Development of High-performance Long-term Stable and Low-cost Flexible Perovskite Solar Cells
Creators: name: ZHANG, JING
Publisher: name: University of Surrey
Year: 2022

description: "Perovskite solar cells (PSCs) have been considered as a promising game-changer in the current photovoltaic (PV) market because the performance of PSCs has been advancing aggressively within the recent decade. Owing to the low-temperature fabrication protocols of PSCs (~100 °C), flexible PSCs (f-PSCs) have been developed and reached a certified power conversion efficiency (PCE) of over 21%. However, two predominant obstacles, namely environmental stability and mechanical robustness, are hampering the practical applications of f-PSCs. For the environmental stability issue, moisture represents a major challenge because it can degrade the perovskite layer from both the top and bottom sides, contributing to the shorter lifetime of f-PSCs while compared with rigid PSCs (r-PSCs). In terms of mechanical durability, which is an exclusive evaluation criterion for f-PSCs, the brittleness of indium tin oxide (ITO) compromises the bendability of the resultant f-PSCs. Additionally, the cost issue is an unavoidable factor regarding commercialization. As estimated, the cost of the ITO layer contributes to ~70% expense of the entire device. To conquer these challenges, here, single-walled carbon nanotubes (SWCNTs), owing to their excellent optoelectronic properties, hydrophobic nature, adjustable work functions, and remarkable mechanical robustness, present a promising way to enhance both environmental stability and mechanical properties of f-PSCs. Unfortunately, neither the fabrication process of SWCNT film for f-PSC application nor the interface between the SWCNT and perovskite materials has been thoroughly examined.
description: In this thesis, we start with the study of SWCNT film as the window electrode to replace ITO via a simple dry transfer process. Through HNO3 doping, not only the work function but also the sheet resistance have been optimized. We then fabricated r-PSCs and f-PSCs with SWCNT films as the window electrode and achieved outstanding PCEs of ~19% for r-PSCs and 18% for f-PSCs, respectively, which are the highest PCEs for ITO-free PSCs to date. Furthermore, due to the excellent mechanical robustness and hydrophobic nature of SWCNT films, both the environmental stability and bendability
description: of SWCNT-based f-PSCs have been enhanced notably. Additionally, we note that the cost of SWCNT is only ~ 10% of that of ITO, which provides a feasible way to make low-cost, long-term stable f-PSCs.
description: We then focus our research on the ETMs in f-PSCs, through the development of a novel ETM combination. The commonly used ETMs, phenyl-C61-butyric acid methyl ester (PCBM), can not block the moisture penetration efficiently. We note that tin oxide (SnO2), which has been widely used in many record-breaking PSCs, has rarely been attempted in inverted structured f-PSCs. Therefore, we developed a hybrid SnO-2/PCBM ETM used in inverted structured f-PSCs, and presented an impressive PCE of over 20%, which is one of the highest PCE for the inverted f-PSCs to date. Encouragingly, the resultant f-PSCs demonstrated better environmental and thermal stability, which is associated with the excellent encapsulating capabilities of the compact SnO2/PCBM layer.
description: Finally, we explore the feasibility of using SWCNTs as the top electrode in f-PSCs, to replace the expensive noble metal electrodes such as Au, Ag, etc. Because these metals could induce degradation of the photoactive layer, threatening the stability of PSCs. Besides, another advantage of using SWCNTs as the top electrode is to enable PSCs to absorb sunlight from both sides to make semitransparent PSCs, which further extends the application of PSCs, e.g. building-integrated PV (BIPV). By employing SWCNT films as both top and window electrodes, we developed all-carbon-electrode f-PSCs, and obtained a PCE of over 15% illuminated from both sides, which is the highest PCE for the ITO-free semitransparent f-PSCs. Additionally, the lifespan and bendability of these new types of f-PSCs have been reinforced considerably. "