Thermal Characterization of Photovoltaic Materials

Solar Energy

Renewable sources of energy –wind power, solar power, hydro-electric power, tidal power, geothermal energy and biomass –are essential alternatives to fossil fuels. Their use reduces our greenhouse gas emissions, diversifies our energy supply and reduces our dependence on unreliable and volatile fossil fuel markets. The sun is the world’s primary source of energy, and solar power systems can harness the sun’s rays as a high-temperature, clean energy source for heat or electricity. Photovoltaic (PV) electricity is emerging as a major power source due to its numerous environ-mental and economic benefits and proven reliability. The use of a typical home PV system to replace fossil fuels could reduce CO2 emissions by approxi-mately 1.2 tons per year

The Advantages of Solar Photovoltaic Electricity

  • The “fuel” is free
  • No moving parts
  • Minimal maintenance
  • Quick installation of modular systems – anywhere
  • Produces no noise, harmful emissions or polluting gases
  • Long lifetime and long-lasting high performance
  • Modules can be recycled
  • It brings electricity to remote areas
  • It can be integrated into buildings
  • Creates employment

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 the emphases will concentrate on

  • the efficiency of PV systems,
  • their lifetime and
  • costs.

This will spur new developments with respect to material use and consumption, device design and production technologies and will drive the development of new concepts for increasing overall efficiency.

The Photovoltaic Effect

The photovoltaic effect is the basic physical process through which a PV cell converts sunlight into electricity.

The energy of an absorbed photon is transferred to an electron in an atom of a semiconductor device. With its newfound energy, the electron is able to escape from its normal position associated with a single atom in the semi-conductor to become part of the current in an electrical circuit. The PV cell has special electrical properties including a built-in electric field (p/n junction) which provides the voltage needed to drive the current through an external load.

Photovoltaic EffectPhotovoltaic Effect
(Alexandre-Edmond Becquerel, 1839).
When photons strike a PV cell, they may be reflected or absorbed, or pass right through. The absorbed photons generate electricity.

PV Technologies

Cells and Modules

PV cells are generally made of crystalline silicon, but they are also often made of thin films. 

Crystalline silicon technology:

The most common technology consists of the crystalline silicon technologies is the formation of cells by slicing thin discs from ingots or castings. These slices are cut either from a single crystal silicon (monocrystalline) or from a block of silicon crystals (polycrystalline). Alternatively, crystalline silicon cells can also be made from grown ribbon sheets. Crystalline silicon technologies therefore consist of the following:

  • Monocrystalline
  • Polycrystalline (multi-crystalline)
  • Ribbon sheets

Thin Film technology:

Thin film modules are created by a chemical process (e.g., a chemical vapor deposit) in which thin layers of photosensitive material are deposited onto a low-cost backing material such as glass, stainless steel or plastic. Currently there are about 4 types of thin film modules available:

  • Amorphous silicon (a-Si)
  • Cadmium telluride (CdTe)
  • Copper Indium/Gallium Diselenide/disulphide (CIS, CIGS)
  • Multi junction cells (a-Si/m-Si)

Other cell types

There are several other types of photovoltaic technologies in development today, some of which are already starting to be commercialized.

  • Concentrated photovoltaic 
    The cells are built into concentrating collectors that use  a lens to focus the sunlight onto the cells. The focus is  to use less of expensive semiconducting PV material  while collecting as much radiation as possible. 
  • Flexible cells
    The technology is based on a similar production process to thin film cells. The difference is that here, the active material is deposited in a thin plastic, allowing the cell to be flexible. This opens the range of applications, especially for building integration (roofs, tiles) and end-consumer applications.

DSC (Differential Scanning Calorimetry) analysis provides valuable information for research and quality control of solar cells:  

  • Analysis of amorphous encapsulantes
  • Information of process temperatures
  • Specific heat for the determination of the thermal conductivity / diffusivity
  • Curing behavior
  • Kinetic Analysis of the curing
  • Quality Control

Simultaneous Thermal Analysis (STA)* generally refers to the simultaneous application of Thermogravimetry (TG) and DSC to one and the same sample in a single instrument. The test conditions are perfectly identical for the TG and DSC signals (same atmosphere, gas flow rate, vapor pressure of the sample, heating rate, thermal contact to the sample crucible and sensor, radiation effect, etc.).

* Differential Scanning Calorimetry & Thermogravimetry→Simultaneous Thermal Analysis

Commercial Module Efficiency


Recommended literature:

Recommended literature:

Detailed Insight Into the World of Thermal Analysis

Detailed Insight Into the World of Thermal Analysis

DSC can be used to analyze 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 (incl. UV curing) behavior
  • Kinetic analysis of the curing behavior

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 fibers, ceramics, glass and metal.

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 behavior of thermo-setting 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 min) or fast ones can be analyzed.

Differential Scanning Calorimetry, Thermogravimetry and Simultaneous Thermal Analysis

DSC Analysis Possibilities

  • Specific heat
  • Melting/crystallization behavior
  • Solid-solid transitions
  • Polymorphism
  • Degree of crystallinity
  • Glass transitions
  • Cross-linking reactions
  • Oxidative stability
  • Purity Determination
  • Thermokinetic Analysis

TG Analysis Possibilities

  • Mass changes
  • Temperature stability
  • Oxidation/reduction behavior
  • Decomposition
  • Corrosion studies
  • Compositional analysis
  • Drying behavior of thick films and inks
  • Thermokinetic Analysis

Thermomechanical Analysis & Dilatometry

Many materials undergo changes to their thermomechanical properties during heating or cooling. The environmental stresses PV systems endure (e.g. temperature, humidity, dust, corrosives, etc.) are also a strong function of the geographic location.

Thermal expansion is an important temperature effect which must be taken into account when modules are designed. The amount of spacing necessary to accommodate for such thermal expansion can be determined as a function of temperature (T) and the thermal expansion coefficients of the glass and the actual cell, as follows:

(αG, αC: expansion coefficients of the glass and the cell respectively, D: cell width; C: cell centre to centre distance).

Typically, interconnections between cells are looped, to minimize 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 fibers, ceramics, glass and metals.

TMA/ DIL Analysis

  • Thermal expansion
  • Coefficient of thermal expansion
  • Expansivity 
  • Volumetric expansion
  • Density change
  • Sintering temperature
  • Shrinkage steps
  • Glass transition temperatures
  • Softening points
  • Phase transitions
  • Creep behavior
  • Influence of additives
  • Rate-controlled sintering
  • Thermokinetics

Dynamic Mechanical Analysis

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.

DMA  Analysis

  • Glass transition temperature (Onset of softening and brittleness)
  • Design data concerning stiffness and damping properties
  • Data on the composition and structure of polymers and polymer blends (compatibility)
  • Curing behavior
  • Aging

Dielectric Analysis

For investigation of the curing behavior 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 is that it can be employed not only in a laboratory scale, but also in process.

For measurement of the ion conductivity, which is calculated from the dielectric loss factor, or its reciprocal value (ion viscosity), single- and multi-channel DEA systems are available for materials with slow (>3 min) and fast curing times.

DEA  Analysis

  • Cure rate
  • Cure state
  • Cure time
  • Degree of cure
  • Glass transition and other polymer transitions
  • Diffusion properties
  • Aging & decomposition effects
Some Application Examples

Some Application Examples

Thermophysical Properties of a Silicon Wafer

In this example, the thermophysical properties of a silicon wafer were measured with the LFA 457 MicroFlash®. In the temperature range from -100°C to 500°C, the thermal conductivity and thermal diffusivity continuously decrease.Determination of the specific heat was carried out with the DSC 204 F1 Phoenix®. The standard deviation of the data points is < 1%.

LFA and DSC measurement of a silicon wafer between -100°C and 500°C.LFA and DSC measurement of a silicon wafer between -100°C and 500°C.

Measurement on a Thin-Film Photovoltaics

CuGaSe2 with its band gap of 1.68 eV is a promising material for thin-film photovoltaics, since it can act as the top cell in a PV tandem device with CuInSe2 as the bottom cell. CuGaSe2 was synthesized 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, which indicates a nonstoichiometric material. The presence of iodine shows that it was used as a mineralizer for synthesis. The presence of Se at temperatures higher than 900°C is due to the thermal degradation of CuGaSe2. Regulation of the Se vapor pressure is required to control the stoichiometry. (measurement with QMS 403/5 SKIMMER®)

STA-MS SKIMMER® measurement of the chalcopyrite CuGaSe2STA-MS SKIMMER® measurement of the chalcopyrite CuGaSe2

Organic Contamination of a Silicon Wafer

In this example, a silicon wafer was measured with the Simultaneous Thermal Analyzer STA 449 F1 Jupiter® coupled to the mass spectrometer QMS Aëolos®. The large sample (1.6 g) was placed into an Al2O3 crucible (volume 5 ml) and heated to 800°C at 10 K/min under synthetic air. Two very small mass-loss steps (0.002% and 0.008%) occur prior to 700°C due to the release of organic components. To ensure clear demonstration, only the mass numbers m/z 15, 51, and 78 are presented here.

STA-MS measurement of a silicon wafer; mass numbers m/z 15, 78 and 51 are correlated to the mass-loss step between 500°C and 800°C.STA-MS measurement of a silicon wafer; mass numbers m/z 15, 78 and 51 are correlated to the mass-loss step between 500°C and 800°C.