K. Ernst

Solar cell with extremely thin absorber on highly structured substrate

We report the optical, structural and photovoltaic properties of an extremely-thin-absorber (eta) solar cell on highly porous TiO2 substrates. Nano-crystalline absorber layers with a local thickness of only 150 nm have been prepared. This small absorber thickness allows good carrier collection even for absorber material with poor transport properties. The morphology of the highly structured TiO2 substrate produces strong internal light scattering, resulting in an enhancement of the optical path length by a factor 5. With 150 nm thick CdTe absorber layers, the eta solar cells produce an open-circuit voltage of 0.67 V and a short-circuit current of 8.9 mA cm−2 for 100 mW cm−2 illumination. Alloying the CdTe with Hg improves the short-circuit current to 15 mA cm−2.

The eta-solar cell with CuInS2: A photovoltaic cell concept using an extremely thin absorber (eta)

Diffusion length of charge carriers within the absorbing material is one of the important restricting properties for the efficiency of solar cell devices. A new cell design using an extremely thin absorber (eta-solar cell) is prepared to obtain an effective separation of charge carriers within the depletion layer. It could be figured out that the properties of CuInS2 (CIS) strongly depend on the porosity of the base layer. Multiple scattering within the porous structure is evident. Moreover, it can be demonstrated that there is a maximum in short-circuit current density for a medium thickness of the absorbing layer.

Hexagonal nanotubes of ZnS by chemical conversion of monocrystalline ZnO columns

Monocrystalline ZnO columns grown in electrodeposition were converted to ZnS using ion exchange reactions in H2S or S vapor. At ∼400 °C the reaction with H2S only affects a thin layer of 10–30 nm thickness at the surface of the ZnO crystallites, and ZnS-coated ZnO columns are produced. Exploiting the large difference in etch resistance between ZnS and ZnO, the ZnO core of the columns can be removed, and a tubular structure of ZnS can be prepared. Typical dimensions of the ZnS tubes are a length of 1–3 μm, a diameter of 100–300 nm, and a wall thickness of 10–30 nm. The ZnS tubes have the same distribution, alignment, and surface morphology as the original ZnO columns. The reaction in S vapor is suitable to produce solid ZnS columns.

Nano-structures for solar cells with extremely thin absorbers

Abstract We have developed several methods to prepare nano-structured semiconductor and conducting oxide layers for large area applications. Due to their unique optical and electrical properties these layers have considerable potential for the fabrication of a novel solar cell type with an extremely thin absorber (eta-cell). Preparation methods, structural and optical properties are reported.

Contacts to a solar cell with extremely thin CdTe absorber

Abstract The concept for a solar cell with extremely thin absorber (eta-cell) comprises a porous TiO2 substrate covered by a CdTe film with a local thickness of 150–250 nm. At the present stage of development this type of cell exhibits a photovoltage between 600 and 700 mV and a photocurrent of 5–9 mA/cm2 under AM1.5 illumination, indicating the feasibility of the concept. The fill factor is, however, only in the range of 20%, due to a strong voltage-dependence of the photocurrent. This paper discusses the influence of the front and back contacts on the limited cell performance and shows that the insertion of a buffer layer at the front contact gives an improved fill-factor.

Comparison of different thin film absorbers used in eta-solar cells

Abstract Alloying with mercury (Hg) has been used in electrodeposition as a means to reduce the bandgap of CdTe absorber films from 1.5 eV for CdTe to 1.07 eV for Cd x Hg 1− x Te (CMT). Alloy films have been incorporated as extremely thin absorbers in solar cells with the structure glass∣SnO 2 ∣microporous–TiO 2 ∣CMT∣Au. The decrease in the CMT bandgap is expected to reduce the conduction band offset at the interface to TiO 2 , which is 0.6 eV for the TiO 2 ∣CdTe interface. This high band offset was believed to be responsible for the low fill-factor observed in the J – V characteristics of the former interface. Lowering the band offset is expected to lead to higher fill-factor values. Our results show that using a CMT as absorber gives place to both higher quantum efficiencies and a spectral response spectrum extended towards longer wavelengths, but not to a fill-factor enhancement.

Characterization of II-VI compounds on porous substrates

We investigate the possibilities for electrodeposition of II–VI compounds on porous substrates. ZnTe and CdTe are deposited on porous TiO2 and characterised by scanning electron microscopy and electrical measurements. It is shown that CdTe can be deposited as a homogeneous film covering a highly structured substrate in conformal manner, while ZnTe is able to fill up the porous structure to a certain extent. However, the ZnTe deposition exhibits a strong tendency for irregular growth, and a smooth top surface has so far not been obtained. These morphological differences can be attributed to different deposition mechanisms. While CdTe growth is characterised by a chemical autoregulation mechanism that allows stoichiometric deposition, ZnTe deposition appears to show a higher dependence on potential and Te concentration. As a consequence, exposed regions of the substrate are more readily covered with ZnTe. The growth near the SnO2 contact is apparently zinc rich, indicating a potential drop over the TiO2 together with a diffusion limitation for Te. Despite inhomogeneous growth, efficient electron transfer from the ZnTe to the TiO2 substrate is observed.

Semiconductor Growth and Junction Formation within Nano-Porous Oxides

We have developed semiconductor growth techniques for the coating and filling of nanopores in ceramic-type substrates. The main idea behind this research is to use the large inner surface of ceramics as a template for the realization of semiconductor heterojunctions with extremely large interface area. As porous substrates we use lightly sintered nanocrystalline TiO2 of 5–10 μm thickness. The pore volume in these substrates is ∼50% and the average pore diameter is 30–50 nm. We are able to establish nanometer thick coatings on the inner surfaces of these substrates or — in a different technique — fill the pore volume with (100 ± 3)% efficiency. The growth techniques involve chemical and electrochemical methods from liquid solutions. Binary, ternary and, most recently, quarternary compounds of the II–VI and I–III–VI material systems were prepared.

Electrochemical texturization of ZnTe surfaces

We report the results of electrochemical etching on ZnTe crystals using the acidic etchant HNO3:HCl:H2O. Under optimized conditions we are able to etch several micrometer deep patterns into 〈111〉 and 〈110〉 surfaces. A comparison between etched and polished crystals shows that a surface enlargement of approximately 50 times can be obtained. The etched surfaces have strongly reduced optical reflectance, high photoluminescence efficiency, and excellent photosensitivity. Needlelike structures can be prepared, which exhibit a blueshift of the excitonic transition energies, indicating that parts of the etch pattern are in the quantum confinement regime.

Surface texturisation of ZnTe crystals and thin films

Electrochemical etching has been employed for the texturisation of crystalline ZnTe surfaces. Under optimised conditions micrometre-deep etching patterns can be obtained. The etched surfaces show a factor-3 decrease in the reflectivity at normal incidence, and a 30% increase in the electrical photosensitivity. When the etching is sufficiently deep, needle-like structures with an aspect ratio larger than 10 appear. These deeply etched surfaces exhibit a blue shift in the excitonic photoluminescence, indicative of electron confinement effects.