Laura Ding

Nanoimprint lithography for high-efficiency thin-film silicon solar cells.

We demonstrate high-efficiency thin-film silicon solar cells with transparent nanotextured front electrodes fabricated via ultraviolet nanoimprint lithography on glass substrates. By replicating the morphology of state-of-the-art nanotextured zinc oxide front electrodes known for their exceptional light trapping properties, conversion efficiencies of up to 12.0% are achieved for micromorph tandem junction cells. Excellent light incoupling results in a remarkable summed short-circuit current density of 25.9 mA/cm(2) for amorphous top cell and microcrystalline bottom cell thicknesses of only 250 and 1100 nm, respectively. As efforts to maximize light harvesting continue, our study validates nanoimprinting as a versatile tool to investigate nanophotonic effects of a large variety of nanostructures directly on device performance.

Environmental Changes in MoTe2 Excitonic Dynamics by Defects-Activated Molecular Interaction.

Monolayers of group VI transition metal dichalcogenides possess direct gaps in the visible spectrum with the exception of MoTe2, where its gap is suitably located in the infrared region but its stability is of particular interest, as tellurium compounds are acutely sensitive to oxygen exposure. Here, our environmental (time-dependent) measurements reveal two distinct effects on MoTe2 monolayers: For weakly luminescent monolayers, photoluminescence signal and optical contrast disappear, as if they are decomposed, but yet remain intact as evidenced by AFM and Raman measurements. In contrast, strongly luminescent monolayers retain their optical contrast for a prolonged amount of time, while their PL peak blue-shifts and PL intensity saturates to slightly lower values. Our X-ray photoelectron spectroscopy measurements and DFT calculations suggest that the presence of defects and functionalization of these defect sites with O2 molecules strongly dictate their material properties and aging response by changing the excitonic dynamics due to deep or shallow states that are created within the optical band gap. Presented results not only shed light on environmental effects on fundamental material properties and excitonic dynamics of MoTe2 monolayers but also highlight striking material transformation for metastable 2D systems such as WTe2, silicone, and phosphorene.

Micromorph thin-film silicon solar cells with transparent high-mobility hydrogenated indium oxide front electrodes

We investigate the performance of hydrogenated indium oxide as a transparent front electrode for micromorph thin-film silicon solar cells on glass. Light trapping is achieved by replicating the morphology of state-of-the-art zinc oxide electrodes, known for their outstanding light trapping properties, via ultraviolet nanoimprint lithography. As a result of the high electron mobility and excellent near-infrared transparency of hydrogenated indium oxide, the short-circuit current density of the cells is improved with respect to indium tin oxide and zinc oxide electrodes. We assess the potential for further current gains by identifying remaining sources of parasitic absorption and evaluate the light trapping capacity of each electrode. We further present a method, based on nonabsorbing insulating silicon nitride electrodes, allowing one to directly relate the optical reflectance to the external quantum efficiency. Our method provides a useful experimental tool to evaluate the light trapping potential of novel photonic nanostructures by a simple optical reflectance measurement, avoiding complications with electrical cell performance.

Optimization of ZnO Front Electrodes for High-Efficiency Micromorph Thin-Film Si Solar Cells

The quest for increased performances in thin-film silicon micromorph tandem devices nowadays requires an increase of current density. This can be achieved with thin cells by combining both robust cell design and efficient light management schemes. In this paper, we identify three key requirements for the transparent conductive oxide electrodes. First, strong light scattering into large angles is needed on the entire useful wavelength range: A front electrode texture with large enough features is shown to grant a high total current (typically >26 mA/cm2 with a 2.4-μm-thick absorber material), while sharp features are reported to allow for high top cell current (>13 mA/cm2) and reduced reflection at the ZnO/Si interface. Second, sufficiently smooth substrate features are needed to guarantee a high quality of the silicon active material, ensuring good and stable electrical properties (typically Voc around 1.4 V). Third, conduction and transparency of electrodes must be cleverly balanced, requiring high transparent conductive oxide mobility (∼50 cm

On the Interplay Between Microstructure and Interfaces in High-Efficiency Microcrystalline Silicon Solar Cells

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Nanometer- and Micrometer-Scale Texturing for High-Efficiency Micromorph Thin-Film Silicon Solar Cells

/V/s) to maintain the sheet resistance below 30 Ω/sq while keeping absorption as low as possible. Optimization of these three key requirements using ZnO electrodes allowed us to realize high-efficiency micromorph devices with 13.5% initial and 11.5% stabilized efficiency.

Hydrogen plasma treatment for improved conductivity in amorphous aluminum doped zinc tin oxide thin films

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 defective material can be mitigated 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 single-junction solar cell.

Fully textured monolithic perovskite/silicon tandem solar cells with 25.2% power conversion efficiency

Optimized transparent conductive oxide front electrodes are vital to further increase the efficiency of thin-film silicon solar devices. We report details on the fabrication of multiscale textured zinc oxide substrates and their implementation in amorphous silicon/microcrystalline silicon tandem (micromorph) devices. Such substrates allow separate optimization of light trapping in the top and bottom cells, and efficient decoupling of transparency and conduction. We show in particular the need for sharp, nanoscale texturing for antireflection and light trapping in the top cell. We also show that smooth, micrometer-scale texturing can efficiently improve large-wavelength light management without degrading the quality of the silicon material grown on the substrate. By combining the appropriate morphologies, high currents can be reached in both the top and bottom subcells, while conserving the optimal electrical properties of the solar cells.

New Generation Transparent LPCVD ZnO Electrodes for Enhanced Photocurrent in Micromorph Solar Cells and Modules

Improving the conductivity of earth-abundant transparent conductive oxides (TCOs) remains an important challenge that will facilitate the replacement of indium-based TCOs. Here, we show that a hydrogen (H2)-plasma post-deposition treatment improves the conductivity of amorphous aluminum-doped zinc tin oxide while retaining its low optical absorption. We found that the H2-plasma treatment performed at a substrate temperature of 50 °C reduces the resistivity of the films by 57% and increases the absorptance by only 2%. Additionally, the low substrate temperature delays the known formation of tin particles with the plasma and it allows the application of the process to temperature-sensitive substrates.

Limits of carrier mobility in Sb-doped SnO2 conducting films deposited by reactive sputtering

Tandem devices combining perovskite and silicon solar cells are promising candidates to achieve power conversion efficiencies above 30% at reasonable costs. State-of-the-art monolithic two-terminal perovskite/silicon tandem devices have so far featured silicon bottom cells that are polished on their front side to be compatible with the perovskite fabrication process. This concession leads to higher potential production costs, higher reflection losses and non-ideal light trapping. To tackle this issue, we developed a top cell deposition process that achieves the conformal growth of multiple compounds with controlled optoelectronic properties directly on the micrometre-sized pyramids of textured monocrystalline silicon. Tandem devices featuring a silicon heterojunction cell and a nanocrystalline silicon recombination junction demonstrate a certified steady-state efficiency of 25.2%. Our optical design yields a current density of 19.5 mA cm−2 thanks to the silicon pyramidal texture and suggests a path for the realization of 30% monolithic perovskite/silicon tandem devices.An optimized two-step deposition process allows the formation of uniform layers of metal halide perovskites on textured silicon layers, enabling tandem silicon/perovskite solar cells with improved optical design and efficiency.