Constructing Orderly Crystal Orientation with Bidirectional Coordinator for High Efficiency and Stable PSCs

Abstract
A well-developed perovskite crystal at the beginning of a crystal lattice facilitates favourable growth orientation for efficient charge transport and the elimination of buried interfaces. However, rapid and uncontrollable crystallization of perovskites poses significant challenges in achieving desired growth orientations and controlling the growth direction during crystallization, necessitating the establishment of optimal substrate conditions. In this study, we propose a bidirectional coordination strategy involving the introduction of cesium trifluoroacetate (CsTFA) onto a tin dioxide (SnO2) surface. Treatment with CsTFA facilitates the passivation of SnO2 vacancies via COOH–Sn while concurrently forming intermolecular interactions with overlying perovskite crystals, manifested as CF3⋯H–N for formamidinium (FA+) and CF3⋯I–Pb, respectively. These interactions initiate the well-established beginning of the perovskite crystals and promote their vertical growth. Consequently, vertically grown perovskite crystals exhibit reduced tensile strain and fewer crystalline defects. Furthermore, a benign buried interface between the perovskite and underlying SnO2 mitigates detrimental damage, thereby suppressing non-radiative recombination losses. This synergetic bidirectional coordination contributes to the fabrication of perovskite solar cells with a maximum power conversion efficiency of 25.60% (certified at 25.39%) and long-term stability under light illumination.

A joint research team from the School of Energy and Chemical Engineering and the Department of Chemistry at UNIST has addressed critical challenges in perovskite solar cells (PSCs), significantly enhancing both their efficiency and stability, which is expected to further bolster their commercialization potential.

Led by Professors Jin Young Kim, Dong Suk Kim, and Geunsik Lee, the team successfully achieved precise control over ion arrangement and reduced structural irregularities by incorporating a bidirectional coordinator between the perovskite photoactive layer and the electron transport layer.

Figure 1. Overall schematic illustration of the study.

Despite their high efficiency and low manufacturing costs, perovskite solar cells have faced obstacles to commercialization due to various defect-related issues. The research team introduced trifluoroacetate (TFA-) ions between the perovskite layer and the tin oxide substrate, which serves as the electron transport layer (ETL), to mitigate these defects.

Figure 2. (a) Cross-sectional SEM images of the perovskite films deposited on the control- and CsX-treated SnO2, configured as FTO/control- and CsX-treated SnO2/Perovskite/Spiro-OMeTAD. (b) GIXRD spectra of the perovskite films deposited on the control- and CsX-treated SnO2 upon varying the grazing incidence angle from 0.2° to 10°. (c) Normalized PL spectra upon varying the thickness of the perovskite films deposited on the control- and CsX-treated SnO2. (d) XRD rocking curves of the perovskite films deposited on the control- and CsX-treated SnO2 (left). FWHM values of XRD rocking curves of perovskite films with different concentrations, 0.4 M and 1.9 M, deposited on the control- and CsX-treated SnO2 (right).

The carboxylate group (-COO-) of TFA- firmly bonds with the tin oxide, enhancing structural stability. Simultaneously, the organic head group (-CF3) effectively reduces defects through bidirectional molecular tuning that interacts with the perovskite layer.

Figure 3. Density functional theory calculation results.

This approach allowed the research team to control the irregular structure of the perovskite thin film, substantially improving charge carrier mobility. The resulting perovskite films, characterized by the absence of buried interfaces and minimized tensile strain, achieved a remarkable power conversion efficiency (PCE) of 25.60%. Moreover, the unencapsulated device maintained over 80% of its initial PCE even under prolonged light exposure after 1000 hours.

Professor Dong Suk Kim remarked, “This groundbreaking bidirectional coordination strategy reveals a promising pathway to enhance high efficiency and confront the persistent challenge of addressing long-term stability concerns.” He added, “This achievement will further enhance the commercialization potential of PSCs.”

This study has been jointly participated by Jaehwi Lee, Dr. Yun Seop Shin, and Dr. Elham Oleiki, as the first author. It has been supported by the Ministry of Science and ICT (MSIT) and the National Research Foundation of Korea (NRF). Their findings have been published in the online version of Energy & Environmental Science in July 2024.

Journal Reference
Jaehwi Lee, Yun Seop Shin, Elham Oleiki, et al., “Constructing orderly crystal orientation with a bidirectional coordinator for high efficiency and stable perovskite solar cells,” (2024).