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Exit Seminar: "The Application of Photoelectron Spectroscopies in Analyzing the Impact of Interfacial Energetics on Perovskite Solar Cells"

Speaker(s) / Presenter(s):
Tuo Liu

Abstract: In recent years, organic-inorganic metal halide perovskites (HP) have garnered tremendous attention in photovoltaic research. This attention is attributed to their low cost and excellent optoelectronic properties, including large absorption coefficients, tunable bandgaps, long charge carrier diffusion lengths, and low trap state densities. Inverted p-i-n architecture perovskite solar cells (PSCs) are of intense interest and are generally regarded as more amenable to low-temperature solution processing. Nevertheless, the development of inverted PSCs is lagging the conventional architecture devices. Imperfect energy level alignments and charge carrier recombination, especially at the interface between perovskite and electron transport layers (ETLs), are two main factors suppressing the power conversion in inverted PSCs.

Organic semiconductors (OSCs), including π-conjugated polymers and small molecules, display distinct advantages in terms of low-cost, lightweight, wide variety, easy solution-processed manufacturing as well as excellent mechanical flexibility. The molecule design produces numerous organic semiconductors with desired properties in application of various advanced organic electronics devices. However, the performance of OSCs-based devices is left behind their inorganic counterparts due to the lower carrier density and mobility. Molecular doping which provides a route to significantly enhance the electric properties, attracts attentions progressively.

Tuo Liu’s work during his PhD focused on the applications of photoelectron spectroscopies on the studies of perovskite solar cells and organic semiconductor-based electronics. The first work carried out how the surface ligands impact interfacial energetics and charge carrier dynamics at methylammonium lead iodide (MAPbI3)/C60 interfaces. The frontier electronic energy levels at perovskite/C60 interface are directly probed by ultraviolet photoelectron spectroscopy (UPS) and low energy inverse photoelectron spectroscopy (LEIPS), providing evidence of interfacial energetics reconstruction caused by surface ligands with different dipoles. Ultrafast absorption/reflectance spectroscopies and transient photovoltage/photocurrent are utilized in comprehensively picturing the charge dynamics in films and devices. The following work reports the doping mechanism in the photoactivated p-doping of hole-transporting material (HTM) to enhance hole extraction for perovskite/textured silicon tandem solar cells, making the device performance less sensitive to the variation of hole transport layer thickness. Last several collaborations works are based on the doping behaviors in different doped organic semiconductors, with applications in organic solar cells and thermoelectrics.

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