PhD Defence Suzana Kralj | Microstructure Control in Transparent Conductive Oxides and Metal Halide Perovskites for Solar Cells

Microstructure Control in Transparent Conductive Oxides and Metal Halide Perovskites for Solar Cells

Suzana Kralj is a PhD Student in the department Inorganic Materials Science. (Co)Promotors are prof.dr.ing. A.J.H.M. Rijnders from the Faculty of Sciences and Technology, University of Twente and prof.dr. M. Morales-Masis from the Eindhoven University of Technology. 

We are living in a time when the demand for energy continues to rise from day to day. The increase is driven by population growth, industrial expansion and increased use of artificial intelligence. Meeting those demands while adhering to international climate goals such as those outlined in the Paris Agreement, requires a rapid transition toward low-carbon and renewable energy technologies. Among all renewable sources, solar energy is by far the most abundant, and photovoltaics (PV) have emerged as one of the impactful technologies. The global installation of PV capacity continues to grow exponentially, demonstrating both its maturity and its central role in our clean-energy future. As silicon PV approaches its practical efficiency limits, emerging materials, such as metal halide perovskites (MHPs), offer a pathway forward, particularly through tandem configurations that can outperform single-junction silicon cells and open the door to new device architectures.

A central thread running throughout this thesis is the influence of microstructure on the functional performance of materials used in perovskite (PVK) and perovskite/silicon (PVK/Si) tandem solar cells. Following a general introduction to the motivation behind this work, photovoltaic technologies, and a focused overview of transparent conductive oxides (TCOs) and MHPs in Chapter 1, as well as vapor-phase deposition methods, pulsed laser deposition (PLD), and characterization techniques in Chapter 2, the subsequent chapters systematically build on this theme. Each chapter examines how controlling microstructure can address key material-level challenges in next-generation PV devices.

Chapter 3, demonstrates that the microstructure of tin-doped-indium oxide, Sn:In₂O₃ (ITO), whether amorphous or polycrystalline, determines nanoscale work-function distributions at TCO/self-assembled monolayer (SAM) interface. Using local surface potential mapping and structural analysis, we show that polycrystalline ITO exhibits grain-orientation-dependent variations that remain even after SAM anchoring, whereas amorphous TCOs or amorphous NiO_x interlayers eliminate these fluctuations entirely. These findings highlight microstructure engineering as a key strategy for achieving more uniform and reproducible electronic contact layers in solar cells, a finding later reflected in the design of state-of-the-art PVK/Si tandems.

Chapter 4, introduces template-assisted growth of cesium-formamidinium lead iodide, Cs_1-xFA_xPbI₃ thin films by PLD. Implementing a lead iodide (PbI₂) + Cs_xFA_1-xPbI₃ tailored template, phase-pure Cs_xFA_1-xPbI₃ films with uniform coverage on both planar and textured substrates were achieved. Compositional analysis via X-ray fluorescence confirmed near-stoichiometric transfer of the inorganic cations (Cs/Pb), with identical Cs_0.2FA_0.8PbI₃ composition and a bandgap of 1.58 eV achieved in templated and non-templated films. However, the template is essential for stabilizing the photoactive cubic (α-) phase and achieving functional devices. The methodology remains robust under more demanding conditions, including increased deposition rates (up to 18 nm min^-1) and application to textured substrates. These results provide valuable insights for the development of scalable, single-source, fully vapor-phase processed MHPs.

Driven by the field’s increasing push toward faster and scalable vapor-phase deposition, Chapter 5, demonstrates the rapid growth of PbI₂/CsBr scaffolds by PLD. These inorganic scaffolds serve as templates for subsequent solution-based conversion into perovskite absorbers and are fully compatible with textured silicon substrates, making them directly relevant for tandem solar cells. Using PLD, we achieve the first vapor-phase growth of PbI₂/CsBr scaffolds on textured silicon at deposition rates exceeding 100 nm min^-1, more than an order of magnitude faster than typical co-evaporation. Our results show that PLD offers a powerful alternative: rapid, single-source deposition of stoichiometric inorganic scaffolds with tunable morphology and conformal coverage on textured interfaces. Of particular significance, we demonstrate the first PVK/Si tandem device incorporating a PLD-derived scaffold, achieving 25.9% power conversion efficiency on 1 cm² devices after solution-phase conversion. These findings position PLD as a promising scalable technique for tandem photovoltaics fabrication.

Chapter 6, addresses sustainability challenges associated with the widespread single use of commercial glass/ITO substrates in perovskite research. Despite the rapid progress in perovskite solar cells (PSCs), most laboratories discard these substrates after fabrication, characterization, and basic stability testing, an unsustainable practice given the scarcity of indium. We present a simple and effective acetone-based protocol for recovering and reusing glass/ITO substrates. The method integrates seamlessly into standard cleaning routines without the need for specialized chemicals or equipment. Solar cells fabricated on reused substrates exhibit performance comparable to those built on fresh ones, even after three reuse cycles. Supporting life-cycle assessment (LCA) analysis shows that even a single reuse cycle reduces greenhouse gas emission relative to single-use practices. Compared with previously reported approaches, our method avoids toxic solvents and offers additional environmental benefits when paired with solvent recovery or larger-batch processing. This work demonstrates that substrate reuse is a practical and impactful strategy for reducing material waste and improving sustainability in perovskite PV research.

Chapter 7 positions the findings of this thesis within the broader context of the field and outlines promising directions for future research. Across all chapters, a common conclusion emerges: microstructure is not a secondary detail but a central design parameter that governs material behavior and device performance in the next generation photovoltaic technologies and beyond.