Mathieu Charrière

Light trapping in solar cells: can periodic beat random?

Theory predicts that periodic photonic nanostructures should outperform their random counterparts in trapping light in solar cells. However, the current certified world-record conversion efficiency for amorphous silicon thin-film solar cells, which strongly rely on light trapping, was achieved on the random pyramidal morphology of transparent zinc oxide electrodes. Based on insights from waveguide theory, we develop tailored periodic arrays of nanocavities on glass fabricated by nanosphere lithography, which enable a cell with a remarkable short-circuit current density of 17.1 mA/cm(2) and a high initial efficiency of 10.9%. A direct comparison with a cell deposited on the random pyramidal morphology of state-of-the-art zinc oxide electrodes, replicated onto glass using nanoimprint lithography, demonstrates unambiguously that periodic structures rival random textures.

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

9.4% Efficient Amorphous Silicon Solar Cell on High Aspect-Ratio Glass Microcones

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Light harvesting schemes for high efficiency 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.

Single to multi-scale texturing for high efficiency micromorph thin film silicon solar cell

High aspect-ratio three-dimensional (3D) a-Si:H solar cells have been fabricated to enhance a light absorption path while maintaining a short carrier collection length. Substantial efficiency enhancement in 3D solar cells was achieved due to the boost in JSC with no degradation of FF which is comparable to FF obtained from 2D solar cells.

Nanomoulding of transparent zinc oxide electrodes for efficient light trapping in solar cells

In Thin Film Silicon (TF-Si) solar cells light harvesting schemes must guarantee an efficient light trapping in the thin absorber layers without decreasing the silicon layers quality and consecutively the p-i-n diodes electrical performance. TF-Si solar cells resilience to the substrate roughness is reported to be possibly improved through optimizations of the cell design and of the silicon deposition processes. By further tailoring the superstrate texture, amorphous silicon / microcrystalline silicon (a-Si:H/μc-Si:H) tandem solar cells with an initial efficiency up to 13.7 % and a stabilized efficiency up to 11.8 % are demonstrated on single-scale textured superstrates. An alternative approach combining large and smooth features nanoimprinted onto a transparent lacquer with small and sharp textures from as-grown LPCVD ZnO is then shown to have a high potential for further increasing TF-Si devices efficiency. First results demonstrate up to 14.1 % initial efficiency for a TF-Si tandem solar cell.

Optimization of thin film silicon solar cells on highly textured substrates

Improving micromorph devices performances nowadays requires a current density increase: good devices typically exhibit an open circuit voltage times fill factor product of one (Voc×FF∼1.4V×0.71=1). Their short-circuit current density (Jsc) 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 Voc 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.