Solar so good
19 Jan 2012 by Evoluted New Media
In order for solar technology to advance we need to fully understand the impacts of different materials on thermal performance. Here, Peter Davies discusses thermal characterisation of photovoltaic materials
The solar energy demand has grown at about 30% per annum over the past 15 years. To meet the growing demand, get products to market faster, and provide critical performance data to support competitive differentiation, research emphasis will be on the efficiency of PV systems, their lifetime and costs. This will spur new developments in material use and consumption, device design, and production technologies, and will drive the development of new concepts for increasing overall efficiency.
Well-proven test methods for the research and development and quality control of materials are already used in the various branches of the photovoltaic industry. Various thermoanalytical methods such as Laser Flash (LFA), Dilatometry (DIL) and Thermomechanical Analysis (TMA), Differential Scanning Calorimetry (DSC), Thermogravimetry (TGA), Simultaneous Thermal Analysis (STA), Dynamic-Mechanical Analysis (DMA/DMTA), and Dielectric Analysis (DEA) provide useful information regarding the thermal, thermophysical and mechanical properties. These can be:
- Heat transfer.
- Thermal conductivity/diffusivity.
- Specific heat.
- Curing behaviour.
- Processing properties.
- Kinetic analyses of curing reactions of encapsulants.
- Softening behaviour of cured encapsulants.
- Creep behaviour.
- Drying behaviour of thick films and inks.
- Thermal stability.
- Contaminations.
[caption id="attachment_26184" align="alignright" width="300" caption="Figure 1: LFA and DSC measurement of a Si sample"]
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Differential Scanning Calorimetry (DSC) is one of the most frequently employed thermal analysis methods. It can be used to analyse nearly any energetic effect occurring in a solid or liquid during thermal treatment. DSC analysis provides valuable information for the research and quality control of solar cells, including: analysis of amorphous encapsulants; information about process temperatures; specific heat for determination of the thermal diffusivity/conductivity; curing (including UV curing) behaviour; and kinetic analysis of the curing behaviour.
Simultaneous Thermal Analysis (STA) generally refers to the simultaneous application of Thermogravimetry (TGA) and DSC to one and the same sample in a single instrument. The main advantage of this is that test conditions are perfectly identical for the TGA and DSC signals (same atmosphere, gas flow rate, vapour pressure of the sample, heating rate, thermal contact to the sample crucible and sensor, radiation effect, etc.). In addition, sample throughput is improved as more information is gathered from each test run.
Many materials undergo changes to their thermomechanical properties during heating or cooling. The thermal expansion is an important temperature effect which must be taken into account when modules are designed. Typically, interconnections between solar cells are looped to minimise cyclic stress. Double interconnects are used to protect against fatigue failure caused by such stress. In addition to interconnect stresses, all module interfaces are subject to temperature-related cyclic stress which may eventually lead to delamination.
Dilatometry (DIL) and Thermomechanical Analysis (TMA) provide valuable information regarding the mechanical properties under load and impact on solar cells. Investigations can be carried out on plastics and elastomers, paints and dyes, composite materials, adhesives, films and fibres, ceramics, glass and metals.
[caption id="attachment_26176" align="alignleft" width="300" caption="Figure 2: STA-MS Skimmer measurement of the chalcopyrite CuGaSe2"]
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Dynamic Mechanical Analysis (DMA or DMTA) allows for quantitative determination of the mechanical properties of a sample under an oscillating load as a function of temperature, time and frequency. The results portray the viscoelastic properties, typically provided as a graphical plot of E’, E’’, and tand versus temperature. DMA identifies transition regions in plastics and resins such as the glass transition and may be used for quality control or product development. In the area of photovoltaics, DMA is used to investigate the degree of curing, post-curing, and the kinetics of the cross-linking process for EVA or other encapsulants.
For investigation of the curing behaviour of thermosetting resin systems, composite materials, adhesives and paints, dielectric analysis (DEA) in accordance with ASTM E2038 or E2039 has stood the test of time. The great advantage of DEA is that it can be employed not only in the laboratory, but also in process. These systems can measure the ion conductivity – calculated from the dielectric loss factor – or its reciprocal value, the ion viscosity. Materials with slow curing times (> 3 minutes) or fast ones can be analysed in single- and multichannel system
[caption id="attachment_26177" align="alignright" width="300" caption="Figure 3: DEA test result of the curing of EVA at 150°C at 1Hz"]
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Investigation of the thermal conductivity and thermal diffusivity is important in overcoming thermal management problems during the PV module operation. The Laser Flash (LFA) technique is a fast and reliable absolute method for determining these thermophysical properties, including the specific heat. This data can then be used for:
- Prediction of the heat transfer and temperature profile as a starting point for the description of the processing behaviour of multi-sheet systems
- Thermal diffusivity and thermal conductivity as input data for numerical simulation.
Figure 1 shows the measurement of the thermophysical properties of a silicon wafer. In the temperature range from -100°C to 500°C, the thermal conductivity and thermal diffusivity (LFA measurement) continuously decrease. The determination of the specific heat was carried out with a DSC system. The standard deviation of the data points is <1%.
The most promising cell types are thin-film solar cells based on direct semiconductors with high absorption coefficients. In fact, the highest laboratory efficiencies are reported for thin-film solar cells based on the chalcopyrite semiconductor system Cu (In,Ga) (S,Se). However, they do also have some disadvantage which lies in their physical-chemical properties and their stoichiometry. This makes crystal growth and/or the manufacturing of epitaxial layers relatively complicated. Here, TG-DSC coupled to a mass spectrometer (MS) can provide important information for studies of physical-chemical properties such as: Melting point, evaporation, characterisation of gas species during heating, determination of impurities and temperature of incongruent melting.
[caption id="attachment_26178" align="alignleft" width="300" caption="Figure 4: First and second heating of a commercially available EVA film (Mitsui Chemicals Fabro)"]
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Figure 2 shows the measurement of CuGaSe2 (band gap 1.68eV) that can act as the top cell in a PV tandem device with CuInSe2 as the bottom cell. CuGaSe2 was synthesised from Cu, Ga, and Se taken in stoichiometric amounts. At 450°C, the evaporation of Se3 is detected by means of the isotope distribution between m/z 230 and m/z 245 what indicates a non-stoichiometric material. The presence of iodine shows that it was used as a mineraliser for synthesis. The presence of Se at temperatures higher than 900°C is due to the thermal degradation of CuGaSe2. Regulation of the Se vapour pressure is required to control the stoichiometry.
One of the most critical steps in PV production is the encapsulation of solar cells. This encapsulation should allow for efficient power generation in any application while protecting the cells from damage or corrosion caused by mechanical shock or the environment (moisture, dust, etc.). Thermal treatment of the polymer resin during the encapsulation process is performed in the laminator in two steps, the lamination and the curing. This treatment effectuates a bonding of the multiple layers of materials with thermo-sensitive polymer films. The most widely used encapsulant is EVA (ethylene vinyl acetate copolymer). As the polymerisation reaction is irreversible, the thermal treatment of the PV cell encapsulation is crucial. The quality and lifetime of the PV modules/arrays depend on the calibre of this production process.
In Figure 3, a DEA test of an EVA sample was carried out in a lab furnace which allowed programming of time and temperature ramps (up to 40 K/min) and the use of disposable comb sensors. The multi-frequency measurement (frequencies between 1Hz and 10000Hz) was carried out, and the ion viscosity (W/cm) was monitored. Presented here is the behaviour of the ion viscosity at 1Hz.
DSC is well suited for performing quality control analyses on commercially available EVA films with respect to their glass transition temperature, melting behaviour and degree of curing. Competitive materials can easily be compared, and the cross-linking process can be investigated as a function of temperature (dynamic tests) or time (isothermal tests).
Figure 4 demonstrates the DSC measurements* on an EVA film sample (approx. 7mg) at heating rates of 10K/min. In the first heating (blue curve), the glass transition temperature at -28°C is followed by an endothermic double peak between 50°C and 100°C. This melting behaviour can be correlated to a lamellar thickness distribution. The exothermic peak at 158°C indicates the exothermic crosslinking reaction. Noticeable is the rather low reaction enthalpy (-14.15 J/g) in comparison to epoxy resins (typically between -400J/g and -500J/g). In the second heating (red curve), the glass transition occurred at almost the same temperature. The endothermic double peak between 40°C and 80°C has changed to a broad shoulder with its maximum at 63°C. The higher the crystal thickness, the higher the melting temperature; therefore, the change from a peak into a broad shoulder is an indication of a distribution of crystals with reduced thickness as a consequence of the thermal treatment in the first run. No exothermic reaction peak is present in the second run, indicating that the crosslinking process was finished after the first heating.
Characterisation, quality control and research and development of solar cells can be well done with the different Thermal Analysis and thermophysical properties testing methods. The tests results can be directly used for example to improve the production process (e.g. kinetic analysis of the crosslinking reaction), optimise crystal growth (e.g. stoichiometry control), investigate curing behaviour of encapsulants, and to study heat transfer mechanism (e.g. determination of the thermal conductivity). The few application examples shown here, already demonstrate that many of the thermal management tasks can be investigated with the help of Thermal Analysis. The clear advantage lies in the fact that these methods are standardised, less complicated in operation and sensitive enough to fulfil the demands of state-of-the-art research and development tasks.
The Author
Peter Davies of NETZSCH-Gerätebau GmbH
*Acknowledgement Our thanks to Dr. W. Stark and M. Jaunich from the Federal Institute for Materials Research and Testing ("BAM") in Berlin for the measurements DSC and discussion. The results and detailed discussion are published in Polymer Testing 30 (2011) 236-242.
