Bulk-heterojunction organic solar cells based on electron donors and acceptors have been considered a promising photovoltaic technology due to their advantages of low-production cost, mechanical flexibility, environmental friendliness and compatibility with roll-to-roll printing processes. Although tremendous progress has been made in this field, state-of-the-art organic solar cells still suffer from inferior photovoltaic performance to inorganic counterparts, especially large voltage loss, and the structure-property relationships of organic semiconductors have not been well understood. In this thesis, I will mainly focus on two areas including both donor polymer design and device fabrication towards high-performance organic solar cells. These studies aim to improve the performance of o...[ Read more ]

Bulk-heterojunction organic solar cells based on electron donors and acceptors have been considered a promising photovoltaic technology due to their advantages of low-production cost, mechanical flexibility, environmental friendliness and compatibility with roll-to-roll printing processes. Although tremendous progress has been made in this field, state-of-the-art organic solar cells still suffer from inferior photovoltaic performance to inorganic counterparts, especially large voltage loss, and the structure-property relationships of organic semiconductors have not been well understood. In this thesis, I will mainly focus on two areas including both donor polymer design and device fabrication towards high-performance organic solar cells. These studies aim to improve the performance of organic solar cells from a material design and device engineering point of view.

In chapter 2, three cases of efficient non-fullerene organic solar cells with small voltage loss are discussed. These cases are based on the donor:acceptor pairs with small energy offset(s). The related optical and electronic characterization results reveal the origins of small voltage loss and provide important guidelines for achieving organic solar cells with both small voltage loss and high efficiencies.

Chapter 3 and 4 focus on understanding the structure-property relationships of donor polymers in non-fullerene organic solar cells. In chapter 3, a new donor polymer named PffBT4T-B is synthesized by introducing a rarely used benzene unit into the repeating units of the previously reported PffBT4T polymer. This PffBT4T-B polymer, although suffering from a twisted polymer backbone and poor charge transporting properties, can form tighter polymer aggregates and expel the ITIC-Th to form its own domains. As a result, both PffBT4T-B and ITIC-Th can maintain their crystallinity in the blend, and thus achieve higher efficiency than that of PffBT4T:ITIC-Th. In chapter 4, two donor polymers with identical polymer backbones but different alky chain regiochemistry are synthesized to investigate the effects of side chains on the properties of donor polymers and the performance of non-fullerene organic solar cells. The morphology characterization results show that the PfBTAZ polymer tends to form large aggregate due to its more planar polymer backbone. Therefore, the devices based on PfBTAZ suffer from excessively larger domain size and thus poorer efficiency than PfBTAZS. Overall, the structure-property relationships revealed in these two cases can guide the donor polymer design in non-fullerene organic solar cells.

Chapter 5 concentrates on non-fullerene tandem organic solar cells. The tandem architecture has been considered an effective approach to further improve the performance of solar cells. However, the previously reported recombination layers used in fullerene tandem solar cells typically require complicated or harsh post-treatments that might not be compatible with non-fullerene tandem solar cells. To address this problem, a recombination layer of PEDOT:PSS/ZnO processed from a novel method is proposed, which merely requires a simple and mild thermal annealing post-treatment at 80 °C. The homo-tandem device based on this recombination layer can achieve a high efficiency of 10.8 % with an extremely high open-circuit voltage of 2.13 V, and this high-voltage tandem devices provide a good platform for other optoelectronic applications like water-splitting.