Solar cell process development in the european integrated project crystalclear

CrystalClear is a large integrated project funded by the European Commission that aims to drastically reduce the cost of crystalline Si PV modules, down to 1 Euro/Wp. Among the different subprojects, the one dealing with the development of advanced solar cells is relatively large (with 11 partners out of the 15 Crystal Clear partners taking part) and has a crucial role. The goal of the subproject is to develop cell design concepts and manufacturing processes that would enable a reduction in the order of 40% of the cell processing costs per Wp. In this paper, we give an overview of all the development work that has taken place in the CrystalClear solar cells subproject so far. World class results have been achieved, particularly on high efficiency cells on Si ribbons, and on industrial-type solar cells on very thin (120 (j.m thick) substrates

Evidence for quantum melting in the two-dimensional electron system on a thin helium film

The real and imaginary parts of the dielectric response of surface state electrons (SSE) on helium films adsorbed on oxidized Si platelets have been measured with a microwave cavity at 10 GHz. Preliminary measurements taken at T=1.2 K show an abrupt increase of the SSE mobility at electron densities near 10(11) cm(-2), which is suggestive of quantum melting of the Wigner solid. Reproducibility of this effect on different Si wafers is discussed

Understanding the rear-side layout of p-doped bifacial PERC solar cells with simulation driven experiments

To investigate the rear side of bifacial p-type Czochraslki-grown silicon PERC solar cells, the present work combines Sentaurus Device simulation - calibrated with extensively characterized samples - and the subsequent fabrication of solar cells according to the simulation findings. The authors investigate the physical alteration of rear-side characteristics in the context of an additional rear-side illumination. The additional injection represents an further factor for the balance of carrier generation, recombination and series resistances which in turn influences the design rules for the rear side layout. Our detailed bifacial simulations include these physical aspects and we derive design solutions for different bifacial illumination scenarios for a bifacial p-doped PERC solar cell. Using an industrial PERC process, solar cells with laser contact openings (LCO) and a rear aluminum grid were produced according to the simulation results with a wide variation in rear si de layout parameters. The PERC batches showed a rather constant medium (front side) efficiency of η = 20.8±0.2% and a bifaciality of 66 to 77% depending on the rear layout, allowing us to investigate the rear-side characteristics in detail and to compare them with the effects predicted by the simulations. We processed an aluminum rear contact grid with finger widths as small as 100 μm and successfully aligned it onto the LCO with 30 mu m contact openings on full-area 156x156 mm2 wafers. We reached good accordance between the monofacial measurements from front and rear side and our simulation model and could thus predict bifacial illumination results by modeling for two issues: 1. Planar rear sides have an advantage over pyramid textured rear sides for 1000 W/m front illumination unless additional rear illumination exceeds 250 W/m

Development and characterization of multifunctional PassDop layers for local p+-laser doping

We present the development of aluminum oxide (AlOx) and boron-doped silicon nitride (SiNx:B) layer stacks for application on the back side of monocrystalline p-type silicon wafers. Two deposition techniques are used for the deposition of the AlOx/SiNx:B layer stacks, atomic layer deposition and plasma-enhanced chemical vapor deposition. Both techniques enable excellent surface passivation with surface recombination velocities of 4 cm/s after firing. Also, heavy local doping with sheet resistances down to 20 Ω/sq is possible by laser processing. We call this concept the PassDop approach. For the laser processed area where the silicon surface is locally boron-doped and the AlOx/SiNx:B passivation layer stack is locally removed, a quite low dark saturation current density of about 900 fA/cm2 is determined. The PassDop approach can be a solution to realize passivated emitter and rear locally doped PERL solar cells by improving their rear side properties while maintaining industrial applicability

Small self-powered grid-connected thermophotovoltaic prototype system

In an earlier paper, we reported on a small grid-connected thermophotovoltaic (TPV) system consisting of an ytterbia mantle emitter and silicon solar cells with a 16% efficiency (under solar irradiance at standard test conditions, STC). The emitter was heated using a butane burner with a rated thermal power of 1.35 kW (referring to the lower heating value). This system produced an electrical output of 15 W, which corresponds to a thermal to electric (direct current) conversion efficiency of 1.1%. In the interim, further progress has been made, and significantly higher efficiencies have been achieved. The most important developments are:- (1) The infrared radiation-absorbing water filter between the emitter and silicon cells (to protect the cells against overheating) has been replaced by a suitable glass tube. By doing this, it has been possible to prevent losses of convertible radiation in the water, and to protect the cells against the flue gasses. (2) Cell cooling has been significantly improved, in order to reduce the cell temperature, and therefore increase the conversion efficiency. (3) The shape of the emitter has been changed from spherical to a quasi-cylindrical geometry, in order to obtain a more homogeneous irradiation of the cells. (4) The metallic burner-tube, on which the ytterbia emitter was fixed in the initial prototypes, has been replaced by a heat-resistant metallic rod, carrying ceramic discs as emitter holders. This has prevented the oxidation and clogging of the perforated burner tube. (5) Larger reflectors have been used to reduce losses of useful infrared radiation. (6) Smaller cells have been used, to reduce the electrical series-resistance losses. A system efficiency of 1.5% was attained by applying all these improvements to the basic 1.35 kW prototype. By using preheated air for combustion (at approximately 370 °C), 1.8% was achieved. In a subsequent step, a photocell generator was constructed, consisting of high-efficiency silicon cells (21% STC efficiency). In this generator, the spaces between the cells were minimized, in order to achieve as high an active cell area as possible, while simultaneously reducing radiation losses. This new system has produced an electrical output of 48 W, corresponding to a system efficiency of 2.4%. This is the highest-ever-reported value in a silicon-cell-based TPV system using ytterbia mantle emitters. An efficiency of 2.8% was achieved by using preheated air (at approximately 350 °C). An electronic control unit (fabricated of components with low power consumptions, and including a battery store) was developed, in order to make the TPV system self-powered. This unit controls the magnetic gas-supply valve between the gas-supply cylinder and burner as well as the high-voltage ignition electrodes. Both the control unit's own power consumption and the battery-charging power are supplied directly by the TPV generator. A small commercial inverter is used to transfer excess power to the 230 V grid. In future systems, the effect of preheating the combustion air will be studied in more detail. Finally, this system will be scaled up to provide self-powered domestic boilers.Thermophotovoltaics Thermophotovoltaic prototype system Self-powered thermophotovoltaic system Grid-connected thermophotovoltaic system

Development and characterization of AlOx/SiNx :B layer systems for surface passivation and local laser doping

This work aims to improve the rear-side properties of p-type monocrystalline silicon solar cells by using the passivated emitter and rear locally diffused (PERL) solar cell concept. To realize the rear side structure, the so-called PassDop approach was used combining both surface passivation and local doping. The concept utilizes a multifunctional, doped AlOx/SiNx:B layer stack; the localized structuring is achieved by local contact opening and doping by a laser process. Using AlOx/SiNx:B PassDop layers, an outstanding effective surface recombination velocity Seff of less than 4 cm/s was achieved after firing at the passivated area. The boron concentration in the PassDop layers did not show any significant influence on Seff. Laser doping resulted in highly doped regions in the silicon with a sheet resistance of below 20 Ω/sq and surface doping concentrations close to 1 × 1020 cm-3. Accordingly, calculations showed that the saturation current density at the laser doped areas can be as low as 900 fA/cm2 for line-shaped contact structures

Low-Ohmic Contacting of Laser-Doped p-Type Silicon Surfaces with Pure Ag Screen-Printed and Fired Contacts

The state-of-the-art low-ohmic electrical contacting of highly boron-doped silicon surfaces is based on the use of screen-printed and fired silver-aluminum (Ag-Al) contacts. For these contacts, metal crystallites with depths of up to a few microns are observed at the interface. For screen-printed and fired Ag contacts on phosphorus-doped surfaces, the observed crystallite depths are much smaller. In this work, low-ohmic electrical contacting of local laser-doped p-type silicon surfaces with commercial pure Ag screen-printing paste are demonstrated. The doping layer is based on the "pPassDop" approach, which serves as a passivation layer on the rear side of p-type silicon solar cells. The specific contact resistances are measured down to 1 mu ohmb cm2 for p-type doping densities of about 3Ã1019cm-3 at the silicon surface and finger widths of around 55 mu m. Microstructure analysis reveals the formation of numerous small Ag crystallites at the interface with penetration depths o f less than 80nm. A first implementation of the "pPassDop" approach on 6-inch p-type Cz-Si bifacial solar cells using solely Ag contacts on both sides results in a peak front side energy conversion efficiency of 19.1%, measured on a black chuck with contact bars on both sides