF.

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