Annihilation of structural defects in chalcogenide absorber films for high-efficiency solar cells

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.In polycrystalline semiconductor absorbers for thin-film solar cells, structural defects may enhance electron-hole recombination and hence lower the resulting energy conversion efficiency. To be able to efficiently design and optimize fabrication processes that result in high-quality materials, knowledge of the nature of structural defects as well as their formation and annihilation during film growth is essential. Here we show that in co-evaporated Cu(In,Ga)Se-2 absorber films the density of defects is strongly influenced by the reaction path and substrate temperature during film growth. A combination of high-resolution electron microscopy, atomic force microscopy, scanning tunneling microscopy, and X-ray diffraction shows that Cu(In,Ga)Se-2 absorber films deposited at low temperature without a Cu-rich stage suffer from a high density of - partially electronically active - planar defects in the {112} planes. Real-time X-ray diffraction reveals that these faults are nearly completely annihilated during an intermediate Cu-rich process stage with [Cu]/([In] + [Ga]) > 1. Moreover, correlations between real-time diffraction and fluorescence analysis during Cu-Se deposition reveal that rapid defect annihilation starts shortly before the start of segregation of excess Cu-Se at the surface of the Cu(In,Ga)Se-2 film. The presented results hence provide direct insights into the dynamics of the film-quality-improving mechanism

Direct Evaluation of the Quantum Confinement Effect in Single Isolated Ge Nanocrystals

To address the yet open question regarding the nature of quantum confinement in Ge nanocrystals (Ge NCs) we employed scanning tunneling spectroscopy to monitor the electronic structure of individual isolated Ge NCs as a function of their size. The (single-particle) band gaps extracted from the tunneling spectra increase monotonically with decreasing nanocrystal size, irrespective of the capping ligands, manifesting the effect of quantum confinement. Band-gap widening of ∼1 eV with respect to the bulk value was observed for Ge-NCs 3 nm in diameter. The picture emerging from comparison with theoretical calculations and other experimental results is discussed

Mobility–Lifetime Products in MAPbI₃ Films

Photovoltaic solar cells operate under steady-state conditions that are established during the charge carrier excitation and recombination. However, to date no model of the steady-state recombination scenario in halide perovskites has been proposed. In this Letter we present such a model that is based on a single type of recombination center, which is deduced from our measurements of the illumination intensity dependence of the photoconductivity and the ambipolar diffusion length in those materials. The relation between the present results and those from time-resolved measurements, such as photoluminescence that are commonly reported in the literature, is discussed

What Is the Mechanism of MAPbI₃ p‑Doping by I₂? Insights from Optoelectronic Properties

Obtaining insight into, and ultimately control over, electronic doping of halide perovskites may improve tuning of their remarkable optoelectronic properties, reflected in what appear to be low defect densities and as expressed in various charge transport and optical parameters. Doping is important for charge transport because it determines the electrical field within the semiconducting photoabsorber, which strongly affects collection efficiency of photogenerated charges. Here we report on intrinsic doping of methylammonium lead tri-iodide, MAPbI₃, as thin films of the types used for solar cells and LEDs, by I₂ vapor at a level that does not affect the optical absorption and leads to a small (<20 meV, ∼9 nm) red shift in the photoluminescence peak. This I₂ vapor treatment makes the films 10× more electronically conductive in the dark. We show that this change is due to p-type doping because we find their work function to increase by 150 mV with respect to the ionization energy (valence band maximum), which does not change upon I₂ exposure. The majority carrier (hole) diffusion length increases upon doping, making the material less ambipolar. Our results are well-explained by I₂ exposure decreasing the density of donor defects, likely iodide vacancies (V_I) or defect complexes, containing V_I. Invoking iodide interstitials, which are acceptor defects, seems less likely based on calculations of the formation energies of such defects and is in agreement with a recent report on pressed pellets

Unveiling the Mechanisms Ruling the Efficient Hydrogen Evolution Reaction with Mitrofanovite Pt 3 Te 4

By means of electrocatalytic tests, surface-science techniques and density functional theory, we unveil the physicochemical mechanisms ruling the electrocatalytic activity of recently discovered mitrofanovite (Pt3Te4) mineral. Mitrofanovite represents a very promising electrocatalyst candidate for energy-related applications, with a reduction of costs by 47% compared to pure Pt and superior robustness to CO poisoning. We show that Pt3Te4 is a weak topological metal with the Z2 invariant, exhibiting electrical conductivity (∼4 × 106 S/m) comparable with pure Pt. In hydrogen evolution reaction (HER), the electrode based on bulk Pt3Te4 shows a very small overpotential of 46 mV at 10 mA cm–2 and a Tafel slope of 36–49 mV dec–1 associated with the Volmer–Heyrovsky mechanism. The outstanding ambient stability of Pt3Te4 also provides durability of the electrode and long-term stability of its efficient catalytic performances

Compositional and electrical properties of line and planar defects in Cu(In,Ga)Se2 thin films for solar cells - a review

The present review gives an overview of the various reports on properties of line and planar defects in Cu(In,Ga)(S,Se)2 thin films for high-efficiency solar cells. We report results from various analysis techniques applied to characterize these defects at different length scales, which allow for drawing a consistent picture on structural and electronic defect properties. A key finding is atomic reconstruction detected at line and planar defects, which may be one mechanism to reduce excess charge densities and to relax deep-defect states from midgap to shallow energy levels. On the other hand, nonradiative Shockley–Read–Hall recombination is still enhanced with respect to defect-free grain interiors, which is correlated with substantial reduction of luminescence intensities. Comparison of the microscopic electrical properties of planar defects in Cu(In,Ga)(S,Se)2 thin films with two-dimensional device simulations suggest that these defects are one origin of the reduced open-circuit voltage of the photovoltaic device