Application of Stable and Efficient Wide bandgap Perovskite on Perovskite/Si Tandem Solar Cells

Abstract

Metal halide perovskites (MHPs) have advanced rapidly as photovoltaic absorbers owing to their strong optical absorption, long minority-carrier diffusion lengths, bandgap tunability, and compatibility with solution processing. Wide-bandgap (WBG, ≈1.63-1.8 eV) compositions are particularly attractive as top cells in perovskite/Si tandems yet inverted (p-i-n) WBG devices remain limited by interfacial non-radiative recombination, extraction barriers arising from imperfect interface energy alignment, and halide-segregation induced instability. Chapter 1 surveys perovskite photovoltaics with an emphasis on WBG devices: fundamental MHP properties (crystal/phase stability, defect tolerance, optical constants, carrier transport), device architectures (single-junction and perovskite/Si tandems), and the specific requirements and pain points of WBG top cells: open-circuit voltage (Voc) deficit, fill-factor (FF) loss from contact resistance and interfacial energy barrier, and light-driven phase segregation. Chapter 2 develops a buried-interface passivation strategy for inverted WBG perovskite solar cells (PSCs). Interlayers (e.g., ammonium halides and conjugated polyelectrolytes) are inserted between the hole transport layer (HTL) and perovskite to promote the perovskite growth, passivate interfacial defects, and mitigate ionic migration. The study combines steady-state photoluminescence (PL), water contact angle measurement, scanning electron microscopy (SEM), transmission electron microscopy (TEM), grazing incidence X-ray diffraction (GIXRD), space charge limited current (SCLC) measurement to evidence the formation of the two-dimensional (2D) perovskite, reduced non-radiative recombination and improved contact quality. Device metrics [JV, stabilized power output, external quantum efficiency (EQE)] and light-stress testing confirm gains in power conversion efficiency (PCE) alongside enhanced operational stability. Chapter 3 advances a complementary energy-level alignment strategy at the perovskite/HTL interface. Self-assembled monolayers (SAMs) with tunable dipoles, exemplified by blending phosphonic-acid carbazole derivatives, are used to deepen the HTL work function (WF) and mitigate the hole-injection barrier introduced by WBG perovskites’ deeper valence bands. Work-function/valence-band alignment is quantified by Kelvin probe force microscopy (KPFM) mapping with improved homogeneous potential distribution. Which aligned with the reduced of the surface roughness as suggested by the atomic-force microscopy (AFM). Which allows void-less perovskite surface observed in the SEM. The interactions between cations and anions with SAMs functionalized with additional methyl groups on the benzene ring were probed using Fourier transform infrared spectroscopy (FTIR), steady-state electroluminescence (EL), PL, and X-ray photoelectron spectroscopy (XPS). The blended SAMs have been incorporated into the device with the quasi-Fermi level splitting (QFLS) suggested negligible loss of carrier transport at the interface. The device modelling COMSOL Multiphysics software results indicate that improper energy alignment at the HTL/perovskite interface creates an unfavourable energy barrier, which hinders efficient charge transfer. A mechanically stacked, four-terminal (4T) perovskite/Si tandem configuration was accomplished using an industrially prevalent Si bottom cell structure, the tunnel oxide passivated contact (TOPCon) Si cell. Device metrics (JV, stabilized power output, EQE) and light-stress testing confirm gains in PCE alongside enhanced operational stability. Chapter 4 synthesizes the findings and outlines future work: co-optimizing buried passivation and dipole engineering; improving ultraviolet (UV)/thermal robustness of HTLs and passivants; and tandem-oriented optical engineering [e.g., refractive-index-matched interlayers and thinner sputtered transparent conductive oxide (TCO)] to minimize parasitic while preserving stability. Collectively, the thesis identifies buried-interface loss as the dominant bottleneck in WBG inverted PSCs and delivers two experimentally validated, complementary routes: defect passivation and energy-level alignment, that reduce interfacial recombination, strengthen carrier selectivity, and improve stability. The resulting design rules are readily transferrable to scalable fabrication and accelerate the deployment of high-performance, stable WBG top cells for monolithic perovskite/Si tandems. Show more