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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 transparent electrodes. Their substitution by equally thin doped nanocrystalline silicon layers has often been argued to resolve these drawbacks. However, low-temperature deposition of highly crystalline doped layers of such thickness on amorphous surfaces demands sophisticated deposition engineering. In this paper, we review and discuss different strategies to facilitate the nucleation of nanocrystalline silicon layers and assess their compatibility with SHJ solar cell fabrication. We also implement the obtained layers into devices, yielding solar cells with fill factor values of over 79% and efficiencies of over 21.1%, clearly underlining the promise this material holds for SHJ solar cell applications.

We introduce passivating hetero-interfaces in single-junction microcrystalline silicon (mc-Si:H) solar cells. We investigate the effect of different i–n layer stacks in thin mc-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 mc-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 mc-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 mc-Si:H devices or mc-Si:H devices with a very high bulk-material quality, well-mastered interfaces and doped layers are of paramount importance for high efficiency.

This 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.

This paper gives new insights into the role of both the microstructure and the interfaces in microcrystalline silicon (μc- Si) single-junction solar cells. A 3-D tomographic reconstruction of a μc-Si solar cell reveals the 2-D nature of the porous zones, which can be present within the absorber layer. Tomography thus appears as a valuable technique to provide insights into the μc- Si microstructure. Variable illumination measurements enable to study the negative impact of such porous zones on solar cells performance. The influence of such defectivematerial can bemitigated by suitable cell design, as discussed here. Finally, a hydrogen plasma cell post-deposition treatment is demonstrated to improve solar cells performance, especially on rough superstrates, enabling us to reach an outstanding 10.9% efficiency microcrystalline singlejunction solar cell.

The challenge for all photovoltaic technologies is to maximize light absorption, to convert photons with minimal losses into electric charges, and to efficiently extract them to the electrical circuit. For thin-film solar cells, all these tasks rely heavily on the transparent front electrode. Here we present a multiscale electrode architecture that allows us to achieve efficiencies as high as 14.1% with a thin-film silicon tandem solar cell employing only 3 μm of silicon. Our approach combines the versatility of nanoimprint lithography, the unusually high carrier mobility of hydrogenated indium oxide (over 100 cm(2)/V/s), and the unequaled light-scattering properties of self-textured zinc oxide. A multiscale texture provides light trapping over a broad wavelength range while ensuring an optimum morphology for the growth of high-quality silicon layers. A conductive bilayer stack guarantees carrier extraction while minimizing parasitic absorption losses. The tunability accessible through such multiscale electrode architecture offers unprecedented possibilities to address the trade-off between cell optical and electrical performance.

We report here on the latest research developments of tandem micromorph (amorphous/ microcrystalline) silicon solar cells in our laboratory. High conversion efficiency for micromorph cells requires both a dedicated light management to keep the absorber layers as thin as possible, and optimized growth conditions of the microcrystalline silicon (μc-Si:H) material. We will show that an improved cell design based on the use of silicon oxide doped layers permits to achieve high efficiencies on substrates that are usually considered as inappropriate for μc-Si:H because of their roughness. Furthermore, recently, new front contacts based e.g. on bi-layers or Ultraviolet nanoimprint lithography were developed, leading to very promising results in micromorph solar cells. While efficiencies of 12.7% initial and 11.3% stable could be achieved with only 1.1 μm of bottom cell on a front rough Low Pressure Chemical Vapor Deposited (LPCVD) ZnO, a remarkable 12% initial was also reached on textured replica (as on the master). Emphasis is also laid in our lab on increasing the deposition rate of mc-Si:H while maintaining high quality material. This is done by reducing the interelectrode gap while working at high deposition pressure, in "powder free" processes at 40 MHz. We could observe that high pressure-low hydrogen dilution process conditions lead to dense high quality material. So far, conversion efficiencies up to 8.5% have been achieved at 1 nm/s for single junction μc-Si:H solar cells with 1.8 μm thick absorber layer. We also report a promising micromorph tandem initial efficiency of 11.9% with the μc-Si:H i-layer at 0.9 nm/s. High efficiency micromorph solar cells could thus be fabricated under conditions that are favorable to industrial, low-cost, fabrication of micromorph modules. Recent results of tandems combining original substrates and improved deposition processes suggest that stabilised efficiencies close to 13% can be expected in a near future.

Improving micromorph devices performances nowadays requires a current density increase: good devices typically exhibit an open circuit voltage times fill factor product of one (V oc ×FF∼1.4V×0.71=1). Their short-circuit current density (J sc ) value thus dictates their efficiency (expressed in %). Maximizing it with reasonable cells thicknesses necessitates therefore combining robust cell design with adequate light management through transparent electrodes and intermediate reflectors engineering. We will first show how record micromorph devices (13.5% initial and >11.5% stabilized efficiencies) are prepared on optimized single-layer ZnO electrodes. Such electrodes requirements will be discussed: 1) Strong and wide light scattering is needed on the entire useful wavelength range. Large features grant high total currents (>26mA/cm2) while sharp ones allow for high top cell currents (>13mA/cm2). 2) Sufficiently small or smooth substrate features permits high quality cell growth, providing good cell design (typically V oc over 1.4V). 3) Good conduction and transparency for electrodes (requiring ∼50cm2/V/s TCO mobility) should preserve sheet resistance close to 20Ω/□ (for FF>70%) with low absorption. We will then focus on pushing further micromorph devices potential. Either textured intermediate reflectors can fulfill the bottom cell needs, or double-texture substrates can be implemented: light scattering at large wavelengths is here achieved via nanoimprint lithography (a versatile approach to glass-texturing), topped by small and sharp ZnO features guaranteeing high top cell current. By combining excellent TCO with smart under-structures, thin devices delivering high currents with excellent efficiencies are within reach.

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.

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.

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.