Shanti Bijani

The industrialization of perovskite solar cells requires adequate materials and processes to make them economically viable and environmentally sustainable. Despite promising results in terms of power conversion efficiency and operational stability, several hole‐transport layers currently in use still need to prove their industrial feasibility. This work demonstrates the use of nanocrystalline nickel oxide produced through flash infrared annealing (FIRA), considerably reducing the materials cost, production time, energy, and the amount of solvents required for the hole transport layer. X‐ray photoelectron spectroscopy reveals a better conversion to nickel oxide and a higher oxygen‐to‐nickel ratio for the FIRA films as compared to control annealing methods, resulting in higher device efficiency and operational stability. Planar inverted solar cells produced with triple cation perovskite absorber result in 16.7% power conversion efficiency for 1 cm² devices, and 15.9% averaged over an area of 17 cm².

Considerable efforts have been devoted to optimizing and controlling the morphology and electronic properties of lead halide perovskites. The defect density of a perovskite layer strongly depends on the processing conditions. Consequently, the fabrication process of high-quality films is often complex, and reproducibility is a challenge. In this work, we present a methylamine gas-based method to recrystallize perovskite layers of any given quality in a controlled way, leading to millimeter-sized domains. Crystallinity significantly increases upon methylamine treatment, and crystal growth follows a preferred orientation. Photoluminescence- and space-charge limited current measurements show that the trap density halves after recrystallization. Conductive atomic force microscopy measurements show a higher surface conductivity and an improved spatial homogeneity after methylamine treatment. When applied in photodetectors, the improved film quality of the recrystallized films leads to increased detectivities of ≈4 × 10¹¹ Jones compared to 3 × 10⁹ Jones of a reference device. The response time falls from 0.1 to 10⁻⁵ s upon methylamine treatment. Our work, thus, presents a promising route to fabricating reproducible, high-quality perovskite films through well-controllable recrystallization.

Nanostructured single-phase magnetite, goethite, and lepidocrocite thin films with variable surface morphology and degree of cristallinity were anodically electrodeposited in a potentiostatic mode onto titanium substrates under different deposition conditions. Dependence of phase composition, degree of cristallinity, preferred crystallographic orientation and surface morphology of these films on applied potential, electrodeposition electrolytic bath temperature and pH was conducted. X-ray diffraction analysis confirmed that the applied potential have a crucial effect on the phase composition of these films so that on degree crystallinity and prefererred crystallographic orientations. Scanning electron microscopy micrograph images indicated that electrolytic bath pH and temperature play an important role on the surface morphology and on the grain size of the deposited materials.

Deviations of the Sedov-Taylor scaling at three different laser ablation regimes (500 mJ in a 0.8 mm spot, 50 mJ in a 0.8 mm spot and 500 mJ in a 2.5 mm spot) were investigated using Schlieren photography in combination with optical scattering and optical emission spectrometry, among others. For each case, the time evolution of the shock front was related to the formation, expansion and properties of the plasma. Both, the time scale of the different radiative processes and that observed for vapor condensation into nanoparticles and sub-micron particles are compatible with the divergences found between the model and experimental data.

Surface and bulk changes in a Nafion membrane as a result of IL-cation doping (1-butyl-3-methylimidazolium tetrafluoroborate or BMIM⁺BF₄ and phenyltrimethylammonium chloride or TMPA⁺Cl⁻) were studied by X-ray photoelectron spectroscopy (XPS), contact angle, differential scanning calorimetry (DSC) and impedance spectroscopy (IS) measurements performed with dry samples after 24 h in contact with the IL-cations BMIM⁺ and TMPA⁺. IL-cations were selected due to their similar molecular weight and molar volume but different shape, which could facilitate/obstruct the cation incorporation in the Nafion membrane structure by proton/cation exchange mechanism. The surface coverage of the Nafion membrane by the IL-cations was confirmed by XPS analysis and contact angle, while the results obtained by the other two techniques (DSC and IS) seem to indicate differences in thermal and electrical behaviour depending on the doping-cation, being less resistive the Nafion/BMIM⁺ membrane. For that reason, determination of the ion transport number was obtained for this membrane by measuring the membrane or concentration potential with the samples in contact with HCl solutions at different concentrations. The comparison of these results with those obtained for the original Nafion membrane provides information on the effect of IL-cation BMIM⁺ on the transport of H⁺ across wet Nafion/BMIM⁺ doped membranes.

The particulate emission during nanosecond ablation of gold targets was investigated at various fluences (10-100 Jcm⁻²) and vacuum levels (0.05-750 Torr). Atomic emission spectra were acquired during the ablation process and post-mortem characterization of particle spatial distribution was performed using scanning electron microscopy. The discussion of the results in the context of existing theoretical models permitted the identification of four distinct mass removal mechanisms. While the presence, shape and intensity of atomic emission lines is a telltale of the nanoparticle formation process, the fluctuations of the emission signal over a number of laser shots was linked to the production of microscopic debris.

In this work the failure analysis carried out in III-V concentrator multijunction solar cells after a temperature accelerated life test is presented. All the failures appeared have been catastrophic since all the solar cells turned into low shunt resistances. A case study in failure analysis based on characterization by optical microscope, SEM, EDX, EQE and XPS is presented in this paper, revealing metal deterioration in the bus bar and fingers as well as cracks in the semiconductor structure beneath or next to the bus bar. In fact, in regions far from the bus bar the semiconductor structure seems not to be damaged. SEM images have dismissed the presence of metal spikes inside the solar cell structure. Therefore, we think that for these particular solar cells, failures appear mainly as a consequence of a deficient electrolytic growth of the front metallization which also results in failures in the semiconductor structure close to the bus bars.

The present work aims to assess Laser-Induced Plasma Spectrometry (LIPS) as a tool for the characterization of photovoltaic materials. Despite being a well-established technique with applications to many scientific and industrial fields, so far LIPS is little known to the photovoltaic scientific community. The technique allows the rapid characterization of layered samples without sample preparation, in open atmosphere and in real time. In this paper, we assess LIPS ability for the determination of elements that are difficult to analyze by other broadly used techniques, or for producing analytical information from very low-concentration elements. The results of the LIPS characterization of two different samples are presented: 1) a 90 nm, Al-doped ZnO layer deposited on a Si substrate by RF sputtering and 2) a Te-doped GaInP layer grown on GaAs by Metalorganic Vapor Phase Epitaxy. For both cases, the depth profile of the constituent and dopant elements is reported along with details of the experimental setup and the optimization of key parameters. It is remarkable that the longest time of analysis was ~10 s, what, in conjunction with the other characteristics mentioned, makes of LIPS an appealing technique for rapid screening or quality control whether at the lab or at the production line.

Undoped ZnO and Ga:ZnO films deposited onto Si wafer substrates have been prepared and characterized in order to investigate the suitability of Ga:ZnO as a transparent and conductive material. A comparative study between ZnO and Ga:ZnO properties using a wide variety of experimental techniques has been carried out. Our results prove the improvement of electrical and optical properties of Ga:ZnO films with respect to an undoped ZnO one. The interface between the film and the Si substrate has been explored in order to detect specific problems that could hinder an optimum electric contact between them. A very thin and abrupt Si/ZnO interface is observed using different characterization techniques, independently of doping. In spite of a dopant enrichment at the interface, the ZnO electronic band structure seems to smoothly adapt to the Si one.