Tin/lead (Sn/Pb) iodide perovskites (PVKs) have emerged as promising absorber layer materials for high-efficiency tandem solar cells due to their low cost, high light absorption coefficients, and narrow band gaps. However, current solvent-based synthesis techniques offer poor scalability, in turn hindering commercialization.

This study introduces sequential thermal evaporation (STE) as a scalable method for producing narrow band gap Sn/Pb iodide PVK absorber layers for application in solar cells. The produced thin films show low doping densities, charge carrier mobilities close to 100 cm2/Vs, and charge carrier lifetimes of over 2 μs. Contrary to the established convention in solvent-based synthesis, no additives were required.

An alloy of precursors (PbSnI4) was used in the deposition, lowering the number of required
sources and increasing the production rate. Optimal annealing temperatures for FAPb0.5Sn0.5I3 and Cs0.05FA0.95Pb0.5Sn0.5I3 produced via STE were determined at 200 ◦C, showing significant improvements in charge carrier mobilities and lifetimes compared to lower annealing temperatures. The drastic increase in performance was ascribed to a recrystallization mechanism. Contrary to spin coating-based research, the introduction of cesium into the PVK structure led to reduced charge carrier mobility and lifetime. The underlying mechanism remains unclear. Addition of tin(II)fluoride (SnF2) led to reduced charge carrier mobilities and lifetimes, with slight improvement in morphology. However, its direct effects were uncertain, questioning its necessity in vacuum deposition methods for Sn-based PVK films.

This works demonstrates the significant potential of STE for the production of near-intrinsic high-performance Sn/Pb iodide PVKs for solar cell applications.

Perovskite solar cell s(PSC) has seen a huge increase in its efficiency, since its introduction in 2009, to 25.5% in 2021.d been introduced in 2009 and had seen a huge increase in their efficiency since then. Improving the perovskite–charge transport layer interface and charge extraction can increase the open circuit voltage (Voc) and thereby the efficiency of the PSC. Studying the charge carrier dynamics in the perovskite-charge transport layer interface can give insight into the various processes such as charge extraction, trapping, etc. taking place which can affect the Voc. In this work, the charge carrier dynamics in a bi-layer system comprising of the commonly used perovskite absorber methylammonium lead iodide (MAPI)/ Spiro-MeOTAD hole transport layer (HTL) and MAPI/ C60 electron transport layer (ETL) using time-resolved microwave conductivity (TRMC) is studied. The bi-layer system is excited by a 650 nm pulsed laser and the TRMC measurements are carried out with and without the presence of continuous bias illumination. The obtained TRMC traces are modeled by differential equations involving the various charge carrier dynamics in the bi-layer system to get quantitative information about various parameters like extraction rates, interfacial recombination, trap density, etc. before and after the continuous illumination.
An efficient charge extraction by C60 and Spiro-MeOTAD is observed in the absence of bias illumination. Under bias illumination, trap or defect states are created in MAPI. While the charge extraction by the C60 is not affected after the bias illumination, the hole extraction by the Spiro-MeOTAD decreases. Curve fitting of the TRMC traces reveals that a larger number of trap states are created in the MAPI/ Spiro-MeOTAD bi-layer system along with an increase in the interfacial recombination, indicating that bias illumination also creates trap states in the MAPI/ Spiro-MeOTAD interface affecting the charge kinetics. Lastly, TRMC measurements carried out on MAPI/ Spiro-MeOTAD bi-layer system at a low temperature of 200 K show that charge extraction and interfacial recombination are no longer affected by continuous illumination. This indicates that the defect states created in the MAPI- Spiro-MeOTAD interface is possibly due to mobile ions in MAPI which is prevented at the low temperature due to quenching of ion mobility.

Perovskite photovoltaic (PV) cells have become one of the most highly researched topics in photovoltaics and have achieved unprecedented increases in device efficiencies, but their commercialization remains hindered by their low stability and high toxicity. The currently best performing perovskite PV cells contain lead, a neurotoxic material whose use is prohibited under many national consumer protection laws, thus impeding adoption by industry. A class of materials called double perovskites offer an elegant pathway to lead-free, low-toxicity perovskites for PV cell applications by replacing the Pb²⁺ cation in the perovskite with a mixture of charge 1+ and 3+ cations. A promising double perovskite, Cs₂AgBiBr₆, was first synthesized in 2016 and has been used in the fabrication of PV devices with efficiencies of ≤2.5%. While numerous research groups have attempted various synthesis routes and produced various final materials, little is known about the dynamics of the double perovskite synthesis or the effect of further metal substitution on the material’s optoelectronic properties. In this work, the solution phase synthesis of Cs₂AgBiBr₆ was studied via Density Functional Theory (DFT) and the optoelectronic properties of Cs₂AgSb_xBi_1-xBr₆ thin films were explored, with antimony substitution presented as a method to lower the band gap to make a more favorable perovskite for PV cell applications.

The synthesis method of the thin films involved mixing all of the precursors in DMSO solvent and spin coating. However, only BiBr₃ and SbBr₃ were found to dissolve individually in solution, indicating a sequential pathway to double perovskite crystallites in solution. Geometry optimizations of Bi-Br-DMSO complexes were performed via DFT using the BLYP functional, with COSMO used to approximate a solution phase system. While COSMO was found to be incompatible with the corrected method of calculating the interaction energy, the relatively low (~11%) basis set superposition error was accepted and the uncorrected calculation method was used to find the most stable Bi-Br DMSO complexes in solution. These complexes were analyzed using TD-DFT and the CAM-B3LYP functional to simulate absorbance spectra and match them to experimental solution spectra. While one of the transitions at ~3.9 eV may be ascribed to a larger cluster of [Bi₄Br₂₀]^8-, the source of the stronger experimental transition at ~3.5 eV could not be determined. The dominant electronic transition of the Bi-Br-DMSO system was a metal-to-ligand charge transfer from the 6푠 orbital of the central bismuth ion to the 3푝 orbital of the bromine ligand.

A facile synthesis method reported in literature was attempted for the synthesis of Cs₂AgSb_xBi_1-xBr₆ thin films, described briefly above. The method was found to produce thin films of high crystallinity but with a tendency to degrade upon exposure to ambient conditions, as evidenced by x-ray diffraction (XRD) measurements. A reduced annealing temperature of 90°C rather than 250°C led to the successful substitution of Sb³⁺ for Bi³⁺ in the double perovksite while simultaneously avoiding material degradation (at the cost of optoelectronic performance). Shifts in the lattice parameter of ~0.05 Å and shifts in the absorbance onset energy of ~0.2 eV were found by XRD and absorbance measurements, respectively, for antimony replacement of up to x = 0.7. The optoelectronic properties of the materials were studied using time-resolved microwave conductivity (TRMC) measurements, and showed a decrease in photoconductance of two orders of magnitude and a reduction of charge carrier lifetime as the annealing temperature was lowered from 250°C to 90°C. Low temperature absorbance measurements combined with TRMC measurements indicated that the peak in the absorbance spectra was most likely the result of an excitonic transition.

Metal halide perovskites have attracted a lot of attention over the last decade as a potential low cost alternative to traditional silicon based photovoltaics. Solar cells based on these materials have already achieved power conversion efficiency (PCE) of 22%. However, these high performing compositions are lead containing which is regarded as a potential risk for humans as well as the environment. A lot of research effort has been put into completely replacing lead with other group-14 elements such as tin and germanium, due to their similar sizes and electronic configuration. These kinds of perovskites have shown promising optoelectronic properties but are highly unstable due to the easy oxidation of tin and germanium.
In this thesis an alternative approach of mixing lead with other smaller divalent metal cations is explored. MAPbI3 is synthesized using lead acetate due to the facile removal of byproducts and its tolerance for mixing with other metal salts. The alternate metal salts were selected on the basis of their solubility in commonly used solvents and the suitability of the crystal structure of the precursor compound for perovskite structure formation. We found manganese to be a suitable substituent of lead and the upper limit for these mixed metal perovskites after geometrical calculations as well as experimental verification is found to be around 30%. Though the mixed metal compositions maintain the tetragonal crystal structure of lead based perovskites, a secondary crystalline phase is observed with increased lead substitution. Efforts are made to identify its composition and to remove it by optimizing the thermal treatment as well as the ratio between the other precursors. The optimized recipe for 30% lead substituted showed phase purity as well as good optical and electronic properties. Detailed compositional analysis revealed that, unlike MAPbI3 synthesized using chloride based precursors, in these mixed metal compositions chlorine is also incorporated in the films containing manganese especially near the substrate interface. This suggests that the smaller metal cation has an affinity for the smaller halide anion and that it plays a key role in the initiation of nucleation in such mixed metal (Pb:Mn) compositions. Finally, solar cells were made as a proof of concept incorporating these mixed metal perovskites and devices with up to 1.45% PCE were obtained.