Photovoltaics

Photovoltaic panels on the roof of Metrohm Headquarters

Light-based technologies will play a crucial role in the future of energy supply.

Photovoltaics has come a long way from powering pocket calculators – and is looking towards a bright future. Arguably, photovoltaic cells will play a starring role in our energy shift to a carbon-free future.

At Metrohm, we want to contribute our part to minimizing our carbon footprint and preserving the environment and natural resources for future generations.

In 2016, we have reached a major milestone on this ambitious journey: the solar panels on the roof of our Headquarters in Herisau, Switzerland, will now cover 25% of our energy demand.

Crystalline silicon: Etch rate monitoring with titration and IC

In total, 80% of all solar cells installed are made of silicon, so-called first-generation solar cells. Every monocrystalline and polycrystalline silicon wafer passes a wet chemical etching step to obtain the desired surface properties to enhance light-trapping. Here Metrohm comes into play.

The etching solutions consist of various acids, stabilizers, wetting agents, or buffers and determine the surface structure and thus the efficiency of the solar cells. Etching continuously increases the amount of dissolved silicon, which influences surface morphology and the etch rates. The composition of the texturing bath is the key for producing efficient solar cells. In the most hostile environments, our instruments analyze the composition of your texturing baths by ...

… thermometric titration

Etch acid mixtures containing HNO3, HF, and H2SiF6 from the etching of silicon substrates can be analyzed in two determinations. The first determination involves a direct titration with NaOH, followed by a back titration with HCl: this determination yields the H2SiF6 content plus a value for the combined contents of nitric and fluoric acid. The second determination consists of a titration with aluminum to determine the HF content. On the basis of these two determinations, the software calculates the individual results for HNO3, HF, and H2SiF6.

> Learn more about the 859 Titrotherm

… potentiometric titration and direct measurement

The total silicon content in the texturing baths can also be determined by potentiometric titration. The determination of total and single acid concentration and of dissolved silicon was carried out using aqueous acid-base titration with NaOH solution.

The Solitrode HF is the appropriate pH glass sensor for potentiometric titration. It has an extraordinary resistance in HF-containing texturing baths.

> Learn more about the Solitrode HF

Alternatively, the total fluoride concentration can be determined with Metrohm’s fluoride-selective electrode (F-ISE). The difference between the total fluoride content and the fluoride bound in hexafluorosilicate provides the hydrofluoric acid content. The content of nitric acid can be calculated from the total acid content.

> Learn more about the F-ISE

… ion chromatography with dual detection

Fluoride, nitrate, sulfate, and acetate are determined by conductivity detection after chemical suppression, while the silicon present in the form of hexafluorosilicate is detected spectrophotometrically as molybdosilicic acid after derivatization in the same analysis.

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Thin film solar cells: Electrolyte analysis with voltammetry

Thin film – or second-generation – solar cells are produced by depositing extremely thin layers of photosensitive materials on various low-cost substrates such as plastic or glass. The idea is anything but new: Already in the 1980s, amorphous silicon (a-Si) solar cells powered our pocket calculators and watches. The low efficiency rate of a-Si is more than compensated by the ease of manufacture, the low costs, and the thermal resistance required during operation in hot climate zones.

Very promising are the two other mainstream PV thin film technologies with the absorber layers of copper indium gallium diselenide (CIGS) and cadmium telluride (CdTe). Thin film technology is the way forward, and electrodeposition is an extremely economical and convenient way to produce solar cells. Voltammetry allows you to analyze the main constituents of electroplating baths and additives with high sensitivity:

CIS cells: Voltammetric analysis of Cd and thiourea in electrolyte baths

Excellent light absorption is achieved with thin layers of copper indium diselenide (CIS) interfaced with the semiconductor cadmium sulfide (CdS) to produce an effective heterojunction. The latter is deposited in a wet chemical process using cadmium acetate and thiourea, the source for in situ sulfide generation. First, Cd2+ is adsorbed on the surface. Subsequently, the sulfide released during the decomposition of thiourea reacts with the adsorbed Cd2+ ions. Cadmium and thiourea concentrations in the bath are precisely controlled using voltammetry.

And there is more: Apart from the CdS heterojunction, voltammetry can be used to determine the concentrations of the metal solutions and additives in the electroplating baths that are used for the electrodeposition of the CIS layer on molybdenum.

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CIGS cell: Voltammetric analysis of Cu, In, Ga, and Se(IV) in electrolyte solutions

By substituting some gallium for indium, copper indium gallium selenide (CIGS) solar cells are created. CIGS passed the 20% efficiency milestone in 2008. CIGS cells are the most important thin film solar cells, consisting of a thin layer of cadmium sulfide (CdS) that forms the p-n heterojunction with the thicker CIGS absorber Cu(InGa)Se2.

Cu(InGa)Se2 thin films are prepared by one-step electrodeposition from low-concentrated metal salt solutions onto molybdenum. Indium, selenium, and copper in the electrolytes are determined by polarography at the dropping mercury electrode (DME). For gallium analysis, anodic stripping at the hanging mercury drop electrode (HMDE) is used. As already mentioned in the CIS chapter, voltammetry also controls the Cd and thiourea content for CdS deposition.

> Learn more about voltammetry

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Electrochemical cell characterization using potentiostats/ galvanostats

Potentiostats/galvanostats from Metrohm Autolab enable straightforward analysis of the electrochemical and photoelectrochemical processes in protocrystalline (nC-Si), black silicon, gallium arsenide (GaAs) multijunction, dye-sensitized (DSC), and organic/polymer solar cells. The validation of all these promising PV technologies involves myriads of electrochemical measurements from current-potential (IV) and power-potential (PV) curves to impedance measurements. Metrohm Autolab provides the necessary instrumentation and know-how.

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Further applications and products 

Optimizing coating parameters with ProcessLab

It doesn’t matter what process step in the texturing process of crystalline silicon has to be monitored – etching, washing, rinsing, or hydrophilization –, the standard parameters pH value and conductivity are crucial for the surface morphology and always have to be monitored. The customized ProcessLab analyzer will do this for you, even in the most hostile HF environments. Besides these direct measurements, ProcessLab allows you to monitor all alkaline and acid texturing, washing, and cleaning steps, rinsing as well as hydrophilization.

See more fields of application Read more about the ADI 2045PL ProcessLab

Surfactant titration in texturing baths

Silicon solar cells require a textured front surface to reduce energy losses due to reflection. Surfactants in the texturing bath (e.g., sodium dodecyl sulfate or cetyl trimethyl ammonium bromide) have a strong influence on surface morphology and coating thickness, which is why their concentration has to be strictly controlled. Metrohm offers several potentiometric determination methods for anionic and cationic surfactants.

Nonionic surfactants Anionic and cationic surfactants

Front end bath control with NIR spectroscopy

The wet chemical process step for alkaline texturing of the wafer surface is crucial. NIRS spectroscopy allows real-time monitoring of bath concentrations of, e.g., hydroxide, silicate, and 2-propanol. The method is very reliable and fast and requires no consumables.

Online NIRS analyzers