Johan Nijs

Partial shadowing of photovoltaic arrays with different system configurations: literature review and field test results

Partial shadowing has been identified as a main cause for reducing energy yield of grid-connected photovoltaic systems. The impact of the applied system configuration on the energy yield of partially shadowed arrays has been widely discussed. Nevertheless, there is still much confusion especially regarding the optimal grade of modularity for such systems. A 5-kWp photovoltaic system was installed at K.U. Leuven. The system consists of three independent subsystems: central inverter, string inverter, and a number of AC modules. Throughout the year, parts of the photovoltaic array are shadowed by vegetation and other surrounding obstacles. The dimensions of shadowing obstacles were recorded and the expectable shadowing losses were estimated by applying different approaches. Based on the results of almost 2 years of analytical monitoring, the photovoltaic system is assessed with regard to shadowing losses and their dependence on the chosen system configuration. The results indicate that with obstacles of irregular shape being close to the photovoltaic array, simulation estimates the shadowing losses rather imprecise. At array positions mainly suffering from a reduction of the visible horizon by obstacles far away from the photovoltaic array, a simulation returns good results. Significant differences regarding shadow tolerance of different inverter types or overproportional losses with long module strings could not be confirmed for the system under examination. The negative impact of partial shadowing on the array performance should not be underestimated, but it affects modular systems as well as central inverter systems.

Advanced manufacturing concepts for crystalline silicon solar cells

An overview is given concerning current industrial technologies, near future improvements and medium term developments in the field of industrially implementable crystalline silicon solar cell fabrication. The paper proves that considerable improvements are still possible, both in efficiency and in production cost. The paper also proves that a lot of effort is being put worldwide on thinner substrates and on thin-film crystalline silicon cells deposited on cheap carriers, in order to save in substrate cost and in order to gain more independence from availability problems of silicon feedback.

Testing the islanding protection function of photovoltaic inverters

A major safety issue in grid-connected photovoltaics is to avoid nonintentional operation in islanding mode when the grid is being tripped. Worst-case conditions under which islanding can occur are specified analytically. The circuit that is commonly used for testing is described. The issue of appropriate test conditions with regard to reactive-power injection to the grid is discussed and the stabilizing impact of rotating machines and resonant circuits is evaluated in detail. Islanding test results for small inverters are presented. They confirm that very simple islanding protection methods that are commonly used, are likely to fail, if inverters are loaded with considerable capacitance. The obtained results support the assessment of the islanding protection function. They emphasize important points when defining new certification procedures for upcoming guidelines and standards.

Self-Standing Porous Silicon Films by One-Step Anodizing

A novel technique for the formation and lift-off of thin porous silicon films from starting substrates in a single step by electrochemical etching in hydrofluoric acid-based solutions is described. Lift-off or separation of porous silicon (PS) film occurs under specific sets of current density and HF concentration, which also determines the PS film thickness that can vary from a few micrometers to a few tens of micrometers. A model based on a diffusion-limited mass transfer of HF molecules from the bulk of the solution to the point of reaction at the pore tip is proposed to explain the lift-off phenomena. Based on this an expression for expected PS film thickness and separation time as a function of current density and hydrofluoric acid concentration has been derived. The experimental results are in agreement with the proposed medel.

Simple integral screenprinting process for selective emitter polycrystalline silicon solar-cells

This letter describes a new simple fabrication process, developed recently for ‘‘blue response’’ improvement in low‐cost polycrystalline silicon solar cells. A selective emitter is created by heavily doping the emitter, followed by a wet etching‐back of the cell area between the fingers. An improvement up to 17 mV in Voc, 1.5 mA/cm2 in Jsc, and 1% (absolute value) in η is obtained. Effective phosphorus gettering, self‐alignment, and application in a low‐cost full screenprinting technology are the main advantages of the proposed process.

Porous silicon in crystalline silicon solar cells: A review and the effect on the internal quantum efficiency

Crystalline silicon (c-Si) is the dominant semiconductor material in use for terrestrial photovoltaic cells and a clear tendency towards thinner, active cell structures and simplified processing schemes is observable within contemporary c-Si photovoltaic research. The potential applications of porous silicon and related benefits are reviewed. Specific attention is given to the different porous silicon formation processes, the use of this porous material as anti-reflection coating in simplified processing schemes and for simple selective emitter processes and its light trapping and surface passivating capabilities, which are required for advantageous use in thin active cell structures. Our analysis of internal quantum efficiency data obtained on both conventional and thin-film c-Si solar cells has been performed with the aim of describing the light diffusing behaviour of porous Si as well as investigating the surface passivating capabilities. An effective entrance angle of 60° is derived, which corresponds to totally diffuse isotropic light, and the importance of a correction for absorption losses in the porous layer is illustrated. Furthermore, photoconductivity decay measurements of freshly etched porous Si on float-zone p-type Si indicate a strong bias-light dependency and a fast degradation of the surface recombination velocity.

Overview of solar cell technologies and results on high efficiency multicrystalline silicon substrates

Fabrication technologies for multicrystalline silicon (mc-Si) solar cells have advanced in recent years with efficiencies of mc-Si cells exceeding 18%. Intense efforts have been made at laboratory level to improve process technology, growth methods, and material improvement techniques to deliver better devices at lower cost. Deeper understanding of the physics and optics of the device led to improved device design. This provided a fruitful feedback to the industrial sector. Both screenprinting and buried-contact technologies yield cells of high performance. An increasingly large amount of research activity is also focussed on the fabrication of thin solar cells on cheap substrates such as glass, ceramic, or low quality silicon. Success of these efforts is expected to lead to high efficiency devices at much lower costs. Efforts are also being put on low thermal budget processing of solar cells based on rapid thermal annealing.

High-low junctions for solar cell applications

A new theoretical model to calculate the effective surface recombination velocity (

Polycrystalline silicon solar cells with a mechanically formed texturization

a) of a high-low junction with an arbitrary impurity distribution is presented. The model is applied to erfc-diffused pp+ junctions using experimental data of bandgap narrowing, lifetime and mobility. Bandgap narrowing is shown to degrade the minority carrier reflecting properties of the high-low junction. Computer results are applied for the design of BSF solar cells and to study other solar cell structures based on high-low junctions. NOTATION diffusivity of minority carriers diffusivity of electrons in a p+ region minority carrier current electron current in a p+ region

Effect of surface recombination on the transient decay of excess carriers produced by short wavelength laser pulses

Polycrystalline (Wacker SILSO) silicon has been mechanically textured using a conventional dicing saw and beveled blades for V‐groove formation. The minimum optical reflectivity achievable is limited by the blade tip radius and surface roughness after damage etching. Solar cells were prepared using a conventional diffusion and screen printed metallization. Grooved cells without an additional antireflection coating (jsc=31.8 mA/cm2, Voc=536 mV, FF=69%, η=11.8%) showed a 20% increase in jsc and a 1.1% absolute efficiency improvement as compared to a nongrooved reference cell with an antireflexion coating (jsc=26.4 mA/cm2, Voc=547 mV, FF=74.1%, η=10.7%). In grooved cells the efficiency is found to be limited mainly by the fill factor due to a nonoptimized front grid design.