A universal co-solvent dilution strategy enables facile and cost-effective fabrication of perovskite photovoltaics

Cost management and toxic waste generation are two key issues that must be addressed before the commercialization of perovskite optoelectronic devices. We report a ground-breaking strategy for eco-friendly and cost-effective fabrication of highly efficient perovskitesolar cells. This strategy involves the usage of a high volatility co-solvent, which dilutesperovskite precursors to a lower concentration (1.4 M) solution. More than 70% of toxicwaste and material cost can be reduced. Mechanistic insights reveal ultra-rapid evaporationof the co-solvent together with beneficial alteration of the precursor colloidal chemistryupon dilution with co-solvent, which in-situ studies and theoretical simulations confirm.The co-solvent tuned precursor colloidal properties also contribute to the enhancement ofthe stability of precursor solution, which extends its processing window thus minimizing thewaste. This strategy is universally successful across different perovskite compositions, andscales from small devices to large-scale modules using industrial spin-coating, potentiallyeasing the lab-to-fab translation of perovskite technologies.

Human and eco-toxicological impacts of organometallic halide perovskites

Ryma Iness Benmessaoud

Solar radiation currently represents the most important source of renewable energy. Existing technologies to collect and distribute solar energy are expensive, difficult to manufacture, non-recyclable and not very cost-effective. Recent advances in physics and materials engineering have pointed to photovoltaic (PV) perovskites (or CH3NH3MI3, where M=Pb, Sn) as the future solution for sustainable energy because of their simple fabrication procedure, low price, and high efficiency. Several companies are already building perovskites-based PV devices for commercialization in the near future. Nevertheless, perovskites contain heavy metals, and safety concerns during PV fabrication and transportation have not yet been addressed, not to mention recycling, and environmental hazards in case of any failure of large-area solar panels. This dissertation documents my investigation of the potential health and environmental effects of perovskites. I have performed a "zoom-in approach" on human cells to study gene expression and biochemical changes, and a "zoom-out approach" in vivo to investigate toxicity at an organism level. The time frame during which living cells can be manipulated on the same substrate is short, therefore we selected concentrations in the range of 50 to 200 µg /ml (from 80 to 400 µM)) to observe a measurable effect in vitro. These concentrations may seem relatively high, however, an accidental exposure scenario with such concentrations may occur during material fabrication if safety measures are not taken properly. In the in vitro study using human cells, I have discovered not only a dose- and time-dependent response to perovskites exposure, but also a cell-type dependent effect. Within 24 hours following perovskites exposure, neuronal cells died whereas lung cells became giant and polynucleated. Complementary assays showed that mitochondria were heavily damaged, with an increase in their activity when compared to untreated cells. Furthermore, genome profile studies showed many differences in gene expression levels. Extra analysis identified the co-regulated gene networks rather than individual coding genes, which indicated that some pathways critical to basic cellular functions were heavily affected by perovskite exposure (such as regulation of the metabolism, cell division, cell signaling etc.). To understand more thoroughly, I applied the Scanning Fourier Transform Infrared Microspecrometry (S-FTIRM) to study cell toxicity. This has never been done before. I investigated the biochemical changes displayed on infrared absorption spectra. The results are complementary to other techniques, which makes the use of S-FTIRMS applicable to real-time toxicity studies because of its submolecular sensitivity and high-resolution. I also explored the eco-toxic effects of photovoltaic perovskites on small model organisms: fruit flies (Drosophila melanogaster) and nematodes (Caenorhabditis elegans). In both models, life span and organism development were reduced by the exposure. Moreover, the fertility of young adults was drastically affected. My in vitro and in vivo studies showed that CH3NH3PbI3 and CH3NH3SnI3 are acutely and chronically toxic compounds. The obtained results are conclusive, and encourage the scientific community not only to conduct further tests on more complex organisms, but to first search for novel solutions with non-toxic properties.

EPFL2017

Perovskite Solar Cells with Carbon-Based Electrodes - Quantification of Losses and Strategies to Overcome Them

Dmitry Bogachuk, Ulf Anders Hagfeldt, Jiajia Suo, Bowen Yang

Carbon-based electrodes represent a promising approach to improve stability and up-scalability of perovskite photovoltaics. The temperature at which these contacts are processed defines the absorber grain size of the perovskite solar cell: in cells with low-temperature carbon-based electrodes (L-CPSCs), layer-by-layer deposition is possible, allowing perovskite crystals to be large (>100 nm), while in cells with high-temperature carbon-based contacts (H-CPSCs), crystals are constrained to 10-20 nm in size. To enhance the power conversion efficiency of these devices, the main loss mechanisms are identified for both systems. Measurements of charge carrier lifetime, quasi-Fermi level splitting (QFLS) and light-intensity-dependent behavior, supported by numerical simulations, clearly demonstrate that H-CPSCs strongly suffer from non-radiative losses in the perovskite absorber, primarily due to numerous grain boundaries. In contrast, large crystals of L-CPSCs provide a long carrier lifetime (1.8 mu s) and exceptionally high QFLS of 1.21 eV for an absorber bandgap of 1.6 eV. These favorable characteristics explain the remarkable open-circuit voltage of over 1.1 V in hole-selective layer-free L-CPSCs. However, the low photon absorption and poor charge transport in these cells limit their potential. Finally, effective strategies are provided to reduce non-radiative losses in H-CPSCs, transport losses in L-CPSCs, and to improve photon management in both cell types.

WILEY-V C H VERLAG GMBH2022

Molecular dynamics simulations of nucleation and phase transition in halide perovskites

Paramvir Ahlawat

Perovskite solar cells have emerged as the most promising cheaper photovoltaic technology. Besides solar cells, halide perovskites have a wide range of applications due to their remarkable optoelectronic properties. Starting from 3.8% in 2009, solar to the power conversion efficiency of perovskite solar cells is now >25%, exceeding market leader polycrystalline silicon. These advancements are mainly due to the improvement in their manufacturing process targeted to make better morphology. To this objective, tens of thousands of experimental studies are conducted where a wide range of additives, compositions, and processing conditions are optimized based on a trial and error methodology. Although this approach increased the efficiency, however, perovskite electronics suffer from the problem of stability and reproducibility. Therefore, to industrialize highly efficient perovskite electronics, one needs to understand better and rationalize their fabrication process, i.e., crystallization of halide perovskites. In this thesis, I perform molecular simulations to understand the formation process of halide perovskites. First, I try to lay out the atomic details of the nucleation process of perovskite from the solution. I find that nucleation proceeds in a two-step manner where an intermediate phase forms before the emergence of perovskite crystals from the solution. I observe that monovalent cations can play an essential role in the nucleation process. On this information, simulation-driven experiments are performed by experimental collaborators to demonstrate that the grain size and crystallinity can be tuned by only varying the concentration of monovalent cations in solution. I also attempted to study the effects of different solvents and pseudo-halide additives used to make high-efficiency solar cells. I calculated that by modulating the solute-solvent interactions, one could minimize raw material losses during the coating process. In comparison, pseudo-halide additives are found to passivate the ionic defects and may slow down the growth process that helps to improve the final morphology. Second, I study the solid-solid phase transition in the popular two-step process. I discover a new path to the low-temperature formation of highly efficient metastable cubic phase of formamidinium lead iodide perovskite. I first validate my simulations with in-situ experiments performed by experimental collaborators. Then simulations are employed to design a practical methodology to successfully make thin films of phase pure cubic formamidinium lead iodide at lower temperatures. Moreover, I study the intrinsic stability of this material. I calculate the energy barrier between the metastable photo-active phase and its photo-inactive thermodynamically stable hexagonal phase. I find that once formed; the photo-active phase might be kinetically stable. Additionally, I identify new polymorphs (perovskitoids) of halide perovskites and also study the structural insights of low-dimensional halide perovskites. Overall, I present a fundamental understanding of the crystallization of perovskites which is helpful to design better experiments to make high efficiency and stable perovskite electronics.