Resumen de Desarrollo de células solares de doble unión de GaInP/GaAs para concentraciones luminosas elevadas (Development of GaInP/GaAs dual-junction solar cells for high light concentrations).
This Thesis deals with the development of the monolithic GaInP/GaAs dual-junction solar cell for high concentration applications, including its theoretical analysis and modeling, and the experimental manufacture and characterization of the needed semiconductor structures grown by metal-organic vapour phase epitaxy (MOVPE), and of the final concentrator solar cell devices. A special emphasis is put on the optimization of the solar cell concentration response, as the most important distinctive characteristic when compared to other GaInP/GaAs dual-junction solar cells developed in other laboratories. In the first part of this Thesis, a theoretical study of the concentrator GaInP/GaAs dual-junction solar cell is presented. The efficiency limit of different configurations of this solar cell, and the intrinsic model of the monolithic, series connected approach are analyzed. The extrinsic properties and distributed effects exhibited by this kind of devices are then studied by using quasi-3D models based on distributed circuit units. The optimum front grid layout for the operation of our GaInP/GaAs dual-junction solar cell under high concentrations is designed, and the effect of non-uniform light profiles is appraised. The second part begins with an introduction to the experimental research carried out in this Thesis, in which the MOVPE technology available at I.E.S. – U.P.M. is described. A review of the semiconductor material and solar cell device characterization techniques used is also presented, emphasizing on the issues specific to the dual-junction solar cell devices. The experimental development of the GaInP top cell, the tunnel junction and the complete dual-junction solar cell is then dealt with. As for the GaInP top cell, the MOVPE growth of the GaInP and Al(Ga)InP materials is first tackled. These materials are then used to build the semiconductor structure of the GaInP solar cell, paying an special attention to the front and back surface passivation. As for the tunnel junction, the material research conducted to obtain very high doping levels in GaAs and AlGaAs is firstly treated. Then the experimental work concerning the study of the main tunnel junction semiconductor structure approaches developed in this Thesis is presented. Both the performance of the tunnel junction devices fabricated and the influence of their MOVPE growth on the growth and quality of the GaInP top cell is assessed in each case. A record peak current density of 10100 A/cm2 and a series resistance at zero bias as low as 1.6•10-5 ohms•cm2 are achieved with a AlGaAs/GaAs tunnel junction doped with carbon and tellurium. In the last chapter of this part, the most representative dual-junction solar cell designs developed in this Thesis are explained and analyzed by means of quantum efficiency, I-V curves and concentration response measurements. The weaknesses of each design are assessed experimentally and theoretically in order to determine the modifications required to improve the performance of the solar cell. An efficiency of 32.6% at 1000 suns is eventually achieved, which is the highest value reported so far for a monolithic dual-junction solar cell. Moreover, the simulations carried out show that an efficiency approaching 34 % is attainable. In the last part, the summary and global conclusions of this Thesis are outlined and some future research activities are envisaged.
