Study on Surface and Interface Engineering for Improving Stability of Perovskite Quantum Dot Solar Cells

Abstract

The power conversion efficiency of perovskite quantum dot (PQD) solar cells shows increase from 10.77% to 16.6% in a short period owing to advances in material and device design for solar cells. However, the device stability of PQD solar cells remains poor in ambient conditions, which requires an in-depth understanding of the degradation mechanisms of PQDs solar cells in terms of both inherent material properties and device characteristics. Along with this analysis, advanced strategies to overcome poor device stability must be conceived. The causes of instability in PQDs have been discussed and classified according to reasons such as structural instability, intrinsic ionic and dynamic properties, and environmental instability. The causes of instability in PQDs have been discussed and classified according to reasons such as structural instability, intrinsic ionic and dynamic properties, and environmental instability. Efforts to overcome this are divided into material design strategies, including surface manipulation and heterostructures, and device design strategies, including charge transport layer (CTL) engineering. Especially, in both strategies, engineering the PQD surface and interface between the PQD and adjacent CTL was a key strategy for improving stability. However, research to comprehensively passivate the the PQD surface and interface between the PQD and adjacent CTL is still insufficient. This is the inspiration and subject of my Ph.D. research. I have participated and led each of studies described in Chapter 3. I conducted everything from the PQD synthesis to film characterization, device fabrication, and stability assessment. The novel semiconducting organic materials to perform this reseach is synthesized by my co- workers D. H. Lee for small molecule (Star-TrCN) and dopant-free polymers (Asy- PDTS, Asy-SPDTS, and Asy-SePDTS). In addition, the DFT calculation based molecular characterization is conducted by S. B. Cho, which is based on simulation using PQD slab model. Chapter 1 provides a brief introduction to PQDs and related photovoltaic development. The cubic-phase instability problem of bulk perovskite, the background of the emergence of PQDs, and the unique characteristics of PQDs are introduced. Afterwards, the origins of PQD instability were classified into structural instability, unique ionic and dynamic characteristics, and environmental instability. The efforts to overcome this summarized into two categories: material design strategies including surface manipulation and heterostructures, and device design strategies including CTL engineering. Inspired by this, the research motivation to produce stable PQD solar cells during his doctoral degree was produced. Chapters 2 and 3 aimed to develop stable PQD solar cells through engineering the PQD surface and interface between the PQD and adjacent CTL. In the first work, chlorine-treated SnO2 (Cl@SnO2) was developed as an electron transport layer (ETL) and applied to PQD solar cells. Cl@SnO2 had relatively low hydrophilicity and photocatalytic activity compared to TiO2 and SnO2, which are widely used as existing ETLs. These characteristics caused the Cl@SnO2-based PQD film to have the best stability in the presence of moisture and light in the aging test. Additionally, solar cell stability under general atmospheric conditions was also the highest in the Cl@SnO2- based PQD devices. In the second work, a stable PQD structure was developed by combining a conductive organic semiconductor and PQDs. To achieve a successful hybrid system, a star-shaped semiconductor material (Star-TrCN) with three- dimensional properties and excellent geometric balance between coplanarity and distortion characteristics was introduced to the PQD surface. The twisted 3D structure of Star-TrCN suppressed self-aggregation and increased compatibility between organic semiconductors and PQDs. Star-PQD hybrid film greatly improved the cubic- phase stability of PQDs by passivating surface defects and preventing moisture penetration. The efficiency of the Star-PQD hybrid PQD solar cell maintained 72% of the initial efficiency even after 1000 hours in atmospheric conditions. In the third work, a dopant-free polymer hole transport layer (Asy-PSeDTS), which can replace the spiro-OMeTAD hole transport layer (HTL) using deliquescent dopants, was applied to PQD solar cells. The selenium (Se) elements in the rigid segment of the polymer acted as a Lewis base for the lead (Pb) vacancies of the PQD surface and showed effective passivation properties. In addition, the polymer HTM showed greatly improved hydrophobicity compared to the conventional spiro-OMeTAD HTL. Through this, the dopant-free polymer-based PQD solar cell maintained excellent stability, maintaining 80% of the initial PCE under atmospheric conditions for 1200 hours. In this study, I conducted step-by-step in-depth research on (1) ETL, (2) PQD surface, and (3) HTL to improve the stability of PQD solar cells, and discussed ways to gradually develop the stability of PQDs. I expect that this result can be applied not only to PQD solar cells but also to other optoelectronic devices using PQDs. Furthermore, I believe that this stidy will be a new strategy that can contribute to the commercialization of PQD in the near future.