Interface and Charge Modulation for Efficient and Stable p-i-n Perovskite Solar Cells


Student thesis: Doctoral Thesis

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Award date4 Jan 2024


The present study delves into the optimization of Perovskite solar cells (PSCs), a promising technology based on halide perovskites. These solar cells have been the subject of considerable attention in recent years due to their low-cost solution processing capability, unique optoelectronic properties, and high conversion efficiency that is on par with most commercial solar technologies. The inverted p-i-n structure of perovskite solar cells, as opposed to the traditional n-i-p structure, offers low-temperature processing capability and promising stability. This makes it an ideal candidate for wearable electronic devices and large-area fabrication. However, despite the significant advancements in the efficiency of inverted perovskite solar cells in recent years, achieving their commercialization still poses several challenges. These include the need for further improvements in the efficiency and stability of the devices, as well as the need for more environmentally friendly processing routes.

In order to break through the current performance bottleneck of p-i-n perovskite solar cells, researchers in recent years have focused on the optimization of hole transport layers (HTL) and interface engineering as key strategies to improve their performance. By optimizing the HTL, the electron transport efficiency between the perovskite absorber layer and the electrode can be improved, minimizing electron recombination and losses, thereby ensuring effective charge collection. Simultaneously, interface engineering allows for the adjustment of energy level alignment between the perovskite absorber layer and the electrode, facilitating the efficient extraction and transport of electrons and holes. Through appropriate interface modulation, the reverse transport and recombination of electrons and holes caused by interface defects and energy level mismatch can be reduced, leading to an improvement in photovoltaic conversion efficiency. Furthermore, optimizing the interface engineering can also minimize interface defects between the perovskite and the electrode, reducing electron-hole recombination and non-radiative processes, thereby enhancing electron and hole transport and collection efficiency.

One of the main focuses of this thesis is on the optimization of hole transport layers (HTLs) and interface engineering to enhance the performance of p-i-n type perovskite solar cells. This research initially delves into the impact of the hole transport layer on the performance of perovskite solar cells. A significant contribution of this work is the exploration of a straightforward polymer HTM design strategy that enables efficient and stable inverted PSCs by introducing pyridine units into a multi-arylamine framework. This innovative approach allows the tuning of the wettability and promotes anchoring. By varying the connection sites of the pyridine units, the properties of the HTM can be effectively modified. In particular, the 3,5-linked PTAA-P1 exhibits molecular structures that interact with perovskite, resulting in high-crystalline perovskite films with uniform back contact and reduced defect density. Inverted PSCs based on undoped PTAA-P1 achieve significant efficiencies of 24.89% (certified value: 24.50%) and 23.12% on small-area (0.08 cm2) and large-area (1 cm2) devices, respectively. Moreover, the unencapsulated devices maintain over 93% of the initial efficiency after maximum power point tracking under simulated AM 1.5G illumination for an impressive 800 hours.

Furthermore, the commercialization of perovskite solar cells (PSCs) urgently requires the development of dopant-free HTMs that can be processed using green solvents. However, strong intermolecular interactions to ensure high hole mobility often compromise solubility and film-forming ability in green solvents. To address this challenge, this paper presents a simple yet effective design strategy to address this trade-off by constructing star-shaped D-A-D structures. The utilization of green solvent, 2-methylanisole (2MA), for processing HTM (BTP1-2) has demonstrated notable enhancements in hole mobility and multiple defect passivation effects. Inverted perovskite solar cells (PSCs) based on 2MA-treated BTP1 have achieved an impressive efficiency of 24.34%, the highest reported value for HTMs processed with environmentally friendly solvents. Additionally, experimental findings indicate that the charge separation of D-A type HTMs at the photoinduced excited state contributes to the passivation of perovskite defects, revealing a novel approach in HTM design.

In the final part of this paper, we optimize the performance of flexible perovskite solar cells through interface engineering. A highly efficient and stable flexible inverted PSC is obtained by modifying the interface between perovskite and the hole transport layer using pentylammonium acetate (PenAAc) molecules. By controlling the synthesis of the anion and cation, it is demonstrated that PenA+ and Ac- possess strong chemical bonding capabilities with the acceptor and donor defects on the perovskite film surface. Flexible PSCs modified with PenAAc achieve a highest conversion efficiency of 23.68% (0.08 cm2, certified value: 23.35%) and have a high open-circuit voltage (VOC) of 1.17 V. Large-area devices (1.0 cm2) also achieve excellent conversion efficiency of 21.52%. The prepared devices exhibit outstanding stability under mechanical bending, maintaining over 91% of the original PCE even after 5000 bending cycles. By introducing interface modification layers, optimizing interface band alignment, and reducing interface defects, we successfully control the efficiency of charge carrier transport, improve carrier selectivity, and reduce nonradiative recombination. These interface engineering strategies significantly enhance the photovoltaic conversion efficiency and stability of the devices.

In conclusion, through the improvement of hole transport layers and interface engineering, we have successfully enhanced the efficiency and stability of perovskite solar cells while exploring green processing routes. These research findings provide important guidance and reference for the further development and commercialization of perovskite solar cells, contributing to the sustainable development of the renewable energy field.