Fraunhofer ISE Research Articles

N‐Type emitter epitaxy for crystalline silicon thin‐film solar cells

AbstractCrystalline silicon thin‐film (cSiTF) solar cells are an attractive alternative to bulk silicon solar cells. At Fraunhofer ISE we follow the concept of the epitaxial wafer equivalent (EpiWE), where 20 µm of silicon are deposited epitaxially by high temperature atmospheric pressure CVD (APCVD) on a cheap silicon substrate. The EpiWE can then be processed using standard industrial cell production. Furthermore, it is possible to simplify the solar cell process when depositing the emitter by epitaxy in‐situ after the growth of the base. The epitaxial emitter could be an alternative method of n‐type emitter processing with adjustable emitter profile and short deposition time. This paper presents results of cSiTF solar cells with epitaxial emitters and photolithographic contacts, which prove the good performance of the emitter and the applicability of the EpiWE to conventional solar cell processes. Included in the discussion is a description of the CVD deposition principle developed at Fraunhofer ISE and the applied clean room solar cell process. The emitter doping profiles and concentrations, as well as the structural quality, are discussed in detail using SIMS and SEM data. The phosphorus out‐diffusion during cooling is prevented by cooling the samples in a PH3/H2 atmosphere. Moreover, a double‐layer emitter is formed having blue sensitive properties. The internal quantum efficiencies (IQEs) show that the emitter can be passivated as well as a ‘standard’ emitter formed by POCl3 diffusion. Efficiencies up to 14·9 and 13·6% for large area cSiTF solar cells on highly doped Cz and mc, respectively, are presented. Copyright © 2007 John Wiley & Sons, Ltd.

High-Efficiency Crystalline Silicon Solar Cells

The current cost distribution of a crystalline silicon PV module is clearly dominated by material costs, especially by the costs of the silicon wafer. Therefore cell designs that allow the use of thinner wafers and the increase of energy conversion efficiency are of special interest to the PV industry. This article gives an overview of the most critical issues to achieve this aim and of the recent activities at Fraunhofer ISE and other institutes.

Silicon CVD deposition for low cost applications in photovoltaics

This paper presents a low cost application of high-temperature silicon CVD depositions for photovoltaics. Crystalline silicon thin-film (cSiTF) solar cells could be an attractive alternative to bulk silicon solar cells. At Fraunhofer ISE we follow the concept of an epitaxial wafer equivalent (EpiWE), where 20 μm of silicon are deposited epitaxially by atmospheric pressure CVD on a cheap silicon substrate. The EpiWE can then be processed in a standard industrial cell production. Furthermore, it is possible to simplify the solar cell process when depositing the emitter by epitaxy in-situ after the growth of the base. The emitter formation made by epitaxy could provide an alternative to the conventional POCl 3 emitter-diffusion for cSiTF solar cells. In this paper the concept of the EpiWE is introduced and the RTCVD deposition reactor is described in detail. The lab-type horizontal CVD reactor was constructed at Fraunhofer ISE to obtain a cheap deposition tool with a sufficient layer quality for solar cells. The deposition process as well as the resulting emitter profiles is discussed with regard to their effect on the solar cells. Solar cell parameters of emitters on n-type Fz and on p-type cSiTF are presented with efficiencies up to 14.2% and 14.8%, respectively. The high open circuit voltages of more than 645 mV show the good performance of the epitaxial emitters.

Statistical simulation of user behaviour in low-energy office buildings

A large number of design guidelines and tools are available for the design of passive cooling systems. Using the underlying thermodynamic models, a certain input (e.g. air change rate, internal heat gains or sun control) results in a certain output (i.e. room temperature). However, in real buildings the room temperature at a given outdoor temperature is a distribution rather than a single value. Therefore, the building engineer should take uncertainties into account, since the actual use of the building, the building physical properties or the user behaviour are statistically distributed. One promising approach to include these uncertainties in the design procedure is the use of statistical models: the design parameter is defined by a mean value and its deviation. From a control theoretical point of view, the deterministic controlled system responds to random disturbance variables by a statistically distributed response function. Considering the institute building of Fraunhofer ISE as example, this study shows how statistical simulations can be applied to the design process of passive cooling in low-energy office buildings.

New concepts for high-efficiency silicon solar cells

This paper gives an overview about recent activities in the industrial application of high-efficiency monocrystalline silicon solar cells. It also presents the latest results achieved at Fraunhofer ISE, especially a new patented process for the formation of back-contact points on dielectrically passivated cells called laser-fired contacts and its application to thin wafers.

06/01894 A methodology for thermodynamic simulation of high temperature, internal reforming fuel cell systems: Matelli, J. A. and Bazzo, E. Journal of Power Sources, 2005, 142, (1–2), 160–168

The conversion of liquid hydrocarbons to a hydrogen rich product gas is a central process step in fuel processors for auxiliary power units (APUs) for vehicles of all kinds. The selection of the reforming process depends on the fuel and the type of the fuel cell.For vehicle power trains, liquid hydrocarbons like gasoline, kerosene, and diesel are utilized and, therefore, they will also be the fuel for the respective APU systems.The fuel cells commonly envisioned for mobile APU applications are molten carbonate fuel cells (MCFC), solid oxide fuel cells (SOFC), and proton exchange membrane fuel cells (PEMFC). Since high-temperature fuel cells, e.g. MCFCs or SOFCs, can be supplied with a feed gas that contains carbon monoxide (CO) their fuel processor does not require reactors for CO reduction and removal. For PEMFCs on the other hand, CO concentrations in the feed gas must not exceed 50 ppm, better 20 ppm, which requires additional reactors downstream of the reforming reactor.This paper gives an overview of the current state of the fuel processor development for APU applications and APU system developments. Furthermore, it will present the latest developments at Fraunhofer ISE regarding fuel processors for high-temperature fuel cell APU systems on board of ships and aircrafts.

Advanced defect and impurity diagnostics in silicon based on carrier lifetime measurements

AbstractAn overview is given on recent developments at Fraunhofer ISE in the field of diagnostic techniques based on carrier lifetime measurements. The status of the different lifetime spectroscopy methods and the diagnostic capabilities of infrared carrier lifetime imaging are outlined. A combination of short acquisition times with high spatial resolution and high dynamic range under (quasi‐)steady‐state conditions gives IR lifetime imaging a high potential for the application in material and process characterization. Measuring in the emission mode provides access to the temperature dependence of lifetime and thus spatially resolved temperature dependent lifetime spectroscopy. Images of injection dependent lifetimes may be measured. For the anomalous increase at low injection a modeling description was developed. Assuming carrier trapping as origin of the increase, spatially resolved trap parameter analysis is possible. A new approach to suppress the anomalous increase by the application of sub‐bandgap light was demonstrated. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

BICON: high concentration PV using one-axis tracking and silicon concentrator cells

BICON is a two-stage concentrator system developed at Fraunhofer ISE which is one-axis tracked. The innovation of this one-axis tracked system is that it enables a high geometrical concentration of 300 × in combination with a high optical efficiency (upto 78%) and a large acceptance angle of ± 23·5° all year through. For this, the system uses a parabolic mirror (40·4 ×) and a three dimensional second stage consisting of compound parabolic concentrators (CPCs, 7·7 ×). For the concentrator concept and particularly for an easy cell integration, rear-line-contacted concentrator (RLCC) cells with a maximum efficiency of 25% were developed and a hybrid mounting concept for the RLCC cells is presented. The optical performance of different CPC materials was tested and is analysed in this paper. Finally, small modules consisting of six series interconnected RLCC cells and six CPCs were integrated into the concentrator system and tested outdoor. A BICON system efficiency of 16·2% was reached at around 800 W/m2 direct irradiance under realistic outdoor conditions. Copyright © 2006 John Wiley & Sons, Ltd.

High-throughput zone-melting recrystallization for crystalline silicon thin-film solar cells

Crystalline silicon thin-film solar cells which shall exceed a conversion efficiency of 15% require a coarse-grained silicon layer of at least 5 μm thickness as an absorber layer, with minority carrier diffusion length and grain diameter at least twice the layer thickness. At Fraunhofer ISE, we use zone-melting recrystallization to recrystallize a silicon layer grown by chemical vapor deposition on high-temperature stable substrates. Our main focus is the development of tools and processes for achieving silicon layers of sufficient electrical quality at high throughput and low cost. A processor for in-line zone melting of 400-mm-wide samples was developed, which has gas curtain load locks and thus offers a theoretical maximum throughput of approx. 10 m 2/h at a melting velocity of 500 mm/min. Zone-melting experiments with varying melting velocity showed that high crystal quality can be maintained even for velocities up to 350 mm/min.

Fuel processors for fuel cell APU applications

The conversion of liquid hydrocarbons to a hydrogen rich product gas is a central process step in fuel processors for auxiliary power units (APUs) for vehicles of all kinds. The selection of the reforming process depends on the fuel and the type of the fuel cell. For vehicle power trains, liquid hydrocarbons like gasoline, kerosene, and diesel are utilized and, therefore, they will also be the fuel for the respective APU systems. The fuel cells commonly envisioned for mobile APU applications are molten carbonate fuel cells (MCFC), solid oxide fuel cells (SOFC), and proton exchange membrane fuel cells (PEMFC). Since high-temperature fuel cells, e.g. MCFCs or SOFCs, can be supplied with a feed gas that contains carbon monoxide (CO) their fuel processor does not require reactors for CO reduction and removal. For PEMFCs on the other hand, CO concentrations in the feed gas must not exceed 50 ppm, better 20 ppm, which requires additional reactors downstream of the reforming reactor. This paper gives an overview of the current state of the fuel processor development for APU applications and APU system developments. Furthermore, it will present the latest developments at Fraunhofer ISE regarding fuel processors for high-temperature fuel cell APU systems on board of ships and aircrafts.

Micro-encapsulated phase-change materials integrated into construction materials

The idea of improving the thermal comfort of lightweight buildings by integrating phase-change materials (PCMs) into the building structure has been investigated in various research projects over several decades. Most of these attempts applied macro-capsules or direct immersion processes, which both turned out to present several drawbacks. Due to these problems, none of these PCM products was successful in the wider market. The new option to micro-encapsulate PCMs, a key technology which overcomes many of these problems, may make PCM products accessible for the building industry. This paper describes the work done at Fraunhofer ISE within a German government-funded project over the last 5 years, extending from building simulations to first measurements of full-size rooms equipped with PCM. The first products are now available on the market.

Development of concentrator modules with dome-shaped Fresnel lenses and triple-junction concentrator cells

The status of the development of a new concentrator module in Japan is discussed based on three arguments, performance, reliability and cost. We have achieved a 26·6% peak uncorrected efficiency from a 7056 cm2 400 × module with 36 solar cells connected in series, measured in house. The peak uncorrected efficiencies of the same type of the module with 6 solar cells connected in series and 1176 cm2 area measured by Fraunhofer ISE and NREL are reported as 27·4% and 24·8% respectively. The peak uncorrected efficiency for a 550× and 5445 cm2 module with 20 solar cells connected in series was 28·9% in house. The temperature-corrected efficiency of the 550 × module under optimal solar irradiation condition was 31·5 ± 1·7%. In terms of performance, the annual power generation is discussed based on a side-by-side evaluation against a 14% commercial multicrystalline silicon module. For reliability, some new degradation modes inherent to high concentration III-V solar cell system are discussed and a 20-year lifetime under concentrated flux exposure proven. The fail-safe issues concerning the concentrated sunlight are also discussed. Moreover, the overall scenario for the reduction of material cost is discussed. Copyright © 2005 John Wiley & Sons, Ltd.

Crystalline silicon thin-film solar cells—recent results at Fraunhofer ISE

Crystalline silicon thin-film solar cells combine the advantages of the stability and high-efficiency potential of crystalline silicon solar cell technology with the low material utilization of the thin-film solar cell technology. At Fraunhofer ISE the wafer equivalent concept is currently pursued. Within this concept, the active silicon layers are deposited on high-temperature stable substrates. The resulting substrate/layer sandwich can be processed into a solar cell using the same techniques that are used in conventional crystalline silicon wafer solar cell processing, hence the name wafer equivalent. In the present paper we report on how we realized wafer equivalents and explain in detail our development work on processors for both large-area silicon deposition and for zone melting recrystallization. An overview is given on the solar cell results achieved in this area.

Design of passive cooling by night ventilation: evaluation of a parametric model and building simulation with measurements

At the new institute building of Fraunhofer ISE, both mechanical and free night ventilation is used for passive cooling of the offices. The results from a long-term monitoring show, that room temperatures are comfortable even at high ambient air temperatures. In two offices, experiments were carried out in order to determine the efficiency of night ventilation dependent on air change rate, solar and internal heat gains. The aim is to identify characteristic building parameters and to determine the night ventilation effect with these parameters. The experiments (one room with and one without night ventilation) are evaluated by using both a parametric model and the ESP-r building simulation programme. Both models are merged in order to develop a method for data evaluation in office buildings with night ventilation and to provide a simple model for integration in a building management system.

Solar thermal-driven desalination plants based on membrane distillation

In arid and semi-arid regions the lack of drinkable water often corresponds with high solar insolation. These conditions are favourable for the use of solar energy as the driving force for water treatment systems. Especially in remote rural areas with low infrastructure and without connection to a grid, small-scale, stand-alone operating systems for the desalination of brackish water from wells or saltwater from the sea are desirable to provide settlements with clean potable water. Fraunhofer ISE is currently developing a solar thermally driven stand-alone desalination system. The aim is to develop systems for a capacity ranging from 0.2 to 20 m3/d. Technical simplicity, long maintenance-free operation periods and high-quality potable water output are the very important aims which will enable successful application of the systems. The separation technique on which the system is based is membrane distillation. The implemented heat source is a corrosion-free, seawater-resistant, thermal collector or a standard flat-plate or vacuum tube collector coupled with a corrosion-free heat exchanger. Laboratory tests under defined testing conditions of all components are very important for the preparation of successful field tests under real conditions.

Analysis of one-sun monocrystalline rear-contacted silicon solar cells with efficiencies of 22.1%

21.4% efficient rear-contacted cells (RCC) with interdigitated contact grids processed at the Fraunhofer ISE on 1.25 Ω cm float-zone (FZ) silicon are analyzed in detail. The comprehensive description does not only include a two-dimensional numerical device simulation, but also a detailed analysis of the optical carrier generation using optical ray tracing and determination of the losses due to distributed metal resistance and perimeter currents employing circuit simulation. Bulk and surface recombination losses are separated, combining carrier lifetime and open-circuit voltage measurements with numerical device simulation. The interface surface recombination velocity of the thermally oxidized emitter covering the front surface is deduced to be 1500 cm/s and the bulk diffusion length within the 1.25 Ω cm FZ silicon base is 1200 μm. Despite this excellent bulk diffusion length, the simulations reveal that at a maximum power point 80% of the total recombination is due to Shockley–Read–Hall recombination in the base. It was determined that losses due to the distributed metal resistance within the contact grid (including nongeneration losses) caused an fill factor decrease of 1% absolute. Loss currents flowing out of the cell perimeter caused an additional fill factor loss of 1.5% absolute. It was predicted that changing the surface concentration of the front and rear emitter diffusion from 5×1018 to 1×1018 cm−3, while keeping the sheet resistance constant, effected a relative improvement of 3% in the efficiency of the RCC structure. In fact, this modification has lead to an increase in the realized cell efficiency from 21.4% to 22.1% (VOC=697.6 mV, JSC=39.8 mA/cm2, and FF=79.4%), i.e., a relative improvement of 3.3%. This cell has an efficiency of 18.6% (VOC=696.5 mV, JSC=33.9 mA/cm2, and FF=78.8%) if illuminated from the rear side.

Crystalline silicon thin-film (CSiTF) solar cells on SSP and on ceramic substrates

CSiTF solar cells are being intensively investigated due to their cost reduction potential compared to silicon wafer solar cells. At the Fraunhofer ISE several high-temperature approaches are under investigation. Our main interest is focussed on low-cost silicon ribbons and on ceramics as substrate material for such solar cells because they match most requirements best. The silicon substrates are being prepared by a ribbon technique, namely the silicon sheets from powder (SSP) technique using silicon powder of different purity as starting material. Onto these substrates silicon layer systems for CSiTF solar cells are deposited by APCVD from SiHCl3 with high deposition rates and recrystallized by a melting step in order to achieve large crystallites in the active layer. The same layer systems have been studied on different kinds of ceramic substrates like SiSiC, SiN, graphite and mullite. To prevent diffusion of impurities from the substrates into the active silicon layer, different types of barrier layers have been used. On SSP substrates made of EG-Si powder conversion efficiencies up to 11.2% on those of UMG-Si 8.6% could be achieved. On ceramic substrates, efficiencies were up to 11% on graphite, up to 9.3% on SiSiC, up to 8.0% on SiN, and up to 8.2% on mullite.

Hydrogen generation from biogenic and fossil fuels by autothermal reforming

Hydrogen generation for fuel cell systems by reforming technologies from various fuels is one of the main fields of investigation of the Fraunhofer ISE. Suitable fuels are, on the one hand, gaseous hydrocarbons like methane, propane but also, on the other hand, liquid hydrocarbons like gasoline and alcohols, e.g., ethanol as biogenic fuel. The goal is to develop compact systems for generation of hydrogen from fuel being suitable for small-scale membrane fuel cells. The most recent work is related to reforming according to the autothermal principle — fuel, air and steam is supplied to the reactor. Possible applications of such small-scale autothermal reformers are mobile systems and also miniature fuel cell as co-generation plant for decentralised electricity and heat generation. For small stand-alone systems without a connection to the natural gas grid liquid gas, a mixture of propane and butane is an appropriate fuel.

Pocket-size PEMs

A recent study by Joan Ogden at Princeton University’s Center for Energy and Environmental Studies tabulated a range of published estimates for the manufacturing costs of mass-produced auto fuel cell systems. The banded-structure membrane fuel cell, from Fraunhofer ISE, draws off voltage at the endplates through an integrated series connection that ties together individual cells. The typical proton exchange membrane (PEM), fuel cell consists of a series of stacked individual cells, with each cell composed of a flow field plate and a membrane electrode assembly. An air breather PEM fuel cell from DCH Technology distributes hydrogen to the flow fields through a central sleeve, while air at atmospheric pressure comes in through the sides. One of the benefits of Manhattan Scientifics’ circuit board approach is the ability of an electrode to act as a ‘preferential molecular filter’. Fraunhofer Institute scientists have been at work on a banded-structure membrane fuel cell. This device holds five-unit cells on a single plate connected as a series circuit.

Comparison of boron‐ and gallium‐doped p‐type Czochralski silicon for photovoltaic application

A set of p-type Czochralski (Cz) silicon materials grown by Shin-Etsu Handotai was used for a comprehensive investigation, including carrier lifetime measurements and fabrication of high-efficiency solar cells at Fraunhofer ISE. The set of different materials consists of gallium and boron doped wafers grown with the Cz method and boron doped wafers grown with the magnetic Czochralski (MCz) method. A clear correlation of the Cz-specific lifetime degradation and the concentration of boron and interstitial oxygen was observed. Thus, gallium-doped wafers with a high concentration of interstitial oxygen of 13·7 ppm showed no degradation. Excellent stable lifetimes of 1098 μs and 862 μs were determined for boron-doped MCz wafers and for gallium-doped Cz wafers, respectively. This high lifetime level was maintained or even improved throughout the cell process optimized for Cz silicon and record efficiencies of 22·7% and 22·5% were achieved for boron-doped MCz silicon and gallium-doped Cz silicon, respectively. Copyright © 1999 John Wiley & Sons, Ltd.