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We focus on the use of mixed-phase phosphorus-doped SiOx (n-SiOx) as advanced n-layer in both the junctions of micromorph tandem solar cells. The material, deposited by plasma enhanced chemical vapor deposition at two pressure values, has been fully characterized. Direct evidence of phase separation into nanocrystalline Si and amorphous oxygen-rich phases is reported, accompanied by information on the optical properties of the material as a whole and the two phases, as gathered by spectroscopic ellispometry. The multifunctional role of the n-SiOx layer, providing for beneficial optical effects at both the intermediate and back reflecting stage in micromorph solar cells, is established. In particular superior performance (improved fill factor and enhanced bottom solar cell spectral response) is demonstrated with use at the rear side of the device in combination with a simple Ag back reflector, with respect to the standard device employing the conventional ZnO/Ag reflector. © 2013 Elsevier B.V. All rights reserved.

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.

Photon management is a key component in the development of efficient solar cells. Especially light-trapping concepts have a high potential to realize enhanced efficiencies. Here, we give an overview over several light trapping concepts for photon management in solar cells. These include basic as well as advanced light-trapping concepts. The theoretical limits of light path enhancement of the different concepts are given and experimental work on these topics is resented. The potential of 3D photonic crystals is discussed in the context of the corresponding approaches as well.

Light management in single and multi-junction solar cells is becoming increasingly important to optimize the optical and electro-optical properties of solar cells. The limits of light path enhancements are discussed for non-resonant and resonant optical nanostructures as well as for angular-selective and energy-selective approaches. The potential of 3D photonic crystals for photon management in solar cells is discussed for the corresponding concepts and the state of the art is reviewed.

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.

The micromorph solar cell (stack of amorphous and microcrystalline cells) concept is the key for achieving high efficiency stabilized thin film silicon solar cells. We introduce a device structure that allows a better control of the light in-coupling into the two subcell components. It is based on an asymmetric intermediate reflector, which increases the effective thickness of the a-Si:H by a factor of more than three. Hence, the a- Si:H thickness reduction diminishes the light induced degradation, and micromorph tandem cells with 11.2% initial and 9.8% stabilized efficiencies (1000 h, 50 °C, and 100 mW/cm2) are made on plastic substrates with Tg

We investigate n-i-p/n-i-p micromorph tandem cells deposited at process temperature below 200°C in order to be compatible with low Tg (

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 270 nm and 1.7 mm 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.

The deposition of a stack of amorphous (a-Si:H) and microcrystalline (µc- Si:H) tandem thin film silicon solar cells (micromorph) requires at least twice the time used for a single junction a-Si:H cell. However, micromorph devices have a higher potential efficiency, thanks to the broader absorption spectrum of µc-Si:H material. High efficiencies can only be achieved by mitigating the nanocracks in the µc-Si:H cell and the light-induced degradation of the a- Si:H cell. As a result, µc-Si:H cell has to grow on a smooth substrate with large periodicity (>1 µm) and the a-Si:H cell on sharp pyramids with smaller feature size (~350 nm) to strongly scatter the light in the weak absorption spectra of a-Si:H material. The asymmetric intermediate reflector introduced in this work uncouples the growth and light scattering issues of the tandem micromorph solar cells. The stabilized efficiency of the tandem n-i-p/n-i-p micromorph is increased by a relative 15% compared to a cell without AIR and 32% in relative compared to an a-Si:H single junction solar cells. The overall process (T

The use of zinc oxide (ZnO) based intermediate reflector (ZIR) in micromorph solar cells using low pressure chemical vapor deposition (LPCVD) was investigated. The influences of deposition temperature and dopant gas concentration on grain size and lateral electrical conductivity measurements are presented. Further ZIR deposition conditions were then directly evaluated in micromorph solar cell devices. Their electrical performances were compared to reference cells and cells incorporating silicon oxide based intermediate reflector. It is shown that both reduced ZIR deposition temperature and increased total flow rate allow for better performing devices with increased shunt resistance, as further supported by lock-in thermography shunt imaging. Relative micromorph efficiency increase of above 7% is shown with thin ZnO layers, along with absence of loss or even small increase of total current in the whole structure compared to cells without intermediate reflector. © 2011 Elsevier B.V. All rights reserved.

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.