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Back surface alternating contacts solar cell

Photovoltaic with better long-term performance in high-radiation environments; higher conversion efficiency at elevated temperatures; and a lighter, more flexible structure for mobile applications improve overall conversion efficiency to more than 30%

Energy

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GaAs-BAC cell model 3D structure. Front-surface and back-surface heterojunctions reduce surface recombination. A thin insulating layer occupies space between the emitter and BSF contacts to prevent electrical shorting.

Solar cells produce electric current and voltage to power an external load, and a primary goal of solar cell design is to increase power output while balancing manufacturing cost.

Due to the widespread availability and low-cost of silicon versus other semiconductor materials, it has remained the overwhelming choice for solar cell manufacturers. But the use of silicon presents problems. Since silicon is an indirect bandgap semiconductor, energy (heat) must be exchanged with the crystal lattice to free electrons. This is not an optimal process.

Further, silicon only uses a portion of the solar spectrum to free electrons. Much of the remaining spectrum energy is absorbed by the crystal lattice, which causes the temperature of the solar cell to rise during normal operation. Additionally, the low surface-state density characteristic of silicon makes it susceptible to radiation damage over time, especially in outer-space applications. Finally, high-energy particles from the sun create intermediate energy states in a solar cell which lead to higher recombination rates and lower efficiency.

The surface-state density of gallium arsenide (GaAs) is much larger than silicon, and the material is inherently harder to total-dose radiation. GaAs is a direct bandgap semiconductor that absorbs photon energy and free electrons without transferring momentum, and less heat is absorbed in the crystal lattice. This generates significant improvements for solar cell design such as lower operating temperatures in a given environment.

GaAs provides additional advantages over silicon including thinner absorbing layers, which improves flexibility and reduces weight. Additionally, GaAs cells maintain performance advantages as irradiance decreases. Generally, high-efficiency GaAs cells produce about 20% more power than high-efficiency silicon cells at room temperature, and about 28% more power at typical operating temperatures. Still, they have not achieved their theoretical limit in power conversion.

Navy researchers have advanced GaAs solar cells by placement of all electrical contacts on the back surface of the solar cell. This improves both the optical and electrical performance of the solar cell since shading is eliminated and robust electrical contacts may be used to decrease serial resistance.

This back surface, alternating contacts (BAC) solar cell featuring p-or-n-type GaAs incorporates alternating p-n junction regions on the back surface of the cell. Various layers of p-or-n-type GaAs are interfaced together to collect charge carriers, and a thin layer of AlGaAs is applied to the front and back surfaces to minimize recombination of charge carriers. Highly reflective, back-surface, metal contacts are used to recycle photons and to further improve optical and electrical performance.

This US patent 9,842,957 is directly related to US patent 9,865,761.

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