Abstract
Perovskite solar cells (PSCs) have garnered considerable attention in recent years due to their low-cost solution processability, unique optoelectronic properties, and high conversion efficiencies that are comparable to most commercial solar technologies. Compared to the traditional n-i-p structure, the inverted p-i-n configuration of PSCs offers higher efficiency, enhanced stability, suitability for tandem devices, and lower processing temperatures, making it a focal point of interest.In p-i-n PSCs, hole transport materials (HTMs) have significantly propelled performance advancements. The initial p-i-n PSCs employed PEDOT:PSS as the HTM, a typical material used in organic photovoltaics at that time. NiOx, also drawing from organic photovoltaic technology, due to its excellent interface hole extraction and transport capabilities, was utilized to replace PEDOT:PSS. Subsequently, PTAA, known for its doping-free nature and outstanding stability, has been widely adopted in p-i-n PSCs. Recently, a novel type of p-i-n PSC utilizing self-assembled monolayers (SAMs) anchored on NiOx as the hole-selective layer achieved over 26% power conversion efficiency (PCE), becoming the mainstream in high-performance p-i-n devices. Conversely, C60 and its derivatives, typically PCBM, have consistently been used as electron transport materials (ETMs) in efficient p-i-n PSCs, as they lack significant viable alternatives.
This study delves into the design and fabrication of interfacial materials for p-i-n PSCs to enhance efficiency and stability, encompassing topics such as (1) boosting the efficiency and stability of NiOx-based p-i-n PSCs through D–A type semiconductor interface modulation, (2) suppressing oxidation at the perovskite–NiOx interface for efficient and stable tin PSCs, and (3) enhancing the efficiency and stability of p-i-n PSCs through solution-processed and structurally ordered fullerene.
Energy level mismatches and a high density of interface defects in NiOx-based p-i-n PSCs typically result in lower PCEs compared to those with high-performance organic HTMs. Additionally, various high oxidation state Ni species (Ni>3+) in the NiOx layer can lead to perovskite degradation. The thesis proposes the use of an imide-based donor–acceptor type semiconductor (BTF14) as an intermediary layer between the perovskite and NiOx, which is beneficial for hole extraction and transport, reducing the defect density at both the interface and the bulk of the perovskite film, and further decreasing the concentration of Ni>3+ substances to stabilize the heterojunction. As a result, the energy conversion efficiency of p-i-n PSCs can be significantly increased from 22.11% with NiOx to 24.20% with NiOx/BTF14. Furthermore, devices based on NiOx/BTF14 also exhibit negligible hysteresis and outstanding long-term stability, maintaining over 77% of their initial efficiency after continuous operation at 60°C under one sun illumination for 1000 hours.
Moreover, this paper identifies the source of the hole transport barrier at the perovskite/NiOx interface and develops a self-assembled monolayer interface modification by introducing (4-(7H-dibenzo[c,g]carbazol-7-yl)ethyl)phosphonic acid (2PADBC) at the perovskite-NiOx interface. 2PADBC, anchored to under-coordinated Ni cations via its phosphonic groups, suppresses the reaction of highly active Ni≥3+ defects with the perovskite, simultaneously enhancing the electron density and oxidative activation energy at the perovskite interface, reducing interface non-radiative recombination caused by tetravalent Sn defects. The device's open-circuit voltage significantly increased from 0.712 V to 0.825 V, the PCE of small-area devices improved to 14.19%, and the PCE of large-area (1 cm2) devices increased to 12.05%. Additionally, 2PADBC modification improved the operational stability of NiOx-based tin PSCs, maintaining over 93% of the initial efficiency after 1000 hours.
In the final part of the paper, we propose a novel solution-processable ETM design by grafting a non-fullerene acceptor fragment onto C60. The synthesized BTPC60 exhibits excellent solution processability and orderly molecular stacking patterns, enabling the formation of uniform, structurally ordered ETM films with high electron mobility. When used as the ETL in p-i-n PSCs, BTPC60 not only demonstrates excellent interface contact with the perovskite layer, enhancing electron extraction and transport efficiency, but also effectively passivates interface defects to suppress non-radiative recombination. The resulting BTPC60-based p-i-n PSCs achieved an impressive 25.3% PCE, and retained nearly 90% of their initial value after aging at 85°C under N2 for 1500 hours. More encouragingly, the solution-processed BTPC60 ETL exhibits excellent film thickness tolerance, achieving up to 24.8% high PCE even when the ETL thickness reaches 200 nm, which is advantageous for its potential application in large-scale manufacturing. Our results indicate that BTPC60 is a promising solution-processed fullerene-based ETM, paving the way for enhancing the scalability of efficient and stable inverted PSCs.
In conclusion, by developing new interfacial materials and engineering, we have successfully enhanced the efficiency and stability of PSCs. These research outcomes provide crucial guidance and reference for the further development and commercialization of PSCs, contributing to the sustainable development in the field of renewable energy.
| Date of Award | 6 Jan 2025 |
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| Original language | English |
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| Supervisor | Zonglong ZHU (Supervisor) |