Evaluation of chemical and electronic properties of photovoltaic materials at interfaces in solar cell devices

Author: Amira Ramadan Alghamdi

  • Thesis download: available for open access on 19 Oct 2024.

Alghamdi, Amira Ramadan, 2023 Evaluation of chemical and electronic properties of photovoltaic materials at interfaces in solar cell devices, Flinders University, College of Science and Engineering

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Organic photovoltaics (OPVs) and perovskite solar cells (PSCs) have come a long way in recent years, reaching power conversion efficiencies of ~19% and 25.7%, respectively, for a single cell test. This advance was brought about by concurrent progress in materials design and synthesis and interfacial engineering. Interfaces formed between different layers in solar cells dictate device characteristics and degradation. This thesis is dedicated to interfacial engineering and the investigation of a range of materials and modifications with OPV and PSC applications front of mind. In particular, the focus is on studying the electronic properties and energy band structures at the interface’s layers in OPVs and PSCs and discovering the impact of the interface on device performance. In this thesis, in terms of OPV structure, a novel potential of organic interface layers was deployed with a range of active materials as a replacement for an inorganic interface layer due to its flexibility, ease of fabrication and potentially more cost-effective design than the conventional inorganic interfaces. The electronic structure and the charge transfer mechanism at the polymer cathode interface layer (CIL)/active layers interface are discussed. A powerful combination of experimental techniques was applied to gain fundamental understanding of the chemical and electronic properties and engineering of the interfaces formed with polymer CIL and active materials. The valance electron structure of new polymer (CIL) of P(NDI3N-T-Br) with TQ1 and N2200 active layers were first investigated where a mixing of phases at the interface was determined. The results show that the dipole formation between the donor, acceptor and P(NDI3N-T-Br) was observed, which enhanced electronic structure at the interfaces and facilitated charge transport over the interface. This energy level alignment meets the expectation of using P(NDI3N-T-Br) as an interface layer in blocking the hole transfer to the interface layer while TQ1:N2200 is used as the photovoltaic active layer. Consequently, another new CIL, P(NDI3N-F8-Br), with PTB7-Th and ITIC is investigated. A comprehensive study of the energy structure of P(NDI3N-F8-Br) with PTB-Th: ITIC deposited in the case of inverted devices is constructed. The results show that P(NDI3N-F8-Br) can work as a hole-blocking layer. Simultaneously, however, the electrons transferring from ITIC were blocked by the P(NDI3N-F8-Br) interface. Investigating the electronic structure of new organic cathode materials thus lead to a better understanding of the charge energetics at the interface and down-selection of the device structure. To achieve a comprehensive understanding, comparison has been made to the interface of PTB7-Th: ITIC–incorporated zinc oxide (ZnO) as a conventional inorganic interface layer. The results show that the active layers worked proficiently with ZnO. As a result, ZnO blocks the holes and extracting the electrons from the ITIC layer, which is desired. Overall, dipole formation was observed at the interfaces of P(NDI3N-F8-Br) and ZnO with active layers. Finally, the distribution of the charge transport component of PTB7-Th: ITIC has been studied to fill the knowledge gap on this type of study in this field. Thus, the component distribution at the surface region of the PTB7-Th: ITIC blend was investigated with the effect of the additive p-anisaldehyde (AA) on the components, which formed one side of the interface of the blend with the MoOx electrode. This finding contributes to an understanding of the interaction between the donor material and the high work function electrode/interface material. Following our research on OPVs, another study on PSCs is conducted. In PSCs, sputtered NiOx (sp-NiOx) is used as hole transport material in PSCs due to the mobility of its holes, compatibility of the stability, easy fabrication and Fermi level position suitable for hole extraction. However, unavoidable defects in sp-NiOx or perovskite films can affect solar efficiency. Thus, self-assembled monolayer (SAM) MeO-2PACz was inserted between the sp-NiOx and perovskite film, which can contribute to reducing defects and improving the device’s performance. The results showed that the MeO-2PACz interface enhances perovskite film quality by reducing charge recombination at the sp-NiOx/perovskite interface. It also passivates defects in the sp-NiOx surface and perovskite layer. The overall outcome resulted in an improvement in the device efficiency from 11.9% to 17.2%.

Keywords: Organic solar cells, perovskite solar cells, interface, energy level and dipole

Subject: Chemistry thesis

Thesis type: Doctor of Philosophy
Completed: 2023
School: College of Science and Engineering
Supervisor: Gunther Andersson