Efraín Ochoa-Martínez

P and Al Diffusion Process for Thin Si Wafers Studied by SEM and EDX

The aim of this study is to optimize processes suitable for production of extremely thin cells focusing in two of the solar cell fabrication steps: P and Al diffusion for pn junction and BSF formation respectively. SEM analysis has been used to monitor BSF junction uniformity and EDX to determine composition of the diffused Al layer formed by screen-printing. The influence of the preparation conditions is studied in order to avoid the formation of an eutectic Al-Si alloy, that is the responsible for the bow of thin wafers. Phosphorus deposition has been carried out by spray coating, a technique that allows in-line implementation and reduces manipulation of the cell. Samples were prepared on 250 and 100 μm silicon wafers, comparing various mixtures of H3PO4 as precursor, the resulting emitters were characterized measuring their surface resistivity and analyzing surface composition by XPS and FTIR.

Modelling of lattice matched dilute nitride 4-junction concentrator solar cells on Ge substrates

Technology Computer Aided Design modeling is used to examine the performance under light concentration of a 4-J solar cell Ge-based that includes a 1-eV MBE-grown dilute nitride subcell. The 1-eV solar cell is modeled and examined by using material parameters extracted from detailed electro-optical characterization prior to be included into a multijunction structure. The modelling reveals the impact of the electric field-assisted collection in the performance of single junction solar cells and its effect when included in a 4-Junction solar cells. This effect is responsible for the lower FF (~15% lower) in the 4J when including the dilute nitride subcell, especially if it limits the photocurrent. Finally, an optimization procedure based on dilute nitrides with higher material quality is performed resulting in a 4-Junction solar cell with an efficiency of 47% for concentrations between 1000-2000 suns direct terrestrial spectrum.

Fabrication and characterization of thin c-Si solar cells by low cost methods

Following the decreasing thickness tendency in PV industry, there have been prepared c-Si solar cells on thin substrates (100 - 120 μm) by means of low cost methods, suitable for industrial application like spray coating and screen printing. Commercial and non-commercial doping sources are tested. A special emphasis is made on contamination tracing throughout the entire process by means of XPS and SIMS analysis. Current-Voltage, spectral response and contact resistance scanning of the cells are presented. Efficiencies of up to 12.2% have been achieved for 4×4 cm cells without antireflective or special passivation technology.

Fabrication and characterization of thin silicon solar cells produced by in-line spray coating

The present study addresses the metallization and characterization of thin (∼110 μm) crystalline silicon solar cells with a sprayed-on emitter prepared from orthophosphoric acid aqueous solutions and commercial spin-on dopants. The feasibility of such process especially for thin cells is analyzed, as well as the characteristics which derivate from the use of each precursor. The use of passivating and antireflective coatings on these devices has been essayed. The performance has been compared with that of cells prepared by an industrial process and featuring a traditional antireflective coating. Shunt resistance has been found to be the main limiting factor in order to increase cell efficiency, probably associated with microcracks due to the fragility of the cell. The characterization and the analytical model suggest that this method could produce cells exceeding 17% efficiency.

Emitter diffusion method for extremely thin silicon wafers

The aim of this study is to optimize a low cost phosphorus diffusion process suitable for pn junction formation on extremely thin (<100um) c-Si solar cells. The deposition of the doping layer has been carried out by spray coating of commercial and non-commercial precursors. There have been monitored sheet resistance, effective lifetime and oxygen content in the prepared emitters.