Bulk and surface recombination losses in planar perovskite solar cells

Abstract

This thesis summarizes the experimental studies on the recombination mechanisms in planar “p-i-n” type perovskite solar cells (PSCs), focusing on the role of both photoactive layer and charge transport layers. The work carried out both materials characterisation and optoelectronic measurements, probing the physical origin charge recombination and assessing their impacts on solar cell power conversion efficiency (PCE). The origin of open-circuit voltage (VOC) is considered, the importance of controlling defects in photoactive layer and controlling doping level of charge transport layer is highlighted, and a couple of design principles for high-performance PSCs are discussed In Chapter 1 an introduction of semiconductor physics relevant to solar-to-electrical energy conversion is presented, followed with a review of solar cells physics, materials and application. Chapter 2 focuses on the principle and data interpretation of the experimental methods employed in this thesis, including X-ray diffraction, electron microscopy, surface probe, photoluminescence spectroscopy and transient optoelectronic measurements. Chapter 3 reports how the crystallinity of thin-film perovskites can be modulated by tuning the stoichiometry of precursor mix in solution processing. This chapter elucidates that both VOC and PCE are governed by electronic trap states in perovskites that are correlated to the crystallinity of perovskite films. In Chapter 4 a facile modification on solution processing is reported that can remarkably remove microstructural defects in perovskite thin films, presenting both detailed microscopic characterisation of these defects and optoelectronic characterisation of their impact device performance. Chapter 5 moves from bulk perovskite to hole transport layers (HTLs), elucidating how the p-doping of HTLs causes surface recombination and is averse to device performance. Finally, Chapter 6, new insights into the fundamental operational principles of PSC are discussed and further work based this thesis are suggested.