Organic electronic devices for solar energy conversion and storage

Abstract: This thesis focuses on two types of organic electronic devices: organic photovoltaic (OPV) devices for solar energy conversion, and photo-capacitors for energy storage.OPVs have been under the focus of research for decades as an effective technique to convert solar energy to electricity. So far, the efficiency of bulk heterojunction OPV consisting donor and acceptor materials is approaching to 18% with non-fullerene acceptor (NFA), which make it close to commercialization. The process of charge generation and recombination are two competing processes in OPVs, since their requirements for the active layer morphology are contradictory. Large donor/acceptor interfaces facilitate charge generation but hinder the transporting pathways for charge transportation. The simultaneously enhanced charge generation and transportation are achieved by using the ternary strategy in my first paper. The fully mixed donors and NFAs are beneficial for the charge generation and fullerene is introduced as an extra electron transport channel. The hierarchical morphology of the blend film is confirmed by the TEM results. The voltage loss analyses indicate that the hierarchical morphology could suppress unfavorable charge transfer state and non-radiative recombination loss. In my second paper, efficient charge generation with low voltage loss are achieved in the solar cells by rational designing a series of NFAs. The detailed voltage losses are discussed in these binary systems, revealing the critical relationship between radiative efficiency and device performance.To harvest photocurrent in OPVs, long lifetime triplet excitons are highly expected to be good candidates. The potential of triplet materials in OPVs has been explored since 1970s. However, the performance of the triplet materials-based OPVs is far behind. The voltage loss in triplet OPVs is intensively studied in my third work. A higher open circuit voltage (0.88 V) is observed for Ir(FOtbpa)3-based devices than those of Ir(Ftbpa)3 (0.80 V) despite a lower charge transfer state energy. To understand above result, the voltage losses through radiative and non-radiative recombination pathways in two devices are quantitively investigated, which indicate a reduced non-radiative recombination loss in the Ir(FOtbpa)3-based devices.The fluctuation of sun irradiation resulting the unstable output power of solar cells. Therefore, it is important to store electricity of solar cells for later use. Integrated photo-capacitor (IPC), combining a solar cell and a super-capacitor by sharing one common electrode, is able to simultaneously realize the energy harvesting and storage. Building upon this advantage, IPC devices received tremendous research attention. In my fourth and last papers, we introduced super-capacitors to construct IPC devices with OPV device or modules. A free standing thick- PEDOT:PSS film is successfully integrated into an all solution-processed IPC device as the common electrode. Resulting devices demonstrate good performance and outstanding stability. With solar PV modules, a higher voltage can be generated and stored by asymmetric supercapacitors, which could be used as a portable power unit.

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