In the full wavelength range of sunlight (300~2500nm), its energy in the visible range (400~760nm) accounts for 43%, the energy in the ultraviolet region (<400nm) accounts for 5%, and in the near-infrared region (>760nm) energy accounts for 52% of the total energy. At present, Spectrophotometer can fully meet the needs of collecting reflectance and transmittance data of ITO films in the full band of 190-2800nm, and is widely used in the photovoltaic industry. In this issue of Millennial Solar, we will explain to you the impact of the resistivity and transmittance of TCO films on the efficiency of SHJ cells.

TCO film

Low resistivity, high transmittance and low-temperature growth are the basic requirements for TCO films in SHJ cells. The high infrared reflectivity of traditional TCO films limits the effective utilization of long-wavelength solar energy in solar cells using it as a transparent electrode.

The resistivity of the TCO film is inversely proportional to the carrier concentration and mobility. By reducing the carrier concentration, the absorption of free carriers can be reduced, thereby increasing the light transmittance.

However, reducing the carrier concentration will cause the resistivity of the film to increase and affect the conductivity.

If the carrier mobility is increased while reducing the carrier concentration, on the one hand, the TCO film can still maintain good conductive properties, and on the other hand, due to the lower carrier concentration and reduced absorption, it can achieve from visible light to near infrared A wide range of high transmittance, which is conducive to improving the efficiency of solar cells.

Improve TCO carrier mobility

There are many ways to improve the carrier mobility of TCO films, including selecting the appropriate deposition method, post-deposition annealing heat treatment, controlling the crystal structure, controlling the impurity concentration, selecting the doping method, introducing hydrogen, and selecting the appropriate dopant. etc. ITO films are widely used in SHJ solar cells due to their advantages of low resistivity, high visible light transmittance, and low-temperature deposition (≤200°C). However, the carrier mobility of traditional ITO is relatively low, which is determined by scattering mechanisms such as ionized impurity scattering. By doping the In₂O₃ system with the following dopants, an In₂O₃-based TCO film with high mobility can be obtained:

■ Hydrogen-doped In₂O₃ (IO:H)

■ Tungsten-doped In₂O₃ (IWO)

Although these two In₂O₃-based TCO films have superior performance than ITO films, their deposition temperatures are generally higher than 250°C. If they are to be used in SHJ, further optimization is required.

Spectrophotometer

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Spectrophotometer can be used to measure the transmittance, reflectance and absorbance of materials.

Spectrophotometer adopts a unique dual-beam optical design, which can perfectly correct the absorbance changes of different sample matrices, thereby stably measuring samples. It has the advantages of wide testing range, high precision and good stability. It also supports the measurement of solar transmittance in a wide range of wavelengths from the ultraviolet region to the near-infrared region, providing strong support for solar cell efficiency analysis.

The low resistivity and high transmittance of TCO films can greatly improve the efficiency of solar cells. Spectrophotometer supports the measurement of reflectance and transmittance of ITO films in the ultraviolet to near-infrared range, helping you open up a new future for optical detection!