This paper reports the proposal, design, and demonstration of ultra-thin GaAs single-junction solar cells integrated with a reflective back scattering layer to optimize light management and minimize non-radiative recombination. According to our recently developed semi-analytical model, this design offers one of the highest potential achievable efficiencies for GaAs solar cells possessing typical non-radiative recombination rates found among commercially available III-V arsenide and phosphide materials. The structure of the demonstrated solar cells consists of an In0.49Ga0.51P/GaAs/In0.49Ga0.51P double-heterostructure PN junction with an ultra-thin 300 nm thick GaAs absorber, combined with a 5 μm thick Al0.52In0.48P layer with a textured as-grown surface coated with Au used as a reflective back scattering layer. The final devices were fabricated using a substrate-removal and flip-chip bonding process. Solar cells with a top metal contact coverage of 9.7%, and a MgF2/ZnS anti-reflective coating demonstrated open-circuit voltages (Voc) up to 1.00 V, short-circuit current densities (Jsc) up to 24.5 mA/cm2, and power conversion efficiencies up to 19.1%; demonstrating the feasibility of this design approach. If a commonly used 2% metal grid coverage is assumed, the anticipated Jsc and conversion efficiency of these devices are expected to reach 26.6 mA/cm2 and 20.7%, respectively.
A pulsed voltage bias method is proposed to eliminate the measurement artifacts of external quantum efficiency (EQE) of multi-junction solar cells. Under the DC voltage and light biases in the EQE measurements, the output current and voltage drops on the subcells under the chopped monochromatic light are affected by the low shunt resistances of the Ge subcells, which cause the EQE measurement artifacts for InGaP/InGaAs/Ge triple junction solar cells. A pulsed voltage bias superimposed on the DC voltage and light biases is used to properly control the output current and subcell voltages to eliminate the measurement artifacts. SPICE simulation confirms that the proposed method completely removes the measurement artifacts.
The optimization of the layout of front metal contact is critical for improving the efficiency of solar cells under high concentrations. In the traditional design approach, the optimum finger spacing and busbar width are determined by minimizing the total power loss associated with the front contact[1-3]. However, there are several assumptions that need to be carefully reexamined. In this work, a rigorous approach to optimize the layout of the front contact is proposed.
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