Polycrystalline gallium indium phosphide (GaxIn1-xP or GaInP) alloy is a promising material system for the top junction of a silicon tandem solar cell, since it has a tunable direct bandgap from 1.35 eV to 2.2 eV. It offers an interesting alternative to the most commonly studied CIGS, CdTe and perovskite alloys for several reasons. Epitaxially grown GaInP alloys have achieved single junction 1-sun efficiencies of over 20%, proving their potential for high efficiencies and stability at room temperature. This is important, since other promising alloys with higher defect tolerance are still trying to overcome this challenge. The band structure of GaInP alloy system along with its various III-V alloys is also well studied for epitaxial growth. This knowledge would be extremely valuable for defect engineering.
This work focuses on optimizing the growth parameters of various polycrystalline GaInP alloys, quantifying the defect characteristics and eventually demonstrating the polycrystalline GaInP solar cells. GaInP alloys with several gallium compositions were grown at various substrate temperatures using molecular beam epitaxy (MBE). Epitaxial growth on polycrystalline substrates was carried out to isolate the effect of interface and surface from that of the grain boundaries. MBE was particularly used to limit impurity segregation at the grain boundaries. This is expected to help better associate composition and structure variation with the grain boundary characteristics. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to study the structural properties, and photoluminescence (PL) spectroscopy was used to assess the optical properties of the grown materials.
XRD was used to calculate the lattice parameters and also confirmed the zincblende crystal structure of the GaInP alloys. Grainsize of 0.5-1μm was characterized using SEM. It is observed that the grainsize and its distribution increases with temperature. Contrarily, the grainsize is found to reduce with higher gallium incorporation. Photoluminescence spectroscopy showed a higher peak intensity for the 10% gallium alloy compared to the 0, 5 and 50% gallium alloys. This result is promising since it suggests a better optoelectronic property of polycrystalline material with bandgap more than 1.43 eV. The discovery has reinforced the promise of this new family of polycrystalline material system to be considered for top junction of silicon tandem solar cells.
Once the growth conditions for various compositions are optimized, temperature and power dependent PL will be used to characterize the nature of recombination in the material system. Time resolved PL and admittance spectroscopy will be used to quantify the minority carrier decay lifetime and defect energy state density respectively. Surface potential across grain boundaries will be measured using Kelvin probe force microscopy. Finally, demonstration of polycrystalline GaInP solar cell will also be presented in the article.