Defect Formation and Healing at Grain Boundaries in Lead-Halide Perovskites

in Journal of Materials Chemistry A (2022)

Polycrystalline perovskite solar cells show high power conversion efficiencies despite the presence of grain boundaries (GBs). The benign nature of GBs on the electronic properties and structural ... [more ▼]

Polycrystalline perovskite solar cells show high power conversion efficiencies despite the presence of grain boundaries (GBs). The benign nature of GBs on the electronic properties and structural stability in metal-halide perovskites contradicts their propensity for point defect formation, a controversy that is far from being understood. In this work, we combine ab initio molecular dynamics and density functional theory calculations on the S5[130] GB of cesium lead iodide, CsPbI3, to shed light on the structural and electronic properties of such GBs. Our results present the first evidence of structural healing of GBs driven by the facile migration of iodine ions, resulting in stabilized GB structures with reduced hole trap states and shallow electron trap states by strain-induced Pb–Pb dimers. Drift-diffusion simulations reveal that shallow electron trap states in GB mainly lower open-circuit voltage by enhanced non-radiative recombination. Finally, we observe the spontaneous formation of iodine Frenkel defects with reduced formation energies compared to the perovskite bulk. Overall, our study reveals a controversy of GBs showing a moderate impact on the electronic properties by structural healing but a detrimental impact on the point defect densities, both being connected to the facile migration of iodine ions in GBs. [less ▲]

Photoluminescence assessment of materials for solar cell absorbers

in Faraday Discussions (2022)

Absolute photoluminescence measurements present a tool to predict the quality of photovoltaic absorber materials before finishing the solar cells. Quasi Fermi level splitting predicts the maximal open ... [more ▼]

Absolute photoluminescence measurements present a tool to predict the quality of photovoltaic absorber materials before finishing the solar cells. Quasi Fermi level splitting predicts the maximal open circuit voltage. However, various methods to extract quasi Fermi level splitting are plagued by systematic errors in the range of 10–20 meV. It is important to differentiate between the radiative loss and the shift of the emission maximum. They are not the same and when using the emission maximum as the “radiative” band gap to extract the quasi Fermi level splitting from the radiative efficiency, the quasi Fermi level splitting is 10 to 40 meV too low for a typical broadening of the emission spectrum. However, radiative efficiency presents an ideal tool to compare different materials without determining the quasi Fermi level splitting. For comparison with the open circuit voltage, a fit of the high energy slope to generalised Planck’s law gives more reliable results if the fitted temperature, i.e. the slope of the high energy part, is close to the actual measurement temperature. Generalised Planck’s law also allows the extraction of a non-absolute absorptance spectrum, which enables a comparison between the emission maximum energy and the absorption edge. We discuss the errors and the indications when they are negligible and when not. [less ▲]

Application of carotenoids in sustainable energy and green electronics

in Materials Advances (2022)

In sustainable development, one of the key factors is the usage of non-toxic and biodegradable natural substances for the development of green energy and technology. Photovoltaics and photoelectrochemical ... [more ▼]

In sustainable development, one of the key factors is the usage of non-toxic and biodegradable natural substances for the development of green energy and technology. Photovoltaics and photoelectrochemical cells are the most promising candidates for renewable solar energy harvesting. Currently, photovoltaics and electronic device technologies are dominated by inorganic semiconductors. Thus, there is a continuous pursuit for alternative eco-friendly semiconductors both for electronics industry and renewable energy technologies. Carotenoids are naturally found coloured pigments which are of semiconducting nature. In nature, they form an integral part of light-harvesting photosystems in photosynthetic organisms, and are also present in several non-photosynthetic organisms playing other functional roles. The structural diversity of carotenoids and their molecular nature make them ideal candidates for devices requiring photo-sensitization, and controlled electrical conduction. Here, we review the potential application of carotenoids, with their advantages, limitations and prospects for further improvement in solar cell technology, photoelectrochemical cells, semiconductor surface modification, and organic electronics in general. We emphasize on the carotenoid based renewable energy production by using carotenoid based dye-sensitized solar cells and water-splitting photoelectrochemical cells. Thereafter, we summarize some of the studies with carotenoid based nanowire, light-emitting diodes, transistors and light sensors. Though, the application of carotenoids in renewable and sustainable technologies are still in nascent stage, prospects are high for carotenoid based solar harvesting and electronic devices that are cheaper, eco-friendly, biodegradable and biocompatible. [less ▲]

Methylamine Gas Treatment Affords Improving Semitransparency, Efficiency, and Stability of CH3NH3PbBr3-Based Perovskite Solar Cells

in Solar RRL (2021)

High bandgap semitransparent solar cells based on CH3NH3PbBr3 perovskites are attractive for building integration, tandem cells, and electrochemical applications. The lack of control of the CH3NH3PbBr3 ... [more ▼]

High bandgap semitransparent solar cells based on CH3NH3PbBr3 perovskites are attractive for building integration, tandem cells, and electrochemical applications. The lack of control of the CH3NH3PbBr3 perovskite growth limit the exploitation of CH3NH3PbBr3-based perovskite solar cells. Herein, a posttreatment is carried out after the initial CH3NH3PbBr3 crystallization based on methylamine gas that drastically enhances the perovskite quality leading to a highly crystalline film with improved average visible transmittance (AVT) close to 56%. Opaque devices showed outstanding results in terms of open-circuit voltage and power conversion efficiency (PCE) reaching 1.54 V and 9.2%, respectively. These achievements are ascribed to a film with reduced morphological defects and better interface quality and reduced nonradiative pathways. For the first time, the fabrication of semitransparent CH3NH3PbBr3-based solar cells is demonstrated reaching a maximum PCE equal to 7.6%, an AVT of the full stack device of 52%, and an excellent light stability at maximum-power point tracking. [less ▲]

Tuning halide perovskite energy levels

in Energy and Environmental Science (2020)

Solar energy is playing a significant role in the development of a world powered by clean energy sources. In this context, halide perovskite solar cells (PSCs) are considered one of the most promising ... [more ▼]

Solar energy is playing a significant role in the development of a world powered by clean energy sources. In this context, halide perovskite solar cells (PSCs) are considered one of the most promising research lines thanks to their high efficiencies and flexibility, combined with an easy and cheap fabrication process. The possibility of combining different materials and compositions is an excellent advantage of PSCs. However, still, a big limit is posed by the need for a proper energy level alignment between the layers of materials comprising devices. Therefore, it is of utmost interest to develop methods allowing to tune the energy levels of the different materials. In semiconductors physics, a common technique to achieve this purpose is to functionalize the surface of the materials with dipolar molecules. Nevertheless, this has been rarely applied to perovskites because of the highly rough surface of the films. In this study, we show that it is possible to use this technique in hybrid organic–inorganic perovskite semiconductors systematically and tune the direction and magnitude of the shift by controlling the deposition process. These findings offer a toolbox to simplify the application of halide perovskites in optoelectronic devices. [less ▲]

Optical frequency comb based on nonlinear spectral broadening of a phase modulated comb source driven by dual offset locked carriers

in Optics Letters (2020)

We demonstrate a versatile technique to generate a broadband optical frequency comb source in the C-band. This is accomplished by nonlinear spectral broadening of a phase modulated comb source driven by ... [more ▼]

We demonstrate a versatile technique to generate a broadband optical frequency comb source in the C-band. This is accomplished by nonlinear spectral broadening of a phase modulated comb source driven by dual frequency offset locked carriers. The locking is achieved by setting up a heterodyne optical frequency locked loop to lock two phase modulated electro-optic 25 GHz frequency combs sourced from individual seed carriers offset by 100 GHz, to within 6.7 MHz of each other.We realize spectral broadening in highly nonlinear fiber after suitable amplification to obtain an equalized, nonlinearly broadened frequency comb.We obtain ~86 lines in a 20 dB band spanning over 2 THz. [less ▲]

Interface Electrostatics of Solid-State Dye-Sensitized Solar Cells: A Joint Drift-Diffusion and Density Functional Theory Study

in The Journal of Physical Chemistry C (2019)

Dye-sensitized solar cells (DSCs) have gained great attention in recent years due to their low-cost fabrication, flexibility, and high power conversion efficiency. In a DSC, due to interfaces between the ... [more ▼]

Dye-sensitized solar cells (DSCs) have gained great attention in recent years due to their low-cost fabrication, flexibility, and high power conversion efficiency. In a DSC, due to interfaces between the dye and the chargetransport materials, the interface electrostatics becomes a key factor determining the overall performance of the cell. Liquid-electrolyte-based DSCs suffer from low stability, electrolyte leakage, and, in some cases, electrode corrosion. Replacing liquid electrolyte with a solid semiconducting material leads to poor interfacial contacts, hence the interface electrostatics becomes one of the limiting factors. In this work, we present a drift-diffusion and density functional theory (DFT) study of solid-state DSCs to investigate the electrostatics at the TiO2/organic dye/Spiro-OMeTAD interface and its impact on the adsorbed dye energy levels, its absorption spectrum, and the related charge injection. In our three-dimensional drift-diffusion model, we solve a set of drift-diffusion equations coupled to Poisson equation for electrons, holes, doping impurities, and interface traps simultaneously. After that, we use first-principles DFT modeling of dye-sensitized interfaces in the presence of the calculated electric fields. We find that interface traps located below the conduction band edge of mesoporous TiO2 influence the accumulation of photogenerated holes and built-in electric field near the interface. The built-in electric field leads to change in the energetics at the dye/TiO2 interface, leading to poor charge injection from excited dye into TiO2. The simulations were carried out for different electronic trap densities in TiO2 and different doping levels in the Spiro-OMeTAD hole-transport layer. This study helps to a better understanding of the interface electrostatics and its role in the charge injection mechanism of solid-state DSCs. [less ▲]

Drift-diffusion and Machine Learning for High Efficiency Perovskite-Perovskite based Tandem Solar Cells

Poster (2019, May)

Simulation of ion migration in perovskite solar cells using a kinetic Monte Carlo/drift diffusion numerical model and analysis of the impact on device performance

in International Conference on Hybrid and Organic Photovoltaics Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV18), Oral Presentation 099 (2018)

Perovskite solar cells have gathered a large interest in the last years as a very compelling and promising photovoltaic technology thanks to many interesting properties such as a wide spectrum of ... [more ▼]

Perovskite solar cells have gathered a large interest in the last years as a very compelling and promising photovoltaic technology thanks to many interesting properties such as a wide spectrum of deposition techniques, a simple integration with both organic and inorganic materials and, most important of all, a high light power conversion efficiency. Perovskite materials have also challenged the scientific community due to the many different physical processes that concur to set the optical and electrical properties: from ferroelectricity [1], to ion migration [2], defects and different recombination processes [3]. An important aspect of perovskite films is the presence of grain and grain boundaries. Although many progresses have been obtained in the quality of the film, still grain boundaries within the perovskite film in fabricated devices are present. The effect of these grain boundaries have been investigated by many groups, we refer here to just one reference [4], but the effect of these grain boundaries to free charges and ion migration is still under debate. In [4] it has been demonstrated as the change in average size of grains have an important impact on cell performance and ionic diffusion. In the present work we theoretically investigate the effect of ion migration with the presence of grain boundaries. We make a multiscale simulation tool based on kinetic Monte Carlo (for ion dynamics) [5] and drift diffusion based on finite elements (for the electrical part) [6] to understand how the different ion diffusion mechanisms can impact the final cell performance. References [1] A. Pecchia et al., Nano Lett., 16, 988 (2016) [2] J. M. Azpiroz et al., Energy & Environmental Science, 8, 2118-2127 (2015) [3] L. M. Herz, Annual Rev. Phys. Chemistry, 67, 65-89 (2016) [4] B. Roose et al., Nano Energy, 39, 24-29 (2017) [5] T. Albes, A. Gagliardi, Physical Chemistry Chemical Physics, 19 (31), 20974-20983 (2017) [6] A. Gagliardi and A. Abate, ACS Energy Lett., 3, 163 (2018) [less ▲]

Solar pumped Nd:YAG laser systems

in Tezpur University (2016, November)

Solar radiation from sunlight is abundant on our earth during daytime. In order to utilise it, here we report a new design and development of solar pumped Nd:YAG laser system. Sun Light is collected using ... [more ▼]

Solar radiation from sunlight is abundant on our earth during daytime. In order to utilise it, here we report a new design and development of solar pumped Nd:YAG laser system. Sun Light is collected using a large Fresnel lens concentrator of diameter 46 cm, which is then used for side pumping of the Nd:YAG laser rod. Horizontal cavity design with a variable cavity length of 14 cm to 25 cm is proposed in which it accommodates mirror, output coupler, laser rod as well the cooling mechanism. Water cooling is employed to remove extra heat generated around the laser rod. The developed system is low cost and portable. Experimental results of the measurement are solar radiation spectrum, laser line width and output power of laser. Successfully Nd:YAG laser emissions with Full Width Half Maximum of about 1.67 nm at 1064 nm wavelength and Output power of the system has been achieved up to 200 mW. The potential applications of the solar powered laser are laboratory experiments, generation of second harmonic and imaging. [less ▲]