Session D45: Energy - Photovoltaics and Photonics

Oxides related to cadmium telluride solar cells

Polycrystalline cadmium telluride (CdTe) is a leading material in photovoltaic technology due to its high absorption coefficient and near-optimum bandgap of 1.44 eV. It is known that CdTe film processing can promote surface oxidation depending on the growth environment and upon exposure to different oxidation conditions. For example, CdTeO₃ forms when the CdTe film is treated in heated dry air, while in humid air, CdTe₂O₅ is detected. These oxides feature tellurium in the oxidation state +4 compared to the +2 state in CdTe. Other possible relevant oxides are CdO, TeO₂, and TeO₃. Using hybrid density functional calculations, we studied the electronic structure of these oxide materials and their band alignment to CdTe, which are essential parameters in the characterization of the interfaces at grain boundaries. The goal is to understand their stability and possible effects on passivating grain boundaries. The results are compared to the available experimental data.   

Defect diffusion in CdTe and CdSe

Recent progress in the development of CdTe solar cells has involved the use of CdTe absorber layers combined with CdSe and/or CdTe_xSe_1-x layers.  One area of particular interest is the interdiffusion between the CdSe layer and the CdTe layer.  As a first step in understanding this, we have carried out molecular dynamics (MD) simulations of interstitial and defect diffusion in both zincblende CdTe and wurtzite CdSe over a range of temperatures using an empirical bond-order potential which has been fitted to experimental measurements and density functional theory calculations.  These results were then used to determine the effective activation barriers and prefactors for Cd, Te, and Se interstitial diffusion and vacancy diffusion.  Since our MD simulations reveal a wide range of different processes, including hopping and exchange, which depend on temperature and the local environment, we have also carried out additional nudged-elastic-band calculations to determine the activation energies for the key mechanisms.  A summary of these results will be presented along with a discussion of their impact on the kinetics of interdiffusion between CdTe and CdSe.     

Electronic and vibrational properties of quaternary chalcogenides and their structural diversity

Quaternary chalcogenides derived from simpler II-VI zinc-blende systems are of great interest to many applications, including photovoltaics, solar cells, and thermoelectricity. Such materials allow for a diverse distribution of their cation atoms, which enables fine electronic properties tuning while maintaining a low thermal conductivity. Here we report electronic structure simulations of CuZn₂InSe₄, taken as a representative from a larger class of chalcogenides. It is found that the structural diversity due to cation and polyhedral arrangements has direct consequences in the electronic structure, especially for the conduction and valence band shifts. The simulations further indicate that hybrid functionals are needed to account for the s-p and p-d orbital hybridization around the Fermi level. We also find that the low thermal conductivity of all phases is mainly attributed to the direct metal-chalcogen bonds making up the lattice. The phonon dispersion further show that the main scattering channel comes from a low frequency optical band hybridized with acoustic phonons.

    

Substrate Effects When Using Photons to Make Perovskite Solar Cells

To shorten annealing time and minimize substrate heating, flash lamps that deliver short, intense light pulses, commonly called photonic curing, are used to replace thermal annealing using hot plates or ovens. This method is different from equilibrium heating in that the radiant energy is selectively absorbed by the film. The high-intensity light pulses produce a high peak temperature in the film. Because most processes follow Arrhenius behavior, reaching high temperatures allow chemical reactions/phase transformations to occur in a short time, significantly reducing the processing time. Often the results are non-intuitive. We will present fabricating flexible halide perovskite solar cells (PSCs) on indium tin oxide (ITO) coated Willow glass. The effect of ITO transmittance on the photonic curing of nickel nitrate sol-gel precursors into nickel oxide to fabricate hole transport layer and consequently the performance of PSCs. Unexpectedly, ITO samples processed by photonic curing show improved optical and electrical properties.   

In-situ buried interface passivation enables efficient and stable inverted perovskite solar modules

Scaling-up perovskite solar cells (PSCs) is a prerequisite to the adoption of perovskite photovoltaics. However, the performance and stability of perovskite solar modules (PSMs) have lagged behind those of lab-scale PSCs. The development o PSMs requires interfacial passivation, yet this is challenging for the buried interface, owing to the dissolution of passivation agents during perovskite deposition. Here, we overcome this limitation with in-situ buried interface passivation – achieved via directly adding a cyanoacrylic acid-based molecular additive, namely BT-T, into the perovskite precursor solution. Classical and ab-initio molecular dynamics (MD) simulations reveal that BT-T spontaneously self-assembles at the buried interface during perovskite film formation. The preferential buried interface passivation results in facilitated hole transfer and suppressed surface recombination. In addition, residual BT-T molecules in the perovskite layer enhance its stability and homogeneity. We report a power-conversion efficiency (PCE) of 20.3% for inverted-structure PSMs. The encapsulated PSM retains 92.5% of its initial PCE (20.2%) following 1817 h maximum power point (MPP) tracking under light illumination at 65 °C, corresponding to a T80 (time to 80% of initial PCEs) of 4500 h. Our demonstration of operating-stable PSMs under accelerated ageing represents a step closer to the commercialization of this emerging technology.   

Phase Stability, Bandgap correction, and Absorption spectra of highly efficient Tetramethylammonium based Perovskite

Modelling Perovskite for solar cell absorbers has made significant progress recently, resulting in improved theoretical photovoltaic conversion efficiency (PCE). This report models the ground state structure of Pb, Sn, Mg, and Ge-based tetramethylammonium perovskite, considering the spin-orbit coupling effect. Similarly, we explore their electronic structure, thermodynamic stability, and optical absorption spectra. The electronic structure calculations were carried out using the Density functional theory and the GW quasiparticle methods. Similarly, the absorption spectra were calculated from the real and imaginary parts of the dielectric tensor obtained from solving the Bethe-Salpeter equation. The phonon dispersion and density of state reveal the thermodynamic stability of Pb, Sn, and Ge-based perovskite. The reported absorption coefficients were in the order of 10⁶ absorption coefficient, and the spectroscopic limited maximum efficiency reported an increased theoretical power conversion efficiency of about 48%. The high absorption coefficient, high spectroscopic limited maximum efficiency, and low transmittance indicate an exciting prospect for highly efficient non-transparent solar cell absorbers.   

Tunneling and recombination processes in silicon heterojunction solar cells (SHJSCs) observed via spatially selective noise spectroscopy

We have employed state-of-the-art cross-correlation noise spectroscopy (CCNS) to study carrier dynamics in silicon heterojunction solar cells (SHJ SCs) composed of a light absorbing n-doped monocrystalline silicon wafer contacted by passivating layers of i-a-Si:H and doped a-Si:H selective contact layers. Using CCNS, we are able to resolve and characterize four separate noise contributions during operation.  First, we observe shot noise with Fano factor close to unity due to holes tunneling through the np-junction.  Second, a 1/f term connected to local potential fluctuations of charges trapped in a-Si:H defects.  Third, a generation-recombination noise with a time constant between 30-50 μs and attributed to recombination of holes at the interface between the ITO and n-a-Si:H window layer.  Finally, we observe a low-frequency generation-recombination term observed below 100 K which we assign to thermal emission over the ITO/ni-a-Si:H interface barrier.  These results not only indicate that CCNS is capable of revealing otherwise undetectable relaxation process in SHJ SCs and other multi-layer devices, but also that the technique has a spatial selectivity allowing for the identification of the layer or interface where these processes are taking place.

 

    

Author not Attending

An efficient photovoltaic power converter is a critical component of laser power beaming systems. We design a photovoltaic cell that can efficiently trap monochromatic light incident from broad incident angles. The proposed design is built on the concept of a one-way coherent absorber with aperiodic multilayer front- and back-reflectors that enable maximal optical absorption in a thin-film absorbing layer. The front- and back-reflector mirrors are inverse-designed to warrant efficient light trapping for omnidirectional incidence. A realistic design is provided based on GaAs absorbing layer and with index-matched III-V materials for the back-reflector with group IV materials as front-reflector. The proposed device can pave the way for efficient optical power beaming systems.   

Author not Attending

An important metric in increasing the efficiency of photovoltaic cells, and in particular intermediate band solar cells (IBSC), is the absorption efficiency in the active layer. Distributed Bragg reflectors (DBR) on one or both ends of the active layer are an attractive choice to recycle photons and increase absorption efficiency. However, traditional DBR's operate at single wavelength and incident angle. We apply optimization techniques for the inverse design of broadband and wide-angle DBR's operating at a desired spectral range, with thicknesses that vary with each layer. The optimization is applied to a type-II quantum dot IBSC, taking into consideration the strain for the choice of materials.   

Designing Dielectric Nanostructures using Numerical Simulations for Reducing Optical Losses

Anti-reflection coatings (ARCs) are used in a variety of structures and devices including windows, lenses, and solar energy conversion devices with a view to minimizing the optical losses. Nanostructured dielectric materials can function as a cheaper and even more efficient alternative to multilayer ARCs. Nevertheless, development of nanostructures for this purpose relies heavily on the ability to find suitable geometries, crystalline nature, feature dimensions and pore structure. Evaluating all materials parameters experimentally and successfully identifying the optimal structure are generally cumbersome. Finite Difference Time Domain simulations can be used to understand the light matter interactions and design desired nanostructures easily; however, a gap is commonly found between experimental and simulation results. In this presentation, we discuss the results of our study on oxide nanotubes using FDTD simulation. We took into account the scattering process in these nanomaterials and obtained results matching closely with those obtained experimentally. Our approach can be extended to other morphologies and employed to develop novel cost-effective coatings for reducing the losses due to light reflection.   

Machine learning-based analysis of finite element simulations for emulating the light-matter interactions in nanostructured, disordered photoelectrodes

Photoelectrochemical (PEC) solar energy conversion applications rely on chemical reactions driven by photogenerated minority carriers (electrons or holes) at a semiconductor-liquid junction. The optical, electronic, and chemical transport processes characteristic of these PEC reactions occur on independent and generally disparate length scales.  Fabricating electrodes with hierarchical structure can optimize the performance for each of these processes simultaneously.  Disordered materials with dielectric contrast on the length scales of the wavelength of light can trap light in localized modes.  The simplicity of the fabrication alone makes this approach a particularly attractive one for engineering light trapping into scalable photoelectrode structures.  One significant issue is that disordered materials can only be defined by ensemble or statistical parameters (pore diameter, scatterer diameter, relative volume fractions) rather than as precise structures, which results in a real, physical variance intrinsic to the ensemble structure.  Simulations of the properties of a given ensemble (local light absorption, for example) require a large number of examples for generating statistically accurate measurements for those properties.  In this talk, we will describe our recent efforts to use neural network emulation to explore light concentration in a simplified, disordered photonic glass. This system provides an apt model for developing mathematical representations for electrode structure (input) and finite element simulation data (output) to interface with machine learning algorithms. We will also discuss an approach to simplify representations for large-scale simulation data based on principal component analysis.  We will outline the practical application of these models and show how algorithmic predictions can be used to identify the most efficient ensemble configuration for a nanostructured semiconductor photoelectrode based on self-organized colloidal composites. 

    

A numerical study of the impacts of photovoltaic system on underneath water evaporation from a salt-production pond

Photovoltaic (PV) agriculture, a combination of photovoltaic power generation and agricultural activities, is attracting more and more attentions. In this study, novel photovoltaic system is designed to harvest photovoltaic electricity and evaporate sea water to harvest salt simultaneously. A three-dimensional (3-D) numerical model is established with solar radiation field, PV panels and an open-air water evaporation pond. The effects of PV panels with different installing heights and coverage area on the water evaporation are evaluated under windless and crosswind conditions. The results indicate the evaporation rate of water decreases with the installation of PV panels under windless condition, since the water vapor accumulation above water surface lowers the evaporation driven potential. However, with the favour of crosswind, the evaporation rate is enhanced as the vapor is blew away. Besides, the installing height of PV panels greatly impact the water evaporation rate. In specific, lower PV panels lead to lower water evaporation rates. But beyond a certain height, the PV panels no longer affects the evaporation rate, when the hydrodynamic boundary layers on both water surface and PV panels are not interfered. On the other hand, the PV panels prevent the water evaporation via the leeward vortex. The more vortices are generated, the more water vapor is reserved. Therefore, the water evaporation rate is decreased with the increasing coverage area of PV panels. This research provides insights of the effects of PV on water evaporation with wind disturbance, prompting design optimizations of photovoltaic-salt co-generation plants.   

A Novel Implicit Model Determines the Photovoltaic Panel Temperature and Environmental Effects

The subject of this study assesses the photovoltaic (PV) module operating temperature's relation to efficiency via a novel numerical heat transfer model and proposes an implicit equation.

The literature has reported that PV modules with higher operating temperatures impact the PV module efficiency [1,2]. There are dozens of explicit and implicit equations to define the PV module temperature. However, they are not universal, and for each application, it is essential to insert correction coefficients depending on environments and boundary conditions [2]. In this study, we propose a numerical model concerning the solar radiation flux, solar ray direction with respect to the ground level, convective heat transfer coefficient, tilt angle, ambient temperature, PV power output, PV panel efficiency, and environmental properties [3].

The result from the suggested model matches the extant empirical works of related literature. The model is capable of covering a more comprehensive range of PV modules. The module efficiency has a linear relationship to the PV module temperature in the suggested model, which is well-known in the literature [4]. It illustrates that the PV module operating temperature has proportional variations with ambient temperature and solar flux, as reported in previous studies. The implicit proposed equation identifies that heat transfer convection changes with the PV module tilt angle, causing the PV module operating temperature effects.