José Rubén Gutierrez

Laser-induced damage for crystalline silicon solar cells

This paper delves into the electrical characterization of the laser induced damage and in the removal of the damage by the chemical treatment. We believe that an accurate etching removes properly the damage so that there could be more pros than cons that give advantage to this isolating method than the inline expensive wet etching isolating method. For that, a fabrication process is established, which helps to demonstrate the potential of the technique. The correlation between the humps and the laser-induced damage has been demonstrated. This phenomenon has been explained based on the increase of the second exponential saturation current in a small area of the device.

Lowly doped emitters for crystalline silicon solar cells

High efficiency silicon solar cells are related to low recombination currents and high open-circuit voltages. The emitter is characterized by the impurity concentration profile. The usual parameters for this purpose are sheet resistance (Rs), surface impurity concentration (Ns) and depth junction. In order to obtain high quality emitter, we look for reduce Joe values, lowly doped emitters with low Ns and Rs values and deep depth junctions. A wet oxidation step is incorporated to minimize the dead layer and the peak surface concentration. Our work has developed a process resulting in ~ 0.8 × 1020 cm-3 and 60 Ω/□.

Study of the effect of different hole sizes on mechanical strength of wafers for back contact solar cells

Drilling process on wafers to produce EWT or MWT solar cells is a critical fabrication step, which affects on their mechanical stability. The amount of damage introduced during drilling process depends on the density of holes, their size and the chemical process applied afterwards. To quantify the relation between size of the holes and reduction of mechanical strength, several sets of wafers have been prepared, with different hole diameter. The mechanical strength of these sets has been measured by the ring on ring bending test, and the stress state in the moment of failure has been deduced by FE simulation.

Defects detection in p-n junction isolation by electroluminescence

The laser isolation of p-n junction is the origin of electrical damage in solar cells. Although several models explain this effect, it is usual to appeal to complex recombination mechanisms to get an accurate representation of their non-ideal I-V behavior. Other models give to an accurate and simple explanation of this damage introducing a connecting resistance of the defect to the rest of the solar cell structure, but its large resistance value is not well understood. This work uses electroluminescence techniques to validate the proposed model by means of finding the recombination paths from the cell volume to highly recombination defects and giving an explanation to this high value of the connection resistance from its two-dimensional behavior.

Effect of thermal oxidation process on emitter profiling and solar cell performance

The control of phosphorus surface concentration is a very important due to that have a strongly effect on solar cell efficiency. Two kinds of phosphorus profiles were simulated Using TSUPREM-4 simulating program, the first emitter profile is obtained with the only pre-deposition process for various diffusion temperature and the second emitter profile by adding a drive-in after introducing thermal dry oxidation step after diffusion process for various diffusion temperature. The first emitters show maximum efficiencies using PC1D simulation program about (η ≈ 16-19%) with emitter surface concentration Ns = (0.8-3) ×1020cm-3 and junction depth (0.35-0.6)μm. For second emitters the surface concentration is reduced with the same diffusion parameters and show improvement of the maximum efficiencies (η ≈ 20-21%) with surface doping concentration Ns = (0.6-2) ×1019 (cm-3) and junction depth (1.3-2.3)μm. The silicon oxide layer thicknesses become thicker for highly surface doped emitters. Silicon solar cell performance and parameters are improved after the thermal dry oxidation process and become more efficient.

Soft and deep phosphorus diffusion for P/Al solar cell structure with selective emitters

In this work, design process combines with conventional technology used for selective emitters and Al-BSF (back surface field) which leads to lowly doped and deep emitters focused on low surface doping concentration and moderate sheet resistance emitters. A deep junction for P/Al structure with selective emitter has been achieved. This process runs in oversaturation condition. Surface concentration values ranging from 3.6×10¹⁹ cm⁻³ to 7.2×10¹⁹ cm⁻³ and depth junction values from 0.52 to 0.71 μm for sheet resistance ∼100 Ω/□ has obtained. Process is industrially feasible, in this process the gettering is higher than conventional process. The estimated saturation current density (Joe) could be around 30–40 fA/cm² with optimization but in present work Joe is under 80 fA/cm².

Upgrading the silicon IBC to the 40% efficiency

A new approach of a tandem silicon technology is presented. That is based on improving a classical IBC structure in which top and bottom cells are jointed on. The electrical connection of this tandem device is not the classical series connected but the voltage matched approach. The final device would be of a maximum efficiency of 39.5% but it is more tolerant to light spectrum, degradation and not optimum materials than conventional series connected and using well known materials for each level an 30 to 32% efficiency level sounds possible.

The IBC Structure as Support for Three Band-Gaps Tandem Devices

The IBC structure is one of the champions in the efficiency competition of silicon devices. Industrial cells are produced with efficiency values surpassing 24%, what is due to an excellent design in terms of separation from the region where carriers are energized and emitters where these carriers are selectively separated. Their position in the back of the cell allows the access to both quasi-Fermi-levels from this surface, and enables the access to their corresponding levels of top or bottom cells from this place, far away from the entrance of light and simplifying the design of complex multi-band-gap structures. This paper will shown several of these structures, many of them achievable with well known materials with technology developed for the microelectronic sector. These solutions would be to cells with efficiencies in the 30-32 %. Using optimum materials, not clear at this moment, we estimate a technological limit of 39.4 % for these new structures.