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Current-voltage and capacitance study of light-induced metastabilities in CuZnSnSSe thin-film solar cells
摘要： White, red and blue light-induced metastabilities in Cu2ZnSnS4(Se4) solar cells were investigated by temperature dependent current-voltage measurements, drive level capacitance profiling, impedance and thermal admittance spectroscopy. A set of devices were studied where white and blue light soaking at room temperature led to degradation of the device performance, while after red light soaking the solar cell efficiency did not change. We observed a significant effect of light soaking on capacitance data measured in both low and high-temperature ranges for these devices. In particular, the net doping concentration extracted from drive-level capacitance profiling substantially increased after light soaking treatments. Low and high-temperature capacitance steps observed in the reference capacitance-frequency spectra were assigned to Fermi level pinning and bulk defects, correspondingly. Light soaking with different-wavelength light led to a shift of both steps toward the high-frequency range, and hence a decrease in the thermal admittance activation energies. A low-frequency “inductive” loop was detected in the impedance spectra after light soaking, regardless of wavelength. It was proposed that the appearance of the “inductive” loop is due to the formation of a negative electric field at the highly defected CdS/Cu2ZnSnS4(Se4) hetero-interface. This result also leads us to conclude that such electric field is responsible for the metastable behaviour of these devices at room temperature, while the low temperature metastable changes might have a different origin. We also discuss the methodology for electrical characterization of the metastable solar cells in detail.
关键词： impedance spectroscopy,metastability,kesterite solar cells,electrical characterization
更新于2025-09-23 15:19:57
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Quantifying Large Lattice Relaxations in Photovoltaic Devices
摘要： Temporal variations of Cu(In, Ga)Se2 photovoltaic device properties during light exposure at various temperatures and voltage biases for times up to 100 h are analyzed using the kinetic theory of large lattice relaxations. Open-circuit voltage and p-type doping increased with charge injection and decreased with temperature at low injection conditions. Lattice relaxation can account for both trends and activation energies extracted from the data are approximately 0.9 and 1.2 eV for devices with lower and higher sodium content, respectively. In these devices, increased sodium content resulted in higher initial p-type doping with greater stability. First-principles calculations providing revised activation energies for the (VSe ? VCu) complex suggest that this defect does not account for the metastability observed here.
关键词： sodium content,lattice relaxations,metastability,CIGS,photovoltaic devices
更新于2025-09-23 15:19:57
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Designing Morphotropic Phase Composition in BiFeO <sub/>3</sub>
摘要： In classical morphotropic piezoelectric materials, rhombohedral and tetragonal phase variants can energetically compete to form a mixed phase regime with improved functional properties. While the discovery of morphotropic-like phases in multiferroic BiFeO3 films has broadened this definition, accessing these phase spaces is still typically accomplished through isovalent substitution or heteroepitaxial strain which do not allow for continuous modification of phase composition post-synthesis. Here, we show that it is possible to use low-energy helium implantation to tailor morphotropic phases of epitaxial BiFeO3 films post-synthesis in a continuous and iterative manner. Applying this strain doping approach to morphotropic films creates a new phase space based on internal and external lattice stress that can be seen as an analogue to temperature – composition phase diagrams of classical morphotropic ferroelectric systems.
关键词： ferroelectrics,metastability,implantation,strain,Morphotropic phases
更新于2025-09-19 17:15:36
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Threshold Effects and Metastability in Solitary Refractive Index Wells
摘要： Threshold effects and trapping subsequent to the collision of a projectile wave packet and a target are ubiquitous in quantum mechanics. In nonlinear optics, the interaction between a soliton (S) and a dispersive wave (DW) exhibits similarities to wave-packet scattering and offers means for all-optical control [1]. However, threshold effects akin to those in quantum mechanical potential scattering when a bound-state eigenvalue is close to the continuum, and generation of metastable states due to scattering resonances, are not directly allowed. Such phenomena are of high interest in the context of optical technologies concerning storage and manipulation of optical data. Here, we exploit an elaborate yet fabricable nonlinear waveguide [2] to realize an all-optical analogue of a quantum mechanical potential well in terms of the refractive index change induced by a solitary wave. Besides threshold effects this allows for an unexpected trapping mechanism with the ability to engineer metastable states with controllable lifetimes. Observation of the above effects requires S and DW to copropagate at nearly identical group velocities. Our setup features an alternating sequence of regions of normal dispersion (N1?N3) and anomalous dispersion (A1, A2), exhibiting group delay and group velocity dispersion pro?les shown in Figs. 1(a,b). We use numerical simulations of an unidirectional propagation equation for the analytic signal [3]. Firstly, we demonstrate threshold effects in the collision of a “projectile” DW in A2 and a “target” soliton in A1, manifested by peaks in the fraction of transmitted DW energy (Fig. 1(c)) and culminating in its re?ectionless transmission at integer potential strengths νeff de?ned by the DW center frequency (Fig. 1(d)). Off-threshold, interference with re?ected DW components causes a polychotomous wave train (Fig. 1(e)). As evident from the spectrograms in Figs. 1(h,i), inside S, DW exhibits a time-domain structure resembling the quasi bound state closest in νeff. Secondly, we show that a projectile DW launched in N3, colliding with a target soliton in A1, experiences a partial down-shift to a frequency range in A2, group-velocity matched to the soliton. The down-shifted frequency components perceive the soliton as time-domain potential-well to which they are con?ned. Nonzero overlap of the trapped state with the adjacent region of normal dispersion causes its decay, resulting in metastability. Trapping and decay of a metastable state (Figs. 1(f,j)) might also lead to a huge time-delay (Figs. 1(g,k)). We further observe re?ectionless transmission, total (bounce-off) re?ection and trapping in the collision of unequal solitons residing in different regions of anomalous dispersion. Besides the fundamental aspect of providing a quantum analog system, this might pave the way for new applications in optical technologies such as all-optical manipulation of data.
关键词： soliton,solitary refractive index wells,dispersive wave,metastability,nonlinear optics,threshold effects
更新于2025-09-11 14:15:04
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Concentration of defects responsible for persistent photoconductivity in Cu(In,Ga)Se2: Dependence on material composition
摘要： Persistent photoconductivity PPC in thin Cu(In,Ga)Se2 films is discussed within a model of relaxing defects acting as donors or acceptors depending on their configurational and charge state. The aim of this work is to identify the factors related to technological processes which affect the magnitude of PPC. We established a method of evaluation of the concentration of metastable defects in thin Cu(In,Ga)Se2 films relating it to the position of the Fermi level in thermodynamic equilibrium and used it to compare and discuss the impact of preparation details on the PPC value. The main result is that deviation from Cu/(Ga+In)Se2 stoichiometry does not change the concentration of metastable defects. Post deposition annealing in selenium affects the PPC depending on the presence of sodium during the treatment, while the impact of sodium itself on the metastable defect concentration apparently depends on whether it is present during the Cu(In,Ga)Se2 deposition process or whether it is supplied during post-deposition treatment.
关键词： Thin films,Photoconductivity,Copper indium gallium selenide,Metastability,Defects
更新于2025-09-09 09:28:46