HJT solar cells have achieved passivation contact on the front and back surfaces, so they have obtained a higher open circuit voltage, which is significantly higher than TOPCon cells and PERC cells. However, there is a serious parasitic absorption in the amorphous silicon layer on the front surface, which makes HJT cells not dominant in short-circuit current. One of the ideas to solve this problem is to use microcrystalline silicon instead of amorphous silicon. From a process point of view, the formation of microcrystalline silicon requires changing the dilution rate of silane and hydrogen, RF power and deposition pressure to improve the crystallization rate of microcrystalline silicon film. Millennial Raman Spectrometer uses a 325nm laser to achieve in-situ testing of amorphous/microcrystalline materials above 5nm; with a spectrometer with high spectral resolution and extremely low stray light, the accuracy and repeatability of spectral data are guaranteed. According to user needs, the crystallization rate fitting software is developed to output the crystallization rate value efficiently and accurately.

According to the differences in crystal structure, silicon thin film materials can be divided into single crystal silicon thin film, polycrystalline silicon thin film and amorphous silicon thin film.

Schematic diagram of the structure of monocrystalline silicon, polycrystalline silicon and amorphous silicon

Amorphous silicon (a-Si:H) is a mature material that has been widely used in the development of large-area thin-film solar cells and thin-film transistors (TFTs) in the fields of photovoltaics and microelectronics. However, due to the short order of its atoms: this order is mainly related to the length and bond angle of the covalent bond, and the bond angle is only maintained between the nearest atoms. Therefore, a-Si:H has poor transport properties, low electron mobility, high dangling bond density, and degrades under light irradiation (Staebler-Wronski effect).

Polycrystalline silicon (poly-Si) has good transport properties, but the high deposition temperature (600°C) required limits its integration on glass and flexible substrates.

Microcrystalline silicon μc-Si:H film

Microcrystalline silicon μc-Si:H is a silicon-based film made by PECVD at low temperature (≤200℃). Unlike amorphous silicon and polycrystalline silicon, microcrystalline silicon μc-Si:H grows in crystal columns with different orientations, and the columns are separated by boundaries composed of amorphous silicon.

Microcrystalline silicon μc-Si:H has a band gap and absorption coefficient different from amorphous silicon, with higher conductivity, higher infrared absorptivity, and higher stability to solar radiation (light absorption). On the other hand, μc-Si:H is deposited at low temperature (200℃) and also shows high carrier mobility, high stability and high conductivity.

Typically, μc-Si:H films are made from a mixture of monosilane (SiH₄) and hydrogen (H₂), but can also be made from a mixture of monosilane (SiH₄), hydrogen (H₂) and argon (Ar). The main parameters for making μc-Si:H film are H₂ dilution, moderate RF power and high deposition pressure. By optimizing these parameters, the crystallization rate Xc can be increased and its performance characteristics can be optimized.

Effect of RF power on thickness, deposition rate, surface roughness and crystallization rate Xc of microcrystalline silicon films

By studying the μc-Si:H films deposited at 200°C in a PECVD reactor with different RF powers (20, 25, 30, 35, 40, 45W) for 30 minutes, we found that different RF powers had a certain effect on the film's thickness, deposition rate, surface roughness, crystallization rate Xc and other characteristics.

Data on the effect of different RF power on the thickness, deposition rate, surface roughness and crystallization rate Xc of μc-Si:H films

The deposition rate is calculated from the average thickness of the film and the deposition time. The following figure shows the deposition rate versus RF RF power. From the figure, it can be seen that the deposition rate decreases with increasing RF RF power.

Deposition rate of μc-Si:H thin films and relation to RF RF power

It is clear that the roughness of the films increases as the RF RF power is increased to deposit the films. According to the literature, larger roughness values in μc-Si:H films are associated with larger crystal size.

The following figure shows 3D images of μc-Si:H films deposited with different RF RF powers. From the measurements we extracted the mean roughness (Sa) and root mean square roughness (RMS) values.

3D images of μc-Si:H films deposited with different RF powers

Raman Analysis

Raman spectroscopy can be used to characterize μc-Si:H thin films. The following figure shows the Raman spectra of the deposited films at different RF power levels. A distinct peak at 520 cm^-1 in the spectrum can be found. The crystallization Xc of all the samples is in the range of 64-75% and the films deposited at 25 W RF power have the maximum crystallization Xc.

For the deposition of μc-Si:H, three conditions are required. During the deposition, the H₂ etching process occurs. The effect of higher RF power promotes the H₂ etching process, which affects the crystallinity Xc of the film.

Why measure Raman spectroscopy?

Raman spectroscopy is used to measure and evaluate the crystallization rate of silicon-based thin films. Raman is a light scattering technique. When high-intensity incident light from a laser source is scattered by a molecule, most of the scattered light has the same wavelength as the incident laser; this scattering is called Rayleigh scattering (elastic scattering). However, there is a very small portion (about 1/109) of the scattered light that has a wavelength different from the incident light, and whose wavelength changes are determined by the chemical structure of the test sample (the so-called scattering substance), and this portion of the scattered light is called Raman scattering (inelastic scattering).

For films prepared by different process parameters, Raman spectroscopy can understand the microstructure and passivation effect of silicon films, and determine the conductivity of the film by characterizing the crystallinity of the film to provide optimization direction for the preparation of high-quality films. Therefore, Raman spectroscopy characterization has become an important part of the research and development process of thin film solar cells.

Raman Spectrometer

E-mail: market@millennialsolar.com

Millennial Galaxy Solar Raman Spectrometer can be applied to test Raman spectra, the equipment has excellent UV sensitivity and excellent spectral repeatability, using 325nm laser, can realize in-situ testing of amorphous/microcrystalline materials above 5nm; with high spectral resolution, very low stray light spectrometer, to ensure that the spectral data of the accuracy and repeatability. The crystallization rate fitting software has been developed according to the user's needs, which can efficiently and accurately output the crystallization rate value.

● UV sensitivity silicon-order peak signal count is better than 1000

● 325nm laser, 1s integration time

● In-situ testing of amorphous/microcrystalline materials above 5nm

●One-button crystallization rate analysis software to dramatically improve test accuracy

For thin film solar cell applications, μc-Si:H has been extensively studied. Currently, a-Si:H/μc-Si:H tandem solar cells (microcrystalline solar cells) have been developed, with a stabilized efficiency of up to 12%. Raman Spectrometer has a powerful adaptation of the crystallization rate test function of the photovoltaic industry, supporting in-situ testing of the process wafer in order to adapt to the R & D and production of the photovoltaic industry, and to help manufacturers face difficulties more easily in the process of manufacturing high-efficiency solar cells. The Raman Spectrometer has powerful crystallization rate testing capabilities for the PV industry.