Towards High Efficiency Gallium Arsenide Phosphide Solar Cells on Silicon.

摘要

Solar technologies with high efficiency and low cost are essential for photovoltaic electricity generation to reach grid parity with traditional fossil fuel sources. While 90% of the solar modules sold in 2014 were based on economical single-junction Si technology, the efficiency has hit an efficiency plateau close to 26% in the past decade. In contrast, III-V compound semiconductor solar cells now hold conversion efficiency records exceeding 45%, though at fir higher manufacturing costs than Si The integration of high efficiency III-V compound solar materials with cost effective Si substrates appear to be a promising path towards high efficiency at a reasonable cost. This concept has the potential to leverage decades of established Si manufacturing processes and infrastructure.;In particular, cascading an epitaxial Gallium Arsenide Phosphide (GaAs yP1-y) top cell with bandgap energy of 1.6-1.8eV onto a 1.1eV Si subcell can enable a theoretical efficiency exceeding 40%, -'10% (absolute) higher than that for Si alone. While Si technology is mature, the GaAs yP1-y material system was relatively unexplored prior to this work. GaP is the III-V material with the closest lattice constant to Si, allowing a convenient compositional grading path from GaP/Si to GaAs yP1-y. Recent advances in GaP/Si template growth have eliminated defects such as anti-phase domains (APDs), stacking faults (SFs) and microtwins (MTs). However, threading dislocations are unavoidable in such a tandem cell due to lattice mismatch, and until recently the threading dislocation density (TDD) of GaAsyP1-y on Si was 108 cm -2 with high bandgapvoltage offsets (Woc) of 0.7-0.8 V, where ~0.4 V is the semi-empirical limit for Woc. GaAsyP 1-y/GaAs solar cells with TDD as low as 3x105 cm -2 have exhibited low Woc values below 0.5 V, close to the semi-empirical limit. Due to the high Woc, the highest efficiencies of GaAs yP1-y/GaP/Si solar cells prior to this work were ~10%. These early results highlight the significance of minimizing TDD due to dislocation-mediated non-radiative recombination. The ability to accurately quantify TDD in the starting GaP/Si templates and final GaAsyP1-y solar cells are extremely important in the efforts to achieve lower TDD and higher efficiency.;In this work, we set out to develop low TDD and low Woc single junction GaAsyP1- y/GaP/Si solar cells to enable a clear path towards high efficiency III-V on Si dual-junction solar cells. In order to meet this goal, we developed defect selective etching (DSE) techniques to quantify TDD accurately for both GaAsyP1-y solar cells and GaP/Si substrates. Additionally, we built a suite of complementary TDD characterization techniques including electron beam induced current (EBIC) and electron channeling current (ECCI) to provide robust feedback to the material growth process.;To reduce the TDD of GaP/Si and GaAsyP1-y/GaP/Si, we explored the effect of different growth parameters in the dislocation dynamics model; growth temperature, growth rate and grading rate. Analysis of the strain relaxation dynamics of 500 nm GaP/Si layers proved that TDD is highly sensitive to the initial growth temperature and growth rate and can be controlled by balancing the effects of dislocation nucleation and glide. Subsequently, we showed the direct benefits of lower starting GaP/Si template TDD on final GaAsyP1-y solar cell TDD and Woc.;The dislocation nucleation and glide regimes observed in GaP/Si were transferrable to GaAsyP1-y/GaP/Si We demonstrate that the GaAsyP1-y/GaP/Si TDD can also be controlled by growth temperature and grading rate. Using the knowledge of strategies for TDD reduction in both GaP/Si and GaAsyP1-y/GaP/Si, we achieved the lowest TDD values on GaAsyP1-y/GaP/Si of 4.18-4.64x10 6 cm-2, with a corresponding low Woc of 0.54-0.55 V, the best reported for GaAsP/GaP/Si solar cells to date. Device design improvements of a thinner emitter and wide-bandgap InAIP window layer enabled Jsc= 13.1-13.3 mA/cm2 and peak internal quantum efficiency, IQE= 98.4%. The overall single junction efficiencies of 11.5-12.0% are the best reported for GaAsyP1-y/GaP/Si single junction solar cells. The results in this work demonstrate a clear path towards high efficiency III-V on Si dual-junction solar cells.