External quantum efficiency is the best tool for creating high-efficiency solar cells

What is the quantum efficiency of solar cells? The definition as defined as how many electrons generated by the incident photons. It can help researchers to judge the quality of the solar cells at each or specific wavelength.

The quantum efficiency QE refers to the external quantum efficiency EQE, also known as the incident photon-electron conversion efficiency IPCE (Incident Photon-Electron Conversion Efficiency).

QE=EQE=IPCE

The external quantum efficiency EQE calculates the number of electrons produced by the total number of incident photons.

An example, suppose there are a total of 10 photons incident on the solar cell, 2 photons are reflected on the surface of the solar cell, and finally 6 charges are generated. Therefore, by definition, the external quantum efficiency of this solar cell is:

How to calculate the quantum efficiency? (The formula of quantum efficiency)

The conversion between spectral response and quantum efficiency can be written as the following formula:

Among them, P(λ) is the incident light energy of each wavelength, in Watts (Watt); I(λ) is the current converted by the solar cell after receiving the incident light, in amperes (Amp), q is electron quantity,  h is Plank Constant,  v is photon frequency, λ is the wavelength of incident photons (nm).

Spectral Response (SR) is an index to evaluate the photoelectric conversion capability of optical radiation detection devices (such as photodetectors, photometers, solar cells, etc.), that is, the efficiency of incident photon-electron conversion efficiency, IPCE.

As described above, the quantum efficiency of solar cells is the electrons generated by the incident photons, which is also called External Quantum Efficiency (EQE). Therefore, the formula of quantum efficiency is:

Figure 1. The conversion between spectral response (SR) and external quantum efficiency (EQE).

Why is quantum efficiency the best tool for creating high-efficiency solar cells?

Quantum efficiency/spectral response reflects the photoelectric conversion efficiency of solar cells at different wavelengths. The conversion efficiency of solar cells is affected by factors such as the material, manufacturing process, and structure of the cell itself, so that different wavelengths have different conversion efficiencies. Using spectral response/quantum efficiency measurement technology to detect and analyze the changes in the conversion efficiency of the solar cells under different conditions, we can analyze the pros/ cons of the process and find out the key factors related to improving efficiency.

The different wavelength ranges represent the structure and manufacturing process of different layers of solar cells. From the results of the spectral response, it is easy to analyze the pros and cons of solar cells in different manufacturing processes, which is a guideline for improving efficiency.

Quantum efficiency/ spectral response/ IPCE spectrum reflects the characteristics of each layer of the solar cell. Taking silicon solar cells as an example, interface reflection will occur at the incident interface. Generally, the loss caused by reflection in the UV and the infrared wavelength band is higher, and the loss in the visible wavelength range is the lowest.

In the 350 nm ~ 500 nm band, the spectral response curve increases as the wavelength increases. Because the penetration depth of long-wavelength photons is deeper, close to the pn junction, the conversion efficiency is improved. Generally, the most efficient part is in the band of the PN junction, because the internal electric field of the pn junction can efficiently disassemble the electron-hole pairs after absorbing photons. Therefore, the highest efficiency is in the 500-800 nm band, which reflects the characteristics of the pn junction layer. The 800~1100 nm wavelength range penetrates to the lowest p-layer. The external quantum efficiency of the single crystal silicon solar cell in Figure 2 can be used to observe the reaction characteristics of each layer.

Figure 2. Schematic diagram of the quantum efficiency spectrum of a silicon solar cell and the response of each wavelength. The illustration shows the component structure of a silicon solar cell.

Figure 3 shows the measured spectral responses A and B of the two silicon crystal cells with two different processes. From the spectral response results, it can be seen that the efficiency of cell A is higher, mainly because of the conversion in the 700~1100 nm band. The efficiency is higher than that of the B cell, and the short-circuit current contributed by it is 0.897 mA/cm2 higher than that of the B cell. But in 300~500 nm wavelength range, the efficiency of A is slightly lower than that of B cell, and the short-circuit current density is 0.675 mA/cm2 lower than that of B cell. Therefore, the overall short-circuit current density of cell A is still higher than cell B (0.897-0.675)=0.222 mA/cm2.

Figure 3. Schematic diagram of solar cell spectral response and AM1.5G under different manufacturing process conditions.

The following figure 4 can be obtained by converting the spectral response into quantum efficiency. The efficiency of A cell is lower than that of B cell at 300 nm ~ 500 nm. To further improve the efficiency of the A cell, it should focus on the process of anti-reflection layer (300 nm ~ 350 nm) and n layer (350 nm ~ 500 nm) as the directions of efficiency improvement.

Figure 4. Quantum efficiency spectra of two cells with different processes.

It is useful to use our product to perform the above test. The ELQ Series developed by INTINS has been gaining attention in the industry for its ability to provide detailed insights into solar cell performance. The ELQ (Electroluminescence and Quantum Efficiency) is a cutting-edge technology for measuring the efficiency and durability of solar cells. The ELQ Series combines electroluminescence imaging and quantum efficiency measurements to provide a comprehensive evaluation of solar cell performance. Above all, it has an affordable price point, a high accuracy and effective testing software, a user-friendly interface that makes the EQE measurement process easier.

Figure 5. The ELQ Series developed by INTINS

In summary, in today's fiercely competitive solar industry, it is important to continuously reduce costs and improve photovoltaic conversion efficiency. The key to improving solar cell conversion efficiency lies in improving manufacturing processes and materials. Measuring the quantum efficiency/spectral response/IPCE of solar cells can understand the photoelectric conversion efficiency of solar cells under different wavelengths of light. Users can quickly find problems in the process and improve them based on the spectral feedback results, which is more beneficial to improving efficiency.