André Augusto

The Influence of Spectral Albedo on Bifacial Solar Cells: A Theoretical and Experimental Study

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.

Analysis of the recombination mechanisms of a silicon solar cell with low bandgap-voltage offset

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.

Thin absorbers for defect-tolerant solar cell design

Thin silicon wafers provide a pathway to lower cost and lower capital intensity PV module manufacturing. They can also produce higher-efficiency devices with less expensive feedstock and crystallization processes because they require shorter diffusion lengths and operate at higher carrier injection. Through simulation, we show that thin Si wafers can be incorporated into high-efficiency cells with greater defect tolerance than thick wafers. Experimentally, we demonstrate the importance of excellent surface passivation to realizing the efficiency potential of thin silicon solar cells and show that such passivation can be achieved in silicon/amorphous silicon heterojunction devices.

All silicon tandem solar cell

Crystalline silicon is consistently the dominant material for commercial photovoltaic devices. Exploiting the direct and indirect bandgap of silicon results in a silicon-silicon tandem solar cells with possible efficiency benefits over standard single-junction silicon solar cells. Epitaxial growth offers a way to make such cells and the resulting devices have higher voltage and lower currents leading to much lower module losses. All silicon tandem devices were modeled in PC1D using precise solar spectrums generated with SMARTS. The optimal layer thicknesses found when the input spectrum is AM1.5G for a silicon-silicon device are: 3.3 μm for the top absorber and 172 μm for the bottom absorber. The modeled device produces an efficiency of 21.3%, a 1.1% relative increase over a model for a commercial silicon cell.

Series connection front-to-front and back-to-back of silicon heterojunction solar cells

Alternating cells with p- and n-type emitters enables direct series connection of equivalent sides, i.e. front-to-front and back-to-back connection of adjacent cells. The challenge is to match the current of cells with p- and n-type emitters. The electrical properties of silicon heterojunction solar cells with front and rear junctions are remarkably similar. The short-circuit current density mismatch between front and rear junction cells is as low as 0.1 mAcm-2. The cells are connected using thin indium coated wires. One-cell and two-cells modules were manufactured, and efficiencies up to 21.2% were reached for one-cell modules. Electroluminescence of the two-cells module is a good indication about the quality of the direct series connection between front and rear junction cells.

Flexible silicon heterojunctions solar cells and modules

In this manuscript is discussed the potential of using silicon heterojunction solar cells to obtain flexible silicon modules. Modules were manufactured using non-metalized silicon heterojunction solar cell with thicknesses down to 70 μm. The modules were rolled around curved surfaces with different diameters. Rolling diameters down to 5 cm were tested. Damages in the solar cells due to the rolling process were inspected using photoluminescence (PL). PL images after the rolling show promising results towards flexibility. Rolling reliability tests were also performed in thin passivated wafers (60-80 μm). The wafers were rolled down to 7 cm diameter. PL images show no evidences of mechanical damage in the wafers due to the lamination or after the rolling process.