Device engineering to enhance the power conversion efficiency of organic solar cells

Abstract

The invention of binary bulk-heterojunction (BHJ) architecture involving a donor (electron-donating) with an acceptor (electron-accepting) material to form a solid thin film with nanoscale domain sizes has been an enormous success in the field of organic solar cells (OSCs). A carefully controlled BHJ nanostructure creates numerous interfacial areas between the donor and acceptor with appropriate donor-acceptor phase separation as required for efficient exciton dissociation and adequate charge transportation. Despite its success, BHJ morphology still faces many challenges in controlling the nanostructure. Hence, in this thesis, I will focus on different strategies to enhance the power conversion efficiency (PCE) of OSCs with better control in morphology through the proper selection of materials and device engineering. First, the task of controlling complex nanoscale BHJ structure using the traditional one-step coating method is considered arduous. Hence, an alternate methodology using sequential processing (SqP) technique has been employed in which the donor and acceptors are cast separately in two steps. However, the current SqP method requires the use of orthogonal solvents, which limits the choice of solvents and donor/acceptor materials that can be used. Hence, to address the problem as mentioned above, an improved version of the SqP method using the temperature-dependent aggregation (TDA) properties of donor polymers (pioneered by our group for efficient OSCs) is utilized. The TDA polymers can be solution-processed at high temperatures. Still, the resulting film becomes completely insoluble at room temperature, which allows for the processing of the overlying acceptor layer from a wide range of solvents. This method overcomes the requirement of orthogonal solvents and enables a much larger window to optimize processing parameters. Second, single-junction binary BHJ OSCs typically has a narrow absorption band; hence, multi-component (involving the mixing of more than two donor and acceptor materials) strategy has been employed for broader and better absorption. Despite the benefits, material selection and morphology control of multi-component BHJ OSCs are incredibly challenging. However, we developed a new type of quaternary blend system consisting of four complementary organic materials in one cell that exhibits a unique “rivers and streams”-type functional hierarchical morphology that allows it to operate exceptionally efficient. By establishing the morphology-device performance relationship, we demonstrate our quaternary blend system provides a synergistic interaction between the four functional components that result in a primarily enhanced performance of the OSCs. Finally, for the first time, we have examined the use of [2.2]paracyclophane for constructing small molecular acceptors (SMAs) for OSC operations. Two novel tetrathienyl [2.2]paracyclophane constitutional isomers, which differ in the functionalization positions of the cyclophane moiety (i.e., ortho- and para-positions), were designed and synthesized. After ring fusion of four PDI wings, two tetrameric perylene diimides (PDI)-based SMAs named oCP-FPDI4 and pCP-FPDI4 were obtained and systematically studied. Our work provides insights into the isomeric constitutional effects of the [2.2]paracyclophane-PDI electron acceptors. It highlights the potential applications of cyclophane derivatives in the design of photoactive materials for OSC devices.

Date of Award	2020
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology