CdTe is one of the most promising photovoltaic materials due to its near optimum band gap and a high absorption coefficient. However, the efficiencies of both champion poly-crystalline CdTe ...photovoltaic cells as well as production line modules are still significantly below the theoretical Shockley-Queisser limit of ~30%. Reduction of non-radiative recombination at grain boundaries is believed to be the key to improving the efficiency of polycrystalline CdTe-based solar cells. Here, atomistic-level characterization, including scanning transmission electron microscopy (STEM) and first principles density functional theory (DFT) modeling, is crucial in developing a fundamental understanding of how grain boundaries affect the solar cells’ efficiency.
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•Sputtered MoS2 film showed misoriented layers and intrinsic defect structure.•Sliding tests of MoS2 showed ultralow friction in N2 but high friction in humid air.•MoS2 transfer ...layers were parallel reoriented with formation of MoO3 embedded.•DFT calculations showed dissociated H2O not increasing interlayer binding energy.•DFT calculations showed physisorbed H2O increase friction by forming H bond with S.
Molybdenum disulfide (MoS2) displays low coefficient of friction (COF) in vacuum and under inert atmospheres, but a higher COF occurs under humid atmospheres. A significant aspect of this increase in COF that is still not well established is the decoupling the effects of dissociated and undissociated water molecules. The MoS2 thin films used in this study with an intrinsic defect structure incorporating misoriented and fragmented layers as observed by HR-TEM, exhibited an increase in COF from 0.007 in N2 with <2% RH to 0.09–0.11 in N2 (and air) with ~40% RH. Sliding induced transfer layers formed on the counterface revealed a reduction in the spacing between MoS2 layers oriented parallel to the interface. Effects of dissociated and undissociated H2O adsorbed between MoS2 layers on the interlayer binding energy (EB) were studied using first principles calculations. Accordingly, H2O can dissociate into 2H and O at a triple vacancy site and form MoOMo bonds, but this process would not change EB and had a minor effect on friction. An undissociated H2O molecule physisorbed between these layers formed H bond with S atoms increasing EB, a tribochemical mechanism that had a pronounced effect on increasing the COF of MoS2.
Impurity energy levels in the band gap can have serious consequences for a semiconductor's performance as a photovoltaic absorber. Data-driven approaches can help accelerate the prediction of point ...defect properties in common semiconductors, and thus lead to the identification of potential deep lying impurity states. In this work, we use density functional theory (DFT) to compute defect formation energies and charge transition levels of hundreds of impurities in CdX chalcogenide compounds, where X = Te, Se or S. We apply machine learning techniques on the DFT data and develop on-demand predictive models for the formation energy and relevant transition levels of any impurity atom in any site. The trained ML models are general and accurate enough to predict the properties of any possible point defects in any Cd-based chalcogenide, as we prove by testing on a few selected defects in mixed chalcogen compounds CdTe 0.5 Se 0.5 and CdSe 0.5 S 0.5 . The ML framework used in this work can be extended to any class of semiconductors.
CdTe based thin film solar cells have shown to be competitive in terms of efficiency and low cost, but the polycrystalline structure and low minority carrier lifetime constrain CdTe based devices ...from reaching the theoretical efficiency limit. In this work, the effect of Se and Cl segregation in polycrystalline CdSeTe photovoltaic devices was studied. We demonstrated using two-photon time-resolved photoluminescence (TRPL) that the Se alloyed CdSeTe absorber layer shows high minority carrier lifetime, and used density functional theory (DFT) calculations to explain the origin of such high lifetime.
Despite challenges to control stoichiometry in the vanadium-sulfur system, template-free growth of patrónite, VS4, thin films is demonstrated for the first time. A novel atomic layer deposition (ALD) ...process enables the growth of phase pure films and the study of electrical and vibrational properties of the quasi-one-dimensional (1D) transition metal sulfide. Self-limiting surface chemistry during ALD of VS4 is established via in situ quartz crystal microbalance and quadrupole mass spectrometry between 150 to 200 °C. The V precursor, unconventionally, sheds all organic components in the first half-cycle, while the H2S half-cycle generates the disulfide dimer moiety, S2-2, and oxidizes V3+ to V4+. X-ray analysis establishes VS4 crystallinity and phase purity, as well as a self-limiting growth rate of 0.33 Å/cy, modest roughness (2.4 nm) and expected density (2.7g/cm3 ). Phase pure films enable a new assignment of vibrational modes and corresponding Raman activity of VS4 that is corroborated by density functional theory (DFT) calculations. Lastly, at elevated growth temperatures, a change in the surface mechanism provides a synthetic route to a second vanadium-sulfur phase, V2S3.
A fundamental understanding of the role of dopants in electronic structure of polycrystalline CdTe may lead to efficiency improvements. In the present work, we investigated effect of Cl, P and S ...doping on CdTe grain boundaries using first principles density functional theory (DFT) calculations. In addition to already known Cl, P and S can segregate in grain boundaries and incorporation of these elements can effectively reduce midgap states to increase the photovoltaic efficiency of CdTe. The methodology we presented can be used to design other alloying elements to CdTe to improve photovoltaic efficiency towards reaching the theoretical limit.
A fundamental understanding of the role of vacancies, interstitials, dislocations and grain boundaries on the electronic structure of CdTe may lead to efficiency improvements. Atomistic-level ...characterization, including microscopy and first principles modeling, is crucial in developing such a fundamental understanding. In the present work, we built atomistic grain boundary and dislocation core models directly from the STEM images using image analysis methods and crystallographic information at the interface. Grain boundaries are modeled using first principles density functional theory (DFT) calculations. Electronic structures of large-scale grain models are also computed with an accurate hybrid functional (HSE06). We report the electronic density of states (DOS) and electrostatic potential profiles of different CdTe grain boundaries to understand charge carrier interactions. Thermodynamics of point defects and pairs of point defects that can exist on or near grain boundaries are studied and pertaining changes in electronic structure are reported. The implications of these electronic structure changes at grain boundaries on photovoltaic performance, and corresponding strategies to improve performance, are discussed.
Grain boundaries (GB) in poly-CdTe solar cells play an important role in species diffusion, segregation, defect formation, and carrier recombination. While the creation of specific high-symmetry ...interfaces can be straight forward, the creation of general GB structures in many material systems is difficult if periodic boundary conditions are to be enforced. Here we describe a novel algorithm and implementation to generate initial general GB structures for CdTe in an automated way, and we investigate some of these structures using density functional theory (DFT). Example structures include those with bi-crystals already fabricated for comparison, and those planning to be investigated in the future.
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