Analysis of photonic structures for silicon solar cells

This work covers the development of an efficient optical simulation formalism for silicon solar cells with structured interfaces that can operate in different optical regimes; the comparison of photonic structures with regard to the optical solar cell performance; the structure fabrication via nanoimprint lithography and the demonstration of enhanced efficiencies on solar cell and mini-module level by integrating exemplary photonic structures

Optical analysis of a III-V-nanowire-array-on-Si dual junction solar cell

A tandem solar cell consisting of a III-V nanowire subcell on top of a planar Si subcell is a promising candidate for next generation photovoltaics due to the potential for high efficiency. However, for success with such applications, the geometry of the system must be optimized for absorption of sunlight. Here, we consider this absorption through optics modeling. Similarly, as for a bulk dual-junction tandem system on a silicon bottom cell, a bandgap of approximately 1.7 eV is optimum for the nanowire top cell. First, we consider a simplified system of bare, uncoated III-V nanowires on the silicon substrate and optimize the absorption in the nanowires. We find that an optimum absorption in 2000 nm long nanowires is reached for a dense array of approximately 15 nanowires per square micrometer. However, when we coat such an array with a conformal indium tin oxide (ITO) top contact layer, a substantial absorption loss occurs in the ITO. This ITO could absorb 37% of the low energy photons intended for the silicon subcell. By moving to a design with a 50 nm thick, planarized ITO top layer, we can reduce this ITO absorption to 5%. However, such a planarized design introduces additional reflection losses. We show that these reflection losses can be reduced with a 100 nm thick SiO2 anti-reflection coating on top of the ITO layer. When we at the same time include a Si3N4 layer with a thickness of 90 nm on the silicon surface between the nanowires, we can reduce the average reflection loss of the silicon cell from 17% to 4%. Finally, we show that different approximate models for the absorption in the silicon substrate can lead to a 15% variation in the estimated photocurrent density in the silicon subcell

Energy yield analysis of textured perovskite silicon tandem solar cells and modules

Perovskite silicon tandem solar cells combine potentially low production costs with the ability to surpass the efficiency limit of silicon single junction solar cells. Optical modeling and optimization are crucial to achieve this ambitious goal in the near future. The optimization should seek to maximize the energy yield based on realistic environmental conditions. This work analyzes the energy yield of perovskite silicon tandem solar cells and modules based on realistic experimental data, with a special focus on the investigation of surface textures at the front and rear side of the solar cell and its implication for reflection as well as parasitic absorption properties. The investigation reveals a 7.3%rel higher energy yield for an encapsulated tandem cell with a textured front side compared with an encapsulated high efficiency single junction solar cell with 24.3% harvesting efficiency for irradiance data of the year 2014 in Freiburg/Germany

Photonic structures for III-V//Si multi-junction solar cells with efficiency >33%

Silicon based multi-junction solar cells are a promising option to overcome the theoretical efficiency limit of a silicon solar cell (29.4%). With III-V semiconductors, high bandgap materials applicable for top cells are available. For the application of such silicon based multi-junction devices, a full integration of all solar cell layers in one 2-terminal device is of great advantage. We realized a triple-junction device by wafer-bonding two III-V-based top cells onto the silicon bottom cell. However, in such a series connected solar cell system, the currents of all sub-cells need to be matched in order to achieve highest efficiencies. To fulfil the current matching condition and maximise the power output, photonic structures were investigated. The reference system without photonic structures, a triple-junction cell with identical GaInP/GaAs top cells, suffered from a current limitation by the weakly absorbing indirect semiconductor silicon bottom cell. Therefore rear side diffraction gratings manufactured by nanoimprint lithography were implemented to trap the infrared light and boost the solar cell current by more than 1 mA/cm2. Since planar passivated surfaces with an additional photonic structure (i.e. electrically planar but optically structured) were used, the optical gain could be realized without deterioration of the electrical cell properties, leading to a strong efficiency increase of 1.9% absolute. With this technology, an efficiency of 33.3% could be achieved

Multiply Bonded Metal(II) Acetate (Rhodium, Ruthenium, and Molybdenum) Complexes with the <i>trans</i>-1,2-Bis(<i>N</i>‑methylimidazol-2-yl)ethylene Ligand

The synthesis and structural characterization of new coordination polymers with the <i>N</i>,<i>N</i>-donor ligand <i>trans</i>-1,2-bis­(<i>N</i>-methylimidazol-2-yl)­ethylene (<i>trans</i>-bie) are reported. It was found that the acetate-bridged paddlewheel metal­(II) complexes [M₂(O₂CCH₃)₄(<i>trans</i>-bie)]_<i>n</i> with M = Rh, Ru, Mo, and Cr are linked by the <i>trans</i>-bie ligand to give a one-dimensional alternating chain. The metal–metal multiple bonds were analyzed with density functional theory and CASSCF/CASPT2 calculations (bond orders: Rh, 0.8; Ru, 1.7; Mo, 3.3)