Smart Wire Connection Technology (SWCT) is an efficient cell interconnection technology that combines many advantages: reduced cell metallization cost, improved module aesthetics, lead-free module.. In addition to the many advantages of this technology, we recently discovered a new opportunity to further boost efficiency of this module technology by overlapping cells. An overlap module is formed by cells overlapped in the string direction. This feature allows to reduce module dimensions and fabrication cost without reducing module power output. An optimal cell overlap of 1mm was determined and large 60 cells bifacial modules were built to compare this concept to regular modules with a cell spacing of few millimetres. Ageing of the module was evaluated and outdoor data presents high performances of overlapped modules in real conditions. So far, a 327Wp bifacial module was build based on this module concept. This module based on 60 heterojunction solar cells measures less than 1.6m2 and has an efficiency of 20.6% with a black back reflector.

Strategies for enhancing the thermal stability of small-molecule organic solar cells are demonstrated and compared with two molecularly engineered additives.

Carrier collection in silicon heterojunction (SHJ) solar cells is usually achieved by doped amorphous silicon layers of a few nanometers, deposited at opposite sides of the crystalline silicon wafer. These layers are often defect-rich, resulting in modest doping efficiencies, parasitic optical absorption when applied at the front of solar cells, and high contact resistivities with the adjacent tra...

Optical and electrical properties of hydrogenated nanocrystalline silicon (nc-Si:H) solar cells are strongly influenced by the morphology of underlying substrates. By texturing the substrates, the photogenerated current of nc-Si:H solar cells can increase due to enhanced light scattering. These textured substrates are, however, often incompatible with defect-less nc-Si:H growth resulting in lower Voc and FF. In this study we investigate the correlation between the substrate morphology, the nc-Si:H solar-cell performance, and the defect density in the intrinsic layer of the solar cells (i-nc-Si:H). Statistical surface parameters representing the substrate morphology do not show a strong correlation with the solar-cell parameters. Thus, we first quantify the line density of potentially defective valleys of randomly textured ZnO substrates where the opening angle is smaller than 130° (ρ<130). This ρ<130 is subsequently compared with the solar-cell performance and the defect density of i-nc-Si:H (ρdefect), which is obtained by fitting external photovoltaic parameters from experimental results and simulations. We confirm that when ρ<130 increases the Voc and FF significantly drops. It is also observed that ρdefect increases following a power law dependence of ρ<130. This result is attributed to more frequently formed defective regions for substrates having higher ρ<130.

We introduce passivating hetero‐interfaces in single‐junction microcrystalline silicon (μc‐Si:H) solar cells. We investigate the effect of different i–n layer stacks in thin μc‐Si:H devices, in which recombination is significant at the interfaces as well as in the bulk material. By applying amorphous silicon passivating layers at the μc‐Si:H i–n interface, we show a device with a high open‐circuit voltage (Voc) of 608 mV, for a standard Raman crystalline fraction of the i‐layer (>50%). This Voc is the highest reported value for a state‐of‐the‐art μc‐Si:H device made by plasma‐enhanced chemical vapor deposition. We also report an efficiency of 9.45% for a solar cell with an absorber layer as thin as 650 nm on an area greater than 1 cm2, and show with a simple crystalline silicon model that for such thin μc‐Si:H devices or μc‐Si:H devices with a very high bulk‐material quality, well‐mastered interfaces and doped layers are of paramount importance for high efficiency.

We study amorphous silicon–germanium (a-SiGe:H) as intrinsic absorber material for thin-film silicon-based triple and quadruple junction solar cells. First, we present the development of a-SiGe:H single junction devices, in particular the Ge-content grading in the absorber layer, the influence of the Ge-content on electrical properties and (infra)red-response, and the influence of using different types of p-layers. We subsequently show the incorporation of optimized single-junction devices in triple junction cells and discuss the interplay between Ge-content and intermediate reflector thickness. For triple junction devices with amorphous silicon (a-Si:H) top cells, a-SiGe:H middle cells and microcrystalline silicion (µc-Si:H) bottom cells, we obtained an initial efficiency of 13.6% and an efficiency of 11.3% after light-soaking. We also present a quadruple junction device with an a-Si:H top cell, a low Ge-content a-SiGe:H second cell, and µc-Si:H third and bottom cells. In this device configuration, we obtained an open-circuit voltage as high as 2.57V. The performance of these cells was limited by not yet optimized current matching, leading nevertheless to an initial efficiency of 10.1%. A brief roadmap towards quadruple-junction devices with stabilized efficiencies of 14% is also outlined.

The use of intrinsic silicon oxide as a buffer layer at the p–i interface of thin-film silicon solar cells is shown to provide significant advantages. For microcrystalline silicon solar cells, when associated with highly crystalline i-layers deposited at high rates, all electrical parameters are improved. Larger efficiency gains are achieved with substrates of increased roughness. For cells with an improved i-layer material quality, there is mainly a gain in short-circuit current density. An improvement in carrier collection in the blue region of the spectrum is systematically observed on all the cells. The presence of a silicon oxide buffer layer also promotes the nucleation of the subsequent intrinsic microcrystalline silicon layer. In amorphous silicon solar cells, the silicon oxide buffer layer is proven to act as an efficient barrier to boron cross-contamination, eliminating the need for additional processing steps (e.g. water vapor flush), while providing a wide bandgap material at the interface. The implementation of silicon oxide buffer layers for both types of cells thus provides a decisive improvement, as it allows extremely fast deposition of the full p–i–n stack of layers of the cell in a single-chamber configuration while providing a high-quality substrate-resilient p–i interface.

When it comes to parasitic absorption in thin-film silicon solar cells, most studies focus on one electrode only, most of the time the substrate (in n-i-p configuration) or superstrate (in p-i-n configuration). We investigate here simultaneously the influence of the absorption in both front and back electrodes on the current density of tandem micromorph solar cells in p-i-n configuration. We compare four possible combinations of front and back electrodes with two different doping levels, but identical sheet resistance and identical light-scattering properties. In the infrared part of the spectrum, parasitic absorption in the front or back electrode is shown to have a similar effect on the current generation in the cell, which is confirmed by modeling. By combining highly transparent front and back ZnO electrodes and high-quality silicon layers, a micromorph device with a stabilized efficiency of 11.75% is obtained.

In this contribution, we investigate the effect of post-deposition treatments on finished non-encapsulated thin-film microcrystalline silicon solar cells and show that annealing in vacuum leads to improved electrical properties of the solar cells, particularly for cells deposited on rough superstrates. Our results suggest that both curing of intrinsic defects in the silicon, which can appear during the deposition of the ZnO back electrode, as well as an improvement of the ZnO back-electrode conductivity itself, occur during an annealing in vacuum, leading to large improvements of the open-circuit voltage and fill factor values. An improvement of the porous zones in the absorber layer, as induced by rough superstrates, is also observed by Fourier-transform photocurrent spectroscopy, implying that these porous zones cannot be considered as being purely bi-dimensional, but have a spatial extension within the absorber layer.

Thin-film silicon solar cells are often deposited on textured ZnO substrates. The solar-cell performance is strongly correlated to the substrate morphology, as this morphology determines light scattering, defective-region formation, and crystalline growth of hydrogenated nanocrystalline silicon (nc-Si:H). Our objective is to gain deeper insight in these correlations using the slope distribution, rms roughness (σ(rms)) and correlation length (lc) of textured substrates. A wide range of surface morphologies was obtained by Ar plasma treatment and wet etching of textured and flat-as-deposited ZnO substrates. The σ(rms), lc and slope distribution were deduced from AFM scans. Especially, the slope distribution of substrates was represented in an efficient way that light scattering and film growth direction can be more directly estimated at the same time. We observed that besides a high σ(rms), a high slope angle is beneficial to obtain high haze and scattering of light at larger angles, resulting in higher short-circuit current density of nc-Si:H solar cells. However, a high slope angle can also promote the creation of defective regions in nc-Si:H films grown on the substrate. It is also found that the crystalline fraction of nc-Si:H solar cells has a stronger correlation with the slope distributions than with σ(rms) of substrates. In this study, we successfully correlate all these observations with the solar-cell performance by using the slope distribution of substrates.

High-efficiency thin-film silicon triple-junction solar cells in p-i-n configuration have been fabricated using amorphous silicon top cell absorber layers, as well as microcrystalline silicon middle and bottom cell absorbers. The triple-junction cells were fabricated on boron doped zinc oxide (ZnO) films with different surface morphologies. To this end, the naturally grown rough ZnO surfaces were flattened using an Ar plasma for three different treatment times. For the shortest time, we achieved a summed current density over 30 mA/cm2 and initial and stabilized conversion efficiencies of 13.5% and 12.5%, respectively. For the medium treatment time, we obtained the highest efficiencies (13.7% initial and 12.8% stable), whereas the longest treatment time led to the highest open-circuit voltage (VOC) of 1.91 V but lower current densities, leading to efficiencies of 12.9% initial and 12.2% stable, respectively. These results were obtained by combining various recently developed features and approaches: first of all, we implemented high-quality μc-Si:H cells with novel buffer layers, leading to very high efficiencies. Second, we applied randomly textured pyramids on the front glass to improve light in-coupling, and finally, we used very thin (~140 nm) top cells that led to a low light-induced degradation (5%-7% relative loss in efficiency).

We compare the performance of two back reflector designs on the optoelectrical properties of microcrystalline silicon solar cells. The first one consists of a 5-mu m-thick low-pressure chemical vapor deposition (LPCVD)-ZnO electrode combined with a white sheet; the second one incorporates an Ag reflector deposited on a thin LPCVD-ZnO layer (with thickness below 200nm). For this latter design, the optical loss in the nano-rough Ag reflector can be strongly reduced by smoothing the surface of the thin underlying ZnO layer, by means of an Ar-plasma treatment. Because of its superior lateral conductivity, the thin-ZnO/Ag back reflector design provides a higher fill factor than the dielectric back reflector design. When decreasing the roughness of the front electrode with respect to our standard front LPCVD-ZnO layer, the electrical cell performance is improved; in addition, the implementation of the thin-ZnO/Ag back reflector leads to a significant relative gain in light trapping. Applying this newly optimized combination of front and back electrodes, the conversion efficiency is improved from 8.9% up to 9.4%, for cells with an active-layer thickness of only 1.1 mu m. We thereby highlight the necessity to optimize simultaneously the front and back electrodes. Copyright (c) 2014 John Wiley & Sons, Ltd.

We report the recent advances and key requirements for high-efficiency “micromorph” tandem thin-film silicon solar cells composed of an amorphous silicon top cell and a microcrystalline silicon bottom cell. The impact of inserting a low-refractive-index silicon-oxide (SiOx) film as intermediate reflecting layer (IRL) is highlighted. We show that refractive indexes as low as 1.75 can be obtained for layers still conducting enough to be implemented in solar cells, and without no additional degradation. This allows for high top-cell current densities with thin top cells, enabling low degradation rates. A micromorph cell with a certified efficiency of 12.63% (short-circuit current density of 12.8 mA/cm2) is obtained for an optimized stack. Furthermore, short-circuit current densities as high as 15.9 mA/cm2 are reported in the amorphous silicon top-cell of micromorph devices by combining a 150-nm-thick SiOx-based IRL and a textured antireflecting coating at the air-glass interface.

Light management is of crucial importance to reach high efficiencies with thin-film silicon multijunction solar cells. In this contribution, we present light-management strategies that we recently developed. This includes high quality absorber materials, low-refractive index intermediate reflectors, and highly transparent multiscale electrodes. Specifically, we show the fabrication of high-efficiency tandem devices with a certified stabilized efficiency of 12.6%, triple-junction solar cells with a stabilized efficiency of 12.8%, recently developed smoothening intermediate reflector layers based on silicon dioxide nanoparticles, and periodic-on-random multiscale textures.

This paper focuses on our latest progress in n-i-p thin-micromorph solar-cell fabrication using textured back reflectors and asymmetric intermediate reflectors, both deposited by low-pressure chemical vapor deposition of zinc oxide. We then present microcrystalline bottom cells with high crystallinity, which yield excellent long wavelength response for relatively thin absorber thickness. In a 1.5-µm-thick µc-Si:H single-junction n-i-p solar cell, we thus obtain a short-circuit current density of 25.9 mA·cm −2 , resulting in an initial cell efficiency of 9.1%. Subsequently, the roughness of the intermediate reflector layer is adapted for the growth of high-performance amorphous silicon (a-Si:H) top cells. Combining bottom cells with high current, an optimal intermediate reflector morphology and a 0.22-µm-thick a-Si:H top cell, we reach high initial open-circuit voltages of 1.45 V, and we obtain a stabilized cell with an efficiency of 11.1%, which is our best stable efficiency for n-i-p solar cells.

In this paper, we investigate tandem amorphous/microcrystalline silicon solar cells with asymmetric intermediate reflectors grown in the n–i–p substrate configuration. We compare different types of substrates with respect to their light-trapping properties as well as their influence on the growth of single-junction microcrystalline cells. Our most promising back reflector combines a textured zinc oxide film grown by low-pressure chemical vapor deposition, a silver film for reflection, and a zinc oxide buffer layer. Grown on this substrate, microcrystalline cells exhibit excellent response in the infrared while keeping high open-circuit voltage and fill factor, leading to efficiencies of up to 10.0%. After optimizing the morphology of the asymmetric intermediate reflector, we achieve an n–i–p micromorph solar cell stabilized efficiency of 11.6%, using 270nm and 1.7μm of silicon for the absorber layer of the amorphous top cell and the microcrystalline bottom cell, respectively. Using this original device architecture, we reach efficiencies close to those of state-of-the-art n–i–p and p–i–n micromorph devices, demonstrating a promising route to deposit high-efficiency thin-film silicon solar cells on opaque substrates.

ABSTRACTThis short communication highlights our latest results towards high‐efficiency microcrystalline silicon single‐junction solar cells. By combining adequate cell design with high‐quality material, a new world record efficiency was achieved for single‐junction microcrystalline silicon solar cell, with a conversion efficiency of 10.69%, independently confirmed at ISE CalLab PV Cells. Such significant conversion efficiency could be achieved with only 1.8 µm of Si. Copyright © 2013 John Wiley & Sons, Ltd.

To further lower production costs and increase conversion efficiency of thin‐film silicon solar modules, challenges are the deposition of high‐quality microcrystalline silicon (μc‐Si:H) at an increased rate and on textured substrates that guarantee efficient light trapping. A qualitative model that explains how plasma processes act on the properties of μc‐Si:H and on the related solar cell performance is presented, evidencing the growth of two different material phases. The first phase, which gives signature for bulk defect density, can be obtained at high quality over a wide range of plasma process parameters and dominates cell performance on flat substrates. The second phase, which consists of nanoporous 2D regions, typically appears when the material is grown on substrates with inappropriate roughness, and alters or even dominates the electrical performance of the device. The formation of this second material phase is shown to be highly sensitive to deposition conditions and substrate geometry, especially at high deposition rates. This porous material phase is more prone to the incorporation of contaminants present in the plasma during film deposition and is reported to lead to solar cells with instabilities with respect to humidity exposure and post‐deposition oxidation. It is demonstrated how defective zones influence can be mitigated by the choice of suitable plasma processes and silicon sub‐oxide doped layers, for reaching high efficiency stable thin film silicon solar cells.