Klaus Schwarzburg

Effect of internal surface area on the performance of ZnO∕In2S3∕CuSCN solar cells with extremely thin absorber

Solar cells with an extremely thin light absorber were realized by wet chemical preparation on arrays of ZnO nanorods. The absorber consisted of an In2S3 layer (∼20nm thickness) and its interface region with a transparent CuSCN hole conductor. By changing the length of the nanorods (0–3.3μm) and keeping the In2S3 layer thickness constant at ∼20nm, the short circuit current increased from about 2–10mA∕cm2. A marked increase of the external quantum efficiency at longer wavelengths is attributed to light scattering and a solar energy conversion efficiency of 2.5% has been demonstrated.

Current-voltage characteristics and transport mechanism of solar cells based on ZnO nanorods/In2S3∕CuSCN

Temperature dependent current-voltage characteristics in the dark and under illumination have been analyzed on up to 3.2% efficient solar cells with extremely thin absorber based on ZnO nanorods/In2S3∕CuSCN structures. The diode ideality factor and the open circuit voltage are strongly influenced on a thermal activation process. Significant enhancement of the devices efficiency by annealing at moderate temperatures has been demonstrated. After this annealing, the activation energy of the saturation current increased from 1.00to1.46eV (in the dark). Transport mechanisms at the In2S3∕CuSCN interface region are discussed.

Formation of the charge selective contact in solar cells with extremely thin absorber based on ZnO-nanorod/In2S3/CuSCN

Annealed and not-annealed solar cells with extremely thin absorber based on ZnO-nanorod /In2S3/CuSCN structures have been compared. Significantly higher external quantum efficiencies have been recorded on annealed devices. The temperature dependent current-voltage characteristics in the dark and under illumination were analyzed in detail. The short-circuit current density increased with the temperature and depended on the light intensity by a power law with a power coefficient of 0.85 that was independent of the annealing or measurement temperature. The temperature dependence of the ideality factor dominated the temperature dependencies of the diode saturation current density and of the open circuit voltage. The activation energies increased strongly after annealing. We propose that the limiting charge selective contact is driven away from the highly defective In2S3/CuSCN interface into the In2S3 layer due to stimulated by the annealing Cu diffusion.

New Efficiency Frontiers with Wafer-Bonded Multi-Junction Solar Cells.

Triple-junction (3J) solar cells will soon be history. The next generation of multi-junction (MJ) devices are now reaching efficiencies far beyond the record levels of 3J cells on Germanium. In this paper we present results of a 4J wafer-bonded solar cell with bandgaps 1.88 / 1.45 // 1.10 / 0.73 eV measured with an improved efficiency of 46.5% at 324x by Fraunhofer ISE. Design for cost has, from the outset, been a priority with the development of engineered substrates to replace costly and low yielding InP substrates, a product building on Soitec’s proprietary SmartCut technology. Wafer bonding enables the electro-mechanical combination of lattice and thermal expansion mismatched materials with electrically conductive, transparent bonds, enabling concentrator solar cells to be built from high quality 2J GaInP/GaAs absorbers lattice-matched to GaAs bonded to high quality 2J GaInPAs/GaInAs absorbers lattice matched to InP that operate well at high concentration.

Current-limiting behavior in multijunction solar cells

Experimental measurements on tandem GaInAsP/InGaAs concentrator solar cells are presented that demonstrate how the short-circuit current can shift from that of the higher current subcell to that of the lower current subcell as irradiance increases. Theoretical modeling illustrates how this can occur when the current-limiting subcell has a noticeably nonzero slope in its current-voltage curve near short-circuit, and should be general to all series-connected multijunction cells of this nature.

Lifetime and performance of InGaAsP and InGaAs absorbers for low bandgap tandem solar cells

Time resolved photoluminescence (TRPL) measurements were used to evaluate the lifetimes of the low bandgap absorber materials InGaAsP (1.03 eV) and InGaAs (0.73 eV) embedded between InP barriers. A low bandgap tandem solar cell based on these absorber materials has been developed. The cell is designed to work below an InGaP / GaAs high bandgap tandem solar cell. Tandem solar cells grown with these absorber materials reached efficiencies above 10% (in-house) below a 4-¿m-thick GaAs filter under 35 suns concentration.

Charge separation and recombination in radial ZnO/In2S3/CuSCN heterojunction structures

A ZnO-nanorod/In2S3/CuSCN radial hetero structure has recently shown promising photovoltaic conversion efficiencies. In this work, the charge separation and recombination in single ZnO/ In2S3 and In2S3/CuSCN interfaces as well as the complete ZnO/In2S3/CuSCN structure were studied by time resolved microwave photoconductivity. Photoconductivity transients were measured for different thicknesses of the In2S3 light absorbing layer, under variation of the exciting light flux and before and after annealing of the ZnO nanorods at 450°C. Upon excitation with 532nm light, a long lived (ms) charge separation at the In2S3/ZnO interface was found, whereas no charge separation was present at the In2S3/CuSCN interface. The presence of the CuSCN hole conductor increased the initial amplitude of the TRMC signal of the In2S3/ZnO interface by a factor of 8 for a 6nm thick In2S3 layer, but the enhancement in amplitude dropped strongly for thicker films. The measurements show that the primary charge separation is located at the In2S3/ZnO interface but the charge injection yield into ZnO depends critically on the presence of CuSCN.

On the benchmarking of multi-junction photoelectrochemical fuel generating devices

Photoelectrochemical solar fuel generation is evolving steadily towards devices mature for applications, driven by the development of efficient multi-junction devices. The crucial characteristics deciding over feasibility of an application are efficiency and stability. Benchmarking and reporting routines for these characteristics are, however, not yet on a level of standardisation as in the photovoltaic community, mainly due to the intricacies of the photoelectrochemical dimension. We discuss best practice considerations for benchmarking and propose an alternative efficiency definition that includes stability. Furthermore, we analyse the effects of spectral shaping and anti-reflection properties introduced by catalyst nanoparticles and their impact on design criteria for direct solar fuel generation in monolithic devices.

Excitation correlation photoluminescence in the presence of Shockley-Read-Hall recombination

Excitation correlation photoluminescence (ECPL) measurements are often analyzed in the approximation of a cross correlation of charge carrier populations generated by the two delayed pulses. In semiconductors, this approach is valid for a linear non-radiative recombination path, but not for a non-linear recombination rate as in the general Shockley-Read-Hall recombination scenario. Here, the evolution of the ECPL signal was studied for deep trap recombination following Shockley-Read-Hall statistics. Analytic solutions can be obtained for a fast minority trapping regime and steady state recombination. For the steady state case, our results show that the quadratic radiative term plays only a minor role, and that the shape of the measured signal is mostly determined by the non-linearity of the recombination itself. We find that measurements with unbalanced intense pump and probe pulses can directly provide information about the dominant non-radiative recombination mechanism. The signal traces follow the charge carrier concentrations, despite the complex origins of the signal, thus showing that ECPL can be applied to study charge carrier dynamics in semiconductors without requiring elaborate calculations. The model is compared with measurements on a reference sample with alternating layers of InGaAs/InAlAs that were additionally cross-checked with time resolved optical pump terahertz probe measurements and found to be in excellent agreement.

Combinatorial Investigations of High Temperature CuNb Oxide Phases for Photoelectrochemical Water Splitting.

High-throughput combinatorial methods have been useful in identifying new oxide semiconductors with the potential to be applied to solar water splitting. Most of these techniques have been limited to producing and screening oxide phases formed at temperatures below approximately 550 °C. We report the development of a combinatorial approach to discover and optimize high temperature phases for photoelectrochemical water splitting. As a demonstration material, we chose to produce thin films of high temperature CuNb oxide phases by inkjet printing on two different substrates: fluorine-doped tin oxide and crystalline Si, which required different sample pyrolysis procedures. The selection of pyrolysis parameters, such as temperature/time programs, and the use of oxidizing, nonreactive or reducing atmospheres determines the composition of the thin film materials and their photoelectrochemical performance. XPS, XRD, and SEM analyses were used to determine the composition and oxidation states within the copper niobium oxide phases and to then guide the production of target Cu(1+)Nb(5+)-oxide phases. The charge carrier dynamics of the thin films produced by the inkjet printing are compared with pure CuNbO3 microcrystalline material obtained from inorganic bulk synthesis.