High-Stable-Efficiency Tandem Thin-Film Silicon Solar Cell With Low-Refractive-Index Silicon-Oxide Interlayer

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/cm(2)) is obtained for an optimized stack. Furthermore, short-circuit current densities as high as 15.9 mA/cm(2) 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.

Optical Layers for Thin-film Silicon Solar Cells

Peter Cuony

In this work we develop and analyze optical layers for use in Micromorph solar cells, a tandem configuration with an amorphous silicon top cell and a microcrystalline silicon bottom cell. The morphology of the front electrode has a decisive role in maximizing the efficiency of a solar cell. To reach a better understanding of the requirements for the front electrode surface, we present a wide range of morphologies that can be obtained with as-grown rough zinc oxide (ZnO) and post-deposition argon plasma surface treatments. We correlate the morphological parameters to light scattering in transmission and reflection, and identify the inclination angles of ZnO pyramids as the most pertinent parameter for thin-film silicon solar cells. We show that there are no reflection losses at the interface between as-grown rough ZnO and silicon, and we quantify the reflection losses for smoother interfaces. With titanium dioxide, produced by reactive magnetron sputtering, we demonstrate that for flat interfaces reflection losses of up to 7.4% can be canceled at 550 nm wavelength. We show that in microcrystalline silicon cells p-type silicon sub-oxide (SiOx) can also be used to reduce reflection losses, and at the same time can act as field-creating window layer. We develop a wide range of mixed-phase SiOx layers and with energy-filtered transmission electron microscopy we reveal the filamentous nanostructure of films produced with high hydrogen dilution of the precursor gas mixture. A partial decoupling of electrical and optical parameters is achieved for such films, where the transparency and the refractive index are determined mostly by the silicon oxide matrix, while sufficient transverse conductivity for the use in solar cells is maintained by few-nanometer-wide silicon filaments. In thin-film silicon solar cells, p-type SiOx~0.5 shows the best results as a window layer with enhanced transparency, while strongly phosphorous-doped n-type SiOx~1 is best suited for use as an intermediate reflecting layer in tandem cells due to a low refractive index of 1.8 with sufficient transverse conductivity. The tunable resistance of doped SiOx layers can be used at various locations in a Micromorph cell to quench shunts or bad diodes in spatially non-uniform devices. Implementation of SiOx with these three functionalities in Micromorph cells has significantly contributed to increasing the best initial efficiency from 11.8% in 2006 to 13.5% in 2010. While rough ZnO front electrodes and SiOx-based intermediate reflecting layers both help to improve Micromorph cell efficiencies, they render the devices more sensitive to water-vapor-induced degradation. With lock-in thermography measurements we demonstrate that water contamination affects mainly the cell borders resulting in non-uniform distribution of the local open circuit voltage. Finally we show how free-carrier absorption and surface plasmon polariton absorption can reduce the reflectivity of the back reflector, and we present preliminary results aimed toward the realization of an absorption-free back reflector making use of polished ZnO and sputtered silver layers.

EPFL2011

Novel Micromorph Solar Cell Structures for Efficient Light Trapping and High-Quality Absorber Layers

Mathieu Gérard Boccard

Key elements involved in the fabrication of Micromorph thin-film silicon solar cells, a tandem device including an amorphous silicon top cell and a microcrystalline silicon bottom cell, are studied in this manuscript. Due to the very short diffusion lengths of photogenerated carriers in both materials, the photoactive layers of both sub-cells of Micromorph devices must be kept thin enough to ensure carrier collection. Due to this limited thickness, not all valuable light (i.e. light of higher energy than the band-gap) can be fully absorbed after one pass through the absorber layers. Advanced light-harvesting schemes are thus mandatory to achieve high conversion efficiencies. Random rough interfaces are typically used to induce light scattering in the photoactive layers, thus elongating the light path through these layers, enhancing their absorption. A simple and analytical way of modeling light harvesting in thin-film solar cells is developed. Its validity is demonstrated by comparing with experimental measurements involving different types of rough interfaces. It is shown that present light-scattering schemes come close to the best theoretically achievable scattering from random rough interfaces. With the morphology of a state-of-the-art rough ZnO layer, 32mA/cm2 can be obtained for a 1-μm-thick μc-Si:H layer, compared to 33.2 mA/cm2 for the Yablonovitch limit. Most of the gains in terms of light management are therefore to be made by making non-active layers more transparent (since these layers are presently responsible of ∼ 7 mA/cm2 of losses for a 1-μm-thick μc -Si:H layer). Parasitic absorption in non-active layers is also shown to be equally detrimental on both sides of the cell in the infrared part of the spectrum, corresponding to the wavelength range for which light trapping is most important. To improve significantly light trapping, complementary strategies to random rough interfaces must therefore be applied. For such a strategy, part of the light has to be prevented to escape from the cell. This can be obtained for example by using an angular filter, transmitting all light up to a certain incidence angle, and reflecting all light of higher incidence angle. An experimental optical setup based on spatial filtering is presented. It is shown to prevent 80% of light from escaping the cell, additionally to other light trapping strategies. A strong absorption enhancement of the complete device is demonstrated, at the cost of a reduction of the acceptance angle. However, most of the spared light is shown to be absorbed in non-active layers. A drastic reduction of parasitic absorption from these layers is therefore identified as a prerequisite to benefit from a better light trapping. Turning then to complete device analysis, the requirements of the front electrode for high- efficiency Micromorph devices are discussed point-by-point, focusing both on optical and electrical requirements. The need for sharp and relatively small features for an efficient coupling of light in the top cell is notably pointed out, as well as the need for features large enough to scatter light of wavelength up to 1100 nm for a high bottom cell current. The impact of sharp and large features on the electrical performances of the cells is also underlined. An optimal morphology is proposed, exhibiting features that are the smallest enabling scattering of light up to 1100 nm, and the sharpest that do not harm the electrical quality of the bottom cell. A device with a stable efficiency of 11.8% could be obtained on such a substrate. To avoid this trade-off, the combination of features with different sizes in a multi-scale elec- trode architecture is studied. It is shown that even though no gain in terms of light scattering is seen compared to a state-of-the-art single-scale morphology, a better electrical quality can be obtained, making the multi-scale approach of interest. A noteworthy 14.1% initial efficiency is demonstrated with such an architecture, with a short-circuit current density of 14 mA/cm2 for a total thickness of the silicon layers of 3 μm. Another way to add a degree of freedom in the design of a Micromorph device is also presented, employing a smoothening intermediate layer between both sub-cells. A separate tuning of the morphology inducing light scattering in each sub-cell can be made. Several routes are explored, out of which promising results are obtained by using a spin-coated lacquer on the top cell. A slight etching uncaps the tips of the top cell surface and allow for electrical conduction, whereas the smoothening effect is preserved. A large top cell current boost (up to+2.5mA/cm2)andopen-circuitvoltageimprovement(+50mV)areobtained,withmany adjustable processing parameters to obtain various morphologies. The newly developed concepts enable a better understanding of the present limitations of Micromorph devices. We believe that, by implementing these concepts in Micromorph tandem devices employing state-of-the-art amorphous and microcrystalline single-junction solar cells, stable 13.5% to 14% efficiency are within reach. Yet, increasing further the efficiency of Micromorph devices will likely require material improvements.

EPFL2012

Amorphous silicon oxide window layers for high-efficiency silicon heterojunction solar cells

Christophe Ballif, Stefaan De Wolf, Antoine Descoeudres, Johannes Peter Seif

In amorphous/crystalline silicon heterojunction solar cells, optical losses can be mitigated by replacing the amorphous silicon films by wider bandgap amorphous silicon oxide layers. In this article, we use stacks of intrinsic amorphous silicon and amorphous silicon oxide as front intrinsic buffer layers and show that this increases the short-circuit current density by up to 0.43 mA/cm2 due to less reflection and a higher transparency at short wavelengths. Additionally, high open-circuit voltages can be maintained, thanks to good interface passivation. However, we find that the gain in current is more than offset by losses in fill factor. Aided by device simulations, we link these losses to impeded carrier collection fundamentally caused by the increased valence band offset at the amorphous/crystalline interface. Despite this, carrier extraction can be improved by raising the temperature; we find that cells with amorphous silicon oxide window layers show an even lower temperature coefficient than reference heterojunction solar cells (-0.1%/K relative drop in efficiency, compared to -0.3%/K). Hence, even though cells with oxide layers do not outperform cells with the standard design at room temperature, at higher temperatures—which are closer to the real working conditions encountered in the field—they show superior performance in both experiment and simulation.