Bernd Bitnar

All-screen-printed 120-µM-thin large-area silicon solar cells applying dielectric rear passivation and laser-fired contacts reaching 18% efficiency

The market need for a lower price per Wattpeak asks for the development of solar cell designs with a low production cost and a high performance. An approach to reach a high efficiency with a solar cell structure containing a diffused emitter on a p-type silicon wafer is the implementation of a PERC structure on the rear side [1]. This structure gets advantageous to the standard screen printed solar cell when its production cost stays comparable to the latter and offers a higher efficiency [2]. Since this technique can inherently be applied to thinner wafers, an additional advantage comes from the reduced material consumption. The purpose of this work is to introduce a production sequence able to create a PERC structure on thin silicon wafers using steps available in the PV industry or at least close to industrial application. Applying this process on Czochralski (Cz) wafers of 120 µm thickness, a stable efficiency of 18.0 % was achieved

Cost Estimates of Electricity from a TPV Residential Heating System

A thermophotovoltaic (TPV) system was built using a 12 to 20 kWth methane burner which should be integrated into a conventional residential heating system. The TPV system is cylindrical in shape and consists of a selective Yb2O3 emitter, a quartz glass tube to prevent the exhaust gases from heating the cells and a 0.2 m2 monocrystalline silicon solar cell module which is water cooled. The maximum system efficiency of 1.0 % was obtained at a thermal input power of 12 kWth. The electrical power suffices to run a residential heating system in the full power range (12 to 20 kWth) independently of the grid. The end user costs of the TPV components ‐ emitter, glass tube, photocells and cell cooling circuit ‐ were estimated considering 4 different TPV scenarios. The existing technique was compared with an improved system currently under development, which consists of a flexible photocell module that can be glued into the boiler housing and with systems with improved system efficiency (1.5 to 5 %) and geometry. P...

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.

Systematic study towards high efficiency multicrystalline silicon solar cells with mechanical surface texturization

The aim of the present work was to optimize a high efficiency process for multicrystalline silicon solar cells (including Al-gettering, oxide and hydrogen passivation, Al-BSF formation, photolithographically defined front metallization) and combine it with the mechanical texturization technique. New cell structures were created, in which only the areas between the front grid are V-grooved. Solar cells were processed on various ribbon and conventional cast silicon materials. IV-characteristics, reflectance and spectral response were measured and analysed by two dimensional device simulation. A comparison with equally processed flat cells shows an increase of cell efficiencies by more than 25% due to the reduction of optical losses before antireflection coating and by an additional 4% due to the enhanced collection probability in the V-groove volumes.

Pilot line processing of 18.6% efficient rear surface passivated large area solar cells

We use the recently introduced Silicon Nitride Thermal Oxidation (SiNTO) process for the industrial fabrication of silicon solar cells that feature a thermal oxide passivated rear surface and local rear contacts. The SiNTO process represents an innovative approach for the fabrication of a passivated emitter and rear cell (PERC), since the front end part from the conventional process sequence is maintained. We apply mostly industrial production equipment using Czochralski silicon wafers that are partly processed in an industrial production line. Conventional screen printing is used for the formation of the front contacts. A stable conversion efficiency of 18.6% (independently confirmed) is achieved for a PERC device fabricated from conventional boron doped Cz-Silicon by means of the SiNTO process. The average efficiency of a batch of 24 SiNTO cells is 18.4%, measured after fabrication (not stabilized). A test module fabricated from 16 SiNTO solar cells features a fill factor of 76.2% and an open circuit voltage of 10.16 V, corresponding to an average of 635 mV per cell.

Record Electricity‐to‐Gas Power Efficiency of a Silicon Solar Cell Based TPV System

In this paper we report on the development and characterisation of a small TPV prototype system, which uses silicon photocells and a rare earth selective emitter. A simulation model of this system was developed, which allows studying the system theoretically. The fabrication of the selective incandescent mantle emitter from Yb2O3 and detailed measurements of its radiation power and emissivity are presented. The maximum emissivity was 0.85 at 1.27 eV. An emitter temperature of 1735 K was obtained for an approximately 75 cm2 large emitter heated by a butane burner. A SnO2 filter tube was developed. The photocell generator is composed of monocrystalline silicon solar cells and a water‐cooling circuit. The prototype system reached, without a selective filter and without preheating of the combustion air, a record electricity‐to‐gas power efficiency of 2.4 %. We compare the experimentally achieved system efficiency with simulations using our model. The possibilities to further increase the system efficiency are discussed.In this paper we report on the development and characterisation of a small TPV prototype system, which uses silicon photocells and a rare earth selective emitter. A simulation model of this system was developed, which allows studying the system theoretically. The fabrication of the selective incandescent mantle emitter from Yb2O3 and detailed measurements of its radiation power and emissivity are presented. The maximum emissivity was 0.85 at 1.27 eV. An emitter temperature of 1735 K was obtained for an approximately 75 cm2 large emitter heated by a butane burner. A SnO2 filter tube was developed. The photocell generator is composed of monocrystalline silicon solar cells and a water‐cooling circuit. The prototype system reached, without a selective filter and without preheating of the combustion air, a record electricity‐to‐gas power efficiency of 2.4 %. We compare the experimentally achieved system efficiency with simulations using our model. The possibilities to further increase the system efficiency are...

New Flexible Photocell Module for Thermophotovoltaic Applications

This paper describes the practical implementation of TPV systems. As a key part of these systems, we developed a new flexible photocell module for TPV applications. The module consists of commercially available monocrystalline silicon solar cells, which were cut into small stripes that are series connected to achieve a high voltage and a reduced current for operation at an illumination density of 5 – 10 suns. For an easy mounting into a heating boiler, the module is highly flexible and can be bent into a cylindrical shape. The rear side of the module is electrically isolated, but achieves a good heat coupling through a covering aluminium foil to the boiler wall. Thus, it can directly be mounted into a water‐cooled boiler housing without the need of an additional water‐cooling circuit. A first test module with an active cell area of 481 cm2 achieved an electricity output of 50 W by irradiation with an Yb2O3 mantle emitter heated with a 20 kW methane burner.

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 <italic>p</italic>-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 AlO<italic>_x</italic>/SiN<italic>_x</italic>:B layer stack; the localized structuring is achieved by local contact opening and doping by a laser process. Using AlO<italic>_x </italic>/SiN<italic>_x</italic>:B PassDop layers, an outstanding effective surface recombination velocity <italic>S</italic>_eff 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 <italic>S</italic>_eff. 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.

TPV Systems — From Research Towards Commercialisation

An overview on the development of thermophotovoltaic (TPV) systems heated with concentrated sunlight (STPV) and with a combustion flame (CTPV) is given. Only a few experimental works on STPV are reported. Lifetime investigations of TPV emitters in solar dish concentrators were carried out. A complete STPV system reported in the literature achieved a system efficiency below 0.1 %. In contrast to these experiments we present simulations, which show that an optimised STPV system with Yb2O3 emitter and high efficient Si solar cells is able to achieve a system efficiency in the order of 30 %. Quite more experimental results are reported for CTPV systems, but a series production and commercialisation of CTPV was not achieved, so far. An application for a CTPV system is a portable electrical power supply. The highest system efficiency reported so far is 6 %. Whether this efficiency is sufficient to successfully compete with generators driven by a gas engine or a diesel motor, remains unclear. Another application...

Small Thermophotovoltaic Prototype Systems

In an earlier paper [1], we reported on a small grid‐connected thermophotovoltaic (TPV) system consisting of an ytterbia mantle emitter and silicon solar cells with 16 % efficiency (under solar irradiance at Standard Test Conditions, STC). The emitter was heated up 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 development steps are: 1) The infrared radiation‐absorbing water filter between emitter and silicon cells (to protect the cells against overheating and against contact with flue gasses) has been replaced by a suitable glass tube. By doing this, it has been possible to prevent losses of convertible radiation in water. 2) Cell cooling has been significantly improved, in order to reduce...