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

The efficiency record of industrial type PERC solar cells exceeded 22% at the turn of the year 2015 to 2016. Our best screen-printed PERC solar cell reached 22.04% efficiency while the best cell batch showed a very narrow efficiency distribution. A detailed electrical and optical loss analysis of those industrial type high efficiency PERC solar cells is carried out which enables further optimization and strategic improvements. A variety of characterization data allows for a recombination current density, resistance and optical loss analysis based on numerical device simulation, analytical calculations and raytracing, respectively. The main recombination losses at maximum power point (MPP) occur in the homogenous and selective diffused regions of the emitter. A series resistance loss analysis is analytically performed. The emitter contribution to the lumped series resistance dominates the series resistance losses. The optical loss analysis performed with raytracing shows main reflection and absorption losses in the rear metal layer which is partly due to the light trapping capability of the PERC cells.

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 side 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 µm contact openings on full-area 156x156 mm 2 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/m² front illumination unless additional rear illumination exceeds 250 W/m². 2. As soon as any rear illumination is added to the front-illuminated PERC solar cell, 100 µm thin fingers at the rear side have an output power advantage compared to 150 µm and 200 µm wide fingers.

Thermophotovoltaics (TPV) was intensively investigated as a technology for heat/electricity co-generation in the last decade of the 20th century. However, a wide-spread commercialisation has not been achieved yet. The world-record system efficiency for a TPV system using silicon photocells of 4% at 50 W electrical output power as well as a maximum electrical output power of 164 W, however at a lower efficiency of 0.84% could be demonstrated by a prototype system operating with an Yb2O3 selective emitter. Related developments of TPV system components such as radiation emitters, filters and photocells are reviewed and theoretical system simulations are compared to experimentally achieved results regarding system efficiency and the electrical output power. Finally, novel TPV applications are suggested and the commercial potential of this technology is discussed.

The authors discuss industry related approaches at Fraunhofer ISE for bifacial p-type silicon solar cells, taking into account the well-known "passivated emitter and rear cell" (PERC), "passivated emitter and rear totally diffused" (PERT) and "passivated emitter and locally diffused" (PERL) architectures. In the case of PERC, challenges in terms of alignment, printability and the importance of bifaciality are addressed. In the case of PERT, a co-diffusion process is utilized to form the emitter and the back surface field simultaneously avoiding also critical shunts that can arise at the edges of such devices. For the PERL technology, the industrial feasible pPassDop approach is discussed. We report on front side energy conversion efficiencies for PERC of 21.4%, PERT of 20.5%, and PERL of 19.8%. Furthermore, bifaciality factors for PERC of 0.7, for PERT of 0.86, and for PERL of 0.89 are presented.

We describe a novel design of a thermophotovoltaic (TPV) system based on thin film photocells. The radiation source is a selective emitter made from Yb 2 O 3 , which allows the use of photocells with a band gap between 1.0 and 1.2 eV. Thin film solar cell materials like microcrystalline Si or Cu(In,Ga)Se 2 (CIGS) can in principle be used for such a system. So far, a special CIGS TPV photocell is not available, but the requirements to the cell for an application in a TPV system, like suitable band gap, small series resistance, flexibility and superstrate configuration, were separately achieved in lab-scale photocells. The proposed thin film TPV system is cylindrically symmetric and consists of a monolithic CIGS photocell module at the outer surface of a glass tube. Due to a direct contact of the whole rear side of the cell with a cooling water film, an efficient cell cooling is achievable. Complicated cooling water plumbing is not necessary. A drastic cost reduction is expected compared to TPV systems that use conventional crystalline Si photocells. Numerical simulations of the novel CIGS TPV system based on an existing system with Si photocells and CIGS photocell parameters from the literature result in a system efficiency of up to 4% without recuperator. We suggest an application of our proposed system for electrically self-powered residential heating systems. For that, a system efficiency of 1–2% is sufficient, so surplus electricity can be supplied into the house grid for external consumption.

A new method has been proposed [W. Durisch, K.H. Lam, J. Close, Behaviour of a copper indium gallium diselenide module under real operating conditions, in: Proceedings of the World Renewable Energy Congress VII, Pergamon Press, Oxford, Elsevier, Amsterdam, 2002, ISBN 0-08-044079-7] for the calculation of the annual yield of photovoltaic (PV) modules at selected sites, using site-specific meteorological data. These yields are indispensable for calculating the expected cost of electricity generation for different modules, thus allowing the type of module to be selected with the highest yield-to-cost ratio for a specific installation site. The efficiency model developed and used for calculating the yields takes three independent variables into account: cell temperature, solar irradiance and relative air mass. Open parameters of the model for a selected module are obtained from current/voltage (I/V) characteristics, measured outdoors at Paul Scherrer Institute's test facility under real operating conditions. From the model, cell and module efficiencies can be calculated under all relevant operating conditions. Yield calculations were performed for five commercial modules (BP Solar BP 585 F, Kyocera LA361K54S, Uni-Solar UPM-US-30, Siemens CIS ST40 and Wuerth WS11003) for a sunny site in Jordan (Al Qawairah) for which reliable measured meteorological data are available. These represent mono-crystalline, poly-crystalline and amorphous silicon as well as with copper–indium-diselenide, CuInSe2 PV modules. The annual yield for these modules will be presented and discussed.

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 16% efficiency (under solar irradiance in standard test conditions, STCs). 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 cell temperature, and therefore increase 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 in useful infrared radiation. (6) Smaller cells have been used, to reduce electrical series resistance losses. Applying all these improvements to the basic 1.35 kW prototype, we attained a system efficiency of 1.5%. 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 500 °C). An electronic control unit (fabricated of components with low power consumption, 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 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.

A crucial parameter for the market penetration of TPV is its electricity production cost. In this work a detailed cost estimate is performed for a Si photocell based TPV system, which was developed for electrically self-powered operation of a domestic heating system. The results are compared to a rough estimate of cost of electricity for a projected GaSb based system. For the calculation of the price of electricity, a lifetime of 20 years, an interest rate of 4.25% per year and maintenance costs of 1% of the investment are presumed. To determine the production cost of TPV systems with a power of 12–20 kW, the costs of the TPV components and 100 EUR kW−1el,peak for assembly and miscellaneous were estimated. Alternatively, the system cost for the GaSb system was derived from the cost of the photocells and from the assumption that they account for 35% of the total system cost. The calculation was done for four different TPV scenarios which include a Si based prototype system with existing technology (ηsys = 1.0%), leading to 3000 EUR kW−1el,peak, an optimized Si based system using conventional, available technology (ηsys = 1.5%), leading to 900 EUR kW−1el,peak, a further improved system with future technology (ηsys = 5%), leading to 340 EUR kW−1el,peak and a GaSb based system (ηsys = 12.3% with recuperator), leading to 1900 EUR kW−1el,peak. Thus, prices of electricity from 6 to 25 EURcents kWh−1el (including gas of about 3.5 EURcents kWh−1) were calculated and compared with those of fuel cells (31 EURcents kWh−1) and gas engines (23 EURcents kWh−1).

Silicon (Si) and germanium (Ge) are semiconducting materials, which are industrially used for the large-scale production of various electronic devices. Solar cells are commonly manufactured from Si. For thermophotovoltaics (TPV) Si has the disadvantage of a high bandgap of 1.1 eV, which requires the use of a spectrally matched selective emitter. Yb2O3 is widely used as an emitter material to illuminate Si photocells. Si concentrator solar cells have been investigated for TPV applications, because they have a high performance at a typical illumination density of 1 W cm−2 in a TPV system. Non-concentrator solar cells achieve lower efficiencies under TPV conditions, but up to now they are more cost effective than concentrator cells. A new Si photocell optimized for TPV has a front side textured with rectangular grooves with vertically evaporated contact fingers and a rear surface mirror to reflect sub-bandgap radiation back to the emitter. Ge photocells have a bandgap of 0.66 eV and can effectively be illuminated by a selective Er2O3 emitter. Their efficiencies are lower than those of photocells from low bandgap III/V materials, such as GaSb. But, due to low free carrier absorption in Ge, an effective rear surface mirror can be formed. A reflectance of up to 82–87% for sub-bandgap radiation and a cell efficiency of 13% for solar air mass 0 (AM0) radiation with a cut-off for wavelengths smaller than 900 nm have been achieved with a Ge TPV cell. SiGe photocells allow the variation of the bandgap as a function of the Ge content. In principle, a SiGe photocell can be matched to a given selective emitter spectrum. A first SiGe quantum dot solar cell has achieved 12% efficiency, but still suffers from a low open circuit voltage. The first TPV systems working with Si photocells have been built. A system efficiency of 2.4% can be achieved. The most promising application of Si-based TPV is likely to be an electrically self-powered residential heating system. For a self-powered operation, a TPV system efficiency of 1–2% is sufficient and Si photocells have the advantage of being inexpensive.

In order to obtain a low band gap photocell based on the widely spread silicon technology, e.g. for thermophotovoltaics, SiGe nanostructures can be introduced into a monocrystalline silicon photocell. Beforehand, it is necessary to know the absorption coefficient of the SiGe quantum wells. On a silicon (1 0 0) substrate multiple Si/SiGe quantum well structures were grown by UHV-CVD. The Ge concentration and the well width were used as growth parameters. To obtain significant absorption, the experiment was set up to allow for 200 internal reflections. The total reflection of the light results in a standing electromagnetic wave. The absorption coefficient was obtained from the experimental data taking the geometry and the electric field distribution in the absorbing layer into account. The influence of well width and germanium content on the absorption was investigated with the goal of maximizing the absorption for photons with energies below the band gap energy of silicon. The measurement results are compared with a theoretical model, which takes the band structure of strained SiGe including confinement effects into account.

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.

We investigate selective radiation emitters made from rare earth oxides suitable for thermophotovoltaic (TPV) systems. Yb 2 O 3 and Er 2 O 3 emitters were fabricated and their radiation power, temperature and emissivity were measured in the entire relevant spectral range. We found temperatures of 1735 K for the Yb 2 O 3 emitter and of 1680 K for the Er 2 O 3 emitter both heated with a 1.35 kW butane burner. The maximum emissivities of the selective peaks were 0.85 at 1.27 eV, and 0.82 at 0.80 eV for the Yb 2 O 3 and the Er 2 O 3 emitter, respectively. The emission spectra show gas emission lines originating from the combustion process in addition to the selective emission bands. An estimation based on a simplified combustion model show that a TPV system with a system efficiency of about 10% can be realised using an Yb 2 O 3 emitter, silicon photocells and a perfect selective filter.

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.

For the first time efficiencies above 11% for solar cells (4 cm 2 ) based on Bayer ribbon growth on substrate (RGS) crystalline silicon have been demonstrated including mechanical V-structuring of the front surface, aluminum-gettering, microwave-induced remote hydrogen plasma (MIRHP) passivation and PECVD SiN/SiO 2 double-layer antireflection coating. MIRHP alone resulted in absolute improvements in the open-circuit voltage of 27 mV, in the short-circuit current density of 2.8 mA cm −2 and in the cell efficiency of 1.9% leading to an open-circuit voltage of 538 mV and an efficiency of 11.1%.

A new method is proposed to produce sheet- or ribbon-like silicon from the melt using a contactless electromagnetic shaping action based on the cold-crucible principle. The ribbon shaping by magnetic repulsion may be limited to a single-side application and used in conjunction with other ribbon growth techniques such as the ribbon growth on substrate method. A growth experiment in a cold crucible resulted in some extrusion of sheet-like shaped silicon in the container openings. This experimental finding supports the view that the proposed ribbon growth technique is indeed feasible.

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.

The introduction of selective emitters underneath the front contacts of solar cells can considerably increase the cell efficiency. Thus, cost-effective fabrication methods for this process step would help to reduce the cost per W p of silicon solar cells. Laser Chemical Processing (LCP) is based on the waterjet-guided laser (LaserMicroJet®) developed and commercialized by Synova S.A., but uses a chemical jet. This technology is able to perform local diffusions at high speed and accuracy without the need of masking or any high-temperature step of the entire wafer. We present experimental investigations on simple device structures to choose optimal laser parameters for selective emitter formation. These parameters are used to fabricate high-efficiency oxide-passivated LFC solar cells that exceed 20% efficiency.

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 for CTPV that was reported is a gas fired stove, which produces electricity in addition to heat. The application of a CTPV system for electrically autonomous domestic central heating systems could probably result in a first commercialisation of TPV. For this application, a system efficiency in the order of 1 % is sufficient. A gas fired TPV system is presented, which uses a novel foam ceramic emitter made from Yb2O3 and commercially available Si solar cells. The proof‐of‐concept of this prototype CTPV system could successfully be furnished. Possible applications for this CTPV system are: electrically autonomous domestic heating systems, parking heating systems for vehicles, heaters for caravans and boats or large industrial burner systems.

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.

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.

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...

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.

We have realised and investigated a thermophotovoltaic generator which uses silicon photocells and an Yb/sub 2/O/sub 3/ selective emitter. Theoretical calculations show that in principle an electrical output power of several hundred watts can be achieved with such a system using 20 kW thermal input power. We have built a small TPV system base on a camping gas lamp which reached an efficiency of 1.9% and an electrical output power of 25 W. Commercial silicon solar cells which are water cooled are used in this system. The efficiency improvements when using a TCO filter instead of the water filter and optimised silicon photocells are discussed. We show measurements of the optical properties of an optimised ZnO filter and the electrical characteristics of PSI high efficiency photocells. To demonstrate the use of the TPV in a residential heating system, a TPV demonstration system with a thermal input power of 20 kW and 161 W electrical output power was developed.

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 1011 cm−2, which is suggestive of quantum melting of the Wigner solid. Reproducibility of this effect on different Si wafers is discussed.

We report on the fabrication and detailed optical characterisation of a selective incandescent mantle Yb/sub 2/O/sub 3/ emitter. A TPV prototype system was built by arranging commercially available monocrystalline silicon solar cells around the emitter. This system reached, without preheating of the combustion air, a record electricity-to-gas power efficiency of 2.4%. A model is presented which allows an accurate simulation of the system properties. First experiments with a demonstration system based on a 20 kW methane burner and a 500 cm/sup 2/ emitter gives evidence that this TPV design can be up-scaled. An optimisation of this system and the integration of a highly selective filter should permit the realisation of a high power (> 1 kW) cost effective silicon TPV system for decentralised generation of electricity in residential heating systems.