Stencil-nanopatterned back reflectors for thin-film amorphous silicon n-i-p solar cells

We fabricated amorphous silicon n-i-p solar cells with two types of nanopatterned back reflectors using stencil lithography. One reflector type has a plasmonic grating that is embedded in the ZnO layer; the other one has a metallic grating patterned on top of the Ag layer. From comparing the short-circuit current densities of the two device types, we conclude that light trapping through grating coupling is more efficient than coupling of light through the excitation of localized surface plasmons. The back reflectors were patterned with dot arrays by evaporation of Ag through millimeter-size stencil membranes. The stencils themselves were patterned by wafer-scale nanosphere lithography. The dot arrays have a periodicity of 428 nm and efficiently scatter light in the near-infrared wavelength range. Both back reflectors types lead to the same morphology for the silicon films. This allows us a fair comparison of the two light coupling mechanisms. We found a 14% and 19% short-circuit current density enhancement for the plasmonic and for the metallic grating, respectively. The external quantum efficiency gains between 550 and 650 nm show similar guided modes resonances for both device types, but the excitation is stronger for the device with the metallic grating.

Stencil-Nanopatterned Back Reflectors for Thin-Film Amorphous Silicon n-i-p Solar Cells

Christophe Ballif, Jürgen Brugger, Franz-Josef Haug, Mona Julia Katharina Klein, Céline Pahud, Andreea Veronica Savu, Oscar Vazquez Mena

Ieee-Inst Electrical Electronics Engineers Inc2013

Optimization of the Asymmetric Intermediate Reflector Morphology for High Stabilized Efficiency Thin n-i-p Micromorph Solar Cells

Christophe Ballif, Rémi Biron, Mathieu Gérard Boccard, Grégory Bugnon, Matthieu Despeisse, Laura Ding, Simon Hänni, Franz-Josef Haug, Sylvain Nicolay, Céline Pahud, Gaetano Parascandolo

This paper focuses on our latest progress in n-i-p thinmicromorph solar-cell fabrication using textured back reflectors and asymmetric intermediate reflectors, both deposited by lowpressure 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.

Ieee-Inst Electrical Electronics Engineers Inc2012

Infrared light management in high-efficiency silicon heterojunction and rear-passivated solar cells

Christophe Ballif, Stefaan De Wolf, Antoine Descoeudres

Silicon heterojunction solar cells have record-high open-circuit voltages but suffer from reduced short-circuit currents due in large part to parasitic absorption in the amorphous silicon, transparent conductive oxide (TCO), and metal layers. We previously identified and quantified visible and ultraviolet parasitic absorption in heterojunctions; here, we extend the analysis to infrared light in heterojunction solar cells with efficiencies exceeding 20%. An extensive experimental investigation of the TCO layers indicates that the rear layer serves as a crucial electrical contact between amorphous silicon and the metal reflector. If very transparent and at least 150 nm thick, the rear TCO layer also maximizes infrared response. An optical model that combines a ray-tracing algorithm and a thin-film simulator reveals why: parallel-polarized light arriving at the rear surface at oblique incidence excites surface plasmons in the metal reflector, and this parasitic absorption in the metal can exceed the absorption in the TCO layer itself. Thick TCO layers-or dielectric layers, in rear-passivated diffused-junction silicon solar cells-reduce the penetration of the evanescent waves to the metal, thereby increasing internal reflectance at the rear surface. With an optimized rear TCO layer, the front TCO dominates the infrared losses in heterojunction solar cells. As its thickness and carrier density are constrained by anti-reflection and lateral conduction requirements, the front TCO can be improved only by increasing its electron mobility. Cell results attest to the power of TCO optimization: With a high-mobility front TCO and a 150-nm-thick, highly transparent rear ITO layer, we recently reported a 4-cm(2) silicon heterojunction solar cell with an active-area short-circuit current density of nearly 39 mA/cm(2) and a certified efficiency of over 22%. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4772975]

American Institute of Physics2013

Infrared light management in high-efficiency silicon heterojunction and rear-passivated solar cells

Christophe Ballif, Stefaan De Wolf, Antoine Descoeudres

American Institute of Physics2013