N. Szabó

Photoelectrocatalysis: principles, nanoemitter applications and routes to bio-inspired systems

An overview on processes that are relevant in light-induced fuel generation, such as water photoelectrolysis or carbon dioxide reduction, is given. Considered processes encompass the photophysics of light absorption, excitation energy transfer to catalytically active sites and interfacial reactions at the catalyst/solution phase boundary. The two major routes envisaged for realization of photoelectrocatalytic systems, e.g. bio-inspired single photon catalysis and multiple photon inorganic or hybrid tandem cells, are outlined. For development of efficient tandem cell structures that are based on non-oxidic semiconductors, stabilization strategies are presented. Physical surface passivation is described using the recently introduced nanoemitter concept which is also applicable in photovoltaic (solid state or electrochemical) solar cells and first results with p-Si and p-InP thin films are presented. Solar-to-hydrogen efficiencies reach 12.1% for homoepitaxial InP thin films covered with Rh nanoislands. In the pursuit to develop biologically inspired systems, enzyme adsorption onto electrochemically nanostructured silicon surfaces is presented and tapping mode atomic force microscopy images of heterodimeric enzymes are shown. An outlook towards future envisaged systems is given.

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.

Measurement of an InGaAsP/InGaAs tandem solar cell under GaAs

We have developed a low band gap tandem (two-junction) solar cell lattice-matched to InP, which is designed to work under a InGaP/GaAs tandem in a four-junction configuration. For the top and bottom subcells InGaAsP (Eg = 1.03 eV) and InGaAs (Eg = 0.73 eV) were utilized, respectively. A new tunnel junction was used to connect the subcells, including thin layers of n-type InGaAs and p-type GaAsSb. The delicate critical interfaces were prepared employing metal organic vapor phase epitaxy (MOVPE) and were monitored with optical in-situ spectroscopy (reflectance anisotropy spectroscopy, RAS). After a contamination-free transfer, the in-situ signals were then benchmarked in ultrahigh vacuum (UHV) with surface science techniques. X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED) revealed that the sharpest InGaAs/GaAsSb interface was achieved, when the GaAsSb layer in the tunnel junction of the solar cell was grown on III-rich (2×4)- or (4×2)-reconstructed InGaAs (100) surfaces.

Time and spatially resolved measurement of interface and bulk recombination of low band gap solar cell material

We have investigated the effect of different metal organic vapor phase epitaxy (MOVPE) preparation routes for the In0.53Ga0.47As/InP interface on the interface recombination velocity and its lateral interface homogeneity. The preparation routines in a MOVPE reactor were varied in order to initiate a lateral homogenous layer growth and to form the InGaAs/InP interface as sharp as possible, which is of major importance for the performance of thin device structures such as tunnel junctions in multi junction solar cells. For the growth characterization, we employed in situ reflectance difference/anisotropy spectroscopy and low energy electron diffraction to depict the favourable interface formation routes. As the minority carrier lifetime is a critical measure for the opto-electronic performance of both bulk and interface quality, minority carrier lifetime dependence in a corresponding InP/InGaAs/InP double hetero structure was measured with spatially resolved and time-resolved photoluminescence using a confocal single photon counting setup.