1064 Nm InGaAsP Multijunction Laser Power Converters

Attributed to their excellent directivity,inherent immunity to electromagnetic interference and low optical transmission loss,wireless laser power transfer systems can be used to supply high power for long distance energy transmission in space with a small receiver area.Laser power converters with high conversion efficiencies are key components of wireless laser power transfer system and have attracted increasing attentions in recent years.High-power 1064 nm laser beam with a small divergence angle and high atmospheric transmittance can be obtained from Nd:YAG(neodymium-doped yttrium aluminum garnet)laser,and has been widely used in the WLPT.Laser power converters(LPCs)for converting 1064 nm laser show a promising prospect in the field of WLPT.In this dissertation,the 1064 nm multijunction LPCs are developed for the first time via the structure design,material growth,and device processing.The main contents and achievements are as follows.1.We selected the InGaAsP lattice-matched to the p-type InP as the active layer and designed triple-and six-junction 1064 nm multijunction LPCs using the absorption coefficient of the active layer at 1064 nm according to the principle of current matching.The output characteristics of the triple-and six-junction InGaAsP LPCs were simulated considering the subcell characteristics by the single-diode photovoltaic cell model.Moreover,effects of the interface recombination on the open-circuit voltage,short-circuit current,minority carrier diffusion current as well as the reverse saturation current were studied.2.The high-quality epitaxial wafers were grown by the low-pressure MOCVD system by optimizing the growth conditions.After optimizing the device processing flow and the relevant parameters,we fabricated the firstly-reported triple-junction InGaAsP LPCs.The open-circuit voltage of the triple-junction InGaAsP LPC reaches 2.17 V,and the conversion efficiency is 34.1%under 0.98 W/cm2 1064 nm laser.Based on the device physics,the efficiency loss mechanism were studied.Besides,the characteristics of the triple-junction InGaAsP LPCs under different laser powers,incident laser angles and bias voltages were also investigated.3.The six-junction InGaAsP LPC demonstrates an open circuit voltage of 4.69 V and conversion efficiency of 41.2%at 1064 nm laser power density of 20.14 W/cm2.Temperature dependent Ⅰ-Ⅴ characteristics of the six-junction LPCs were measured,and we found that the short-circuit current decreases with increasing temperature and is mainly restricted by the bottom subcell 6.The temperature coefficient of the open-circuit voltage is fitted to be-9.77 mV/K at a laser power of 0.174 W,which is the sum of the subcell temperature coefficients in terms of the open-circuit voltage and decrease slightly with increasing the laser power.Furthermore,the temperature coefficient of the subcell open-circuit voltage also increases with the increasing subcell thickness.The conversion efficiency decreases with the temperature at a rate of-0.09%abs/K independent of laser power.Finally,the temperature dependent wide-voltage-range Ⅰ-Ⅴ characteristics were measured and the worse performances of the six-junction InGaAsP LPC incorporating the p++-InAlAs/n++-InP tunnel junction were explained.4.The open-circuit voltages of the six-junction InGaAsP LPCs of different sizes were investigated under different laser power densities,proving that the metal electrode shading area is an important factor affecting the open-circuit voltage.The open-circuit voltages of the LPCs of different sizes as functions of the laser irradiation time were studied,and the correlation between LPC heat dissipation capacity and LPC area were determined.Based on the heat conduction equation,we established a transient two-dimensional temperature field distribution model and numerically calculated the temperature-rise as well as the temperature field distribution of an LPC under laser illuminationd.