Development of High Efficiency Low Cost III-V Solar Cells ‎



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We have investigated the integration of high efficiency solar cells with ‎crystallographically compatible large grain Ge thin films (fabricated on glass) for cost ‎reduction purposes. We have combined defect tolerant III-V dilute nitride (quantum ‎engineered) solar cells and tunnel diode technologies with low cost poly-Ge substrates ‎‎(developed by Al-Induced Crystallization method). Hence, it was required to allocate our ‎research into three complementary paths, the fabrication of defect tolerant tunnel diodes, ‎implementation of defect tolerant high efficiency quantum structured solar cells and ‎development of low-cost Ge substrates. ‎ First, we theoretically and experimentally evaluated the impact of moderate to ‎large ‎dislocation densities on the performance of GaAs tunnel ‎diodes. We have shown for ‎excessive dislocation ‎densities (ND>1×109 cm-2) I-V characteristics ‎undergo a severe ‎degradation. However, for devices with moderate ‎dislocations (about 2×108cm-2), above ‎what has been perceived as acceptable for PV applications, there is even ‎an improvement ‎in term of peak current densities. ‎ Next, we studied ultra-thin defect tolerant solar cells compatible with highly defective ‎low-cost substrates. A 1eV quantum engineered design was investigated for elevated ‎absorption capability to compensate device thickness reduction. Dilute nitride III-V ‎materials were used for their ideal bandgap in 4-junction solar cells (lattice matched with ‎Ge), high absorption coefficient and mere valence band offset with GaAs (as an ideal ‎case for quantum engineering). A resonantly coupled multi-quantum well and ‎supperlattice system were theoretically and experimentally studied for the validation of a ‎thermo-tunneling concept. The results suggest a world record Voc beyond the radiative ‎limit (0.4 eV below bandgap). ‎ In the last step, we have empirically investigated a low temperature ‎‎(<400C) ‎Al ‎induced ‎crystallization of amorphous Ge on glass. We were initially able to achieve ‎polycrystalline Ge, mainly oriented towards [111] direction, with grain sizes as large as ‎‎200µm. Further, we studied experimental parameters favoring Ge crystallization towards ‎‎[110] or [100] crystals direction, suitable for solar cells application. The AlOx layer, ‎between Al and Ge layer, have been found a critical parameter facilitating Ge ‎crystallization. The thickness of Al layer has been known as determining parameter in ‎selecting [110] cubic crystals orientation or [100] rhombohedral Ge structure.‎



Solar cells, III-V, Dilute nitride, Quantum wells, Ge, Al Induced Crystallization, Thin films, Tunnel Diode