We present the design and performance results of a MW-sized photovoltaic (PV), battery energy storage system (BESS), and diesel genset hybrid system, which has been now in operation for over 2 years. Built in two phases, the power generation of the island features a cumulated diesel genset power of 5.3 MVA, a PV system size of 4.15 MWp, and battery of 5 MVA and 5.9 MWh. The hybrid system reaches a renewable penetration over 46%, setting a milestone in the energy transition from fossil fuel towards MW-sized renewable-based power generation. The usage of voltage-controlled grid-forming inverters allows to switch off thermal power generation for 9 to 11 hours daily depending on the amount of solar irradiation and load featuring diesel savings of more than 1.7 million liters of diesel per year. During nighttime, the battery system is still active and has served already to avoid over 10 grid blackouts and automatically control the genset-based generation. Furthermore, the hybrid system allows for substantial reduction in operational expenditures, spare parts, and maintenance intervals. The system also features a relevant impact on frequency, grid stability, and harmonics, hence increasing the power quality of the island. Thanks to appropriate system design based on experience and proprietary software, it is possible to predict the performance accurately and provide projections on savings.

In this paper, the authors study the surfactant-mediated epitaxy (SME) of relaxed Ge films on (001)-orientated porous silicon wafers, which could be a first step towards a cost-efficient lift-off technique for lightweight high-efficiency multijunction Ge/III-V solar cells. Transmission electron microscopy (TEM) and high-resolution X-ray diffraction (XRD) investigations show full relaxation and high structural perfection of the epitaxial Ge. An abrupt interface showing no evidence for intermixing is formed between Ge and Si. Similar to SME of Ge on standard Si(001) wafers the lattice mismatch is compensated by a periodic array of full edge dislocations parallel to the interface. XRD measurements indicate that part of the silicon is under a small tensile strain. The authors attribute this to partial strain accommodation in the top silicon layer of the porous substrate which acts as a compliant substrate.

We investigate the power losses in back-contact back-junction monocrystalline thin-film silicon solar cells. The cells are made from epitaxial layers grown on and separated from porous Si (PSI process). We combine two-dimensional finite element modeling with a resistance network simulation. The simulated and measured current-voltage characteristics agree. Free energy loss analysis reveals that the main limiting loss mechanism of the best cell with 13.5 % efficiency is the high saturation current density at the metal-silicon interface of 5×104 fA cm2, causing 2.5 % absolute efficiency loss.

ABSTRACTWe present a both‐sides‐contacted thin‐film crystalline silicon (c‐Si) solar cell with a confirmed AM1.5 efficiency of 19.1% using the porous silicon layer transfer process. The aperture area of the cell is 3.98 cm2. This is the highest efficiency ever reported for transferred Si cells. The efficiency improvement over the prior state of the art (16.9%) is achieved by implementing recent developments for Si wafer cells such as surface passivation with aluminum oxide and laser ablation for contacting. The cell has a short‐circuit current density of 37.8 mA cm−2, an open‐circuit voltage of 650 mV, and a fill factor of 77.6%. Copyright © 2011 John Wiley & Sons, Ltd.

The literature describes various techniques for fabricating thin monocrystalline Si films without the need of sawing. Layer transfer using epitaxy on porous Si (PSI) and subsequent layer separation from the growth substrate is one particular attractive option. A 40 μm thick epitaxial Si cell from this so-called PSI process is capable of saving about 80 % of the Si that is consumed by a 180 μm-thick Si wafer plus 100 μm Si kerf loss. In addition, the two crystallizations of growing the poly-feedstock and of growing the ingot are replaced by a single crystallization: the epitaxial growth of a thin Si layer. Applying the PSI process, we demonstrate an independently confirmed aperture efficiency of 19.1 % for a 4 cm2-sized layer transfer cell that has a thickness of only 43 μm. This is the highest efficiency ever reported for a thin-film (<;50 μm) crystalline Si layer transfer cell. We achieve this record with a passivated emitter and rear contact (PERC) structure. The passivation layer is Al2O3 from atomic layer deposition (ALD). The contacts on the rear side are formed by laser ablation using ultrashort ps pulses (LCO). The ISFH roadmap that our PSI process development is a part of is also discussed in this contribution. Our program aims at developing thin-film/wafer hybrid technologies that shall combine the high efficiency potential of thin monocrystalline Si films with the low cost per area of the Si-thin-film world. 19%-efficient and less then 50 μm-thick layer transfer cells mark a large step forward towards the realization of such thin-film/wafer hybrid technologies.

We demonstrate reproducible formation of mesoporous germanium layers suitable for solar energy applications by electrochemical etching in highly concentrated electrolytes. For long anodization times or thick layer formation a porosity gradient is observed leading eventually to high porosity regions and cavity formation at the bottom of the porous layer. A 30 min annealing step at a temperature of 575 ºC in hydrogen atmosphere allows for reorganization and subsequent lift-off. The mean surface roughness increases from 0.31 nm for unprocessed Ge up to 7.85 nm for reorganized Ge as measured by atomic force microscopy. µ-Raman confocal spectroscopy analysis confirms that etching and annealing do not affect the crystalline structure.

High efficiency solar cells require high generation and low recombination rates. High bulk lifetime, well passivated surfaces, and lowly doped thin emitters allow for low recombination rates. Thin passivated emitters should be contacted locally in order to avoid excessive contact recombination. This is common practice for front junction solar cells but is also advantageous for back junction cells. We analyze a novel layer selective laser ablation process. From a passivating stack composed of 70nm silicon nitride that we deposit on top of 35nm of amorphous silicon we selectively ablate the silicon nitride layer. Transmission electron microscopy investigations confirm the full ablation of the silicon nitride layer. After the ablation process, a 17 nm-thick amorphous silicon layer remains on the substrate. The crystalline silicon substrate shows no dislocations after the process. Evaporating aluminum on top of the locally ablated nitride layers forms local contacts of the aluminum to the silicon.

We demonstrate reproducible formation of mesoporous germanium layers suitable for solar energy applications by electrochemical etching in highly concentrated electrolytes. For long anodization times or thick layer formation a porosity gradient is observed leading eventually to high porosity regions and cavity formation at the bottom of the porous layer. A 30 min annealing step at a temperature of 575°C in hydrogen atmosphere allows for reorganization and subsequent lift-off. The mean surface roughness increases from 0.31 nm for unprocessed Ge up to 7.85 nm for reorganized Ge as measured by atomic force microscopy. μ-Raman confocal spectroscopy analysis confirms that etching and annealing do not affect the crystalline structure.

We present the reproducible fabrication of porous germanium (PGe) single- and multilayers. Mesoporous layers form on heavily doped 4 '' p-type Ge wafers by electrochemical etching in highly concentrated HF-based electrolytes with concentrations in a range of 30-50 wt. %. Direct PGe formation is accompanied by a constant dissolution of the already-formed porous layer at the electrolyte/PGe interface, hence yielding a thinner substrate after etching. This effect inhibits multilayer formation as the starting layer is etched while forming the second layer. We avoid dissolution of the porous layer by alternating the etching bias from anodic to cathodic. PGe formation occurs during anodic etching whereas the cathodic step passivates pore walls with H-atoms and avoids electropolishing. The passivation lasts a limited time depending on the etching current density and electrolyte concentration, necessitating a repetition of the cathodic step at suitable intervals. With optimized alternating bias mesoporous multilayer production is possible. We control the porosity of each single layer by varying the etching current density and the electrolyte. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Uniform mesoporous Si double layers are formed on 4 inch p-type <100> wafers with an off orientation of 6º towards <111> by means of electrochemical etching in ethanoic-based HF electrolytes. These substrates are of interest for the epitaxial growth of III–V compound semiconductor stacks on their top for the production of multi-junction solar cells and very thin electronic devices. We demonstrate transfer of porous layers after an annealing process in hydrogen atmosphere. Electron Back-Scatter Diffraction analysis confirms that the substrate orientation is conserved during the etching and annealing steps. Confocal μ-Raman spectroscopy analysis shows a decrease in the Raman signal intensity after etching and a subsequent increase after annealing while no shift is observed. By means of Atomic Force Microscopy, analysis the surface appearance after the etching and annealing steps can be visualized. The mean surface roughness varies during the process from 0.55nm for the unprocessed wafers to 0.27nm after etching and 0.78nm after annealing. The decrease of average roughness after etching is caused by an electropolishing step prior to porous formation. Despite of slight increase of mean surface roughness after annealing the samples are still appropriate for high quality epitaxial growth and subsequent lift-off.

We investigate sintering and reorganization of electrochemically etched mesoporous Ge layers. Sintering in nitrogen, forming gas, or argon atmospheres always yields a reorganized and oxidized porous layer. Water-insoluble Ge native oxides produced during etching do not hinder thermal reorganization, but evolve to water soluble oxides during annealing. Reorganized porous layers hence dissolve in subsequent water or HF dips. Sintering in hydrogen atmospheres removes native Ge oxides and prevents oxidation of porous layers. Porous layers with porosities less than 30% exhibit a compact closed surface after sintering, whereas porous layers with porosities exceeding 50% collapse. The porosity decreases linearly in a range of 57%–45% for electrolyte concentrations and in a range of 35–50wt% for a specific resistivity of 0.013Ωcm and an etching current density of 5mAcm−2. We obtain porosities below 30% by using substrate resistivities in the 0.015–0.030Ωcm range, etching current densities below 2.5mAcm−2 and an electrolyte concentration of HF 50wt%.

Mesoporous GaAs double layers with different porosities and thicknesses up to 7 µm are formed on highly doped p‐type 4 inch GaAs substrates by means of electrochemical etching in highly concentrated hydrofluoric acid electrolytes. Small 〈111〉 oriented pyramids form at the interface between porous layer and substrate if etching current densities below 7.5 mA cm−2 are applied. Porous double layers with different porosities are obtained by varying simultaneously the etching current density and the electrolyte concentration. The porosity of the porous GaAs layers decreases nearly linearly from 69 to 53% with electrolyte concentrations increasing from 30 to 50 wt.%, respectively. The etching process increases the mean roughness of the porous layer surface from 0.15 to 0.24 nm.

Monocrystalline, mesoporous GaAs double layers with controlled porosities are formed by means of electrochemical etching on p-type GaAs substrates using highly concentrated HF-based electrolytes. Variations in the electrolyte concentration and etching current density lead to changes in the porosity, morphology, thickness and etching rate of the porous layers. The porous layer is composed of micro and mesopores with a diameter in the range of 1 nm to 38 nm and a mean value of less than 10 nm. The etching rate of the porous layer lies in a range of 1.7 nm/sec to 1725 nm/sec. Hundred nm sized < 111 > oriented pyramids form at the interface between porous layer and substrate if etching current densities below 7.5 mA/cm(2) are applied. Mesoporous layers with thicknesses of up to 7 mu m form reproducibly. Porous layers thicker than 7 mu m automatically lift-off from the substrate. We demonstrate the spatially homogenous formation of mesopores on GaAs wafers with 4 '' in diameter. The etching rates and thicknesses values achieved indicate that etching of GaAs mesopores may be applicable to the industrial production of space solar cells.

Uniform thick mesoporous germanium layers are reproducibly formed on 4 in. p-type Ge wafers by electrochemical etching in highly concentrated HF electrolytes. Pore formation by anodic etching in germanium leads to a constant dissolution of the porous layer. The growth rate of the porous Ge layer is therefore given by the difference between the etch rate at the porous layer/substrate wafer interface and the dissolution rate at the electrolyte/porous layer interface. The growth rate lies in the range of 0.071-2.7 nm/min for etching current densities of 0.1-80 mA/cm(2), while both the etch rate and the dissolution rate lie in the range of several micrometers per minute. We define the substrate usage as the ratio of the growth rate and the etch rate. This substrate usage determines the growth efficiency of the porous layer and lies in the range of 0.2-2%. Thus, the substrate wafer is thinned substantially during anodic porous layer formation. Constantly alternating from an anodic to a cathodic bias prevents the thinning of the substrate. The dissolution rate decreases, and the usage increases up to 98%.

In the present paper we investigate the impact of different substrate reconditioning methods on the electrical quality of the subsequently epitaxially grown silicon layers (epi-layers). We show that the removal of the residual porous layer on the substrate after lift-off is crucial for the quality of subsequent epitaxial growth and compare different wet chemical treatments for removal. Based on the results we choose KOH to demonstrate the successful 13-fold re-use of a single substrate. We determine Lbulk of the epi-layer grown after thirteen re-uses of the same substrate to correspond to the fourfold of the epi-layer thickness. Resultant epi-layers grown after several re-uses of the same substrates are still suitable for a sufficient current collection in a thin solar cell device.