Characterization

Nowadays, research and development is indivisible connected to characterization of specific samples under investigation. At all times the question is: why does one specific sample feature the desired properties, but another seemingly similar sample shows worse properties? And what can the researcher learn from the experiment to improve performance of further samples?

Sometimes apparently simple techniques are used, sometimes more sophisticated ones. But interpreting those techniques may always lead to very fundamental non-trivial questions.

The characterization group with its various tools and methods is steadily present during the manufacturing process of solar cells and beyond. This shall be illustrated with regard to a typical solar cell.


It all begins with the wafer

Starting with the delivery of wafers, integral or spatial resolved minority carrier lifetime measurements (via photoconductance decay, micro wave reflection, and luminescence) are used for quality control of the material. These methods may also be used for process monitoring during the manufacturing process and tracking down lifetime related problems especially due to unwanted contamination during high temperature steps.

The next step in the manufacturing process is at least a surface cleaning treatment, or more likely a texturing process providing better light trapping capabilities of the solar cell, simultaneously removing a defect-rich surface layer. But the microscopic structure of the surface is extremely sensitive to the specific process and its parameters, and thus there are hardly any two similar textures. A closer look on these structures may be worthwhile either by regular optical microscopy or even higher resolved by atomic force or scanning electron microscopy. In contrast to monocrystalline silicon which often features a rather monotone image, multicrystalline silicon features a variety of different grain orientations, sometimes not even directly visible prior to etching. Spatially resolved electron back scattering provides data about the various grain orientations. But regardless of the exact microscopic structure, a measurement of the reflection in the interesting visual and near infrared spectral range shows how effective the light trapping of the structure is, despite the absence of an antireflection coating.


The wafer becomes a solar cell

The diffusion of a strongly doped emitter layer enables a reasonable photoelectrical energy conversion, either an n-type emitter on a p-type base material or vice versa. The achieved sheet resistance can be determined by means of a 4-point-measurement. A deeper insight of the diffused layer may be provided by the ECV (electrochemical capacitance voltage) method, measuring a profile of the electrical active dopant atoms. This may be applied to a carrier separating pn-junction as well as to an electrical reflector layer as it is given in a back- or front-surface-field. The strongly doped region can also be visualized in cross sections by means of SEM (scanning electron microscopy). The QSSPC (quasi steady-state photoconductance) method can be used to evaluate the saturation current density of the emitter under investigation. Confocal Raman microscopy as well as Kelvin probe force microscopy allow for investigations of local doped regions.

An antireflection coating (ARC) made of silicon oxide, silicon nitride, silicon carbide or amorphous silicon defines the typical bluish color of the almost completed solar cell. A measurement of the spectral reflection proves whether the applied ARC allows for an optimized light coupling and ensures a significant improvement of the current density of the solar cell. As the observable color is caused by an interference effect, the color impressions carries only limited information about the structure and optical properties of the layer itself. A measurement of the optical properties by means of spectral ellipsometry allows for the evaluation of the refractive and extinction indices as well as for the true thickness of the layer. Even if the thickness of the dielectric ARC layer is just about 50-100 nm, noticeable absorption may occur in the blue spectral range, leading to a decreased blue response which is often mistaken not as an optical loss, but an electrical loss in the emitter region. Furthermore, measurements of infrared absorption via FTIR (Fourier transformed infrared absorption) provide information about the statistical atomic bond structure. This allows for example a quantification of the hydrogen content in widely-used hydrogenated silicon nitride ARC layers, which is a major issue for volume passivation.

Using non-metallized symmetrical samples at this stage allows also for an evaluation of surface passivation quality. Lifetime measurements provide vital information about the surface recombination velocity (for samples without pn-junction) or emitter saturation current density (for samples with pn-junction), respectively.

One major issue is whether the contact formation was successful, mainly influencing the fill factor. The TLM (transfer length method) measurement is used to exactly quantify the metal-silicon contact resistance. The destructive Corescan method provides a quick spatially resolved overview of the series/contact resistance distribution hardly influenced by the silicon quality. Non-destructive electroluminescence imaging can be used, resulting as well in spatially resolved images of series resistance and saturation current. A more basic technique is the measurement of the finger conductance by four point probe technique. In combination with a cross-section analysis by surface profiling, atomic force microscopy or optical 3D microscopy, the specific conductivity of the used metal can be determined. Local contact sites can be analyzed by scanning acoustic microscopy as well. An advanced analysis may be carried out by electron microscopy with the possibility of an element specific EDX (energy dispersive X-ray) analysis.


The fully operational solar cell

After metallization of the now completed solar cell many techniques can be used for detailed characterization. Commonly, I(V)-measurement under constant illumination (1 sun equivalent) or during a flash determines the electrical properties of the solar cell, especially the short circuit current density jsc, the open circuit voltage Voc, the fill factor and finally the conversion efficiency η. Quantitatively reliable values are guaranteed by a crosscheck with independently confirmed calibration cells. Any serious problems in the manufacturing process will show up here at the latest by reduced electrical parameters. A more detailed analysis includes an I(V)-measurement in the dark and potentially an Isc/Voc or Suns/Voc measurement. Combining these three characteristics allows for an evaluation of the solar cell, e.g. on the basis of the two diode model. The used parameters are commonly diode saturation current densities and ideality factors, as well as series and parasitic parallel resistance. Each parameter contains more or less disjunct information, in which process step problems may have occurred.

However, the parameters of the two diode model do not reasonably explain the obtained short circuit current density jsc. This issue may be investigated by spectral response or quantum efficiency measurements. Analysis of the generated photocurrent and reflection under monochromatic illumination in the visible and near-infrared range shows which parts of the spectrum contribute to the short circuit current. The partially missing current in specific spectral regions provides information about front surface, emitter, bulk or back surface properties. And finally, the determined quantum efficiency may be used to calculate the true jsc regarding the AM1.5G spectrum, as all solar simulators exhibit only an approximated AM1.5G spectrum (spectral mismatch).


From integral to spatially resolved measurements

If a sample exhibits lateral inhomogeneity either due to quality distribution (e.g. multicrystalline silicon) or manufacturing inadequateness, spatially resolved or imaging techniques gain in importance.

The camera based electroluminescence imaging is a fast spatially resolved characterization technique. Even if silicon features an indirect band gap and is little-known for radiative recombination, the few emitted photons carry information of the local carrier concentration or the applied voltage, respectively. Reduced material quality, increased series resistance or a local short circuit of the pn-junction, each effect shows a certain pattern in a luminescence image when the sample is biased. In the case of photoluminescence imaging the sample is excited optically. This provides the advantage to gain information on material quality without the typical series resistance limitation of electroluminescence imaging.

Another spatially resolved technique for characterization of completed solar cells is lock-in thermography. Externally excited, the deposited energy is dissipated into heat by certain loss mechanisms. Cell areas with electrical losses above or below average stand out in the heat image. Each loss mechanism features a specific pattern and may be identified. If optical excitation is chosen, the sample does only require a pn-junction, making this technique also suitable for process control.

The LBIC (laser beam induced current) technique abandons the advantage of full spectral information of the spectral response technique but becomes spatially resolved, focusing only on few discrete wavelengths. The determined photocurrent maps taken at different wavelengths in combination with reflection maps allow for quantum efficiency or effective diffusion length maps.

A variation of the LBIC technique is the EBIC (electron beam induced current) technique using a high energy electron beam instead of laser light to excite electron-hole pairs in silicon. Depending on the electron’s kinetic energy, penetration depth may be varied and, as part of an electron microscope, it features an unbeatable theoretical resolution, concurrent SEM and crystallographic imaging. Furthermore, measurements can be performed in a broad temperature range down to liquid nitrogen (77 K).


Beneath the surface

Defects responsible for material quality decrease can be investigated in more detail, too. Larger defects like inclusions may be visualized by means of infrared transmission microscopy; smaller defects require more advanced techniques like scanning electron or transmission electron microscopy.

Last but not least, controlled breakage of solar cells may sometimes yield information. Broken cells in an industrial production line cause delay and may break even more cells. Systematic breakage tests of solar cells or wafers are used to identify process steps that lower the breakage force below a critical value which is encountered in production. This accounts also for the adhesion force of the soldered connectors between cells in modules, which should not fall below a critical limit. Acoustic microscopy gives insight into the soldered connections, revealing e.g. cold solder joints.