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29th European Photovoltaic Solar Energy Conference and Exhibition; 53-57, We propose to use metal nanogrids to enhance light absorption in ultrathin GaAs solar cells with 120 nm-thick and 220 nm-thick absorbers. Thanks to the transfer of the cells on a silver back mirror leading to Fabry Perot resonances in the semiconductor cavity, the short circuit current is increased by 40%. With an additional antireflection coating and a nanostructured back mirror, we obtain Jsc=17.4 mA/cm2 and 21.2 mA/cm2 for 120 nm and 220 nm-thick absorbers, respectively. It represents a two-fold enhancement of the Jsc compared to the non-transferred solar cells. We also show that is possible to realize really absorptive semiconductor devices within a few tens of nanometers. As a proof of concept, a super-absorber device consisting of a front nanogrid deposited on a 25 nm-thick GaAs layer absorbs more than 80% of the light over the 450-830 wavelength range.

29th European Photovoltaic Solar Energy Conference and Exhibition; 58-59, We propose a novel approach for light trapping in ultra-thin solar cells. It is based on absorption through multiple resonant modes in the critical coupling regime (A=100%). The model is briefly described. Numerical results are presented for c-Si and GaAs absorbers. The results (absorption efficiency, short-circuit current as a function of the absorber thickness) exceed the previous theoretical limits for light trapping in solar cells.

28th European Photovoltaic Solar Energy Conference and Exhibition; 2225-2227, Using a standard RF-PECVD reactor, silicon layers are epitaxially grown at 175°C. Few μm thick layers can be efficiently used as an absorber in the wafer equivalent approach: p++c-Si(wafer) / epi-Si(~μm) / n+aSi:H(12nm) has excellent diodes characteristics with FF of 80.5 % and 8,8% efficiency. By dilution of GeH4 in SiH4/H2 plasma, we show for the first time epitaxial SiGe alloys, with variable Ge fraction, grown at such low temperature. Structural and electronic properties of these materials has well has solar cells parameters are investigated using spectroscopic ellipsometry, Raman, TEM, J(V) characteristics and EQE. Using stress induced lift off technique ultra-thin Si layers of few cm2 are transferred on inexpensive substrates. Material quality is kept constant during the transfer process as shown by Raman FWHM of 5.3 cm-1, and ultra-thin heterojunctions solar cells are processed on foreign substrates. Preliminary results suggest lifetimes in higher than 67 μs for passivated 3,2μm thick transferred epitaxial PECVD grown layers.

28th European Photovoltaic Solar Energy Conference and Exhibition; 166-169, We propose to integrate 2D plasmonic nano-gratings in ultrathin GaAs solar cells and demonstrate light absorption enhancement on a broad spectral range. We present several geometries fulfilling this goal. First, we describe the multi resonant broadband absorption mechanism achieved in a 25 nm-thick GaAs layer thanks to a metallic nanostructure array. We then describe two novel architectures to trap light inside ultrathin (25 to 110 nm) semiconductor layers. Light trapping in these two designs is based on the use of metallic nanogrids on the front or the back side of the device. Thanks to numerical simulations, we show in both cases an increase in absorption, reaching more than 80% on average on the 550-900 wavelength range for a 25 nm-thick GaAs layer. We calculated a corresponding theoretical short circuit current density of 21.6 mA/cm2. For a 110 nm-thick p-i-n heterojunction complete solar cell design, we achieve a theoretical Jsc of 22 mA/cm2, representing a rise of 23% compared to the device without nanostructures. Along with these simulations, first experimental validation of these light trapping concepts is presented.

27th European Photovoltaic Solar Energy Conference and Exhibition; 2244-2248, We propose new solutions for broadband light trapping in ultrathin (≤100 nm) CIGS solar cells. By introducing metallic nano-arrays embedded in the ZnO:Al front contact layer, we show that multi-resonant optical absorption of light can be achieved in solar cells with 100 nm-thick CIGS layer. We analyze the origin of this broadband light trapping by numerical simulations. We predict a short-circuit current of 30 mA/cm², which independent of the angle of incidence and the polarization of the light. These results represent a breakthrough in the conception of CIGS solar cells, allowing a ten-fold reduction in CIGS thickness as compared to conventional CIGS solar cells, with potential conversion efficiency up to 17 %. We further describe the key steps of the fabrication of nano-structured solar cells with ultrathin and flat CIGS layers.

27th European Photovoltaic Solar Energy Conference and Exhibition; 2132-2136, We propose a novel design with a nano-patterned metallic layer embedded in the front window of an ultra-thin a-Si:H solar cell. Broadband light trapping is achieved under both light polarizations for a structure with a one-dimensional silver grating placed above the active layer. Losses in the metal are kept low over the visible range. The silver grating is also used as a front contact, allowing us to replace the conventional indium tin oxide layer by a non-absorbing material thereby reducing the absorption losses at short wavelengths. A theoretical short-circuit current of 14.6 mA/cm2 is predicted for a 90 nm-thick a-Si:H absorber layer. This multi-resonant approach has been experimentally validated with the fabrication and optical characterization of solar cells with an ultra-thin a-Si:H absorber layer.

26th European Photovoltaic Solar Energy Conference and Exhibition; 398-401, Advanced light trapping in thin absorber layers is a breakthrough for next-generation low cost and efficient photovoltaic (PV) systems. We propose a design based on an ultrathin amorphous silicon solar cell with an embedded sub-wavelength nanopatterned metallic layer. Light absorption enhancement in the active layer is achieved through the contributions of different mechanisms. A short-circuit density current of 14 mA/cm2 is theoretically predicted for a cell with a 90 nm-thick a-Si:H absorber layer. Preliminary experimental results of plasmonic structures with an ultrathin a- Si:H layer are presented.

26th European Photovoltaic Solar Energy Conference and Exhibition; 2863-2866, Conventional light trapping techniques are inefficient at the sub-wavelength scale. This is one of the main limitation for the thickness reduction of thin-film solar cells. We propose a novel architecture for broadband light absorption in a 100 nm-thick CIGS active layer. We show that this ultra-thin CIGS solar cell could preserve a constant short-circuit current while decreasing the active layer thickness by at least one order of magnitude. We focus on numerical optimisation to achieve a good short-circuit current with tunable multiple resonances. Fabrication issues are discussed.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 265-268, Next generations of solar cells require optical management at the sub-wavelength scale. In this regime, conventional light trapping techniques based on anti-reflection coatings and scattering structures are no more efficient. In this paper, we propose a multi-resonant ultra-thin solar cell with 25 nm-thick GaAs absorber. Strong optical confinement is achieved by metallic nanostructures. High absorption efficiency and low metal absorption losses are demonstrated numerically in the 500-800 nm wavelength range. The predicted efficiency of the ultra-thin solar cell is 17 %, with a nearly 100-fold decrease in absorber thickness compared to most thin-film photovoltaic devices.