Device architecture engineering for stable and efficient perovskite solar cells

Abstract

Perovskite solar cells (PSCs) have been considered one of the most promising photovoltaic technologies, with a certified power conversion efficiency (PCE) of up to 25.5%, comparable to silicon solar cells. However, the stability issue of PSCs remains a major challenge that impedes their commercialization. Device architecture plays a critical role in determining both stability and PCE of the PSCs. For instance, mesoporous metal oxide scaffolds such as TiO2, Al2O3, and ZrO2 can act as protective barriers to reduce the moisture ingression from the ambient air into the perovskite layer. A thick carbon layer can also be used as the electrode to replace the expensive noble metal electrodes, as well as to replace the moisture-sensitive hole transport layers (HTLs) to enhance the stability of the PSCs significantly. However, using these device architectures can introduce interfacial defects that reduces the PCE of the PSCs. This work of thesis aims to enhance the stability without sacrificing the PCE of the PSCs through appropriate device architecture engineering. It started with the study of the impact of TiO2 architecture as the electron transport layer (ETL) on the PCE and stability of the PSCs. A systematic comparison between the PSCs based on the planar and mesoscopic TiO2 architectures with various thicknesses was made to understand how the ETL architecture and thickness affect the device performances. It was found that, the presence of a mesoscopic TiO2 architecture contributes to an improved charge collection efficiency and can act as a battier to protect the infiltrating perovskite from moisture ingression, resulting in a reduced hysteresis behavior and enhanced stability of the PSCs. The mesoscopic PSCs could retain 85% of initial PCEs after storage in ambient air for ~670 h (RH= 60–70%), while a retention of 75% was shown in the planar devices upon the same aging conditions. The optimal thickness of the mesoporous TiO2 layer was found to be around ~140 nm. On the other hand, a thicker mesoporous TiO2 layer (e.g., ~220 nm) was found to be detrimental to the device PCE due to its negative impact on the crystallization of the perovskite grains and the poor electrical conductivity. Based on these initial findings, a bilayer TiO2/Al2O3 architecture was proposed, where mesoporous Al2O3 layer (~150 nm) was deposited on top of the mesoscopic TiO2 (~150 nm). It was found that the mesoporous Al2O3 layer acts as a scaffold barrier between the ETL and HTL without any negative impact on the PCE of the PSCs simultaneously, providing better protection for the infiltrated perovskite layer compared to the device based on the mesoporous TiO2 only. As a result, the devices assembled in humid air (RH>65%) based on the bilayer TiO2/Al2O3 mesoscopic architecture delivered the PCEs up to 16.8%, with an average retention of 82% of their initial PCEs after storage in actual ambient air for 2000 h (~83 days) without any encapsulation. In comparison, PSCs based on a single layer of mesoporous TiO2 retained only 57% of their initial PCEs after the same aging test. To further enhance the device stability with the device architecture engineering as well as consider from a low-cost perspective, it was proposed to replace the moisture-sensitive and expensive HTLs such as Spiro-OMeTAD and PTAA and rare metal electrodes to form a new type of low-temperature-processed HTL-free carbon-based PSCs (LTC-PSCs). Encouraged by the success of the laser-induced graphene (LIG) technique, this study demonstrated a one-step in-situ laser synthesis of graphene flakes decorated with ultrafine NiOx nanoparticles (~14 nm). The NiOx-decorated LIG material was then used as HTL-free carbon electrodes in LTC-PSCs. To our knowledge, this is the first time that LIG has been used in the field of PSCs. The carbon electrode constituted by the dense LIG stacks shows stronger coherent interfacial contact with the perovskite layer than the commercial graphite, resulting in improved i

Date of Award	31 Dec 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Zhu Liu (Supervisor) & Michele Curioni (Supervisor)