A NEW PARADIGM FOR > 50% PHOTOVOLTAIC EFFICIENCIES
Marina S. Leite, Harry A. Atwater
Currently, there is a critical need for a photovoltaic design that will convert sunlight into electricity with practical efficiencies higher than 50 %. Multijunction solar cells (MJSCs) are one of the most promising options to achieve ultra-high efficiencies. In such devices, the subcells are arranged in a tandem configuration, where the highest energy photons are absorbed by the top subcell and the remaining photons are progressively absorbed by the subsequently lower band gap subcells. III-V compound semiconductors are generally used to fabricate MJSCs; however, limitations imposed by the lattice constants of available substrates strongly restrict which materials can be used for high-quality epitaxial growth.
Detailed balance calculations show that an optimized lattice-matched triple-junction solar cell (1.93 eV InAlAs/1.06 eV InGaAsP/0.73 eV InGaAs) can achieve an efficiency > 50 % under merely 50-suns illumination, surpassing the conventional Ge- and GaAs-based approaches. The optimized design corresponds to a lattice spacing equal to 5.80 Å. Here we demonstrate the fabrication of the single crystalline template used as a starting point for the epitaxial growth: 50 mm diameter dislocation-free fully relaxed single crystalline InxGa1-xAs layers with lattice parameter equal to the bulk value. The elastic strain of originally coherently-strained films is relieved upon substrate removal and the crystal assumes its bulk lattice parameter, as confirmed by X-ray diffraction and transmission electron microscopy. The biaxial in-plane distortion affects the energy band gap of the alloy, as observed in optical measurements. These templates can be used as a building block for epitaxial growth at a variety of lattice parameters, overcoming the limitations imposed by the existence of only a few bulk substrates.
As a key proof-of-concept on the band gap and material combination, we have fabricated each independent subcell lattice-matched to InP - InAlAs(1.47 eV)/InGaAsP(1.06 eV)/InGaAs(0.74 eV). The subcells were designed to be current-matched (12.0 mA/cm2), allowing for the evaluation of the overall device performance. The fabricated individual single-junction InAlAs, InGaAsP, and InGaAs subcells show cell efficiencies equal to 5.6 %, 8.0 %, and 9.4 %, respectively, under AM 1.5 g 1-sun illumination. The independently-connected four terminal solar cells showed an open circuit voltage (Voc) equal to 1.8 V, demonstrating the tandem activity of the device. The combination of the experimental results with device modeling analysis for a 5.80 Å lattice-matched optimized MJSC design, indicate that these monolithic 3-junction solar cells can be successfully implemented as a new alternative for ultra-high efficiency MJSCs.