Luminescent and electronic properties of perovskite solar cells

Perovskite solar cells show great promise to serve as an alternative for traditional silicon-based solar cells, however several problems present obstacles for their commercialization. Standard perovskite cells contain lead, which is toxic to humans and harmful to the environment. Alternative perovskite compositions offer the opportunity to replace lead with non-toxic metals. Lead can be directly replaced by substituting it for tin. Unfortunately, tin-based perovskite solar cells do not show the same efficiency or stability as their lead-based counterparts. Using numerical modeling, a study of a current optimized, tin-based solar cell was performed to understand its efficiency limitations and where the most gains can be acquired. While the short-circuit current of the device is high, its limiting factors arise from those already known, primarily the alignment of the electron transport layer with the perovskite conduction band.Another option for replacing lead is by using a double perovskite. This class of perovskites replaces lead with two different metals that maintain charge neutrality. While providing more compositional freedom compared to the direct replacement of lead, double perovskites offer their own challenges. To better understand transport phenomenon between the transport layers and the perovskite, an investigation into the temperature dependent photoluminescence of Cs2AgBiBr6 with different hole and electron transport layers was performed. No clear trends appear allowing a comparison of the transport layers, but some speculations are provided to explain the results common amongst all the samples.Lead is not the only roadblock for perovskites though, as they also show instability across multiple domains. Two of the most prominent ways instability shows up are closely linked -- ionic motion and reversible degradation. A continuation of an insightful paper by Domanski \emph{et. al.} was performed to further elucidate the effects of ionic movement on reversible degradation processes. We show that these processes can be controlled by the application of a bias voltage and that the enhancement of these processes by illumination is not significant. We continued the examination of these reversible processes by studying their effects under real world conditions. Several perovskite solar cells were subjected to real world conditions in a controlled environment during which we tracked their performance. By comparing the measured data to those calculated if the cells had remained pristine we are able to quantify and separate the reversible from irreversible processes.Finally, a more fundamental investigation was made into the internal mechanisms of the perovskite cells. A temperature dependent study correlating photoluminescence spectra and current-voltage curves was performed revealing different two regimes operation. The potential mechanisms underlying each regime are explored.

Investigation of Lead-free perovskites for Optoelectronic Application

Alexander Fedorovskiy

The search for new energy sources and materials is at the foundations of scientific and technological progress. These two, often interconnected, fundamental elements open a way to create novel designs and improve existing ones for all types of machines and devices.Perovskite solar cells (PSCs) are a great example of this kind of system. Achievements in halide perovskite materials research made a remarkably rapid growth of energy conversion efficiency in PSC technology possible. In turn, a manufacturing process for the technology is potentially cheaper and more straightforward (relative to Si-based). As a result, the PSCs can be a solid background for producing the next generation of solar cells.However, unfortunately, PSCs are still not ready for a market, primarily because of three problems: large-scale production, stability, and toxicity. The first problem is mainly a result of the rawness of the technology, requires mostly resolving engineering tasks, and is not a fundamental barrier. The second problem â stability has a deeper connection to the properties of the material itself. The nature of degradation processes in PSCs is not completely clear. Still, different studies have found some promising ways to stabilize PSCs for a range of thousands of hours. Thus, a combination of engineering and material research can solve this problem, despite its deeper basis.The third problem, toxicity, is a fundamental property of the high efficient PSCs because of its lead-based compound. Lead is one of the critical components of halide perovskites used in the technology, and at the same time, this metal has confirmed toxicity. The water solubility of lead in halide perovskites (primarily as PbI2 and PbBr2) complicates the situation for the whole lifecycle of the devices - from production to recycling. The toxicity of the materials used in PSCs can affect one of the main goals of solar energy: to be produced safe, clean, and sustainable.Taking into account these facts, I focused this thesis on searching for lead-free alternatives for PSCs.Initially, I focused on searching for a model to predict promising vacancy-ordered double perovskites' formability as part of initial screening. I demonstrated the applicability of the geometrical approach based on Goldschmidt's Tolerance Factor for a simple, easy-to-use formability prediction technique for this class of materials. During this part of the study, I also demonstrated a difference between using the method for classic and vacancy-ordered perovskites.In the next part, I used a novel approach based on deep learning to design a new model of perovskite formability. As a result, I successfully forecast perovskite formability with an accuracy of over 98% for the best case. Moreover, I found that the resulting models were able to find some inaccuracies in the original data.Following, I synthesized and researched the structure and properties of tellurium-based vacancy-ordered perovskites. During the research, I found an unexpected complex situation of the optical gap determination for the materials. To solve this challenge, I demonstrated different approaches of absorption spectra fitting and cross-referenced it with other experimental data.

EPFL2021

Goldschmidt's Rules and Strontium Replacement in Lead Halogen Perovskite Solar Cells: Theory and Preliminary Experiments on CH3NH3SrI3

Ulf Anders Hagfeldt, Tor Jesper Jacobsson

During the past few years, organic lead halogen perovskites have emerged as a class of highly promising solar cell materials, with certified solar cell efficiencies now surpassing 20%. Concerns have, however, been raised about the possible environmental and legalization problems associated with a new solar cell technology based on a water-soluble lead compound. Replacing lead in the perovskite structure: with a less toxic element, without degrading the favorable photo physical properties, would therefore be of interest. In this paper, the possibility of replacing lead with other metal ions is explored by following the replacement rules of Goldschmidt together with additional quantum mechanical considerations. This analysis provides a conceptual toolbox toward replacing lead, as well as additional insights into the photo physics of the metal halogen perovskites. This approach is exemplified by focusing on strontium in particular, which is nontoxic and relatively inexpensive. The ionic radius of Sr2+ and Pb2+ are almost identical, suggesting an exchange could be made without affecting the crystal structure. Couple cluster calculations on the metal ions and their halogen salts give the bonding patterns to be sufficiently similar and density functional theory (DFT) revealed the strontium perovskite, CH3NH3SrI3, to be a stable phase, despite the difference in electronegativity between lead and strontium. This is further supported by the existence of binary PM, and SrI2 compounds and the beneficial formation energy of the strontium perovskite. The electronic properties of both CH3NH3SrI3 and CH3NH3PbI3 were simulated and compared, revealing a higher degree of ionic interaction in the metal halogen bound in the strontium perovskite. This is a consequence of the lower electronegativity of strontium, which, together with the lack of d-orbitals in the Valence of Sr2+, results, in a higher band gap. The band gap for the strontium perovskite was estimated to 3.6 eV, which unfortunately is too high for an efficient photo absorber. Initial investigations on experimental synthesis of the strontium perovskite, using wet chemical methods, revealed it to be harder to produce than the lead perovskite This is explained as a:consequence of different bonding patterns in the metal iodine salts, which obstruct the methylammonium intercalation pathway utilized for forming the perovskite. Vapor phase methods are instead suggested as more promising synthesis routes.

American Chemical Society2015

Contact Design for Silicon Heterojunction Solar Cells

Luca Massimiliano Antognini

Today more than ever the world needs clean energy sources and thus a fast deployment and scaling up of the photovoltaic industry. In this context improving solar cell efficiency plays a major role. In order to achieve the maximum single junction efficiency for this material, technologies featuring carrier selective passivating contacts are foreseen to be the next successors of actual industrial cells. Among them, the silicon heterojunction (SHJ) solar cell is a promising technology which demonstrated efficiencies up to 26.7%. Its main specificity consists in the use of intrinsic and doped amorphous silicon (a-Si:H) to provide surface passivation and carrier selectivity to the c-Si, which allows to reach very high operating voltages in comparison to other c-Si based technology. The major drawbacks of SHJ solar cell is the large parasitic absorption occurring in those layers. To address this challenge, a strong focus on alternative non-silicon based thin films with wider band gap has built up in the literature. A first part of this thesis focuses on reviewing the fundamental process behind carrier selectivity and discuss the ability of different models from the literature to draw informations directly from the peculiar shapes of IV curves commonly observed in the solar cells featuring newly developed materials. We first discuss the simple four ideal diodes model of Roe et al to explain the occurrence of S-shapes depending on the contact quality. We discuss from a theoretical point of view how to improve the model by modifying the ideal diodes by other circuit elements as well as adding bulk recombinations. We test the theory experimentally and identified modifications that should be incorporated in the model.In a second part of the thesis, we explore the development of doped nano-crystalline silicon (nc-Si) layers to replace classical amorphous silicon layers. We investigate the influence of TMB and BF3 as dopant sources on the transparency and contact properties of nc-Si:H(p) layers as well as their integration in solar cells. We expose the roles of both gas to modify the crystallinity of the layer and find different optimum for both of them to lead high efficiencies, illustrated by a certified 23.9%-efficient solar cell. We report also on further optimization steps of this front junction architecture resulting in certified efficiency of 24.44%.We explore next the n-type nc-Si layers for application as window layer. In particular, we perform thickness and doping series to unravel the layer properties along its growth direction. In order to improve the low doping of the nucleation zone of nc-Si:H(n) at the i/n interface, a thin a-Si:H(n) buffer layer is introduced and shown to improve the passivation, selectivity and contact resistivity. Finally, we also report on the beneficial effect of an additional SiOx capping layer which improves both the reflection properties as well as all contact properties, broadening the optimal parameter window. The absorption loss in the front silicon layers remains however the highest share of losses, and therefore we explore theoretically the opportunity of using thinner front side layers or using thin TCO / silicon nitride bilayers to reduce the absorption losses. We finish the discussion by presenting an optimization roadmap, showing a possibility for double-side contacted SHJ cells to reach efficiencies above 26% without requiring any patterning or localization step.

EPFL2022