We have investigated the influence of the spectral albedo on the power output of bifacial solar cells. We adapted the Shockley-Queisser radiative flux balance framework to account for a variation of the spectrum and intensity of the incoming light. We find that the ideal band gap and the maximum efficiency depend on the spectral albedo of the surroundings and that optimal cell performance cannot be assessed when only accounting for a spectrally independent albedo. With a spectral albedo model, we predict that the power output for a bifacial silicon solar cell surrounded by green grass is 3.1% higher than for a wavelength-independent albedo, and even 5.2% higher for white sand. We experimentally verify this trend for silicon heterojunction solar cells and we derive the ideal spectral albedo.
The mathematical dependence of bandgap-voltage offset on Auger and radiative recombination is derived. To study the recombination near the intrinsic limit, we manufacture thin silicon heterojunction test structures designed to minimize surface recombination, and to measure voltages and effective lifetimes near the Auger and radiative limit. Open-circuit voltages over 760 mV were measured on 50-μm-thick structures, leading to bandgap-voltage offsets at open-circuit down to 0.35 V. The Auger and radiative recombination represents over 90% of the recombination at open-circuit. This dominance also holds at the maximum power point, giving pseudo-fill factors of 86%. We demonstrate the potential of thin silicon devices to reach high voltages, and bandgap-voltage offsets in line with the best reported for direct bandgap materials such as gallium indium phosphide and gallium arsenide.
For commercially-viable solar-grade silicon, thinner wafers and surface saturation current densities below 1 fA cm−2, are required to significantly increase the practical efficiency limit of solar cells.
Tilting of carbon encapsulated metallic nanocolumns in carbon-nickel nanocomposite films by ion beam assisted deposition Appl. Phys. Lett. 101, 053112 (2012) Displacive radiation-induced structural contraction in nanocrystalline ZrN Appl. Phys. Lett. 101, 041904 (2012) Electron beam evaporated cadmium sulphide thin films for solar cell applications J. Renewable Sustainable Energy 4, 043104 (2012) Epitaxial GaN films by hyperthermal ion-beam nitridation of Ga droplets J. Appl. Phys. 111, 113521 (2012) Elevated temperature dependence of energy band gap of ZnO thin films grown by e-beam deposition This work reports for the first time results on MgO tunnel junctions prepared by ion beam. The MgO barrier was deposited from a ceramic MgO target using an assisted beam, following the deposition and assisted beam phase diagram which relate the beam profile with the current and energy. The deposition rate for MgO is calculated with and without assisted beam, and compared with the experimental values. The MgO film growth on Ta/ CoFeB / MgO simple stacks was optimized aiming at a ͑002͒ preferred orientation for the MgO growth, measured by x-ray diffraction. The optimum assist beam energy was tuned for each deposition beam condition ͑+800, + 1000, + 1200 V͒, using assist beams of 40 mA ͑ϳ130 A / cm 2 ͒ with 0 to + 600 V. Without assist beam, no texture is observed for the MgO, while the ͑002͒ orientation appears for assisted deposition. The optimum range of assist voltages is large, being limited by the onset of etching at high voltages, reducing the deposition rate. Magnetic tunnel junctions were deposited with the structure Ta 50 Å / Ru 200 Å / Ta 50 Å / Mn 78 Ir 22 150 Å / Co 90 Fe 10 30 Å / Ru 8 Å / Co 56 Fe 24 B 20 40 Å / MgO t / Co 56 Fe 24 B 20 30 Å / Ru 30 Å / Ta 50 Å, with the MgO barrier deposited with the conditions optimized by x rays. The effect of the assist beam energy on the junction properties ͑magnetoresistance and magnetization͒ are discussed. Tunnel magnetoresistance values up to 110%, with RA products of 100-400 ⍀ m 2 , for 11 Å thick MgO barriers are obtained using assisted deposition with a +100 V assist beam, which is a major improvement of the ϳ30% of TMR, if no beam is used.
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