University thesis:
Dissertation, Universität Freiburg, 2021
Footnote:
Description:
Abstract: Reducing the greenhouse gas emissions of the energy sector is essential to curb the anthropogenic climate change. The present dissertation introduces a novel, in-situ solar cell and module design for photovoltaic energy conversion that enables to closely approach the lower boundary of the CO2-eq footprint given by the glass substrate. The potential of this technology to further reduce the degree non-sustainability of the global PV industry is outlined in detail in this dissertation. On this basis, the dissertation focusses on the scientific challenges of realizing highly efficient in-situ PV device in particular the specific crystallization kinetics and physical phenomena at the interfaces. <br>The fully printable solar cell architecture is based on a monolithic, mesoporous electrode stack which is effectively glass-encapsulated by a durable glass-frit side-encapsulation and a glass front- and back-pane. The high-efficient perovskite photoabsorber is injected in liquid form as a last processing step and crystallizes in-situ inside the prefab cell or module.<br>This novel perovskite solar cell (PSC) design approach poses a range of challenges: First, the presence of the electrode stack, including a thick opaque carbon-graphite layer and glass panes, hampers the characterization of the physical processes during crystallization and device operation. Second, while ample studies have been carried out on perovskite crystal formation from liquid precursors in free-standing films, little reports exist on perovskite crystal formation in mesoporous networks and the control of perovskite crystallization in thin (10 µm) capillaries as represented by the glass-glass encapsulation in the in-situ concept. <br>A main focus of this dissertation was the development of several advanced characterization methods and physical models to study and understand the perovskite crystallization, the interface formation with the charge extraction layers, and the charge carrier recombination. The acquired understanding served as a feed-back to inspire the development of new strategies for the formation of high-performing perovskite crystal layers. <br>With the aim to shine light on the crystallization kinetics, a method to observe for the first time the evolution of the photocurrent of a semiconductor during crystal formation in real-time was developed. This is made possible by the mesoporous layer stack, which enables measuring the electric current throughout all stages of crystallization from the liquid precursor. By combining photocurrent and photoluminescence measurements, the different crystallization stages could be clearly identified, including a so-far overlooked crystallization stage during which the electrical contact between perovskite and charge extraction layer is formed. <br>Complementary to this approach, to analyze the charge extraction of the perovskite layer at high spatial resolution, a method of potentiostatic photoluminescence imaging (PPI) was developed. Therefore, photoluminescence microscope images are recorded at different electrical bias such as open and short circuit. This fast imaging method is especially interesting for liquid-processed solar cells like perovskites. The PPI method allows to detect and distinguish two main challenges for crystal formation of homogenous perovskite film morphology and the charge extraction via the contact layers. <br>Besides charge extraction, interfacial recombination represents the most detrimental loss mechanism in PSC. Therefore, a 2D model to study the interfacial charge-carrier recombination of the perovskite at the mesoporous titania (m-TiO2) interface and the graphite back electrode was developed. The model was verified by a drift-diffusion simulation and complementary experimental measurements. It was found that the conventional 1D energy-band model is not sufficient to adequately represent perovskite/m-TiO2 layers. Moreover, the model provides an explanation for the remarkably high photovoltages of perovskite solar cells with graphite back electrode despite the non-ideal charge selectivity of pristine graphite. <br>Building on the acquired knowledge, a solvent-free perovskite precursor formulation was developed, as conventional, solvent based precursors are not suitable for crystallization in capillary structures. As an alternative, purified perovskite powder was liquefied by exposure to methylamine gas, forming a “molten salt”. This enables dense crystallization inside the mesoporous stack as well as homogeneous crystal layer formation in the in-situ capillary channels. <br>With this approach, the fabrication of printed, graphite-based perovskite solar cells with a certified, stabilized efficiency of 12.6 % could be achieved, which represented the world record for fully printed PV devices. For glass-encapsulated in-situ devices, a certified, stabilized record efficiency of 9.3 % could be obtained. This demonstrates the proof-of-principle of the in-situ approach and paves the way for further up-scaling of the technology