Impact of doping and interfacial band bending of charge transport layer in inverted perovskite solar cells

Yiu, Wai Kin (2025) Impact of doping and interfacial band bending of charge transport layer in inverted perovskite solar cells. PhD thesis, University of Glasgow.

Abstract

This thesis investigates the n-doping reaction mechanism and interfacial band bending of charge transport layers (CTLs) in inverted perovskite solar cells (PSCs). CTLs, comprising electron transport materials (ETMs) and hole transport materials (HTMs), play a crucial role in determining the efficiency of PSCs by facilitating efficient charge extraction and transport while minimising recombination losses. However, device performance is often hindered by challenges such as low intrinsic conductivity of organic ETMs. To address these challenges, this work explores the n-type doping in non-fullerene organic ETMs, with a focus on improving conductivity and studies self-assembled monolayers (SAMs) as HTMs, understanding the influence of fermi levels on high efficiency of inverted PSCs.

The study begins by examining functionalized bisflavin (BF) derivatives and naphthalenediimide (NDI) derivatives with glycol and alkyl side-chains as nonfullerene ETMs due to bio-inspired nature and more straightforward synthetic process, compared to conventional ETM such as [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Due to inherently lower conductivity of these pristine derivatives, n-type doping was performed to enhance the conductivity using the dopant to generate free radical, as confirmed through electron paramagnetic resonance (EPR) measurements. UV-vis absorption spectroscopy and conductivity studies revealed that derivatives with polar glycol side-chains (BFG and NDI-G) facilitated faster doping reactions compared to the non-polar alkyl counterparts (BFA and NDI-EtHx). This behaviour was attributed to the polarity compatibility between the glycol side-chains and the dopant, which promoted molecular interactions and enhanced the doping efficiency.

Interestingly, the BF and NDI systems exhibited distinct responses to doping effects. While the doped BF derivatives show limited improvement in charge transport, the doped NDI derivatives demonstrated significant conductivity enhancements. Optimised NDI-G doped materials achieved a conductivity exceeding 10-2 S/cm, resulting in improved photovoltaic performance. Density functional theory (DFT) calculations explained these observations by highlighting the formation of charge transfer complexes (CTCs) with strong binding energies. The alignment of energy levels between CTC and neutral molecules was found to be critical for effective electron transfer and the generation of free charges. Based on these findings, a detailed doping mechanism is proposed in this work.

Additionally, bulk defects such as ion vacancies caused the surface recombination in the PSC system, it is necessary to decouple the charge accumulation from recombination. In here, we investigate using a novel stabilization and pulse (SaP) measurement technique to decouple the ionic feature with electronic effect, studying the influence of SAMs on Fermi-level alignment in PSCs. SAMs with varying dipole moments (MeO-2PACz, Me-4PACz, and 2PACz) were studied, revealing distinct flat ion potentials (Vflat) that affected charge extraction efficiency. An optimal Vflat of approximately 0.8 V was identified, while higher values were associated with interfacial barriers and reduced performance. Supporting evidence from Kelvin probe microscopy (KPFM) and time-resolved photoluminescence (TRPL) further confirm this hypothesis.

In summary, this thesis contributes insights into the charge transport and recombination through the n-type doping of non-fullerene organic ETMs and the interfacial band bending of SAM-based HTMs in inverted PSCs. The findings underline the strategic importance the doping mechanism and the critical role of interfacial engineering in enhancing photovoltaic performance. These results have broader implications for advancing efficient perovskite-based solar technologies.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Subjects: Q Science > QD Chemistry; T Technology > TP Chemical technology
• Colleges/Schools: College of Science and Engineering > School of Chemistry
• Supervisor's Name: Cooke, Professor Graeme and Docampo, Dr. Pablo
• Date of Award: 2025
• Depositing User: Theses Team
• Unique ID: glathesis:2025-85488
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 02 Oct 2025 09:19
• Last Modified: 02 Oct 2025 09:21
• Thesis DOI: 10.5525/gla.thesis.85488
• URI: https://theses.gla.ac.uk/id/eprint/85488

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