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Encapsulation of Solar Cells
Contact: Axel Boden Solar cells that are to be in operation for a long time need to be protected from environmental factors such as dampness. To achieve this, the upper side of the solar cells is covered with a transparent covering layer. This can be glass or plastic sheet. The covering layer is glued to the surface of the solar cells with a plastic that networks when heated.

Diagram of a vacuum laminator

In the laboratory, encapsulation takes place in a vacuum laminator (see above diagram). The solar cell is placed face up inside it on a flat heater plate. EVA (ethyl vinyl acetate) foil is laid on it and, on top of that, the transparent protective covering (glass or plastic sheet). The lid of the laminator is then closed. In this lid, there is a membrane, which now rests on the solar cell, dividing the laminator chamber into two sealed parts. At the beginning of the process, both of these spaces are evacuated and at the same time, the solar cell is heated up. On reaching a predetermined temperature (e.g. 80°C), air is let in to the space above the membrane. This presses the membrane onto the stack consisting of solar cell, EVA foil and covering layer, creating a continuous contact between the individual layers over the entire surface. Upon increasing the temperature further, the EVA foil polymerises, at about 150°C, becoming a transparent, thermally stable film and creating a strong bond between the surface of the solar cells and the covering layer.

The photo shows the encapsulation station with the vacuum laminator open on the right and control equipment on the left. The measuring instruments in the middle record the parameters to be examined during the process of lamination.

"Photovoltaic Energy Systems" Experiment PE1

Solar-Modules
Rodrigo Guido Araújo Dr. Stefan Krauter Kim Onneken Dr. Volker Quaschning

________________________________________________________________

UFRJ-COPPE-EE TU-Berlin Contents
1 Solar cell, equivalent circuit and characteristics 4 1.1 I-V characteristic of the solar cell 7 1.2 MPP-Power, efficiency, fill factor 9 1.3 Influence of irradiance on the I-V curve 10 1.4 Influence of temperature on the I-V curve 10 2 Solar module 11 2.1 Influence of shadowing 12 2.2 Bypass-diodes 12 3 Measurement of the solar irradiance and the temperature 14 3.1 Measurement of the solar irradiance 14 3.2 Measurement of temperature 16 4 Preparation exercises 17 5 Experimentation 17 5.1 Setting up the experiment 17 5.2 Recording of I-V characteristics at different temperatures 18 5.3 Recording of the I-V characteristics for different levels of irradiance 18 5.4 Recording of the I-V characteristics for different states of shadowing 19 5.5 Different type of module 19 6 Protocol 19 7 List of equipment 20 8 Literature 20 Prologue Electricity generated from photovoltaic (PV) systems produces zero emissions, is modular, and can produce energy anywhere the sun shines. Investment in PV my be cost-effective in certain distributed generation and

grid-support applications where PV output tends to coincidence with local peak demands. Small stand-alone PV systems have proven practical in many areas of the country where connection with the local distribution was too costly or impractical. Such systems, when coupled with a storage battery, can serve loads such as homes, radio stations, control systems, telephone repeaters and lighting systems. PV modules may be installed in buildings as part of the roofing, walls, and/or windows.

Subject of experiment
Tests of PV-Modules at different illumination levels, module temperatures and shadowing.

Aims
• • • • • •

Setting-up of the experiment to trace I-V-characteristics Tracing of I-V-curves at different 3 illumination levels Tracing of I-V-curves at different 3 temperature levels Tracing of I-V-curves at different states of shadowing. Determination of all module data (e.g. temperature coefficient for power output, MPP). Comparison of different types of modules (mono- and multicrystalline)

Introduction
In applied photovoltaics, 20 to 80 solar cells are connected in series to a "Solar Module” to get a applicable voltage of 10-40 V. A PV solar module consists of a sandwich with a front cover of highly transmittive glass solar cells, and either a back cover of glass or of a laminated foil. The cells and the sandwich are sealed by a soft transparent plastic layer, so no moisture could harm. The plastic is often EVA (ethylene-vinyl-acetate) which is melted in a vacuum laminator to avoid airbubbles in the sandwich. The power output of a solar module is given by the manufacturer in Wp (Watt peak), which says that the module was rated at the so called " Standard test conditions”(STC), which mean an illumination level of 1 000 W/m² (which is bright sunshine), a spectrum equivalent to AM 1,5 and 25°C module temperature at the test. During real application most of the times the illumination is much lower than 1 000 W/m² (average is 200 to 500 W/m² - to measure the actual illumination, a so called "pyranometer” is used) and the cell temperature is at 40-60°C. Irradiation is seldom at AM 1.5 nor perpendicular to the module surface. Also a part of the module is sometimes shadowed by trees or buildings or direct dirt (e.g. from birds) accumulation on the front surface. To observe the influence of most of the upper parameters on the electrical performance (as I-V-characteristic, efficiency and power output) this experiment was designed for. 1. Solar cell, equivalent circuit and characteristics

This chapter an overview of the different equivalent circuits of the solar cell is given, without discussing the physical processes in a profound way. 1. Equivalent Circuits The simplified equivalent circuit of a solar cell consists of a diode and a current source which are switched in parallel. The current source generates the photo current IPh, which is directly proportional to the solar irradiance E. The p-n transition area of the solar cell is equivalent to a big diode (in admissive orientation) which is also integrated in the picture.

Figure 1: Simplified equivalent circuit of the solar cell. The V-I equation of the simplified equivalent circuit could be derived from Kirchhoff's current law (first law of Kirchhoff: all in- and outgoing currents at point add up to zero): with IPh Photo current ID Diode current IS Diode reverse saturation current ( 1.1)

m Diode "ideally factor" m = 1...5VT Thermal voltage: ; VT = 25,7mV at 25°C. -23 -1 k constant of Boltzmann k = 1,380658 · 10 JK T absolute temperature; [T] = K (Kelvin) 0 K = -273,15°C e charge of an electron e = 1,60217733 · 10-19 As As given by the name, the simplified equivalent circuit doesn't give an optimal representation of the electrical process at the solar cell. At real solar cells a voltage loss on the way to the external contacts could be observed. This voltage loss could be expressed by a series resistor RS . Furthermore leakage currents could be observed, which could be described by a parallel resistor RP.

Figure 2: Equivalent circuit with one diode of a solar cell.. Derived from Kirchhoff´s first law the equation for the extended I-V curve could be achieved. 0 = IPh - ID - Ip - I. with follows

( 1.2) This implicit equation could not be solved to I and U in such an easy way as the equation from the simplified equivalent circuit. Therefore a numeric method (e.g. as Newton-Raphson (see Stoer 1993) have to be applied.. An even more exact modeling could be achieved by the Two-Diode-Model. Here two different diodes with different diode ideally factors m connected in parallel. At the equations of the diode it was always taken for granted the there is no breakthrough at operation in the inhibited direction of the diode, but at high negative voltages a breakthrough at the solar cell could be observers. This was modeled at the following figure by a variable current source I(VD).

Figure 3: TwoDiode-Model of a solar cell with a second current source to simulate the breakthrough of the diodes at high negative voltages.. The I-V curve of the equivalent circuit can also be derived from the node-law of Kirchhoff. The supplementary term of the equation models the breakthrough at high negative voltages.

( 1.3) with V, I terminal voltage and current at the solar cell IPh photo current IS1,IS2 saturation current of the first, respectively the second diode RS serial resistance RP parallel resistance m1,m2 diode factor of the first, respectively the second diode ( m1 1 (ideal), m2 2) VT temperature voltage (see page 4) VBr breakdown voltage (VBr -15V..-50 V) a correction factor (a = 0..1 -1) n exponent for avalanche breakdown (n = 1..10) 1. I-V characteristic of the solar cell In the figure shown below measurements and calculated I-V characteristics of a multicrystalline solar cell (10 x 10 cm) are compared. The simplified model (Fig.1) with an ideal diode (m=1) shows still large deviations, while a much better fit could be achieved by modeling with a real diode. ( m>1). Even more accurate results could be completed by the "full" one-diode-model (Fig.2). A modeling by the twodiode model only makes sense, when more precise measurements as done in this experiment are carried out. The uncertainty region of the measurement values are marked as a beam.

Figure 4: I-V curve of a multicrystalline solar cell (10 x 10 cm), irradiance E=430 W/m², temperature T=300 K. In the figure below the dark current I-V curve of a solar cell for a wide range of voltage is shown. To measure a dark current curve an external voltage has to be applied to the solar cell. At the positive voltages the diode is in permitting stage. At negative voltages the diode blocks up to -15 V and then breaks through slowly. At this stage a high power dissipation occurs in the cell which warms it up. At a current of 2 A the heat dissipation of the dark cell is already 30 W. When the temperature rises too much the cell could be destroyed by so called "hot spots".

Figure 5: Dark I-V curve of a solar cell for an extended voltage region. 1. MPP-Power, efficiency, fill factor If a solar cell gets short circuited, a short circuit current Isc occurs which is about equivalent to the photo current (Isc IPh), the terminal voltage is zero. If no load is applied, the so called open circuit voltage Voc could be measured at the terminal. In both cases the electrical output power is 0 W. An irradiated cell is providing power output for a voltage region between 0 V and Voc . A point of operation where output power is at its maximum is preferred. This specific operation point is called MPP (maximum power point), while voltage times current is maximal here. The efficiency of a solar cell could be calculated from the power at MPP PMPP, the cell area AC and the irradiance E as follows: ( 1.4) Also another value plays a role in photovoltaics, the so called fill factor FF. It is defined as follows: ( 1.5) If the I-V curve would be rectangular (that is the ideal case), the fill factor would be 1. The fill factor is a quality consideration, for real cells it is between 0.75 and 0.85. 1. Influence of irradiance on the I-V curve By increasing the irradiance level also the amount of electron-hole pairs getting separated and inducing the photo current are increasing. The photo current is therefore proportional to the irradiance, which could be seen on the I-V curves. Also the output power in increasing

.

. ( 1.6) 1. Influence of temperature on the I-V curve

Most of the parameter of the solar cell show a temperature dependency. The general equation to calculate the temperature coefficient TC for a value y is: ( 1.7) If there is a linear connection between the size y and the temperature T, so the temperature coefficient TC is: ( 1.8) The short circuit current is increasing a little bit at rising temperatures, while losses of the open circuit voltage is about ten times higher (-0.4%/K). Therefore the power output is decreasing for increasing temperatures. The power loss factor is around 0.3-0.5 % per degree Celsius, so for an increase of 30°C in temperature the power is decreasing by 9-15 %. 1. Solar module At a solar module many cells are connected in series in order to achieve a higher voltage. Most commercial modules have between 36 and 40 cells..

Figure 6: Series connection of 36 cells. The current through all cells is identical. (I1 = I2 = ... = I36). The voltage of the module consists of the single voltages Vi over the n cells: ( 2.1) If the electrical parameters are the same for all cells as well as the temperature and the irradiance the Voltage of the module is: V = n·V i ( 2.2) The I-V curve of the module is composed by the I-V curves of the single cells (addition of voltages at the same current).

Figure 7: I-V characteristics of PV-module (36 cells) at E = 400 W/m², T = 300 K 26°C. 1. Influence of shadowing The I-V curve is affected decisively when cells are irradiated at a different level. This will be explained at an example. At a module consisting of 36 monocrystalline cells (10 x 10 cm) one cell is shadowed by 75%. All other cells are irradiated completely. The following figure explains the creation of one point at the final I-V curve (1): For a given current the voltage is calculated by the addition of the shadowed cell (1a) and 35 times the voltage of a irradiated cell (1b). The final I-V curve is also drawn in the figure: It could be seen that the module power output is reduced drastically by this single cell shadowing. While there was shadowed only 2% of the module area, the power output at MPP is reduced by 70 %. The shadowed cell acts as a load. The maximum power dissipation - at this cell is 12.7 W occurring at short circuit of the module. The points of the maximum power P1 and P2 are marked in the curves of identical power output..

Figure 8: I-V curve of module with one cell shadowed by 75 %, irradiance E = 407 W/m², temperature T = 300 K. 1. Bypass-diodes At the shadowed cell there is the threat of overheating already at this experiment. Therefore the manufactures of PV-modules switch bypass diodes to the cells or strings of cells. If the cells are operated at negative voltages, the current passes through the bypass diode. The voltage at the cells is limited to the threshold of the passing operation of the diode.

Figure 9: Simplified equivalent circuit of a solar cell with a bypass diode If a bypass diode is switched to every cell as shown in Fig.9, the module output power is reduced only by the power of the shadowed cell in case of shadowing (plus the losses of the diode). At most commercial modules bypass diodes are not implemented for each cell, but for each string of 12-24 cells. A destruction of a cell

by "hot spots" could be avoided by this, but the losses of output power are significantly higher.

Figure 10: IV curve of solar module (1) completely irritated (2) one cell shadowed entirely with a bypass diode at the cell (3) with a bypass diode at each sting of cells only (4) without bypass diode.

1. Measurement of the solar irradiance and the temperature
2. Measurement of the solar irradiance To measure solar irradiance different measurement systems for different purposes exist. The advantages and disadvantages are shown in the table. In our experiment we use a pyranometer because it gives us a spectral independent value of the irradiance while the slow reaction time is less important for our purpose. The pyranometer is covered by two glass spheres. The reflection losses of a sphere are more independent from the direction of the incidencing irradiance than plane surfaces. After the solar radiation passed the spheres, it hits an absorber plate which is warming up by that. The higher the intensity, the higher the temperature of the black absorber plate. The temperature difference is measured by a thermocouple which uses either a absorbing area (e.g. painted in white) or a massive metal body as a reference. The thermocouple generates a small voltage which is proportional to the temperature difference. This voltage, which is also proportional to the global irradiance (direct and diffuse irradiance), could be measured by a very sensitive Voltmeter.

Figure 11: Schematic of a pyranometer. A pyranometer allows high accuracy for long periods of measurements. It is relatively slow, so for fast changes of irradiance (e.g. at small, fast moving clouds) the results differ form reality. In our experiment we us a so called star-pyranometer which has got black-and-white areas on the absorber area which are thermally isolated from each other, in order to get temperature reference points for the thermocouple. Table 1: Overview over the different types of instruments to measure the solar irradiance E Name Type of receiver Application Properties pyranometer hollow radiometer silicon sensor thermocouple open air measurement E slow, some aging, good accuracy

electrically calibrated open air calibration E very good accuracy thermal receiver solar cell open air measurement E lab measurement E robust, fast, limited accuracy robust, fast, exact (if spectral mismatch was corrected)

reference cell photo diode pyro electrical radiometer thermal receiver

solar cell photo element pyro electrical thermocouple column

spectral measurement very accurate E() spectral measurement slow, exact E() spectral measurement slow, very accurate E()

1. Measurement of temperature

At the experiment three different kinds of temperature measurements are considered, they have different properties, advantages and disadvantages. 3.2.1 Thermoelectric voltage The thermoelectric voltages of a thermocouple junction (each consisting of two different metals solded together) at the measuring point (the solar cell) and at a reference point (e.g. ambient) are measured and compared. The voltage difference indicates the temperature at the measuring point. Therefore this kind of measurement requires a reference temperature and a quite sensitive voltmeter it is not used here. 3.2.2 Thermal resistance A temperature dependent resistor (as the Pt 100) is used. The resistance of the resistor increases as its temperature increases, e.g. at a temperature coefficient of TCR =0,00385 K-1 for a platinum resistor. The Pt 100 resistor has got a reference value of R(T0) = 100 at T0 = 0 °C. For different temperatures the according resistance is given as follows:
( 3.1)

To get accurate values for the temperature, the measurement of the resistance has to be carried out very precisely (e.g. by a so called four-wire-measurement which eliminates the resistive losses of the cables from the sensor to the Ohmmeter) . If done properly the results of this kind measurement are very accurate (deviation less than 0,1 K). 1. Temperature measurement by NTC or PTC. A semiconductor as silicon shows a temperature dependency of its conductivity, this effect is used to use it as a temperature sensor. Although the temperatureresistance dependency is non-linear, for a certain range of temperatures it could be approximated as a linear function and expressed by a temperature coefficient. NTC devices show a negative temperature coefficient, while PTC show a positive temperature coefficient of the electrical resistance towards the temperature. As a recent development integrated circuits with an attached amplifier to get an active, linearized device (output e.g. 5 mV/K) are often used.

1. Preparation exercises
The theory has to be known as well that all experiments could be done quickly without hesitation. If the preparation is not good enough the students are obliged to do the experiments another time. • From Fig. 8 short circuit current, open circuit voltage, power at MPP, the efficiency and the fill factor have to be determined for both, completely irradiated and shadowed condition.


Which current and voltage are at a resistor of a) 1 and b) 20 connected to the module of Fig. 7 ? The output power of the module has to be calculated for both loads.



Which voltage could be read at the millivoltmeter connected to the pyranometer at an irradiance of E = 200 W/m² ? Which voltage has got the thermocouple at a temperature of 40 °C ?



1. Experimentation 2. Setting up the experiment The experiment to record the I-V curves is set up according to Figure 12. The irradiance is done by the halogen lamps. The Pyranometer is to be placed at different positions of the solar module because the distribution of the irradiance is not equal. The position with the lowest irradiance is taken as the reference point. For the I-V plot the following adjustments should be taken: • V: 1 V/cm • I: 2 mV/cm It is to be calculated which amplitude (in cm) occurs at a current of 1 A (with the suggested shut resistor of 3A/60mV). 1. Recording of I-V characteristics at different temperatures The lamps are switched on. The irradiance is to be measured by the pyranometer at different positions of the module area (calibration factor is 15.5 mV at 1000 W/m2) and the I-V characteristic to be traced by varying the electrical load resistor. Also the cell or module temperature is to be measured. The results are recorded by the computer. Due to the irradiance the temperature of the module is rising. The IV curves should be made at every 10 K temperature difference, starting from ambient temperature up to a limit when a thermal equilibrium is reached and temperature stops rising (about 50-60°C). The first measurement has to be carried out quickly therefore the module is heating up very fast at the beginning.

Figure 12: Experiment for recording the I-V characteristics or the PV-module 1. Recording of the I-V characteristics for different levels of irradiance

The irradiance level should be increased in steps by 100 W/m², starting at 100 W/m², going up to the maximal irradiance (about 800 W/m 2). The irradiance could be varied either by changing the voltage of the lamps (unfortunately this changes also the spectrum of the lamps) or keeping the voltage of the lamps constant and using different kind of neutral filters in front of the lamps or in front of the module. For doing that, the irradiance according to the readings by the millivoltmeter has to be calculated: Very often the low inner resistance of the millivoltmeter influences the measurement, this deviation has to be taken into account. Then the I-V characteristics could be recorded: The measurements should be done quickly to keep the temperature almost constant for all measurements. The temperature has to be recorded too in order to determine the temperature coefficients later. 1. Recording of the I-V characteristics for different states of shadowing The average irradiance on the module area is fixed at 500 W/m². For the different shadowing states the according I-V curves are recorded. These states have to be examined: • no shadowing • an entire cell at the center of the module • an entire cell at the corner of the module • ¾ cell at the corner of the module • ½ cell at the corner of the module • ¼ cell at the corner of the module • 2 neighboring cells at the corner of the module • 2 cells at different corners of the module 1. Different type of module The same procedure has to be carried for a different kind of module. After doing the experiments the students are obliged to dismantle the experiment for the next group to come. 1. Protocol The protocol should include the following points: • Circumstantial answering of the preparation exercises • Results of the experiments: Curves, axis and scales have to be marked and described. Minimum and maximum of irradiance on the module area has to be noted, as well as ambient and module temperature. The I-V characteristics are to be discussed. • Determination of power at MPP, efficiency and fill factor for all I-V curves • Graphical representation of power at MPP, efficiency and fill factor as a function of temperature and irradiance. • Determination of the temperature coefficients of open circuit voltage, short circuit current and power at MPP.



Determination of number of bypass diodes of the module.

1. List of equipment o 1 PQ10/40 and 1 SM 55 solar module o temperature sensors with measurement transformer and amplifier, if necessary o 4 Halogen lights, each 500W, 130 V o 1 Pyranometer with amplifier and Millivoltmeter 0...20 mV o 1 Shunt-resistor 3A, 60mV or an equivalent magnetic sensor with instrument amplifier o 1 Potentiometer 200 Ω, 3 A o 1 I-V-curve tracking device (X-Y-Plotter or a computer with a A/D converter, printer) o connection cables o 1 digital multimeter for check o paper for plotter or printer o different cover materials: black mosquito net (to cover module and to reduce irradiance) black carton (to cover some solar cells for the shadowing experiment). 2. Literature




Wenham, S. R. Green, M. A., Watt, M. E.: Applied Photovoltaics. Center of Photovoltaic Systems and Devices, University of New South Wales, Sydney, Australia, 1994. "Solar-Server" on the Internet http://emsolar.ee.TU-Berlin.DE

Flexible polymer barrier films for the encapsulation of solar cells

Initial situation Renewable energies, such as photovoltaics (PV), do not produce carbon dioxide or other dangerous green house gases and thus, have a strongly positive influence on the climatic balance in the atmosphere. PV can also partly replace nuclear energy which always entails hazardous risks. Thin film solar cells have the potential of low cost, high efficiency and low material demand, which will be decisive for future dissemination of PV. The encapsulation of thin film solar cells is made by a vacuum lamination process using an encapsulating polymer (ethylene vinyl acetate, EVA) and a second sheet of glass (Figure 1). The disadvantages of this technology are high weight and the gap between the two sheets of glass at the edges, where moisture and gas can enter into the photoactive layers and cause degradation. The process can not be easily automated and needs a lot of time, energy and material. Besides, it is not possible to produce flexible modules in this way. Therefore, the development of new encapsulation

Figure 1 Detailed scheme of a thin film solar cell of the stateof-the-art (CIGS module), with kind permission of ZSW

Figure 2 Requirements on the barrier layer technology. Products on top have the highest requirements.

Figure 3 Scanning transmission micrograph of a thin hybrid polymer coating on SiOx deposited on a flexible PET film

processes for thin film solar cells is necessary to reduce costs and to increase competitiveness of PV. The project Eight partners joined in the EC funded project HIPROLOCO in order to overcome the disadvantages of the double-sided glass encapsulation for thin film solar cells. It was the aim of this project to develop a new automated, stepby-step or continuously operating encapsulating process for rigid and for flexible thin film solar cells. Therefore, new encapsulants which are based on flexible polymer materials had to be developed. To protect the solar cell from water vapor in the atmosphere, an encapsulation system with very high barrier properties is necessary (Figure 2). Currently available polymer films do not meet these high requirements; an additional system of barrier layers on the polymer film is necessary. It was the task of Fraunhofer ISC to develop new barrier coating materials based on ORMOCER®s, which together with the inorganic vapor deposited SiOx layer could guarantee sufficient protection to ensure a long durability of the encapsulated solar cells. The barrier system is only one part of the whole encapsulation system. Altogether, the new polymer film based encapsulants had to include an adhesive/sealing layer, a barrier system against water vapor and gases and an outside layer for weatherability. All these functions had to be combined in one composite („one component encapsulant“). In this way the overall cost reduction for encapsulation should reach about 50 per cent. Solution The barrier system With hybrid polymers very good barrier properties regarding permeation of gases and vapors can be obtained. For the application envisaged here, the barrier properties of the encapsulating polymer film had to be improved by means of at least one inorganic vacuum deposited barrier layer in combination with a barrier coating system based on hybrid polymers, ORMOCER®s. Through the combination of barrier layers (Figure 3) high and ultra-high barrier properties can be achieved. The barrier improvement of the hybrid polymers depends to a great deal on the quality of the vapor deposited oxide layers (evaporation, sputtering etc.). If the hybrid polymers are applied on inorganic oxide layers deposited by physical or chemical vapor processes (PVD, CVD), water vapor and oxygen transmission rates (WVTR, OTR) below 0.1 [g/m2d] and 0.1 [cm3/m2dbar] are accessible. Combined with sputtered layers the hybrid polymers usually result in ultrabarriers which show transmission rates below 0.005 [g/m2d] for water vapor and 0.005 [cm3/m2dbar] for oxygen (barrier structure: PET/Al2O3 sputtered/ORMOCER®). Sputtered layers have been found to give the best barrier properties, whereas electron beam coatings (PVD) allow the production of transparent barrier layers at relatively low cost. Fraunhofer ISC and Fraunhofer IVV have cooperated with Alcan Packaging Services in order to develop the high barrier systems needed for the encapsulation composite (Figure 2). The barrier systems were produced by the combination of SiOx (PVD) and ORMOCER® barrier layers on a special PET 36 µm film which was optimized for vacuum coating. Fraunhofer ISC was responsible for the development of ORMOCER® barrier layers. These barrier layers were applied by roll-to-roll coating processes at Fraunhofer IVV. The SiOx layers were produced by Alcan Packaging Services. Different layer structures were tested (Figure 4). For the four-layered barrier system the OTR

Figure 4 Barrier properties of the barrier systems produced by roll-to-roll processes (OR: ORMOCER®)

Figure 5 Encapsulation films for solar modules, with kind permission of IVV

Figure 6 Encapsulation system for solar modules: encapsulation material for a) the front side and b) the rear side of the solar cell

was measured below 0.01 cm3/m2 d bar while the WVTR reached the value 0.01 g/m2d. These results are in the range of the necessary barrier requirements for the encapsulation of solar cells. Therefore, this layer structure was used for the new encapsulation process tested at the end of the project. The design of the encapsulation system Besides the barrier film laminate additional layers with different properties have to be used for both the front and the rear side of the solar cell to reach the requirements mentioned above (Figure 6).The front side material, which has to be a transparent film composite, consists of the following single layers: an ETFE (ethylene tetrafluoro-ethylene copolymer) film which gives mechanical stability and weatherability, the SiOx layer working also as a primer for a UV absorber containing adhesive and additionally working as a first barrier layer, the barrier system (multilayer structure) which provides the required high barrier properties against moisture and gases. For the rear side of the solar cell there is no need for protection against UV-light. Light transparency of the back layer is not an important issue. Therefore, the design of the rear side material is less complicated (Figure 6b). The process of encapsulation The composite encapsulating materials were produced by Isovolta according to Figures 5 and 6. They were used for the production of test modules at Kloepper Maschinentechnik GmbH, ZSW and Free Energy Europe SA. The suitability of the new process of encapsulation has been proved. Some optimization work with regard to scaling up and adaptation of the single process steps has still to be carried out. This is planned in the frame of a further project. Customer benefits The flexible nature of the encapsulants results in an optimized encapsulation process and especially in good protection of the edge area. The new material can be used from the roll, thus providing an easy handling and automation in the production of flexible modules. Thin barrier layers, like SiOx, combined with ORMOCER® based barrier coatings, will save material and energy consumption in the production of the new encapsulating material. Therefore, the result of this project is a material and energy saving encapsulation technology. This will help to promote the application of thin film solar cells in the building industry and to increase the number of users of solar cells. Especially flexible thin film solar cells cover broader fields of application because they can be adapted to non-planar surfaces. That means, PV could be a standard integrated part of construction components, in e. g. roofing materials, façades, balustrades. Thus, a larger number of potential users could benefit from the solar technology. Upcoming and future products require flexible films that have high barrier or even ultra-high barrier properties (Figure 2). The encapsulation film for solar modules is one important example which illustrates that there is still a great need for the development of improved polymer barrier systems. Cooperation The work described was carried out in the HIPROLOCO project funded by the European Commission (ENK5-CT-2000-00325). We thank the EC for funding and all partners for their cooperation. Partners of Fraunhofer ISC: Fraunhofer IVV, Germany; Isovolta österreichische Isolierstoffwerke AG, Austria; Zentrum für Sonnenenergie- und Wasserstoff-Forschung BadenWürttemberg ZSW, Germany; Kloepper Maschinentechnik

GmbH, Germany; Alcan Packaging Services AG, Switzerland; Free Energy Europe SA, France; Slovak University of Technology, Slovakia. Your contact Dr. Sabine Amberg-Schwab Phone +49(0)9 31/41 00-6 20 Fax +49(0)9 31/41 00-6 98 E-Mail: [email protected] E-Mail: [email protected] Ulrike Weber Phone +49(0)9 31/41 00-6 21 Fax +49(0)9 31/41 00-6 98 E-Mail: [email protected] E-Mail: [email protected]

© 2005 Fraunhofer-Institut für Silicatforschung ISC

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