Juan Carlos Jimeno

Inverted ITO-free organic solar cells based on p and n semiconducting oxides. New designs for integration in tandem cells, top or bottom detecting devices, and photovoltaic windows

We report organic photovoltaic devices in which the standard ITO transparent electric contact has been substituted by lower cost ultrathin metallic electrodes. Solution and vacuum processable n and p-type semiconductors provide the electrode with the rectifying behavior of the diode. We are in this way able to invert the built-in electric field at wish and make the device deliberately either top or bottom sensitive with the same efficiency depending on the application. Taking advantage of these new generation electrodes we furthermore report devices with fill factors over 70%—to our knowledge, the largest published to date for an organic photovoltaic cell—and power conversion efficiencies over the state-of-art with 3.5% in inverted P3HT:PCBM devices, ITO free designs over 2.5% and (semi)transparent photovoltaic devices with conversion efficiencies close to 2.6%. This breakthrough could once and for all trigger the fabrication of organic tandem solar cells and photovoltaic windows.

Impact of Extended Contact Cofiring on Multicrystalline Silicon Solar Cell Parameters

During the temperature spike of the contact cofiring step in a solar cell process, it has been shown that the concentration of lifetime-killer dissolved metallic impurities increases, while adding an annealing after the spike getters most of the dissolved impurities toward the phosphorus emitter, where they are less detrimental. The contact cofiring temperature profile, including the after-spike annealing, has been called extended contact cofiring, and it has also been proposed as a means to decrease the emitter saturation current density of highly doped emitters, thus benefiting a wide range of materials in terms of detrimental impurity content. The aim of the present work is to determine the effect of performing this additional annealing on contact quality and solar cell performance, looking for an optimal temperature profile for reduction of bulk and emitter recombination without affecting contact quality. It presents the effect of the extended cofiring step on fill factor, series resistance, and contact resistance of solar cells manufactured with different extended cofiring temperature profiles. Fill factor decreases when extended cofiring is performed. Series resistance and contact resistance increase during annealing, and this happens more dramatically when the temperature peak is decreased. Scanning electron microscopic images show silver crystallites in contact with silver bulk before the annealing that allow a direct current path, and silver crystallites totally surrounded by glass layer (>100 nm thick) after annealing. Glass layer redistribution and thickening at low temperatures at the semiconductor-metal interface can be related to the series resistance increase. Degradation of series resistance during the temperature spike, when it is below the optimum one, can also be attributed to an incomplete silicon nitride etching and silver crystallite formation. To make full use of the beneficial effects of annealing, screen-printing metallic paste development supporting lower temperatures without a thick glass layer growth is needed.

Module to module monitoring system, M3S, a new strategy for PV-system monitoring

A new system for PV-plant monitoring is presented. The system allows the fault detection in each module of a PV plant. For that, electronic circuits are connected in parallel with each PV module sensing voltages around all the components. An extra current generator can modify slightly the current in one string introducing current in parallel to a selected module. A simple and efficient module model, not physically linked, allows the detection of a fault in each module obtaining the real output power, their availability power in its maximum power point and the estimated power in the case of an adequate running of each module. The system has been designed in terms of simplicity, easy implementation and low cost, not having extra irradiance or temperature sensors.

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.

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.

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.