Interface engineering for organic−inorganic hybrid perovskite solar cells

• Main Author: Foo, Shini
• Other Authors: Huang Yizhong
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Engineering::Materials::Energy materials
• Online Access: https://hdl.handle.net/10356/154961
• Institution: Nanyang Technological University

Laboratory efficiencies of current state-of-the-art organic−inorganic hybrid perovskite (OIHP) solar cells are on par with the well−established crystalline silicon−based solar cells. Despite the ideal intrinsic optoelectronic properties as efficient and relatively economical light-absorbing layers, the commercialization of OIHP materials is greatly impeded by issues such as poor stability and reproducibility. Comprising of various functional layers stacked on top of one another, interfaces often serve as transport barriers since defects tend to aggregate between the layers, impeding the photovoltaic performance, stability, and reproducibility of solar cells. To address these challenges, intensive developments have been made recently to interfacial engineering so as to reduce the amount of undesirable trap states in OIHP solar cells. Although significant progress has been made to improve the device performance and stability of OIHP solar cells using interfacial engineering, the preferable properties of passivation material utilized at various interfaces are often overlooked. Therefore, this thesis focuses on the importance of passivation materials at interfaces found in OIHP solar cells, whereby properties tailored and profitable for various interfaces are highlighted. Specifically, modifications were conducted to the electron transport layer (ETL), as well as the ETL/ OIHP and OIHP/ hole transport layer (HTL) interfaces. Hence, this thesis hopes to encourage the commercialization of OIHP solar cells by using premediated passivation materials at various interfaces for the creation of highly efficient, reliable, and stable of OIHP solar cells. Work I. Despite the wide range of possible electron transport materials, titanium dioxide (TiO2) continues to be the most utilized semiconductor in OIHP solar cells due to its good repeatability. However, the intrinsic properties of TiO2 are incompatible for efficient electron transport since compact−TiO2 (c−TiO2) ETL has relatively low electron mobility and poor electrical conductivity. Dopants are commonly used to tune the properties of semiconductors. Here, aluminum (Al) and indium (In) dopants are co−doped into the conventional c−TiO2 to form the AlIn doped TiO2 (AITO) ETL. When used in the mesoporous structure, superior photovoltaic performance and stability were demonstrated in OIHP solar cells. Systematic characterization of the individual dopants revealed that the improved short−circuit current (JSC) in the devices employing the Al-doped TiO2 ETL was due to the reduced optical losses while the enhanced open−circuit voltage (VOC) and fill factor (FF) of the devices utilizing the In doped TiO2 ETL allowed better energy level alignment and conductivity. When used in the mesoporous structure, superior photovoltaic performance and stability were demonstrated. All in all, this study shows a facile yet effective co−doping method to improve several cell parameters at once, consequentially, resulting in high overall power conversion efficiency (PCE). Work II. As seen in the previous work, the compact AlIn doped TiO2 (AITO) ETL employed in the mesoporous structure led to improved stability and efficiency in OIHP solar cells. However, planar OIHP solar devices exhibit ease in scalability due to the simple processing procedures. Yet, certified PCEs of planar OIHP solar devices remain inferior to that of the mesoporous configuration. In this study, the previously optimized compact AITO layer was utilized together with the SnO2 ETLs to form the heterogeneous ETL. It is revealed, through this paper, that the use of both AITO and SnO2 ETLs in a single device provides synergistic benefits to the overall performance, long−term stability, and reproducibility of the planar OIHP solar devices. Specifically, the addition of SnO2 at the AITO/OIHP interface provided reduced recombination occurrences due to the passivating nature of SnO2 films, as well as enhanced electron transport due to its high electron mobility. On the other hand, the AITO ETL deposited below the SnO2 layer prevented shunting pathways and encourage stability. Despite the improved reproducibility, stability, and efficiency, the overall performances of the as−prepared OIHP devices are still unsatisfactory compared to commercially available silicon solar cells and can be improved further. Work III. As can be seen in the second work, passivation at the ETL/OIHP interface using the heterogeneous ETL showed significant improvement in device reproducibility although further enhancements in device stability and efficiency are desirable. Owing to ionic and soft bonding characteristics of OIHP materials, defects such as under−coordinated ions and dangling bonds tend to concentrate at the OIHP/HTL interface. In addition, the hydroscopic deposited adjacent to the OIHP layer could accelerate degradation, reducing the stability of the OIHP solar device. Here, modifications were made to the previously optimized heterogeneous ETL-based device, whereby 2−hydrozybenzophenone (HBP) molecules are deposited on top of the annealed OIHP layer. It is hypothesized that the HBP molecules passivate the under−coordinated lead (Pb2+) defects accumulated at the interface by promoting favourable Lewis acid-base adduct formation. Through this work, it is verified that the use of an optimized Lewis base passivation layer enhances the overall device performance and stability attributed to the efficient hole transport across the OIHP/HTL interface and reduced recombination tendencies in the HBP passivated solar device. Therefore, this thesis demonstrates the effectiveness and simplicity of interfacial engineering to which synergistically improvement in device efficiency, reproducibility, and stability of OIHP solar cells can be attained. The thesis further shows the importance of using well–designed passivation materials, tailored for various interfaces of OIHP solar cells.