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In this work, the impact of different Cu(In1‐xGax)(Se1‐ySy)2 solar cell structures on the shift of the dominant recombination region at varying light intensities was investigated. A new parameter was proposed to account for the dominant recombination region relative to the minimum band gap location in a graded absorber. Additionally, the influence of shunt resistances on the dominant recombination location for different CIGSSe solar cell structures was modeled. Within the investigated illumination range, correlations between the dominant recombination location and solar cell parameters as well as their temperature dependence were discussed. [ABSTRACT FROM AUTHOR]

The metastable behavior of the fill factor temperature coefficient (βFF,rel) in Cu(In,Ga)(S,Se)2 solar cells is investigated for absorbers with a different Ga content in the front surface, and for different buffer materials. The buffer/absorber interface region is suggested to be the location which causes the metastability in βFF,rel. Temperature dependent current–voltage curves, light‐biased external quantum efficiency, and low temperature capacitance–voltage measurements are performed to characterize the electrical properties and metastable defects in the investigated solar cell devices. The presence of Ga in the absorber front surface, together with the buffer layer material plays a significant role in the metastable behavior. Additionally, energy dispersive X‐ray spectroscopy indicates that the InxSy buffer layers allow for elemental interdiffusion from absorbers with a Ga‐richer front surface, whereas a Zn(O,S) buffer shows to be rather resilient to atomic diffusion. The elemental interdiffusion between the buffer and absorber layers is proposed to be the cause of the defects causing the observed βFF,rel metastability. [ABSTRACT FROM AUTHOR]

The luminescence properties of polar ZnO/(Mg,Zn)O quantum wells (QWs) are determined, besides confinement effects, by a redshift caused by the Stokes shift and the quantum-confined Stark effect (QCSE). We present a comprehensive study of photoluminescence and optical transmission measurements to separate these two effects. Single QW structures have been prepared by pulsed laser deposition on a-plane sapphire exhibiting excitons with and without a distinct QCSE. The QCSE leads to a redshift of the QW luminescence maximum beneath the free exciton energy in ZnO as well as a change of the dynamics from a single exponential decay function to a non-exponential one, well described by a stretched exponential decay function. The internal electric field was evaluated to 0.66 MV/cm. In the absence of an internal electric field, the Stokes shift was determined to drop from 64 meV down to 24 meV with increasing well width. Additionally, with increasing temperature, the QCSE vanishes as the internal electric field is screened by free charge carriers, leading to a drop of the redshift above 150 K to a value comparable to the Stokes shift determined for the QWs without a distinct QCSE. The radiative decay time increases linearly at low temperatures for both types of QWs, indicating free exciton emission as the major recombination channel. [ABSTRACT FROM AUTHOR]

The carrier dynamics of donor-bound and free excitons, localized in the alloy disorder potential, were investigated for MgxZn1-xO (0.08≤x≤0.33) thin films. The measured transients show a fast decrease in the luminescence intensity within the first nanoseconds, followed by a slow, strongly nonexponential decay. Shortly after the excitation pulse, the time-delayed spectra are dominated by the (D0,X) recombination. With increasing time, the free exciton recombination becomes visible on the high-energy side, dominating the spectra at large times after the excitation pulse. By fitting the transients with nonexponential model decay functions, we can deconvolve the luminescence spectra. As expected, the mean decay time of the excitons localized in the alloy disorder potential significantly increases with increasing Mg content. [ABSTRACT FROM AUTHOR]

We report on the experimental determination of the spontaneous polarization of wurtzite- (Mg,Zn)O by examination of the recombination dynamics of polar ZnO/(Mg,Zn)O quantum wells (QWs). The thickness-dependent decay time of the unscreened single-exciton states inside the QWs was modeled by a self-consistent solution of Schrödinger- and Poisson-equation to deduce the total polarization across the QW for different Mg-contents inside the barriers. By the separation of the piezoelectric components of the polarization, a linear increase in spontaneous polarization with increasing Mg-content x of P/x=(0.15160.015) C/m² was determined for Mgx Zn1-xO. [ABSTRACT FROM AUTHOR]

Fresnoite Ba2TiSi2O8 (BTS) thin films were grown and crystallized in situ using pulsed laser deposition (PLD) with CO2 laser direct heating of the a-plane sapphire (1 1 0) substrates up to 1250 °C. Starting with 775 °C growth temperature, (0 0 1)- and (1 1 0)-textured BTS and BaTiO3 phases, respectively, could be assigned in the films, and the typical fern-like BTS crystallization patterns appear. For higher process temperatures of 1100 to 1250 °C, atomically smooth, terraced surface of the films was found, accompanied by crystalline high-temperature phases of Ba–Ti–Si oxides. HAADF micrographs taken in both scanning transmission electron microscopy and energy-dispersive x-ray spectrometry mode show details of morphology and elemental distribution inside the films and at the interface. To balance the inherent Si deficiency of the BTS films, growth from glassy BTS × 2 SiO2 and BTS × 2.5 SiO2 targets was considered as well. The latter targets are ideal for PLD since the employed glasses possess 100% of the theoretical density and are homogeneous at the atomic scale. [ABSTRACT FROM AUTHOR]

We report on recombination dynamics and screening effects due to photogenerated carriers in poiar ZnO/(Mg,Zn)O single quantum wells. For small well widths (⩽2.5 nm), the unscreened transition energy and decay time can be determined directly since the influence of the internal electric field is negligible due to strong confinement of the carriers. The energetic dependence of the decay time was reproduced by a relaxation model to describe the localization within potential minima due to well width fluctuations. For larger well widths, screening effects have to be taken into account even for moderate excitation densities in the 1 W/cm-2 range. It is very hard to determine the unscreened single-exciton state of these wells with strong quantum-confined Stark effect within reasonable time scales and a significant signal-to-noise ratio. We present an extended relaxation model to determine the decay time of the unscreened single-exciton state. A difference in polarization between ZnO and MgxZn1-xO of ΔP/x = 0.026 C/m² is concluded from the dependence of the decay time on the well width. [ABSTRACT FROM AUTHOR]

A strong quantum confined Stark effect (QCSE) was observed in wedge shaped MgZnO/ZnO quantum wells (QWs) grown by pulsed laser deposition. A reduced laser fluence of 1.8 J/cm2 was used. Reference samples grown at higher standard fluence 2.4 J/cm2 showed only a negligible QCSE. Using off-axis deposition without substrate rotation, a constant composition of the barriers was maintained while varying the well width in a wedge shaped QW. A redshift of the QW luminescence with increasing QW thickness up to 230 meV below the ZnO emission was found, accompanied by an increase in the exciton lifetime from 0.3 ns up to 4.2 μs. [ABSTRACT FROM AUTHOR]

One of the advantages of Cu(In,Ga)(S,Se) 2 (CIGSSe) thin film solar cells is the lower temperature sensitivity of the solar cell parameters as compared to conventional silicon-based technologies. To further improve and to tailor CIGSSe temperature behavior for different environments, it is important to gain a detailed understanding of the microscopic mechanisms involved. The goal of the current study is to elucidate the impact of the individual functional layers within the CIGSSe solar cell stack on the temperature dependence of the overall cell performance. For this purpose, temperature-dependent current-voltage measurements were performed on a selected set of CIGSSe cells, containing a systematic variation of functional layers. The experimental results demonstrate that the absorber layer exhibits the largest influence on the temperature dependence of the power efficiency. An analysis of the contributing parameters revealed that the temperature-dependence of the power efficiency is mainly governed by a change in the open-circuit voltage. The temperature coefficient of the open-circuit voltage is analyzed in terms of the cell's physical parameters, including their minimum band gap and the temperature-dependence of the dark saturation current. • CIGSSe absorber layer is the most influencing layer on the temperature sensitivity of solar cells. • V oc temperature coefficient has the largest impact on the temperature coefficient of P mpp for all studied CIGSSe structures. • Sodium post deposition treatment improves greatly the temperature coefficient of V oc. • V oc deficit is a parameter governing the temperature coefficient of V oc , for a similar gallium grading profile. [ABSTRACT FROM AUTHOR]