Andrej Yu. Kuznetsov

Nanostructure Formation and Passivation of Large-Area Black Silicon for Solar Cell Applications

Nanoscale textured silicon and its passivation are explored by simple low-cost metal-assisted chemical etching and thermal oxidation, and large-area black silicon was fabricated both on single-crystalline Si and multicrystalline Si for solar cell applications. When the Si surface was etched by HF/AgNO(3) solution for 4 or 5 min, nanopores formed in the Si surface, 50-100 nm in diameter and 200-300 nm deep. The nanoscale textured silicon surface turns into an effective medium with a gradually varying refractive index, which leads to the low reflectivity and black appearance of the samples. Mean reflectance was reduced to as low as 2% for crystalline Si and 4% for multicrystalline Si from 300 to 1000 nm, with no antireflective (AR) coating. A black-etched multicrystalline-Si of 156 mm × 156 mm was used to fabricate a primary solar cell with no surface passivation or AR coating. Its conversion efficiency (η) was 11.5%. The cell conversion efficiency was increased greatly by using surface passivation process, which proved very useful in suppressing excess carrier recombination on the nanostructured surface. Finally, a black m-Si cell with efficiency of 15.8% was achieved by using SiO(2) and SiN(X) bilayer passivation structure, indicating that passivation plays a key role in large-scale manufacture of black silicon solar cells.

Palladium Schottky barrier contacts to hydrothermally grown n-ZnOand shallow electron states

Schottky barrier contacts have been formed by electron beam evaporation of Pd on the (0001¯)-face of hydrothermally grown n-type single-crystalline ZnO. The contacts can be operated under reverse bias voltages up to −5V and in the temperature range between 130 and 350K. A barrier height of 0.83eV is deducted at room temperature, which is in reasonable agreement with the value predicted by fundamental theory. The ideality factor for the current-voltage characteristics varies between 1.01 and 1.03 in the voltage range of −5.0to+0.5V, indicating that thermionic emission is a dominant mechanism for charge-carrier transport. Thermal admittance measurements were performed between 15 and 350K, and two prominent levels were resolved at 0.05(1) and 0.33(2)eV below the conduction band edge, respectively. The origin of these levels is not known, but possible candidates are complexes involving hydrogen, Zn interstitials, and vacancy-oxygen centers.

Probing Defects in Nitrogen-Doped Cu2O

Nitrogen doping is a promising method of engineering the electronic structure of a metal oxide to modify its optical and electrical properties; however, the doping effect strongly depends on the types of defects introduced. Herein, we report a comparative study of nitrogen-doping-induced defects in Cu2O. Even in the lightly doped samples, a considerable number of nitrogen interstitials (Ni) formed, accompanied by nitrogen substitutions (NO) and oxygen vacancies (VO). In the course of high-temperature annealing, these Ni atoms interacted with VO, resulting in an increase in NO and decreases in Ni and VO. The properties of the annealed sample were significantly modified as a result. Our results suggest that Ni is a significant defect type in nitrogen-doped Cu2O.

Oxygen vacancies: The origin ofn-type conductivity in ZnO

Oxygen vacancy (VO) is a common native point defects that plays crucial roles in determining the physical and chemical properties of metal oxides such as ZnO. However, fundamental understanding of VO is still very sparse. Specifically, whether VO is mainly responsible for the n-type conductivity in ZnO has been still unsettled in the past fifty years. Here we report on a study of oxygen self-diffusion by conceiving and growing oxygen-isotope ZnO heterostructures with delicately-controlled chemical potential and Fermi level. The diffusion process is found to be predominantly mediated by VO. We further demonstrate that, in contrast to the general belief of their neutral attribute, the oxygen vacancies in ZnO are actually +2 charged and thus responsible for the unintentional n-type conductivity as well as the non-stoichiometry of ZnO. The methodology can be extended to study oxygen-related point defects and their energetics in other technologically important oxide materials.

Maskless inverted pyramid texturization of silicon

We discovered a technical solution of such outstanding importance that it can trigger new approaches in silicon wet etching processing and, in particular, photovoltaic cell manufacturing. The so called inverted pyramid arrays, outperforming conventional pyramid textures and black silicon because of their superior light-trapping and structure characteristics, can currently only be achieved using more complex techniques involving lithography, laser processing, etc. Importantly, our data demonstrate a feasibility of inverted pyramidal texturization of silicon by maskless Cu-nanoparticles assisted etching in Cu(NO3)2 / HF / H2O2 / H2O solutions and as such may have significant impacts on communities of fellow researchers and industrialists.

Back-illuminated Si photocathode: a combined experimental and theoretical study for photocatalytic hydrogen evolution

Si is an excellent absorber material for use in 2-photon photoelectrochemical hydrogen production. So far nearly all studies of silicon photoelectrodes have employed frontal illumination despite the fact that in most water-splitting 2-photon device concepts the silicon is the “bottom” cell in the tandem stack and therefore illuminated from the back with respect to the electrolyte. In the present work, we investigate back-illuminated Si photoelectrodes experimentally, as well as by modelling, the dependence of induced photocurrent on various parameters, such as carrier diffusion length (Le) and surface recombination velocity (vs) to quantify their relative importance. A bifacial light absorbing structure (p+pn+ Si) is tested under back-illumination conditions which mimic the actual working environment in a tandem water splitting device. The thickness of the absorbing Si layer is varied from 30 to 350 μm to assess the impact of the diffusion length/thickness ratio (Le/L) on photocatalytic performance. It is shown how the induced photocurrent (JL) of a back-illuminated sample increases as wafer thickness decreases. Compared to the 350 μm thick sample, a thinned 50 μm thick sample shows a 2.7-fold increase in JL, and consequently also a higher open circuit voltage. An analytical model is developed to quantify how the relative Le/L-ratio affects the maximum JL under back-illumination, and the result agrees well with experimental results. JL increases with the Le/L-ratio only up to a certain point, beyond which the surface recombination velocity becomes the dominant loss mechanism. This implies that further efforts should to be focused on reduction of surface recombination. The present study is the first experimental demonstration of a Si wafer based photocathode under back-illumination. Moreover, the comparative experimental and theoretical treatment also highlights which photoabsorber properties merit the most attention in the further development towards full tandem water splitting devices.

Recombination centers in as-grown and electron-irradiated ZnO substrates

Optical detection of magnetic resonance (ODMR) was used to study defects in ZnO substrates irradiated with 3 MeV electrons at room temperature. The Zn vacancy and some other ODMR centers were detected. Among these, the Zn vacancy and two other centers, labeled as LU3 and LU4, were also commonly observed in different types of as-grown ZnO substrates. The LU3 and LU4 are related to intrinsic defects and act as dominating recombination centers in irradiated and as-grown ZnO.

Interface Engineering of High Efficiency Organic-Silicon Heterojunction Solar Cells

Insufficient interface conformity is a challenge faced in hybrid organic-silicon heterojunction solar cells because of using conventional pyramid antireflection texturing provoking the porosity of interface. In this study, we tested alternative textures, in particular rounded pyramids and inverted pyramids to compare the performance. It was remarkably improved delivering 7.61%, 8.91% and 10.04% efficiency employing conventional, rounded, and inverted pyramids, respectively. The result was interpreted in terms of gradually improving conformity of the Ag/organic/silicon interface, together with the gradually decreasing serial resistance. Altogether, the present data may guide further efforts arising the interface engineering for mastering high efficient heterojunction solar cells.

Optical activity and defect/dopant evolution in ZnO implanted with Er

The effects of annealing on the optical properties and defect/dopant evolution in wurtzite (0001) ZnO single crystals implanted with Er ions are studied using a combination of Rutherford backscattering/channeling spectrometry and photoluminescence measurements. The results suggest a lattice recovery behavior dependent on ion dose and involving formation/evolution of an anomalous multipeak defect distribution, thermal stability of optically active Er complexes, and Er outdiffusion. An intermediate defect band occurring between the surface and ion-induced defects in the bulk is stable up to 900 °C and has a photoluminescence signature around 420 nm well corresponding to Zn interstitials. The optical activity of the Er atoms reaches a maximum after annealing at 700 °C but is not directly associated to the ideal Zn site configuration, since the Er substitutional fraction is maximal already in the as-implanted state. In its turn, annealing at temperatures above 700 °C leads to dissociation of the optically active Er complexes with subsequent outdiffusion of Er accompanied by the efficient lattice recovery.

Engineering of optically defect free Cu2O enabling exciton luminescence at room temperature

Cu2O is an interesting semiconductor with extraordinary high exciton binding energy, however exhibiting weak room temperature excitonic luminescence. The issue was addressed in literature emphasizing a detrimental role of native point defects responsible for optical quenching. Resolving the problem, we propose a method to manipulate the Cu and O vacancies contents opening a gateway for optoelectronic applications of Cu2O. Specifically, applying oxygen lean conditions, we observe a remarkable suppression of VCu enabling strong room temperature exciton luminescence, while manipulating with VO reveals no impact on the signal. As a result, the excitonic signature was interpreted in terms of phonon assisted transitions.